API RP 550 Part 1 (1965) - Manual on Installation of Refinery Instruments and Control Systems. Part I- P

- - March i965

MANUAL ON

I l I IINSTALLATION OF REFINERY INSTRUMENTS

AND CONTROL SYSTEMS

PART I-PROCESS INSTRUMENTATION AND CONTROL

I ~ I Y publication is distributed "as is" and is no longer a current publication of the American Petroleum Institute. It is furnished solely for historic purposes and some or all of the information may be outdated. API MAKES NO WARRANPI OF ANY KIND, EXPRESS OR IMPLIED, AND SPECIFICALLY THERE IS NO WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR ISSUE.

AMERICAN PETROLEUM INSTITUTE Division of Reíining

1271 Avenue of the Americas

New York, N.Y. 10020

Price $4.00

COPYRIGHT 2003; American Petroleum Institute

Document provided by IHS Licensee=Fluor Corporation/2110503105, User=, 06/26/2003 09:19:53 MDT Questions or comments about this message: please callthe Document Policy Management Group at 1-800-451-1584.

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March 1965

MANUAL ON

INSTALLATION OF REFINERY INSTRUMENTS AND CONTROL SYSTEMS


publication is distributed "lts is" and is no longer a current publication of the American Petroleum Institute. It is furnished solely for historic purposes and some or ail of the information may be outdated. API MAKES NO WARRANlY OF ANY K I N D , EXPRESS OR IMPLIED, AND SPECIFICALLY THERE IS NO WARRANTY OF ME3CHANTABIUTY OR FITNESS FOR A PARTICULAR ISSUE.

AMERICAN PETROLEUM INSTITUTE Division of Refining



Price $4.00



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API RP 550 Second Edition iMarch 1965

LIBFARY AMERICA4N PETISOLEUM IIYSIITUTE

MANUAL ON


PART 1-PROCESS INSTRUMENTATION AND CONTROL

Ris- publication is distributed "as is" and is=.r: longer a current publication of the American Petroleum Institute. It is furnished solely for historic purposes and some or all of the information may be outdated. API MAKES NO WARRANTY OF ANY KIND, E X P R E S S OR IMPLIED, AND SPECiFICAUY THERE IS NO WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR ISSUE.



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Copyright @ 1965 American Petroleum Institute



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FOREWORD

This manual is based on the accumulated knowledge and experience of en,' 'meers in the petroleum industry. Its purpose is to aid in the installation of the more generally used measuring. and control instruments and related accessories in order to achieve safe. continuous. accurate. and efficient operation with minimum maintenance. Although the information contained herein has been prepared primarily for petroleum refineries. much of it is applicable without change in chemical plants. gasoline plants. and similar installations.

This second edition of the manual. which is being published in two parts. represents the latest suggested or generally used practices in the installation of all the devices covered in the first edition. plus additional information based on revisions suggested by many individuals and several organizations. (The first edition of the manual was issued in 1960. )

Part í assays the installation of the more commonly used measuring and control instruments. as well as protective dcvices and related accessories. Part I l presents a detailed discussion of process stream analyzers. Thcsc discussions are supported by detailed information and il!ustrations to facilitate application of the recommendations.

The information contained in this publication does not constitute. and should not be construed to be. a code of rulcs ur regulations. Furthermore. it does not grant the right. by implication or othernise. for nianuf;icturc. sale. or use in connection wi th a n y method. apparatus. o r product covered by Icttcrs patent: nor does it ensure anyone against liability for infringcnicnt of Icttcrs patcnt.

Phers of this manual ;ire reminded that i n thc rapidly advancing held of instrumentation no publication of this type can bc complctc. nor can any written document be substituted for qualified engineering ;inaIysis.

Certain instruments :ire not cowred herein bccausc of their very specialized nature Lind limited use. When one of these deviccs (or classes of deviccs) gains gcnernl usage Lind installation rcachcs ;I fair degrce of standardization. this manual will be revised to incorporate such additional information.

Suggested revisions arc invited and should be submitted to the dircctor of the Division of Refining. American Petrolcum Institute, I27 I Avcnuc of the Americas. New York. N. Y. 10020.



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CONTENTS PAGE

Section I-Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Section 3-Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Section 3-Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Section 4-Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Section 5-Automatic Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Section 6-Control Valves and Positioners . . . . . . . . . . . . . . . . . . . . . . . 62 Section 7-Transmission Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Section S-Seals . Purges . and Winterizing . . . . . . . . . . . . . . . . . . . . 79 Section 9-Air Supply Systems . . . . . . . . . . . . . . . . . . . . . . . . . 86 Section 1 O-Hydraulic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Section I i-Electrical Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Section 13-Instrument Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Section 13-Alarms and Protective Devices . . . . . . . . 113 Outline of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119



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COPY PROVIDEO FOR HISTORICAL PURPOSES MLY

INTRODUCTION

Successful instrumentation depends upon a workable arrangement which incor- porates the simplest systems and devices that will satisfy specified requirements. Sufficient schedules. drawinss. sketches. and other data should be provided to enable the constructor to install the equipment in the desired manner. The various industry codes and standards. and laws and rulinps of regulating bodies should be followed where applicable.

For maximum plant personnel safety, it is recommended that transmission systems be employed to climinats the piping of hydrocarbons. acids. and other hazardous or noxious materials to instruments in control rooms.

In the installation o£ an instrument, the various components must be accessible for efficient maintenance and certain of these elements must be readable €or good operation. Orifices. control valves. transmitters. thermocouples. level gages. and local controllers. us well as analyzer sample points. generally should be readily accessible from grade. permanent platforms, or fixed ladders. In this manual. special consideration is given to the location, accessibility, and reaùability of the elements.

Proper installation is cssential in order to utilize the full capabilities which are built into the instrument systems and to realize the greatest return on investment. In many instances. the instrument difficulties encountered h.ave been traced to incorrect installation.



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PAART I-PROCESS INSTRUMENTATIt IN AND CONTROL

SECTION

1.1 CONTENT Recommended practices for the installation of dif-

ferential pressure instruments, area flowmeters, and other commonly used flowmeters for indicating. recording, transmitting, and controlling Huid flow are presented in this section. Other types of flow instruments, not as widely used and not covered herein, are:

1. Positive displacement meters. 2. Sight flow indicators. 3 . Weirs or head area meters (seldom used in re-

finery services except for waste water disposal, sewage, etc.). 1. 5 . 6. 7 . 8. 9.

10.

Sonic or ultrasonic flowmeters. Thermal-type Rowmeters. Solids How devices. Self-actuating flow regulators; see Sect. 6. Mass flowmeters. Metering pumps. Kinetic manometers.

These devices are used only where special rlow problems are encountered and should be installed in accordance with the manufacturer’s instructions or in accordance with a specially engineered installation which meets specific requirements.

1.2 GENERAL a. Differential Pressure Instruments

The differential head type of instrument measures flow inferentially from the differential pressure caused by flow past a primary element which generally is one of the following types: I . Orifice: Usually the thin plate concentric orifice, but may be eccentric, segmentai, or of some other special form depending upon application. 7. Flow 17Oz:ie: Used in installations where higher velocity and moderately better pressure recovery are required than are available with an orifice plate. Flow nozzles are better suited for gas service than for liquid service. 3. Venturi tribe: Used in installations where high capacity and good pressure recovery are required, or where the measured stream contains a constant per- centage o€ solids. 4 . Flow tirhe: Used in installations where low pressure loss is a major consideration, or where piping configurations are restrictive. 5. Pitot tube: Generally used in installations where no appreciable pressure drop can be tolerated on high-

1-FLOW

volume flows, such as on cooling water. These devices measure velocity. The accuracy of the rate of flow depends upon the determination of the average velocity from the velocity distribution. 6. Elbow raps: Used in installations where the velocity is sufficient and where high accuracy is not required.13 However. its repeatability is good. A water velocity of 17 fps will produce a water differential of approximately 100 in. Some test data are available from the University of Illinois.’

The differential pressure across the primary elements described herein usually is measured by one of the following devices : 1. Manometer. 2. Mechanical mercury meter. 3. Bellows meter. 4. Diaphragm transmitter.

I). Area Flowiiieters

most commonly used meters in the area class. For refinery service, rotameters are probably the

e. Turhiiie or Propeller Flowiiieters

caused by flow past a turbine or propeller. Turbine meters measure flow from the rotation

(l. Velocity or Target Flowiiieters

Velocity or target meters measure flow inferentially from the force imposed on a target suspended in the flow path.

e. Electromagnetic Flowmeters

The average flow velocity is measured inferentially from a voltage which is generated by the measured fluid moving through a uniform magnetic field. The flowing fuid must have some degree of electrical conductivity.

1.3 DIFFERENTIAL PRIMARY ELEMENTS

u. Thin Plate Orifices I . CONCENTRIC ORIFICE PLATES

The sharp-edge concentric orifice plate is the most frequcntly used primary clement because of lower cost. Hexibiiity, and availability of accurate coefficients,

I’ Figures refer t o KEFEKENCES on p. 30.

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For most services, orifice plates are made of corrosion-resistant materials, usually Type 304, 316, or 430 stainless steel. Other materials sometimes are required for special services.

The upstream face of the orifice plate should be as flat as can be obtained commercially. Any plate which does not depart from flatness along any diameter by more

than 0.010 in. per in. of dam height, -- , may be

considered flat. The upstream face of the orifice plate must be smooth and have a finish at least equivalent to that obtained in commercial cold-finished sheet stock.

The thickness of the orifice plate at the orifice edge should not exceed (minimum requirements governing in all cases):

D - d * 2

D D - - or preferably -- - (one-fiftieth of pipe diameter) 30 30 d - (one-eighth of orifice diameter) s D-d

s (one-fourth of dam height)

In some cases, the thickness of the orifice plate will be greater than permitted by the limitations for the thickness of the orifice edge! in which case the downstream edge shall be counterbored or beveled at an angle of 45 deg or less to the required thickness at the orifice edge. The word Upstream or Inlet should be stamped on the orifice tab on the square-edge side of the plate. Dimensions for orifice plates are shown in

Bores must be round and Concentric. Practical tolerances for orifice diameters, as liven in AGA Report No. 3.:: are shown in Table 1-1.

The upstream e d y of an orifice should be square and sharp. I t is usuaily considercd sharp if thc reHec- tion of a beam of light from its edge cannot be seen without niagnification. The edge radius should not

Fig. 1-1.

.

TAULE 1-1-Prticiical 'l'olwances for Orifice Diameters

Orifice Size (Inches) 0.2500 . . . . . . 0.3750 . . . . . . . . . 0.5000 . . . . . . . . . . . . . . 0.6250 . . . . . . . . . . . . 0.7500 . . . . . . . . . . . . . . 0.8750 . . . . . . . . . . . . 1 .o000 . . . . . . . . . . . . 1.7500 . . . . . . . . . . 1.5000 . . . . . . 1 .7 5 O 0 . . . . . . . .

Over 5.0000 . . . . . . . ;1.oooo [O 5.0000 . . . . . . . .

- .

:: D = insicle diameter OC pipe. d = orifice diameter.

Tolerance Plus or Minris

(Inches) . . . . . 0.0003

. . . . . . 0.0005

. . . . . . . 0.0006

. . . . . . . 0.0008

. . . . . . . 0.0009

. . . . . . . 0.0010

. . . . . . . 0.0012

. . . . . . 0.0011 . . . . 0.0017

. . . . . . 0.0020 . . . . . . 0.0075 . . . . . . . 0.0005 per in. of

diameter

exceed 0.0004 times the bore diameter. It should be maintained in this condition at all times. For two- way flow both edges should be square. Orifice plate details and schedule of thicknesses are shown in Fig. 1-1. Detailed tolerances are discussed in AGA and ASME *. .i publications.

In wet-gas or wet-steam services, where the volume of condensate is small, a weep hole flush with the bottom of the orifice run may be used to prevent a buildup of condensate in horizontal lines. The weep hole serves as a drain to prevent freeze-up during shutdown pen- ods. A weep hole flush with the top of the pipe also can be used to pass small quantities of gas in liquid streams. A %-in. weep hole with a 1.5-in. orifice diameter will give m approximate error of 1 percent.

Because test information is more readily available for thin plate orifices than for other primary devices, it is possible to design orifice installations to good accuracies. However, field installations are not always designed to obtain the best accuracy. In installations which are used only for control purposes some higher order of inaccuracy is acceptable than is required in installations which are used for accounting, material balance, or buying and selling. Common errors are those which result from improper tap location, round edges. viscosity variations. and lead-line head differences. Orifice plates should be kept clean and free from accumulations of extraneous material.

2. d/D ( p ) RATIO Orifice diameters should be selected so that the ratio

of orifice diameter to actual internal pipe diameter, d/D. does not exceed 0.75 for liquids and 0.70 for gas or steam and preferably is not less than 0.20. A d / D ratio between 0.4 and 0.6 is considered best. Because of the danger of plugging by pipe scale and other foreign material, the minimum orifice bore should not be smaller than 0.5 in. diameter on all but the cleanest services.

3. OTHER ORIFICE PLATES Eccentric or segmental orifices may be used in hori-

zontal runs €or special services where concentric orifices cannot be used. Eccentric orifices are useful for mixed- phase gas-liquid services. Segmental orifices are recommended for slurry services. because of the low cost. in- sensitivity to changes in liquid-solids ratio. and relatively satisfactory accuracy (approximately 2.3 percent for plate calculations) ."

The eccentric orifice usually is placed n i t h its edge tangent to a circle of a diameter 0.9s of that of the pipc. The point of tangency is at the top vertical center line for liquids containing some vapor, and at the bottom vcrticni center line for vapors containing some liquids. Cocficients also are available for ecccntric orifces at 90 or 180 deg from the point of tangency. Eccentric and segmental orifice plates arc shown in Fig. 1-2. The segmental orifice usually is constructed with a circle

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FLOW

kW-; THIS INFORMAT!ON SHOULD

>BE SIDE OF THE ?LATE STAMPED ON UPSTREAM

SHARP

COUNTERBORE ON DOWNSTREAM CORNER WHEN t = T OPTIONAL b'8"HOLE IN ORIFICE

L1NES:BOTTOM OF LINE FORGAS OR HYDROCARBON LIQUIDS CONTAINING TRACES OF WATER.TOP OF LINE COR LIQUIDS CONTAININGTRACES OF VAPOR OR NONCONDENSABLES

"LATES INSTALLED IN HORIZONTAL SECTION A-A

Material: Type 316 stainless steel or other suitable material (All iMeasurements in Inches)

Outside Diameter Tab

L W 1 ' 2 4 "4 4 4 4 "4 6 1 6 1 6 1 6 I 6 I 6 1 6 1 6 1 6 1 6 1

., ,

' "4 0 ,

. , I

Notes: 1. ?lie outside diameter (OD) of the orifice plate is that required to fit inside the bolts of standard ASA flanges. The outside

2. Sizes 1 in., 1:- in., and 272, in., should be avoided. diameter is equal to the diameter of the bolt circle less nominal diameter of bolt +O in. -I::. in.

FIG. 1-l-Concentric Orifice Plate.

diameter (D) between 0.97 and 0.98 pipe ID and generally is used in services which require that it be placed at the bottom of the line. For best accuracy, the tap location should be 180 deg from the center of tangency. However, to avoid gas bubbles in the taps, the location may be anywhere within the sector shown in Fig. 1-2.

The quadrant-edge (or quarter-circle) orifice is a device in which the upstream edge is rounded to form a quarter-circle. The thickness of the plate near the orifice is equal to the radius of the quarter-circle.

The quadrant-edge orifice is attractive for the flow measurement of viscous streams because of its relatively constant coefficient over a wide range of low Reynolds numbers. It is of special value where the viscosity is high and also variablc. ( In contrast, the square-edge orifice coeficicnts show increasing dependence on orifice Reynolds numbers. R,,.* bclow 100.000. Squarc-edge

i:: In some data I< , , . the Keynolds number for the pipe, is given: in other dat:i Ki. the Reynolds number for the restriction, is used. The distinction between these two numbers is not always ciearly set forrh: RI, = p k , .

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API RP 5 5 0 - P ~ ~ ~ r 1

,TAPS MAY BE ?-' / LOCATED WITHIN L ' THIS SECTOR

PIPE IO-( /

INTERSECTS CHORD

CENTER OF TANGENCY- ~ OD

--,o.se 2 AND ARC OF ORIFICE-' /ic 0 -

. I 2

ECCENTRIC SEGMENTAL

FIG. I-2-Ewciiiric :<t i t1 Sc~giiit~rit;il Or¡ fice i 'l:~tt**.

orifice coefficient correction factors ;ire aviiilable for R,, down to approximately 35.000.)

The quadrant-edge orifice niay be used when the line Reynolds numbers ( RI, ) range from 100.000 or more down to 3.000 to S.000 (depending upon the p ratio) with ;i coclficient accuracy of about 0.5 percent. When R I , is bclow thc 3.000 to 5.000 range, the coeîhcient curve shows ;I hump. and the longer the upstream run. the higher the hump. This hump may bc suppressed. even at values of RI-, bclow 1.000. by taking the How straight o u t of a vessel nozzle through a meter run of only a few pipc diamcters ahcad of the orificc, or by using a scrcen. such ;is il muhiplate Ilow straightener. a few diamcters upstrcani.7 I ' '

Readings for flows which cxcccd thc maximum Rey- nolds number limits may be appreciably in error. The machining of quadrant orifice plates should be of high quality becausc the dimensions of thc cdge and its shape and smoothness arc most important.

Other special orifice forms also have been designed; for a detailcd discussion see the current literature.'

4. SIZING O R I F I C B S Usually, orificcs arc sizcd for a I 00-in. water column.

dry calibration, maxiniuni diîfcrcntial for liquids. This allows an increase or decrease in meter range for different rates of flow without changing the orifice plate. For gas or stcam flow a good rule of thumb. even if a range of less than 100 in. is required. is that thc meter range. in inchcs of water, should not exceed thc flowing pressure. in pounds pcr square inch absolute.

The procedures for computing orifice sizes and flow through orifices are given in various publications.'. ~ : - 2 . l 2 Special slide rules are availablc for orifice computations.l These slide rules are especially valurihie for checking longhand or computer computz\- tions and for prcliiiiinary orifice sizing. Orifice caicula- tions can be purchased from the mmufacturers o f orifice platcs o r tlownieters. Occasionally. only approximate physical propertics of the flowing fluid arc known bcforc startup; in such cases, a flow siidc rule niay be used to dctcrniine orifice size. Computations can be inadc a t a

latcr period using actual flow conditions, or corrections can be applicd to preliminary computations made with the approximate values.

I). Flow Nozzles

Flow nozzles are used less frequently than orifice plates. Their principal advantages are better pressure recovery and approximately 65 percent higher flow capacity for a given diameter than can be obtained under the same conditions with orifice plates. Flow nozzles are better suited for gas (vapor or steam) service than for liquid service. They should only be considered €or liquid service when f i o ~ must be increased in an existing installation beyond the limits for a normal orifice installation. If a How nozzle is required for liquid in a new installation the size of the line should be ques- tioned. Flow nozzles may be used in light slurry service if the How is downward in ;i vertical run. However, accuracy is poor below 6-fps line velocity. In general, the meter run requirements. Hange ratings, and tap requirements are the same as for orifice installations. However. because the d,/D ratio for the same flow and line size is smaller, a shorter meter run may be used where the run length is based on the minimum run for the actual d / D ratio. A typical flow nozzle is shown in Fig. 1-3. There are several forms of flow nozzles. one of thc most common being the ASME long-radius form.'. -, Properly installed How nozzles are equal in accuracy to sharp-edge orifices. Calculation procedures and Coefficients for How nozzles have been published in the literature.', ', ',

c. Venturi Tiilws aiid Flow Tubes

Venturi tubes and flow tubes are infrequently used in refinery operations. Head loss for these devices is lower than for other constricting primary elements; venturi and flow tubes should be considered for all applications where minimizing head loss is an important factor. These primary devices are more costly than orifice or flow nozzle installations, the long-form venturi being the most expensive. The venturi tube and flow tubes are shown in Fig. 1-3.

1. VENTURI TUBES

Venturi tubes give a much lower head loss than orifices or flow nozzles. For a long-form venturi tube. the approximate head loss will be between 10 and 14 percent of measured differential. dependent upon the d/D ratio. Minimum runs usually are shorter than for orifice plates or flow nozzles. As a rule. the manufacturer of the venturi tube can supply the minimum length meter run data (which varies with pipins con- iìguration) .

Although coefficients are available for the calculation of flow through vcnturi tubes,'. i. -, thc nianiifac- turer may specify the flow for LI given diffcrential. Ven-

l o



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FLOW

+.-- t--+-t

%!!!i- FLOW NOZZLE

LO-LOSS TUBE

U U

GENTILE TUBE DALL TUBE

FIG. 1-3-Flow Nozzle. Venturi Tube. ;id Flow Tube$.

turi tubes give an accuracy equal to that of a thin plate orifice. Venturi tube flow coefficients are relatively stable over a wider range of Reynolds numbers than sharp-edge orifices or now nozzles. When properly purged. they are suitable for metering streams which contain solids! provided the solids-liquid ratio remains constant. An increase in the solids-liquid ratio will cause a higher reading, and vice versa. The cost of venturi tubes is high and they are not widely manufactured of steel; however, steel or alloy venturi inserts for line installations are available in some sizes at lower cost.

2. DALL TUBES The Dall tube is a primary flowmetering device

which has been used in England for some time. It is now available as a fabricated line insert. The Dall tube is approximately two diameters long. The static pressure tap is in a line-size section which is followed by ;i sharp shoulder and a steep conical entrance to a short cylindrical section which has an annular slot, followcd by a 15-deg conical discharge throat tcrmi- nating with a shouldcr.

Examination of the Dall tube gives the impression that a Huid flow through it would be subject to a very high head loss. Actually, the Dall tube head loss is only about to 6 percent of the measured differential ;is

compared to 10 to 14 percent for the same flow in a long-form venturi. The coetficient may vary for line Reynolds numbers below 500.000. Rounding of the sharp edges will cause slight variations in the coefficients.

Unless it is purged. the Dall tube should not be used for slurries- or fluids which contain suspended solids because the annular throat slot is subject to plugging. The minimum meter run is longer than that required for a venturi tube.

3. GENTILE TUBES The Gentile tube has impact- and suction-type

piezometer openings to increase the measured differential. The outstanding characteristics are that it gives a good differential with a relativsly small amount of constriction and is short (approximately one and one- half diameters) : its coefficients are the same for now in either direction; and the cost is less than for a venturi tube. However. it is very susceptible to line roughness. It is claimed that the Gentile tube can be used with a shorter meter run than a venturi or a Dall tube. The coefficients are said to be relatively constant for line Reynolds numbers from 100.000 to 800.000. Until sufficient data have been accumulated on the effect of manufacturing tolerances and upstream piping configurations on its accuracy. a Gentile tube should be calibrated for any application ivhere high accuracy is important.'

4. Lo-Loss TUBES The lo-loss tube I ' has a pressure recovery compara-

ble to that of the Dall tube. Also. properly purged. it is quite suitable for handling fluids with suspended solids.

CI. Pitot Tubes aiid Pitot Ventiiris

Pitot tubes and pitot venturis are used where the pressure drop os power loss through other devices cannot be tolerated and where accuracy is not of prime concern. These devices frequently are used for measurement of high air and water Row rates. Pitot venturis are useful in applications where an ordinary pitot tube does not give satisfactory differential. Pitot venturis should not be used at greater than 9 fps in liquid service if dissolved gases are present: higher velocities cause cavitation. and gas bubbles collect in the meter connecting lines. For good measurement a traverse is required unless there is sufficient straight upstream run to obtain a uniform velocity profile. Fig. 1-4 shows pitot tubes and a pitot venturi.

Proper design will permit the installation or removal of pitot tubcs and pitot venturis from lines which are in servicc. They lire not suitable for use in hot oil or other hazardous service except in fixed installations designed to be leakproof. The relative cost of pitot tubes and pitot venturis with respect to othcr primary elements decreases as the line size increases. A typical pitot tubc installation is shown in Fig. 1-5.

i

!

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-~

L O W - PRESSURE CONNECTIOPI-

HIGH-PRESSURE CONNECTION

RESSLJRE ECTION

GATE VALVE

IMPACT

FLOW c--

STATIC c__

PITOT TUBES

VENT L'ALJES ---

iEi,lT DOT -- -

HIGH- PRESSURE (IMPACT) 8 CONNECTION.

-Lc-i FLOW

I

T Y P I C A L PITOT V E N T U R I

FI(;. 1 4 P i t o t Tubes aiitl Pitot Vc-iitiiri.

h TO DIFFERENTIAL ri INCTRUFAENT

-IMPACT

- R E D U C E D ?RESSURE

e. Metering Runs

1. ORIFICE TAPS Orifice taps may be of several types, as shown in

Fig. 1-6. Flange taps usually are preferred for refinery use. Vena contracta taps and pipe taps sometimes are used; however, vena contracta taps cannot be used with some sizes and pressure ratings of welding-neck flanges because one or both taps may fall in an undesirable location in the flange hub or weld.

Radius or throat taps (those located one pipe ID upstream and one-half pipe ID downstream) can be used. The downstream tap for the radius or throat tap sometimes falls in. or partially in, the flange hub.

Corner taps are being used by some refiners, particularly on small lines, where flange taps may be at the wrong location in the pressure profile. Some data are now available on corner tap installations.'" One type of corner tap orifice flange arrangement is shown in Fig. 1-7.

Pipe taps or full-flow taps (those located two and one-half diameters upstream and eight diameters downstream) measure the permanent pressure loss: therefore, they require a higher permanent pressure drop for a givcn rnetcr differential than flange, vena contracta, radius, or corner taps. Pipe taps may be used to meas- urc a higher flow rate than can be measured by flange



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FLOW

FLOW - FLANGE TAPS

VENA CONTRACTA TAPS

w ?IPE TAPS

ß or d /D . . . 0 . 3 0.3 0.4 0.5 0.6 0.7 Distance from orifice

plaie ( in ID’S) 0.84 0.80 0.74 0.65 0.56 0.45

Tolerance is approximately 20.1 D.

Pipe Taps. FIG. 1-&Flange Taps. Vena Contracta T a p . aiid

taps. and the like. in u given line size for a given differential.

Orifice flanges, with flange taps as shown in Fig. 1-7, generally are supplied with %-in. taps and with a minimum 300-psi ASA raised face rating. These flanges have a minimum thickness of 1 ? h in. which, in the smaller sizes, is thicker than the standard 300-psi flange. Each tap should be positioned 1 in. from the nearest face of the orifice plate. It is important to allow for compressed gasket thickness.

The American Gas Association has published curves on allowable variations in pressure tap hole location versus ß ratio. It is recommended that the tolerances for p ratio of 0.70 be used. For pipes smaller than 4 in., the tolerance is 0.025 in.; this tolerance increases to 0.065 in. at ß of 0.40 or smaller. For pipes 4 in. or larger. the tolerance is 0.05 in.: this increases to 0.125 at ß of 0.40 or smaller.

Some refiners use 35 -in. taps as a standard, others only in hot or corrosive services. In such cases, minimum flange thickness should be 1% in. Where piping speci- tications exclude threaded joints in primary piping, socket. or fillet weld. taps may be used with socket weld block valves. If secondary piping may be screwed. the block valves may be socket weld on one end and threaded on the other. Screwed taps may be seal- welded: however. some contend that this gives none of the desirable characteristics of either screwed or welded joints. i t is recommended that nipples to the first block valves be at least Schedule 160. Between adjacent lines sufficient space should be provided for orifice taps, block valves. and connecting piping. Consideration should be .-

,, ,--7 I ” t na - i/z”OR 3/4” NPT- PRESSURE TAPS

b’2”DRILL FOR 4” OR LARGER 3/g”DRILL FOR 3” OR 31/*’’ I/‘”DRILL FOR 2v2” OR SMALLER

THICK SHEET GASKETS OR 0.135” THICK SPIRAL WOUND G A S K E T S d ’

FLANGE TAP RAISED FACE ORIFICE FLANGES (WELDING NECK) (SLIP ON)

Nure: To provide adequate clearance in l%-in. and smaller pipe sizes, pipe flange-clearance to plate is then compressed gasket thickness.

FIG. 1-7-Orifice Flanges.

13

0.2D OR LESS

CORNER TAP FLANGES

end is often made flush with face of raised face



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

~~

API R P 550-PART I

given to room required for rodding or drilling out taps. Special orifice plate holding. fittings are available

which facilitate orifice plate changing. Some of these devices permit changing the orifice plates while the line is under pressure; these types require regular lubrica- tion and maintenance.

2 . MINIMUM LENGTH OF METER RUNS Meter runs l 7 should be designed with not less than a

minimum length * of straight pipe preceding and following the orifice (see Fig. 1-8 through 1-13). It should be noted that these charts show minimum lengths of run; these runs should be increased. if practicable.'. ;i. ll.

Where pipe taps are used. the upstream run should be increased by two pipe diameters and the downstream run increased by eight pipe diameters. I t is recommended that the meter run lengths shown in Fig. L-S through 1-1 3. based on a d / D ratio of 0.70. be used

::: This length is tistially given in nominal pipe diameters. _ _ _ _

Diameter Ratio ( d / D I IV0 t 1,.v:

I . These curves should be used for tees. crosses. or "Y's" wh ich arc >ingle-cntrance littings with the other openings closed.

7. When a tee. cross. or "Y" i s used for multiple-inlet. or inlet :incl outlet. see Fig. 1 - 1 i. curves A ancl 1% should be used.

3. Tees. crosses. and "Y's" should be considered as disturbing fittings rcgnrdless of which connections are used.

J. Straight pipe X should he at least six pipe diameters when preceded by other fittings in the same plane. Otherwise the di5t;iiiccs given in Fig. 1-11 \hotilcl be t i~ed.

2. When X is preccclccl by Iittings in ;I different plane. Fig. 1 - 1 7 should be tised.

h. Whcn S is preceded by a valve covercd by Fig. 1-10, the minimiini total dimension X+A should he equal to dimension A in Fig. 1-10.

FI(;. 1-8-!Miniiiiiiiii Ixiigth Meter Riiiia. Singilt. Fittings 1 Jp>irc.:iiii.

TRAIGHTENING VANES

Diameter Ratio (d/Ol

.Vorc.s: 1 . Thebe curves should be used only where gate valves or

cocks are to be wide open. For partially closed valves use Fig. 1-10,

7. Where straight pipe 9 preceding the gate valves or cocks i s preceded by other fittings or throttling valves, 1+.4 should be equal to dimension A taken from the curve appropriate to the fitting or valve.

3 . Straightcning vanes will not reduce the lengths of straight pipe <\. and >hould he used only because of other fittings preceding the open gate valves or cocks, if the conditions determined bv Note 7 cannot be met.

4. I f straightening vanes are required. they mav be installed citlicr downstre:im of the gute or cock using curve A' or iip- stream using curve A. Vanes located upstream shall be distance A ' - C Lahen from the curve appropriate for the preceding -fitting.

Fi(;. 1-9-Opeii Gaie Valves and Cocks.

whcrevcr practicable. even though the actual d/'D ratio is smaller. A brief schedule for meter run requirements, based on 0.70 and 0.75 d/D, is shown in Fig. 1-74. I f t'or other reasons it is necessary to use runs designed for less than 0.70 d/D. special consideration should be given to the possibility of a future increase in d / D requirements. Straightening vanes should be avoided because of the possibility of their loosening and working downstream. The magnitude of error caused by insufi- cient upstream meter run with a ,B ratio of 0.37 is shown in Fig. 7-15. which is representative of a family of curves for other ,ß ratios. The curves indicate the increase in error caused by encroachment on upstream meter run length.

3. ORIENTATION OF M E T E R RUNS Horizontal orifice runs avoid the head error which is

caused by taps being located at different levels. Vertical

14



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW - . -. -

........... . . . . . . . -. . . . . . C/ .- ...... __ . . . . . .

5------ --s .... . . . . . . . . . . . .

- __ - . . . . . .

. . . . . . . . .

. . . . __ .

25 ! . . . Ø

. . . . . . . . . . . . . . . . . . . . . .

I . . . . . . . . . . . . . ....... . . . I

__ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . . . . . . . . . . . . . . . . . . . . . . . . .

O 2 3 4 5 6 7 8

Diameter R.+t,n idlDL

Note: These curves apply to control, check, or globe valves and to cocks and gates which are used for throttling.

FIG. l-lO-C<mtiol, Check. or Glolw Valve>.

20 r ! I ' ! I I ,

Diameter Ratio ( d / D l

Noies: 1. Where the straight pipe S is preceded by other fittings or

throttling valves, S+;i shall be at least equal to A taken from the curve appropriate to the fittings or valves.

7 . Straightening vanes will not redoce lengths of straight pipe A and should be used only because of required lengths which cannot be met under Note 1.

3. If required. straightening vanes may be installed downstream of the reducer using curve A' or upstream using curve .-i. The straight run of pipe preceding the straightening vanes or swage. whichever is upstream. shall be A' - C taken from the curve appropriate for the preceding fitting.

FIG. 1-1 1-Reducers or Inerenrers Upstream.

15



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

ORIFICE-' ELLS OR LONG RADIUS BENDS

_--- ----??-i ORIFICE-' ORIFICE-'

STRAIGHTENING VANES

, , ! . . . Ì , , ! 25

. - .. . . . . . . . . . . . . . . . # i

__ . . . . . . . ~- ~ . . . . .

- . - . . . - 20

!..L. , .

-.

. . . - .

$, I5 I 1 i

. . . . . . . . . . . . . . . . . . . . . . . . .

.. J m

"l

IO

I .... ................. . . 1 . . _. . . .~ .. ~ ... . . . .

__ -. . . . . . . . . . . . . :-. 1 . . . . . . . . . . .. .

~ -7- l ~* , , .. , .& _i-. .- I ,

.2 3 4 .5 .6 7 .8

Diameter Ratio f d / D I

Notes: 1. Where straight pipe X is preceded by fittings in a dif-

ferent plane use Fig. 1-13. 7. Where X is preceded by a valve covered in Fig. 1-10, the

minimum total length X+A shall be equal to A taken from the curve in Fig. 1-10.

FIG. I-lZ-Miiltil>lo Fittitig? i n Saina: I'laiìe.

F CE 1'

ELLS OR LONG X- RAD'US BENDS

i -A2-04

?- IO

)ELLS OR LONG M/N n / RADIUS BENDS

'TEE,CROSS.OR LATERAL 'STRAIGHTENING VANES

40

35

30 - 2 E 25 0

20 0. .. 5 15 2 "l

IO

5

O .I 2 .3 .4 .5 6 7 .8

Diameter Ratio i d / D )

Notes: 1. Tees, crosses, or ''Y's'' with multiple inlets and outlets

should be considered as disturbing fittings regardless of arrangement of entrances and exits.

2. Where straight run X preceding the fittings is preceded by a valve covered bv Fig. 1-10. X+A should be at least equal to A from Fig. 1-10.

FIG. 1-1 S-3Iultiple Fittings i n Different Planes.

16



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

COCK OR GATE

OPEN 0.3 100.8 ;&

L--2 I

* Suggested minimum distance; test data not available. Notes: i . Lengths are for :I maximum ratio of 0.70 and. in paren-

theses. 0.75. For minimum requirements for other p ratios and other configurations see Fig. 1-8 through 1-13.

2. Where vena contracta or pipe taps are used, the lengths as shown shall be taken from their respective differential taps and not from the orifice plate.

3. Where two flow disturbances exist and the length between them exceeds one-third the lengths herein for that type of combined disturbance. classify as a single disturbance and use length of straight run for the second disturbance.

4 . Where more than two disturbances exist and the distances between them are less than as given herein for two flow disturbances. the length of straight run shall not be less than 31 pipe diamerers.

5 . Where two or more disturbances exist. the distance between anv disturbance and the flow-measuring connection shall not be less than as illustrated herein.

FIG. 1-1 &Straight Runs of Pipe for Flow*-Measuring Installations.

orifice runs are often preferred for gas or stream flows which contain appreciable amounts of condensate. and for liuuids which contain vapor. If vertical runs are used. flow should be downward for wet gases or steam and upward for volatile liquids. The potential error in vertical lines can be minimized by proper manifolding. as shown in Fig. 1-1 6 and 1-2 1. or by the use of seals or purges. For steam. the condensate pots must be at the same level as shown in Fig. 1-17. Slurry should flow downward through a vertical line if a flow nozzle or a venturi tube is used as the primary element.

4. MlNJbív>l DIAMETER OF METERING RUNS Metering runs for orifices should be ?-in. diameter

nominal pipe size or larger. In lines smaller than 2 in.? it is advisable to swage the line up to the ?-in. size for the metering run. or to use rotameters, calibrated meter runs. or other special devices. Errors caused by the roughness of pipe wails become more pronounced in smaller sized orifice runs. Small-size orifices are subject to plu9ging in a11 but the cleanest service.

5 . S,i'.\.ric PRESSURE A N D TEMPERATURE MEASURE-

It is recommcndcd. when metering gases, that a static prcssrire tap be installed in the main line near the

A I E N T LOCATIONS

~ ~

O IO 20 311 JO 50 ô0 PIPE DIAMETERS FROM ORIFICE Ï 3 DISTURBANCE

i-Double-plane ell. $ 7-Single-plane ell. $. 3-Regulator upstream. 4-Gate valve upstream.

Note: These curves were derived from tests run on a 0.37 ,9 orifice.

FIG. 1-15-Error~ Caiiaed 1~ Inriiíñ<.ieiit Cpatreai i i Meter Runs (Typical Effect of Distiirbance Without Vanes).

primary measuring device. Either an upstream or a downstream pressure tap can be used. but the appropriate expansion factor must be used for the tap which i s selected. The downstream tap is recommended because a given change in differential pressure causes less variation in the value of the expansion factor based on downstream pressure than in the value of the expansion factor based on upstream pressure.l However, the upstream tap may be used if variations in the expansion factor arc to be neglected. although tap location is sometimes specified by regulatory agencies in some custody transfer installations. As a point of caution. neither the upstream nor the downstream tap of flange taps nor the downstream tap of vena contracta taps gives a true measurement of line pressure. Measurement of the static pressure is required in order to correct the apparent reading to the. actual flow.

It may also be desirable to measure the temperature of the flowing fluid, especially gas, to make required corrections in the apparent flow value. Generally, temperature is measured on the side of the orifice where the static pressure is measured. Thermowells, if used, should be inserted in the line a sufficient distance from the primary element to prevent flow disturbances from affecting the measurement. On the upstream side, tiierniowclls should precede the orifice by at least 20 pipe diamctcrs. If straightening vanes are used, thermowells should be placed at least 10 pipe diameters upstream of the inlet edge of the vanes. Downstream therniowells should not be located closcr than distance B i n Fig. 1-8 throupli 1-1 3.

17



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 5 5 0 - P ~ ~ - r i

I r_ I I

, ! , , \ ' I

<,'\',,--: < i ,,'

' - 2 ; A '-,i, B , . / , \ I,

INSTALLATION -OR HORIZON 1 4 1 AND VERTICAL LINES WhERE PRESSURE CONNECTIONS ARE TAKEN AT TUE ORIFICE FL4NGE

OPTIONAL

INSTALLATION 'A'hE': VENA CONTRACTA T i ? S ARE USED

1 . The meter preferably is mounted below the line.

TYPICAL'WHEN METER IS LOCATED ABOVE LINE. VAPOR TRAPS SHOÜLD 3 E LOCATED AT HIGh POINTS BE-

'::\. ,.< g,J .- IypIcAl I Ic'"c. = - * , Fc. , & /

TWEEN MFTFR AND CONNECTIONS I V I V - J C H L C " , / Y - I . . , . .r . . * ? A , ,..- - - -

STALLATIOIU w n t n ~ C H L I I Y ~

_IOU10 SEE SECT 8 =OR ALTERNATE SEAL INSTALLATIONS

HEAVIER THAN LINE G A S I N ONE METER LEAD CAN I N T R ~ DUCE ERROR VENT PE910DICALLr

3. A slope of at least 1 in. per ft downward to the meter 7 . Piping from the flange to the meter manifold to he 'h in.

Meter nianii'olcl piping to be !5 in. Tubing and multiple-port valves may he used wherever desired.

should be provided on all horizontal lead lines.

may be present in meter leads. 4. Meter zero should be checked from orifice taps if water

k ï G . l-l(>-Dilr'i.rc.iitinl F l o w nieters i t i Liquid Service.

-1NSUL

A INSTALLATION FOR METERS O N VERTICAL L I N E S . INSU- LATE LOWER TAP UP TO L E V E L OF CONDENSATE POT

r

. -OPTIONAL FOR STEAM TRACING AND/OR

.ATE

PRESSURE PEN I"OR LARGER PIPE

? -THICK INSULATION

RECORDER OR TRANSMITTER < I

(TYPICAL A L L v i ~ w s ) I ,??Y>, j j i

\ \ \ , I I ' ' I ;*y\ 1 i / '?. I

--.;;

I $ u. ' I

~ .Y-' ,

!,--TO PRESSURE PEN

, , ' . ?-TO TRAP

B INSTALLATION WHEN METER MUST BE MOUNTED ABOVE INSTALLATION FOR METERS THE PIPELINE. VERTICAL RISERS SHOULD BE AT LEAST :" ON HORIZONTAL L I N E S INSULATED PIPE. FLANGE TAPS ARE NOT RECOMMENDED

DUE TO THE NEED FOR LARGER CONNECTING ,?¡PE. USE EITHER VENA CONTRACTA OR PIPE TAPS. 57N3ENSATE .?Ois ARE AT SAME LEVEL

Notes: 1. The meter should be installed below the line wherever

2. Piping to be same size as for liquid Aow except as noted

3. Use condensate pots (see seal chambers. Sect. X ) to h. Condensate pots must always be a t the same level.

4. Multiple-port valves may be used in the meter manifolds

5. Meter leads should be insulated and steam traced if am- possible. if desired.

in c. condcnsc the vapor and providc two equal liquid he:& on the metcr chambers. 'II 1ty.

bient temperatures may go below freezing.

7. Condensate pots shown oversize in these drawings for :

FIG. l-lf-Difft*reriti;il Flowtiit-ters for Steniii ;itid Other Coiideiiseblc Viipors.

18



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

6. INSTALLATION AND INSPECTION OF METERING RUNS

Meter run pipe or tubing should be carefully selected for a uniform internal surface which is free of internal striations and grooves but not polished. It should also be selected for roundness and for conformance with published diameters. Some refiners prefer to buy specially selected pipe or tubing for meter runs; others prefer to buy preassembled meter runs complete with orifice flanges for installations where accuracy is important. Out-of-roundness tolerance varies with the d/D ratio. When the d ;D ratio is 0.70. the out-of- roundness tolerance is 0.5 percent for the upstream sections and 1 percent for the downstream sections. For tolerances for other d / D ratios see AG.+ Report N o . 3. ' I t is recommended that 1111 meter runs be designed ;is if for 0.70 d / D ratio. I f published orifice coefficients are used. the diameters of the pipe should match published diameters within 0.5 percent for flange trips. and 0.3 percent for pipe taps.

Flange tap orifice flanges are either o€ the screwed. slip-on. or welding-neck type. If slip-on or threaded Han9c.s are used. care must be taken to see that all burrs are removed after drilling the taps throuch the pipe. When slip-on flanges are used. additional care musr be taken to see that all weld splatters are removed from the Hange face. Reduction of the diameter of the pipe or distortion caused by welding should be eliminated. If welding-neck flanges are used. it is essential that the flange bore be the same as the pipe internal diameter and that the bore be concentric and parallel with the pipe. If there is any internal roughness at the weld it

should be ground smooth. I t is desirable to use ;1 ta- pered mandrel to position the welding-neck flange during welding. Flange taps should be properly oriented during installation.

Before installation, a11 orifice run fabrications should be inspected for dimensions. straightness. absence oi burrs and welding deposits. and internal roundness. Where welding-neck Hanges have been used. concentricity of the pipe with the Hange neck should be checked. It is essential that the flange bore be the same as the internal diameter of the pipe. For gas mc'asure- ments, the tolerances should be in accordance \vith AGA Report No. 3.':

For liquid service? where the taps are horizonial. sufficient clearance should be avaiiable betwe2n adjacent lines for installation of block valves and fittings.

Before installation, orifice plate bores should be inspected for concentricity, roundness, sharpness. and absence of burrs and nicks. The bore should be measured with a micrometer and the reading checked againsr that stamped on the paddle handle. If a be\el-edge orifice plate is to be installed, the beveled edss m u s t facc downstrillim. Thc quadrant-edge orifice plate. on the other hand. is installed with tlie rounded suse iip- stream. The flat orifice plate must be positioned cari'- fully between raised face Ranges to ensure that the bori' is concentric within 3 percent of the inside diameter OI

the meter run. The inside diameter of the gasket must not be smaller than the inside diameter of the pipe anci the gasket must be positioned concentrically. Oriiice plates supported in ring-type joint holders will be posi-

E . YSTA~- ;T~ON =on DPY 'NSTALLATION FOR WET 1NSTALLATION FOR WET I ' í P I C A L INSTAL--. \

;is wi-u C ' V E - JALVE ?.I A N IF; - 3

F t F VERTICAL i '.: ZRESSURE "EN CI'. 'JECTION JPTlOh- ..

OR 3 R Y G A S WITH M E T E R ABOVE THE LINE 4NC 3QESSURE PEN CONNEC- LINE TION OPTIONAL

GAS CERVICE WHENwF L.iErER 15 BELOW i , -

i . The mcter niay be motintecl cither above or below the line. I f the gus is wct or contains corrosive subsrunccs. or both, a liqiiid seal or an air or gas purge (not shown) should be provided. ( Rcfer to Sect. X. 1 i f desired.

2 . In gas service the pressure taps should be located on the top of the pipe.

3. On the bellows nieters, tlie lower connection 'iioiild be

1. Multiple-port valves may be used in the meter riinnifolds

5. Piping to be sanie size as for liquid Row.

iiscd to prevent liquid :icciimuloti»n in nieter body.

FI(;. 1-18-Diffc~rc~ntiaI Flowii ivtws i i i (;as Svrvie-e.

19



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

APl R P SSO-PAKT I

tioried within the conccntricity tolerances OE the ring groove Lind the orificc bore within the ring.

Some reiìners rcquire that installation of orifice plates be postponed until aftcr the lines have been flushed out. The reason for this is to prevent trash from piling up in front of the orifice plates and to prevent any trash which might be dislodged during initial circulation from damaging the edges of the orifice plate.

7 . ACCESSIBILITY OF PRIMARY ELEMENTS I t is advisable to locate the orifice. or other primary

element. bo that i t is accessible from grade. walkway. platform. or movable platform even if this requires rerouting the process lines.

1.4 DIFFERENTIAL RIEASURIIVC, DE\ICES

Several types of measuring devices are used to determine the diffcrential produced by the primar): element. It is difficult to maintain accuracy at low-flow readings because flow is proportional to the square root of the diffcrential pressure. These devices are used where the rangc in tlow is 3 to i . or less.

All the devices listed herein, except the glass manometer which is an indicating device only. are available in m y combination as indicating, recording. transmitting, and controlling instruments. Transmitters are available for pneumatic or electric transmission.

For flow recorders. the charts most generally used are the "O to i O" square root charts. Square root charts are available with various linear secondary scales for recording pressure. level. or tempcrature on the same chart. A suitable mctcr factor is multiplied by the reading to give the actual How. By judicious sizing of the orificc. meter factors can bc obtained in round figures. However. it is much more convenient to change meter factors than to change the orificc platc or the meter rangc whcnevcr the physical properties of the flowing strcam change. Few rctincrs use special charts to read Bow directly. Total flow may be obtained by plani- metering tlow charts or by equipping the meter with an integrator. Corrections must be applied for changes in condition of the flowing stream.

Some of the dcvices mentioned in following paragraphs usually are supplied as blind transmitters without direct tlow scales; in this case an output indicator with a O to I O square root or other suitable scale should be furnished so that !low may be read at the transmitter or control valve location. This device should not be used to calibrate the transmitter. For calibration in service, a test gage or manometer should be used for pnciimiitic instruments. A tcst grade meter should be uscd for c:ilibrriting clcctric .transmitters.

;I. JIwli;iiiit.nl Merciiry Mtttvrs

iMeclianica1 mercury meters, in the past, have been stiindard in the industry but are rapidly being replaced by the úry-type meters.

Ranges most generally used by refiners are 20 in., 50 in., 100 in., and 200 in. of water (dry calibration); other ranges are also available. Although mechanical meters are available with the square root extracting type of range tubes or float tubes, most meters are read directly in differential or in flow on a square root scale.

Seal chambers or condensate pots frequently are used with mechanical mercury meters. (For further discussion. see Sect. 8.)

Mechanical mercury meter installations are shown in Fig. 1-1 6 through 1-1 8. Usually, these meters are yoke-mounted on pipe pedestals with provisions, such as spherical seated unions. so that the meters can be leveled to the close tolerances necessary for good measurement.

i>. Bellows Meters

In the bellows-type meter, the bellows is opposed by a calibrated spring system and is filled to prevent rupturing when overpressured and to provide pulsation dampening. Temperature compensation is also provided. l n addition. meters with ordinary-type bellows which usually are used only on applications of low difsrential are still available.

Bellows meters can be either line-mounted or mounted at grade or on platforms. Seal chambers or condensate pots are not used generally. A 1 %-in. tee has sufficient volume for a liquid seal or as a condensate pot for steam or condensable vapor service for instruments which displace less than 1 CU in. with full-scale deviation. However. if the displacement is much greater than 1 CU in.. or it' the differential of the instrument is low in comparison to the column displacement. regular condensate pots should be used. Typical meter piping is shown in Fig. 1-1 6 through 1-18.

Bellows meters have both top and bottom body connections. The top connections are used for liquid flow installations and the bottom connections for gas flow installations to avoid the error caused by trapping. gas or liquid, respectively, in the meter body. It is desirable to use M-in. connections, which may require rotating the body chambers in some cases where both M-in. and ?4-in. connections are provided. It is suggested that the alternate tapped opening can be used as a drain or vent.

c. 3laiioiiieters

The simplest measuring device is the glass manometer which m a y vary in form from the simple U-tube to the more hislily developed single-tube devices. These are of little use in refineries except as test devices rind as indicators on nonhazardous low-pressure streams. A manometer with nianifold is shown in Fig. 1-19.

(1. Diapliragiii Transmitters

Force-balance diaphragm and filled-diaphragm types of difrerential meters are used extensivclv on refinery

20



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW

EDGE OF ORIFICE FLANGE-.,

FIG. 1-19-Class Tube Jfaiioiiieter.

units. These instruments generally are used without seal or condensate pots because of their corrosion-resistant construction and low displacement. Line mounting is preferred if the location is accessible and the vibration level is not too high. Gas meters are mounted slightly above the line to allow liquids to drain back. Liquid meters are mounted below the line to allow gas bubbles to work back to the line. I f leads are short enough. the transmitter may be mounted level with the center of the line. With this arrangement. it makes little difference in error if the opposite legs of the connecting piping contain liquid or vapor in different amounts.

Piping arrangements for diaphragm transmitters are shown in Fig. 1-20 and 1-21. If mounted farther from the orifice than as shown in these illustrations. the piping may be similar to that shown in Fig. 1-16 through 1-1 S.

1..5 CONNECTING PIPING

II. Meter Locatioii

Flow recorders. indicators. transmitters. or controllers should be mounted at a convenient height, usually 4 f t or 5 f t above grade, platforms, or walkways. On flow control installations the transmitter or transmitter output cage should be visible from the control valve and control valve bypass to facilitate emergency local and manual control. For a close-coupled meter. which is the preferred installation, the process line should be routed to a convenient height above. grade, or near a platform. \valkway. or other permanent means of access to facilitiitc maintenance and for making a zero check with a manometer or with a test gage. If free access is available to the space below the primary element, the use of n rolling platform or ladder of moderate height may be satisfactory. Many flow transmitters are sus- ceptiblc to damage or malfunctioning if subjected to vibration. The mounting location, therefore, must be carefully sclectd.

l /gSCHEDüLE 80 PIPE 3-WAY MANIFOLD VALVE

"/2" TUBING TUBE TO PIPE

GAS OR LIQUID SERVICE

1 - 1

/ - 3 / 4 " GATE VALVE SWAGED/SCREWED

"a\ --- l/2" TUBING

3-WAY MANIFOLD VALVE WITH INTEGRAL TUBING F I T T I N G S OR l/<TUBE X 1/2" M A L E ADAPTER TUBE TO PIPE

"/2"T'JSE X ' /2"MALE ADAPTER T U B E TO PIPE

W FOR U S E CN LIQUID SERVICE

FIG. I-20-Typical Close-Coupleti Manifold Arrangeiiieiits for Diaphragm Flow Transmitters.

b. Meter Leads

Meter leads should be as short as possible. For locally 'mounted indicators or recorders the upper limit should be not more than 50 ft to the meter. The leads should slope at least 1 in. per f t downward from the orifice taps for liquid measurement. For close-coupled meters, horizontal leads may be used to eliminate the effect of fluid density variation in leads. For gas measurement the leads should slope at least 1 in. per ft upward from the orifice taps. or downward toward the drain pots if the meter must be mounted below the orifice run. On wet-gas service, where it is necessary to locate the meter below the orifice taps, knockout pots should be located directly above the block valves so that condensate will drain back into the line. It is also advisable to use drain pots below the meter in order to collect lhe additional condensate which may enter the meter piping.

Meter piping should be designed and installed in accordance with the piping specification for the service involved. Usually !A-in. Schedule 80 or heavier pipe. depending upon the corrosion allowance required, is used for meter leads. Some refiners prefer to use stainless steel tubing and tube fittings. Copper tubing, usu-

21



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FOR GAS SERVICE IN VERTICAL LINES L

FOR STEAM AND LIQUID SERVICES IN VERTICAL LINES

LIQUID(SEE dkp NOTES 3 AND4)

STEAM (SEE NOTE 3 )

FOR GAS, STEAM, AND LIQUID SERVICES IN HORIZONTAL LINES Notrs: 1. Differentinl pressure transmitter shall be located as close

to the pipe and prcsstire taps as practic;ible with the length of thc iiibing kcpt to I I i i i inimtini.

2. For gases, locate the differential pressure transmitter so the lines slope down a minimum of 2 in. per ft to the pressure taps.

3. For steam and liquids. rhe lines from the connections a t the differential pressure transmitter shall run horizontally to the JS-dcg elbows.

cc3 VALVE NORMALLY OPEN

H LALVE NORMALLY CLOSED 9 PIPE TO TUBE CONNECTOR

& MULTIPORT VAL\/€

-E CROSS -I- UNION

4 TEE 6 VENTPLUG

5 x a o w W PLUG

1 STREET ELBOW

'0 05O ELBOW %=E TO TUBE CONNECTOR

4. For liquids requiring sealing, slope the lines down 1 in. from the 45-deg elbows to the connections at the differential pressure transmitter.

5. Nipples and gate valves against orifice fianges shall conform to main-line specifications. Other piping materials shall conform to instrument pressure-piping material specifications.

~

d l y !'i! in . OD. is sometimes used for steam meters to avoid corrosion and plugging problems. Steam meters should be provided with valves for blowdown. All locally mounted instrumcnts and lead lines handling water or process fluids which may freeze, become excessively viscous. or form hydrates in cold weather should be instailcd i n accordance with Sect. S.

Attention should be given to meter-connecting piping and manifolding as a source of meter inaccuracy. There is a possibility that the liquid head in one meter lead may differ from the head in the other lead because of differences in specific gravity, temperature, or amount of gas or water in the leads. For example, if the meter is 100 in. below the orifice with one side filled with water and the other side filled with a liquid of 0.65 sp gr, the zero error will be 35 percent of full scalc for a 100-in. meter range. It should be noted that, at times, most hydrocarbon streams will contain water.

The possibility of error because of gravity differences or vapor binding is eliminated by mounting the meter o r transmitter close-coupled to and level with the meter taps.

If it is not possible to mount the meter close to the primary element, seals or purges should be used unless it is certain that water will not be present and that vapor bind will not be a problem. In the case of vapor bind-

22

ing on light hydrocarbons, steam tracing may be used to eliminate liquid in the lead lines.

v. Meter Mariifolcls

Manifolds are necessary on all differential-measuring devices for checking zero and for putting the meter into or out of service. This is especially important on mercury-filled meters and those which cannot take full-line pressure on one side only. Manifolds usually are classed as 3-valve manifolds, 5-valve manifolds, or 3-valve manifolds with drains (see Fig. 1-16 through 1-18). Generally. 3-valve manifolds are used in liquid service and with close-coupled transmitters (see Fig. 1-20 and 1-2 i ) . When the transmitter is close-coupled. the tap block valves may serve as two of the three valves of the mcter manifold unless double blocking is required for removing the instrument while the line is in service. The 5-valve manifold installation frequently is used with liquid sealed meters and with meters in gas service. Generally, 5-valve manifolds are used on sales meters.

Piping and valves for manifolding should be the i /2 -in. size. However. on sealed metcrs %-in. manifold piping or tubing of suitable material, such as stainless steel, can be used between the seal and meter. Special manifolds with integral valves also can be obtained.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW

(l. Seals. Chicleiisate Pots, unci Knockout POIS In some services it is necessary to protect certain

types of meters from the process fiuid or to reduce potential errors caused by water or vapor in a meter lead. In such cases, seal chambers should be installed as recommended in Sect. S. In steam service. condensate pots are necessary to maintain an equal liquid head on each side of the meter regardless of meter displacement. For the force-balance diaphragm transmitter, the displacement is sufficiently low so that condensate pots usually are not required.

e. PurFing

Purging occasionally is necessary in order to prevent plugging of meter leads if the flowing fluid contains solids, is corrosive to meter parts, is highly viscous: or if water or condensate cannot be tolerated in the meter or meter piping. It is necessary to restrict the purge flow so that it is uniform on both sides of the meter and does not cause a false differential. Restriction orifices. purge rotameters (preferably armored type), needle valves. or drilled gate valves are commonly used to control the volume of purge fluid. The drilled gate valve is desirable if frequent blowing back is required. The purge fluid should be clean, dry, and compatible \vith the process fuid. (For additional information. see Sect. 8.1

1.6 -4REA METERS a. General

Area-type meters (rotameters) are being used for flow measurement where the following requirements exist: 1 . Wide range of flow rate (as high as 10 to I l . 2. Good linearity (for flexible proportioning of two or more Aows) or simple correlation of a flow s i p a l with other linear response variables. 3. High-viscosity immunity. 4. Liquefied petroleum gas (LPG) or other volatile liquid measurement (gasification in lead lines). 5 . Freezing or congealing liquids (strong chemicals. waxes. and asphalts). Steam-jacketed meters are usually required. 6. Slurries or streams with suspended solids, within reasonable limits. 7. Low-flow quantity (purge meters, blending service). 8. Frequent on-of service.

Area meters are available as indicators, transmitters, recorders, and controllers in any combination. They are relatively expensive in the large sizes. Although not used as frequently as orifice meters, they are valuable for unusual or special service conditions, as noted herein. Perhaps the greatest area of use is in low-flow service, especially for flow quantities below the normal range of the orifice meter.

Glass tube meters. unless contained in suitable armor. should not be used t'or general hydrocarbon or other hazardous service.

I). Installation

1. LOCATION

The meter should be installed in a location free from vibration and where sufficient clearance is available for occasional foat removal for inspection or range changes. It should be located so that i t is visible and readily accessible for operation and maintenance. In general. when a meter is to be used in regulating service, it should be placed as close as possible to a throttling point. preferably with the valve located at the outlet ntting.

3. MOUNTING

Rotameters always should be mounted vertically with with the outlet (downstream) connections at the top of the meter and the inlet (upstream ) connections at the bottom. that is. with the hishest scale graduation and the largest part of the metering tube at the top. A plumb bob. or similar device. should be used to check vertical alignment. Should the rotameter not be mounted in a vertical position. both accuracy and sensitivity will be affected.

Rotameters may be mounted on either side of a panel or directly installed in either a horizontal or a vertical pipeline. When panel mounting is used, the panels should be rigid, vertical, and installed in a location free from severe vibrations.

3. PIPING

Most variable area flow measurement is practically independent of upstream piping arrangements. Elbows. globe or throttling valves. and other fittings have no effect on measurement accuracy if they are no closer than iive diameters upstream.

Where connections are interchangeable (for vertical or horizontal connections), horizontal connections are recommended if at all practicable in the overall piping arrangement. Horizontal connections permit the use of the plugged vertical openings as convenient cleanout ports. The design of most rotameters permits the end fitting to be rotated in 90-deg increments, allowing a convenient variety of connection arrangements. Rotam- eter piping connections are shown in Fig. 1-2?( A j .

All piping should be properly supported to prevent sagging caused by the weight of the meter. Care must be taken so that the piping arrangement does not impose any strain on the meter body. Sometimes it is advisable to install a brace bctwcen the inlet and the outlet piping. A check valve should be installed downstream on installations where backflow muy be expected. in gas service. or wherc thc possibility of liquid hammer may exist.

23



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

PURGE METER

(IF REQUIRED)

INDICATOR, TRANSMITTER, RECORDEROR CONTROLLER

O R RECORDE"

INDICATOR.

t HORIZONTAL VERTICAL INLET

LINES THROUGH A SPOOL

AIR SUPPLY

I l I I CHECK VALVE

A E FIG. 1-22-Piping Connections for Rotiiiiieter.

3. BYPASS PIPING If the meter is installed in a service which may re-

quire opening the meter for servicing, and if shutdown is undesirable, block and bypass valves may be provided to permit process operation while the meter is being serviced. Some service conditions which may necessitate opening the meter are: dirty fluids, slurries? corrosive or erosive media, fluids with a high-solidification point, or pipe scale in the case of small meters.

The bypass line and valves should be main-line size. Block valves should be installed upstream and downstream of the rotameter. A drain valve should be installed between the inlet block valve and the meter. A typical bypass arrangement is shown in Fig. 1-22 ( B ) .

When a rotameter installation includes a bypass, care must be taken to be sure that the bypass valve is tightly closed when the rotameter is in service. Only the downstream block valve may be used for throttling when Hashing might be encountered.

5 . STR.-\INERS In smaller sizes, except on slurry services. it is some-

times aivisable to locate a strainer upstream of the metcr to prevent the float from being jammed with pipe scale or other foreign material.

6. PL'RGE FLUID [n installations where purging is necessary, the purge

Buid may be injected at the top of the extension tube, as shown in Fig. 1-22(B), or at other connections provided in the instrument. Whcre the main-line pressure

or purge fluid supply pressure may vary over short periods of time. it is advisable to use the purge rotameter-differential regulator combination for automatic control of the purge rate of flow; see Sect. S.

7. STARTUP When the meter is put into operation. the valve should

be opened slowly to prevent flow surges which might damage the float or other meter components. If the meter is purged. the purge flow must be started first.

1.7 TARGET FLOW TRANSMITTERS

a. General The target flow transmitter is a fluid flow measuring

transducer which generates an output signal directly proportional to the force applied on a "target" suspended in the fluid stream. Flow is measured as the square root of this force. and therefore. the square root of the transmitted signal.

The unit is installed directly in the flow line. eliminat- ing the need for pressure tap connections.

The meter is contained in a body with short pipe sections extending upstream and downstream. A circular. square-edge, or a shaped metal target is secured to a beam or "force bar" which is held in the center of and perpendicular to the fluid stream. The flow is through the annular orifice around the target. The force bar ex- tends through a seal-flexure to a transmitter mechanism. Fig. 1-23 shows the schematic arrangement of a target- type meter. The transmitting mechanism may be the

24



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW

..c)

FORCE M E A S U R E M E N T

B E A M OR FORCE BAR

S E A L

FIG. 1 -23-Targe t-Type Me ter-Schemat ic Arrangement.

force-balance type or a detlection-measuring type. such as those with strain gage sensors.

The force-balance transmitter develops optionally a pneumatic or electrical output signal. The strain gage deflection sensor gives an electrical output which may be measured by a bridge-type instrument.

The target-type How transmitter is suitable for use on 1 viscous or dirty hydrocarbon strcams. It crin also be

used on other liquids as well as on gases and vapors. Typically, flows can be measured to accuracies of = 3 percent of full scale within the flow ranges generally considered applicable to orifice meters.

Pressures as high as 1.500 psi at temperatures up to 750 F can be handled when the transmitter is welded into the line. For meters furnished with firinges. the normal flange pressure versus temperature rating tables must be adhered to; however. pressure and temperature should never exceed the manufacturer‘s meter body rating.

1). Installation

1. LOCATION AND MOUNTING The targct How transmitter can be installed in either

horizontal or vertical lines. It should be located where it is accessible from grade, platform. or ladder.

The targct-typc mctcr is line-mounted. It should be oriented with thc dircctional arrow in accordance with How direction. For bctter cooling on hot horizontal lincs. thc metcr should bc mounted with thc head to .the side. Ail piping should be suficiently supportcd to prcvent undue stress.

2. PIPING Standard oritice metcr piping practice should be

followed, using mctcr run values of 0.70 d / D for target-

25

type How transmitter installations. This includes the optional use of straightening vanes where necessary to reducc the run of straight pipe (see Fig. 1-8 through

If the meter is installed in a scrvice which may require zero adjustment of the meter or opening the meter for calibration or servicing, and if a shutdown is undesirable. block and bypass valves may be provided to permit process operation while the meter is being serviced. Upstream and downstream blocks should be line size and located in accordance with orifice meter practice.

3. STARTUP

The instrument should be zeroed before flow is started. The flow should be started gradually to prevent damaging the instrument. The bonnet should be vented to remove gas if the instrument is in liquid flow service.

1-14).

c. Calibration

The target-type meter may be adjusted to zero by stopping all flow in the line (usually by bypassing) and adjusting the output to correspond to zero flow. Range adjustment is normaIIy accomplished by removing the meter from the line and applying weights to the force bar in accordance with the manufacturer‘s instructions.

1.8 TURBINE OR PROPELLER METERS

a. General

As its name implies. the turbine or propeller meter (Fig. 1-24) is a volumetric. Auid flow measuring transducer which measures flow inferentially from the rotation of a turbine. propeller. or other type of rotor located in the stream. The rate of rotation is determined by the average velocity of the fluid and, hence, by the quantity

FIC. l-Z.’Turliiiic: Moter.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-P,.w-r I

of material Howing through the meter. I t may be used for either liquids or gases. Turbine riictcrs arc ¡low sensing devices only and require additional equipment to provide indication, transmission. rccording, and/or control in any combination.

Prcscntly mailable ;ire turbine meters designed for usc within an approximate temperature range from minus 430 F to plus 1,000 F and a pressurc range. depending upon size. up to 50,000 psig.

Pressure loss varies approximately as the square of the How rate. Manufacturer's pressure loss curves should be consulted t'or each application. it is generally wi th in the rnngc of 2 psig to 7 0 psig at rated i-tow depending upon mctcr dcsign and fluid service.

Generally. the metcrs ;ire constructcd of stainless steel and arc adaptable for usc in mildly corrosivc services. They can be steam-jacketed i f required.

Tui-bine meters arc being used for How measurement whcrc the following rcquircnients exist: I. High rriq,wrl>iliry: Flow rate rangeability varies from about 7- I to 25- I dependent upon flow. viscosity. Ruid meter sizc. accuracy requirements. and static pressure. For :in Livcrngc application. :i turbine Howmetcr can be cxpectcd to meter volumctric flow to accuracies of

percent o r better of actual !low. over flow ranges varying from 5- L to 20- 1 o r niore. depending upon the mctcr design and s i x Lind the i.iscosity of the liquid. 1. Hi~~~li- irc.c. i i i -r ic~~~ c u d high-reprLircihilii~ i h * meu.~iire- ment: Turbine nicters arc ~iscful for high-accuracy metering :ipplications. such ;is blending. for mcasurc- ment ;it low ílow rates. for conscrvation of expensive materials. Lind For custody transfer. Accuracies LIP- proaching O. I percent with excellent repcatability can bc attained whcn :ill factors which ;ire known to influ- cncc thc pcrformuncc of the metcr are controllcd.

Liquids with viscosities above 5 ccntistokes present problems t h a t must bc studied. because they drive this iypc of mctcr into the nonl inear rcpion. Inaccuracies result whcn thc highcr viscosity is variable o r combined with thc Inrgc inllucncc of tcmpcratiirc upon viscosity. Although this may seem to be ;I considerable problem. it can frequently be solved when suitable correction factors arc applied. Some meters ;ire equipped with viscosity compensators to eliminate the inaccuracy of overrunning caused by viscous drag. The compensator is elTectivc up to approximately 5 0 centistokes. 3. Good litiewiiy; The output is directly proportional to ilow over the usable range of the meter. 4 . Lic/irefieti perroleiirn gus o r ollirr Iiigli-\,irpor-pre.s.srir~ liyrritl rne~i.siirernet7t: I f rangeability or accuracy can be sacriticcd. flashing can usually bc prevented by over- sizing the meter to reducc pressure drop. 5. Hi~<dr-/iow rares for u xiveti-,vi:e meter: Size for size. thc turbine metcr will nicasiire a highcr How rate than a diífcrcntial hcnd Howiiictcr, but not ;is high as il magnetic I1 o w ni e !er . f i . Low-floiv rfires: Minimum size of meter is limited only by machining techniques and probability of plug-

______ ..__

t r in<r o C'

~ _ _ _ - _ _

7. Rupiri response: Pulsating How can be measured within limits. provided it is within the mechanical durability limitations of the meter. Time constants may bc as short as 5 to 7 milliseconds.

I). Iiistallatioii

1. LOCATION AND MOUNTING

The meters are installed directly in the lines. Elec- trical-pulse-type meters should be kept a reasonable distance from unassociated electrical equipment. As a rule of thumb. the line should be relatively free from frc- quencies of vibration exceeding 30 cps. If the meter is a direct-reading type. the register should be in such a position that it can be easily read.

Turbine meters should be mounted with the inlet connections upstream exccpt for some types which measure Row in either direction. No special considerations are necessary for the absolute levelness of the installation.

The meters are generally installed in horizontal lines. Some makes may be installed in vertical pipe. but calibration for the position may be necessary. However. in some meters, special thrust bearings should be specified for vertical mounting to prevent undue wear. i t is usually necessary to specify the position for which the meter is to be calibrated.

2 . PIPING

Accuracy and repeatability of most turbine flowmeters are dependent upon the upstream and downstream piping arrangements. Effects on calibration have been noted when the straight run of pipe upstream of the meter is less than 5 or 10 diameters. or the downstream run is less than 295 diameters. Where turbine flowmeters are used in services in which highest accuracy is required. upstream and downstream distances recommended for orifice metcrs of large d /D ratio should be maintained as shown in Fig. 1-43 through 1-14. The usual straightening vanes may be used with this type of meter. should it be necessary to shorten the meter run.

Care should be exercised in the installation of flanged meters to sec that the gaskets do not interfere with the flow pattern.

Piping should be adequately supported and arranged to prevent undue stress on the meter body.

The need for a bypass is determined by application. I f it is necessary to isolate or open the meter. and if it is in continuous service where shutdown is considered undesirable, block and bypass valves may be provided to permit process operation while the meter is being serviced. Some of the conditions which may necessitate opening the meter are: damagc caused by foreign matcrial. wcar. or solids buildup. If bypassed. the meter should be in the main run. and block valves should be line sizc and placed at least 1 O pipe diameters upstream and 5 pipe diameters downstream of the meter (see Fig. 1-8 and 1-9). The bypass valve should be capable o f positive shutoff to prevent measurement errors.

26



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

-.

3. STRAINERS A N D ACCESSORIES In almost all turbine meter installations. strainers are

required to prcvent foreign material from blocking or partially blocking meter How passages or lodging between the rotor and the meter body. The strainer must also be capable of removing particles of such a size that might injure the rotor. Typical screen size for this type of protection is 200 mesh for %-in. meters. 120 mesh for M-in. to %-in. meters. 80 mesh for I-in. to 3-in. meters, and 60 mesh for %in. or larger meters. The strainer should be located i 0 or more pipe diameters upstream of the meter.

Where high-accuracy liquid metering is to be accomplished. means must be provided for the automatic removal of air and/or gas which may be in the stream. Gas entrainment can cause errors in repeatability and accuracy of the meter. When used in vacuum service. turbine meters should be installed so that they will have a positive head of liquid upstream. This head should be at least equivalent to the anticipated pressure drop through the meter. To minimize cavitation problems in vacuum service. a liquid trap downstream should be provided to maintain a downstream pressure of at least one foot of liquid.

4. OVTPUT Meter output may be mcchiinical or electrical. Me-

chanical output is usually accomplished by reduction gearing and a magnetic coupling driving a low-drag register or transmitter. or both.

Meters which producc electrical outputs usually sense the turbine rotation by means of a magnetic coil or a tuned RF circuit. As each signaling point of the rotor passes a pick-off unit. an electrical impulse is produced. The normal output of this meter will be an alternating electrical signal. The impulses forni digital information. with each pulse representing a discrete volume of fluid, so that the accumulated-pulse total represents the total volume measured, and the frequency represents the volumetric flow rate. The alternating signal can be displayed as pulse count, voltage. or frequency.

If the amplitude of the signal is relatively small, it is highly susceptible to noise pickup. and shieldin, <J IS . recommended to reduce spurious counts. i f transmission distance exceeds I O ft. an amplifier should be placed near the meter. The higher signal-to-noise ratio of the output of the amplifier is less susceptible to interference. Meter. amplifier. and leads should be segregated from other electrical equipment. For specific information concerning wire size and auxiliary electrical equipment the manufacturer should be consulted.

5. STARTUP Turbine-type flowmeters should be placed in servicc

only after the operating unit has been flushed and hydrostatically tested. I f strainers are used, they should be cleaned after flushing, and periodically during operation. Plugged strainers may break loose and sweep

FLOW

downstream. demolishing the meter internals. Flow should be introduced slowly to the meter. especially when in liquid service. to prevent overspeeding. The impeller blades can be shearcd off ;is ;i result o f ;I sudden hydraulic impact.

c. Calilwation

_ _ _ _ - - ~ _ _ _ _ _ _ -

The calibration factor. expressed in electrical pulses generated per unit volume of throughput. is normally called a "K" factor by the manufacturer.

No adjustments are available on the primary sensor to change voltage magnitude or frequency of the electrical signal. which is ;I function of the rate of rotation of the meter and. hence. a function of flow. Any adjustment for correction must be made on the receiving equipment or applied as a meter correction factor.

Calibration of turbine meters may be accomplished through either of two basic schemes. One method is to perform the calibration in the shop: this consists of takins standard volumes through the meter (with appropriate temperature compensation ) and comparing with the recorded count obtained from the meter for the specified volume. The second method is the use of an in-line system either upstream o r downstream of the turbine meter. Connected to the bypass piping is either a permanent or a mobile ball or pig prover. When .the stream is blocked the flow is diverted through a calibrated meter run and the output signal of the meter is imposed upon a counter. the counter being actuated during the time the ball or pig moves through the calibrated volume. A P I Srmdcirtl I I O I : Measirrrmenr oj Petro- iertm. Liquid Hytirocarhons Dy Positive Dispilicement Merer ' ' may be consulted for meter-proving procedures.

1.9 MAGNETIC FLOWMETERS

u. General The magnetic flowmeter measures volume rate of flow

of liquids which have some measure of electrical eon- ductivity.

Unfortunately, most petroleum hydrocarbons do not have sufficient conductivity to be satisfactorily measured with the magnetic flowmeter. For this reason its use is restricted to auxiliary services-e.g.. water: emulsions, such as acid sludge; water solutions of acids, caustics, monoethanolamine (MEA), diethanolamine í DEA) ~

and additives; water slurries; and some catalyst com- plexes such as those containing aluminum chloride. Crude oil which contains salt water may show sufficient conductivity for measurement by thc magnetic meter, but the possibility exists that there will be separation so that the measurement will not be satisfactory.

The mctcr consists of two parts, a magnetic flow transmittcr (installed directly in the process line) and an electronic receiver. The transmitter generates signal proportional to volume Aow and sends it to the receiver, which may be as far as 2,000 f t away.

27



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

The magnctic flow transmitter operates on thc principle of an electrical power gcncrator. This is bascd on Faraday’s law of electromagnetic induction. This prin- ciplc. stated simply, is that when u moving conductor cuts across lines of force in a magnetic field, a voltage is induccd in it. Moreover. if thc dimensions of the conductor (line diaincter) remain constant, then the induced voltage is directly proportional to the velocity of the conductor. Mathematically, this principle can be stated as:

Where: E = CBDV

E = generatcd voltage. C = dimensionless constant. H = magnetic field strength. D = length of the conductor (diameter of the meter). V = velocity of the conductor (velocity of iluid).

As shown in Fig. 1-25. the flowing liquid itself is a series of nioving conductors. Conductor iensth, D, is constant as deterniined by the inside diameter of the transmittcr tube. A uniform magnetic field, B. is generated mutually perpendicular to the axes of the meter tube and the electrodes by a n electromagnet encircling the tube. The output voltage. E . therefore. varies directly and exclusively with velocity, V , of thc Ilowing liquid. Thc constant. C . corrects for other eficcts. Inas- much as thc tube arca is constant, the generated signal is also directly proportional to volumetric How ratc as long as the magnetic field is constant.

System accuracy ( including transmitter. connecting cables. and transducer o r receiver) is typically 1 percent of full scrilc throughout thc range. Special higher accuracy systems arc optionally available. Sensitivity is approximatcly 0.0 I percent.

The major characteristics of the magnetic flowmeter ;ire:

1. Transmitter is instnllcd directly in line. prcscnts no obstruction, thus, causcs no clogging or loss of head. 7. It rcsponds only to velocity and, thus. is not affected by changes i n density, viscosity, or line pressure. 3. Since it avcrages thc vclocity profile between the electrodes. long runs of pipe or straightening vanes ~isually arc not needed. Thc only requirement is that the velocity prolile between the electrodes be reprcscntativc of tlic Ilow. 4. The transmittcr output is linear with velocity or flow and no square root extraction is necessary. 5 . It can Iiandlc a wide varicty of dificult liquids. 6. Bidirectional Row may bc measured.

I, . lilsiallatioil

The magnetic liownictcr is primarily Lin electrical dcvicc which should not be treated :IS ;I pipe spool. In- stallation of this type of nicter in the piping rcquires k I: owl edge and ca re. The in a II II fac t u re r ‘s ins tallat ion rcconiiiicndations should bc followcd, remembering that

MAGNETIC COILS \\

Typicul Magnetic I’low Transmitter.

Schematic of Operation.

FIG. 1-2.3-Miigii~~iic Flnwiiieter.

although the transmitter is built on a rugged piece of pipe. it should be handled us a precision instrument.

Transmitter tubes ;ire made of nonmagnetic materials -such us stainlcss htcel. Inconcl. or tiberglas pipe. The nonmetallic tubes arc used unlined. but the metal tubes must be lined with ;I nonconductin: material-such as fluorocarbon. synthetic rubbcr. or glass to prevent short- circuiting the signal. Each tninsmittcr assembly has definite limits as to opernting conditions. Major limitations are the liquid pressiirc. tcmpcrature, corrosive and

28



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FLOW .. -

erosive properties. The operating conditions must not exceed the limits for the particular transmitter construction as given in the manufacturer’s specifications. The weakest zone may be the seal for the external connection to the electrodes. Any leakage at the electrodes will enter the electrical section; this is particularly undesirable in hydrocarbon bearing service.

1. HANDLING OF TRANSMITTER

’

The following precautions should be observed: a. Care should be used in lifting the transmitter to

avoid liner damage (see Fig. 1-26). If the liner is damaged it should be replaced or repaired, using an approved procedure. before installation.

b. Dropping the transmitter or subjecting it to impact, particularly on the flange face. should be avoided.

c. The transmitter should be kept in its shipping crate, and protective end covers should be kept over the flange faces until ready to install.

cf. The manufacturer’s storage recommendations should be observed.

LOCATION A N D ORIENT.ATION O F TRANSMITTER 2 . The following points should not be overlooked: C I . The magnetic flow transmitter tube may be in-

stalled in any position (vertical. horizontal. or at an angle j but it must run full of liquid to measure accurately.

b. If the meter is mounted vertically. the flow should be upward. Both electrodes must be in contact with the flowing liquid. The transmitter should be installed so that the electrodes are not in a vertical plane. A chain of bubbles moving along the top of the flow line could prevent the top electrode from contacting the liquid.

c. The transmitter should be accessible from grade or from a platform, with enough space around it so that at least the top housing could be removed if necessary. At the very minimum, sufficient access room should be available to remove any handhoie covers or inspection plates.

THIS W I L L RUIN THE LINER (FSEFERABLY. BOLT VALVES OR SPOOL PIECES IN PLACE FIRST)

l*.l(;. 1-26-ll;iiiilliiig 3Iugiicîic Flowiiicier.

d. If thc transmitter is to be underground or in a pit that might become flooded. provision should be made to prevent i t from being submerged, unless it is equipped with ;i special housing to permit operation while submerged.

e. Vertical mounting and a straight run upstream. with Row upward, is especially recommended if an abrasive slurry is being measured. This dis- tributes wear evenly.

f. The transmitter should not be installed in services where the temperature is high enough to damage the liner or insulation.

3. FLOW DIRECTION A signal will be obtained with flow in either direction

through the transmitter. If flow direction changes, or if the meter tube has been installed in the wrong direction. the correction can be made electrically. It is not necessary to reverse the transmitter in the line.

4. PIPING CONNECTIONS TO TRANSMITTER The following precautions should be observed: a.

b.

C .

d .

e.

f.

During installation. care should be exercised to prevent overheating the transmitter tube and/or liner from nearby heat sources. such as welding. if a metal tube transmitter has its liner brought out over the flange faces. the liner should not be forced between adjacent Hanges. bumped, or subjected to anything else that might damaze it. On new pipe installations, it is desirable to bolt the adjoining pipe fittings or valves to the transmitter before installing it in the line. to avoid liner damage. If this is not possible. it should be bolted in continuity from upstream to downstream piping. if piping is already installed. it is advisable to remove one or both adjoining pipe sections. In installations where there are no block valves or bypasses. it may be desirable to make up and install a flanged spool piece on each end of the transmitter. For those applications which require frequent cleaning of the How lines, the transmitter can be installed with block valves and a bypass valve to permit access to the tube interior without shutting down the process. Possible piping arrangements are shown in Fig. 1-27. The bypass valve should be capable of positive shutoff to prevent measurement errors. To permit checking the meter for zero flow, it is necessary to install the transmitter so that flow can be stopped with a full tube. For most continuous processes this will rcquirc a block and bypass arrangemcn t .

Transmitters up to i 2 in . in sizc rcquirc no support othcr than that required for an equal Icngth of pipc. Thc transmitter should not be used to support the adjaccnt piping. For largcr sizcs, a support structure may bc ncccssary, depending upon size. con:;truction. Lind manufacturer’s rccom me ndn t io n s.

29



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

The piping should be dcsipcd for sufticient flexibility to prevent excessive forces from being trans in i t tc d t o t he e lec t r i ca 1 I y i n s u I a te d li ange faces. Particular attention should be paid to installations in vertical lines to assure that the wcight of the transmittcr or piping is not applied to the Ilangc lacing in cxcess of its durabilit!..

g. Several diffcrcnt types of tlangc connections Lire iiscd. The general rule for all types is to make sure that thc tlange and its djncent mating flange arc properly :iligncd and that the bolts are tightened cvcnly. Bolt toryuc wrenches should be tised and recommended torque values should be o bserved.

5 . E L E C T KicA i. JNSTALLATION

Poit~ev: Powcr should be supplied at a voltage and frequcncy within the tolerances specified by the manufacturer.

Wiring and S i g r i d Leads: Special low-capacitance cable is used to carry the gcnerated signal from transmitter to rcccivcr. It must not be installed close to power cable or in the same conduit as the power supply. The manufacturer’s recommendations should be consulted. The cable should bc supported so that i t does not osci I1 a te.

Gvoutiditig: The importance of proper grounding cannot be overcmphasizcd. i t is necessary for personnel safety and satisfactory flow measurement. The nianu- fact urer‘s i n s t r uc t io ns o n ground i ng and j II m pc r arr a n ge- ment should be carefiilly followed. Piping should always be grounded. A continuous clcctricril contact to the

same ground potcntial is necessary between the flowing liquid. the piping. and the transmitter. This is especially important if the conductivity of the liquid is low. How this is achieved depends upon the transmitter construction. the adjacent pipe (unlined metal, lined metal, or iionmetallic 1. and the transmitter manufacture. Jumpers from the meter body to the piping are usually required for metallic piping. If the meter is installed in nonmetallic piping it is usually necessary to make a grounding connection to the liquid by means of a metallic spacer ring betwecn the flanges unless internal grounding has been provided in the transmitter. This is extremely important and must be done as recommended i f the system is to operate properly.

Seding the Trcinsmirter: Most transmitters have their signal and power connections enclosed in dripproof. splashproof. or explosionproof housings. The connections should be sealed in accordance with the manufacturer‘s instructions.

6. STARTUP

The instrument jhould be adjusted to zero with the tube full before Ilow is started. After flow is started. if thc receiver output does not read upscale. the polarity of the power or the signal connections should be reversed in accordance with the manufacturer’s instructions.

REFERENCES

I-. K. Spink. Priric,iples t r i i r l P r ( ic~ ice oí Flott. . \ le le i E / i , ~ i - r ie~ , r i r~ ,g , 8th cJn.. T h e Foxboro Co.. Foxboro. Mass. i 1958) .

W. JM. Lansford. ”The Use of an Elbow in a Pipe Line for Determining the Kate of Flow in the Pipe.” Univ. Illinois Eng. Exp. Sta.. Urbnna. 111.. Bull. No. 189 í 1936).

:: 0riIic.e iLlercririg of Ntrrirrcrl Gtis, Gas Measurement Com- mittee Itcport No. 3. Am. Gas Assoc.. New York. Apr. [ 1955).

’ PTC 19.5.4: I I I . S ~ I . I I I ? I C ‘ ~ I ~ , S r i r i c l A ppccrcitris, supplement to A S M E Pott,er T ~ N Codes, Am. Soc. Mech. Engr.. Xew York, Feh. (1959) .

Flrritl MPiers: Their Theory (riid Applicritioii. report of ASME Research Committee on Fluid Meters, 5th edn.. Am. Soc. iMech. Engr.. New York (1959).

‘I S. R. ßeitler and D. J. Masson. “Calibration of Eccentric and Segmental Orifices in 3- and 6-In. Pipe Lines.” ASME T r ( / / i . ~ . . 11 751-6. Oct. ( 1949).

i M. Bogema. B. Spring. and M. V. Ramamoort Edge Orifice Performance-Effect of Upstream tribution.“ ASME Tr(rri.s.. J . ßtrsic Eris. 84 Ser. D [41 415-8. Dec. ( 1962). ’ It . E. Sprenkle and N . S. Cocirtright. “Straightmins Vanes

for Flow Measurement.“ Mrcli. €rig. 80 [21 71-3. Feb. (1958) . ’’ J . A. Landstra. “Qiiarter-Circle Orifices,” T r ~ r i s . 1Ii .u. Clierir.

I’’ M. Bogema and P. L. Monkmeyei-. “The Quadrant Edge Orilicc-i\ Fluid M e t e r for Low Reynolds Ncimhers.“ .-iSiLJE

” l i . F. Steams, R. K. J o h n s o n . I i . M. Jackson. and C. A. Larson. f . ’ l o i i - Metr.sirrc’riierit with Oriliiz hlc,trrs. D. Van Nomand Co.. Inc.. New York. 350 (195 I ) .

”. T. J . Fiiban and W. A. Grilìin. “Smell-Diamrt~r-Orifice Metcring.” A S M E Tr(iri.s.. J . ßrrsic Eng. 82 Ser. D [3 ] 735-40. Sept. i 1960).

Eli:.. 38 [ l I 26-32 ( 1960).

Ti.<//i.~.. J . ß<i.\ic Eii,~,,. 82 Sei-. D [ 3 1 779-34, Sept. ( 19601.

30



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW

I:' I. O. Miner. "The Dall Flow Tube." ASME Tram. 78 'G Coinprit~itioit of Ori/iw Bores U s i i i g Cor i l r r Coiiiiecrioils.

Barton Instrument Corp.. Monterey Park. Calif. ( 1954). " L. J. Hooper. "Calibration of Six Beth Flow Meters at '' R. E. Sprenkle, "Piping Arrangements for Acceptable Flow

Alden Hydraulic Laboratory." A S M E Trnris. '72 1099. Nov. Meter Accuracy." A S M E Trtrirs. 67 345, July ( 1945). í 1950). API Std. 1101: Meusiireiiiclnt of Petroleiiiii Liyrrid HydrO-

L. J. Hooper, "Design and Calibration of the Lo-Loss crirboiis b y Positive Displtrcenieiit Meter , Ist edn.. Am. Petrol. Tube," A S M E Truiis. 84 Part II' 46 1-70. Dec. (1962). Inst.. New York. Aug. (1960).

1 475-9. Apr. ( 1956).

31



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

COPY PROVIDED FOR HISTûiUCM PURPOSIS ONLY

SECTION

2.1 COIVTENT Recommended practices for the installation of the

more commonly used instruments and devices for indicating, recording, and controlling liquid levels and liquid-liquid interface levels normally encountered in petroleum refinery processes are described in this section.

Instruments and devices excluded from discussion in this section are: 1. Solids level instruments. on which standard installation procedures and information have not yet been fully established. 2. Electronic types of level instruments which utilize probes to detect changes of capacitance or conductivity. 3. Level instruments which involve the use of photo- electric cells or radioactivity.

Because of the special nature of these instruments, the installation of each is a particular problem. For example. probe-type instruments should be considered in the light of their intrinsic safety, a field not covered in this manual: the use of radioactivity in instrumentation imposes health and safety considerations, subjects also not discussed hercin except as they apply to certain analyzers: SCC Part I t of the manual.

2.2 GEiVERAL Certain general procedures, practices, and precau-

tions apply to practically all instruments discussed in this section: where applicable, the generalities covered in items ( b ) through ( h ) should be considered a part of, or a preface to, the text of each of the subsequent discussions.

u. T y i ~ s Covere(l

The following types of instruments are covered: i . Locally mounted indicating gages (Par. 2.3). including tubular gage glasses, transparent and refex gage glasses, float-and-cable (automatic) tank gages, hydrostatic he3d pressure gages, differential pressure ievel indicators, m d miscellaneous types of gages. 2. Lcvcl transmitters (Par. 2.4), including displacers, ball I ioats. differential pressure types, hydrostatic head types. and electric and clectrunic types. 3. Locally mounted controllers (Par. 2.5). including displaccrs. ball foats. differential prcssurc types, direct expansion types. aiid altitude valves (static-head types). 4. Remote o r board-mountcd reccivcrs (Par. 7.6). 5. Levcl alarms (,Par. 2 . 7 ) . 6 . .4cccssorics ( Par. 7.8 ) . including seals Lind purges, I L -:i ce - p I ;I ~r s i I1 u 111 in ;i tors, and we:\ thc r protect ion.

1). All locally mountcd liquid levcl instruments, includ-

ing gage gliisscs. should be readily accessible from gradc.

;\cct.s?ii It i li t';

2-LEVEL

platform, fixed walkway, or fixed ladder. (It is not considered good practice to mount instruments where they can be reached only from a portable ladder. A rolling platform may be satisfactory if free access is available to the space below the instruments.)

c. Visibility

In all applications where a liquid level is regulated by a control valve, some indication of the level-gage glass, pressure gage, or other-must be clearly visible from the control valve location to permit manual control when necessary.

d. Connections to Vessels

Level instrument connections must be made directly to vessels and not to flow lines (continuous or intermittent) or flow nozzles. The connections and interconnecting piping should be installed so that no pockets or traps can occur. Where such pockets are unavoidable, drain valves should be provided at the lowest points.

e. ')lultil)le-Iiistruineiit Mounting

When two or more instruments, including gage glasses, are required for any application, such as gage glass and controller or gage glass and alarm switch, they should be mounted in such a way as to keep the number of openings in the vessel to a minimum. Suggested methods are the use of tees. as in Fig. 2-6 and 2-7, or the use of a common standpipe, as in Fig. 2-2(R) and 2-8.

f . Block Vulves

1. SIZE A N D MATERIAL

Ail block valves must have materials of construction, rating, and type of connections conforming to the minimum specifications for the equipment on which the instruments are connected. With the exceptions 3s noted in the following item 2. full-nozzle-size block valves should be installed at the vessel nozzles. Fittings or piping should not be installed between the nozzles and block valves.

2. EXCEPTIONS Block valves for tubular gage glasses, gages mounted

on standpipes, and hydrostatic head instruments and pressure switches, such as used for level alarm signals, should be installed as close as possible to the points of conncction to the vessel with no fittings other than pipe nipples or nozzles between each connection and the block valve. Where connected to nozzles, block valves should not be sized smaller than 1 in.; sce Fig. 2-1 (C) a n d ( E ) . Where connected to nipples or to standpipes,

32



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

e - ' . :~ _- . ... y ,. . I LEVEL

2.3 LOCALLY .MOUNTED INDICATING GAGES

< 1 . . . - __ __ - -__ ___ * , L.

block valves may be a minimum of %-in. size; see Fig. 2-1 ( D I .

Dual block salves for parallel instruments, connected by tees mounted directly on nozzles. as shown in Fig. 2-6. are permitted by some companies. Although this arrangement is a space-saver, and in many cases more economical, it makes the problem of calibrating a transmitter in operation more difficult: see also Par. 2.4 (a-5).

Some flange-mounted diaphragm-type level transmitters are designed to be mounted flush with the wall of the vessel. thus. block valves are impracticable.

a- Strain Relief

Connections between vessels and heavy gages, controllers, or transmitters should be relieved of strain by properly supporting such instruments (and seal pots, where used) and by installing offsets or expansion loops where necessary to provide for thermal expansion.

li. \'ihratioii

Many level instruments are susceptible to damage or malfunctioning if mounted in locations where they are subject to vibration. Care must be exercised in the selection of locations for mounting such devices.

!"BLOCK VALVE. . ¡"NOZZLE !"SLOCK VALVE ( G A Ï E ) "EDUCING (GATE) \,

D E

."' NOZZLES /,,

a C

PREFERRED METHOD OF INSTALLATION

Notes: 1. Nipples between block valves and gage cocks in A, C, and

E should be I in. by in., Schedule 160 reducing nipples. In B and D. they should be :h in.. Schedule 160 short nipples. Gage cocks are :;4 in. npt.

2. Block valve and nozzle sizes shown are minimum.

FIG. 2-1-Gage Glass Connections to Vessels.

Locally mounted indicating devices include tubular gage glasses. transparent (through-vision) and reflex gage glasses, float-and-cable (automatic) tank gages, hydrostatic head pressure gages, and differential pressure level indicators.

a. Tihilar Gage Glasses

1. APPLICATION Many companies discourage the use of tubular gage

glasses on process units. Where applicable. however. these gapes provide the simplest and most direct method for level measurement. This type generally is used in services where the temperature is below 300 F. the pressure is below 50 psig. and the material in the vessel is nontoxic and nonhazardous. Tubular gage glasses in short lengths occasionally are used in low-pressure steam service.

2. CONNECTIONS Gage glass connections to the vessel should be made

by means of block valves and automatic gage cocks. as shown in Fig. 7-1 í-iì. Some companies permit the use of automatic gage cocks only, as shown in Fig. 2-1 ( B ) . whereas other companies use block vulves and non- automatic page shutoff cocks, as shown in Fig. 3-1 ( C ) .

Where a connection is made to a nozzle 1?h in. or largcr, any reduction in pipe size should be accomplished in one step between the nozzle and the block valve, as shown in Fig. 2-1 ( E ) . 3. MASIMUM LENGTHS

Tubular gage glasses used in steam service should have a maximum length of 18 in.. and in other services 30 in. Where greater ranges of level are to be observed, overlapping gage glasses should be used.

4. PROTECTION The tubular gage glass should be protected by guard

rods and preferably should be mounted on the side of the vessel away from any source of damage, such as roadways, work areas, or mobile equipment lanes. How- ever! the gage must be visible to the operator at all times and especially in the event of an emergency. Where greater protection is required because of unavoidable exposure to such hazards, slotted sleeves or screen-type cages are sometimes preferred, even though they interfere with good visibility.

1). Transparent (Through-Vision) and Reflex

1. APPLICATION Transparent or reflex gages are used in most services

where the temperatures exceed 200 F; the pressures exceed 50 psig; and/or the vessels contain toxic, flammable, or otherwise hazardous fluids.

Gage Glasees

33



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

Trcrtispciretrt gcigc's should be ~ised in installations involving acid. caustic. o r dirty (o r dark-colorcd) nia- tcrials; in high-prcssure steam applications: and for liquid-liquid interfacc servicc; also in any application whcrc it is necessary to illuiiiinate the glass from the rear.

Reflex gtrges preferably should be used on all other clean service applications includinc C,l and heavier hydrocarbons. They may also be used on C ; and lighter hydrocarbons if the product does not dissolve the paint or other coating from inside the gage. thereby leaving a bare metal backwall which in turn reduces the effective- ness of the prisms.

2. GAGE ASSEiMßiIES

Gage columns arc made up of one or more standard- length scctions and should be connected to the vessel as recommended in Fig. 1-2 ( -4 ) . For greatest accuracy and safety. gagc columns should be limited in length to four sections or 5 ft between connections. and to three No. Y ( I2 in . ) sections in services at 400 F o r higher. In noncritical lcvcl applications. where errors in true Icvel can be tolerated and where the temperatures are less than 400 F. longer gage columns arc sometimes used. I n such cases, additional supports arc required. and expansion and contraction. which result from tem-

l',$OR 2- NOZZLE

4DRA!N/*' u- ALTERNATE" ..- DRA,N., 4'

A B '/;'GATE VALVE [/;OR 3/4GATE VALVE

SINGLE GAGE TWO OR MORE COLUMN GAGE COLUMKZ

FIG. 2-2-l~ge Coluiiiti As~eìiil>lies.

peraturc changes. must be taken into account by installing offsets o r expansion loops.

3. MULTIPi .E-GAGE MOUNTING Larze ranges of level preferably are observed by

mounting overlapping gage columns on a standpipe as shown in Fig. 3-2(B). Automatic gage cocks ?A in. in size senerally are used on multiple gages. Many refiners have found that the maintenance required on the ball checks of automatic gage cocks is so p e a t that they prefer to use individual block valves and pipe tees. Both types of installations are shown in Fig. 3-3(B). Vent and drain valves should be installed as illustrated.

Iiiterfrrce observation requires the use of transparent gage glasses. Fig. 2-3 shows two commonly used and recommended methods of mounting multiple gages on horizontal vessels where both liquid-liquid and liquid- vapor interfaces are to be observed. Connections to the vessel must be arranged so that there is always one in each phase of each interface being measured. Also. it is preferable to overlap multiple gages where possible.

4. PROTECTION AGAINST ETCHING On transparent gage columns to be installed where

the liquid or vapor will attack glass-e.g.. on steam drums or in applications involving hydrofluoric acid. amines. or caustic-a thin sheet of mica. Teflon. Kel-F, or other material which will withstand attack is sometimes inserted between each piece or between the gage glass and gage gasket to prevent etching of the glass.

I A L T E R N A T E I

BULL PLUG .- 34



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL ___._ . ... . -~ .

I t should be noted that sunlight will discolor somc plastics. therefore carc should be used i n selecting the matcrial for the shield. Such shields cannot be used in reflex gages as they will render the prisms ineffective.

c. Float-ancl-(:al,le (Automatic) Tank Gages

This type of gaging is the most common method of indirect level indication. Float-and-cable tank gages are used primarily on large storage tanks. The types of cable and tape, floats and guides are varied, and the indicating or transmitting devices are even more varied between gages manufactured by difl'erent makers. Thus, each installation presents it$ own individual problems. These gages should be installed by the manufacturer or in strict accordance nith his recommendations.

The reliability and continuing accuracy of a tank gage installation is directly dependent upon the condition of the tank on which it is installed. Old and in- correctly erected tanks-particularly those with unstable bottoms. shells, or roofs-will introduce appreciable amounts of error and variation which no gage, however carefully installed, can correct. In general, the following practices and precautions apply to all types of tank gage installations: 1. Automatic tank gages should be located in close proximity to the gaging hatch yet sufficiently distant from the suction and filling lines to minimize the disturbing effects of eddies, currents, or turbulence arising from these sources. 2. Either the ground-level or tank-top reading device should be at a convenient height or distance from the ground or the gaging platform to assure easy and correct readings, thus avoiding errors. 3. The entry point of the automatic gage tape should be such as to eliminate errors caused by roof movement. 4. Where turbulence-caused by high emptying and high filling rates or mechanical agitators-can affect the float or sensing element. i t is usually necessary to enclose the measuring element in a stilling well. Where high-viscosity matcrials are encountered. it may be desirable to provide heating for the stilling well. 5. All gages must be mounted securely to the tank shell, with a sufficient number of brackets properly attached and adequately spaced to hold the gage rigidly in place and in proper alignment at all points. The top horizontal tape conduit (extension arm) murt be braced by support members from the top angle oniy. 6 . N o floating-roof gage installation should have any tape exposed outside the tail pipe, as this can cause errors because of wind drift. 7. Float guide wires should be installed plumb, properly centered. free of kinks or twists, and pulled taut under proper spring tension. Y . Tank gage units of this type are sometimes used in conjunction with a remote indicating gaging system; where maximum accuracy is required. gravity-com- pcnsating dcviccs are available and should be uscd.

35

9. Some of the practices mentioned herein are outlined in API Standard 2500: Measirriiig, Surnpliiiy, rind Test- ing Crude Oil, which covers the installation and use of automatic tank gages; for further details reference should be made to this publication.

d. Hydrostatic Head Pressure Gages

1 . APPLICATIONS A N D LIMITATIONS Level indication by this means is limited to tanks or

vessels not under pressure. The height of a liquid above a pressure gage can be inferred from the pressure gage reading (hydrostatic hcad). providcd the density of the liquid is known. However. where bpecific gravity changes are large. this type of level indicator mill be highly in- accurate if read under one condition of calibration.

2. INSTALLATION Instruments used for reading head pressure are stand-

ard pressure instruments of relatively low range and should be installed in accordance with the recommendations outlined in Sect. 4. Pressure gage arrangements. illustrated in Fig. 2-4, include the direct hydrostatic head type, see arrangement A: the diaphragm and the bell (trapped air) types, see arrangernent B: and the air bubbler system with either remote or local gage. \ee arrangement C.

3. PRECAUTION Great care must be taken to prevent dirt. scale. or

sediment from entering the lead lines or tubing. as these instruments ordinarily have small (Y2 in. or ?Q in.) process connections and are easily plugged.

e. Differential Pressure Level Iiitlicators

For level indication alone. differential pressure instruments arc scldom used exccpt as transmitters. Where they are used, however, the installation of this type of instrument is the same as the installation of the differential pressure transmitter for level transmission. see Par. 2.4(d).

f. Miscellaneous Gages

Devices other than the aforementioned are sometimes used to detect level in certain special cases. Among the more common devices are: 1. Frost plugs, shown in Fig. 2-5íA). are used to detect the level of butane and other liquids which will boil in an ordinary gage glass. With sufficient humidity in the air, frost will form on all plugs which are below the surface level of the liquid. 3. Ram's horns, shown in Fig. 2-5iB). arc used especially in dirty, waxy, heavy black or coking oil services which are too severe for most types of float instruments. The large-size, nonclogging, curved pipe permits the product to flow up through it into the



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

,,PRESSURE GAGE FOR DIRECT HYDROSTATIC HEAD

ALTERNATE WITH

GAGE COCK

PRESSURE GAGE OR

MANOMETER

DIAPHRAGM BOX-HEAO TYPE

B

PRESSURE GAGE OR

MANOMETER

SECTION

BELL-TYPE TRAPPEOAIR SYSTEM

PRESSURE GAGE (LOCAL. REMOTE, OR BOTH) BLOCK VALVE- ,-REGULATOR

4 T \ORIFICE OR ALTERNATE. Il SIGHT FEED BUBBLER,

straight horizontal pipe, where it will cover a thermocouple and indicate by a temperature change that the level is at that point. These units are placed one above the other on a vessel at desired points where the liquid level is to be checked.

2.4 LEVEL TRANSMITTERS Transmitters include pneumatic and electrical systems

which have measuring elements of the displacer, bail Hoat. differential pressure, and hydrostatic head types. Some transmitters are equipped with dual pilots, one with an adjustable throttling range for control and one with a fixed band for transmission of level indication. In all cases, the transmitter should be located so as to be visible from the control valve whenever possible; the transmission circuits should be installed as outlined in Sect. 7.

a. Die placer Transmitters 1. TYPES AND FUNCTIONS

Displacer transmitters may be either blind or of the local-indicating type. They may be used as transmitters or as locally mounted controllers.

2. LIMITATIONS Because the displacer itself has relatively little motion,

its use should be avoided when heavy black, waxy oils

A

THERMOCOUPLE

THERMOCOUP

FIG. 2-5-Miseellaneous Gages: Frost Plug ( A ) : Rain’s Horn (B) .

or (especially) coking oils are encountered. or where material will settle out on the displacer or in the chamber. When it is necessary or perhaps desirable to use a displacer in such service. a liquid purge should be used.

3. MOUNTING OF EXTERNAL CAGE DISPLACERS ON VESSELS

For external cage displacer installations, connections to vessels should be made by means of nozzles, block valves, and pipe fittings selected for the service. Trans- mitters and controllers usually are piped with gage columns in parallel as shown in Fig. 2-6 through 2-8. Occasionally, however, it is advantageous to have an additional set of taps on the vessel for independent indication of level.

4. CONNECTIONS TO VESSELS When screwed connections are permitted. the nozzles

and piping may be 1% in. with unions placed as shown in Fig. 2-6. In most process applications, however, and especially where viscous fluids are involved, level transmitters and controllers should have 2-in. flanged connections; vessel nozzles and piping from nozzles to controllers should be 2 in. with flanged connections. Drain valves (gates) %I in. in size always should be provided and if a vent or vents are required or desired, they should be gate valves %i in. or 95 in. in size installed as

36



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

CONNECTIONS

UOZZLE-SIZE 3LOCK VAL'/E

??ESSURE VESSEL

(GATE)

$"AUTOMATIC GAGE COCK OR TEE

UNIONS WHEN THREADED CCNNECTiONS ARE USED

GAGE COLUMN I&i A X I

SIDE AND BOTTOM SIDE AND SIDE TOP AND SIDE CONNECTIONS CONNECTIONS CONNECTIONS

FIG. 2-C-Esternal Cage Displacer with Parallel Gage Glass.

indicated in Fig. 2-6. (Some companies permit the use of %-in. gate valves in both these locations. particularly in clean services.)

5. MOUNTING OF EXTERNAL CAGE DISPLACER AND STANDPIPE

Where greater ranges of level are encountered. e.g., over 4 ft or 5 ft, a standpipe and overlapping gage glasses may be used as shown in Fig. 2-7 and 2-8. The standpipe, usually of 2-in. or 3-in. pipe, serves as a mechanical support for the instruments and as a surge chamber to prevent turbulence or foam from interfering with the operation of the transmitter. In addition, the arrangemcnt in Fig. 2-8 permits direct calibration of zero and span of a transmitter or controller (with the vessel either in or out of service) by properly manipulat- ing the block, drain, and vent valves in such a way as to run the level of the fluid up and down in the gage column and transmitter in parallel. In cases where levels of considerable range are to be transmitted, it may be

,-3OOO-L8 COUPLING

COR :'')EN $VENT.+ NOZZLE-SIZE

BCOCKVALVEI

NOZZLE-SIZE T, SLOCK VALVE

i DRAIN If "

I l''(?a ->"x :"SWAGED NIPPLE

TYPICAL INSTALLATION VSING CUAL BLOCK VALVES AND ONE bAGE. :VlTr WELDED E i 9 0 W CONSTPUCTION

ALTEPNATE - WITH STANDPIPE (WELDED ELBOW TYPE ) FOR

N o res: I . Some companies require the third block valve ( a t the

nozzle) in this type of assembly. 7. Controller may be piped with side and bottom. side and

side. or top and side connections: as shown in Fig. 2-6. 3. Nozzle spacing on the vessel is critical on close-coupled

installations, especially where side and side connections are used. because of differential expansion of vessel and controller. Double or reverse elbow connections are sometimes used on the upper side connection to minimize trouble from this sotirce.

FIG. %;-External Cage Displacer with Parallel (;age or Standpipe.

preferable to use a differential pressure transmitter, see (d ) following.

6. PURGING In some installations, for example on crude oil unit

steam strippers where condensing steam can drip into hot oil in the displacer cage, it is sometimes necessary to purge the top of the displacer cage with gas. Purging installations are described in Sect. 8.

7. INTERNAL DISPLACERS Occasionally, and particularly when it is desired to

avoid steam tracins, it is preferable to mount the displacer inside the vessel rather than in an outside cage. When mounted on top of a vessel, the vessel nozzle and the head casting of the instrument must be provided with mating flanges of the type and specification requircd by the service.

Ample overhead clearance must be provided for removal of the float and rod. When a side mounting is required, the vessel nozzle and the torque-tube housing

37



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

PRESSURE VESSEL 2"OR LARGER NOZZLE

GATE VALVE (VENT) ___t

OR PLUG t 2" GATE VALVE

GATE VALVE

-AUTOMATIC GAGE COCKS OR TEES

2"OR 3" STANDPIPE

GATE VALVE(VENT)

I!j"OR LARGER SCREWED OR FLANGE CONNEC- T IONS

I g O R LARGER

,)"COUPLING - moo LB TAPPED ONE END ONLY

\+''OR LARGER GATE VALVE

THESE ASSEMBLIES MAY BE ELBOWS A S SHOWN IN --- FIG, 2-7

REDUCER TO PIPE

Nofe: Controller may be piped with side and bottom or side and side connections a s shown in Fig. 7-6.

FIG. 2-8-S1aiidpipe with Extenial Cage Displacer Cotitroll~tr :irid Miiltiple Sight Cages.

of the instrurrient should be provided with the proper type of mating flanges and provision should be made for access to the lloat-torque-tube connection, e.%., a man hole.

S. T N T E R N A L DISPLACER GUIDES i n many internal displacer installations, guides are

rcquircd. A stilling well for side-mounted displacers, as shown in Fig. 2-9, usually is provided for this purpose, although rod or ring guides are sometimes used. Ring guides are particularly suitable for emulsion service.

9. SIGNAL TRANSMISSION

Where the signal is transmitted to a remote controller or board-mounted instrument, the transmission should be accomplished as outlined in Sect. 7. When the displacer instrument. mounted in any of the foregoing ways. is of the electrical type, as for alarms or protective devices, it should be piped as described herein. The electrical wiring should conform to the electrical code applicablc; see. also, Sect. 13.

1,. Caged Ball Flout Trarisriiitters

This type of transmitter is most generally used in clean services for the direct operation of valves or electrical switches for alarms or pump motor controls. Where they are installed directly on vessels, connections

WELDED TOSHELL ~

6 WELDED TO PIPE/

1

FIG. 2-9-Typical Stilliiig Well.

should be made as described for the installation of displacer transmitters in (a ) preceding.

r. Internal Ball Float Transmitters

1. APPLICATION This type of instrument is sometimes used for heavy

black, waxy or coking oil service, or where the liquid contains particles or materials which tend to settle out and which would eventually block the float action in an external cage type of instrument. On severe coking applications. it may be desirable to use a steam or flushing oil purge to keep the shaft free and the packing in suitable condition. The trend in such applications is toward the use of dip-tube, purge-type. or differentiai-pressure type levei transmitters and controllers, where possible.

2. INSTALLATION Where an internal ball float is considered necessary.

either the rotary-shaft type (Fig. 2-10) or the 10-in. flange-mounted type (Fig. 2-11) may be used. Where the float will be subjected to turbulence within the vessel. shielding. guiding, or other provision should be made to eliminate the effects of turbulence on the float. Pneumatic piping or electrical wiring to such transmitters should be in accordance with the recommended practices for transmission as outlined in Sect. 7.

3. SUPPLEMENTAL INDICATOR In severe services, as outlined in the preceding para-

graph, it is recommended that the transmitter or con-

38



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL

4- A-- c

THIS TYPE CF i N 5 Ï A L L A - TION SHOULD NOT BE USED FOR VESSELS WITH AN INSIDE DIAMETER OF LESS THAN 4‘6”

PLAN VIEW FIG. 2-lO-In~tallation of Internal Rotary Shaft Ball Float

Level Iiiatrnnient in Vertical Vebsel.

troller be supplemented by another type of instrument to serve as a check indicator, e.g.. a hydrostatic head instrument. a through-vision gage column. or a set of try cocks or ram‘s horns as described in Par. 2.3 ( f-2).

tl. Differential Pressure Transmitters

There are three commonly used types of differential pressure transmitters-force balance. motion balance

b- 40”MAX-

PLAN VIEW

‘ANGLE OF FLOAT TRAVEL^ ,--\

* W I T H PILOT VALVE: 3O0MAX

ELEVATION VIEW

FIG. 2-11-Inrtalliitioci of Interrial Float Flnii,Rt.-Ill~,iiiiic:tl- TyI),. Level Iiirtrniiient.

(bellows and diaphragm ) , and mercury. In general, differential pressure transmitters for level transmission will be most accurate where the measured fluid is of fairly constant specific gravity.

1. FORCE-BALANCE TRANSMITTERS

Applications of force-balance transmitters include local control. remote control. and remote recording of wide ranges of liquid level. Such instruments are commonly known as differential pressure (DP) cells, or converters.

Connections to the vessel may be made by means of pipe fittings of the material and rating recommended for the service, or by means of Vz-in. tubing and tubing fittings. The transmitter should not depend upon its own piping for support. but should be yoke-mounted or bracket-mounted. Typical installations are shown in Fig. 2-12. views ( A ) and ( F ) .

Constant head may be maintained on the external or reference leg of the transmitter when condensables are present by means of a seal pot. as shown in Fig. 2-L2(B).

Temperature compensation may be accomplished automatically and a constant head maintained by the method shown in Fig. 2-13. There are numerous other methods of heating or cooling to keep the reference leg at the same temperature as the vessel liquid; however, because they are not generally used, they are not described herein.

2 . MOTION-BA LANC E TRAN SM ITT ERS

Applicmions are generally the same as for force- balance transmitters. They normally provide local indication independent of the transmitter mechanism.

Connections to the vessel may be made in the same manner as for force-balance transmitters. However, because of the liquid volume displacement. installation details as shown in Fig. 2-12(C) and (D) should be ïollowed.

Constant l i e d may be maintained on the reference le; of the transmitter when condensable are present by means of a constant-head pot as shown in Fig. 2-12 ( B ) .

Tempercitiire cornpensation may be accomplished by the method shown in Fig. 2-13 with the addition of a constant-head pot.

3. MERCURY

Mercury manonieters are not extensively used for Ievcl transmission. their place having becn taken by the force- and motion-balancc transmitters. However. where they arc used, the installation is the same as for the other types, cxccpt that the mercury manometer requires seal pots and a different and more earcïul weatherproofing treatment.

39



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

STANDARD TYPE OF INSTALLATION FOR INSTALLATION. WITH FORCE-BALANCE CONSTANT HEAD POT, TRANSMITTERS FOR MERCURY OR

MOTION-BALANCE TRANSMITTERS

I1 I \ TRAN S MITT ER TRANSMITTER

I ANOTHER TYPE

MANIFOLD

\ /TRANSMITIER/ 1 WHERE NECESSAR" TO CHECIIZEROANDSPANOF TRANSMITTER WHILE IN SERVICE, THISCHECK BYPnSS 8 ARRANGEMENT IS

SOMETIMES USED

LIAY ALSO @E USED FCR THIS SAME

TRANSMITTERS PtJRPOSE )

a SEALPOT --

UNIONS IF LEADS AR€ O F PIPE

iNSTALL UNIONS NEAR MANIFOL D

MANIFOLD

TRANSMITTER

C 0 DIFFERENTIAL PRESSURE TRANSMITTER LOCATED ON PLATFORM ABOVE TOPCONNECTION

TY VARIABLE

LEVEL MEASUREMENT WITH DIFFERENTIAL PRESSURE INSTRUMENTS APPLYING PURGE GAS

F

FIG. 2-1 2-Typic.nl Installations of Differential Pressiiri: Level Instruments.

VESSEL

CUP AÓDED WHEN USED WITH MOTION-BALANCE TRANSMITTERS

INTERNAL PIPE

EXTERNAL PIPE

1

TRANSMITTER

FIG. 2-13-Differc.riti:ii L t : w i Arrangenient to Compensate for Temperature.

e. Hvdrostatic Head Transmitters

1. INSTALLATION Hydrostatic head may be transmitted by means of a

bubbler tube and differential pressure transmitter as shown in Fig. 2-14(A) and ( B ) , or by means of a diaphragm- or bellows-actuated air pilot or differential- pressure-type transmitter mounted directly on the vessel as shown in Fig. 2-14(C). The latter type should be mounted on a flanged nozzle at such a point that it will not be subject to blocking by sediment. (It should be pointed out that some makes of the diaphragm- or bellows-actuated pneumatic pilot are nonlinear in the lower 20 percent of their range.)

2. PRECAUTIONS Bubbler tubes must be sized to prevent pressure-drop

errors, which result from purge gas flow. They must be installed so that sediment cannot block the open ends, and must be supported, if necessary, so that turbulence or mechanical strains cannot bend or break them. In addition, for greatest accuracy the connecting leads mirst be leakproof.

f. Electric and Electronic Level Transmitters

Several types of electric and electronic level transmitters are available, some of which are actuated by floats, others by hydrostatic head or differential pressure. In all cases, the scnsing device is installed in

40



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL

/' ' , \ #7 I

ALTERNATE: WITH SIGHT FEED NEEDLE A

BUBBLER VALVE

PURGE GAS

SAME A S A EXCEPT A 5

USED ON PRESSURE

VESSEL PRESSURE B

I ) .

I . PNEUMATIC A N D ELECTRIC TYPES

types of transmitters in Par 2.4(b) and (c ) .

2. MECHANICAL TYPES These are most generally used in water service and

consist of a mechanically actuated valve connected by a shaft or lever linkage to either an external caged float or an internal float. installation of the float mechanism is the same as that for a pneumatic or electric ball float instrument. Care must be taken to see that nothing blocks the action of the float and that the float is protected from turbulence. Furthermore. the valve and the piping must be installed and supported so that there is no strain on the valve or packing gland and no interference with linkages or levers which might prevent full travel of the float and valve.

Caged aiicl Internal Ball Float Controllers

Installation is the same as outlined for the equivalent

c . Differential Pressiire Controllers TANK OR I VESSEL OPEN OR

VENTEDTO ATMOSPHERE I C VESSEL

UNDER PRESSURE

-CONSTANT HEAD LEG

DIAPHRAGM OR BELLOWS ACTUATED PNEUMATIC PILOT. OR FLANGE-

SAME AS c EXCEPT AS USEDON PRESSURE

MOUNTED DIFFERENTIAL VESSEL PRESSURE TDANSMITTEQ

FIG. 2-1PHvdrostatic Head and Differential Pressure Level Transmitters.

accordance with the practices outlined in preceding paragraphs. and the transmission of the signal is accomplished as described in Sect. 7.

Transmitters or transducers for electronic instruments should not be located too close to hot lines. vessels. or other equipment. Ambient temperatures which exceed 140 F are likely to result in calibration difficulties and rapid deterioration of electronic components. Suscepti- bility of mechanical or electronic components to vibration should be ascertained and, where necessary, adjustments should be made in the mounting.

2.5 LOCALLY MOUNTED CONTROLLERS

Locally mounted controllers used on all pressure vessels include the following types: displacer, caged bail float, internal ball float, differential pressure, and direct expansion. The altitude valve or other static-head type of locally mounted controller is used for vessels or tanks not undcr pressurc.

a. Diaplacer Controllers

Recommended practices for the installation of displacer controllcrs are the same as for equivalent types of transmitters outlined in Par. 2.4(a).

1. APPLICATION These arc basically the samc as transmitters and.

without accessory devices. are 1 00-percent proportional band ( nonadjustable) controllers. The 100-percent proportional band unit can be used together with a valve positioner to attain a system which can be applied as an adjustable proportional band controller or an on-off controller. This unit gives a wide latitude of adjustment.

2. INSTALLATION

types of transmitters. see Par. 2.4(d-1. -2 ) . The installation is basically the same as for equivalent

d. Direct Expansion Controllers

1. APPLICATION Direct expansion controllers are used on steam gen-

erators or in applications where a single boiling-point liquid is being vaporized so that expansion of the tube or thermostat is different in the liquid and vapor regions. Generally the expansion of the tube operates either a control pilot or a valve directly to control the flow of fluid into the vessel.

2. INSTALLATION

Care must be exercised in installation to assure freedom of motion of the free end of the thermostat tube. The tube must nof be painted. If mounted where it is exposed to the weather, it must be housed or shielded to protect it from rain and snow.

Piping leads to the expansion tube should be weil insulated. Fig. 2-15 shows a typical arrangement of such a controller operating a direct-connected valve in the feedwater line to a boiler.

41



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

,IN SOME INSTALLATIONS, A DIRECT-CONNECTED VALVE 15 MOUNTED HERE. IN OTHERS, THIS IS A PILOT VALVE

MOUNTED ON SIDE

BOILER DRUM

EXPANSION TUBE OR THERMOSTAT’

TENSION RELIEF LINK

DRAIN

TED VALVE

FEEDWATER

\ IN SOME INSTALLATIONS THIS IS A DIFFERENTIALLY LINKED VALVE POSITIONED BY BOTH BOILER WATER LEVEL AND STEAM FLOW

FI(;. 2-15-Espeiision Titlie System.

e. -Altitiirle Valve ( Static-Head Type) Controller

A standard piping arrangement for an altitude valve type of controller is shown in Fig. 2-16. A self-contained. cclf-regulating level controller of this type may be mountcd at almost any distance from the (nonpressurized) vessel or tank provided the pilot valvc is considerably lower than the lowest desired level of the liquid. Because the line fluid is the actuating medium, the altitude valve generally is used in water service.

2.6 RERIOTE OR BOARD-MOUNTED RECEIVERS

Although level is sometimes recorded by transmitting to a second pen on a flowmeter or pressure recorder, it may also be indicated, recorded, and/or controlled by individual levei instruments actuated by transmitted. signals. Receivers may be either pneumatic or electric.

a. Installation

Recommended practices for the installation of remote or board-mounted receivers may be found in Sect. 4. 5. 7, and 11. Design of the installation should be such that a high level causes the pointer or pen to move upscale or toward the outside of round charts. (Instruments which read in the reverse of normal are apt to cause confusion and be misread, particularly during upset conditions when it is most important that they be read easily, quickly. and correctly.)

1). Range

ments is O to 100. representing percent of maximum. Recommended scale or chart range for level instru-

2.7 LEVEL ALARMS

Basic instruments for initiating high-level or low- level alarm signals are, with the possible exception of the float size, the same as the float-type controllers discussed in Par. 2 .5(b) . Other types are sometimes used. e.g., pressure switches at the receiver in pneumatic transmission systems, hydrostatic head pressure-actuated switches on nonpressurized tanks, and differential pressure-actuated switches on pressurized vessels. (For a detailed discussion of alarms and protective devices, see Sect. 13.)

a. Installation of Float Alarms

The installation of float alarms is the same as for the equivalent types of transmitters covered in Par. 2.4(a), (b) , and (c). A typical installation of high-level and low-level alarm switches with parallel gage glass is shown in Fig. 2-17.

1). Installation of Other Alarms

Pressure switches in pneumatic transmission circuits are installed without block valves. A sensitive pressure actuated switch or differential pressure-actuated switch mounted directly on a tank or vessel to signal high- or low-hydrostatic head should be located at a point on the tank or vessel which is not subject to blocking by sediment.

c. Signal Traiisniissioii

Installation practices are discussed in Sect. 7 and 13.

42



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL

ARRANGEMENT FOR A L A R M SWITCHES

J H1”NOZZLE OR BOSS

ON VESSEL

GAGE COCKS OFFSET TYPE

ALTERNATE ARRANGEMENT

FOR ALARM S W ITCHES

PURGE OR TEST CONNECTION f

FIG. 2-1 í-Arraiigemeiit of High- and Low-Level Alarm Switches with I’arallel Gage Glass.

2.8 -ACCESSORIES

a. Seals and Purges

Occasionally it is necessary to use seal pots or purges in connection with liquid level instruments. The application of seals and purges is discussed in Sect. S.

b. Gage Glass Illuminators

Where it is necessary to back-illuminate transparent gage columns, it is recommended that lighting fittings, made for the purpose and suitable for the service conditions, be purchased and installed in accordance with applicable codes and the manufacturer’s recommendations.

c. Weather Protection

1. GENERAL All locally mounted instruments and lead lines

handlins water or process fluids which may freeze, form hydrates. or become excessively viscous in cold weather should be heated and insulated. In addition, transmitters and locally mounted instruments other than gage glasses should be suitably protected by housings or other protective shielding to prevent improper instrument performance or excessive maintenance as caused by the effects of weather. Frost shields should be used on transparent and reflex gage glasses if the operating temperatures rire below 32 F. Heated gage glasses and jackcted gage cocks (see Fig. 2-18) are availablc from some manufacturers and are used in

\ Y

L4 I l I l I I DETAIL

\ --. .

... ‘. ‘.

HEATING f TUBE

CROSS-SECTION (REFLEX GAGE)

FIG. 2-18-Heated Gage Glass (or “Stearii-(;uttecl”).

many applications which require heating of gage glasses. They should be installed as shown in the view depicted by Fig. 3-18.

2. STEAM TRACING Steam tracing is commonly used for protection of

both instruments and lead lines. A correctly installed steam-tracing system must have an individual shutoff valve and a trap on each individual lead. Where the process fluid in the lines or instruments being steam- traced has a boiling point lower than the steam temperature, care must be taken to separate or insulate the steam tracer to prevent the possibility of causing the fluid to boil; see Sect. 8, Par. 8.4(b).

.

3. OTHER METHODS OF HEATING In some climates it is satisfactory to use steam con-

densate for tracing. In isolated cases, particularly in nonhazardous areas, electrical heaters are sometimes used to heut gage glasses, instrument cases, and short lead lines.

4. WINTERIZING For complete coverage of steam-tracing practices,

seals and purges, and winterizing in general. refer to Sect. 8.

43



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

COPY PROVIDED FOR HISTORICAL PURPOSES ONLY

SECTION 3-TEMPERATURE

3.1 CONTENT

This section presents common practices for instaila- tion of devices for measuring temperature in reñnery process services and for: 1. Indicating the temperature at the point of measurement. 2. Utilizing the temperature for local control of the process variable. 3. Transmitting the temperature to a remote location for indication or control at that point.

Included in the discussion are the more common types of measuring devices: thermowells, glass and dial thermometers, filled-system instruments, thermocouples, and resistance thermometers. Self-acting temperature controllers are also included insofar as the temperature actuation is concerned. The installation of automatic control equipment is discussed in Sect. 5 . Only brief mention is made of methoás for the transmission of teniperature measurements. Transmission systems arc

. discussed in Sect. 7.

3.2 ?Y?HERMOWELLS

a. Gtweral

It is not usually possible to expose the temperature- sensing device to the process fluid. In spite of the thermal lag introduccd. thermowells (see Fig. 3-1 through 3-7) arc employed in temperature measurement to protect the thermal elements and to permit removal o f these elements during plant operation. i t is important to maintain good contact between all temperature-sensing clcmcnts and their wells.

1,. Tlierniowell Insertion Length Thc inccrtion length. U. is the distance from the free

end of the temperature-sensing element or well to, but not including, the external threads or other means of attachment to a vessel or pipe (see Fig. 3-1 ) .

.ENGTH

c. Tlierriiowell Immersion Length

The immersion length, R, is the distance from the free end of the temperature-sensing element or well to the point of immersion in the medium, the temperature of which is being measured; see Fig. 3-1. (Normally, this point would coincide with the inner wall of the vessel or pipe.) The immersion length required to obtain optimum accuracy and response time is a function of mechanical factors such as: type of sensing element. available space. well-to-fluid container mechanical connection dcsign. and well strength requirement. Optimum immersion depth also depends upon heat transfer considerations as determined by the physical properties of the measured fluid. flowing velocity, temperature difference between fluid measured and well head. and material and mass distribution of the well and the sensing element. A detailed review of these factors rarely ever is justified for petroleum refinery distillation units. Com- mon considerations are:

I . The entire heat-sensitive length of a bulb (whether a bimetallic element; gas-, liquid-, or vapor-Illled bulb: or resistance thermometer elemcnt) must be immersed in the heat zone to be measured. 3. For thermocouples : ?Ten times the outside diameter of the protecting tube is the recommended minimum immersion; this value should be increased where space permits. With flowing liquids, six diameters immersion may be used if the pipe and the external portion of the protecting tube are well insulated.? l a 3. On small lines. where adequate immersion cannot be obtained by ;I thermowell inscrtcd perpendicular to the line, the usual practice is to insert the well at a 90-deg bend in the line; a less preferred method would be to enlarge a short section of the line to accommodate the thermowell. These practices are seldom required on lines 4 in. or larger in size. Typical installations are shown in Fig. 3-5.

CI. Thermowell Materials

The materials selected must be suitable for the temperature and corrosion environment encountered. For general services, up to 1,200 F, the minimum quality material usually specified is Type 304 or Type 3 16 stainless steel. Therrnowells in certain corrosive services, such as dilute acids, chlorides, and heavy organic acids, require well materials suitable for the specific corrosive media.

e. Tlierniowell Construction

Typical thermowell construction and installation details arc shown in Fig. 3-1 through 3-7. Thermowells may be screwed as shown in Fig. 3-2 and 3-6. How- ever, where frequent inspection, special materials (e.g.,

:? Figtircs refer to REFERENCES on p. 50.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

TEMPERATURE

e-------- -3-

q-+&UNiON OPTIONAL

JAN STONE TYPE FLANGE

FIG. 3-3-1 y2-Iti. Flanged T h e m u w e l l V a n Stone.

I i

! !

i

~

3” _ _ -I,

*-* 8

FIG. 311-1-111. Groiiiitl-Joint Therniowell.

glass coating) ~ or pressure and temperature limitations require, flanged wells, such as shown in Fig. 3-3 and 3-41 are commonly installed. The thermowell shown in Fig. 3-7 has been found convenient when flanged construction is desired for erosive service. It is important to maintain good contact between all temperature-sensing elements and their protective wells. When expeli- ence indicates that rapid temperature response is necessary, thermowells for temperature controller installations should be constructed with wall thicknesses as thin as operating conditions will permit.

Caution: Where atomic hydrogen may permeate the thermowell, it can lead to the destruction of a filled thermal system unless the well is vented to the atmosphere.

3.3 THERMOMETERS FOR LOCAL TEMPERA- TURE MEASUREMENT

a. General

Because local temperature-measuring devices are exposed to accidental contact and possible damage, they must be reasonably rugged and at the same time provide the necessary accuracy.

1>. Mercury in Glass “Industrial” Thermometers

These instruments use mercury or other liquid of low freezing point; they are mounted in metal frames

45



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

t FIG. 3-3-Thermowell Iiisiallation for Small Lines ( 3 In. or Smaller).

and usually :ire provided with glass covers. The thermometer bulb is inserted in the thermowell. A desiccant sometimes is used inside the case to prevent condensation on the glass. Glass thermometers generally have been superseded by dial thermometers, see (c ) , because of the quick readability, resistance to breakage, and other merits of the dial-type construction.

C. Dial 'rlic,riiioiiiett.rs

These are the most common thermometers in industrial LISC. They arc frcqucntly of the bimetallic type with circular dials and are available in a wide range of temperature scales and styles. Dial-type thermometers which use tilled systems are also available (sec Par. 3.4).

TEMPERATURE EULE

RECORDER OR CONTROLLER, INDUSTRIAL THERMOMETER, DIAL THERMOMETER. ETC)

(BULB-TYPE TEMPERATURE

' I

I l

STANDARD PIPE THREAD(USUAL

IPS THREADED CONNECTION

3.4 FILLED-SYSTEM TEMPERATURE INSTRU- MENTS

a. General

This type of instrument consists of a temperature- sensing bulb, a capillary tube, and a pressure-sensing device. Filled systems utilize a liquid, gas, or vapor. depending upon the requirements and temperature range of the system. Ambient temperature conpensation is often required. Available overrange protection varies with diffcrcnt types and may also influence the selection. Where practicable. an overrange of at least 50 percent is desirable. The usc of filled-system devices is limited by the length of capillary tubing which may be employed and by the maximum temperature to which the bulb may be exposed.

l',ASA 5:: 32 TAPEDES ? PE TEREAD I" SC-EDIJLE 80

36



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

TEMPERATURE

Special consideration should be given to the installation of narrow-\pan. force-balance temperature transmitters with uncompcnsatcd capillury. 50 that capillary length is held to ;I minimum. Normally, the manufacturer's standard is 5 ft: on outdoor installations. it is advisable that capillary length not exceed 10 ft.

h. Self-Acting Teiiiperatiire Controllers

Where precise control is not essential, self-acting temperature controllers are used frequently. These devices utilize thermal expansion systems and direct-operated valves. In operation. an increase in temperature expands the liquid in the system and thereby operates the valve. Because of the many different fluids beins uscd. bulb sizes and filling fluids vary with the temperature range. As with other temperature-sensing instruments. bulbs should be protected by thermowells.

Valve operators are in bellows form. The bellows may operate either a valve (directly) or a pilot valve which controls line Huid for actuating power to operate the main valve. Temperature indication in the form of ;?

dial mounted on the top of the valve and operated by the same thermal system is available from some manufacturers. Some form of temperature indication is always desirable with these self-contained devices.

4 s with all capillary types of thermal systems. care must be taken to protect the bulb and capillary from any damage which may cause fluid leaks or impede the flow of fluid. Also, i t is desirable that the installation be made so that the valve can be removed for inspection or service without damage to the capillary.

c . Temperature Traiisiiiitters

Temperature transmitters may utilize any one of several types of filled systems. together with pneumatic or electronic transmitting and amplifying devices. to con- vert the measured temperature into an air or electrical signal. In addition to covering a wide range of temperatures. some instruments can be obtained with the following additional features:

1. SUPPRESSED RANGE A very accurate measurement of temperature can be

obtained by selecting a transmitter with full range of air pressure output over only a portion of the operating temperature limits of the instrument.

2. THERMAL LAG COMPENSATION

in some instruments it is possible to obtain a pneumatic device (dcrivative action directly applied in the measuring systcm ) which. when properly adjusted. will compcnsatc for thermal lag with a resultant high speed of responsc. Some companies use this compensation for transmitters associated with controllers but not for transmitters for recording instruments alone.

I

a I. In all installations of filled-system temperature instru-

ments! it is necessary to protect the bulb and capillary tubing from mechanical damage. It is usually desirable to use armored capillary tubing and to support the tubing run between the bulb and controller or transmitter in such a manner as to protect it from accidental damage. It is essential that the capillary tubing be not cut or opened in any manner.

3.5 THERMOCOUPLE TEMPERATURE

a. iipplications

Temperature instruments utilizing thermocouples are now the most generally applied of all temperature-measuring devices. They are applicable for a wide range of temperatures with reasonably good accuracy. Typical installations Lire shown in Fig. 3-8.

1). General

1. MATERIALS AND RANGES

thermocouple materiais are:

Preruii t i o t i s

IRSTRUMENTS

In the petroleum industry the most commonly used

Usual Temperature

Range Thermocouple ISA (Degrees

Materials Symbol Fahrenheit) Iron-constantan o to 1,200 Chromel-Alumel * _ . . . . . . . . . . . K 800 to 2,000 Copper-constantan . , . , . . . . . . . . . T -300to 200

, . . . . . . . . . . . . . . J

Because thermocouples normally are installed in thermowells. the couple usually is selected for the temperature range and the well material is selected suitable for the measured media. Where thermocouples are installed without protective thermowells, it is common practice to use iron-constantan in reducing atmospheres and Chromel-Alumel in oxidizing atmospheres.

2 . F.ABRIC.ATION Fabrication details for thermocouples are covered in

;I publication:: of the Instrument Society of America (ISA).

Recently, increased use has been made of metal- sheathed. mineral-insulated thermocouple assemblies. These assemblies are made by insulating the thermocouple conductors with a high purity, densely packed ceramic insulation (usually magnesium oxide') and en- closing the assembly in an outer metal sheath. These thermocouples are available with outside diameters ranging from 0.04 in. to 0.84 in. and thermocouple wire sizes from No. 36 gage up to NO. 8 gage. Outside sheath material is available in a variety of stainless steels, in- conei. moncl, titanium. tantalum. platinum. or any workabic metal. Thermocouples are available in lengths

:: Trademark: products of other manufacturers may also be -

iiscd.

J

47



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

APl R P 550-PART I

L . > 6 da- 12 =I

“c r w E THERMOCOUPL,F , 1 ” VALE CONDUIT FITTING, HEAD - i” \ ?/a - .. {UNION I

@-+: ::

OPTIONAL WITH RUBBER RUSHING

/ Í ‘ x I/: REDUCER

OPEN DUPLEX WIRE SCREWED WELL, 3/4” MALE THREAD FLANGED WELL, ‘/2” FEMALE THREAD

L TYPE THERMOCOUPLE /I” MALE ~,,, HEAD - I ” X 3 / 4 ’ ’ 1 . .

\\ I,

CONDUIT FITTING,

REQUIRED

- 3/$‘ FLEX I ELE

‘,‘Tx 3”STEEL NIPPLE

OR

JACKET(SEE NOTE)

FLEXIBLE CONDUIT SCREWED WELL, 3/4” MALE THREAD FLANGED WELL, /2” FEMALE THREAD

iVofr,: Where “TW” jncketcd flexible steel conduit is used. it should be vented to relieve the pressure in case of thermocouple wcll failure.

FI(;. : ~ - X - T h < . r i i i ~ i < < > i i ~ ~ l e - t ~ ~ - ~ < i t i ~ l i i i t Coiiiiectioiir.

up to 40 ft or longer. Threc typcs of measuring junctions (see Fig. 3-Y) are available: 1 . A is the standard construction with grounded tip, weldcd or silver soldered for fast response. 3. ß has a n exposed tip for extremely fast response. 3. C has an isolated tip (electrically isolated from the sheath) with slower rcsponse.

3. INSTALLATION For field assembly, thermocouples may be connected

as shown in Fig. 3-10. More commonly, however, thermocouples will be purchased ready for installation.

THERMOCOUPLE

INSULATION CROSS-SECTION

FIG. 3-9-Metal-Sheathed, Mineral-Insulated Thermocoiiple Assemblies.

4s

IRON-CONSTANTAN THERMOCOUPLE WITH PORCELAIN BEAD INSULATION

IRON-CONSTANTAN THERMOCOUPLE WITH GLASS INSULATION

These thermocouples are installed in thermowells as discussed in Par. 3.2. I t is essential that a thermocouple be in contact with the well to minimize temperature lags. Metal-sheathed, mineral-insulated couples are sometimes installed as bare elements without wells on special applications as shown in Fig. 3-1 1.

c. Tube Temperature Measurement

A special application of thermocouples is the measurement of “skin-point” temperature of furnace tubes. Such installations require careful attention to be certain that the thermocouple is properly attached to the tube and is shielded from furnace radiation. Care must be exercised to avoid adding mass at the point of measurement which may assume a temperature different from that of the relatively cool tube wall to which it is attached. Many companies have their own standards for this ap-



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

TEMPERATURE

,WELD

METAL-SHEATHED, MINERAL- INSULATED THERMOCOUPLE

n STUFFING BOX

GATE VALVE

THERMOCOUPLE

TUBE F I T T I N G

VESSEL OR PIPE WALL

FIG. 3-1 1-Iii~iallaiioii o f Thermocouples Without \\-ell-..

plication. These installations have been costly and complex and not entirely reliable. The development of mineral-insulated couples gives promise of simpler installations with greater reliability.

One design for attaching this type of thermocouple to heater tubes is shown in FiS. 3-12(A). The attachment block is ;ici in. by ? { 6 in. by 3 in. in size and is made of the same metal as the heater tubes. TWO holes are drilled through the block at an angle of 30 deg for the insertion of the thermocouple. The two holes permit the replacement of the thermocouple at least once without the necessity of welding on a new block. This is important with alloy tubes as the stress-relieving required after welding is both expensive and time-consuming. The thermocouple is inserted in the hole in the block and tightly peened after the block has been welded on the tube. The thermocouple is then held firmly to the tube with stainless steel bands.

Another method of installing metal-sheathed. mineral- insulated thermocouples to heater tubes is shown in Fig. 3-12(B). In this design, the thermocouple assembly consisting of a thermocouple welded to a small curved stainless steel pad is purchased complete from the vendor. The stainless steel pad with the thermocouple attached is then welded to the heater tube and the thermocouple strapped to the heater tube with stainless steel bands.

d. Extension Wires

Thermocouple extension wires from the thermocouple head to the remote cold junction and temperature in-

BLOCK

i I 17-

I DETAIL OF

BLOCK THERMOCOUPLE

STAINLESS S T E E L B A N D

A

y METAL-SHEATHED, T H E R M O C O U P L E PAD MINERAL-INSULATED

THERMOCOUPLE

METAL-SHEATHED, T H E R M O C O U P L E PAD MINERAL-INSULATED

S T E E L BAND HEATER STAINLESS

S T E E L BAND

THERMOCOUPLE WELDED TO PAD

S T A I N L E S S S T E E L PAD CURVED TO FIT HEATER T U B E

DETAIL OF THERMOCOUPLE PAD ASSEMBLY

B Arraiigeiiieiit h shows thermocouple on heater tube. Arruiigeiiient B shows thermocouple on pad.

FIG. ~J-1P-Installatioii o f Thr:riiioroiiples.

strument should be installed as described in Sect. 7. Materials for thermocouple extension wires may be selected as follows: Thermocouple I SA

Iron-constantan . . . . JX Iron-constantan.

Materials Symbol Extension Wire Materiais

KX Chromel-Alumel f o r . lowest error. Iron-cupronel for slightly lower cost and generally satisfactory

Chromel-Alumel . . ~ i wx I applications.

Copper-constantan . . TX Copper-constantan.

e. Temperature Iiistruments

Temperature measurements normally will be determined from board-mounted potentiometers, either of the indicating type or recorders and controllers. For convenience in checking and servicing, it is desirable that extension wires be brought to appropriate terminal strips, either outside or inside the instrument. on which the temperature point numbers are indicated.

The following points should be considered in paralleling temperature instruments: 1. The resistance of the thermocouples and extension wires may permit erroneous reading. 2. The interaction between the instruments involved may give temporarily erroneous readings.

49



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API RP 550-PART I .. .

In view of these factors, most companies do not permit paralleling other instruments on the signal circuit of a temperature controller.

Interactions between paralleled instruments (recorders, indicators, lo_cgers) can be reduced to a minimum with the use of high-impedance amplifier instruments. Amplifiers for industrial control applications are available with impedances from 500 ohms to more than 7,000 ohms. However, only those with impedances greater than 5,000 ohms can be considered high-impedance instruments. The higher the impedance, the lesser the interaction. To avoid this interaction, parallel extension wircs may be riin to the temperature point. and connected to: 1. Dual thcrinocouples in the same wcll. if perii-iitted by plant prnctices o r in important service, or 2. Thermocouples in separate wells at the same location.

f. Ref’cwwce Jiiiivtioiis

The reference junction. sometimes called the cold junction, usually is locatcd in the instrument. In some instances-whcre especially accurate temperature meas- u re nient s a re recl u i red. or where the temper at u re ins t r u- ment is subjccted to varying temperatures-the reference junction is cxternal. Also. when a number of very long leads arc rcquircd. ;I nc,iicompensatin= cable is uscd Lind ;I rctcrcncc junction compcnsation device is located at thc tcrmination point of the conventional extension wire. Such cxtcrnal reference junctions nia? be buricd t o :i depth where constant tcmpcraturc prevails o r thcy may bc installed in Lin cnclosure where the tc ni pc ra t i1 rc i s t lie r m o s t at ical I y co n t roi le d. I n an y c ve n t. it should bc noted that the accuracy of tcmpcrature incilsurement is no bcttcr than the constancy of the reference junction tcnipcraturc o r its compensation in the i ns t ru in en t.

3.6 RESISTANCE TI-lERR1OMETER INSTRCT- >TENTS

A 1’ 1’ 1 i (*:It ion u. Rcsistance thermomctcrs can provide more accurate

measurement of temperatures than is possible with

thernlocouple installations. Accordingly, resistance thermometers are used in many installations where their greater accuracy is warranted, such as in the case of low-differential temperature measurement. Jn order to obtain the greater accuracy and sensitivity inherent in a resistance thermometer and to minimize thermal lag, it is important that optimum thermoweil dimensions (for the particular resistance element) be employed in order to maintain good contact between the element and the well. For this reason. wells for resistance thermometers frequently are provided with the resistance elements as matched units.

h. Signal Trrinsmission

individual extension wires (usually three ) from resistance thermometer elements are frequently brought to field terminal strips, from which they are continued to board-mounted resistance thermometer instruments in multiconductor cable. Installation practices are discussed in Sect. 7.

c. Connections

Connections to resistance thermometer instruments usually will be made directly on the instrument or at terminai strips of prefabricated consoles. To obtain the advantages of resistance thermometers. care must be taken to ensure that proper extension wire resistances are achieved and that connections are clean and tight.

REFERENCES

I S A R P I . /-.7: T / i r r i i i o ~ o i i / > / e . ~ wid T/ierr~iocoirp/r Esrrnsiori Wires, Instr. Soc. Am.. Pittsburgh, Pa. (1959): see Sect. 1.6: ”Installation.”

zlbit/., Sect. 1.1: “Coding of Thermocouple Wire and Ex- tension Wire.”

lbid.. Sect. 1.3: “Fabrication.”

50



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COW PRB)VIDED FOR MSTMItAt PURPOSES ûMY

SECTION ..'PRESSURE

4.1 CONTENT This section covers the installation of the more com-

monly used instruments for measuring, indicating, recording, and transmitting pressure. Included in the discussion are such devices as pneumatic and electronic pressure transmitters, receivers for pneumatic signals, and pressure switches. Excluded from discussion are pressure instruments which, by inference, measure other variables. e.g., rate of flow. Such variables are discussed in other sections of this manual.

4.2 GENERAL

a. Precautions Hydrocarbons, or any other material which may be

hazardous to personnel in the event of leakage, should not be piped to any instruments located in a central control room. It is customary to transmit the pressures of such materials, either electrically or pneumatically, to receiving instruments. It is advisable also to transmit pressures in cascs where long piping connections would present problems. Esamples include instances where solids present in the process Ruid coiild cause plugging difficulties and where differences in elevation could result in a liquid head problem. In order to eliminate the, necessity for insulation and heating, except at the process connection, it may be desirable to transmit when the Ruid involved would freeze or solidify at low atmospheric temperatures.

1,. Viiwatioii Many pressure instruments are susceptible to damage

or malfunctioning if mounted in locations where they are subject to vibration. Care must be exercised in the selection of locations for mounting such devices.

4.3 PIPING

a. Size Pressure piping should be designed and installed in

accordance with the piping specification for the service involved. Piping runs to instruments should be no smaller than E-in. pipe, or ".b-in.-OD annealed steel or stainless steel tubing where permitted by the piping specification. Where instruments have connections smaller than E - i n . pipe size. the line size should be reduced at the instrument or its adjacent manifold.

1). Cleaning

clean of cuttings and other foreign material.

c. Short C:oiiiit.c:tioiis

From an opcrating standpoint. it usually will bc most satisfactory. as well as most economical. to install a

All pipe should be reamed after cutting and blown

pressure device as close to the pressure connection as possible consistent with accessibility and required visibility. This practice requires less material. eliminates liquid and vapor traps in the piping. eliminates liquid head problems, and reduces the chances of plugging. A close-connected pressure instrument is shown in Fig. 4-1.

d. Long Connections

If a long connection is necessary. the piping should be sloped between the pressure tap and instrument to minimize the number of traps for vapors or liquids. Where high points cannot be avoided. vents should be installed: scale traps or drains. or both. should be provided at low points in piping. Where the shutoff valve is not readily accessible from the instrument location. an additional valve should be installed at the instrument.

e. Flexiidity

Instrument pressure piping should be installed and supported so that the forces developed from the expansion of hot piping or vessels cannot result in a piping or instrument failure. Some refiners use thin-walled annealed steel or stainless steel tubing to provide flexibility in pressure leads.

f. Pulsation

Pressure instruments which measure pulsating pressures of reciprocating pumps. Compressors. and the like should be equipped with pulsation dampeners to prevent rapid failure of the gage movements or the pressure elements. or both. Some users prefer to employ needle valves for this purpose; see Fig. 4-3 ( A ) .

g. Purging and Sealing

Where there is a possibility of pluggins with solids or viscous liquids, or where corrosive materials are pres-

-- , ' i \ GAGE IF LOCAL i ) INDICATION IS

I ' x ' I i&y:::&~ I UNION TO PERMIT REMOVAL OF INSTRUMENT I

'REDUCE AT INSTRUMENT IF NECESSARY

I PRESSURE CONNECTION (BLOCK VALVE MUST MEET REOUIREMENTS OF PIPING SPECIFICATION)

FIG. Idl-Cloa<.-Colliicct~<¡ I'ressure Instriiiucnt.

51



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

i i PULSAIION DAMPENER

31APHRAGM -TYPE GAGE - PROTECTOR FOR CORfiOSlVE

A 6

FI(; . 4-2-l’iping for I’uisatiiig and Corrosive Cervices.

ent, the pressure lead may be sealed, purged, or protected by a diaphragm seal as shown in Fig. 4-2(B). This method is limited primarily to pressure gage protection. Gage manufacturers list a large selection of diaphragm materials which provide protection against most substances; furthermore. they usually supply the gagc scalcd. The possibility of error resulting from diaphragm stress or thermal expansion of seal fluid should bc rccognizcd if it is necessary to use a seal protector w i t h ;I I:irgc-capacity pressure element-such as ;I spiral o r Iiclical hourdon. The seal diaphragm dis- piaccmcnt volume should be sufficiently large to avoid the intrc?diiction of an crror. The use of approximately 5 ft of tlcxiblc capillary betwcen seal and instrument will pcrmit location of the instrument away from a high- tcmpcratiirc \,cssel or line. and reducc the probability of transmitting vibration to the instrument clement. Current practice is to purge the pressure connections on vcsscls ancl lines in the catalyst system on process units of the fluid-solids type. The purge medium is introduced through ;i restriction orifice or needle valve at a point close to the vessel or line. The nature of the purge niecliuni must be such that no hazard is introduced during normal, abnormal, or startup operations. For information on seals, purge quantities, and purging mcthods :<cc Sect. S.

4.4 INDICATING GAGES

a. Connec-tion Location and Sizes

Indicating pressure gages for flush mounting on instrument panels should be back-connected. Gages for field iiiounting should be the bottom-connected. surface- mounted type. Standard gage connection sizes are % -in. and !/‘-in. pipe size. However, in order to reduce the number of small connections, there is a trend toward the exclusive use of the %-in. size.

11. Stpports

Gages up to and including the 4?/”-in. size may be supported by their own pipe connections unless the lines or equipment involved are subject to severe vibration. Gages subjected to vibration should be supported inde-

pendently. In some cases this can be done best by supporting the piping close to the gage. Typical examples of gage supports are shown in Fig. 4-3.

c. Safety Devices

Pressure gage cases should be provided with disk inserts or blowout backs designed to pop out when the case pressure rises to a pound or more. These are protective devices installed to prevent bursting of the glass window in front of the gage dial or case in the event of pressure element failure. Some users require this feature on ail instrument cases which contain process pressure elements. Gages can be obtained also with safety glass or plastic windows as an additional safeguard. Gage supports should be designed so that the functioning of the safety disks is not prevented. Because a coat of paint almost always will prevent the functioning of the safety disks, gage cases should not be field-painted. Care must be taken to make certain the disks are un- covered when gage cases are insulated or traced for heating. Gage cutouts. with or without velocity checks, are available for limiting overrange.

c l . Siplioiis

Siphons or “pigtail” condenser seals should be provided in connections to close-mounted gages in steam or other condensable vapor service to maintain liquid in the pressure element and to prevent overheating; see Fig. 4-4.

e. Boiirtloii Tube Material

It is important that bourdon tube materials be selected for the service conditions included. For example, a number of failures of “standard alloy steel” tubes (AISI Type 4130 is typical) have occurred in hydrocarbon services where sulfur compounds or hydrogen were present. The frequency of these failures led several refiners to adopt the 4ISI Type 316 stainless steel tubes for general use. The use of other alloys may be justified in specific applications.

52



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

PRESSURE . ___-

UNION IVHERE NECESSARY 9 29 A B

SIPHON FOR i CONDENSING VAPORS

CONNECTION (BLOCK’ MEET REQUIREMENTS

OF PIPING SPECIFICATIONS)

C D FIG. U R e n i o t e l y Lorated Instriinients for Pressure

Piping.

4.5 INSTRUMENTS OTHER THAN GAGES

a. Supports

Recorders, transmitters, controllers, pressure switches. and the like normally should be supported independently of the pressure connection. The type of support depends upon the make of instrument, the location. and the user’s preference. When installing pressure instruments. care should be taken to avoid the possibility of imposing stresses from the pressure piping, conduit: etc.. \\.hich may cause malfunction.

b. Local Indication

Pneumatic transmitters which have no mechanical indication í the so-called blind type) normally are supplemented with pressure gages connected directly to the process; see Fig. 4-1. Provision for the installation of a test gage should be made at the output air connection of the transmitter.

c. Electroiiir Instruments

Wiring for pressure transmitters or transducers for electronic instruments should be installed in accordance with Sect. 7.

Prrssurc transmitters or transducers for electronic instruments should not bc located too close to hot lines. vessels. o r 0 t h equipment. Ambient temperatures which rxcccd 140 F are likcly to result in calibration difficulties and rapid deterioration of electronic components. Susceptibility of mechanical or electronic components to vibration should be ascertained and, where necessary, adjustments should be made in the mounting.

d. Receivers Each receiving device used in conjunction with a

pneumatic transmission system can be classified as 3

pressure instrument. The ranges of such receivers depend upon the transmission system in use, the most common being 3 psig to 15 psig. These receivers may be indicators, recorders, relays, pressure switches, or controllers. Normally, the air pressure is supplied to the receivers through %-in. tubing; see Sect. 7.

e. Pilot Pressure Regulators The pilots of certain back-pressure and pressure-

reducing regulator valves can be classified as pressure instruments. These simple proportional controllers generally are mounted either on top of the diaphra, om case? or on the yoke or spring barrel of the valve. The air connection between the controller and the valve diaphragm normally is installed with tubing at the factory. The connection to the process pressure tap should be made as shown in Fig. 4-5.

f. Differential Pressure Differential pressure may be measured with an instru-

ment of the same type as is used for fiow measurement. Instruments for this service are available in a variety of ranges. Such devices may be of the mercury or so- calied dry type. One method which uses an instrument of this type to measure the differential pressure across a catalyst bed or a fractionating tower is shown in Fig. 4-6. The process must be able to tolerate the purge gas. which is necessary to keep the long lead to the lower pressure connection free of liquid. Occasionally. very low gage pressures are measured with a differcntial pressure instrument by leaving the appropriate connection of the device open to the atmosphere.

T E S S U R E CONNECTION (BLOCK VALVE MUST MEET f--- REOUIREMENTS OF PIPING SPECIFICATIONS)

SECOND BLOCK VALVE DESIRABLE IF FIRST BLOCK 15 NOT ACCESSIBLE -.

/’ I ”

2 -

I

GAGE IF PRESSURE INSTRUMENT IS “BLIND”

,-(, CORRECT FOR STATIC HEP.0 ‘ C

UNION TO PERMIT REMOVAL OF INSTRUMENT

FIG. 4-5-l’r<:ssurc: Iiistruniont I’ipiiig.

53



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API RP 550-PART I

g. Draft Gages Low-pressure instruments of the slack-diaphragm

type (called draft gages) are employed for the measurement of firebox and breeching pressures in steam gen- erators or fired-process heaters. Typical pressure connections at the firebox or breeching are shown in Fig. 4-7. Provision usually is made for a plugged access opening for “rodding” out soot deposits. The piping or tubing between the pressure tap and gage generally is M in. or larger and should be installed so that there are no pockets for condensation. Usually, the connection at the draft gage is a short length of %-in. tubing. A three- way cock is provided to vent the gage element to atmosphere for zero checking.

M+--PLUGGE> TEE --. FIG. -l.-C+Oiie Meihod of Jleasuring Differential Pressure

Across ii Rcicior or Fractionator.

Q

I WELD HALF C3,F’LING OR NIPPLE AND DRILL WALL

REAM HOLE BREEChING

A RRICK SETTING

6 STEEL RREECHING

FIG. 4-’i-Draft Gage Coiiiiectioiis.

54



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COPY VROVIDED FOR HISTIlílX&l PURPOSES ONLY

SECTION 5-AUTOMATIC CONTROLLERS

.5.1 CONTENT

This section discusses the installation and application o€ automatic controllers. Because installation details vary with the application. much of this section necessarily will be general in scope.

The material presented herein applies to both pneumatic and electronic controllers. It should be noted that there are some differences between pneumatic and electronic instruments. in addition to those discussed herein. which are only differences of degree and do not change the basic requirements for a sood installation. Users should install all control instruments in accordance with the manufacturer's recommendations and such additional requirements as are needed. in the judgment of the design engineer. to satisfy the particular conditions of the individual installation.

For a discussion of air and power supply systems. refer to Sect. 9 and 1 i ~ respectively.

3.2 FQRMS OF CONTROL

has been de- Lioted to the analysis of control systems and ehe application of the various control functions ( proportional. reset. and rate action) to these systems. Such work has included simulation studies and mathematical calculations. Valuable information has been obtained from these complex procedures. and such methods show considerable promise.

In seneral, however, the majority of control function requirements are determined on the basis of experience and judgment. For example, in order to minimize controller dead band.: controllers actuated by narrow-range transmitters would require a low gain l i (wide proportional band) with automatic reset. Tower pressures and temperatures have been controlled very satisfactorily in this manner.

In recent years. considerable study

a. Oil-Off

On-off control is the most elementary form of control. It is suitable for alarms and protective device actuation. for automatic startup or shutdown of individual items of equipment, and €or a limited number of refinery applications where intermittent regulation of the controlled variable is not objectionable.

I L Proportioiial

Control!crs which use proportional action only are of value where on-off control is inadequate and where. with load change. a moderate offset (deviation from the set point) can be tolerated. It is to be noted that wherc a high-gain (narrow proportional band) setting is used, the offset which results from load change will not be as

Figures refer to REFERENCES on p. h l

great as where low-gain (wide proportional band) settings are used. Many pressure applications are satisfied by high-gain settings. and most level applications require low-gain settings.

c . Proport ional Plus Reset

The proportional plus reset control action is more widely used in refinery practice than any other form. It is required to correct the deviations of the controlled variable from the set point resulting from load changes. The addition of reset to a controller can. however. cause difficulties if the controller is in intermittent operation. On most proportional plus reset action controllers. when they are not controlljng. the reset action accumulates and drives the output to a limit. This limit may be beyond normal controller and valve operating values. Con- trollers on batch operations exemplify applications where this characteristic should be considered. Another example is preferential control applications wherein two or more controllers actuate the same valve: one or more of the controllers may be :it a reset limit. Hence. on a switch of controllers. a process upset could be e nco un te red.

d. Proportioiial Plus Rate

Proportional plus ratc control is of value whcrc lags. other than dead time. cause a significant delay in recog- nition of. and correction for. a change in the process variable. This form of control i s also of value where other control forms would permit too large an otfset or would permit harmful or undesirable oscillations or overpeaking of the controlled variable. Fired heaters. with large process and measurement time lags, and batch operations are examples of applications where the proportional plus rate action form of control could be used to avoid the undesirable effects of some of the other types of control forms.

e. Proportional Plus Reset Plus Rate

The proportional plus reset plus rate control is of value where the accumulated deviation from the set point must be kept to a minimum. This would occur on applications having high process gain and one or more large lags. Opinion varies as to where this type of control is best applied. Some believe that this form is necessary in most temperature-control applications. However. caution is advised when considering the use of this form since. occasionally, one might find that, even with properly adjusted rate action. equipment performance is upsct. .Also. for intermittent servicc-type applications, it would be well to consider using controller circuit designs wherein the location of the rate action unit climinates the reset-action-inspired overpeaking of the controlled variable.

55



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f. Prolwriioiial Phis Reset Plus Inverse Rate

This form of control differs from the preceding type only in the eRect of adjusting the proportional action; it is not a new combination. The proportional plus reset plus inverse rate action is of value where an initial change in the measured variable may be misleading, or where equipment time constants are such that a cor- rective change should not be made too quickly. For example. a sharp incrcase in steam demand from a boiler may cause a rise in the drum level. due to “swell” or the momentary displacement of water from the boiler tubes on thc temporary reduction in pressure. The inverse rate action in the drum level controller could prevent decreasing the fcedwatcr flow at the wrong time.

5.3 -APPLICATION OF CONTROL FORMS

a. Required Ranges of Adjustment

In selecting a controller for any application, it should be realized that the ranges of adjustment should be sufficiently broad to permit necessary settings of control actions as required. I f a proccss requires a gain setting of 1. then the instrument selected must have a range of gain adjustment which will include 1. Accordingly, the following discussion of ranges of adjustment is included:

1. PROPORTIONAL ACTION

Some instrumcnts, including simple pressure and temperaturc controllers. use only proportionai action rind normally havc gains adjustable down to 10, 5. or even 2.

Othcr instriiments-norm all!^ having reset or rate action. or both. in combination with proportional action- have gains adjustable down to :it Icast 0.4. i f not lower. [For certain cxccptions. see ( e ) . ] Although many applications of this sccond-group of instruments do not require this range of adjustment. most controllers have adjustable ranges at least this wide which pcrmit inter- changeability of control equipment.

2. RESET ACTION

Thcrc arc two general ranges of reset action-fast and srtuitlarti. Because spans are not standardized between difcrcnt manufacturcrs, these terms arc necessarily general. Fast reset can be adjusted within a range of approximately 0.1 or 0.2 to 100 or morc repeats per minute. whereas standard resct can be adjusted within a range of approximately 0.02 or 0.05 to 10 or Icss rcpeats per minutc. It will be noted that there is considerable overlap. and cxact figures arc not as important as thc orders of magnitude and rangcability. It should bc mcntioncd that somc manufacturers use ”minutes per repeat” (sometimes shortened to “minutcs” or “min”) which is the reciprocal of “rcpeats per minute.” Others mercly usc an arbitrary scale value which may o r may not be convcrtcd to actual timc o r rntc values.

3. RATE ACTION As with resct, sometimes there is an option as to

available ranges of rate action. However, there is no standardization among the various manufacturers on units of measurement, nor on approximate ranges. Some manufacturers supply a single range of rate action, others have various ranges available. When rate action is required. it is recommended that the range be determined by discussion with the vendor.

1). Flow

For flow applications in general. gain adjustable down to 0.5 and adjustable reset are desirable. Fast-reset range should be used with force-balance diaphragm transmitters. and standard-reset range with mercury manometer transmitters. One exception to this practice is a special form of pneumatic controller used on liquid flow with a force-balance diaphragm transmitter. This type of controller is mounted directly on the diaphragm air motor of the control valve. It is provided with fixed proportional and rate actions and an adjustable very fast reset action. This controller has been found sitisfactory on liquid Aow and on some gas flow applications. but it is not recommended for other services.

c. Pressure

For pressure applications, gain adjustable down to at least 0.5 and standard reset are desirable.

t l . Teniperature

For temperature applications, gain adjustable down to at Icast 0.5 and standard reset are desirable. In addition. adjustablc rate action may also be necessary.

e. Level

Level controllers usually fall into two general groups and different gain requirements are necessary for each. The distinction between the groups is based on the effect of level change on other process variables. Dif- ferentiation is probably best illustrated by the following examples:

Example 1 : Consider a fractionating tower from which the bottoms product flows to storage through a cooler, where the flow is controlled by the level in the base of the tower. Some variations in tower level are permissible. and this fact can be used to advantage by not rcquiring automatic reset and by setting the gain as high as practicable to keep the level within acceptablc limits. in this case. level changcs will cause fluctuations in the flow rate until stabilization is reached but will not cause any harmful results.

Example 2: Next, consider the same example, except that bottoms product is going as feed to another tower

56



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AUTOhlATIC CONTROLLERS

or through a heat exchanger which preheats feed to the tower. Now, i f the control in the first example were used, the changes in flow rate of the bottoms product would cause upsets in other parts of the process. A level controller with low gain and slow reset action will change bottoms product llow rates slowly and thus reduce. if not avoid. such upsets. For this case. the gain should be adjustable down to at least 0.4.

It should be noted that most unit processes have very little surge capacity other than change in level in process vessels designed for other purposes. To take advantage of available surge capacity, the gain setting in control examples 1 and 2 should be as low as practicable.

5.4 CASCADE AND RATIO CONTROL

a. Cascade Control

In a cascade control system. one controller. commonly called the primary or master, is actuated by the process variable which is to be regulated to a constant value. and a secondary. or slave. controller is actuated by a variable that can be used to cause changes in the first. or primary' variable. The primary controller output adjusts the set point of the secondary controller which. in turn. operates a control valve or other final control element. This is not to be confused with the system wherein a remote manual loading station is used to adjust the set point of a controller which has been mounted adjacent to its control valve to improve control by reducing the etfective time lag.

Cascade control is of value because. if properly applied and installed: 1. It can reduce the effect of time constants in the loop even when the primary loop constant is of the order of 3 to 5 or more times the secondary loop constant. 2. i t can eliminate the effect of disturbances in the secondary variable before the disturbances can enter the process loop. 3. In some instances. it can accomplish the effects of both items 1 and 2; Fig. 5-1 illustrates such an application.

Cascade control should not be attempted unless it is required for any of the preceding conditions; otherwise: cascaded systems may add unnecessary complications to the control circuit as well ils higher cost.

The control forms for the individual controllers can be dctcrmiried as described in Par. 5.2 and 5.3. Very often both the primary and secondary controllers are provided with proportional plus reset action.

Some applications or combinations of controllers may require a gain sctting of the primary controller lower than is normally available from the instrumcnt. One mcthod of overcoming such a situation is to pass the primary controller output through a ratio relay.

Also, for protcction of the process, it may be necessary to limit the amount of set point adjustment in the

I

=RIMARY MEASUREMENT (DESIRED TRAY TEMPERATURE)

I I Note: Setting the pressure controller to a desired value

reduces the effect of the reboiier time constant. and at the same time the pressure controller corrects for stem1 pressure variations.

FIG. ~-l-Piieliniatic-'T~pe Cascade Control System.

secondary controller. This can be accomplished in any of several ways: 1. A limiting device may be installed within the primary controller to limit its output signal. 2. A separate relay may be mounted externally but must be connected in between the primary and secondary controllers to limit the set point signal. 3. Mechanical set pointer travel stops inside the secondary controller.

To illustrate a need for such limiting. consider a cascade control system wherein a primary controller is to adjust the set point of a compressor speed controller. The speed controller should never be directed to operate the compressor at its stalling speed or at the speed where the overspeed trip mechanism will be triggered.

I,. Ratio Control In ratio control. two variables are measured and .&he

secondary or "controlled" variable is regulated to maintain a predetermined ratio to the primary or "wild" variable; the ratio between the two variables is usually adjustable. It is sometimes desirable to have the ratio adjusted by the output of another controller or a transmitter. Occasionally, this is done on fractionating column applications and is sometimes called "three-element control." This is not the same as'the combination of instruments (which is generally similar in purpose) that is used on boiler applications and is also referred to as "three-element control." ( I n boiler feedwater control. additional adjustments permit compensation for "swell" and "shrinkage" of water in a boiler dmm resulting from changes in steam ratc; this is not a true ratio system. 1

When ratioing two flows, at least the following two points should Aways bc considered: 1 . The ratio unit should bc "squared" in a zero wild How; zero wild ílow calls for zero controlled flow. 2. Both signals should be of the same characteristics. i t is not possible to ratio a linear signal with a square root signal.

57



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Ratio control applications requiring extremely wide ratios may prcscnt sonic problems; the required change in ratio may bc grcatcr than the ratio change iivailable in the control mechanism. In this case, either one or both of the tlow transmitter ranges may be changed to help achieve the desired ratio relationship.

5.5 LOCATION OF CONTROLLERS a. General

Controller location requires careful study. Controllers can be mounted on a control room panel, either integral with or detached from a recording or indicating instrw ment: near the point of measurement and/or control: or directly on the control valve operator. Set points of controllers can be adjusted directly or remotely.

The number of possible instrument combinations makes i t dilEcult to set up definite recommendations. Neverthelcss. the need for installation standards lias led to the growth of a number of necessary working practices. Some of the more important practices are discussed in tlic succeedin5 paragraphs.

I, . Fac.toi.5 .11T1.C.iiii~ ~ ~ o l l t r o l l t ~ r LocYItion

Thc following points (not listed in order of importance ) siioiilci bc considercd when deciding on the location tor ;I controller:

i . Convenicnce to opcrating personnel-ease of reading. crise of changing set point. approximate place in Ilow schenic. ctc.

2. Convenience to maintenance personncl-accessibility for servicing. frcyuency of need for servicing, etc. 1 5 .

4. 5. 6.

the 7. 8. 9.

10. 1 I . 12. 13. 14. 15.

Installed cost because of location. Siil'cty of personnel rind equipment. Vibration eflccts on equipment and performance. Corrosion cnuscd by surrounding atmosphere and

Weatherproofing and winterizing, where necessary. Instrument 1 ag. Explosionproofing, where required. Protection from fire. Accessibility in the event of firc. Protcction from nicchanical damage. Ani bien t temperature. Radiation from sun or hot equipment. Conipanv policy with rcspcct to types of instruments

process Illlid.

purchased and their location. 16. Inhtrument error due to transmission system characteristics.

c. Lop

1. WIT13 ELECTKONIC I N S T K U M E N T S

Process. measurement, and equipment response lags arc common to d l control systems. Electronic control systems are essentially free from transmission lags.

3. WITH PNEUMATIC INSTRUMENTS

Pneumatic transfer lag can be reduced by proper controller location. Transmission lags of both transmitted and controlled air signals are affected by tubing size and length, by pilot capacity, and by volume of air to be handled through the tubing. The situation is further complicated by the fact that a given air-transfer time lag which will introduce no control problems in one application will be entirely unsuitable in another. The following points are worthy of mention for, although self-evident, they are often overlooked: 1. Lag is greater with longer tubing runs. 2 . Lag is greater with very small tubing sizes (because of friction) as well as with very large tubing sizes (because of volume). 3 . Lag is greater when air is flowing through the tubing to a large-volume end device (e.g., a valve motor). 4. Lag is smaller when air is flowing through the tubing to a small-volume end device (e.g., a receiver bellows). 5 . Difference in lag is not significant between commonly used tubing sizes í?4 in. and ?A in.) of moderate lengths.

(1. (hiiderations i r i YIiiiiiiiizirig Pneuniatic. Lag

For general scrvice applications where transmission lag is not ordinarily critical. the total length of tubing from the transmittcr to the controller plus that from the controller to the control valve, or its pocitioner. should not cxcced 400 f t ; neither run should exceed 250 ft. Bascd on the uscr's judgment and experience. longer runs may be tolerated for these applications if control valves having small-volume diaphragm heads are used. When control valves Lire more than 125 f t from their controllers. use of valve positioners or relays may be helpful in reducing pneumatic lag. Such devices. however. may be lcss beneficial overall since they can intro- ducc dead time, worsen frequency response of the loop, create a phase shift. and so forth.

For the applications where transmission lag can be harmful, it is suggested that tubing runs be limited to approximately one-half of the values noted herein. Also, mounting the controllers adjacent to their valves will help considerably in reducing lag. Installing high-capacity air pilots in the controllers is another method of reducing pneumatic lag.

e. Centralizatioii of Control Stations

One major consideration affecting controller set point and instrument location is operator convenience. This is of considerable importance because centralization of instruments results in more efiìcient and safe operation of process equipment. I t is desirable to locate at a cen- trai point, usually in the control room, a suîììcient number of instruments to permit control o€ all major process variables from this one point.

sx



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AUTOMATIC CONTROLLERS

f. Locally Mounted Pneumatic Coiitrollers

Frequently, controllers are mounted locally because of the lack of justification for control room mounting and to reduce pneumatic transfer lag. When reduction of pneumatic transfer lag is the reason for local mounting, it may be desirable to have the tubing run to the control room. A number of combinations are shown in Fig. 5-3 through 5-5. The system shown in Fig. 5-5. which uses four tubes. is the most desirable; however: it cannot always be justified economically.

Note: The panel-board-mounted controller is connected by two tubes-one to the transmitter and one to the valve. Only locally mounted controllers are shown in Fig. 5-2 through 5-5.

5.6 MOUNTING

a. Geiieral

Some controiiers are mounted on or within recording or indicating instruments; others are installed separately.

I t is beyond the scope of this section to cover the mounting of instruments on panel boards. This topic is ùiscussed in Sect. 13.

1). Moiiiitiiip oï Local Coiiirollere

iMethods of mounting local controllers include flush, surface, bracket, and yoke. Also, some controllers can be mounted on the valve yoke or operator of a control valve.

The principal considerations in mounting are rigidity, accessibility for service or maintenance, freedom from excessive temperatures. design of the instrument, fire

RffiULATION PROCESS

MEASUREMENT w '1'.

TRANSMITTED MEASURED VARIABLE

REMOTE RECEIVER & IND. AND/OR REC.

This type of controller generally is known as a I-tribe system. The transmitter must be of the proper type for the particular

process variable (temperature, flow, pressure, level, gravity, etc.) and may be indicating, recordins, or neither.

The controllcr contains a manual set point: it must be of the receiver type for the kind und range of signal from the transmitter. and may be indicating, recording. or neither. The controller mounting should be of the type most suitable for the application.

The remote receiver should be of the type to suit the requirements of the application.

FI(;. 5-2-Fictld Controller with Reiiiote Rewivc-r.

PROCESS REGULATION

f TRANSMITTER , :ZG+Z)

TRANSMITTED MEASURED VARIABLE-

t I 1 REMOTE RECEIVER 1

IND. AND/OR REC. 1 I I

4 SET POINT

'I I REMOTE LOADING 1 STATION

I 1

This type of controller generally is known as a 2-tube system. The transmitter must be of the proper type for the particular

process variable (temperature, flow, pressure! level, gravity, etc.) and may be indicating, recording. or neither.

The controller must be of the receiver type for the kind and range of signal from both the transmitter and remote loading station, and may be indicating, recording. or neither. The controller mounting should be of the type most suitable for the applica tion.

The remote receiver and the remote loading station may be set up as separate items or combined into a single unit. In either case. both should be of the type to suit the requirements of the application.

FIG. .5-:3-Fielrl Controller with Reiiiote Receiver and Loading Statioii.

PROCESS REGULATION

I UEASUREMENT

REMOTE RECEIVER REMOTE LOADING -1 IND. AND/OR REC. F] J""! INDICATOR


process variable (temperature, flow, pressure, level, gravity, etc.) and may be indicating, recording, or neither.

The controller must be of the receiver type for the kind and range of signal from both the transmitter and remote loading station. and may be indicating, recording, or neither. The controller mounting should be o? the type most suitable for the application.

The remote receiver, the remote loading station, and the controller output indicator may be set up as separate items but most often they are combined into a single unit. In any case, all should be of the type to suit the requirements of the application.

FIG. 5-4-Field Controller with Remote Receiver. Loading Station, and Controller Output Indicator.

59



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PROCESS REGJLATION

I I

TRANSMITTED MEASURED VARIA EL E

REMOTE CONTROL STATION “AVING A RECEI‘IER. SET OC U T ADJUSTER AND

AUTOMATIC- MANUAL 7 4 N S F E R ZWITCH

This type of controller generally is known as a 4-titbe system. The transmitter must be of rhe proper type for the particular

process variable (temperature. How. pressure, level, gravity, etc.) and may be indicating. recording. or neither.

The controller must be of the receiver type for the kind and range of signal from both .the transmitter and remote loading station. und may be indicating or recording. although usually it is neither. The controller mounting should be of the type most suitable for the application.

The remote control station usually is installed on the instrument panel in a control room and consists of n receiver which may be either indicating or recording, and contains a set point adiiister and n 3-position transfer switch for automatic- manual type operations.

FIG. .5-.5-Fii-ld (:ontrollci. w i t h Rvinote Control Station.

protection. and the desirability of freedom from vibration and mechanical damage. Normally, the requirement for vibration-free mounting precludes mounting a controller on a control valve. but sometimes this is not important with controllers designed for this purpose.

c. Rack o r Wnll Jloiintiiig

A number of controllers can be mounted on racks, walls. or other surfaces. Other than adequate attachment. there is no particular problem in mounting because the surfaces are usually rigid and free from excessive vibration.

5.7 JIISCELLANEOISS CONTROL REQUIRE- JIEYVTS

a. B y a s s Facilities

Controller bypass facilities should be furnished with every controller used on major process variables. This enables remote manual control in the event of unsatis- factory operation or failure of automatic control. Inas- much as snitches arranged for bumpless transfer are available in a number of combinations, recommended practices for their selection are: 1. Thrce- or four-position transfer switches and regulators should be supplied for a11 temperature controllers, critical control applications, and ali controllers used in cascade control systcms. 2. For otlicr applications requiring bypass facilities,

60

two-position transfer switches and regulators are satisfactory.

JI. Control Requirements of Transmitters

Requirements for transmitters ordinarily are not complex. I n some instances (as in level applications), the same instrument can be used either as a transmitter or as a controller. It should be pointed out that when any instrument is used as a transmitter, the proportional band setting should be at 100 percent with no reset and ordinarily no rate action. Any change in the proportional band should be made at the receiver-controller and not at the transmitter. The use of an instrument set at 100-percent proportional band to act simultaneously as a transmitter and controller is not recommended. The applications requiring transmission and control should be provided with dual pilots or secondary controllers. Some filled-system temperature transmitters are available with rate action to compensate for measurement or transmission lags.

c. Local Indication of Measurement for Manual Control

Frequently, transmission of measured variables to a control room is required, either for operating guides or for controlling. The point of measurement usually is some distance from the control room, and, if a control valve is involved, it, too, is mounted some distance from the control room. In order to facilitate manual operation of a control valve handwheel, or the bypass around a control valve, it becomes quite desirable to have an indication of the controlled variable (temperature, level, flow, etc.) near the control valve manifold. Frequently, it is possible to provide a local indicator so installed that it can be read from both the trammitter and the control valve. Also, some transmitters are available with integral indicating devices. When additional indication is desired, it is usually possible to install other indicators actuated from the transmitter output signal.

d. Controller Protection

Normally, it is necessary to protect controllers (and other instruments) from extreme changes in ambient temperature, against physical damage, and from accidental painting.

Frequently, an instrument is of such rugged mechanical design that external protection against physical damage is unnecessary. However, some controllers require a cover or shield to protect against paintins or accidental turning of exposed dials or adjustment screws. In some locations shields are required to protect ap ins t excessive heat from either thc sun or radiating equipment. Locations should be selected which will reduce the need for protection from accidental mechanical abuse, or suitable guards should be provided. Winteriz- ing requirements arc discussed i n Scct. 8.



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REFERENCES ‘T. J. Williams and V. A. Lauher, Aiuonirrric Control of

C h i n i c d rind Petrolciini Processes, Gulf Publishing Co.. Houston ( 196 1 ).

Process Iristriimenrs mid Controls Hnndbook, D. M. Con- sidine (ed.) . McGraw-Hill Book Co.. Inc., New York (1957).

G. K. Tucker and D. M. Willis, A Siniplified Technique of Control System Engineering, Minneapolis-Honeywell Regulator Co., Brown Instrument Div.. Philadelphia (1958).

‘L. bî. Zoss and B. C. Delahooke, Theory cind Applicritions of Intlitsrriril Process Control, Delmar Publishing, New York (1961). ’ ASA CS5.1: Terminology for Aiitonintic Conrrol, Am. Std.

Assoc., New York í 1963). a S A M A Std. RC IS-12: Markings for Adjiistnient Meuns

in Automnric Controllers, Sci. App. Makers Assoc., New York ( 1960).

61



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SECTION CíCONTROL VALVES AND POSITIONERS

6.1 CONTENT

This section presents recommended practices for the installation of regulators and control valves, including air and hydraulic valve positioners, booster valves, and other associated relays.

A number of instruments can be mounted on control valves and regulators; reference should be made to other sections of this manual for installation and piping practices covering these devices.

For additional information on control valve manifolds. rcfcrcncc may be made to thc ISA RP 4.2: Rec- ommerirlrtl Prtictice foi Srniidri id Cotinol V h e iMani- fold De.sigt1.s (Cr1t.hon SICPI l i t i l i * c 3 O d y ) .

6.2 GENERAL

a. Acreasil)ility

1\11 control valves should be installed so that they are readily acccssiblc for maintcnance purposcs.

Thcy shoiild be located ;it gradc unless prcssurc or othcr dcsign conditions niakc such an arrangement impracticable. Whcn located above gradc. control valves should be installed so rhat they arc readily accessible from ;I pcrmnncnt platform or walkway with ample clcaranccs lor maintenance opcrations.

I,. Lo<*alioti

Where tlicrc is a choice of location, it is desirable to have the control valve installed near the piece of operating equipment which must be observed while on local nianual control. It is also desirable to have indication of the controllcd wriablc visiblc from thc control valvc.

c. (:lc:uruiit*c*

Sufficient clearance should be provided above and below the control valve so that the bottom flange and plug, or the topworks and the plug, may be removed with the valve body in place. Extra clearance is required where heat-radiating fins or other accessories are used.

On large valves which closc on air failure, it is often advantageous to use reverse-acting topworks so that the valve may be located a minimum distance above grade or platform and still provide accessibility and prevent interference with overhead piping.

tl . Prcctiiitioiis

Control valves which handle combustible fluids should be kept away from hot pumps, lines, or equipment. This practice rcduccs the possibility of liquids splash- ing upon hot lines or equipmcnt should thc control valves be rcmovcd and the line bctwecn thc block valves drained.

Siniilrirly, control valves used in process lines or fuel

62

lines, or both, to fired heaters should be located outside the firewall around the heater. If no firewall is provided, the control valves should be located on the sides of the heater away from the burners or at a sufficient distance from the heater so that the control valves may be removed and the line drained without danger of a flashback. An alternate method is to pipe the drain or bleed connections a safe distance from the heater.

In order to prevent premature failure of diaphragms and electric or clectronic components, control valves should be located so that the topworks are not adjacent to hot lines or cquipment.

During startup of any new facilities, care should be taken to keep scale, welding rods, and so forth from plugging control valves. One method which is sometimes used is to remove the valve and substitute a spool piece during flushing operations.

The control valve actuators should be selected so that on failure of operating medium the valve will '<fail safe." i.e.. lock in position or take the position (either opened or closed) which will result in the least upset to .the unit.

6.3 CONTROL VALVE TYPES

Control valves can be classified according to body design. Thc selection of a valve for a particular application is primarily a function of the process requirements. and no attempt will be mude herein to cover this subject. Some of the more common types of control valve bodies are discussed in the following paragraphs.

a. Two-Way V n h e

The globc body control valve with top- and bottom- o widing or skirt-guiding, provided with single or double seats, is the most commonly used type of control valve.

A variation of the two-way valve is the angle valve which is used primarily in coking or slurry service.

Another variation is the split-body vaive which is available in both globe and angle patterns. In this valve the seat ring is clamped between the two body sections which makes it readily removable for replacement. The split-body valve is used a great deal in chemical plants.

I). Three-Way Valve

The three-way valve is a special type of valve primarily used for splitting or mixing services.

c. Butterfly Valve

Thc butterfly valve is a rotatine-vane t!pe OF valve uscd in applications whcrc low-pressure drop in the fully open position is essential, and wherc size and light wcight must be considered. It is availablc with grease scals, pressurized neoprcnc. or various types of syn-



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

C O N T R O L V A L V E S AND POSITIONERS

thetic rubber seating surfaces when tight shutoff in the closed position is essential.

d. Miscellaneous Valve Body Types

There are many other types of control valves used in control service, such as gate, plug cock, slide, Saun- ders, and rubber pinch valves. These valves use the same types of actuators as the control valves mentioned herein; however, considerably more power or torque usually is required.

6.4 CONTROL VALVE ACTUATQRS

There are many types of actuators for stroking control valves. The selection of a particular actuator is a function of: 1. Operating inedia available. 2. Thrust requirements. 3. Length of stroke. 4. Speed of stroke. 5 . Control valvc body type.

are described in the succeeding paragraphs. Some of the more commonly used types of actuators

a. Diap11r:igni

The diaphragm actuator, with air as the operating medium, is the most commonly used type of control valve operator. Diaphrasm actuators can be either the spring-opposed type. springless, or pressure-balanced type.

1). Self-.Actuatetl Regulators The self-actuated regulator is a variation of the

diaphragm actuator and normally uses the process fluid as the operating medium. For pressure applications, some self-actuated regulators use bellows instead of diaphragms for the actuator; for temperature applications. bellows with a filled system and bulb are used instead of diaphragms.

c. Piston Actuators Piston or cylinder actuators are used usually where

valve designs with long strokes are required. The piston or cylinder can be operated hydraulically or with air or gas.

(1. Motor Actuators Motor actuators for control valves can be electrically

powered. or a vane or nutating-disk type of air- or gas-driven motor can be used as the power source. A variation of this type is the electrohydraulic operator which uses a continuously running electric motor to drive a pump and supply hydraulic pressure for a self- contained piston.

6.5 CONTROL VALVE 3IANIFOLDS

Current practices for the installation of block valves vary, but normally block valves are installed before and after the control valve. and a bypass with valve is installed around the assembly.

u. Block aiid Bypass Valves

Where the greatest flexibility is to be provided for future expansion, the block valves upstream and downstream of the control valve should be line size. In situations where the control valve is two or more sizes smaller than line size. the block valves may be one size smaller than line size.

Tt is often necessary that bypass valves be full-line size. or not more than one size smaller. in order to have the necessary capacity for filling and emptying the unit in a reasonable length of time. This is especially true under gravity flow conditions. Also. where a small control valvc is installed in a large line. the larger bypass valve gives the necessary mechanical strength to the manifold.

Tn selecting and sizing block and bypass valves. the installed cost should be considered. In some cases. the installed cost of line-size valves is less than the cost of one size smaller valves plus the swages. welding. and labor required for installation.

Bypass valves are usually globe or gate valves in sizes up to and including 4 in. For larger sizes, gate valves normally are used; in special cases plug cocks with gear operators are used.

Recommended minimum sizes for block and bypass valves are given in Table 6-1.

I). Swages ut COiit i .01 Valves

Where a screwed control valve is used. the union connections are placed at the large end of the swage with the smaller end screwed directly into the control valve. Minimum Schedule 80 swages should be used to provide adequate support with minimum restriction to the flow. However, even heavier swages may be required to meet line specifications.

Where a flanged control valve smaller than line size is used, swages are placed adjacent to the control valve flanges except where additional pipe nipples are required to permit boit removal.

Eccentric swages are often used in place of concentric swages to allow ready draining of the line and to prevent buildup of deposits in the pockcts formed by the concentric swages. Tlic lise of swages at control valves is illustratcd i n Fig. 6-1.

c. Piping Wiilioiit Block and Bypass Valves

Block and bypass valves sometimes are not used.

63



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TABLE (>-l-Rccoiiiiiiendetl hliiiiniuiii Ellock : I I I ~ I5ypass Valve Sizing (All Sizes in Inches)

3'4 1 1 Y2 2 3 4 6 8

Valve Valve Valve Valve Valve Valve Valve Valve Line Size:

Control Valve Size

!4 M

1 I Y2 2 3 1 6 8

10 12

Y2

Valve

. . . . . . . . . . . . . . . . . . . . % ?í 1 1 1% 1YZ YI ?h 1 1 1 Y Z l Y 7 2 . . . . . . . . . . . . . . . . . 1 I 1?4 1 % 2 2 2 2 . . . . . . . . . . . .

l ! / ? l ! h 2 2 2 2 3 3 . . . . . . . . 7 2 3 2 3 3 4 4 . .

3 3 4 3 4 4 6 6 . . 1 4 6 4 6 6

6 6 8 6 . . 8 8

. . . . . .

10 Valve

. .

. . .

8 8 8 8

10 8 I O I O

12 Valve

. . . .

10 10 10 10 12 10 12 12

Instances where these valves are not always necessary are : 1. With control valves in steam lines to pump drives or turbine clrivcs sparing motor drives. 2. Whcre it is desirable to reduce the sources of leakage of hazardous fluids, such as hydrogen, phenol, or hydrofluoric acid. 3. In slurry lines where it is difficult to introduce purge fluids or when there is a possibility that deposits may build up in any passage where the flow is not continuous. 4. In clean services where the operating conditions are mild, especially when 3-in. or larger valves are used and omission of the manifold will not jeopardize the safety or operability of the unit.

In all cases where the block and bypass valves are not used, the control valve should be equipped with a continuously connected, side-mounted handwheel.

d. Manifold Piping Arrangements

The manifold piping should be arranged to provide flexibility for removing control valves, particularly where

ring-type joints are used. Flexibility of piping is also necessary to keep excessive stresses from being induced in the body of the control valve.

Arrangements for vent and drain valves are shown in Fig. 6-2. Nipples for such connections are usually %-in. or I-in. minimum. Schedule 80 or heavier as requircd to meet line specifications. Such connections may be used for:

1. Drains. 2. Telltale indicators to determine absence of pressure when removing control valves.

3. Vents.

4. Bleeds.

5. Flushing. 6. Extra pressure taps.

7. Sample connections.

The piping around control valves should be self-supporting or should be permanently supported so that when the control valve or block valve is removed, the

Arraiigemcnt A: Swages scrcwed into the control valve;

Arrangement B: Flanges to match the control valve; weld-

Arrangenieiit C: Extra pipe nipple used between the swage

Arrangement D: Eccentric swages often used to permit complete drainage of line and prevent buildup of deposits in concentric swage.

Arrangement E: Reducing ells may be used in place of welding ells and swages where space is limited.

union a t the large end of the swage.

ing tee or eil used at the large end of the swage.

and the flange to permit easy removal of flange bolts.

FIG. 6-1-Swagt:s at Control Valves.

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CONTROL VALVES AND POSITIONERS

T L7 ALTERNATE f ?

J l -

ALTERNATE X Notes:

1. Vent or drain connections may be placed in the line or in the swage as shown,

2. Nipples and valves are sometimes replaced by plugs or caps.

3. Various combinations of vents and drains are used success- fully depending upon the requirements of the service.

FIG. 6-2-Locations of Vent and Drain Valves.

A VOTE C A N BE ROTATED INTO A N Y

D L A N E , KEEPING CONTROL VALVE VERTICAL

lines will remain in place without the necessity of providing temporary supports.

Control valve manifold piping arrangements are shown in Fig. 6-3. Possible piping arrangements for a pressure-balanced valve in fuel gas service are shown in Fig. 6-4. Piping arrangements for steam to pumps or turbines are shown in Fig. 6-5 and 6-6. Possible piping arrangements for emergency operation of control valves are shown in Fig. 6-7.

6.6 PIPING AND WIRING TO CONTROL VALVE ACTUATORS

This discussion covers the installation of piping and wiring for the valve-actuating medium as well as control signal piping or wiring to the actuator.

The followins codes and standards, as well as refer-

OR - A - -

D E ' NOTE : FLOW SHOULD BE U P

UNDER P L U G ;OR HIGH A P

-1 A P

F NOTE: FOR USE ONLY WHERE

LOW A P IS ESSENTIAL

Arr;ingt!mt,nt A ( I S A 1<P 4.2. Type 1) is preferred because Arrangement D (ISA RP 4.2. Type 3 ) is preferred because manifold is compact, control valve is readily accessible for the control valve is readily accessible. Bypass is self-draining. maintenance, and the assembly is easily drained. Arrungcineiit E restilts in a compact manifold. but control

readily accessible. Arrnngt:ment F is preferred hecxise bypass is self-drnining:

valve is scl f-tl rai ni n g.

side of the control valve are not shown. nor :ire the supports.

Arrniigcnitint B is preferred because control valve is more

Arrangt:iiit:lit C is often used with angle valves. Control

valve may not be io0 accessible.

however, requires greater space.

Note: Block Lind bypass valves should be installed close to tees. as shown, to minimize pockets. Drain and vents on either

FIG. 6-3-Control Valve Manifold Arrangements.

65



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

REGULATOR

FUEL GAS TO B U R N E R S TO B U R N E R S

FROM CONTROLLER- RESTRICTION

FROM A I R SET- OUTPUT CONTROLLEQ + OUTPUT

P S V MAY BE REQUIRED IF DIAPHRAGM OR TOPWORK IS UNSUITABLE FOR MAXIMUM SUPPLY PRESSURE, OR THRUST ON STEM AT MAXIMUM PRESSURE IS EXCESSIVE. V E N T PSV TOSAFE LOCATION

A B Arr:iiigc-iiieiit A diows pressure-halnnced valve with isolating Arraiigenicut B shows a pilot regulator which is suggested

relay and restriction orifice for maintaining minimum fires. where minimum fire maintained by restriction would cause liestriction oritice is sized for the niiinufactiirer's recommended furnace tube failure on process flow stoppage. Isolating relay is minimuni turndown capacity for the burners. To prevent the used to prevent gas flow into controller on diaphragm leakage control valve l'rom closing completely. travel stops may be used or breakage. Ratio relay may be used to permit air loading to in lieu o f the rchtriction orifice. match fiiel gas burner pressure.

1:IG. 64-Pipiiig Arraiigeiiiciits for Pressure-Balanecd Valve at Process Heaters.

enccs to sections of this manual. should be followed as they apply to the installation of equipment: 1 , N F P A Uiilletiii !Vu. 70: Noriorial Electriccii Code. 3. '4 1'1 R I > 500: liecomnieiirle:l Prrictice for Classifica- r im of A reris for Elcctricui Iiistallritions iii Petroleum R e fin cries.

3 . Sect. 7. Trciiismissiori Sysretns. 4. Sect. 9 , Air Siipplx Systems. 5. Sect. 10. Hxdrml ic Sxsrems. 6 . Scct. 1 I . Electrical Power .ïiipply.

TO

S E E FIG 6 - ó FOR P I P I N G NOTES

a. Power Silppiy

A power supply (air. gas. hydraulic fluid. electricity) at suitable Icvel and of adequate capacity should be provided for the valve actuator. To assure operation of the control valve under the most severe conditions. ca- pacities should be based on the most stringent requirements of the actuator.

I L Di up li rag iii -Act iia t ors

Diaphragm xtuators may be operated directly by the controllcd air signal or through positioners or

A B Arrangenient A is used where one control valve is employed Arraiigcnient B is used where two normal pumps have a

for normal pump, or turbine, and spare. common spare.

FIG. 6-C-Stenrii Piping to Spiire Pump or Turbine.

66



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

CONTROL VALVES A N D POSITIONERS --___ __._ .~ ~ -

booster relays. Booster relays may be ~iscd ro provide faster movements OE the control vrilvc if thc controller air output is not adequate ior the application.

Pneumatic piping to diaphragm-actuated control valves is shown in Fig. 6-8 and 6-9. Piping arrangements with booster relays installed are illustrated in Fig. 6-10.

C. Piston or Cylincier Actuaiors PICOT

On cylinder actuators. manually operated four-way valves may be provided to locally control the operation of the valve if so desired.

A bypass valve also should be provided between the two ends of the cylinder actuator to equalize the cylinder pressure should manual operation of the valve be required.

THAN LINE LIQUID

),/FROM PURGESYSTEM

FIG. 6-i-Piping at Regulator Valves or Pres9ure Pilota.

ALLOW QQOt.4 FOR ?LUG SEMOVAL AS WELL. AS TOPWOR6, REMOVAL STEAM

HEADES

TU43iNE

A 'OTRAP

TURBINE

B

3 -WAY TOPUMP VALVE OR

TURBINE "n L

TURBINE TOTRAP D TOTRAP

Arrangement A makes the control valve readily accessible; the piping is self-draining. Steam inlet may be above minimum distance required for plug removal.

Arrangenient B may be used where the bypass can be installed at turbine or pump inlet. Coctrol valve may be too high for easy xcess: piping is self-draining

Arrangement C is often used to make control valve more accessible because the control valve can be located at the same elevation as turbine or pump steam inlet.

hraiigenieiit D is often used for emergency start, or stop, of pump or turbine.

Notes: 1. Bypass and bypass valve should not be so located as :o

form a pocket: they should be self-draining. 2. Location of separators. strainers. and traps not shown

because company standards differ on use and placement. The control valve should be located as low as possible for easy access.

FIG. f í - ~ ~ ( h i i t r ( d Valve Arraiigriiients for Swam to Piinips or Turbines.

REVERSIIUG RELAï FRrlLA

1 n F R O M ,

CONTROLLE!? OUT?JT

FROM ' A'RSET

A CONTROLLEX OUTPLNT

B

C

POSITIONER VALVE *JZ+ REVERSING

D Arrangement A shows nominal control valve piping with

gage for reading diaphragm pressure. Arrangement B shows control valve with reversing relay.

The reversing relay is used to reverse controller output to valve. where two valves with different action are connected to one controller, or is used to provide the same action when on automatic or manual control.

Arrangement C shows control valve with positioner. Arrangement D shows control valve with positioner and

reversing relay. The reversing relay is the same as used in B with standard control valve; in addition, it allows the positioner to be bypassed. A reverse-acting positioner may be used hut i t may not be bypassed.

FIG. CA-Pipiiig at Control V u i v ~ .

67



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

ELECTRIC LEADS TO EMERGENCY SLVITCS

FROM AIR SET

FROM AIRSET

PNEUMATIC LEAD TO EMERGENCY SWITCH

PNEUMATIC VALVE

A B Arrangement A shows control valve with emergency

solenoid actuator. Arrangement B shows control valve with emergency

pneumatic actuator.

FIG. 6-9-Control \’&e Piping with Emergency Control Action.

o FROMCONTROLLEROUTPLJT

BOOSTER RELAY

I-FROM AIR SET

VENTS

OPENBYPASSF7G FOR MANUAL LEVER OR HANDWHEEL- BYPASS OPERATION

MANUAL VALVE FOR OPERATED LOCAL 4-WAY

CON TROLLED HYDRAULIC

HYDRAULIC SUPPLY FLUID

PILOT-RELAY OR MAY BE PNEUMATIC ORSOLENOID ACTUATED4-WAY VALVE FOR 2-WAY SERVICE

CONTROLLER OUTPUT

VID RETURN B

-&- Arrangenient i\ shows the cylinder aciuator with provision

Arrangement 13 shows the cylinder or piston actuator with- for local nan nu al hydraulic operation.

out local hydraulic operation. Arrangvniivit \how\ control v:iive w i t h booster relay. Block

viiives and h y p a h h v:iIvcs are for booster wrvicing.

FIG. 6-Lü-lhwrtcr 1 ~ v l : i ~ - Piping. FIG. í i i2-t l~tIraulic ( :y l i i i t lw i)r Piston :\ctuator.

EXHAUST CYLINDEP

ACTUATOR AIR SUPPLY

PNEUMATIC ACTUATED OR SOLENOID OPERATED 1 k!U ‘\ 4-WAY VALVE

tj

i 9 0 M &CONTROLLER + OUTPUT

REVERSING VALVE POSITIONER PISTON OR

CYLINDER ACTUATOR

A ‘\., C

CONTROLLER FROM AIR SE’

VENTED LOADING

REGULATOR

OUTPUT

”ISTON OR CY LINDER ACTUATOR

OPEN BYPASS VALVE FOR MANUAL LEVER OR HANDWHEEL OPERATION

r PISTON OR CYLINDER ACTUATOR

FROM

OUTPUT

EXHAUST I LIS=- 41R SUPPLY I .# : > POSITIONER PILOT RELAY

I v

B D Arr:ingen~ctit A shows the actuator used with four-way valve

Arrangement B shows the actuator with constant pressure

Arrangement C shows the actuator with reversing relay. Arrangement D shows the actuator with pilot relay for in two-position service.

loading on one side. throttling service.

FIG. 6-1 1-Air Cylindor o r Pision Actiiators.

68



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ELECTROPNEUMATIC I F F E O M i . lQ SE; TRANSDUCE9

)----i---- ----- 2

FROM CONTROLLER

T z,C? 5 : G N A i LEADS F3OM CONTROLLER

AIR SET

CONDUIT CONNECTION FOR MOTOR POWER LEADQ

L----------i---i- ELECTRO- "Y DRAULIC rFL ACTUATOR

L--- -- --- --- ---- - 9 1 CONDUIT CONNECTION FOR

CONTROLLER SIGNAL LEADS FOR

A B C Arrangement A shows the electropneumatic transducer Arrangement B shows the electropneumatic valve positioner

Arrangement C shows the electrohydraulic valve actuator. mounted at pneumatic control valve. on valve.

Note: Provide seals in electrical leads as required by electrical specifications for area in which actuator is installed.

Fi(;. 6-1:3-Elrctroptieumatic and Electrohvdraulic Actuators.

Special considerations for hydraulic cylinders are :

1. If the hydraulic manifold is rigidly piped, it should be connected to the hydraulic fluid supply and the return headers by Hexible metallic hose. 2. To assure a continuous suppl!. of hydraulic fluid to positioner pilots. it is advisable to provide an oil filter. or strainer, and spare suitably valved and piped so that either unit may be removed and cleaned ivithout shutting off the supply to the pilot. 3. Vent valves should be provided at high points in the hydraulic fluid system.

4. The use of automatic. Huid-trapping valves to lock the hydraulic fluid in the cylinders to prevent valve movements on failure of the hydraulic system may or may not be required. depending upon nhether the valve will move i f the hydraulic oil pressure is lost. Fluid- trapping valves should be considered for large valves.

Pneumatic piping to cylinder or piston actuators is shown in Fig. 6-1 1. Piping arrangements for hydraulic actuators are shown in Fig. 6-12.

Some of the electrohydraulic actuators have self- contained reservoirs, pumps, and power cylinders. Such units must be mounted in an upright position to permit proper functioning of the hydraulic system. Because pump motors are operating continuousllr, these units should have adequate ventilation to prevent overheating. Typical piping arrangements for electropneumatic and electrohydraulic actuators are illustrated in Fig. 6-1 3.

CI. Motor Actuators

Electric motor-driven actuators should be mounted so that the motor is above the gear box; this arrangement prevents gear oil from saturating the motor windings.

Air motors of the air turbine or nutating-disk typc normally require an aspirator-type lubricator in the air supply line. Pneumatic piping to air motor-actuated valves is shown in Fig. 6-14.

-FROM CONTROLLEROUTPUT

EMERGENCY S'NITCH -

AUXILIARY

o.:

AIR MOTOR

VALVE

VALVE

L U 0 R i C A T O R ~ 9 B

Arrangement A shows air motor actuator with position

Arrangement B shows air motor actuator with auxiliary transmitter.

equipment to close valve on emergency. FIG. 6 - l L A i r Moior Actuators.

69



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

7.1 CONTENT Suggested practiccs for thc installation of pneumatic,

electrical. and hydraulic measurement and control signal systems are outlined in this section.

Transmitting or receiving instruments, controllers, regulators. control valves, valve positioners, and the installation of these devices are not covered in this section, except as they may affect or be affected by the type or characteristics, or both, of the transmitting system or medium.

Radio transmission is not discussed herein. Trans- mission and control lines supplied and installed as part of the control panel are described in Sect. 12.

7.2 GENERAL a. Basis

The installation methods described are those generally used in the United States and are based on the requirements of applicable codes.

The approach is based on the assumption that transmitters. receivers. and other devices are correctly installed.

I ) . Design aiid Coiistruction Considerations The correct installation of measurement and control

signal transmission systems becomes increasingly important as oil refining units grow larger and more complex. Personnel safety. unit performance, and the length of runs may be jeopardized if fast. reliable signals are lacking during either normal or emergency operation. Careful design and construction are necessary to obtain satisfactory transmission systems at acceptable costs. Consideration must be given to such items as: 1. Installation of leads so as to reduce possibility of damage by fire or mechanical shock.

. 2. Minimum use of piping carrying viscous, unstable, corrosive. toxic, or freezing fluids, slurries, and crystals. 3. Exclusion of hazardous or noxious fluids from control rooms; exclusion of high voltages (above 150 volts) from control rooms. 4. Reliability of air and power supplies. 5 , Provisions for manual control, testing, and ready access to instruments.

c. Location ancl Routing of Wireways or Tiihing Runs

In choosing locations and routing for wircways or tubing runs. facts to bc considcred are: 1. Overhead conduit must be routcd to rcducc accidental iiicchanical abuse and possible damage to wircs or tubes h m tire or overheating. Conduit should not be located closc to hot lines. 2. Routing ovcrhead runs along pipe racks may simplify support problems.

70

3. Undcrground runs often may be shorter and better protected against fire or mechanical damage than overhead runs. Howcvcr, they should be avoided in locations where Hooding with hydrocarbons or corrosive liquids is probable. The following considerations also apply:

a.

b.

C.

Underground ducts must be protected against accidental excavation or crushing by passage of heavy equipment over them. Frequently, this protection is accomplished by embedding the ducts in a concrete envelope. Where ducts pass beneath roadways. suitable reinforcing should be provided; for easy identification the concrete is colored. usually red. Parkway-type installations are often protected by a concrete slab. Special attention must be given to the location of pull points in underground duct systems; replacement of damaged wires or tubes may be difficult and extremely costly. Intermediate pull points may be eliminated in bundled tubing and multiconductor cables by inclusion of sufficient spares. If underground routing is expected. the instrument program should be firmly established as early in the project as practicable to avoid costly changes or additions.

Fig. 7- 1 through 7-6 illustrate comparative pneumatic and electronic arrangements for a typical installation.

d. Signal Types Signal transmission systems in refineries usually are

electric or pneumatic; hydraulic systems have only a limited use for precise operation of large valves or dampers.

7.3 P3EUMATIC SYSTEMS

a. Stantlard Pneumatic Traiisniissiori Ranges

Industry practice has been to abide by either of two principal transmission system pressure ranges-3 psig to 15 psig or 3 psig to 27 psig-recognized by the In- strument Society of America (ISA) and the Scientific Apparatus Makers of America (SAMA). Refiners USE- ally arrange to supply air at a pressure of 20 psig for instruments having 3 psig to 15 psig transmitted air range. This is recommended for new installations. However, 35 psig air pressure is used with springless, diaphragm- type operators which stroke single-seated valves, and 50 psig or 100 psig air pressurz frequently is used for actuating piston operators furnished with large valves or dampers. To cnsure operation in emergcncics. one re- finer specifies that large actuators shall dcvelop design thrust at SO pcrccnt of nominal supply pressure and shall be nicchanic:illy capable of continued operation without darnagc at I20 pcrccnt of nominal. For rccom- mendations on air supply and instrument pancl bourd piping, see Sect. 9 and 12. rcspectivcìy.



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. . - . -

E-21 . _-

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m Q Lo N

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TRANSMISSION SYSTEMS

I,. Tuhing

Pneumatic transmission lines are usually %-in. or %-in. OD. The %-in. size is preferred. Tubing is supplied in copper, aluminum, and plastic, either in single strands or in bundles.

1. COPPER TUBING Copper tubing is more widely used than either alu-

minum or plastic tubing. Copper tubing is supplied annealed or half-hard.

2. ALUMINUM TUBING The use of aluminum tubing has become common.

In some locations. where sulfur attack may be expected because of atmospheric conditions, aluminum has shown better corrosion resistance than copper. However, it has not worked well in seacoast locations nor in some petrochemical plants. Also, it seems to be somewhat less resistant to vibration than copper. When installing aluminum tubing care must be taken to provide sufficient insulation between the aluminum tubing and other metals to prevent electrolytic corrosion. Currents caused by arc-welding in the vicinity of the tubing have been found to leave small pinholes in the tubing. Spray or rebound from “guniting” also has been found to produce small pits in the tubing surface; therefore, in areas where considerable guniting is probable, some permanent form of protection should be provided. Aluminum tubing must be protected from any contact with magnesia insulation.

3. PLASTIC TUBING Plastic tubes are light, corrosion-resistant. and have

sufficient resilience to prevent damage from occasional freezing; however, because there is some tendency for the tubing to “cold flow” and develop joint leaks. care must be used in the selection of fittings. Also, because some plastics burn and most plastics lose strength when the temperature exceeds 200 F, the possibility of overheating or fire damage limits application in outdoor processing areas. unless the tubing is suitably protected; see (d) .

c. Tube Fittings

Many types of tube fittings are used. Where plastic tubing is used because of atmospheric corrosion, it is good practice to apply a colored plastic coating (acrylic) to the fittings. The color helps to make coating damage visible.

Brass fittings are used with copper tubing and anodized aluminum fittings are used with aluminum tubing; the anodizing prevents mechanical seizure between the tube and fitting.

ci.

The problem of supporting small lines and protecting them against mechanical damage or overheating is diffi-

Support and Protection of Tubing

71

cult and costly to solve. There is no agreement as to the best method of installation. The types of installation in common use are:

1. Support on racks (see Fig. 7-7). 2. Support on expanded metal troughs (see Fig. 7-7). 3. Prefabricated multiple tube bundles (see Fig. 7-8 and 7-9).

A number of manufacturers offer prefabricated supports and attachments for both individual tubes and tube bundles.

1. RACK SYSTEMS Rack systems have the advantage that any tube can

be individually traced, repaired, or replaced. They have the disadvantage of highest installed cost, because they must be assembled by field labor. Also, they provide

/STRUCTURAL ANGLE CLAMP

INVERTED CHANNEL

” <CLAMP

TYPE 1 TYPE 2

END VIEW TYPE 3

SIDE VIEW

COLUMN (SHOWN) WALLOROTHER SUITABLE SUPPORT

TYPE 4 TYPES Types 1 and 2 may be used for one or more lines with short

spans. These types are suitable for either indoor or outdoor applications.

Type 3 shows raceway for a large number of lines. Only one bank of lines is permitted. Raceway may be trussed with diagonal bars if load conditions require. This type is suitable for both indoor and outdoor use, including tunnels.

Type 4 shows raceway carried on angle brackets. Covers are optional.

Type 5 illustrates a two-tier raceway for a greater number of lines than in Type 4. Design and application are the same as in Type 4.

FIG. 7-7-Methods o f Siipportiiig Instrunieiit Tubing.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

JNDI

FIG. 7-8-Detail Showing Wavs to Break Out Leads froiii Armored Multiple Tubes.

the least protection against mechanical damage to tubes unless provided with covers. Some companies assemble sections of vertical racks o n the ground before erection. The use of prcfabricated support bars permits reductions in engineering and field costs.

2. TROUGH SYSTEMS Trough systems, like rack systems, have the advantage

that any tube can be individually traced, repaired, or replaced. In addition, they provide somewhat greater protection against mechanical damage to the tubes than do rack systems. Installed cost is generally somewhat lower because less field fabrication work is required.

3. BUNDLED TUBING Several manufacturers offer bundled tubing in a vari-

ety of materials and protective coatings: Copper or uluminum tubing is furnished in parallel

or spirally wrapped layers, one tube in each layer being color-coded for identification. The tube nests are wrapped with plastic-impregnated cloth tape. or an extruded plastic coating is applied. Interlocking galvanized steel, aluminum, or other metallic armor can be obtained which will give protection equal to that of “parkway-cable” electrical wire which is used extensively for direct burial.

Plustic tubing is available either with an extruded

plastic sheath or with armor sheathing where needed. Where fire damage is a consideration, the tube bundle may be covered with pipe insulation. Bundled plastic tubing also is available with a heat-resistant asbestos jacket over the extruded sheath and covered with a plastic protective jacket. This material can be used underground and in conduit.

Field installation of bundled tubing can be made very simple if thorough engineering is done and cable termination points are carefully selected. Copper or aluminum tube bundles have high-tensile strength and may be bent on short radii. Bundles may be run in expanded metal troughs, on steel messenger cable. or supported by suspension from pipe stanchions. Bundles may also be attached to building walls or structural members with pipe clamps or special fasteners.

Junction details are shown in Fig. 7-8 and 7-9. Ter- mination may be made easily with standard bulkhead tube fittings. Taps also can be made readily.

4. PRECAUTIONS

bundles are:

identified in any given length.

Some precautions to be observed in installing tube

Tubes must be color coded or their ends permanently

Spuues must be determined and installed initially. Tubing system must be laid out carefully with all re-

quirements stated because the bundles must be ordered precisely.

5. PLASTIC JACKETING Plastic jacketing of metal tubes appears advantageous

in preventing corrosion; single tubes as well as bundled tubes are available with these protective jackets.

6. PROVISION OF SPARE TUBES AND RACK SPACE In bundled tube systems it is good practice to initially

provide spare tubes, at least 10 percent or not less than two tubes per bundle. Also, spare rack space should be provided initially in all systems. Space for a 25-percent increase in leads is suggested.

e. Connections to Instrument Equipment

Inasmuch as air supply details are covered in Sect. 9, panel board details in Sect. 12, and other special items elsewhere, only two precautions will be discussed.

1. TRANSMITTERS Some refiners install a valve or cock in the signal

connection from each transmitter to permit blocking off the signal line when testing it for leaks. This valve is located between the transmitter and the tee connected to the output air pressure gage, so that the gage may be used to indicate line leakage.

2. RECEIVING DEVICES It is good practice to install a shutoff valve in the air

lead to each transducer or other receiving device to

72



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I !I

THIS ASSEMBLY IS FREQUENTLY ENCLOSED IN A JUNCTION BOX

nnn n

permit taking the device out of service without disturbing other instrumentation or control.

7.4 ELECTRICAL SYSTEMS a. Background

It is essential that those responsible for electrical design and installation in refinery areas be thoroughly familiar with the National Electrical Code (NEC) and API RP 500 definitions, and with the nature of explosionproof enclosures. Recommended installation methods are described in API RP 540: Recommended Prac- tice for Electrical Installations in Petroleum Refineries.

The specification for equipment, enclosures, wiring, and installation methods should fully conform to the requirements of the latest edition of the NEC, the National Electrical Safety Code (NBS Handbook H30) , and the regulations of authorities having jurisdiction at the job site.

b. Plant Areas Where Atmosphere May Contain

It is recommended that designs of electrical installations, where there may be some hazard from the pres- ence of volatile flammable liquids, gases, or vapors in explosive or ignitible quantities in the atmosphere, be

Flammable Vapors

DETAIL A

Tubing.

based on the requirements of Art. 500, NFPA Bulletin No. 70: National Electrical Code, and API RP 500: Recommended Practice for Classification of Areas for Electrical Installations in Petroleum Refineries. The latter is intended as a guide in classifying potentially hazardous refinery areas and in determining their extent.

1. TYPES OF AREAS

These publications recognize three types of areas: Division I areas: The criterion for these locations is

that they are likely to be hazardous. Division 2 areas: The criterion for these locations

is that they are likely to be hazardous only under abnormal conditions, such as the failure or rupture of equipment.

Nonhazardous areas: These are locations which cannot be classified as Division 1 or 2.

2. INSTALLATIONS FOR DIVISION 1

Division 1 areas require the use of explosionproof equipment which is designed so that operation or failure of any portion of the electrical system, even though causing vapor ignition within the housings, will not release flame or hot gases so as to ignite the surrounding atmosphere.

73



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

3. INSTALLATIONS FOR DIVISION 2 Division 2 areas require the use of equipment ar-

ranged so that full operation of the electrical system (including arcing devices such as open contact switches, relays, etc.) may occur without providing a source of ignition under normal conditions. Complete protection is not provided against electrical breakdowns, inasmuch as these occur very rarely and the equipment usually is de-energized promptly in such cases.

c. Intrinsic Safety

Intrinsically safe equipment and associated wiring are covered in Art. 500 of the NEC. Such approved equipment and construction may be used in any hazardous location for which they are approved; certain other provisions of Art. 500 and 510 will not apply to such installations. Intrinsically safe equipment and wiring are incapable of reIeasing sufficient electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture. Abnor- mal conditions include accidental damage to any part of the equipment. wiring or insulation, or other failure of electrical components, application of overvoltage, damage during adjustment and maintenance, and similar conditions.

1. AP PARATUS A N D CIRCUITS The British have developed intrinsically safe appa-

ratus and circuits based on the principles set forth in the preceding paragraph.

2. APPLICATIONS Although intrinsically safe circuits are limited in ap-

plication, they have been used in England and Canada for some time. However, their use is just beginning to be accepted in the United States.

3. ~NSTALLATION

Because there are no accepted standards of design nor methods of test for intrinsically safe equipment, it is necessary to install each circuit so that any faults which may develop in the circuit, either within or outside the classified area. cannot create a spark of sufficient energy to ignite a combustible mixture within the classified area.

d. Support ancl Arrangement of Conduit Systems

The following methods are suggested for the support and arrangement of conduit systems, recommended in the absence of electrical specifications covering the entire plant: I. Rigid conduit should be supported at least once every 7 ft; other types should be supported at closer intervals. 2. Conduit should never be supported from piping, because the piping may have to be removed for inspection or replaced.

74

3. Provision should be made for thermal expansion or movement of supports, such as swaying of towers in high winds. 4. Conduit should be fastened with pipe clamps or U-bolts. not tack-welded. Substantial steel hangers should be provided for groups of conduits where it is not practicable to clamp directly onto building walls or structural members. 5. The distance between pull points in a conduit system should not exceed 250 ft for straight runs. Sixty feet should be deducted from straight-run length for each 90 deg of bend, or Y3 ft per deg of bend on other than 90-deg bends. Pull fittings should be used to avoid more than 270-deg total bends between pull points. Where manufacturer's ells. or plastic or synthetic rubber wire insulations are used, it may be necessary to reduce the distance between pull points. 6. Drains and seal fittings must be provided in accordance with the NEC. 7. Where fire damage is possible, flame barriers (often of asbestos sheeting) should be provided. Insulation of conduit should be avoided unless possible electrical overheating of wire insulation has been checked. For thermocouple or other low-current signal wires, however, jacketing conduit with suitable pipe insulation (85 percent magnesia, fiberglas, etc.) will help to protect against flame. 8. Junction conduit fittings shculd be installed near furnaces or other locations where wire insulation damage seems probabIe.

e. Conduit Materials

Conduit materials should be as specified in ASA Standard CSO. I : Specification for Rigid Steel Conduit, Zinc Coated.'"

f. Wire Insulation

Because refinery conditions vary considerably, no one type of insulation has been found to be suitable for a majority of installations. When selecting insulation, the possibility of temperature extremes or of chemical or electrolytic action should be considered. The NEC gives temperature and current limits as well as corrections for most grades of wire. The following types of insulation are used: I . Nutrira1 rubber, brciid-covered: Type R code rubber. Immersion in petroleum liquids will cause rapid deterioration of rubber-base materials. 2. Neoprene: Neoprenes are affected by aromatic hydrocarbons and have high coefficients of friction. 3. Polyvinyl plastic: Type TW is quite popular for general use. This wire should be carefully selected heavy-duty grade rather than building grade. 4. Polyethylene plastic: Polyethylene will burn readily unless compounded with a flame retardant. 5. Enamel Lina' felted trsbestos or spun glass: Frequently

" Figures refer to REFERENCES on p. 78.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---


specified in hot locations. Such insulation deteriorates rapidly on flexing, especially after being heated. Also, it is subject to moisture deterioration. Often it is preferable to splice to a more impervious material for leads extending beyond the hot location. 6. Type MI: Metal-jacketed, magnesium oxide insulated wire. This material has excellent heat resistance, except that in hot locations the method of sealing terminations or splices will limit the maximum allowable temperature. The magnesium oxide, which is very hygro- scopic, must be kept dry. Cut ends of the cable must be sealed immediately to prevent absorption of moisture. This may be done by immediately installing the terminal fittings, properly packed with sealing compound, or by temporarily sealing the ends by suitable waterproofing. 7. Weatherproof braid over rubber: Used by many refiners for general service; others prefer the superior resistance to hydrocarbons (other than aromatics) of polyvinyl plastic. 8. Lead sheathing: Used for underground wiring; however. lead is attacked by caustic solutions. hence, Type TW often is more suitable.

g. Signal Transmission Circuit Wiring from measuring and analyzing de-

vices differ in their requirements for shielding, insulation. twisting of leads. coaxial cable. or exclusion of measuring and power leads from the same conduit. Manufacturer's requirements should be checked carefully before specifying circuit arrangement or materials.

1. SPARE LEADS Usually, spare leads are installed initially; often one

spare pair is provided for a minimum of four leads of each type and for each multiple of eight leads. 2. THERMOCOUPLE WIRING

Sparking hazard: Thermocouple wiring usually is considered as offering negligible sparking hazard. In- dustry experience has shown that failure of the potentiometer measuring system, in a manner which would permit a sparking hazard, is unlikely in normal service.

Conduit and fittings: Thermocouple extension wire usually is run in conduit and vaporproof fittings are installed (see Fig. 7-10). When attaching conduit to thermocouple wells, some sort of flexibility is necessary. As a rule, this is arranged with a loop of flexible conduit, although where the extension wire insulation is weatherproof and chances of mechanical damage are slight, some companies run the wire in the open for the last foot or so. The possibility of well breakage must be provided for in situations where the sheath is connected to the thermocouple head. Usually, the conduit is sealed off and a vent hole is left on the thermocouple side of the seal. Seal fittings are used to isolate the flexible conduit entrances and, also, to seal off conduit entranccs to control rooms (see Fig. 7-10). Seals also should be provided where the conduit enters field instruments.

Electrical signals

IRAINAGE- LADDER AND SAFETY BASKET

30"LLNGTH OF STO 3/4" FLEXIBLE METALLIC

TOWER OR

-._____--

6" NIPPLE ,AND PERFORATED PIPE CAP OR DRAIN FITTING

I I FIG. 7-10-Conduit Arrangement for Thermocouple

Extension Wires.

Protection in control room: Thermocouple leads are often run in sheet metal wireways inside control rooms. Exterior conduits terminate in a junction box and leads are carried in wireways to panels. Thin-wall conduit and flexible conduit (Greenfield) are acceptable between wireways and instruments in nonhazardous areas. or in Class I. Division 2 areas if properly sealed and grounded.

Extension wire: Thermocouple extension wire is usually No. 16 Awg minimum, solid duplex. Multicon- ductor 20-gage cable with plastic jacketing is frequently used. For sizes and types of thermocouple and fully compensated extension wires, see Sect. 1.1, 1.2, and 1.7 of ISA R P 2.1-.7: Thermocouples and Thermo- couple Extension Wires.z For the correct combinations of thermocouple and extension wires needed for accurate measurements, refer 'LO Sect. 3.

Jirnctions: Junctions in thermocouple extension wire should be made with labeled, insulated terminal strips in conduit fittings or junction boxes (see Fig. 7-1 1 ) . A system arrangement for use where several thermocouples are involved is shown in Fig. 7-12. Watertight outdoor junction boxes and gasketed sheet metal indoor junction boxes are preferred. Some companies avoid the use of junctions (other than splices) in thermocouple leads, particularly those going to controllers. These companies believe that temperature diferences at junctions can cause errors in measurement.

Connections t o instruments: Thermocouple wires to controllers usually are not connected to any other

75



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API RP 550-PART I

TERMINAL SLOCK NOS 1, 4.. 7 \

?NO., TERMINALSCF FOR EACH BLOCK

SCHEMATIC ÇO R

THERMOCOUPLE JUNCTION BOX

A

1EWS

A

I @-@I I

TERMINAL BLOCK Notes: 1. Each junction box should have a nameplate mounted on

the outside of the cover, or door, marked ?Thermocouple Junction Box A, o r B, or C,? etc.

2. The terminal blocks should be numbered so that No. 1 block is in the upper left corner of each junction box, No. 2 block directly below it, etc.

3. T h e top pair of terminal screws on each block should be numbered 1, with pair No. 2 directly below it, etc. 4. A pair of terminals might be referred to as in the follow-

ing example: ?A-5-8? indicates junction box A, terminal block No. 5. terminal No. 8.

5. The tagging strip should show the number of each thermocouple; the service descriptions for each thermocouple should be inscribed on a heavy card secured in a frame attached to the inside of the box cover.

FI(;. 7-1 I-Coniieetiuiis in Thermornuple Junction Boxes.

instrument. It is considered good practice, however, to connect at least one thermocouple from each recorder to an indicator to permit checking instrument readings. Many companies connect all recording (but not controlling) couples through the indicator to the recorder.

Cold junction compensation: Cold junction compensation should be accomplished in accordance with the recommendations in Sect. 3.

3. RESISTANCE THERMOMETER WIRING Circuit classification: Resistance thermometer circuits

usually cannot be classed as intrinsically safe, therefore wiring methods must conform to the classification of the area as specified in Art. 500 of the NEC.

Connections: Resistance thermometers usually are connected in a manner similar to thermocouples, except that extension wires are copper. Where bridge-measuring circuits are used, three wires usually are run to each thermometer bulb to place the bridge arm junction at the element.

4. ANALYZER SIGNALS For analytical equipment. it is universal practice to

not run measurement leads in the same cable or conduit

with power leads. As with other transmission circuit conduits between units in which component failure might allow hazardous process fluids to enter the conduit system. analyzer signal and control conduits should be doubly-sealed with an adequate vent between.

5. OTHER CONTROL WIRING

All other signal wiring should meet NEC requirements, particularly those of Art. 500 which apply to the area classification wherein wiring is located. A suggested electronic transmitter installation is shown in Fig. 7-1 3. For general guidance on the proper handling of refinery wiring installations, see API RP 540.

For Division I areas rigid conduit, installed with explosionproof fittings and flexible connectors, is required. All arcing devices must be completely enclosed in explosionproof housings and isolated from the conduit with sealed fittings.

For Division 2 areas rigid conduit, installed with vaporproof fittings, generally is used outside control rooms, except that arcing devices are enclosed and sealed as for Division 1 areas. In all cases. the conduits should be sealed at both ends and vented between seals.

Sheet metal wireways of substantial construction are employed occasionally for multiple runs, particularly inside control rooms, where acceptable to local code- enforcing authority.

Intrinsically safe instrument leads may be installed in a similar fashion to thermocouple leads provided manufacturer?s requirements pertaining to static or inductive pickup are met.

It is well to remember that line fluid may be released while checking or servicing flow, pressure, and similar electrical transmitters; therefore when installing these instruments particular care should be taken with e!ec- trical wiring.

6. MOISTURE PROTECTION .

insulation to prevent entrance of moisture. Care must be taken to seal connections, wiring, and

7.5 HYDRAULIC TRANSMISSION SYSTEMS FOR CONTROL

The principal advantage of hydraulic transmission is that equipment requiring considerable force to actuate it can be quickly and very accurately operated.

The principal disadvantage is the relative bulk of the measuring, controlling, and storage equipment. and of hydraulic medium supply systems.

a. Types

There are two types of hydraulic control systems- local and central. Requirements especially for the central system are covered in Sect. 10. Connections to actuators are described in Sect. 6.

76



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THERMOCOUPLES

ONE COMMON BOX MAY BE USED IF A.B.AND ARE ON THE SAME

LOCATED ON OPERATING PLATFORM

PLATFORM

JUNCTION BOXES

LOCATED ON ûNE COMMON BOX MAY BE USED

IS NOT EXCESSIVE

TEMP IND. NO. T I -2

, EMP CONT. NO TUC-5

I I

Thermocouple Instrument No. Terminal Location No. Service -

1 No. 4 cracking coil, oil to heater TI-2 TR-8 B-1-1 C-1-1 E-1-1 2 No. 4 cracking coii, oil from heater control T R C J D-1-1 F-1-1 E-1-2 3 No. 4 cracking coil, vapor from No. 1 tower TI-2 TR-8 B-1-2 C-1-2 E-1-3 3 No. 4 cracking coil, vapor from No. 2 tower TI-2 TR-8 B-1-3 C-1-3 E-1-4 5 No. 4 cracking coil, vapor from No. 3 tower TI-2 TR-8 B-2-4 C-1-4 E-1-5 6 No. 4 cracking coil, tar from No. 1 tower TI-2 TR-8 B-1-5 F-1-5 E-1-6

8 No. 4 cracking coil. oil from heater TI-2 A-1-1 C-2-1 E-2-2 7 No. 4 cracking coil, tar from cooler TI-2 TR-8 ::;B-2-6 C-1-6 E-2-1

:P Esanipie, 8-2-6: B denotes terminal junction box. 2 denotes terminal block number. 6 denotes terminal number. ilrotes: I . At temperature controller locations. two thermocouples

are often placed in the same thermowell with one connected to the indicator.

2. Junction boxes A, B, C, D, E. and F are required only in the vicinity of towers, heaters. reactors, or major vessels, or where the total conduit run exceeds 500 ft.

3. All junction boxes should be mounted so that the top of each box is approximately 5 ft O in. above grade or platform. There should be a %-in. drain in the bottom of each box.

FIG. í-12-Thermocouple Wiring Arrangement.

1. LOCAL SYSTEM The local system is usually a “packaged operator”

for a valve, damper, or regulator? which receives a signal from an electric or pneumatic system and positions the controlled element. It consists of a pump, usually elec- tricaliy driven, a fluid storage chamber, a pilot-operated regulator, an actuating cylinder, and sometimes a pressure tank. 2. CENTRAL SYSTEM

The central system supplies several valve actuators with fluid under pressure from a common tank often maintained at pressures within a range of 300 psig to 500 psig. Fluid is returned to atmospheric storage.

ORIFICE i‘.

kDRAIN (FITTING)

-MUST BE EXPLOSION PROOF EQUIPMENT IN CLASS I AREA

- PREFERABLY RIGID CONDUIT,

MAY BE USED IN DIVISION 2 BUT VAPOR TIGHT FITTINGS

AREA

DRAIN y

[FIT TING) O . Pipe Material TO RECEIVERS IN CONTROL ROOM NEAR CONTROL VALVE

INDICATOR IS USUALLY LOCATED

Seamless or fusion-welded pipe should be used. Sup- ports should be in accordance with good piping practice, FIG. 7-13-Iiistallatiuii of Electronic Transniitter.

77



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I . LOCAL CONTROLLER Some refiners specify %-in. Schedule 80 pipe for a

local controller. Other refiners prefer ?h-in.-OD tubing. For fast valve positioning, larger tubing or higher pressures should be used.

2. CENTRAL SYSTEM For the central system, a %-in. minimum iron pipe

size, Schedule 80 wall thickness is often specified. Fit- tings should be of forged steel. either socket-welded or screwed and back-welded. Connections are usually 3/4 in.; valves are often of the bolted bonnet integral-gate type. Piping usually is pickled for scale removal after installation.

7.6 INSPECTION AND TEST

This section is based on the assumption that the user has satisfied himself that all measuring and control equipment is suitable for his service, adequately fastened in place. properly connected, fully capable of operation, and properly calibrated.

I . Piieuiiiatic Transmission Systems

Pneumatic transmission systems should meet the requirements o€ ISA RP 7.1: Pneumatic Control Circuit Pressirre Test.

1,. Elertrirul Transmission Systems

Electrical transmission systems should meet the following specifications: 1 . “Megger” wire-to-wire insulation resistance o€ transmission leads and wire-to-ground insulation. Instrument must be disconnected. Any readings below 1 megohm line-to-line or 1 megohm line-to-ground may indicate faulty insulation.

2. Check all shielded circuits, and all circuits whose connected instruments may be sensitive to inductive pickup, by putting 60-cycle alternating current (or other normal frequency) on all other circuits in the same conduit with circuit being checked to see whether any shift in instrument reading occurs.

c. Hydraulic Transmission Systems

Hydraulic transmission systems should be flushed clean and hydraulically tested to 150 percent of maximum operating pressure but to not less than 100 psig.

REFERENCES

ASA CSO. 1: Amcrictiti Stcrnticrrd Specifictition for Rigid Steel Coiiclrrit. Zinc Corited, Am. Sid. Assoc., New York ( 1959).

ISA R P 12 series, Instr. Soc. Am., Pittsburgh, Pa.: 11. ISA R P 12.1: Elecìrictrl Instrrrinents in Hozcirdous

h. ISA R P 12.4: Iizstrritnent Pirrging for Redrrction o f

c. Companion Recommended Practices due for publi-

Atinosplieres ( 1960).

ffrrxirdori.~ Aren Clrissificarion ( 1960).

cation include:

Designrition Subject RP 12.3 Intrinsic Safety RP 13.3 Explosion-Proof R P 13.5 Sealing and Immersion RP 12.6 Wiring Practices

’ ISA RP 1.1-.7: Tlirrtiiocoiiples ni id Tlierniocoirple Esten-

r i . Sect. 1.1: “Coding of Thermocouple Wire and Ex-

b. Sect. 1.7: “Coding of Insulated Duplex Thermo-

c. Sect. 1.7: “Temperature-EMF Tables for Thermo-

‘ ISA R P 7.1: Pnerrniutic Control Circirit Pressirre Test,

sion Wires. Instr. Soc. Am., Pittsburgh, Pa. ( 1959).

tension Wire.’’

couple Extension Wires.”

couples.”

Insti. Soc. Am.. Pittsburgh, Pa. (1956).

78



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SECTION 8-SEALS, PURGES, AND WINTERIZING

8.1 CONTENT

This section presents guides for sealing, purging, and winterizing measuring instruments and their connecting lines from the process to ensure reliable instrument performance. Measuring instruments include such types as flow. level, and pressure.

Seals and purges are the means of preventing the measured material from entering the instrument or instrument lines and causing improper operation or damage to the instrument through vaporization, condensation, viscosity effects. corrosion or sedimentation.

Winterizing refers to the methods used to ensure proper performance of instrument systems at low- ambient temperatures.

Special cases of analyzer winterizing are covered in Part II of this manual.

8.2 SEALS

a. General

In the past, the use of seals has been a troublesome feature in refinery instrumentation, especially with flowmetering equipment. The use of force-balance and other types of negligible-displacement transmitters, which are close-coupled to the point of measurement, eliminates much of the need for seals. Where sealing is necessary, these transmitters permit sealing without chambers and complicated pipins arrangements.

DIAPHRAGM SEAL PIPING SEAL PIPE FILLED WITH SEALING LIQUID TOTEE. SEALING LIQUID TO BE HEAVIER THAN LINE

SEAL FLUID

FLEXIBLE DIAPHRAGM

4

SEAL CHAMBERS

I? x ---$jpEF I:.

SEAL LIQUID 1 @ LIGHTERTHAN SEAL LIQUID HEAVIEi) THAN LIQUID MEASURED LIQUID MEASURED

8 8 SEE SECTION 4 FOR PIPING DETAILS AND GAGE SUPPORTS

FIG. 8-l-Seals for Pressure Gages.

h. Diaphragm Seals

One method of sealing is to use a flexible diaphragm or bellows to hold a seal liquid in the instrument and to mechanically separate the sealing medium from the measured material. This method, shown in Fig. 8-1, primarily is limited to pressure gage protection (refer to Sect. 4).

c. Liquid Seals

A common method of sealing instruments makes use of a liquid that is immiscible with the measured process fluid. The immiscible liquid should have a density differ- ing from the density of the measured process fluid. With this method, instruments of negligible displacement, which may range from minimum to maximum without appreciable movement of liquid in the connecting lines, may be readily sealed in the piping. Instruments with appreciable displacement require seal chambers and special piping arrangements for control of the seal level in order to prevent hydrostatic errors. Typical seal installations are shown in Fig. 8-1 through 8-3.

CI. Seal Chainhers

Through general practice a few types and sizes of seal chambers. or condensate pots, have been developed which meet most sealing problem requirements. Details for two of the most popular types of seals used are shown in Fig. 8-4. Seal chambers are available from manufacturers or may be made in the field. The materials and fabrication should comply with ISA RP 3.1: Flowmeter Installations, Seal and Condensate Chnm bers.

e. Sealing Liquids

Water or ethylene glycol and water mixtures have the requisites for most sealing operations in the petroleum

SE ALTO TEE,,^ i>a c

SEALTO TEE BOTH SIDES

REDUCE PIPE LL SIZE IN-'

TO FLANGE TAPS RISER

ABOVE SEAL

SEESECTION I FOR PIPING DETAILS

TYPE A - F L O W

SEE SECTION 2 FOR PIPING DETAILS

TYPE B -LEVEL

FIG. 8-2-Se:ils for Force-Balaiice Iiistriiiiieiits.

79



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API RP 550-PART 1

ONE VALVE BYPASS OPTIONAL

b-

-4 O

-60

BYPASS OPTIONAL-

TO METER MANIFOLD ro METER MANIFOLD

TYPE A -LIQUID FLOW TYPE B -LIQUID FLOW SEAL HEAVIER THAN LINE LIOUID SEAL LIGHTER THAN LINE LIOUID

GLYCOL GRAVITY 10 I O 1 6

20 I 0 3 0 30 1.044 40 I 0 5 8 5 0 6 0

~-

\ I I

l I ~

YI w LINES SELF-ORAINING TO ORIFICE TAPS >

TYPE C - GAS FLOW

A B c

+! USE OF SIDE TAPS MAKES VAPOR TRAP OF TOP OF CHAMBER ORSEDIMENT TRAP OF BOTTOM

a"iiPE SIZE 6"PiPE SIZE

14" 16"

21/;"MiN 3"MIN

a

TO INSTRUMENT

TYPE D - LEVEL

FIG. 8-R-Seals for Mercury Meters.

LIOUID CAPACITY THRU c hl!JST EXCEED INSTRUMENT DISPL4CEMENT I F SiOE TAPS ARE USED

ENDS FORMED, CAPPED OR F L A T

CONNECTIONS ~ ~ ' ' N P T 6 0 0 0 - L B H A L F COUPLING OR WELD PAD

TYPE A

TYPE B Note: ASA 831.1: Code for Pressiire Pipirzg governs ma-

terial and fabrication. Refer, also. to ISA RP 3.1: Flowtrieter Iiistnlliirions, Seal ni id Cotideiistite Clrtrinbers.

FIG. 8J 'Seal Chambers.

IO 20 30 40 50 60 7 0 "/a VOLUME OF ETHYLENE GLYCOL

Note: Curve does not represent true freezing point of ethylene glycol and water solution. It gives recommended mixtures which assure the proper operation of a sealed instrument.

FIG. X-5-Ethylene Glycol and Water Solution.

industry and are used almost to the exclusion of any others. Ethylene glycol should be the inhibited type to prevent it from becoming corrosive. Characteristics of ethylene glycol and water mixtures are given in Fig. 8-5. Other sealing liquids and their properties are given in Table 8-1.

8.3 PURGES

a. General

Some instrument applications are made possible by the use of purge fluids which may be liquid or gas. These fluids are introduced into the instrument lines and flow out through the instrument taps. The purge liquid serves to seal the instrument and to sweep the lines clean of the measured material which tends to enter the instrument lines. Typical purging arrangements are shown in Fig. 8-6 and 8-7.

1). Purge Fluids

Purging of instrument lines requires a suitable purge liquid or gas at a pressure sufficiently high to ensure continuous and even flow of the purge into the instrument lines. For example, purge oil should be clean, free of a tendency to flash, noncontaminating to products, and available at a pressure greater than that of the measurement. Purge gases must be clean, dry, and compatible with process conditions. Generally, the purge is fed continuously at a controlled rate. Restriction orifices or sight flow indicators with adjustable restrictions are used to determine and limit flow. They may be combined with differential relays for controlling the flow. The point of entry of the purge into the instrument lines should be as near the instrument tap as possible in order to hold the

80



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

. . . . < .

VI VI

O d m

O 3 I

2

VI

2

2 8 ci d

s O

2

O W W

O l- m

o 3

2 z E E rj

. . . . . .

. . . . . .

. . .

d I

m 9 W

N

V-IWW +VIS I I I I

. . . . . .

. . <

. .

W N O w l - w O O O O I - . . . - - e

3 Q! -

8 1 COPYRIGHT 2003; American Petroleum Institute


--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

GAGE OPTIONAL 2 I " GATE VALVE FILTER OR STRAINER

'\PURGE CONTROL ORIFICE IN SCREWED UNION OR SIGHT FLOW INDICATOR OR FLOW CONTROLLER

PIPE OFFSET TO PREVEN; DRAIN BACK. TURN 180" IF SEAL IS HEAVIER THAN FLUID MEASURED

TO INSTRUMENT

TYPE A

MEASUREMENT FLOW RATE CATALYST 30 TO 1 2 0 S C F H LEVEL(IN VACUUM 1.0 TO I .5 SCFH

SERVICE)

TURN LINES AT A IF INSTRUMENT IS BELOW TAP

VALVES

\PURGE UNIT, RESTRICTION ORIFICE. A OR DRILLED GATE VALVE

' A I

TYPE 8

Type A is suitable for purging pressure and vacuum gages. Type B is suitable for catalyst measurement as well as for

level in vacuum columns or low-pressure vessels.

FIG. 8-(>-Purge Iiistailations.

TURN LINES UP IF METER IS ABOVE ~ P

I - I" TYPE A

PURGE LIGHTER THAN LINE FLUID WITH SURGE POT

TURN LINES UP IF METER 15 ABOVE,

*TO METER

TYPE 6 PURGE HEAVIER THAN LINE FLUID WITH SURGE POT

Note: Where the restriction (R) is an orifice plate, the fol-

Orifice Flange

lowing quantities may be used for calculations:

Drilling Gas Flow Liquid Flow (Inch) (Cubic Feet per Hour) (Gallons per Hour)

!h 1 .o 3.0 Y8 2.0 5.0 ?!2 5.0 8.0

FIG. 8-7-Purges for Flowmeters.

pressure drop in the lines, as a result of flow, to a minimum.

c. Rate of Flow The rate of flow to be established in any purge in-

stallation has a range of variance limited on the low side by extremely small orifices or restrictions, which become difficult to maintain, and on the high side by excessive ñow-producing instrument errors. There are too many factors involved to attempt to set high flow limits. Usually, if errors exist because of purge flow they are made apparent by momentary interruption of the purge flow. Care should be exercised in applying purge rates to orifice flanges, as the orifice tap is bottom-drilled with either a %-in.: %-in., or %-in. drill. The %-in. orifice drilling may prove restrictive for the higher purge rate.

The purge rotameter is the most convenient device to determine and establish purge flow. One company offers a standard purge rotameter with ranges of 0.38 to 3.8 gph of water and 0.2 to 2.0 std CU ft per hour of air at 1 O psig. These ranges are satisfactory for purging against clean liquids; however, where the material measured tends to clog or deposit sediment. the ranges should be extended. Another company offers a rotameter range of 0.1 to 8.0 gph of water and 0.1 to 30.0 std CU f t per hour of air at 10 psig. These ranges cover most purge applications, except those used with catalyst measurements.

Restriction orifices properly sized and installed give reliable service. Fig. 8-7 suggests quantities to be used in calculating purge orifices for limiting Aow through orifice flange taps.

d. Surge Chambers

. Where the rate of purge flow is not greater than the displacement of fluid brought about by the operation of the instrument, surge chambers, or pots. should be installed as shown in Fig. 8-7.

8.4 WINTERIZING

a. General

The need for winterizing and the methods of protecting instruments vary with the severity of the winters in a particular locality. To a great extent, winterizing requirements are influenced by individual plant practices.

1. LINES The need for and the degree of winterizing required

for the lines will vary with the material being measured. Lines which contain dry, nonviscous, and nonfreezing fluids, with pour points below the minimum temperatures encountered, can be installed without any protection. Lines which contain liquids that can freeze, set up, or carry traces of moisture should be protected by steam tracing or some form of heating and insulation.

82



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SEALS. PURGES. AND WINTERIZING

2. INSTRUMENTS HEAVY TRACING LIGHT TRACING WEATHERPROOFING

PIPE COVERING 7/0" MAGNESIA OR

Instruments may require protection by heating and insulation, or housing and heating, or sealing. CONTACTS EQUIVALENT

ENT LINES EST05 PAPER

b. Steam Tracing and Heating

is generally used as the heat medium for winterizing,

usually is trouble-free. However, it can be a possible source of instrument error if overheating or uneven heating results from improper application. Steam tracing and heating arrangements are shown in Fig. 8-8.

Two methods of steam tracing generally are recognized. One method, called heavy tracing, refers to steam tracing in direct contact with the connecting lines or instrument. Where maximum heating effects are desired, the tracing may be cemented to the instrument or connecting lines with a thermal transfer cement. The other method, called light tracing, refers to steam trac-

ing or by insulation, to prevent the higher heat input o€ direct contact (see Fig. 8-9). When the instrument requires heating, it is generally recommended that it be provided with a housing which can be heated. Addi- tional details for steam tracing of lines and instruments are shown in Fig. 8-8 through 8-12.

c. Electrical Heating

METER LINES

TRACER As steam is usually readily available in a refinery, it

Properly applied steam heating always is effective and STEAM TRACING AND INSULATION METHODS FOR INSTRUMENT LINES

ing placed away from the equipment it protects, by spac- R WITH DISCONNECT

BLOCK ASBESTOS' The advantage of electrical heating is that heat input can be tailored to the application and can be controlled

TYPICAL FOR GAGES STEAM TRACING ANO INSULATION METHODS

FOR PRESSURE INSTRUMENTS

Note: Insulation must not be applied in a manner which HEA<;.NG STEAM Q.EAûER

KEEP TRACING F2EE OF POCXCTS 1; .-CSâ.SLE. 'F ?OCKETS EXIST L IMIT TOTAL SUM O F HE!GHTOF"OCKETS TO A'",3.'*'=tOoh O F will obstruct blowout protection features.

FIG. 8-9-Steam Tracing and Insulation Methods for HEATING STEAM HEADER PRESSURE, PSIG

Iiistrument Lines and Pressure Instruments.

thermostatically at a reasonable cost. When selecting heating elements, care should be exercised to assure that they are not potential sources of ignition. MI cable with low surface temperature is frequently used €or this purpose. Reference should be made to N F P A Bulletin No, 70: National Electrical Code, Art. 500, Sect. 5013-b-2. The increasing use of electronic control instruments should expand the use of electrical heating by providing a convenient source of electricity for heating purposes; see Fig. 8-13(A).

d. Hot-Water Heating

satisfactory in mild climates.

e. Process Flow Heating

Where there is a continuous flow through a line of a material with a suitable temperature, the heat available therefrom may be utilized for instrument protcction. Fig. 8-13(B) is representative of such an installation.

INSULATED VOUSING

HEATING COIL MOUNTED ON BOTTOM OR YSIDE OF IOUSING. ANCHOR AND SHIELD TO

PREVENTACCIDENTCONTACTANDBJRNS.

COPPES TUBING. USE 3/8"OR LARGER COIL MAY BE MADE IN FIELD FROM

TUBING. RADIATING SURFACE REQUIRED VARIES WITH CLIMATE, SIZE OF HOUSING, HOUSING INSULATION,AND STEAM PRESSURE

STEAM TRAP OR HAND VALVE OR USE LIQUID EXP4NSION THERMOSTATIC VALVE Hot-water or steam-condensate heating may prove FOR CONTROL OF HOUSING TEMPERATURE

TYPICAL METHOD OF STEAM TRACING AND HOUSING FOR FINNED HEATING COIL FIELD-MOUNTED PRESSURE RECOMMENDED FOR HOUSING INSTRUMENT

FIG. 8-8-Steam Trociiig aiid IIeliting.

83 COPYRIGHT 2003; American Petroleum Institute


--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

APC RP 550-PART I

TRACER I/4“0R 3/8”TUBING PROVIDE TRACER SHUTOFF VALVE AT SOURCE I

I I

I TOTRAP I TO TRAP

,,,WEATHERPROOFING PIPE COVERING

REFLEX TRANSPARENT GAGE GAGE

- I /A” OR 3/s” COPPERTUBE TRACER

TYPICAL GAGE GLASS TRACING

I TRACER ‘/a’‘ OR 3/<TUBING PROVIDE SHUTOFF VALVE AT SOURCE ;

METHOD OF RUNNING TRACER FOR HEAVY TRACING OF TRANS- PARENT GAGE

WRAP TRACER AROUND LEVEL FOR HEAVY

LIGHT TRACING

FOR INSTRUMENT

:TO TRAP

GAGE GLASS AND LEVEL TYPICAL TRACING TRACING FORCED-

BALANCE LEVEL

N o r e : Refer to Fig. 8-9 for tracing and insulation methods.

FIG. Il-IO-Steam Tracing and Insulation for Level Instruments.

TRANSMITTER GAS SERVICE

TRACER Y/OR 3/a”TUBlNG PROVIDE SHUTOFF VALVE To AT SOURCE TRACING ON

TRANSMITT

PR IN RE MI

SEALED TRANSMITTER TRANSMITTER OIL LINE TO SEALTRACING

Notes: 1. All tracers have shutoff valve at source and steam trap

2 . Refer to Fig. 8-9 for tracing and insulation methods.

FIG. 8-11-Steam Tracing and Insulotion for Force-

OR WATER SERVICE

or valve at termination for condensate disposal.

Balance Flow Instruments.

SEALED METER STEAM METER OR OR

TRANSMITTER TRANSMITTER

TRACE AND INSULATE LINES -.,,f TOGETHER TO PREVENT GRAVITY ERRORS FROM UNEVEN HEATING. WEFER TO FIG. 8 - 9 FOP IbISULATiNG METHODS

‘\ I

. 1

OIL OR WATER METER , 4;’ OR TRANSMITTER , -,=y,- ,_I

C? c

---.‘y/ Note: All tracers have shutoff valve at source and steam

trap or valve at termination for condensate disposal.

FIG. 8-12-Steam Tracing and Insulation for Flowmeters.

A R Arrangement A shows an electrical heating installation. Arrangement B shows a process flow heating installation.

This type of protection can be used where there is continuous flow and line temperature ranges between 60 F and 150 F. Line remains bare inside the housing. Sides of housing are made of block insulation or metal coated with insulating paint.

FIG. 8-13-Electricai Heating alid Heating by Process Flow.

84



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SEALS, PURGES, AND WINTERIZING ~~ ~ -

f. Housings

Instruments of weatherproof design can, in many instances, work through ambient temperature changes and give satisfactory results without additional housing. Even so, users still have to design or buy a variety of instrument housings to protect instruments. Features which should be considered in determining the type and design of housings are:

1. WORKING SPACE AND ACCESS The free space around the instrument. inside its hous-

ing, should be adequate for routine maintenance procedure and for the removal of the instrument. Properly sized and positioned access doors are necessary. Ob- servation windows may be a desirable optional feature.

2 . LINEENTRY Entry is preferably through the sides or the bottom

of the box. Entries should be located to minimize piping and fittings.

3. INSULATION The inside of a housing may be lined with foil-faced

fiberglas or suitable insulating material securely attached to the walls. Insulating paint may also be used inside or outside the box with satisfactory results.

4. MOUNTING

attached to the instrument support. Housings may be self-supporting, wall-anchored. or

5 . VISIBILITY OF INSTRUMENT

Instruments may be flush-mounted on housing walis or behind windows in the door or side of the housing.

6. WEATHERPROOFING Housings should be rainproof and dustproof with line

entries sealed. Metal housings should be galvanized or painted. or both. The hardware and assembly bolts and screws should be corrosionproof.

Additional details for instrument housings and mountings are given in Fig. 8-14.

LEGS ANCHORED

B ' BOTTOM OF HOUSING CLAMPS

BETWEEN FLANGES OF INSTRUMENT SUPPORT

>

ALTERNATE MOUNTINGS FOR ARRANGEMENT A

Arrangement A shows a typical instrument housing and mountings. Line entry point is optional; door in both front and back: window in front door. Minimum thickness for metal housing is 22 gage except for the bottom plate which is Y8 in.: sheet metal should be galvanized.

Arrangement B shows self-supporting housing; instruments are flush-mounted.

Arrangement C shows wall- or line-supported housing. Back of box is %-in. metal. It is bolted to the wall or line bracket and supports the instrument.

Note: Housing insulation is %-in. foil-faced fiberglas, celotex, or insulating paint.

FIG. 8-?&Instrument Housings and Mountings.

g. Air Supply System

This system should not require winterizing if air dryers are supplied in accordance with Sect. 9 and the dryers are of adequate capacity to maintain a safe dew point.

Extensive winterizing of the air supply system and of instruments which use air may be required if dryers are not used.

85



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SECTION 9-AIR SUPPLY SYSTEMS

9.1 CONTENT

installation of instrument air supply systems. This section presents common practices for the

9.2 GENERAL

For proper instrument operation, instrument air should be oil- and dust-free and sufficiently dry to prevent condensation of water.

a. Compressors

Compressors in instrument air systems normally are two-stage compressors with an air or water cooler between stages. Compressors which use no oil in the parts exposed to the compressed air are advisable. For required capacity see Par. 9.3. The driving power for the compressor normally will be either electricity or steam; see ( c ) following.

IL Treatment Facilities

To clean, dry, and prevent freezing of instrument air, it should, after compression, pass through an aftercooler and a water separator to remove the major portion of the water. If oil-free compressors are not used, the air should then pass through an adsorber (oil pre- filter) which will selectively remove any oil vapors. The air should then be dried to a dew point of at least 10 F below the lowest known local ambient temperatures. The Heating, Ventilating, Air-Conditioning Guide la

may be used as a guide for ambient temperatures. Where there is a possibility of adsorbent fines entering air lines, a dust collector or filter should be used following the dryer, in addition to the individual filters shown in Fig. 9-1 and 9-2.

c. Staiidhy Provisions

For reliability, a standby compressor powered from a different source should be provided to supply a u in the event the primary source fails. If the normal air supply is derived from an electrically driven unit, a steam driver

TYPE 0 (BRASS)

HIGH- PRF 5SllRF HFADFR TYPE A LOW-PRESSURE HEADER WITH MASTER FILTER AND REDUCING VALVES

_ _ _ _ - - _ _ WITH ihi6iViDUAL FILTER REGULATORS. I F L E S S I H A N TENCONSUMERS ON PANEL

FIG. 9-2-Piping for Controllers on Back of Instrument Panel.

should be used for the standby unit. Automatic startup instrumentation should be provided for the standby compressor. The capacity of the standby unit should be sufficient for the entire instrument load. Both centrifugal and reciprocating compressors are used for this service, depending upon size, economics, and preference of the user.

d. Arrangements

A typical arrangement of a primary source of compressed air used with a plant air compressor is shown in Fig. 9-3. If a separate source of instrument air is desired, the system might be arranged as shown in Fig. 9-4. The connections as shown by broken lines could be used if, upon failure of the instrument air supply, automatic makeup from plant air is desired. If an instrument air-cleaning and air-drying system is installed at a location remote from the plant air compressors, a positive shutoff check valve should be installed in the plant air line to prevent a backflow when the standby compressor is operating; see Fig. 9-5.

e. Safety Valve

Figure refers to REFERENCE on p. 90. Instrument air systems normally are designed for pressures up to 125 psig and should be protected by safety valves set for the design pressure.

f. High-pressure Air

If small quantities of high-pressure dry air are required, booster compressors (which may require separate drying and cleaning facilities) should be supplied. Booster compressors should take suction from the air receiver. as shown in Fig. 9-3 and 9-4. The high- pressure system design should follow the same principles as the design for the regular instrument air system.

TO CONTROL VALVE OR RECEIVER

v4"COPPER TUBING

L-+To SECOND INSTRUMENT IF NOT FURNISHED IN INSTRUMENT

FIG. <)-l-Piping for Inalrument in Field.

86



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AIR SUPPLY SYSTEMS

Note : Symbols are in accordance with ISA RP 5.1.

FIG. 9-3-Instrument Air Supply System with Refinery Compressors Used as Primary Source.

g. Precautions Even under emergency conditions, such as plant

startup or standby service, a compressor which will contaminate the air lines with oil should not be used. Once the air system becomes contaminated, it will continue to contaminate clean air.

It is essential that a good filter be supplied to remove adsorbent fines. Otherwise, instrument troubles will develop.

9.3 CAPACITY The capacity of ail components of an instrument air

system should be based on the total requirements of all connected loads, assuming all instruments operate simultaneously. Where accurate figures are not available, a figure of 1 cfm should be used for each instrument component. Allowances for air motors: piston positioners, purge and blowback requirements vary from 3 cfm to 10 cfm. At least 10-percent extra capacity should be included to allow for the capacity loss of dryers during operation, leaks in the distribution system, and future expansion.

9.4 DRYING AND CLEANING a. Compressor Aftercooler

The compressor should be supplied with an aftercooler to cool the air to within 10 F of the incoming cooling fluid.

87

b. Water Separator

to discharge the disengaged liquids. The water separator should have an automatic trap

c. Oil Vapor Aílsorher The oil vapor adsorber should have a capacity of at

least 2 lb of oil vapor for each 100 std CU ft per min of design capacity. Block and bypass valves should be installed to permit replacing the adsorbent.

d. Air Dryer The dryer should be the adsorptive type and should

use silica gel, activated alumina, or the equivalent to remove water vapor. A refrigerative type may be used if the required dew point conditions can be met.

e. Permissible Pressure Drop The pressure drop throughout the entire drying and

cleaning system, which consists of an aftercooler, water separator, oil vapor adsorber, and air dryer, should not exceed 10 psi.

9.5 DISTRIBUTION SYSTEMS

a. Types of Systems The two basic types of instrument air distribution

systems are the loop and the radial. In the loop



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

I FROM PLANT I AIR SYSTEM

CHECK VALVE .LA

i AIR OR WATER

-., COOLED

f"R * CHECK VALVE

INSTRUMENT AIR

COMPRESSORS

AIR OR WATER

= $ , COOLED 2"

FILTER

A UTOM AT1 C START U P

INSTRUMENT AIR - -

COMPRESSORS GLOBE (EMERGENCY) 1" BLEED VALVE

iVofc: Sviiibols are in accordance with ISA R P 5.1.

i: DEW POINT RECORDER

i (OPTIONAL)

DRY ERS w qJJ REMOVER

L(IGH-PRESSURE AIR

DRYING SYSTEM

I L- - - - - - - - - - - - - -~COMPRESSORS AND

FIG. gil-Instrument Air Supply System Used with Separate Compressors.

FROM PLANT AIR SYSTEM

DEW POINT

DUST COLLECTOR OR FILTER WHEN NEEDED

(SEE PAR. 9.21~)

AUTOMATIC

COMPRESSORS GLOBE 1"BLEED VALVE

Note: Symbols are in accordance with ISA RP 5.1.

FIG. 9-LInstrument Air Supply System for Remote Location.

88



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

type, each process unit receives air along the distribution main from either direction and the main describes a loop through all process units. In the radial type, each process unit receives air along the distribution main from only one direction and the main does not describe a closed loop.

b. Line Sizing

Lines in the distribution system should be sized so that the maximum pressure drop does not exceed 5 psi between the dryer outlet and the most remote consumer when all consumers are taking air at maximum rates. A minimum pipe size of 95 in. should be used for takeoff s to individual consumers except where many instruments are in close proximity and connected to one header, such as on a control panel. In this case a smaller pipe size may be used.

e. Instrument Supply Piping Air supply piping details for instruments should be

similar to Fig. 9-1 and 9-2. It should be noted that for installations in which a large number of instruments are installed, it will probably be more economical to install a single- or a dual-master filter with the reducing valve. If air consumption is less than 10 cfm to a panel, individual filters are often used (see Fig. 9-2 Type B ) . An air header is required for Types A and B.

9.6 STANDBY SYSTEM CONTROLS

If a standby compressor is supplied. it should be equipped to start automatically if the outlet pressure of the dryer falls below the desired value.

Additional safeguards against loss of instrument air, such as automatic cutback of noninstrument air users or automatic cut-in of plant air. should be considered. Typical systems are shown in Fig. 9-3 through 9-5.

a. Symbols in Fig. 9-3

PIC 1 : This is a dual-pilot instrument. The first pilot starts the standby compressor when air pressure falls below a safe minimum. A lockup device maintains it in a running condition until this device is manually reset. This system is used to prevent the instrument from attempting to control the system pressure by throttling the steam supply. Pressure control is incor- porated in the compressor through unloading valves. The second pilot is set to control at a pressure lower than that of the first pilot. It throttles the cutback valve to maintain this air pressure.

The control valve in the steam line normally will open as air pressure decreases. It will have either a 15-psig signal (compressor shut down) or a 3-psig signal (compressor running) transmitted to it. Alarm PA 1 is set to operate when this control pressure falls below 12 psig, i.e., when the compressor starts.

PA 1 :

PA 2: The control valve in the plant a u system normally will close as air pressure decreases. Alarm PA 2 is set to operate when the pressure to the valve falls below 14 psig, indicating that the plant air system is being cut back.

b. Symbols in Fig. 9 4

PIC 1: This is a dual-pilot instrument. The first pilot controls the standby compressor as described in (a) . The second pilot has a set point below the first and is adjusted to open the control valve and admit plant air if the instrument air pressure falls below the set point.

PA 1: PA 2:

Same as PA 1 described in (a). The control valve in the plant air line nor-

mally will open with decreasing air pressure. Alarm PA 2 is set to operate when the control pressure to this valve falls below 14 psig, indicating that plant air is being used as an emergency makeup.

c. Symbols in Fig. 9-5

PIC 1: This is the automatic startup pressure controller which opens the steam valve to start the standby compressor when the instrument air pressure falls below a safe minimum.

PA 1 : Same as PA 1 described in ( a ) .

d. Alarms

Pressure alarms PA 1 and PA 2 should consist of pressure-sensing devices, alarm lights, and either separate howlers or a common howler. The alarm lights and howler should be located in the control house.

e. Control Valves

The automatic control valve in the steam line to the driver of the standby compressor should be line size, arranged to open when an air failure occurs. In addition, a 1-in. bypass globe valve should be used to bleed sufficient steam to keep the driver warm. The automatic control valve in the plant air line, as shown in Fig. 9-3, should be line size, arranged to close when an air failure occurs. The automatic control valve in the plant air line, as shown in Fig. 9-4, should be line size, arranged to open on instrument air failure,

9.7 AIRDRYERS

The air dryers. as shown in Fig. 9-3 through 9-5, are set up for manual control of regeneration. However, most systems are designed for automatic regeneration on a strict time basis. The regeneration cycle may be based on the dew point of the dried air (Le., regeneration starts when dew point rises to some set point), if

89



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

desired. If automatic regeneration is provided, switching valves should be used which will not interrupt the flow of air even if stopped in some intermediate position. Regeneration normally takes place either at line pressure

loss of instrument air pressure for even a short period of time.

REFERENCE Heating, Ventilating, Air-conditioning Guide, American

soc. of Heating, Refrigerating and Air-conditioning Engineers, Inc., New York (1960).

or a t atmospheric pressure.- If atmospheric pressure regeneration is used, provision should be made to have both dryers pressurized during switchover to prevent

90



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COPY PROVIDED FIR HISTMICAL PURPOSES ONLY

SECTION 1O-HYDRAULIC SYSTEMS

10.1 CONTENT

This section discusses the installation of central hydraulic pressure systems which utilize hydraulic cylinders to move slide valves, dampers, and similar types of equipment.

10.2 BASIS OF DESIGN

Most hydraulic supply systems are designed in such a way that the liquid is pumped from a vented storage drum to a pressure drum and held under pressure by a blanket of inert gas. From the pressure drum the liquid flows to the actuated devices and returns to the vented storage drum; see Fig. 10-1.

10.3 PUMPS

Two pumps should be supplied, one with an electric motor drive and one usually with a steam drive. Each pump should be sized to supply the normal requirements of the actuated devices and should be capable of supplying 200 percent of the anticipated leakage of all pilots plus pistons, or 4 p m per pilot-whichever is greater. The inert gas pressure will supply the power to circulate the abnormally high quantities required under emergency conditions. Pumps should be designed for the normal operating pressure required [see Par. 10.5 (b ) ] and suitable for the liquid used. over the ambient temperature range anticipated.

=9011’ C Y C NDEæ5

TO CYL.NDiRS

C0NNECT:ON ‘OR PERiO2.C INERT GAS INJECTION -.

STOR-AGE ORQM

N O R M A L RANGE GF LIQUID LEUEL AT

FILL CONNEC-

TION

3 R A l N

hYDRAULiC - PRESSURE -

P U M P S Mi ._

S T E A M SUPPLY A= MINIMUM DISTANCE DERMITTED EY PRESSURE VESSEL OESIGN CONSIDERATIONS

FIG. 10-l-Hydraulic Supply System.

10.4 DRUMS

a. Pressure Drum

A pressure storage drum should be supplied in accordance with Unfìred Pressure Vessels, Sect. VIII, of the ASME Boiler and Pressure Vessel Code as well as other regulations which may be required by the govem- ing authority at the place of installation. The vessel usually is designed for a maximum allowable working pressure of 110 percent of the normal operating pressure [see Par. 10.5(b)], or normal operating pressure plus 25 psig, whichever is greater. In installations where normal operating pressure is below 250 psig, a maximum allowable working pressure of normal operating pressure plus 15 psig sometimes is used to obtain a substantial decrease in cost.

1. PRESSURE STORAGE DRUM The pressure storage drum should have a total vol-

ume, above the takeoff line to the cylinders, equal to the output of one pump for 5 min plus 10 times the total volume of all hydraulic cylinders.

2. LOW-LEVEL ALARM The low-level alarm should be at the level where the

volume of liquid above the takeoff line to the cylinders is equal to the output of one pump for 5 min plus twice the total volume of all hydraulic cylinders.

3. HIGH-LEVEL ALARM The high-level alarm should have a minimum setting

at the level where the volume of liquid above the low- level alarm is equal to twice the volume of all the hydraulic cylinders.

4. REQUIREMENTS The conditions in the three preceding paragraphs

combined with that of Par. 10.5(b) should ensure that sufficient liquid and energy will be stored to meet emergency conditions which will occur when both pressurizing pumps are out of operation. These quantities normally should be sufficient to move the actuated devices two full strokes (from one end to the other and back) at the emergency speeds necessary to meet process requirements.

b. Storage Drum

A vented storage drum should be supplied equal in size to the pressure drum and designed for a minimum of 15 psig.

10.5 PRESSURE Pressures referred to in this section are those at the

top of the pressure drum. All regulators, safety valves, and alarms located at any other elevation should have

91



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their set points adjusted to compensate for the effects of elevation. Typical pressure relationships for a hydraulic system are shown in Table 10-1.

a. Minimum Pressure The minimum pressure in the pressure drums should

not be less than that required to operate any actuated device against its maximum load, plus the static head of liquid between the highest actuated device and the drum, plus the pressure loss caused by friction in the pipelines when all actuated devices are operating at their emergency speeds. Experience has shown that this pressure usually will not exceed 165 psig.

1). Normal Operating Pressure The normal operating pressure should be at least one

and a half times the minimum operating pressure. This normal operating pressure should allow sufficient energy from gas expansion during emergencies to operate the hydraulic cylinders [see Par. 10.4( a-4)]. This pressure usually will not exceed 250 psig.

303-

290-

265-

240-

c. Inert Gas Pressure is maintained by nitrogen trapped in the

pressure drum and compressed by the hydraulic liquid

Safety valve set point [Par. 10.6(b)]

Pump relief valve discharging at rated capacity [Par. 10.6(a)]

M<ixit?iritn tillorvohie ii,orkiiig pressirre

Pump relief valve set point [Par. 10.6(a)] High-pressure alarm set point [Par. 10.8( b ) ]

Nortticil operiititzg pressure [Par. 10.5( b ) ] Pressure drum pressure regulator set point

[Par. 10.4ía)l

[Par. iO.iO(a)]

TAULE IO-l-Typical Pressure Relatioiisliips for u Ilydraulic Sycteni

Pressure (Pounds per Square Inch

3 17-1 Maximum pressure accumulation resulting from Gage)

160-

145-

I fire

Low-pressure alarm set point [Par. 10.8(aì]

M i t i i t ~ w t n pressirre [Par. 10.5(aì]

when the level falls to the minimum liquid volume point [see Par. 10.4(a-2)1.

15-

being pumped into the drum by the pressurizing pumps. Other inert gas may be used provided it does not adversely affect the hydraulic liquid. Air is not used because of its corrosive effect on the hydraulic system components and because of the hazardous conditions which would exist if air came into contact with combustible hydraulic fluids.

The quantity of inert gas is usually such that at normal operating pressures the liquid level should be below the level of the high-level alarm, and above a point half-way between the levels of the high- and low-level alarms.

All additions or withdrawals of inert gas necessary to maintain this condition must take place when the pressure in the pressure drum is the normal operating pressure.

Minimum design pressure for storage drum [Par. 10.4 b ) I

10.6 SAFETY AND RELIEF VALVES

The pressure drum should be protected from overpressure in accordance with Unfired Pressure Vessels, Subsection A, ?General Requirements.?

a. Pump Relief Valves

A relief valve should be supplied for each pump with a capacity equal to that of the pump. These relief valves should discharge into the pump suction piping. They should be set to open when the pressure in the top of the drum reaches the maximum allowable working pressure.

h. Safety Valve

A safety valve should be supplied for the pressure drum to provide overpressure protection in case of fire, This valve should be installed on the top of the pressure drum in accordance with API R P 520: Design and Instcrllation of Pressure-Relieving Systems (see Part I) and all applicable codes.

10.7 LEVEL ALARMS

a. Storage Drum Low-Level Alarm

A low-level alarm (LA-1, Fig. 10-1) should be installed on the vented storage drum, located and set to signal when the level falls to one-third of the drum capacity.

c. Pressure Drum High-Level Alarm

A high-level alarm (LA-3. Fig. 10-1) should be in- 0-1 stalled on the pressure drum, located and set to signal

92



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HYDRAULIC SYSTEMS

10.12 PIPING when the level rises to the maximum liquid volume point [see Par. 10.4(a-3)].

10.8 PRESSURE ALARMS

a. Pressure Drum Low-Pressure Alarm

A low-pressure alarm should be installed on the pressure drum and set to signal when the pressure falls to within 10 percent of the minimum pressure [see Par. 10.5(a)].

1). Pressure Drum High-pressure Alarm

A high-pressure alarm should be supplied on the pressure drum and set to signal when the pressure rises to the maximum allowable working pressure [see Par. 10.4(a)].

10.9 PRESSURE GAGES AND GAGE GLASSES

Pressure gages and gage glasses should be installed as shown in Fig. 10-1. For a detailed discussion on level and pressure instruments see Sect. 2 and 4? respec- îively.

10.10 PRESSURE REGULATORS

a. Pressure Drum Pressure Regulator

A self-acting pressure regulator (PCV-2, Fig. 10-1) should be supplied for bypassing the liquid from the pressure drum to the vented storage drum. This regulator should be set to maintain the normal operating pressure at the top of the drum when the constant-speed electric pump is operating. The capacity of this regulator should be equal to that of the electric pump.

il. Steam Pump Pressure Regulator

A self-acting pressure regulator (PCV-1, Fig. 10-1 ) usually is installed for throttling the steam to the steam pump. This regulator should be set to maintain a pressure 10 percent below the normal operating pressure at the top of the drum. The pressure tap should be taken directly from the drum vapor space.

10.11 STRAINERS

a. Locations

Dual strainers should be supplied in the bypass line from the pressure drum to the storage drum and in the line from the pressure drum to the cylinders, as shown in Fig. 10-1.

11. Type

Strainers should be 100 mesh or finer and have a maximum pressure drop of 1 psi at twice the capacity of one pump [see Par. 10.31.

93

a. Sizing

Piping sizes should be determined by the requirements of the cylinders under emergency conditions.

b. Header Connections

Connections between hydraulic devices and headers should have car-sealed open block valves at the headers.

10.13 FLUID

a. Types

There are many types of hydraulic fluids used. These range from hydrocarbons, such as special lubricating oils, to synthetic compounds and mixtures of water and ethylene glycol.

b. Temperate Climates

For locations where ambient temperatures below 32 F are not anticipated, the hydraulic liquid may be water with a water soluble inhibitor added to prevent rust and oxide formations.

c. Cold Climates

For locations where ambient temperatures may fall to 32 F and below. the hydraulic liquid may be a mixture of inhibited ethylene glycol and water. The percent by weight of the inhibitors in ethylene glycol and the quality of the chemicals on which the percentages are based (in parenthesis) are shown in the following tabulation:

Quality of Percent Inhibitor

Inhibitor by Weight (Percent) Diethylethanolamine . . . . . . . . . . . . . . 3.0 (100) Phosphoric acid . . . . . . . . . . . . . . . . . . 1.2 ( 8 5 ) Sodium mercaptobenzothiazole . . . . . 0.4 ( 50) Ethylene glycol . . . . . . . . . . . . . . . . . . 95.4 (100)

1. ETHYLENE GLYCOL

The inhibited ethylene glycol should be mixed with water, in the proportions given in Fig. 10-2, to ensure protection from the minimum expected ambient temperature. The percentages of inhibitors in the ethylene glycol should remain constant irrespective of the amount of water in the total mixture.

Note: Under no circumstances should the percent by volume of inhibited ethylene glycol in the total mixture exceed 60 percent because the freezing point of the mixture will increuse rapidly if the concentration of ethylene glycol becomes greater than this amount. I t is recommended that the concentration of inhibited ethylene glycol in the total mixture should not exceed 56 percent by volume.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

MINIMUM EXPECTED AMBIENTTEMPERATURE. F FIG. 10-2-Required Percent by Volume of Inhibited

Ethylene Glycol iii Hvdraulic Fluid for Ambient Tempera- tures 32 F and Lower.

2. PROPERTIES

The properties of the mixture, 56 percent by volume and with a pH of 8.8 to 9.0, will be approximately:

Temperature Viscosity (Degrees Fahrenheit) (Centipoises)

1 O0 2 O 22

-20 55 -40 175

The specific gravity will be approximately 1.073 at 68 F.

3 . LOW-TEMPERATURE SERVICE

In extreme low-temperature service ( - 10 F to -40 F) some separation of inhibitor may occur. If separation takes place, an aromatic of low boiling range should be added to ensure miscibiliîy.

94



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SECTION 11-ELECTRICAL POWER SUPPLY

11.1 CONTENT

This section is presented as a guide to those responsible for furnishing electrical power supplies to refinery instrument and control installations. Safe, reliable,

2. Methods and sequence of events in handling process feed and products. 3. Effect upon equipment and piping. 4. Effect upon other refining mits and Plant facilities.

well-designed instrumentation is dependent ’ upon a power supply having these same characteristics. Recom- mendations include what to install as well as how to install paver supplies. Wiring methods (covered in Sect. 7 and 12) will not be discussed in this section. For additional information and for guidance in the proper handling of refinery electrical installations in general, reference should be made to API R P 540: Recom- mended Practice for Electrical Installations in Petro- leum Refineries.

It is good practice to determine the sequence of events as well as the risk to personnel and plant, assuming both power dips and extended power failures to instrument systems. Thought should be given to the effects of shutting down equipment on other related refinery units or facilities. The possibility of fire or other hazards deserves full consideration. Difficulty of starting or stopping equipment, the maintenance involved, and the disposal of feedstocks or off-specification products are among the many factors to be considered.

11.2 GENERAL b. Plant Power Considerations

needed is dependent upon many variables, such as: 1. Type and size of processing unit. 2. Sources and reliability of plant power supply. 3. Type and size of instrument system.

The particular type of instrument power supply The plant power supply system should be studied with particular emphasis on the probability and frequency of power dips and interruptions, and the effects of such occurrences on plant operations. It is customary to provide energy from two or more separate sources where service interruption could cause serious problems.

The demands on modern instrument systems often necessitate the installation of an emergency power supply to ensure safe, continued operation. When a plant power failure occurs, an emergency source is needed to permit normal shutdown of process equipment. Considering all safety factors, the total cost of an emergency power supply should be compared to the accumulated savings afforded by maintaining full or partial production during normal power failure. Infrequent power failures of short duration which are not likely to cause damage, complete shutdowns, or long delays to full onstream operations may not justify the expense of emergency service. For example, catalytic reformer units provided with all-electric drives and electronic instrumentation have been designed with “fail-safe” controls which have eliminated the need for emergency power. A power dip (a failure for a period of a few cycles to several seconds) poses special problems. Some alarm relays, solenoid valves, and equipment safety devices may be affected and cause process upsets of serious consequence, even though plant drives, lighting, electronic recorders, and some controllers are unaffected. These problems will be discussed in (a ) through (c) following. Designs should provide for periodic testing of emergency power systems without upsetting plant operations; see Par. 11.8 (c) .

a. Type and Size of Processing Unit

When considering the possibilities of power failures, process and utility flow diagrams should be thoroughly studied, keeping the following points in mind: 1 . Safety aspects.

95

1. PURCHASED POWER (DUAL SERVICE) A reliable power supply from a public utility usually

includes two or morc supply feeders from separate sources operating in parallel; see Fig. 11-1. Their individual reliability. routing, exposure. and type of switching (circuit protection. response of automatic relaying, bus ties, etc.) are of prime importance. If. €or example, two or more supply feeders. continuously connected to the refinery bus, are relayed so that a faulty feeder is promptly disconnected at the refinery, the voltage dip during the switching operation generally will be of very short duration. Troubles from such a dip usually can be overcome by incorporating a time delay in the instrument, safety. or interlock devices. This assumes that small voltage and frequency variations are not con-

7 SUPPLY FEEDERS T

Note: Supply feeders operate in parallel. Each supply feeder circuit breaker is relayed to trip on failure of its feeder. Failure of one supply feeder can be sustained with no power interruption to the refinery bus.

FIG. Il-l-Piir~-h:iwtI Power, ParaIlcl-Feeder Operniion.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

APT R P 550-PART I

sidered problems. A different dual-feed arrangement with load transfer from a normal supply feeder to an emergency feeder at the refinery bus presents a more serious problem. see Fig. 11-2. Frequently, the switching time from the normal to the emergency feeder is long enough to require an emergency instrument power

2 . GENERATED POWER supply.

Any degree of reliability may be built into systems where the plant generates all or part of its own electricity. The time delay when starting standby generating equipment may be an important consideration in supplying energy to the instrument circuits. Regulation of voltage and stabilization of frequency usually are more difficult with plant-generated power than with purchased power.

c. Type and Size of Instrument System

The characteristics of an instrument system also dictate the reliability needed from the power supply. If a process unit is operated with fully automatic electrical instrumentation or an elaborate alarm system with safety features, or both, the power supply usually will have to be very reliable. Conversely, a small processing unit, which uses pneumatic or semiautomatic electrical controls with fail-safe features, may operate acceptably on a single circuit. The use of electrical instrumentation with fast response and fine control calls for power supplies of uninterrupted stability. A discussion of these points follows.

Most instruments, controls, alarms, and safety circuits operate from a single-phase, 1 15-volt, alternating-current ( a-c) supply. Motor-operated valves usually operate on single- or three-phase voltages between 115 and 550 volts. Particular attention must be given to the recommendations of the instrument manufacturer. Some requirements may involve special voltage or current

NORMAL EMERGENCY SUPPLY FEEDER SUPPLY FEEDER

T F NORMALLY

CLOSED rJ-- NORMALLY

Y OPEN I I

I i

Note: On failure of normal supply feeder. relays operate to open the normal supply feeder breaker and close the emergency supply feeder breaker; thus power to the load is restored after ;I momentary interruption.

FI<:. Il-Z-Piirvhused Powvr. Siiigir-Fer-der Opsraiioii wi th Emergent-y Qtaiitlby Feeder.

100 80 60 40 20 LOAD POWER FACTOR IN PERCENT LAGGING

FIG. ll-3-Effect of Load aiid Power Factor on Output Voltage at 115-Volt Input.

limitations. Other requirements stipulate rigid specifications on frequency and harmonic distortion of wave form. Electronic instruments frequently require voltage stabilizers with suitable filtered outputs. With some equipment, all components of a control loop must receive energy “in-phase.” A common supply source will take care of this requirement; see Par. 11.3(c). It is important to note the wide variation in output from voltage regulators when applied frequency, power factor, and loading vary; see Fig. 11-3 and 11-4.

In the past, refinery units were controlled principally by pneumatic systems often actuated, in the case of temperature, by slide wire potentiometers. Power dips and short power failures had little effect on these systems. The same outages on automatic electronic controls with all the safety interlocks and emergency trip circuits on heaters, compressors, and so forth must be

Y, = cycles per second.

FíC. I I L E f f e c t of Frcrqiiriiry oii Oiityiit Voltage.

96



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ELECTRICAL POWER SUPPLY

considered more carefully. For example, if the normal movement of a control valve is through only a fraction of its range, deposits from the process fluid may build up on the stem or plug. A voltage drop may cause this valve to stick in an off-normal position. The application of rate action in controls can cause wide momentary swings in control valve position (following smail power disturbances) to the extent of actuating the interlock or trip devices.

Instrument circuits which are critically affected by power dips should have an alternate power supply arrangement to overcome the difficulty.

Note: In certain cases where air-operated control valves are affected, solenoid valves can be installed in the controlled air line. Closing on power failure, these solenoid valves trap diaphragm air and hold a valve position through the dip or short power failure. thus minimizing an upset condition. Return of the solenoid valves to normal can be immediate or can be delayed to permit the electrical equipment to warm up.

11.3 POWER SUPPLY CIRCUIT ARRANGE- MENTS

In the United States today, instrument electric power generally is supplied at 1 15 volts, single-phase, 60-cycle alternating current (ac). Wattage requirements are low enough to permit the use of small conventional lighting transformers. They may be connected across one phase of the process unit’s secondary three-phase power supply (220 to 550 volts). The transformer is located near or in the control house with the secondary power supply (low-voltage side) connected to an instrument panel board. A simple arrangement of a power supply circuit is shown in Fig. 11-5.

a. Isolation of Instrument Circuits

Many potential control and metering problems can be avoided by carefully isolating instrument supply and signal circuits. Isolation of instrument circuits is helpful in reducing the effects on instrumentation of power system load changes, switching transients or human switching errors, short circuits, unintentional grounds, stray ground currents or “ground loops,” and inductive and capacitive pickups. First consider supply circuits. Because only instrument loads are to be supplied from the 480-volt circuit breaker at the unit bus, as shown in Fig. 11-5, the effects of load changes, switching transients, shorts, or grounds originating in noninstrument circuits are minimized. Also, human mistakes should less frequently affect controls or metering. Use of an isolating type of voltage stabilizer may help to minimize stray ground currents through ground loops.

Also, thought should be given to the proper isolation of signal circuits, as outlined in Sect. 7, and to the proper grounding of instrument cases, conduit, and the like. This will help to minimize unwanted inductive or capacitive pickup, as well as to avoid metering errors

(SEE FIG.11-8AND % - T T 11-9)

I IF A 3-POLE BREAKER IS FURNISHED IN LOAD CENTER. 3 R D POLE SHOULD NOT BE USED TO FEED OTHER LOADS

ALTERNATE

\-

Clocks and clock drives. ~

Miscellaneous relays. Solenoid valves. Analyzers (such as infrared

types) with matched or built-in voltage regulators.

Instruments not voitage-sensitive.

’ - 3 ~ ~~ ~

Potentiometer-type temperature indicators, electronic recorders, and controllers.

Analyzers requiring stable voltage (paramagnetic, flaming filament, thermoconductivity, pH, electrical conductivity, etc. 1.

FIG. ll-5-Power Supply.

because of varying ground potential in separated plant areas.

b. Grounding Provisions

Every petroleum processing unit should have a good grounding system, consisting of multiple grounds (rods, water line ground connections, etc.), all interconnected with large stranded cable. It is customary to bring cable stubs up to several convenient points for connections. Neutrals and other grounds from circuit breaker panels, transformer cases, and the like should be directly connected to these ground connection points with wire adequately sized to carry fault currents. It is not good practice to loop a ground wire from, say, a circuit breaker panel to a transformer and then to the ground connection point.

One side of each instrument supply circuit should be grounded for safety reasons and to ensure reliable performance of certain devices. It is best to make this ground connection at the instrument panel. Transform- ers can then have fuliy insulated leads, which facilitate testing and which free the circuit connections for inspection. The steelwork of each instrument panel board should have its own “private” ground wire to the process unit ground system connection point; it should not be included with breaker panels, transformers, or similar equipment. Where the panel board is nonmetallic, each instrument housing mounted on it should be wired to the panel board ground (or nearest panel steelwork as a minimum).

97



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It should be noted that many voltage stabilizers have no electrical connection between primary and secondary circuits. This factor should be field checked by continuity testing in doubtful cases. Secondary circuit grounding, as shown in Fig. 11-5? nearly always will be desirable. Conversely, where the alternate stabilizer circuit is used (also shown in Fig. 11-5), some instruments may require the installation of special isolating transformers.

c. Single-Power Source Usually, in a process area it is best to supply all parts

of the instrument system from a single source to preserve voltage and power factor relationships. Sometimes this is essential. Where power-consuming devices are used at the point of measurement, the instrument power supply circuit should be extended rather than make use of a nearby lighting circuit.

(l. Alarm Circuits ancl Interlocks

Interconnected wiring frequently occurs in control room alarm circuits. A common bell or horn (with a common silencing button) may serve all of these circuits. A branch circuit from the instrument breaker panel should be reserved exclusively for alarms as shown in Fig. 1 1-5. A typical alarm arrangement is shown in Fig. 1 1-6. Circuit variations sometimes include 3-pole disconnect switches with the third pole breaking the horn circuit as shown in Fig. 11-7. In cases where “bouncing liquid levels” or other repetitive upsets cause repeated alarms, a separate horn disconnect switch may be desirable to avoid making other alarms ineffective, or the horn may be provided with a 3-sec or 4-sec time delay.

Interlock circuits should be designed to prevent feed- back or parasitic supply to, or from, interconnected power and control circuits. In addition, the design should be such that voltage “dips” and momentary outages do not cause unwanted control or alarm action. Experience must be combined with good judgment in the selection of relay and other device contact configura-

MOTOR3RIVEEi

VI

z 3 œ W

+ O k

t

r

ALARM ACTUATING CONTACT OPEN DURING NORMAL UNIT OPERATION

FIG. ll-C>-Alarm Connection.

HORN

E

PANEL BOARD

TO INDIVIDUAL ALARM UNIT

FIG. 11-í-Multiple Wiring of Alarm Units.

tions. Where atmospheric and environmental conditions permit, it will usually be advantageous for such contacts to remain in the closed position during normal operating periods, with abnormal process conditions causing the contacts to open. Fail-safe circuitry is inherent with this practice; however, there will be cases where it is more practical to use contacts which close when the abnormal condition occurs. The corrosive effects of atmospheric contamination, along with such factors as heat and vibration, must be evaluated in making this important selection. If mercury switches are used, possible effects of vibration (slopping mercury around) should be considered.

e. Disconnect Switches The arrangement for disconnect switches on panels

is discussed in Sect. 12. Where locally mounted instruments are to be sup-

plied, the electrical circuit function may be considered in determining the number of disconnecting switches to be provided. Where a local transmitter‘ receives power through a separate circuit from the main instrument panel, field disconnect switches are seldom needed and should be considered as possible sources of trouble. (Even in remote locations, it is well to consider running a pair of wires for “sound power” telephone communi- cation rather than insert an undesirable disconnect switch into a control loop.) Where electrical power is used only for chart drives, one disconnect switch may serve as many as four instruments, as discussed in Sect. 12, see Par. 12.8(d). Disconnect switch enclosures for field use should be explosionproof in Class I areas. Cun- sideration also should be given to other environmental factors, such as humidity, rainfall, dust, and salt air. The choice of 2-pole and 3-pole types is discussed in Par. 12.8(d).

11.4 POWER SUPPLY TO LOADS a. Regulated Voltage Necessary

The supply voltage to some instruments must be regulated if accurate, dependable performance is required.

98



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ELECTRICAL POWER SUPPLY -~

For example, the life of electronic tubes, capacitors, and the like may be seriously shortened by overvoltage. Electronic equipment usually is designed to tolerate the voltage and frequency fluctuations which may be expected from purchased electrical power. However, refinery instrumentation designers should not overlook the probability that plant voltage fluctuations may be more severe than a public utility power company would tolerate-the utility company must attempt to minimize lamp flicker. Plants have been troubled with undependable operation of some types of analyzers and voltage-sensitive electrical equipment, particularly those plants generating their own electricity. The system shown in Fig. 11-5 represents good practice.

h. Regulated Voltage Unnecessary In general, clocks, chart drives, relays, contactors, and

solenoid valves will be unaffected by wide voltage varía- tions. It is usually simple to avoid coil burnouts by co- ordinating specifications for transformer ratio and tap connections with device voltage ratings. I t is economical to supply such equipment independently of the main voltage stabilizer. Devices needing regulation will work better if their stabilized supply is not affected by the intermittent changes in loading which solenoid or relay action might entail. The performance of many potentiometer instruments is not seriously affected by minor voltage variations.

Some instruments, such as infrared analyzers, require close voltage regulation obtained through matched or built-in voltage stabilizers. Such instruments should not be connected to regulated branch circuits. The harmonic content in the output of the main voltage stabilizer may adversely affect their operation. Refer to Part II of this manual.

c. Characteristics of Saturable Reactor and Ca- pacitor (Resonant) Voltage Stabilizers

Stabilizers of this type closely regulate output voltage with input variations of i- 15 percent voltage. In doing so, the output wave form is distorted. Third-harmonic content may vary from 3 to 35 percent and smaller percentages of fifth or seventh harmonics may appear. Wave shape is not critical in most instruments. Some instruments which employ a-c signal circuits require expensive stabilizers equipped with harmonic filters. The cutput voltage of resonant stabilizers, as shown in Fig. 11-3 and 1 1-4, varies with load. power factor, and frequency. The following guides may be used for better results: 1. Preferably, supply stabilized voltage to those instruments (or parts of instruments) presenting a steady load. Avoid loads which are frequently switched on and off. 2. If the power factor is low, oversize the stabilizer, see Fig. 1 1-3. 3. Use a frequency-compensated regulator where the frequency is likely to vary more than 1 percent. This

should be preferred practice on refinery-generated power systems (note that a noncompensating 60-cycle stabilizer cannot work well on 50-cycle or other fre- quencies). 4. Install voltage stabilizers in weii-ventilated indoor locations; they have high operating temperatures and develop strong external magnetic fields. Stabilizers should be physically separated from instruments which have high-gain audio-frequency circuits. 5. The resonant type of voltage stabilizer is current limiting; therefore, its power supply circuit breaker will not trip if the secondary is shorted (after an initial rise to twice normal, output current will drop to between 1 and 1 i/2 times full load). Although this limits damage to shorted instruments, the resulting output voltage drop will affect other devices. This effect should be considered when providing protection on the output side.

CI. Other Voltage Stabilizer Characteristics

Other voltage regulators, free of harmonic distortion in their output, may require a few cycles to reach stability after a change of input voltage.

11.5

a. Two Nornial Circuits Frequently, two power sources are available which

have sufficient capacity, reliability, and voltage and phase-angle stability to permit connecting them both to the instrument supply circuits most of the time. They can be provided with reverse power-relaying protection arranged to trip a faulty feeder as shown in Fig. 11-8.

LOAD SUPPLY FROM TWO SOURCES

b. Spare Circuit

Where only a spare (less reliable) power source is available for emergency use and a momentary power dip is not objectionable, an automatic transfer switch may be used to transfer instruments from the normal to the alternate source of power when the normal source fails. The switch automatically transfers the load back to the normal circuit when voltage is restored. A typical transfer scheme is shown in Fig. 11-9. The switch may be arranged to transfer load at 70 percent of normal voltage and restore normal service at 90 percent of normal voltage (after a short time delay to prevent chattering).

NORMAL FEEDERS FROM SEPARATE SOURCES

1 DIRECTIONAL RELAYING (OR EQUIVALENT ) d p)

f TO INSTRUMENT PANEL BOARD (SEE FlG.11-5)

FIG. l l -8-Two Normal Circuits.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

J-, “SPARE” d,

TO INSTRUMENT PANEL BOARD (SEE FIG. 11-5)

FIG. 11-9-Normal Circuit and Spare Circuit Used for Emergencies.

11.6 SUPPLYING ELECTRICITY DURING POWER DIPS

Assuming power dips are of short duration, one frequently used method of maintaining instrument power supply is to normally furnish power from a small generator which is electric-motor-driven through a flywheel. During a power dip, the flywheel inertia is sufficient to maintain voltage levels. However, the flywheel should not be expected to maintain voltage levels for a period exceeding approximately 15 sec.

11.7 SUPPLYING EMERGENCY POWER TO INSTRUMENTS - A N D ASS OC1 ATED LOADS

When a potential power plant interruption involves enough risk to warrant providing an emergency power source, it is customary to make the source capacity big enough to supply power to some gage, control room, and area lights. Occasionally, electric motor valve operators and safety protective circuit loads are supplied from the emergency sources. Unless a spare emergency circuit of sufficient capacity and reliability can be provided, these requirements nearly always involve an emergency power generator driven by one or a combination of the following: steam or gas turbine, internal-combustion engine, or battery and motor.

a. Emergency Generator Characteristics

It is best to arrange the generator so it can pick up load rapidly. It should be of simple design with as few accessories as practicable and should require a minimum of maintenance. The package type of generator unit with a direct-connected exciter and built-in voltage regulator is often furnished. An enlarged terminal box may contain the control cabinet with voltmeter and am- meter. Unless a Class I enclosure or outdoor installation is necessary, the generator units are provided with dripproof protection. For simplicity a “static” (rectifier type) voltage regulator is often used. Operating voltages may be 120/208, 240, or 480 volts, single- or three- phase, 60-cycle ac. The higher voltages are used where vaive motor operators are included in the system. Gen- erator drivers generally are direct connected.

b. Steam Turbine Drives

Simple steam turbines usually are arranged for atmospheric exhaust, and inlet steam at less than 250 psig, and 550 F (total temperature). A constant speed governor is furnished, together with an overspeed trip (set as high as permissible). A typical starting scheme is shown in Fig. 11-10. The turbine should be designed to start without warmup. Where a slight delay in load pickup cannot be tolerated, either continuous turbine operation or the methods described in Par. 1 1.8 (a ) may be considered.

c. Gas Turbine Drives

Small, rapid-starting, gas-turbine-driven emergency sets have been developed, primarily for military use. Usually, they are designed to use diesel or jet fuel, and require a battery and a motor or other means to start them. Gas turbine drives can pick up load in 15 sec to 40 sec. Refinery experience with the gas turbine drive is very limited at the present time, probably because available ratings are 50 kw and above. More detailed information is available from several manufacturers.

d. Gasoline (and Diesel) Engines

Gasoline engines are usually 4-cycle, with built-in centrifugal flyball governors. Engines which will run on natural gas or propane fuels also are available. The latter fuels may be cleaner, and will not lose volatility in standby use as gasoline may. High-tension magneto- ignition systems are commonly furnished. Battery and starting motor may be provided. The gasoline tank should be elevated, or a half-gallon tank containing starting fuel should be provided above the engine unit. Cold climates require additional precautions in order to keep the engine warm and ensure a prompt fuel supply. In the sizes usually applicable for plant emergency use, gasoline engines are cheaper and often have better starting characteristics than diesel engines. Fuel costs and efficiencies have little bearing when considering standby service. With the engine-driven standby, there will be a significant time lapse before the driver can pick up the load.

I l COMBINATION PRESSURE 3-WAY SOLENOID VALVE

REDUCING VALVE AND FILTER,

PRESSURE GAGE

AIR SUPPLY

STEAM TO

TURBINE

CUT-OFF AND STEM SLOTTED FOR SCREW DRIVER ADJUSTMENT

FIG. 11-10-Emergency Control for Steam Standby.

1 O0



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11.8 COMBINATION DRIVES FOR POWER DIPS AND INTERRUPTIONS

a. Motor-Generator-Engine Combination

Where both voltage dips and power interruptions are experienced and a reliable instrument power supply is required, it is practical to provide a motor-flywheel generator as mentioned in Par. 11.6. It should be connected through a centrifugal or magnetic clutch to one of the engines (or directly to one of the turbines) described in Par. 11.7. This combination should be able to hold voltage within I 5 percent and frequency within 5 2 cycles. A typical installation might include the following equip ment :

A 115-volt, 60-cycle a-c generator of suit-

A synchronous electric motor. A gas, gasoline, or diesel engine with cen-

able rating with heavy flywheel.

trifugal clutch.

The electric motor, powered from the refinery supply, would continuously drive the generator and flywheel. The generator would feed 1 15-volt power continuously to the electronic instrumentation and safety interlock equipment. This might amount to approximately 20 percent of rated load. On power failure of more than % sec, the electric motor would be disconnected from its reñnery power supply and simultaneously the engine drive would start automatically. Its centrifugal pawl clutch would engage the generator drive shaft after com- ing up to speed. The inertia of the flywheel will keep driving the generator during this period. Because the engine can start and speed up unloaded, it would pick up the rated load in approximately 3 sec. Experience indicates that the generator voltage would drop slightly as the load is applied but not enough to affect the equipment.

Undervoltage time delay relays of Y2 sec should be provided in the refinery main power supply to the electric motor drive and to the emergency lights to preclude their dropping out during power dips. A longer power failure will cause loss of power to the motor and switch the emergency lighting from the refinery power supply to the generator. Provision also should be made for switching to the refinery power supply if the motor-generator-engine set becomes inoperative. This provision rarely is expected to be used; however, it could cause a dropout during the switchover.

Startup of the engine should be checked periodically to ensure reliable operation. The engine is started automatically by a phase failure relay in the starter circuit; it can also be manually started. A simple sketch of this installation is shown in Fig. 11-1 1.

TO EMERGENCY LIGHTING

UNDER VOLTAGE RELAY WITH TIME DELAY

TO INSTRUMENTATION AND ALARM SYSTEM

I 1_1 ,CLUTCH

-FI ,

NI" I "m NITH PHASE

FAICURt RELAY IN STARTER CIRCUIT

FIG. 11-11-Instrument and Emergency Power Supply.

b. Motor-Generator-Battery Combination

An alternate system, which operates continuously, consists of an a-c motor, a-c generator, and a direct- current (d-c) unit (motor or generator) all on the same shaft. Storage batteries with a control cabinet complete the needed equipment. Normally the a-c motor drives the a-c generator and d-c unit. The d-c unit, which acts as a generator, maintains the batteries in a fully charged condition. Upon power failure, the d-c unit acts as a motor to drive the a-c generator. Such a system has a potential weakness which should be guarded against. Battery capacity limits emergency operation time. An alarm should warn the operator of the condition in sufficient time to take appropriate action. The running time allowed by this system, however, is adequate for most power failure conditions. A simple sketch of this system is shown in Fig. 11-12.

NORMAL TO INSTRUMENT AND REFINERY -EMERGENCY LIGHTING

POWER SUPPLY PANEL BOARD r 4

SYNCHRONOUS A-C GENERATOR D-CUNIT WITH MOTOR BATTERIES

FIG. 11-12-Continuously Operating Emergency Power Siipplv with Motor, Generator, and D-C Unit-Battery Com- bination.

101



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API RP 550-PART 1

c. Testing Emergency Power Supplies type which may be called upon to start cold from a non- operating condition be frequently tested. This should include a complete operating cycle. The complexity and reliability demanded of the system determines the frequency of testing, operation once a week being CUS- tomary in some plants.

Emergency power supplies for instrumentation should be listed on the maintenance schedules for checking during unit turnarounds or as the need dictates. It is of prime importance that emergency power supplies of the

102



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SECTION 12-INSTRUMENT PANELS

12.1 CONTENT This section presents common practices for the con-

struction and installation o€ various types o€ instrument panels. It is current industry practice to purchase most major instrument panels as completely wired and piped assemblies from a panel fabricator. However, the information contained in this section is applicable to both the prefabricated panel and the field fabricated panel.

12.2 GENERAL The purpose of any instrument panel is to aid the

unit operating personnel in maintaining efficient and safe performance of the unit from a central location. There- fore, the instruments generally found on the panel board include remote recorders or controllers, or both, as well as those indicators which are important for the control of the unit. All controllers and significant recorders should be located in the most convenient and accessible location. An attempt usually is made to have the panel layout follow the actual physical arrangement of the unit as closely as possible. A system which will enable an operator to quickly identify any particular instrument is desirable and should be considered in the panel layout. Nameplates, color codes. or symbols frequently are used. Spare panel space. about 10 percent. usually is provided in the panel layout for future expansion.

12.3 CONVENTIONAL PANEL

A conventional panel is defined as a panel with miniature or large case instruments, or both, mounted in horizontal and vertical rows. Large case instruments are mounted either two or three instruments high. Minia- ture instruments usually are mounted a maximum of four or five instruments high. The distance between these rows of instruments depends upon the type of instrument and accessibility for maintenance and adjustment. Such items as individual alarm lights and miscellaneous indicators normally are mounted above the top row of instruments. Alarm cabinets for annun- ciator systems sometimes are considered large case instruments and are mounted accordingly. In other arrangements they are mounted above the top row of instruments. Typical layouts for the conventional panel are shown in Fig. 12-1 and 12-2.

a. Panel Clearance Clearance from face of panel to control room wall be-

hind the panel in most cases is from 5 ft to 6 ft. This distance allows the subassemblies with auxiliary equip ment to be mounted on the wall. Where special equipment, such as for data loggers, is to be mounted behind the panel, this distance may have to be increased.

13. Instrument Arrangement 1. MOUNTING HEIGHTS

Normally, a limitation is placed on the maximum and minimum heights for mounting instruments on the panel. These heights vary with different users but generally are based on visibility and accessibility.

2. DENSITY The density of instruments varies with the type of

panel. complexity o€ the process. and preference of the user. From a cross-section of users, average panel density for the various types of panels based on recording instruments per running foot is as follows:

Conventional panels . , , , . , , . . . i to 1.5 large case instruments.

( 3 to 5 miniature instruments. Graphic panels . . . . 1.5 to 3.3 miniature instruments. Semigraphic

panels . . . . . , . . . . 3 to 5 miniature instriiments. Consoles , , . . . , . . . . 3 to 5 miniature instruments.

12.4 CONSOLE-TYPE CONTROL CENTER

A console is any free-standing cabinet-type enclosure, usually incorporating a desk or sloping area on the front of the cabinet. It is usually lower and more compact than the conventional panel. It can be designed to include a flow plan, with the instruments mounted in the sloping area, or with many other arrangements.

Consoles are becoming more popular because of the ever increasing amount of data being presented to the operator. Most console panels are fabricated as cubicles with all wiring and piping completely enclosed. Two typical consoles are shown in Fig. 12-3.

I o o o

FIG. 12-I-Cunvontioiiai Panel w i t h Large Case Instrumeiits.

103



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l i

FIG. 12-2-Conventional Panel with Grouped Miniature Instruments.

12.5 GRAPHIC PANEL A graphic panel normally includes a simplified flow

plan of the unit as a visual aid to the unit operators. The layout of the flow plan should present as much infonna- tion as possible but with a minimum amount of label or nameplate reading. Locally mounted recorders and controllers generally are omitted from the graphic presentation.

a. Types of Graphic Panels

1. FULL GRAPHIC A full graphic panel is defined as a panel with minia-

ture instruments mounted in a simplified flow plan of the unit depicted on the face of the panel. End or “wing” panels are usually added for specialized instruments. Also, those instruments which do not lend them- selves to incorporation in the flow plan layout usuaily are mounted on the end panels or below the graphic representation. Instruments such as the large case electronic temperature instruments, analyzers, and occasionally those instruments provided for utility or data records are included in this category. A full graphic panel is illustrated in Fig. 12-4.

2. SEMIGRAPHIC The semigraphic panel combines the compactness of

a conventional panel (using miniature instruments) with the flow plan feature of the graphic panel. Such a panel has a simplified flow plan of the unit located above grouped instruments. A typical semigraphic panel is shown in Fig. 12-5.

b. Panel Arrangement and Layout 1. MAJOR EQUIPMENT

The flow plan layout usually includes towers, drums, furnaces, and other major equipment. However, an attempt should be made to eliminate all but the equip-

ment necessary to present a complete picture of the process and its associated control system. For example:

a. Minor knockout drums or pumpout pumps usually are not shown.

b. A turbine-driven pump with steam to the turbine regulated by a panel-mounted instrument generally is shown.

c. A heat exchanger influencing the process control system should be shown.

Actual scale size of equipment and vessels usually is ignored, but vessel shapes are represented as closely as feasible.

2. EQUIPMENT INTERNALS Internals of equipment, such as trays, baffles, or pip-

ing, usually are not shown in the flow plan layout unless their inclusion wiil improve understanding of the process. For example:

a. The trays in a debutanizer or stabilizer generally are not shown.

6. The trays and baffles in a crude oil still with many sidestream and pumparound circuits are shown.

c. The tubes in a furnace may be shown if there are important temperature-indicating points on cross- overs or if it is desired to show complex crossover lines.

d. The tubes in a simple reboiler furnace generally are not shown.

e. The shell and tube sides of heat exchangers usually are indicated for clarity.

3. PROCESS PIPING Process piping which is connected directly to the

panel instrumentation or which will improve understanding of the process normally is shown in the flow plan layout. Arrowheads generally are shown on lines to indicate flow directions. Lines are continuous wherever possible. Where two lines cross, the horizontal line normally is continuous and the vertical line is broken, except that instrument lines break if they cross process lines. Crossing of the same color lines is avoided. Lines originating or terminating off the panel usually have nameplate designations, such as “GAS OIL TO TANK- AGE.”

4. INSTRUMENTS In a console panel the instruments are usually

mounted conveniently around a miniaturized process flow plan of the unit. An effort is made to mount the instruments so that they are visible and accessible with a minimum amount of travel on the part of the operator. Many consoles are U-shaped to accomplish this purpose.

’ In a full graphic panel the instruments can be mounted to one side of the process line, in the process lhe, or inside of the vessel symbol provided this would not interfere with the graphic display of the vessel internals. When instruments are mounted in the vessel

104



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.. .

1 os COPYRIGHT 2003; American Petroleum Institute


--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

FIG. 12+Full Graphic Panel (Front).

FIG. 12-5-Layout o f Seniigraphic Panel.

106



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INSTRUMENT PANELS

symbol, the symbol usually is larger than the instrument case. Generally, an effort is made to maintain horizontal and vertical alignment of instruments on the panel.

In a semigraphic panel an attempt is always made to mount the instruments below the section of the flow plan to which they pertain. Generally, they are identified with the flow plan by use of symbols which incorporate both a color code and a shape, i.e.? red squares. green tri- angles. and the like.

5. FLOW PLAN SYMBOLS

Equipment symbols have been fabricated from many materials including sheet aluminum, brass, and plastic. Process piping and instrument lines have been fabricated of aluminum, brass. or plastic strips of rectangular or half-oval cross-section. Ordinarily, narrower width strips are used for instrument measurement, transmission, and control lines. The symbols are attached to the panel by use of adhesive material or studs. or a combination of both. If adhesives are used exclusively, the symbol material should have a coefficient of thermal expansion similar to that of the panel material. An alternate method is to have the flow plan painted on the face of the panel. This method generally is considered inferior to the other methods because it is less resistant to damage. Usual practice is to have spare symbols, lines, and paint furnished for panel repair or modifica- tion. Some typical flow plan symbols are shown in Fig. 12-6.

I U CONTROL CONTROL CENTRIFUGAL RECIPRO-

VALVE VALVE PUMP CATING (3 WAY) PUMP

V LOGGED TEMPERATURE PRESSURE ANALYZER POINT LOGGER AND POINT

I ND ICATOR

COLOR OF INDICATOR RECORDER ?DA%%ToR RECORDER

CONNECTING PIPING

HEAT EXCHANGERS TEMPERATURE INSTRUMENTS

DISPLACE-

METER ROTAMETER ORIFICE MENT VENTURI

FIG. 124-Typical Graphic Panel Symbols.

6. COLOR CODE A color code is used in flow plans to distinguish be-

tween process piping, instrument lines. wiring, etc. A typical color code is as follows:

a. Panel background-light gray or light green. b. Large equipment symbols (towers, vessels, etc.)

-dark gray or dark green. c. Small equipment symbols (pumps, exchangers.

control valves, etc.)-black. d. Instrument measurement and transmission lines-

black. e. Instrument control lines-silver. f. Water -dark blue. g . Steam-light blue. h . Process air-silver. i. Hydrocarbon vapor and gas-light green. i. Hydrocarbon liquid-dark green.

k. Tower bottoms-dark brown. 1. Reflux and top pumparound-light yellow. m. Other pumparound-gold. n. Fuel gas-light brown. o. Fuel oil-red.

c. Nameplates Nameplates normally are provided for instruments:

vessels, equipment, and process line terminations. These nameplates are mounted either within or adjacent to the item covered. The most common material for nameplates is a laminated bicolor plastic. In engraving this type of plate the top layer is cut through which allows the letter to show in the second color (see Fig. 12-7). Other methods of lettering include the printed legend with a protective overlay or a back-engraved legend in clear plastic with a painted back contrasting with the lettering.

12.6 STRUCTURAL Panels are fabricated from a number of materials or

combinations of materials. The main requirements for

BEVEL EDGE TO SHOW WHITE LAMINATION

I 30 I Notes: I . Typical full-size nameplate. 2. Material for nameplate: %-in. thick bakelite. laminated

white core. contrasting surface, dull finish.

FIG. 12-7-Li11iii1iated Plasii(: Naiiiel>late.

107



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API RP 550-PART I

selection are rigidity, safety, smoothness of surface, and durability. On panels using flow plan layouts. butt joints normally are used in order to have as even and smooth a surface as possible for the flow plan. Individual panel sections usually are removable without disturbing other panel sections. Fiiler panels sometimes are inserted above the main panel. Clocks, utility gages, and alarm indicators can be mounted on the filler panels if desired.

a. Panel Materials

Panels have been fabricated from steel. formica, ma- sonite, and such combinations as formica overlay on a metal or fireproof plywood panel. In fabrication, care should be taken to have the faces of panels flat and smooth.

b. Panel Framework

Panel frameworks are of several types, such as box frame, cubicle, and self-supporting, and usually are of steel construction. The main purpose of this framework is to support the instruments, certain auxiliary equipment (such as instrument power switches), and the interconnecting piping and wiring on the panel. It is also used to support or hold the panel in position. In some instances, it is necessary to add stiffeners to the panel itself in order to keep the face of the panel flat and relatively free from vibration. Typical framework construction is illustrated in Fig. 12-8 and 12-9.

c. Subassemblies Such auxiliary items as pressure switches (for alarms)

and miscellaneous pneumatic devices normally are fabricated as subassemblies. They are mounted directly on the panel framework, or are wall-mounted behind the panel board. Because the trend is toward wall-mounting, subassemblies normally are completed with their own wireways, pneumatic bulkheads, and supporting framework. A typical subassembly is shown in Fig. 12- 10.

d. Panel Base

Panels have been placed directly on the floor of the control house but more often they are mounted on some type of base. This base is normally a flush, recessed, or extended curb. The curb construction usually is of concrete or steel, or both. A typical extended curb with a steel channel and concrete construction is shown in Fig. 12-1 1.

e. Panel Erection

Panels normally are bolted to the base and, if required, suitably braced to the wall behind the panel. The extent of bracing will depend upon the self-supporting characteristics of the panel. In many cases, a drop-

ceiling will provide any additional support necessary and will add to the appearance of the panel. On open-end panels it is customary to provide a turnback or other means to enclose the end panels.

f. Panel Tolerances

Common practice permits a tolerance in the width of any individual panel, usually & '/4 in. However, this individual panel tolerance should be compensating so that the overall tolerance for a panel group, for erection in any one plane, does not exceed a given amount. This overall tolerance normally is limited to 2% in. A tolerance in panel height of 2% in. is usually permitted. These tolerances will affect the location or tolerance in mounting holes in the panel and panel base.

g. Instrument Nameplates

Identifying instrument nameplates are desirable on both the front and back of the panel. The nameplate on the back of the panel and/or instrument should include power switch identification.

12.7 PAINTING

a. Face of Panel

Formica panels normally do not require any painting. Care should be taken with steel panels to have the face of the panel flat, smooth, and free of mill scale and foreign materials before painting. When the face of the panel is painted, the number of primer and intermediate coats will vary depending upon the panel material. How- ever, the final coat normally is a satin finish. Usual practice is to have additional paint supplied for panel repair.

b. Backof Panel

The backs of panels and interior surfaces of a console or cubicle normally are painted with a light color for visibility. The structural members of the panel framework and subassemblies generally are painted before erection and are not necessarily a light color. As a rule, the backs of panels and most structural members require a primer coat and a finish coat only.

12.8 ELECTRICAL INSTALLATIONS

Electrical installations should be in accordance with the latest edition of National Electrical Code and local codes. In classifying areas, the latest edition of API RP 500: Recommended Practice for Classification of Areas for Electrical Installations in Petroleum Refineries should be followed. The discussion herein is based on a Class I, Group D, Division 2 control house or area; see Art. 500 of the NFPA Bulletin NO. 70: National Elec- trical Code.

108



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INSTRUMENT PANELS

FIG. 12-10-Subassemb1y (Pressure Switches).

a. Electrical Supply

The most common type of electrical supply to an instrument panel is 120-volt, 2-wire, grounded, single- phase alternating current. Some refineries use an un- grounded electrical supply.

1). Wiring

Practice in connection with electrical wiring is varied. The methods used employ sheet metal wireways. rigid or flexible conduit. or combinations of these. The minimum conduit size normally used is 95 in. In all cases, the union or connector should be located at the instrument for safety and maintenance reasons. General practice is to provide spare knockouts. or conduit fittings,

SLOTTED HOLES FOR FON. BOLT5

- ! GROUT HOLES

DETAIL OF BASE CHANNEL

8" II 51

INSTRUMENT PANEL BASECHANNEL ANCHOR BOLT NUT AND WASHER MACHINE BOLT WELDED

\FILL UNDERSIDE OF BASE CHANNEL WITH GROUT

TYPE I TYPE2 * [ =CHANNEL

FIG. 12-11-Iiistrumeiit Patiel Board Bases.

to allow for future additions to the panel. For cubicle oc enclosed panels, the wiring generally is bundled together or run in wireways. The enclosure is considered sufficient protection. Power supply wiring should be run separately from the electrical transmission or thermocouple wiring.

c. Wiring Test

Wiring and electrical equipment should be thoroughly tested for proper installation and operation. Insulation resistance. measured from line-to-line and line-to- ground. should not be less than one megohm with the instrument disconnected. Also, transmission circuits should be tested as described in Sect. 7, Par. 7.6(b). Chart drives should be run for at least one hour to check their operation. Alarm systems, push buttons, pressure switches, and other simple equipment should be tested under simulated operating conditions. Sequencing systems should be given operational tests by simulating field conditions with relays and switches. This operational check is superior to a point-to-point or continuity check but places the responsibility on the purchaser to provide the fabricator with sufficient information to make such a test.

d. Disconnect Switches

Disconnect switches on power supply to instruments normally are 2-pole and are arranged to disconnect both leads. Alarm unit disconnect switches usually are 3-pole and are arranged to break the connection to the howler. It is common practice to have one disconnect switch serve as many as four instruments when only chart-drive power is involved. For electronic instruments and potentiometers where other than chart-drive power is required, one disconnect switch is used per instrument. For cabinet-type annunciators with multiple-alarm units, some users require more than one power disconnect switch for servicing requirements. Each disconnect switch should be clearly labeled to identify the particular instruments or alarm units served by that switch.

e. Terminal Blocks

Terminal blocks usually are provided on panels and subassemblies for power supply wiring, alarm system wiring, and electrical transmission lines. Normally, no terminal blocks are permitted for thermocouple extension wires, nor for some types of analyzers (pH, etc.). It is preferred that these be run directly to the receiving instrument. The terminal blocks should be clearly identified with engraved or embossed numbers. Normally, terminal blocks are of the enclosed type. They can, however, be the open type if they are contained in an enclosed panel, such as a console or cubicle. or in a wireway.

111



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12.9 PIPING Practice regarding instrument air supply to panels is

varied. Some refiners prefer a separate pressure-reducing regulator-filter for each instrument. Others use a header system with a master pressure-reducing regulator and separate filter to supply the header, with each instrument having a separate takeoff from the common header.

a. Supply Header

In cases where each instrument has its own pressure- reducing regulator-filter, the header is usuaily of gai- vanized or black iron pipe. For the reduced-pressure header system, the header is normally of brass or copper. TO eliminate scale and rust formation encountered in the use of steel pipe, the piping downstream of the filters should be of brass, copper, or aluminum. The header should be properly sized for the capacity of air required, with some allowance for future expansion. Individual takeoffs for the instruments are normally brazed or threaded to the top of the header. Generally, at least 10 percent spare valved takeoffs are provided for the future addition of instruments. For the reduced- pressure header system, adequately sized dual filters and pressure-reducing regulators are provided for reliability (see Fig. 12-12). These regulators usually incorporate downstream overpressure relief features. Take- off connections are normally a minimum of V i in. For long panels, the header usually is joined between panel sections with a flanged or union type of connection to facilitate field installation. It is also normal practice to furnish a valved drain connection on the bottom of the header farthest from the air supply source.

11. Interconnecting Piping

Interconnecting piping between instruments and from instruments to bulkhead is normally ?A -in.-OD copper tubing. The tubing usually is supported by clamping it to panel structural members and is made up with a minimum of joints and arranged for easy access.

FIG. 12-12-Dual Pressure-Reducing Regulator and Filter Set.

c. Bulkhead Connections

Control and transmission lines and interconnecting lines between panels and subassemblies normally are brought to bulkhead fittings, usually located at the top of the panel; see Fig. 12-9. The simplest form of bulkhead consists of a steel plate, mounted vertically, with suitable fittings on either side to join the tubing from the field to the tubing from the panel instruments. In a normal prefabricated panel, the connections from, or to, the field instrument are the only ones required to be made by the purchaser during installation of the panel. Each bulkhead connection should be clearly labeled with the designation of the particular instrument or connection it serves. It is common practice to include a takeoff connection in the piping from the bulkhead connection to the panel instruments for testing or for future connection to a logging system or to other instruments. It is also usual to include spare bulkhead connections for the future addition of instruments.

d. Testing

Testing of the air supply header and signal. tubing normally is accomplished with air. Each joint should be tested with a soap-and-water solution and should be absolutely tight. Instruments should be tested in the manner prescribed by their manufacturer.

112



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March 1965

MANUAL I

ON



inis publication is distributed "as is" and is no longer a current publication of the American Petroleum Institute. It is furnished solely for historic purposes and some or ail of the information may be outdated. API MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, A N D SPECIFICALLY THERE IS NO WARRANP/ OF MERCHANTABILITY OR FITNESS FOR A PARTI CUIAR ISSU E.

AMERICAN PETROLEUM INSTITUTE Division of Refining



Price $4.00



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API RP 550 Second Edition March 1965

MANUAL ON



Ris- publication is distributed "3s is" and is-?? longer a current publication of the American Petroleum Institute. It is furnished solely for historic purposes and some or ail of the information may be outdated. API MAKES NO WARRANW OF ANY KIND, EXPRESS OR IMPLIED, AND SPECIFICAUY THERE IS NO WARRANTY OF MERCHANTABIUlY OR FITNESS FOR A PAFlTICUIAR ISSUE.



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FOREWORD

This manual is based on the accumulated knowledge and experience of en,' oineers in the petroleurn industry. Its purpose is to aid in the installation of the more generally used measuring and control instruments and related accessories in order to achieve safe. continuous. accurate. and efficient operation with minimum maintenance. Although the information contained herein has been prepared primarily for petroleum refineries. much of it is applicable without change in chemical plants. gasoline plants. and similar installations.

This second edition of the manual. which is being published in two parts. represents the latest suggested or generally used practices in the installation of all the devices covered in the first edition. plus additional information based on revisions suggested by man!' individuals and several organizations. (The first edition of the manual was issued in 1960. )

Part I assays the installation of the more commonly used measuring and control instruments. as wvsll as protective devices Lind related accessories. Part I L presents a detailed discussion of process stream analyzers. These discussions ;ire supported by detailed information and il!ustrations to facilitate application of the recommendations.

The information contained in this publicatior? does not constitute. and should not be construed to be. a code of rules or rcgulations. Furthermore. it does not grant the right. by iniplication or otherivise. f o r nianufacture. sale. or use in connection with any method. apparatus. o r product covcrcd by Icttcrs patent: nor does it ensure anyone against liability for infringcnit'nt of letters patent.

L'!ers of this manual ;ire reminded that i n the rapidly advancing field of instrumentation no publication of this type ciin be cornpicte. nor can any written document be substituted for qualiîicd engineering uiiilysis.

Certain instruments ;ire not CO\ ercd hcrcin bccausc of their very specialized nature and limited use. \Wien ont' of these devices ( o r classes of devices) gains ccncrnl usage Lind installation rcriclie~ ;i fair degrce of standardization. this manual will be rcviscd to incorporate such additional information.

Suggested revisions are invited and 3hoiilJ be submitted to the director of the Division of Rctining. American Petroleum Institute. I27 I Avenue of the Americas. N e w York. N . Y. 10070.

i



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CONTENTS PAGE

Section 1-Flow . . . . . . . . . . . . . . . 7 Section ?-Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Section 3-Temperature . . . . . . . . . . . . . . . . . . . . . . . 44 Section 4-Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Section 5-Automatic Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Section 6-Control Valves and Positioners . . . . . . . . . . . . . . . . . . . . 62 Section 7-Transmission Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Section Y-Seals. Purges. and Winterizing. . . . . . . . . . . . . . . 79 Section 9-Air Supply Systems . . . . . . . . . . . . . . . . . 86 Section I O-Hydraulic Systems . . . . . . . . . . . . . . . . . . . . 91 Section I 1-Electrical Power Supply . . . . . . . . . . . . . . . . . . . . . . . 95 Section i ?-Instrument Panels . . . . . . . . . . . . . . . . . . . . . . . . 103 Section 13-Alarms and Protective Devices. . . . . 113 Outline of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119



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COPY PROVIDiD FOR HISTORICAL PURPOSES OHLY

INTRODUCTION

Successful instrumentation depends upon u workable arrangement which incor- porates the simplest systems Lind devices that will satisfy specitied requirements. Sufficient schedules. drawings. sketches. and other data should be provided to enable the constructor to install the equipment in the desired manner. The various industry codes and standards. and laws and rulings of regulating bodies should be followed where applicable.

For maximum plant personnel safety. i t is recommended that transmission systems be employed to eliminate the piping of hydrocarbons. acids. and other hazardous or noxious materials to instruments in control rooms.

In the installation of an instrument. the various components must be accessible for efficient maintenance and certain of these elements must be readable €or good operation. Orifices. control valves. transmitters. thermocouples. level sages. and local controllers. as well as analyzer sample points. generally should be readily accessible from grade. permanent platforms. or fixed ladders. In this manual. special consideration is given to the location. acccssibility, and readability of the elements.

Proper installation is mentia1 in order to utilize the full capabilities which are built into the instrument systcnis and to realize the grcatest return on investment. In many instances. the instrument difficulties encountered hpve been traced to incorrect installation.



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P-4RT I-PROCESS TNSTRUMENTATI( IN AND CONTROL

SECTION

1.1 CONTENT Recommended practices for the installation of dif-

ferential pressure instruments, area flowmeters. and other commonly used Howmeters for indicating. recording, transmitting, and controlling fluid flow are presented in this section. Other types of How instruments. not as \videly used and not covered herein, are:

1. Positive displacement meters. 7 . Sight flow indicators. 3. Weirs or head area meters (seldom used in re-

finery services except for waste water disposal, sewage, etc.). 1.

6. 7 . S. 9.

1 o.

3 .

Sonic or ultrasonic flowmeters. Thermal-type flowmeters. Solids flow devices. Self-actuating flow regulators: see Sect. 6. Mass flowmeters. Metering pumps. Kinetic manometers.

These devices are used only where special ¡low problems are encountered and should be installed in accordance with the manufacturer's instructions or in accordance with a specially engineered installation which meets specific requirements.

1.2 GESEILAL a. Differential Pressure Instruments

The differential head type of instrument measures flow inferentially from the differential pressure caused by flow past a primary element which generally is one of the following types: I . Orifice: Usually the thin plate concentric orifice, but may be eccentric, segmental, or of some other special form depending upon application. 2. Flow n o x i e : Used in installations where higher vclocity and moderately better pressure recovery are required than are available with an orifice plate. Flow nozzles are better suited for gas service than for liquid service. 3 . Venturi tribe: Used in installations where high capacity and Sood pressure recovery are required, or wherc the measured stream contains a constant per- csntase of solids. 4 . Flow riihe: Used in installations where low pressure loss is a major consideration, or where piping configurations are restrictive. 5. Pitot tube: Generally used in installations where no appreciable pressure drop can be tolerated on high-

1-FLOW

volume flows. such as on cooling water. These devices measure velocity. The accuracy of the rate of flow depends upon the determination of the average velocity from the velocity distribution. 6. Elbow taps: Used in installations where the velocity is sufficient and where high accuracy is not required.'" However. its repeatability is good. -4 water velocity of 17 fps will produce a water differential of approximately 100 in. Some test data are available from the University of Illinois.-

The differential pressure across the primary elements described herein usually is measured by one of the following devices: 1. Manometer. 2 . Mechanical mercury meter. 3. Bellows meter. 1. Diaphragm transmitter.

1). Area Flowmeters

most commonly used meters in the area class. For refinery service, rotameters are probably the

c . Turl)iiie or Propeller Flowmeters

caused by flow past a turbine or propeller. Turbine meters measure flow from the rotation

d. Velocity or Target Flowiiieters

Velocity or target meters measure flow inferentially from the force imposed on a target suspended in the flow path.

e. Electromagnetic Flowmeters

The average flow velocity is measured inferentially from a voltage which is generated by the measured fluid moving through a uniform masnetic field. The flowing fluid must have some degree of electrical conductivity.

1.3 DIFFERESTIAL PRIMARY ELEMENTS

u. Thin Plate Orifices 1. CONCENTRIC ORIFICE PLATES

The sharp-edge conccntric orifice plate is the most frequently used primary element because of lower cost. Hexibility, and availability of accurate coefficients.

:' Figtires refer to REFERENCES on p. 30.

7



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API RP 55O-PART I

For most services, orifice plates are made of corrosion-resistant materials, usually Type 304, 316, or 430 stainless steel. Other materials sometimes are required for special services.

The upstream face of the orifice plate should be as flat as can be obtained commercially. Any plate which does not depart from flatness along any diameter by more

than 0.010 in. per in. of dam height, C-c, may be considered flat. The upstream face of the orifice plate must be smooth and have a finish at least equivalent to that obtained in commercial cold-finished sheet stock.

The thickness of the orifice plate at the orifice edge should not exceed (minimum requirements Soverning in all cases):

2

D or preferably

d - (one-eighth of orifice diameter) S

(one-fiftieth of pipe diameter) 30 50

- (one-fourth of dam height) S In some cases, the thickness of the orifice plate will

be geater than permitted by the limitations for the thickness of the orifice edge. in which case the downstream edge shall be counterbored or beveled at an angle of 15 des or less to the required thickness at the orifice edge. The word Upstream or Inlet should Òe stamped o n the oritice tab on the square-edge side of the plate. Dimensions for orificc plates are shown in

Bores must be round and concentric. Practical tolerances for orifice diameters, as given in AGA Report No. 3.:: are shown in Table 1-1.

The upstream edge of an orifice should be square and sharp. I t is iisuriily considcrcd sharp if thc reHec- tion of ;I beam of light from its edge cannot be scen without magnification. The edge radius should not

Fig. 1-1.

.

Orifice Size (Inchesì O 2500 0.3750 O . j O O 0 0.6150 0.7500 0.87.50 1 .o000 1.2500

1.7500

Over 5 0000

I .5nm

:.moo to 5.onoo

- . <: D = inside diamctcr of pipe.

d I orifice diamctcr.

Tolerance Plus or llintis

( Inches ì 0.0003 0.0005 0.0006

0.0009 0.00 1 o 0.00 I4

0.0008

0.00 I 2

o on17 o o w n 0 . 0 0 3 0.0005 pcr in. of

diameter

exceed 0.0004 times the bore diameter. It should be maintained in this condition at ail times. For two- way flow both edges should be square. Orifice plate details and schedule of thicknesses are shown in Fig. 1-1. Detailed tolerances are discussed in AGA and ASME 4 . publications.

In wet-gas or wet-steam services. where the volume of condensate is small, a weep hole flush with the bottom of the orifice run may be used to prevent a buildup of condensate in horizontal lines. The weep hole serves as a drain to prevent freeze-up during shutdown periods. A weep hole flush with the top of the pipe also can be used to pass small quantities of gas in liquid streams. A Yi-in. weep hole with a 1.5-in. orifice diameter will give an approximate error of 1 percent.

Because test information is more readily available for thin plate orifices than for other primary devices, it is possible to design orifice installations to good accuracies. However, field installations are not always designed to obtain the best accuracy. In installations which are used only for control purposes some higher order of inaccuracy is acceptable than is required in installations which are used for accounting, material balance, or buying and selling. Common errors are those which result from improper tap location, round edges. viscosity variations. and lead-line head differences. Orifice plates should be kept clean and free from accumulations of extraneous material.

2. d /D (/3) RATIO Orifice diameters should be selected so that the ratio

of orifice diameter to actual internal pipe diameter. d/D. does not exceed 0.75 for liquids and 0.70 for gas or steam and preferably is not less than 0.20. A d / D ratio between 0.4 and 0.6 is considered best. Because of the danger of plugging by pipe scale and other foreign material. the minimum orifice bore should not be smaller than 0.5 in. diameter on all but the cleanest services.

3. OTHER ORIFICE PLATES Eccentric or segmental orifices may be used in hori-

zontal runs for special services where concentric orifices cannot be used. Eccentric orifices are useful for mixed- phase gas-liquid services. Segmental orifices are recommended for slurry services. because of the low cost. in- sensitivity to changes in liquid-solids ratio. and relatively satisfactory accuracy [approximately 2.2 percent for plate calculations ) ."

The eccentric orifice usually is placed \vitIl its edge tangent to a circle of a diameter 0.98 of that of the pipc. Thc point of tangency is rit the top vertical center linc for liquids containing some vapor, and at the bottom vcrtical ccnter linc for vapors containing some liquids. Cocficients also are available for ccceiitric orifices at 90 or 180 deg from the point of tangency. Eccentric and scgmcntal orifice plates arc shown in Fig. 1-2. The segmcntal orifice usually is constructcd with a circle

8



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FLOW

S H A R P i SQUARE EDGiS

O M i T CHAMFED C 9 COUNTERBORE ON DOWNSTREAM CORNER w n E ~ t = T

OPTIONAL HOLE LE IN ORIFICE PLATES INSTALLED IN HORIZONTAL LINES;BOTTOM OF LINE FORGAS 09 3YDROCARBON LIOUIDS CONTAINING TRACES OF WATER TOP O F , ~ N F

SECTION A-A

=OR L auim CON?AINÏNG?R~C<S

Material: Type 316 stainless steel or other suitable material

ÜF VAPOR OR NONCONOENSABLES

(All Measurements in Inches)

Out side Dia meter

I

Tab

L W 4 ' ,X I - 4 i I 4 "4 6 1 6 1 6 1 6 1 6 i 6 1 6 1 6 1 6 1 6 1

"4

','& n ,

..I

Notes: 1. The outside diameter (OD) of the orifice plate is that required to fit inside the bolts of standard ASA flanges. The outside

1. Sizes 1 in., If4 in., and 214 in.. should be avoided. diameter is equal to the diameter of the boit circle less nominal diameter of bolt +O in. - - l : : ~ in.

FIG. 1-1-Concentric Orifice Plate.

diameter (D) between 0.97 and 0.98 pipe ID and generally is used in services which require that it be placed at the bottom of the line. For best accuracy. the tap location should be 180 deg from the center of tangency. However. to avoid gas bubbles in the taps, the location may be anywhere within the sector shown in Fig. 1-2.

The quadrant-edge (or quarter-circle) orifice is a device in which the upstream edge is rounded to form a quarter-circle. The thickness of the plate near the orifice is equal to the radius of the quarter-circle.

9

The quadrant-edge orifice is attractive for the flow measurement of viscous streams because of its relatively constant coefficient over a wide range of low Reynolds numbers. It is of special value where the viscosity is high and also variable. ( In contrast, the square-edge orificc coelficients show increasing dependence on orifice Rcynolds nurnbcrs. R,l." below 100.000. Squarc-edge

:li In somc data RI,. the Reynolds number for the pipe, is given: in other data Kqi. the Reynolds number for the restriction, is used. The distinction between these two ntimhers i s not iilways clearly set forth: KI, = p k .



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API RP S5O-PART I -

- -10 * o 98 INTERSECTSCHORO ' ' ' 4 N D A R C O F ORIFICE- 1 io- CENTER OF TANGENCY- --- 2 2

ECCENTRIC 5 EGM EN TA L

oriticc coetticient corrcction factors arc avnilable for R,, down to approximately 25.000.)

The quadrant-edge orifice may be used when the line Reynolds nuinbcrs ( RI, ) range from 100.000 or more down to 3.000 to 5.000 (dcpending upon the /j ratio) with ;I coctficicnt accuracy of about 0.5 perccnt. When R I , is bclow the 3.000 to 5,000 range, the coeílicient curvc shows ;I hump. Lind the longer the upstrcani run. the higher thc hunip. This hunip niay bc. suppressed. even ;it values of R I , below 1.000. by taking the How straight o u t of a vesscl nozzle through a meter run of on ly a few pipe diameters ahcad of the orifice. or by using :I scrccn. such ;IS ;L niultiplate Ilow straightener. a few diaiiictcrs upstream.; I' '

Readings for tlows which cxcced thc maximum Rey- nolds number limits may be appreciably in error. The machining of quadrant oriticc plates should be of high quality bccausc the dimensions of thc edge anci its shape and smoothncss arc most important.

Other special orifice forms also have been designed; for a detailcd discussion see the current 1iterature.l

4. S I Z I N G O R I F I C ~ S

Usually. orificcs arc sized for a 100-in. watcr column. dry calibration, maxiniuni diffcrcntial for liquids. This allows an increase or decrease in meter range for ditfcr- ent rates of tìow without changing the orificc plate. For gas o r stcam flow a good rule of thumb. even i f a range of less than 100 in. is required. is that the metcr range. in inches of water, should not cxcced thc !lowins pressure. in pounds per square inch absolute.

The procedures for computing orifice sizes and flow through orifices are given in various publications.'. .:-i. Il. 1: Special slide rules are availablc for orifice computations.' These slide rules are especially valuable for chccking longhnnd or computer coiiiputa- tions and for prcliiiiinary oriticc sizing. Orifice calculations can be piirchascd from the manufacturers of orificc plates o r Hownicters. Occasionally. only approximate physical propcrtics of the Howing tluid arc known bcforc startup; in such cases, a How slidc rule iiiay bc used io dctcrniine orifice size. Computations can be iiiadc at a

latcr period using actual flow conditions, or corrections can be applied to preliminary computations made with the approximate values.

h. Flow Nozzles

Flow nozzles are used less frequently than orifice plates. Their principal advantages are better pressure recovery and approximately 65 percent higher flow capacity for a given diameter than can be obtained under the same conditions with orifice plates. Flow nozzles are better suitcd for gas (vapor or steam) service than for liquid servicc. They should only be considered for liquid service when flow must be increased in an existing installation beyond the limits for a normal orifice installation. i f a flow nozzle is required for liquid in a new installation the size of the line should be ques- tioned. Flow nozzles may be used in light slurry service if the How is downward in a vertical run. However, accuracy is poor below 6-fps line velocity. In general, the meter run requirements. Hange ratings. and tap requirements are the same as for orifice installations. However. because the d.:D ratio for the same flow and line size is smaller. a shorter meter run may be used where the run length is based on the minimum run for the actual d / D ratio. A typical flow nozzle is shown in Fig. 1-3. There are several forms of flow nozzles. one of thc most common being the ASME long-radius forni. ' , y, Properly installed flow nozzles are equal in accuracy to sharp-edge orifices. Calculation procedures and coefficicnts for flow nozzles have been published in the literature.'. '.

c. ittiituri T i i l ~ ~ aiitl Flow Tubes

Venturi tubes and flow tubes are infrequently used in rcîincry operations. Head loss for these devices is lower than for other constricting primary elements; venturi and flow tubes should be considered for all applications where minimizing head loss is an important factor. These primary devices are more costly than orifice or flow nozzle installations, the long-form venturi being the most expensive, The venturi tube and flow tubes are shown in Fig. 1-3.

I . VENTURI TUBES

Venturi tubes give a much lower head loss than orifices or flow nozzles. For a long-form venturi tube. the approximate head loss will be between 10 and 14 percent of measured differential. dependent upon the d / D ratio. Minimum runs usually are shorter than for orifice plates or flow nozzles. As a rule. the manu- t'acturcr of the venturi tube can supply the minimum length meter run data (which varies with pipin, '7 con- iiguration j .

Although coefficients are available for the calculation of flow through venturi tubes.'. '. ;i the nianufac- turer may specify the flow for a given differential. Ven-

lo



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW

%!!!i+ FLOW NOZZLE

LO-LOSS TUBE

VENTURI TUBE

u U GENTILE TUBE DALL TUSE

FIG. 1-3-Flow Nozzle. Venturi Tube. aiid Flow Tube>.

turi tubes give an accuracy equal to that of a thin plate orifice. Venturi tube flow coefficients are relatively stable over a wider range of Reynolds numbers than sharp-edge orifices or flow nozzles. When properly purged. they are suitable for metering streams which contain solids. provided the solids-¡¡quid ratio remains constant. An increase in the solids-liquid ratio will cause a higher reading, and vice versa. The cost of venturi tubes is high and they are not widely manufactured of steel; however. steel or alloy venturi inserts for line installations are available in some sizes at lower cost.

7. D.ALL TUBES The Dall tube is a primary flowmetering Jevice

which has been used in England for some time. I t is now available as a fabricated line insert. The Dall tube is approximately two diameters long. The static pressure tap is in a line-size section which is followed by ;i sharp shoulder and a steep conical entrance to a short cylindrical section which has an annular slot. followed by a 15-deg conical discharge throat tcrmi- natinp with a shouldcr.

Examination of the Dall tube gives the impression that ;i fluid fow through it would be subject to a very high head loss. Actually, the Da11 tube head loss is only about to 6 percent of the measured differential as

compared to 10 to 14 percent for the same How in a long-form venturi. The coetticient niay vary for line Reynolds numbers below 500.000. Rounding of the sharp edges will cause slight variations in the coefficients.

Unless it is purged. the Dall tube should not be used for slurries- or fluids which contain suspended solids because the annular throat slot is subject to plugging. The minimum meter run is longer than that required for a venturi tube.

3. GENTILE TUBES The Gentile tube has impact- and suction-type

piezometer openings to increase the measured differential. The outstanding characteristics are that it gives a good differential with a relatively small amount of constriction and is short (approximately .me and one- half diameters) : its coefficients are the same for flow in either direction; and the cost is less than for a venturi tube. However. it is very susceptible to line roughness. It is claimed that the Gentile tube can be used with a shorter meter run than a venturi or a Dall tube. The coefficients are said to be relatively constant for line Reynolds numbers from 100.000 to 800.000. Until sufficient data have been accumulated on the effect of manufacturing tolerances and upstream piping configurations on its accuracy. a Gentile tube should be calibrated for any application where high accuracy is important.1

4. Lo-Loss TUBES The lo-loss tube I-, has a pressure recovery compara-

ble to that of the Dall tube. Also. properly purged. it is quite suitable for handling fluids with suspended solids.

( I . Pitot Tulies aiitl Pitot Vetitiiris

Pitot tubes and pitot venturis are used where the pressure drop or power loss through other devices cannot be tolerated and where accuracy is not of prime conccrn. These devices frequently are used for measure- nicnt of high air and water How rates. Pitot venturis are useful in applications where an ordinary pitot tube does not give satisfactory differential. Pitot venturis should not be used at greater than 9 fps in liquid servicc i f dissolved gases are present: higher velocities cause cavitation. and gas bubbles collect in the meter connccting lines. For good measurement a traverse is required unless there is sufficient straight upstream run to obtain a uniform velocity profile. Fig. 1-4 shows pitot tubes and z pitot venturi.

Proper design will permit the installation or removal of pitot tubcs and pitot venturis from lines which are in servicc. They are not suitable for use in hot oil or other 1iaz:irdous service except in fixed installations designed to be leakproof. The relative COS& of pitot tubes and pitot venturis with respect to other primary elements decreases as the line size increases. A typical pitot tube inst:illnti»n is shown in Fig. 1-5.

11



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L O W - PRESSURE CONNECTIGPi -L- El HIGH-PRESSURE

CONNECTION

RESSURE ECTION

,_- PIPE W A L L

PITOT TUBES

TO DIFFERENTIAL INSTRIJi.tENT

- F L O W -REDUCED PRESSURE

w TYPICAL PITOT VENTURI

1-l-Pitot Tubrs aiid Pitot Vvniixri.

e. Metering Runs

1. ORIFICE TAPS Orifice taps may be of several types. as shown in

Fig. 1-6. Flange taps usually are preferred for refinery use. Vena contracta taps and pipe taps sometimes are used: however. vena contracta taps cannot be used with some sizes and pressure ratings of welding-neck flanges because one or both taps may fall in an undesirable location in the flange hub or weld.

Radius or throat taps (those located one pipe ID upstream and one-haif pipe ID downstream) can be used. The downstream tap for the radius or throat tap sometimes falls in. or partially in. the flange hub.

Corner taps are being used by some refiners, particularly on small lines, where flange taps may be at the wrong location in the pressure profile. Some data are now available on corner tap installations.“* One type of corner tap orifice flange arrangement is shown in Fig. 1-7.

Pipe taps or full-flow taps (those located two and one-half diameters upstream and cight diiimetm downstream) measure the permanent pressure loss: therefore, thcy require a higher permanent prcssure drop for a givcn metcr differential than Hange. vena contracta, radius, or corner taps. Pipe taps may be used to measure a higher flow rate than can be measured by flange



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLANGE TAPS

FLOW - VENA C3NTRACTA TAPS

PIPE TAPS

.?or d/D . . 0.25 0.3 0.4 0.5 0.6 0.7 Distance from orifice

plate ( i n ID'S) 0.84 0 . 8 0 0.74 0.65 0.56 0.45

Tolerance is approximarely = O . 1 D.

Pipe Taps. FIG. 1-h-Flange Taps. Veiia Contracta T a p . aiid

taps. and the like. in a given line size t'or ;i given differential.

Orifice Hanges, with Hange taps as shown in Fig. 1-7, generally are supplied with %-in. taps and with a minimum 300-psi ASA raised face rating. These Hanges have a minimum thickness of 1% in. which, in the smaller sizes, is thicker than the standard 300-psi flange. Each tap should be positioned 1 in. from the nearest face of the orifice plate. It is important to allow for compressed gasket thickness.

The American Gas Association .; has published curves on allowable variations in pressure tap hole location versus /j ratio. Tt is recommended that the tolerances for ß ratio of 0.70 be used. For pipes smaller than 4 in., the tolerance is 0.025 in.; this tolerance increases to 0.065 in. at ß of 0.40 or smaller. For pipes 4 in. or larger, the tolerance is 0.05 in.: this increases io 0.125 at ß of 0.40 or smaller.

Some refiners use %-in. taps as a standard, others only in hot or corrosive services. In such cases. minimum Hange thickness should be 1% in. Where piping specifications exclude threaded joints in primary piping, socket. or fillet weld. taps may be used with socket weld block valves. If secondary piping may be screwed. the block valves may be socket weld on one end and threaded on the other. Screwed taps may be seal- welded; however. some contend that this gives none of the desirable characteristics of either screwed or welded joints. It is recommended that nipples to the first block valves be at least Schedule 160. Between adjacent lines sufficient space should be provided for orifice taps. block valves. and connecting piping. Consideration should be

0.20 OR LESS

FLANGE TAP RAISED FACE ORIFICE FLANGES (WELDING NECK) (SLIP ON) CORNER TAP FLANGES

Noie: To provide adequate clearance in l?<-in. and smaller pipe sizes, pipe end is often made flush with face of raised face ñange-ciearance to plate is then compressed gasket thickness.

FIC. 1-7-Wifirr Flanges.

13



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API RP 550-PART I

given to room required for rodding or drilling out taps. Special orifice plate holding fittings are available

which facilitate orifice plate changing. Some of these devices permit changing the orifice plates while the line is under pressure; these types require regular lubrica- tion and maintenance.

3. MINIMUM LENGTH OF METER RUNS Meter runs l 7 should be designed with not less than a

minimum length * of straight pipe preceding and following the orifice (see Fig. 1-8 through 1-13). It should be noted that these charts show minimum lengths of run: these runs should be increased. if practicable.'. ::.

Where pipe taps are used. the upstream run should be increased by two pipe diameters and the downstream run increased by eight pipe diameters. It is recommended that the meter run lengths shown in Fig. 1-S through 1-13. based on a d. D ratio of 0.70. be used

This length is ustially given in nominal pipe diameters. ___ -

I I

.. = =,-,'.V. TEE OR CROSS -A'-~-L 1; ;:-*.-7- --c u

,- : : : : - E x r u ORIFICE ~ 1 ORIFICEJ'

'J?UM. TANK OR SEPARATOR 'STRAIG~TENING VANE

. . . . . . I . . . , . .

. , . . . . , . , . . . . . I . - . . . .

I . . . _. . -. . . .. 1 . .- . . . . ,...... : . ; . . . . I 1 I : . ; . 1 .- - . . . .

2 . 3 .4 5 6 7 8 o 1

Diameter Ratio (d /D) .VOtl,S: I . Thew curves shoiild be used For tees. crosses. or "Y's"

wl i i c l i arc \inglc-cntr:incc fittings with the other openings ~1ow.i. 2. When ;i tee. c ros . or "Y" is used for niiiltiple-inlet. o r

inlet :ind outlet. we Fig. 1-13. curves ..\ iind 1% shoiild he used. 3. Tees. crosses. and "Y's" should be considered :is disturbing

fittings regardless of which connections are used. 4. Straight pipe X should he ;it least six pipe diameters when

preceded h y other fittings in the same p1:inc. Otherwise the Ll ihtai iccs given in Fig. 1-12 \houid he iisccl.

5 . When S is preccclcd hy Iittings in ;I different plane.

O . When S is preceded hy a valve covered by Fig. 1-10. the minimiini total dimension X+A should be equal to dimension ,\ in Fig. 1 - 1 0 .

FI(;. 1-8-iWiiiiiiiiiiiii 1.viigth Jlvtrr Riii i~. Siiigic Fiitings 1 ,¡plriwin.

Fig. 1-1.: \h«iild he iised.

__c na '---oRiFic~

GATE VALVES OR COCKS(W1DE OPEN)

AS REQUIRED BY PRECEDING FITTINGS (SEE NOTES)

Diameter Ratio (d l01

.Vot<~s: I. These curves should he used only where rate valves or

cocks are to be wide open. For partially closed valves use Fig. 1-10.

2. Where straight pipe S preceding the gate valves or cocks is preceded hy other fittings or throttling valves. K - h should be equal to dimension A taken from the curve appropriate to the fitting or valve.

3. Str;iightening vanes will not reduce the lengths of straight pipe ..\. Lind shotild be used only because of other fittings preceding the open gate valves or cocks, if the conditions determined hy Note 2 cannot be met.

4. if straightening vanes are required. they mav be installed ciiher downstre:im of the gate or cock using curve A' or iip- stream u i n g curve A. Vanes located upstream shall be distance A ' - C taken from the curve appropriate for the preced- ins -fittin-.

FI(;. 1-9-Opeii Gaie Valve* anri Cocks.

whcrcvcr practicable. even though the actual d/'D ratio is smallcr. A brief schedule for meter run requirements, bascd on 0.70 and 0.75 d/D, is shown in Fig. 1-14. If for other reasons it is necessary to use runs designed for less than 0.70 d/D. special consideration should be given to the possibility of n future increase in d / D re- quircments. Straightening vanes should be avoided because of the possibility of their loosening and working downstream. The magnitude of error caused by insuffi- cient upstream meter run with a p ratio of 0.37 is shown in Fig. 1-15. which is representative of a family of curvcs for other ,ß ratios. The curves indicate the increase in error caused by encroachment on upstream mctcr run length.

3. ORIENTATION OF METER RUNS Horizontal orifice runs avoid the head error which is

caused by taps being located at different levels. Vertical

14



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

50

45 . . . . . . . . . . .

. . . . . -

-- ......... .- . . . . . . ._ . . . . . c /

....... - . . . . . . 9

. . . . . . . .... -. . . . . . . . . . . .

- . ._ . - . . . . . . . . - . . .

..........

. . . . . . . . . . . . . . . . . . . . . . . . . - - - . . . . . . . . . . . . . . . . . . . ...... O

2 3 .+ 5 5. 7 8

D ; ~ ~ ~ ~ ~ ~ a.tmo

Note: These curves apply to control, check, or globe valves and to cocks and gates which are used for throttling.

FIG. 1-10-Cotitiol, Chcvli. or G l o l ~ e V a l v r - .

- X d b A - 0 - ,L=a3 bZl[Ej ORIFICE-'

' AS REOWIRED BY ¡ ,* PRECEDING FITTINGS-

(SEE NOTES)

vSTRAIGhTENING VANES (SEE NOTES)

' AS REWIRED 3ï FPRECECING F~TTÏNGS- (SEE NOTES)

20, I 1 I I i

Diameter Ratio ( d / D l

:\'ores: 1. Where the straight pipe I is preceded by other fittings or

throttling valves: X-.4 shall be a t least equal to .-i taken from the curve appropriate to the fittings o r valves.

2. Straightening vanes will not redoce lengths of straight pipe A and should be used only because of required lengths which cannot be met under Note 1.

3. If required. straightening vanes may be installed downstream of the reducer using curve A' or upstream using curve .4. The straight run of pipe preceding the straightening vanes or swage. whichever is upstream. shall be A' - C taken from the curve appropriate for the preceding fitting.

FIG. 1-1 1-Reducers or Increowrs Upstre:iiii.

15



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

i + - - A 7 B - - l

ELLS OR LONG SADIUS BENDS ,-LONG RADIUS -A-B- aENDS

LONG RADIUS 3ENDS b-

ORIFICE-’

Diameter Ratio I d /D l

Notes: 1. Where straight pipe X is preceded by fittings in a dif-

ferent plane tise Fig. 1-13. 2. Where X is precrcied by a valve covered in Fig. 1-10, the

minimum total length X+A shall be equal to A taken from the curve in Fig. 1-10,

FIG. 1-12-~1~ l t ip l t~ Fiitiiigr in ?iuiiit* I’luti+~.

IO IJ ‘ X- X L f R A > ,S OP BENDS LONG ,DOIN n :‘/ELLS OR LONG

I RADIUS BENDS

Diameter Ratio [ d / D I

Notes: 1. Tees, crosses, or “Y’s” with multiple inlets and outlets

should be considered as disturbing fittings regardless of arrangement of entrances and exits.

2. Where straight run X preceding the fittings is preceded by a valve covered by Fig. 1-10. X+A should be at least equal to A from Fig. 1-10,

FIG. l - l3-~lul t iple Fittings iii Different Planes.

16



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

COCK OR GATE

OPEN 0 . 3 T O O ô :-a

LA--.

I

* Suggested minimum distance; test data not available. Notes: 1. Lengths are for a maximum 9 ratio of 0.70 and. in paren-

theses. 0.75. For minimum requirements for other p ratios and other configuraiions see Fig. 1-S through 1-13.

3. Where vena contracta or pipe taps are used, the lengths as shown shall be taken from their respective differential taps and not from the orifice plate.

3. Where two flow disturbances exist and the length between them exceeds one-third the lengths herein for that type of combined disturbance. classify as a single disturbance and use length of straight run for the second disturbance. 4. Where more than two disturbances exist and the distances

between them are less thnn ns given herein for two Row disturbances. the length of straight run shall not be less than 31 pipe diameters.

5. Where two or more disturbances exist. the distance between anv disturbance and the flow-measunng connection shall not be less than as illustrated herein.

FIG. I - 1 I S t r a i g h t Runs of Pipe for Flow-3íea'uriiig Installations.

orifice runs are often preferred for gas or stream Rows which contain aDpreciable amounts of condensatc. and for liuuids which contain vapor. If vertical runs are used. How should be downward for wet gases or steam and upward for volatile liquids. The potential error in vertical lines can be minimized by proper manifolding. ;is shown in Fig. 1-1 6 and 1-21. or by the usc of seals or purges. For steam. the condensate pots must be at the same level as shown in Fig. 1-1 7. Slurry should flow downward through a vertical line if a flow nozzle or ;1 venturi tube is used as the primary element.

4. Metering runs for orifices should be ?-in. diameter

nominal pipc size or larger. In lines smaller than 2 in.. it is advisable to swage the line up to the ?-in. size for the metering run. or to use rotameters. calibratcd mctcr runs. or other special devices. Errors caused by the roughness of pipe walls become more pronounced in smaller sized orifice runs. Small-size orifices are subject to plugging in all but the cleanest service.

MINIMUM DIAMETEK OF METERING Rvsc

5 . S.i-.\.íiC PRESSURE AND TENPERATURE h'fE:\SURE- 1 1 E N T LOCATIONS

It is recommcndcd. whcn metering gases. that a static pressure tap be installed in the main line near the

O IO 20 32 GO 50 PIPE DIAMETERS FROM CRIFICE T3 DISTURBANCE

1-Double-plane eil. f I-Single-plane ell. > ?-Regulator upstream. +Gate valve upstream.

Note: These curves were derived from tests run on a 0.37 f l orifice.

VIC. 1-15-Errnw Gaud 1):- Ii i3i i f f ie i t : i i t I:psïrraiii Meter Rutis (Typical Effect of Disturbance Without Vanes).

primary measuring device. Either an upstream or a downstream pressure tap can be used. but the appropriate expansion factor must be used for the tap which is selected. The downstream tap is recommended because a given change in differential pressure causes less variation in the value of the expansion factor based'on downstream pressure than in the value of the expansion factor based on upstream pressure.l However, the upstream tap may be used i f variations in the expansion factor arc to be neglected. although tap location is sometimes specified by regulatory agencies in some custody transfer installations. As a point of caution. neither the upstream nor the downstream tap of flange taps nor the downstream tap of vena contracta taps gives a true measurement of line pressure. Measurement of the static pressure is required in order to correct the apparent reading to the actual flow.

I t niay a!so be desirable to measure the temperature of the flowing fluid, especially gas, to make required corrections in the apparent flow value. Generally, temperature is measured on the side of the orifice where the static pressure is measured. Thermowells, if used, should be inserted in the line a sufficient distance from the primar', element to prevent flow disturbances from affecting the measurement. On the upstream side, tlierniowclls should precede the orifice by at least 20 pipe diamctcrs. If straightening vanes are used, tiiermowells should be placed at least I O pipc diameters upstream of the inlet edge of the vanes. Downstream tiiermowells should not be located closer than distance I3 in Fig. 1 - 3 throush 1-13.

17



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API RP 55O-PART I

INSTALLATION =OR PORIZON T A L AND VERTICAL ,INES WhERE JRESSURE CONNECTIONS I R E TAKEN ST :YE 3RIFaCE FLANGE

Notes: 1. The meter prct'crably is mounted below the line. 1. Piping f rom the flange to the meter manifold to be !,1 in.

Meter m:inifold piping to be !'i in. Tubing and miiitiple-port valves may he used wherever desired.

3. A slope of a t least 1 in. per ft downward to the meter

4. Meter zero should be checked from orifice taps if water should be provided on all horizontal lead lines.

may he present in meter leads.

kï(;. l-l~>-DilF<.rc.:iti;il Flowi:ic-icw i i i Liquid Service.

A INSTALLATION =GR METERS ON VERTICAL L I N E S . INSU- LATE LOWER TAP UP TO L E V E L OF CONÜENSATE POT

( T

RECORDER OR TRANSMITTER

YPICAL A L L VIEWS)

.;; '+TO T?PP

5 INSTALLATION FOR ,VETE25 ON HORIZONTAL LINES

iV0tc.c I . The meter should be installed below the line wherever

possible. 2. Piping to be same size as for liquid fow escept as noted

in C. 3. Use condensate pots (see seal chamhers. Sect. 8 ) to

condense the vapor and provide two cqiinl liquid heads on the meter chambers.

t"OR LARGE? SIP5

- -THICK INSL'LATION

INSTALLATION WHEN METER MUST BE MCUNTED ABOLE,, THE PIPELINE , VERTICAL RISERS SHOULD BE AT LEAST : tNSULATED PIPE. FLANGE TAPS ARE NOT 2ECOMMENDED W E TO THE NEED FOR LARGER CONNECTING ?IPE. USE E!THER VENA CONTRACTA OR PIPE TAP+. Z?NlENSATE ?OTS ARE A r SAME LEVEL

J. Multiple-port valves may be used in the meter manifolds

5. Meter leads should be insulated and steam traced if am-

6 . Condensate pots must always be a t the same level. 7 . Condensate pots shown oversize in these drawings for

i f desired.

bient temperatures may go below freezing.

c la r i t y.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

6. INSTALL:iTION -\ND INSPECTION O F METERING RUNS

Meter run pipe or tubing should be carefully selected for a uniform internal surface which is free of internal striations and grooves but not polished. It should also be selected for roundness and for conformance with published diameters. Some refiners prefer to buy specially selected pipe or tubing for meter runs: others prefer to buy preassembled meter runs complete with orifice flanges for installations where accuracy is important. Out-of-roundness tolerance varies with the d / D ratio. ' When the d . D ratio is 0.70. thc out-of- roundness tolerance is 0.5 percent for the upstretim sections and 1 percent for the downstream sections. For tolerances for other d . D riitios sec .AG.-\ Report No. 3:' It is recommended that ali meter runs be designed LIS if for 0.70 d,'D ratio. I f published orifice coefficients are uscd. the diameters of the pipe should match published diameters within 0.5 percent for fange taps. and 0.2 percent t'or pipe taps.

Flange tap orifice flanges are either of the screwed. slip-on. or welding-neck type. I f slip-on or threrided fiangcs are used. carc must be taken to see that dl burrs nre removed Lifter drilling the taps through the pipe. When slip-on flanges are used. additional care must be taken to see that all weld splatters are removed from the fiange face. Reduction of the diameter of the pipe or distortion caused by welding should be eliminated. If welding-neck Hanges are used. it is essential that the flange bore be the same as the pipe internal diameter and that the bore be concentric and parallel with the pipe. If there is any internal roughness at the weld it

should be ground smooth. I t is desirabie to u w a ta- pered niandrel to position the \veiding-ncck flange Ju r ing welding. Flange taps should be properly oriented during installation.

Before installation, all orifice run fabrication5 should be inspected for dimensions. straightness. absence oi burrs and welding deposits. and internal roundness. Where weldins-neck Hanges have been used. conccntricity of the pipe with the flange neck should b2 checked. It is essential that the flange bore be the same as the internal diameter of the pipe. For = "as measur+ ments. the toierances should be in accordancc \vith AGA Report No. 3."

For liquid service, where the taps are horizonial. sufficient clearance should be available between adjacent lines for installation of block valves and fittings.

Before installation, orifice plate bores should be inspected for concentricity, roundness. sharpness. and absence of burrs and nicks. The bore should be meas- wed with a micrometer and the rending checked against that stamped on the paddle handle. If 3 bc.\-ctl-zd,ue oritice plate is to be installed, the beveled edgs inus; lace downstrcam. Thc quadrant-cdge oritice Fiate. on the other hand. is installed \vi th the rounded edge upstream. The Hat orifice plate muht be positiond cari'- fully between raised face Hanges to tnsure that ;he bors is concentric \vithin 3 percent of the inside diameter « I the meter run. The inside diameter of the gasket must not be smaller than the inside diameter of the pipe and the gasket must be positioned concentrically. Orifice plates supported in ring-typc joint holders will be posi-

I . Tiir meter niay be mounted cither :hove or below the line. I f t he gas i s wct or contains cortohive substïnccs. or both. a liqiiid seal or ;in a i r or gris piirgc (not shown) .jhouid be provitlcd. 1 RcFci- fo Seci. X.)

7. In gas servicc ihe pressure taps should be located on the top of the pipe.

3. On the bellows nietcrs, the lower connection >iiouId be

1. Multiple-port valves may be tised in the meter riimifolds

5 . Piping to be same size as for liquid flow.

i i sd to prevent liqiiid ncciiinulation in meter body.

if desired.

FI(;. 1-1X-DiíTvrc~iiii:il Flowiiic-ïvrs i i i Gas Se*rvic.c.. .

19



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

timed within the concentricity tolerances of the ring groove and the oriticc bore within the ring.

Some refiners rcquirc that installation of orifice plates be postponed until after the lines have been flushed out. The reason for this is to prevent trash from piling up in front of the orifice plates and to prevent any trash which might be dislodged during initial circulation from damaging the edges of the orifice plate.

7. ACCESSIBILITY OF PRIMARY ELEMENTS I t is advisable to locate the orifice. or other primary

clement. so that it is accessible from grade. walkway. platform. or movable platform even if this requires rerouting the process lines.

1.4 DIFFEREKTIAL 3IE.ASURIIVG DE\ICES

Several types of measuring devices are used to determine the diffcrential produced by the primary element. It is difficult to maintain accuracy at low-flo\v readings because ilow is proportional to the square root of thc diffcrcntial pressure. These devices are used whcrc thc range in flow is 3 to I . or less.

All the deviccs listed herein, except the glass manometer which is an indicating device only. are available in a n y combination 3s indicating, recording. transmitting, and controlling instruments. Transmitters are available for pneumatic or electric transmission.

For flow rccordcrs. the charts most generally used are the "O to I O" square root charts. Square root charts are available with various linear secondary scales for recording prcssurc. Icvcl. or temperature on the same chart. A suitable mctcr factor is multiplied by the rsad- ing to give thc actual tiow. By judicious sizing of the oriticc. metcr factors can bc obtiiincd in round figures. Howcvcr. i t is much more convcnient to change meter factors than to change thc orificc plate or the meter range whcnevcr the physical properties of the fowing stream change. Fcw rctincrs use special charts to read flow directly. Total How may be obtained by plani- metering fow charts o r by equipping the meter \vith an integrator. Corrections must be applied for changes in condition of the flowing stream.

Some of thc dcvices mentioned in following paragraphs usually are supplied as blind transmitters n i tn- out direct flow scalcs: in this case an output indicator with a O to I O squarc root or othcr suitable scale should be furnished so that flow may be rcad at the transmitter or control valve location. This device should not be used to calibrate the transmitter. For calibration in service. a test gage or manometer should be used for pncumutic instrumcnts. A tcst gradc mctcr should be i i s d for cal i brat i ng c Icctr ic transmit tc rs .

a. Jiwliuiiic*ul Mcrciiry 1Ic:tc.rs

Meclianical mercury mctcrs, in the past. have been stmdnrd in thc industry but are rapidly being replaced by tlic dry-typc inetcrs.

Ranges most generally used by refiners are 20 in., 50 in.. 100 in., and 200 in. of water (dry calibration); other ranges are also available. Although mechanical meters are available with the square root extracting type of range tubes or float tubes, most meters are read directly in differential or in flow on a square root scale.

Seal chambers or condensate pots frequently are used with mechanical mercury meters. (For further discussion. see Sect. 8.)

Mechanical mercury meter installations are shown in Fig. 1-16 through 1-18. Usually. these meters are yoke-mounted on pipe pedestals with provisions, such as spherical seated unions. so that the meters can be leveled to the close tolerances necessary for good measurement.

11. Bellows Meters

In the bellows-type meter. the bellows is opposed by a calibrated spring system and is filled to prevent rupturing when overpressured and to provide pulsation dampening. Temperature compensation is also provided. I n addition. meters with ordinary-type bellows which usually are used only on applications of low differential are still available.

Bellows meters can be either line-mounted or mounted at grade or on platforms. Seal chambers or condensate pots are not used generally. A I Y2-in. tee has sufficient volume for a liquid seal or as a condensate pot for steam or condensable vapor service for instruments which displace less than 1 CU in. with full-scale deviation. However. if the displacement is much greater than I CU in.. or if the differential of the instrument is low in comparison to the column displacement. regular, condensate pots should bc used. Typical meter piping is shown in Fig. 1-16 through 1-18.

Bellows meters have both top and bottom body connections. The top connections are used for liquid flow installations and the bottom connections for gas flow installations to avoid the error caused by trapping gas or liquid, respectively, in the meter body. It is desirable to use M-in. connections, which may require rotating the body chambers in some cases where both ?h-in. and ?4-in. connections are provided. It is sug- gcstcd that thc alternate tapped openins can be used as a drain or vent.

c'. %iiu iiie ters

The simplest measuring device is the glass manometer which may vary in form f r o m the simple U-tube to the more highly developed single-tube devices. These are of little use in refineries except as test devices rind as indicators on nonhazardous low-pressure streams. A nianomctcr with nianifold is shown in Fig. 1-19.

11. I)iiiplirnpiii Transmitters

Force-balance diaphragm and filled-diaphragm types of dilierential meters rire uscd extensivclv on refinery

2 o



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW

EDGE OF ORIFICE FLANGE-.

FIG. 1-19-Giass Tube Maiioiiieter.

units. These instruments generally are used without seal or condensate pots because of their corrosion-resistant construction and low displacement. Line mounting is preferred if the location is accessible and the vibration levei is not too high. GAS meters are mounted slightly above the line to allow liquids to drain back. Liquid meters are mounted below the line to allow gas bubbles to work back to the line. If leads are short enough. the transmitter may be mounted level with the center of the line. With this arrangement. it makes little difference in error if the opposite legs of the connecting piping contain liquid or vapor in different amounts.

Piping arrangements for diaphragm transmitters are shown in Fig. 1-20 and 1-21. If mounted farther from the orifice than as shown in these illustrations. the piping may be similar to that shown in Fig. 1-16 through 1 - 1 S.

1.5 CONNECTING PIPING

il. Meter Loration

Flow recorders. indicators. transmitters. or controI- lers should bc mounted at a convenient height. usually 4 ft or 5 ft above grade, platforms, or walkways. On flow control installations the transmitter or transmitter output gage should be visible from the control valve and control valve bypass to facilitate emergency local and manual control. For a close-coupled meter. which is the preferrcd installation, the process line should be routed to ;I convcnicnt height above grade. or near a-platform. \\.alkway. or other permanent means of access to facilitiitc maintenance and for making a zero check with a iiirinonieter or with ;1 test gage. If free access is available to the space below the primary element, the use of n roiling platform or ladder of moderate height may be satisfactory. Many flow transmitters are sus- ceptiblc to damage or malfunctioning if subjcctcd to vibration. The mounting location. therefore. must be carefully selected.

MANIFOLD VALVE

' /2'>TUF - 3/4'MALE P

TUBETC i>c L . ,DAPTER' I/< TUBING I PIPE

GAS OR LIQUID SERVICE

( P 18"

hPPROX)

3/4" GATE VALVE SWAGED/SCREWED l / r "TUBE x %"MALE ADAPTER

TUBE TO PIPE

1/2" TUBING

3-WAY MANIFOLD VALVE W I T H INTEGRAL TUBING

l/2" MALE ADAPTER TUBE TO PIPE

FITTINGS OR !/;TUBE x

3/4" GATE VALVE SWAGED/SCREWED l / r "TUBE X %"MALE ADAPTER

TUBE TO PIPE

3-WAY MANIFOLD VALVE W I T H INTEGRAL TUBING

l/2" MALE ADAPTER TUBE TO PIPE

FITTINGS OR !/;TUBE x

il \ "1/2" TL'SE X I / I "MALE ADAPTES

TUBE TO PIPE

FOR USE ON LIPUID SERViCE

FIG. l-20-Typical Close-Coupled .\latiifold .Irraiigeiiit.iits for Diaphragm Flow Transmitters.

1). Meter Leads

Meter Ieads should be as short as possibie. For locally 'mounted indicators or recorders the upper limit should be not more than 50 ft to the meter. The leads should slope at least 1 in. per ft downward from the orifice taps for liquid measurement. For close-coupled meters, horizontal leads may be used to eliminate the effect of fluid density variation in leads. For gas measurement the leads should slope at least 1 in. per ft upward from the orifice taps. or downward toward the drain pots if the meter must be mounted below the orifice run. On wet-gas service, where it is necessary to locate the meter below the orifice taps. knockout pots should be located directly above the block valves so that condensate will drain back into the line. It is also advisable to use drain pots below the meter in order to collect the additional condensate which may enter the meter piping.

Meter piping should be designed and installed in accordance with the piping specification for the service involved. Usually %-in. Schcdule 80 or heavier pipe. depending upon the corrosion allowance required, is used for meter Ieads. Some refiners prefer to use stainless steel tubing rind tube fittings. Copper tubing, usu-

21



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FOR STEAM AND LIQUID SERVICES IN VERTICAL LINES FOR GAS SERVICE IN VERTICAL LINES

VALVE NORMALLY OPEN \ STREET ELBOW

M \ALVE NORMALLY ZL3SED 9 PIPE TOTUSE CONNECTOR & .E CROSS MULTIPOP.TVACiE

p.J TEE 6 VENTPLüG

3 :Laow 8 PLUG

> AS'ELaO?:' ?:=E TOTbBE CONNECT73 &p ,' -!- UNION

LIPUID(SEE NOTES 3 AND41 STEAM (SEE NOTE 3)

FOR GAS, STEAM, ANO LIQUID SERVICES IN HORIZONTAL LINES N O f ~ . Y :

1. Differential pressure transmitter shall be located as close to the pipe and prc.sstire taps i15 pr:iciic:ihle with the Ienpih cil the iiihing hcpt to il mininitini.

2 . For gases, locate the differential pressure transmitter so the lines slope down ;I minimum of I in. per ft to the pressure taps.

3. For steam and liquids. the lines from the connections at the differential pressure transmitter shall run horizontally to the 45-dcg elbows.

4. For liquids requiring sealing, slope the lines down 1 in. from the 45-deg elbows to the connections at the differential pressure transmitter.

5. Nipples and gate valves against orifice flanges shall conform to main-line specifications. Other piping materials shall conform to instrument pressure-piping material specifications.

F I ( ; . 1-2 1 -I.rucl-l, i II<. A rrniigt-ni tnn I I for Din ph razni Flow- Tria n.4111 i t t c rs .

___

ally !h in. OD. is sometimes used for steam meters to avoid corrosion and plugging problems. Steam meters should be provided with valves for blowdown. All locally mounted instruments and lead lines handling water or process fluids which may freeze. become excessively viscous. or form hydrates in cold weather should be installcd i n accordance with Sect. Y.

Attention should be given to meter-connecting piping and manifolding as a source of meter inaccuracy. There is a possibility that the liquid head in one meter lead may differ from the head in the other lead because of differences in specific gravity, temperature, or amount of gas or water in the leads. For example. i f the meter is 100 in. below the orifice with one side filled with water and the other side filled with a liquid of 0.65 sp gr, the zero error will be 35 percent of full scale for a 100-in. meter range. It should be noted that, at times, most hydrocarbon streams will contain water.

The possibility of error because of gravity differences or vapor binding is eliminated by mounting the meter or transmitter close-coupled to and level with the meter taps.

If it is not possible to mount the meter close to the primary element, seals or purges should be used unless it is certain that water will not be present and that vapor bind will not be a problem. In the case of vapor bind-

22

ing on light hydrocarbons, steam tracing may be used to eliminate liquid in the lead lines.

c. Meter Manifoltls

Manifolds are necessary on all differential-measuring devices for checking zero and for putting the meter into or out of service. This is especially important on mercury-filled meters and those which cannot take full-line pressure on one side only. Manifolds usually are classed as 3-valve manifolds, 5-valve manifolds. or 3-valve manifolds with drains (see Fig. 1-16 throueh 1-18). Generally. 3-valve manifolds are used in liquid service and with close-coupled transmitters (see Fig. 1-20 and 1-3 I ) . When the transmitter is close-coupled. the tap block valves may serve as two of the three valves of the mcter manifold unless double blocking is required for removing the instrument while the line is in service. The 5-valve manifold installation frequently is used with liquid sealed meters and with meters in gas service. Generally, 5-valve manifolds are used on sales meters.

Piping and valves for manifolding should be the 95 -in. size. However. on sealed meters %-in. manifold piping or tubing of suitable material, such as stainless steel, can be used between the seal and meter. Special manifolds with i n t e p l valves also can be obtained.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW

il . Seals. Coiitleiisate Pots. aiitl Knockout Pots

In some services it is necessary to protect certain types of meters from the process fluid or to reduce potential errors caused by water or vapor in a meter lead. In such cases. seal chambers should be installed as recommended in Sect. S. In steam service. condensate pots are necessary to maintain an equal liquid head on each side of the meter regardless of meter displacement. For the force-balance diaphragm transmitter, the displacement is sufficiently low so that condensate pots usually are not required.

e. Purgiiip

Purging occasionally is necessary in order to prevent plugging of meter leads if the flowing fluid contains solids, is corrosive to meter parts, is highly viscous. or if water or condensate cannot be tolerated in the meter or meter piping. It is necessary to restrict the purge flow so that it is uniform on both sides of the meter and does not cause a false differential. Restriction orifices. purge rotameters (preferably armored type), needle valves. or drilled gate valves are commonly used to control the volume of purge fluid. The drilled gate vaive is desirable if frequent blowing back is required. The purge fiuid should be clean. dry, and compatible \vith the process fuid. (For additional information. see Sect. S . i

1.6 -4RE.A METERS il. General

Area-type meters (rotameters) are being used for flow measurement where the following requirements exist: 1 . Wide range of flow rate (as high as 1 O to 1 1 .

3. Good linearity (for flexible proportioning of t\vo or more flows) or simple correlation of a flow signal \vith other linear response variables. 3. High-viscosity immunity. 4. Liquefied petroleum gas (LPG) or other volatile liquid measurement (gasification in lead lines). 5 . Freezing or congealing liquids (stronp chemicals. waxes, and asphalts j . Steam-jacketed metcrs Lire usually required. 6. Slurries or streams with suspended solids, within reasonable limits. 7. Low-[low quantity (purge meters, blending service J .

S. Frequent on-off service.

Aren meters are available as indicators, transmitters, recorders, and controllers in any combination. They are relatively expensive in the large sizes. Although not used as frequently as orifice meters, they are valuable for unusual or special service conditions, as noted herein. Perhaps the greatest area of use is in low-flow service, especially for flow quantities below the normal range of the orifice meter.

Glass tube meters. unless contained in suitable armor. should not be used for general hydrocarbon or other hazardous service.

I). Installation

1. LOCATION The meter should be installed in a location free from

vibration and where sufficient clearance is available for occasional float removal for inspection or range changes. It should be located so that it is visible and readily accessible for operation and maintenance. In general. when a meter is to be used in regulating service. it should be placed as close as possible to a throttling point. preferably with the vaive located at the outlet ntting.

2. MOUNTING

Rotameters always should be mounted vertically with with the outlet (downstream) connections at the top of the meter and the inlet (upstream) connections at the bottom. that is. with the highest scale graduation and the largest part of the metering tube at the top. .A plumb bob. or similar device. should be used to check vertical ;ilignment. Should the rotameter not be. mounted in a wrtical position. both accuracy and sensitivity will be :iff ec t ed .

Rotameters may be mounted on either side of a panel or directly installed in either a horizontal or a vertical pipeline. When panel mounting is used, the panels should be rigid. vertical. and installed in a location free from severe vibrations.

3 . P I P I N G

Most variable area îlow measurement is practically indcpendent of upstream piping arrangements. Elbows. globe or throttling valves. and other fittings have no cHect on measurement accuracy i f they are no closer than jive diameters upstream.

Where connections are interchangeable (for vertical or horizontal connections 1, horizontal connections are rcconiniended i f at all practicable in the overall piping arrangement. Horizontal connections permit the use of the plugged vertical openings as convenient cleanout ports. The design of most rotameters permits the end fitting to be rotated in 90-deg increments: allowing a convenient variety of connection arrangements. Rotam- eter piping connections are shown in Fig. 1-22(A).

A11 pipin2 should be properly supported to prevent sagging causcd by the weight of the meter. Care must be taken so that the piping arrangement does not impose m y strain on the metcr body. Sometimes it is advisable to install a bracc bctwcen the inlet and the outlet piping. A check valve should be installed downstream on installations whcre backílow may be expected. in gas service. or whcre the possibility of liquid hammer may exist.

23



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APL RP 55U-PART I

PURGE METER

(IF RMUIRED)

ioi

HORIZONTAL L INES

INDICATOR, TRANSMITTER,

t VERTICAL INLET

THROUGH A SPOOL HORIZONTAL LINES WITH METER BYPASS

A B FI(;. .2-22-Piping Conneciinils for Roiniiieter.

4. BYPASS PIPING or purge fluid supply pressure may vary over short I f the meter is installed in a which may re-

quire opening the meter for servicing, and if shutdown is undesirable. block and bvpass valves may be provided

periods of time. it is advisable to use the purge rotameter-differential regulator combination for automatic

Of the purge rate Of flow; see Sect. ’- - - to permit process operation while the meter is being 7 . STARTUP seriiced. Some service conditions which may necessitate opening the meter are: dirty fluids, slurries. corrosive or crosive media. fluids with a high-solidification point, or pipe scale in the case of small metcrs.

The bvpass line and valves should be main-line size.

When the meter is put into operation, the valve should be opened slowly to prevent flow surges which might damage the float or other meter components. If the meter is purged. the purge flow must be started first.

1.7 TARGET FLOW TRANSMITTERS

a. General The target flow transmitter is a fluid flow measuring

transducer which generates an output signal directly proportional to the force applied on a “target” suspended in the fluid stream. Flow is measured as the square root of this force. and therefore, the square root of the transmitted signal.

The unit is installed directlv in the flow line. eliminat-

Block vilves should be installed upstream and downstream of the rotameter. A drain valve should be installed between the inlet block valve and the meter. A typical bypass arrangement is shown in Fig. 1-22 ( B ) .

When a rotameter installation includes a bypass, care must bc taken to be sure that the bypass valve is tightly closed \\hen the rotametcr is in service. Only the downstream block valve may be used for throttlin_o whcn Hashing might be cncountcred.

5. STR.-\INERS In sniallcr sizcs. cxccpt on slurry scrviccs. it is some-

times advisable to Iocatc a strainer upstream of the mctcr to prcvcnt thc float from bcing jammed with p i p scale or othcr forcign matcrial.

6. PCRGE F L U I D

In installations whcrc purging is necessary. the purge Auid may be injected at the top of the extension tube, as shown in Fig. 1-22(B), or at other connections provided in thc instrumcnt. Whcre thc main-line pressure

ing the need for pressure tap connections. The meter is contained in a body with short pipe sec-

tions extending upstream and downstream. A circular. square-edge, or a shaped metal target is secured to a beam or “force bar” which is held in the center of and pcrpendicular to the fluid stream. The flow is through the annular orifice around the target. The force bar ex- tends through a seal-fexure to a transmitter mechanism. Fig. 1-23 shows the schematic arrangement of a target- type meter. The transmitting mechanism may be the

24



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

- FORCE MEASUREMENT

BEAM OR FORCE BAR ;I &---SEAL

FIG. 1-2S-Target-Type Jleter-Schematic Arraiigeiiient.

force-balance type or a detlection-measuring type. such as those with strain gage sensors.

The force-balance transmitter develops optionally a pneumatic or electrical output signal. The strain gage deflection sensor gives an electrical output which may be measured by a bridge-type instrument.

The target-type How transmitter is suitable for use on viscous or dirty hydrocarbon streams. It can also be used on other liquids as well as on gases and vapors. Typically. flows can be measured to accuracies of = 7 percent of full scale within the flow ranges generally considered applicable to orifice meters.

Pressures as high as 1.500 psi at temperatures up to 750 F can be handled when the transmitter is welded into the linc. For meters furnished with flanges. the normal flanse pressure versus temperature rating tables must be adhered to; however. pressure rind temperature should never exceed the manufacturer's meter body rating.

1,. Installuiioii

1 . LOCATION ;\ND MOL'XTING

The target flow transmitter can be installed in tither horizontal or vertical lines. It should be located where it is acccssiblr from gradc, platform. or ladder.

Thc targct-typc metcr is line-mounted. It hhould be oriented with the dircctional arrow in accordancc with How direction. For better cooling on hot horizontal lincs. rhc meter should bc mounted with thc herid to .the side. All piping hhould be sufhciently supportcd to prevent unduc stress.

2 . PIPING Standard oriticc metcr piping practice should be

followed, using rnetcr run values of 0.70 d / D for target-

type flow transmitter installations. This includes the optional use of straightening vanes where necessary to reduce the run of straight pipe (see Fig. 1-8 through

If the meter is installed in a service which may require zero adjustment of the meter or opening the meter for calibration or servicing, and if a shutdown is undesirable. block and bypass valves may be provided to permit process operation while the meter is being serviced. Upstream and downstream blocks should be line size and located in accordance with orifice meter practice.

3. STARTUP

The instrument should be zeroed before flow is started. The flow should be started gradually to prevent damaging the instrument. The bonnet should be vented to remove gas if the instrument is in liquid flow service.

1-14).

c. Calibration

The target-type meter may be adjusted to zero by stopping all How in the line (usually by bypassing) and adjusting the output to correspond to zero flow. Range adjustment is normally accomplished by removing the meter from the line and applying weights to the force bar in accordance with the manufacturer's instructions.

1.8 TURBIIVE OR PROPELLER METERS

a. General

.4s its name implies. the turbine or propeller meter (Fig. 1-23) is a volumetric. fluid flow measuring trans- duccr which measures flow infercntially from the rotation of a turbine. propeller. or other type of rotor located in the stream. The rate of rotation is determined by the average velocity of the Auid and, hence. by the quantity

25



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 55O-PnKT I

of material Ilowing through the meter. It may be used for cithcr liquids or gases. Turbinc nieters arc Ilow sensing dcviccs only and rcquire iidditional equipment to provide indication. transiiiission. rccording. and/or control in any combination.

Prcscntly available are turbine metcrs designed for use within an approximate temperature range from minus 130 F to plus 1.000 F and a pressure range. depending upon size, up to 50.000 psig.

Pressure loss varies approximately as the square of the How rate. Manufacturer's pressure loss curves should be consulted for each application. It is generally within the rangc of 2 psig to 2 0 psig at rated How depending upon meter design iind fluid service.

Generally. tlic meters ;ire constructed of stainless steel and ;ire adaptablc t'or use in mildly corrosive services. They c m be ste:im-jacketed i f required.

Turbinc nictcrs arc hcing used for tiow measurement where thc following requirements exist: I . High rutigethi/ify: Flow rate ranpbi l i ty varies from about 7- I to 25- I dependent upon flow. viscosity. Huid iiietcr sizc. iiccuriicy requirements. and static pressure. For ;in avcrngc application. ;i turbinc flo\vmetcr can be expected to meter volumctric tlow to accuracies of !,i pcrccnt o r better of :ictual iìow. w e r ílo\v ranges varying from 5- i to 20- 1 or niore. depending upon the mctcr design and size aid the i k o s i t y of tlic liq~iid. -7. Hi~~~lr-tr<,c.iirtic.?. trt i t l l i i ~ l i - ~ e ~ e ~ i ~ ~ r l ) i ¡ ì ~ ~ ! i o i c . uieusiiw- metri: Turbine meters ;ire useful for high-accuracy metering xpplications. such ;IS blending. for mcasurc- mcnt ;it iow iiow ratcs. for conservation of cxpcnsive materials. Lind For custody transfer. Accuracies ap- proaching O. i percent with excellent repeatability can be attuincd whcn ;ill factors which rire known to inHu- cncc the performance of the metcr arc controlled.

Liquids with viscositics above 5 ccntistokcs present problcms that must he studied. Ixxaiisi: they drive this type of meter i n t o the nonlinear region. Inaccuracies result whcn thc higher viscosity is variable or combined with the Inrgc iiillucncc of temperature upon viscosity. Although this may sccm to be ;i considerable problem. it cnn freqiicntly be solved whcn snitablc correction factors arc applied. Some mcters are equipped with viscosity compcnsntors to eliminate the inaccuracy of overrunning causcd by viscous drag. The compensator i s clTcctivc up to iipproximately 50 centistokcs. 3. Good /iwxirifJ*: The output is dircctly proportional to Ilow over the usable range of the metcr. 4. Liqiie/ìd peiroieitm yu.s or olìier ì i ì ~ l i - ~ ~ ( i ~ ) ~ ) s - ~ r e . ~ . ~ i i r e iiyriiri rne~i.sitremeti~: I f rangeability or accuracy can be sacriticcd. flashing can usually be prevented by over- sizing the meter to reducc pressure drop. 5. Hiyìi-How rules for u g i v e t i - s i x meter: Size for sizc. the. turbine nicter will iiw;isure a higher How rate than a clilfcrcntial head Howiiictcr, but n o t ;IS high ;IS ;I ningnetic ilow nie tc r. f i . Low-/foic* rtifes: Minimum size of meter is limited only by iiiachining techniques and probability of plug- t r j n t r C C'

7. Rapid response: Pulsating How can be measured within limits. provided it is within the mechanical durability limitations of the meter. Time constants may be as short as 5 to 7 milliseconds.

I,. Iiistallatioii

1. LOCATION AND MOUNTING

The meters are installed directly in the lines. Eiec- trical-puise-type meters should be kept a reasonable distance from unassociated electrical equipment. As a rule of thumb. the line should be relatively free from fre- quencies of vibration exceeding 70 cps. If the meter is a direct-reading type. the register should be in such a position that it can be easily read.

Turbine meters should be mounted with the inlet con- ncctions upstrcam except for some types which measure How in either direction. No special considerations are necessary for the absolute levelness of the installation.

The meters are generally installed in horizontal lines. Some makes may be installed in vertical pipe. but calibration for the position may be necessary. However. in some meters. special thrust bearings should be specified for vertical mounting to prevent undue wear. It is usually necessary to specify the position ?or which the meter is to be calibrated.

2 . PIPING Accuracy and repeatability of most turbine flow-

meters are dependent upon the upstream and down- weam piping arrangements. Effects on calibration have been noted when the straight run of pipe upstream of the meter is less than 5 or 10 diameters. or the downstream run is less than 31/- diameters. Where turbine Howmeters are uscd in services in which highest accuracy is required. upstream and downstream distances recommended for orifice metcrs of large d /D ratio should be maintained as shown i n Fig. 1-8 through 1-14. The usual straightening vanes may be used with this type of metcr. should it be necessary to shorten the meter run.

Carc should be exercised in the installation of flanged meters to see that the gaskets do not interfere with the flow pattern.

Piping should be adequately supported and arranged to prevent undue stress on the meter body.

The need for a bypass is determined by application. I f it is necessary to isolate or open the meter. and if it is in continuous service where shutdown is considered undesirable. block and bypass valves may be provided to permit process operation while the meter is being serviced. Some of the conditions which may necessitate opening the meter are: damage caused by foreign material. wcar. or solids buildup. lt' bypassed. the meter should be in the main run. and block valves should be line sizc and placed at least I O pipe diameters upstream and 5 pipe diameters downstream of the meter (see Fig. 1-8 and 1-9 1. The bypass valve should be capable of positive shutoff to prevent measurement errors.

26



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3. STRAINERS A N D ACCESSORIES In almost all turbine meter installations. strainers are

rcquircd to prevent foreign material from blocking or partially blocking meter flow passages or lodging between the rotor and the meter body. The strainer must also be capable of removing particles of such a size that might injure the rotor. Typical screen size for this type of protection is 200 mesh for %-in. meters. 170 mesh for '/Z-in. to %-in. meters. 80 mesh for I-in. to 3-in. meters, and 60 mesh for %in. or larger meters. The strainer should be located I O or more pipe diameters upstream of the meter.

Where high-accuracy liquid metering is to be accomplished. means must be provided for the automatic removal of air andior gas which may be in the stream. Gas cntrainment can cause errors in rcpeatability and accuracy of the meter. When used in vacuum service. turbine meters should be installed so that they will have a positive head of liquid upstream. This head should be at least equivalent to the anticipated pressure drop through the meter. To minimize cavitation problems in vacuum service. a liquid trap downstream should be provided to maintain a downstream pressure of at least one foot of liquid.

4. OUTPUT Meter output may be mechmical or electrical. Me-

chanical output is usually accomplished by reduction gearing and a magnetic coupling driving a low-drag register or transmitter. or both.

Meters which producc electrical outputs usually sense the turbine rotation by means of a magnetic coil or a tuned RF circuit. As each signaling point of the rotor passes a pick-otf unit. an electrical impulse is produced. The normal output of this meter will be an alternating electrical signal. The impulses forni digital information. wi th each pulse reprebentins ;i discrete volume of fluid. so that the accumulated-pulsc total represents the total volume measured. and thc frequency represents the volumetric flow rate. The alternating signal can be displayed as pulse count. voltage. or frequency.

If the amplitude of the signal is relatively small. it is highly susceptible to noise pickup. and shielding is rec- ommcnded to reduce spurious counts. If transmission distance exceeds I O ft. a n amplifier should be placed near the meter. The higher signal-to-noise ratio of the output of the amplifier is less susceptible to interference. Meter. amplifier. and leads should be segregated from other electrical equipment. For specific information concerning wire size and auxiliary electrical equipment the manufacturer should be consulted.

5. STARTUP Turbine-type flowmctcrs should be placcd in servicc

only after the operating unit has been flushed and hydrostatically tested. I f strainers are used, they should be cleaned after flushing, and periodically during operation. Plugged strainers may break loose and swcep

downstream. demolishing the meter internals. Flow should be introduced slowly to thc meter. especially when in liquid service. to prcvent ovcrspeeding. The impeller blades can be sheared otf as ;I rcsiilt of ;I sudden hydraulic impact.

c. Calilwation

The calibration factor. expressed in electrical pulses generated per unit volume of throughput. is normally called a "K" factor by the manufacturer.

No adjustments are available on the primary sensor to change voltage magnitude or frequency of the electrical signal. which is ;i function of the rate of rotation of the meter and. hence. a function of flow. Any adjustment for correction must be made on the receiving equipment or applied as a meter correction factor.

Calibration of turbine meters may be accomplished through either of two basic schemes. One method is to perform the calibration in the shop: this consists of taking standard volumes through the meter (with appropriate temperature compensation ) Lind comparing with the recorded count obtained from the meter for the specified volume. The second method is the use of an in-line system either iipstrcam o r downstream of the turbine meter. Connected to the bypass piping is either a permanent or a mobilc. ball or pi," prover. When .the stream is blocked the flow is diverted through a calibrated meter run and the output signal of the meter is imposed upon a counter. the counter being actuated during the time the ball or pig moves through the calibrated volume. A PI Stcititltird 11 O 1 : Measurement oí Petro- leum Liqiiid Hydrocarbons by Positive Displwement :Mefer may be consulted for meter-proving procedures.

1.9 MAGNETIC FLOWJIETERS

u. General The magnetic flowmeter measures volume rate of flow

of liquids which have some measure of electrical conductivity.

Unfortunately. most petroleum hydrocarbons do not have sufficient conductivity to be satisfactorily measured with the magnetic flowmeter. For this reason its use is restricted to auxiliary services-e.g.. water: emulsions. such as acid sludge; water solutions of acids. caustics. monoethanolamine ( MEA ), diethanolamine f DEA). and additives; water slurries; and some catalyst com- plexcs such as those containing aluminum chloride. Crude oil which contains salt water may show sufficient conductivity for measurement by thc magnetic meter, but the possibility exists that thcre will be separation so that the measurement will not be satisfactory.

Thc mctcr consists OF two parts. a magnctic flow transmittcr (installed directly in the proccss line) and an electronic reccivcr. The trammitter generatcs a signal proportional to volurne flow and scnds it to the receiver, which niay be as far ;is 2.000 f t away.

27



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The magnctic flow transmitter operatcs on thc principle of an electrical power pcnerator. This is based on Faradíiy's law of electromagnetic induction. This prin- ciplc, stated simply. is that when a moving conductor cuts across lines of force in u niagctic tield, u voltage is induccd in it. Moreover. if thc dimensions of the conductor (line diameter) remain constant, then the induced voltage is directly proportional to the velocity of the conductor. Mathematically, this principle can be stated as:

Where: E = CBDV

E = generated voltage. C = dimensionless constant. R = magnetic field strensth. D = length of the conductor (diameter of the meter). V = velocity of the conductor (velocity of fluid).

As shown in Fig. 1-25. the flowing liquid itself is a series of moving conductors. Conductor length, D, is constant as determined by the inside diameter of the transniittcr tube. A uniform magnetic tield, B . is generated mutually pcrpendicular to the axes of thc meter tube and thc clcctrodes by :in elcctromagnct encircling the tube. Thc output voltagc. E . thcrcfore. varics directly Lind exclusively with velocity, V , of thc Howing liquid. The constant. C. corrects for other cffects. Inas- much as thc tube arca is constant. the generated signal is also directly proportional to volumetric flow ratc as long ;is the ningwtic ticld is constant.

System accuracy ( including transmitter. connccting cables. and trmsduccr or reccivcr) is typically 1 percent of full scale throughout the rangc. Special higher accuracy systems arc optionally available. Sensitivity is approximately 0.0 I pcrccnt.

Thc major characteristics of the magnctic Howmeter ;ire : 1. Transmitter is instnllcd dircctly in line. prcscnts no obstruction. thus, causcs no clogging or loss of head. 3. I t rcsponds only t o velocity and. thus. is not ali'ected by c t i a n p in density, viscosity, or line pressurc. 3. Sincc it avcragcs ttic vclocity profile betwccn the clcc- trodcs. long r u n s of pipc or straightening vanes usually arc not nccdcd. The only requirement is that the velocity prolilc bctwccn thc electrodes be ieprescntativc o f tlic llow. 4. The transrnittcr output is lincar with velocity o r Row and no square root extraction is neccssiiry. 5 . I t can handle ;I widc varicty of difficult liquids. 6 . Bidircctional Now may bc measurcd.

I L Ili~iallatioii

Tlic inagnctic Hownictcr is primarily ;in electrical device which should n o t be trcatcd ;is ;I pipc spool. In- stnlliition of this typc of nictcr in the piping rcquires ktxiwlcdgc and carc. The innriiifncturer's installation rcconinicndations should bc followcd. remembering that

MAGNETIC

Typical Magnetic I'low Transmitter

C O R F

CORE

although the transrnittcr is built on u rugged piece of pipe. it should be hiindled as a prccision instrument.

Transmitter tubes ;ire made o f nonmagnctic niaterials -such as stainless htccl. Inconcl. or tiberglas pipe. The nonmetallic tubes arc uscd unlined. but the metal tubes must be lined with :i nonconducting matcrial-such as Huorocarbon. synthetic rubbcr, or glass to prevent short- circuiting the signal. Each transmitter assembly has definite limits as to operating conditions. Major limitations are the liquid prcssurc. tcmpcrature, corrosive and

28



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erosive properties. The operating conditions must not exceed the limits for the particular transmitter construction as given in the manufacturer's specifications. The weakest zone may be the seal for the external connection to the electrodes. Any leakage at the electrodes will enter the electrical section; this is particularly undesirable in hydrocarbon bearing service.

1. HANDLING OF TRANSMITTER The following precautions should be observed: a. Care should be used in lifting the transmitter to

avoid liner damage (see Fig. 1-26). If the liner is damaged it should be replaced or repaired, using an approved procedure. before installation.

b. Dropping the transmitter or subjecting it to impact. particularly on the flange face. should be avoided.

c. The transmitter should be kept in its shipping crate, and protective end covers should be kept over the Hange faces until ready to install.

tf. The manufacturer's storage recommendations should be observed.

3. LOCATION .AND ORIENT.-\TION O F TR.ASSMITTER The following points should not be overlooked: CI. The magnetic How transmitter tube may be in-

stalled in any position (vertical. horizontal. or at an angle ) but it must run full of liquid to measure accurately.

b. If the meter is mounted vertically. the flow should be upward. Both electrodes must be in contact with the Rowing liquid. The transmitter should be installed so that the electrodes are not in a vertical plane. A chain of bubbles moving Liions the top of the flow line could prevent the top electrode from contacting the liquid.

c. The transmitter should be accessible from grade or from a platform. with enough space around it so that at least the top housing could be removed if necessary. At the very minimuni. sufficient access room should be available to remove any handhole covers or inspection plates.

00 2

'LANGES "

d. If the transmitter is to be underground or in a pit that might become Hooded. provision should be made to prevent it from being submerged, unless it is equipped with a special housing to permit operation while submerged.

e. Vertical mounting and a straight run upstream. with flow upward. is especially recommended if an abrasive slurry is being measured. This dis- tributes wear evenly.

f . The transmitter should not be installed in services where the temperature is high enough to damage the liner or insulation.

3. FLOW DIRECTION A signal will be obtained with How in either direction

through the transmitter. If How direction changes, or if the meter tube has been installed in the wrong direction. the correction can be made electrically. It is not necessary to reverse the transmitter in the line.

4. PIPING CONNECTIONS TO TRANSMITTER

The following precautions should be observed: a. During installation. care should be exercised to

prevent overheating the transmitter tube and/or liner from nearby heat sources, such as welding.

O. If a metal tube transmitter has its liner brought out over the flange faces. the liner should not be forced between adjacent Hanges. bumped, or subjected to anything else that might damage it.

c. On new pipe installations. it is desirable to bolt the adjoining pipe fittings or valves to the transmitter before installing it in the line. to avoid liner damage. I f this is not possible. it should be bolted in continuity from upstream to downstream piping. if piping is already installed. it is advisable to remove one or both adjoining pipe sections. In installations where there are no block valves or bypasses. it may be desirable to make up and install a Hanged spool piece on each end of the transmitter.

ti. For those applications which require frequent cleaning of the How lines. the transmitter can be installed with block valves and a bypass valve to permit access to the tube interior without shutting down the process. Possible piping arrangements arc shown in Fig. 1-27. The bypass valve should be capable of positive shutoff to prevent measurement errors.

e . To permit checking the meter for zero flow. it is ncccssary to install the transmitter so that flow can be stopped with a full tube. For most continuous prccesscs this will rcquirc a block and bypass ar r:i n p i c n t.

f . Transmitters up to I Z in . in sizc rcquirc n o support other t h a n that rcqiiircd for an equal Icngth of pipc. The transmitter should not bc used to support the xijaccnt piping. For larger sizes. a support structurc may be ncccssary, depending upon sizc. con:;truction. and manuïxtiirer's rcc- ommcndntions.

79



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API RP 550-PART I

same ground potential is ncccssary between the flowing liquid. the piping, and the transmitter. This is especially important it' the conductivity of the liquid is low. How this is achieved depends upon the transmitter construction. the adjacent pipe (unlined metal. lined metal. or iionmetallic ) . and the transmitter manufacture. Jumpers t'rom the meter body to the piping are usually required for metallic piping. I f the meter is installed in nonmetallic piping it is usually necessary to make a grounding connection to the liquid by means of a metallic spacer ring betwecn the flanges unless internal grounding has been provided in the transmitter. This is extremely important and must be done as recommended i f the systeni is to operate properly.

.'+ding the Triinsrnitter: Most transmitters have their signal and power connections enclosed in dripproof. splashproof. or explosionproof housings. The connections should be sealed in accordance with the manufacturer's instructions.

Tlic piping should be dcsigncd for sutticient Hcxibility to prcvcnt cxcc ve forces froni being transtiiittcd t o the elect ally insulated Hange frices. Particular attention should be paid to in- stiillations in vertical lines to assure that the weight of the transmitter o r piping is not applied t o the Ilangc facing in cxccss of its durabilit!.

s. Scvcrai diHcrcnt types of Hringc connections are used. The gcncral iule t'or all types is to make surc that thc flange and its adjacent mating flange arc propcrly aligned and that the bolts are tightened cvcnly. Bolt torque wrenches should be used and recommended torque values should be observed.

5 . ELEc,rruc.\i- 1 NSTALLATION

Power: Power should be supplied at a voltage rind frcqucncy within the tolerances specificd by the manu- fact u re r .

Wiritig crtiil Sistitrl Leads: Special low-capxitance cable is used to carry the gcncrated signal from transmitter to rcccivcr. It must not be installed close to power cable or in the same conduit as the power supply. The mrinufactiirer's rccommcndations should bc con- sultcd. The cable should bc supported s o that i t does not oscillate.

Groritrdiiig: Thc importancc of proper grounding cannot be overcmphasizcd. I t is ncccssary for personnel safety and satisfactory flow merisurement. The manufacturer's instructions o n grounding and jumper arrangement should be carefully followed. Piping should always be grounded. A continuous clcctrical contact to the

30

6 . STARTUP

The instrument should be adjusted to zero with the tube full before How is started. .4fter flow is started. if the receiver output does not read upscale. the polarity of the power or the s i p a l connections should be reversed in accordance with the manufacturer's instructions.

KEFEKENCES

i I.. K. Spink. Principles w i e / Pr<rciii.e o,/ Fluii. . \ / L , I c ~ Eii,<~i- /iceiiri,v. 8th edn.. The Foxboro Co.. Foxboro. Xlass. i 19581. ' W. ,M. Lansford. "The Use of an Elbow in a Pipe Line for

Determining the Kate of Flow in the Pipe." Univ. Illinois Eng. Exp. Sta.. Urbana. 111.. Bull. No. 189 1 1936).

:: Ori / i~c , ,Maroiri,q of Notiirri/ G~is. Gas Measurement Com- mittee lieport No. -?. Am. Gas Assoc.. New York. AFr. 1 1955 ) .

' PTC 19.5.4: f I I , s c r I i I i i e m wid App<ir<itirs. supplement to ASME f'rJiiYi. Test Cuclcs, Am. Soc. Mech. Engr.. New York, Fcb. 11959).

.' Fliricl h.lc~rci:v:. Their Theory <irzt/ Appliccrtiori. report of .4SME Research Committee on Fluid Meters, 5th edn.. Am. Soc. iMech. Engr.. New York í 1959).

'I S. l i . ßeitler and D. I. Masson. "Calibration of Eccenrric and Segmental Orifces in 4- and 6-In. Pipe Lines." A S M E

M. Bogema. B. Spring. and M. V. Ramamoorthy. "Quadrant Edge Orifice Performance-Effcct of Upstream Velocity Dis- tribution." ,JSM& 7'rcrri.s.. J . Bcisic Eil,<,. 84 Ser. D [1] 415.8. Dec. (1962) .

'R . E. Sprenkle and N. S . Courtright. "Straightening Vanes for Flow Meas~irement."iMrc/i. 0i .g. 80 [21 71-3. Feh. 11958).

'' J . A. Landstra. "Quarter-Circle Orilices." Tr(i1i.s. fjrsr. Cliciii.

'"M. I3ogcma Lind P. L. Monkmeyer. "The Q t i d r a n t Edge Orilice-A Fluid Meter for Low Iieynoids Nunihers." , - í S h f E

" l i . F. Stearns, l i . li. Johnson. l i . M. Jackson. rind C. A. Larson. f'loii- Mm.s/ iwi i i i> i i t i i - i t l i Ori,/icz A f c i i ~ r - ~ . D. Van No.\tiand Co.. Inc.. New York. 350 (1951 ).

.I'. T. J . Filbnn and W. A. Grilhn. "Small-Di;imrter-Orifi~e Metering." ASME Tr<i/is.. J . ßcr.vic f3 i .q . 82 Ser. D [ 3 ] 735-40. Sepi. í 1960).

Ti.<iit.~.. 71 751-6. Oct. ( 1949).

f x y . 38 [ i 1 26-32 I 1960).

T ~ ( ~ I I S . . J . ß<r.tic Eii.4,. 82 Ser. D r 3 I 729-34. Sept. I 1Y60 ) .



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW

"'I. O. ,Miner. "The Dall Flow Tube." A S M E Traiis. í 8 'li Coinpiriariori of Orilice Bores Llsiir,y Corirri Coiriieciioiis.

475-9. Apr. í 1956). Barton Instrument Corp.. Monrerey Park. Calif. ( 1954). " L. J. Hooper. "Calibration of Six Beth Flow Meters a t '' R. E. Sprenkle, "Piping Arrangements for Acceptable Flow

Alden Hydraulic Laboratory." A S M E Trnrrs. 72 1099. Nov. .Meter Accuracy." A S M E Tiriiis. 67 315. J u I ~ ( 1945). (1950). A P I Std. i l O I : Meusrrreineiir of Petroleirrli Liqirid iìvdi-o-

Ir> L. J. Hooper. "Design and Calibration of the Lo-Loss crrrhoirs by Positive Displacci?ieirr Alerer, Ist edn.. Am. Petrol. Tube," ASME Trriris. 84 Part II. 16 1-70. Dec. ( 1962). Inst.. New York. Aiig. í 1960).

31



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COPY PROVIDED FOR HISlMCM WRPOSES ONLY

SECTION

2.1 CONTENT Recommended practices for the installation of the

more commonly used instruments and devices for indicating, recording, and controlling liquid levels and liquid-liquid interface levels normally encountered in petroleum refinery processes are described in this section.

Instruments and devices excluded from discussion in this section are: 1. Solids level instruments. on which standard installation procedures and information have not yet been fully established. 2. Electronic types of level instruments which utilize probes to detect changes of capacitance or conductivity. 3. Level instruments which involve the use of photo- electric cells or radioactivity.

Because of the special nature of these instruments, the installation of each is a particular problem. For example. probe-type instruments should be considered in the light of their intrinsic safety. a field not covered in this manual: the use of radioactivity in instrumentation imposes health and safety considerations, subjects also not discussed herein except as they :ipply to ccrtain analyzers: sce Part I I of the manual.

2.2 GESERitL Certain general procedures, practices, and precau-

tions apply to practically all instruments discussed in this section: where applicable. the generalities covered in itenis í b ) through í h ) should be considered il part of, or a prefacc to, the text of each of the subsequent discussions.

a. Tyw5 C o v w e d

The following types of instruments are covered: i . Locally mountcd indicating gages (Par. 2.3). including tubular gage glasses, transparent and reflex gage glasses. float-and-cable (automatic) tank gages. hydrostatic head pressure gages, differential pressure ievel indicators. rind miscellaneous types of gages. 2. Level transmitters í Par. 2.1). including displacers, ball Iloats. difcrcntial pressure types, hydrostatic head typcs. Lind electric and electronic types. 3. Locally niountcd controllers (Par. 2 . 5 ) . including ciisplaccrs. ball Houts. differential prcscurc types. direct expansion typcs. and altitude valves (static-head types). 1. Remote o r board-mounted reccivcrs ( Par. 1 . 6 ) . 5. Lcvcl iilarms ( P a r . 2 .7 ) . (i. .-ìcccssories ( Par. 2.8 J . including seals and purges? gagc glii3s illuiiiinators. and wentlicr protectio!.

1,. ..\cce';.;ilIili ty

All loc;illy niountcd liquid levcl instrunicnts. including gags gliisscs. shoiild be rcadily accessible from gradc.

2-LEVEL

platform, fixed walkway, or fixed ladder. ( I t is not considered good practice to mount instruments where they can be reached only from a portable ladder. A rolling platform may be satisfactory if free access is available to the space below the instruments. )

c. Visibility

In all applications where a liquid level is regulated by a control valve, some indication of the level-gage glass, pressure gage, or other-must be clearly visible from the control valve location to permit manual control when necessary.

d. Connections to Vessels

Level instrument connections must be made directly to vessels and not to flow lines (continuous or intermittent) or flow nozzles. The connections and interconnecting piping should be installed so that no pockets or traps can occur. Where such pockets are unavoidable: drain valves should be provided at the lowest points.

e. ~Iultiple-Iiistriiiiieiit Mounting

When two or more instruments, including gage glasses. are required for any application, such as gage glass and controller or gage glass and alarm switch, they should be mounted in such a way as to keep the number of openings in the vessel to a minimum. Susgested methods are the use of tees. as in Fig. 2-6 and 2-7. or the use of a common standpipe, as in Fig. 2-2( B j 2nd 2-8.

f . Bloc*li Valves

1. SIZE AND MATERIAL All block valves must have materials of construction,

rating, and type of connections conforming to the minimum specifications for the equipment on which the instruments are connected. With the exceptions as noted in the following item 2. full-nozzle-size block valves should be installed at the vessel nozzles. Fittin, us or piping should not be installed between the nozzles and block valves.

2 . EXCEPTIONS Block valves for tubular gage glasses, gages mounted

on standpipes, and hydrostatic head instruments and pressure switches, such as used for level alarm signals, should be installed as close as possible to the points of conncction to the vessel with no fittings other than pipe nipples or nozzles between each connection and the block valve. Where connected to nozzles, block valves should not be sized smaller than 1 in.; sce Fig. 9-1 ( C j Lincl ( E ) . Where connected to nipples or to standpipes,

32



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i

block valves may be a minimum of %-in. size; see Fig. 2-1(D) .

Dual block valves for parallel instruments, connected by tees mounted directly on nozzles. as shown in Fig. 2-6. are permitted by some companies. Although this arrangement is a space-saver. and in many cases more economical. it makes the problem of calibrating a transmitter in operation more difficult: see also Par. 2.4 (a-5).

Some flange-mounted diaphragm-type level tmns- milters are designed to be mounted flush with the wall of the vessel. thus. block valves are impracticable.

g. Strain Relief

Connections between vessels and heavy gages, controllers. or transmitters should be relieved of strain by properly supporting such instruments (and seal pots, where used) and by installing offsets or expansion loops where necessary to provide for thermal expansion.

li. Vilwatioii

Many levei instruments are susceptible to dama, “e or malfunctioning if mounted in locations Lvvhere they are subject to vibration. Care must be exercised in the selection of locations for mounting such devices.

-- 4’’ ‘.iOZZCES ’ a C

0 E

PREFERRED METHOD OF INSTALLATION

Notes: 1. Nipples between block valves and gage cocks in A, C, and

E should be 1 in. by :)i in., Schedule 160 reducing nipples. In B and D. they should be in.. Schedule 160 short nipples. Gage cocks are in. npt.

2 . Block valve and nozzle sizes shown are minimum.

FIG. 2-1-Gage Glass Cuiiiicctions to Vessels.

2.3 LOCALLY MOUNTED INDICATING GAGES Locally mounted indicating devices include tubular

gage glasses. transparent (through-vision) and reflex gage glasses, float-and-cable (automatic) tank gages, hydrostatic head pressure gages, and differential pressure level indicators.

a. Tiibiilar Gage GIasses

1. APPLICATION Many companies discourage the use of tubular gage

glasses on process units. Where applicable. however. these gages provide the simplest and most direct method for level measurement. This type senerallv is used in services where the temperature is below 200 F. the pressure is below 50 psig. and the material in the vessel is nontoxic and nonhazardous. Tubular gage glasses in short lengths occasionally are used in low-pressure steam service.

7 . CONNECTIONS Gage glass connections to the vessel should be made

by means of block valves and automatic gage cocks. as shown in Fig. 1-1 í-4‘). Some companies permit the use of automatic page cocks only. as shown in Fig. 2-1 ( B ) . whereas other companies use block valves and non- automatic zage shutoff cocks. as shown in Fig. 7-1 ( C ) .

Where a connection is made to a nozzle I?+ in. or larger. any reduction in pipe size should be accomplished in one step between the nozzle and the block valve, as shown in Fig. 2-1 ( E ) . 3. M ~ s i m ~ a r LENGTHS

Tubular gage glasses used in steam service should have a maximum length of 18 in.. and in other services 30 in. Where greater ranges of levei are to be observed, overlapping gase glasses should be used.

3 . PROTECTION The tubular gage glass should be protected by p a r d

rods and preferably should be mounted on the side of the vessel away from any source of damage, such as roadways! work areas. or mobile equipment lanes. How- ever: the gage must be visible to the operator at all times and especially in the event of an emergency. Where greater protection is required because of unavoidable exposure to such hazards, slotted sleeves or screen-type cages are sometimes preferred, even though they interfere with good visibility.

1). Transparent (Tlirougli-Vision) and Reflex Gage Glasses

1. APPLICATION Transparent or reflex gages are used in most services

where the temperatures exceed 200 F; the pressures exceed 50 psig; and/or the vessels contain toxic, flammable, or otherwise hazardous fluids.

33



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APL R P SSO-PART I

Tr~ituparerit ga~y:rs should be used in installations involving acid. caustic. or dirty (or dark-colorcd ) nia- tcrials; in high-prcssure stcani applications; and for liquid-liquid interfacc servicc; also in any application whcrc it is neccssary to illuminate the glass from the rear.

Refles p g e s preferably should be used on all other clean service applications including C., and heavier hydrocarbons. They may also be used on C : and lighter hydrocarbons if the product does not dissolve the paint or other coating from inside the gage. thereby leaving a bare mctal backwall which in turn reduces the effective- ness of the prisms.

2 . G A G E ASSEMRLIES Gage columns arc made up of one or more standard-

length scctions and should be connected to the vessei as recommended in Fig. 1-2 ( A ) . For greatest accuracy and safety. gage columns should be limited in length to four sections or S ft between connections. and to three No. Y ( I2 in. ) sections in services at 400 F or higher. In noncriticnl Icvcl applications. where errors in true Icvcl can be tolerated m d whcrc the tempcratures are less than 300 F. longer gage columns arc sometimes used. In such cases, additional supports arc required. and expansion and contraction. which result f r o m tem-

',,EZtEL :',tg OR 2" NOZZLE

I/; VENT (WHEN USES)

I" OR LARGER 4UTOMATIC NOZZLES AND ,;AGE COCil BLOCK VALVES I

A CINGLE GAGE

COLUMN

GAGE

GAG5 COLU!AN

*DRA;:: P VALVE 1 / 2 " ~ ~ ~ ~

B TWO OR MOR:?

GAGE COLUMha

FIG. 2-2-Gngc Coluiiiii Asscnil)lic*s.

perature changes. must be taken into account by installing ofscts o r expansion loops.

3. MULTIPLE-CACE MOUNTING Large ranges of level preferably are observed by

mounting overlapping gage columns on a standpipe as shown in Fig. 2-2 ( B ) . Automatic gage cocks M in. in size generally are used on multiple gages. Many refiners have found that the maintenance required on the ball checks of automatic gage cocks is so great that they prefer to use individual block valves and pipe tees. Both types of installations are shown in Fig. 2-2(B) . Vent and drain valves should be installed as illustrated.

Interface observation requires the use of transparent gage glasses. Fig. 2-3 shows two commonly used and recommended methods of mounting multiple gages on horizontal vessels where both liquid-liquid and liquid- vapor interfaces are to be observed. Connections to the vessel must be arranged so that there is always one in each phase of each interface being measured. Also. it is preferable to overlap multiple gages where possible.

4. PROTECTION AGAINST ETCHING On transparent gage columns to be installed where

the liquid o r vapor will attack ,olas-e.g.. on steam drums or in applications involving hydrofluoric acid. amines. or caustic-a thin sheet of mica. Teîion. Kel-F. or other material which will withstand attack is sometimes inserted between each piece or between the gage glass and gage gasket to prevent etching of the glass.

ALTERNATE

F

SlGnT GAGES -.

3/4"OR LARGER 1 / BLOCK VALVES-/

(GATE) I LIOUID-VAPOR INTERFACE ----_-

-- - _ _ _ _ _ _ _ SIGHT

I

34



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL ~ . ~ . . ~ - . .

It should be noted that bunlight will discolor some plastics. thcrcforc care should be used in selecting the material for thc hhicld. Such shields cannot be L I S C ~ in rellex gages as they will rcnder the prisms ineîfectivc.

c. Float-antl-Calde (Autoinatic) Tank Gages

This type of gaging is the most common method of indirect level indication. Float-and-cable tank gages are used primarily on large storage tanks. The types of cable and tape, floats and guides are varied, and the indicating or transmitting devices are even more varied between gages iiianufactured by different niakers. Thus. each installation presents its own individual problems. These gages should be installed by the manufacturer or in strict accordance with his recommendations.

The reliability and continuing accuracy of a tank gage installation is directly dependent upon the condition of the tank on which it is installed. Old and in- correctly erected tanks-particularly those with unstable bottoms: shells, or roofs-will introduce appreciable amounts of error and variation which no gage. however carefully installed, can correct. In general, the following practices and precautions apply to all types of tank gage installations: 1. Automatic tank gases should be located in close proximity to the gaging hatch yet sufficiently distant from the suction and filling lines to minimize the disturbing effects of eddies, currents, or turbulence arising from these sources. 2 . Either the ground-level or tank-top readins device should be at a convenient height or distance from the ground or the gaging platform to assure easy and correct readings, thus avoiding errors. 3. The entry point of the automatic sage tape-should be such as to eliminate errors caused by roof movement. 4. Where turbulence-caused by high emptying and high filling rates or mechanical agitators-can affect the float or sensing element. i t is usually necessary to enclose the measuring element in a stilling well. Where high-viscosity materials are encountered. it may be desirable to provide heating for the stilling well. 5. All gages must be mounted securely to the tank shell, with a sufficient nuniber of briickets properly attached and adequately spaced to hold the gage rigidly in place and in proper alignment at all points. The top horizontal tape conduit íextcnsion arm) must be braced by support members from the top angle only. 6. N o floating-roof gage installation should have any tape exposed outside the tail pipe, as this can cause errors because of wind drift. 7 . Float guide wires should be installed plumb. properly centcred. free of kinks or twists, and pulled taut under proper spring tension. 8. Tank gage units of this type are sometimes used in conjunction with a remote indicating gaging system; where maximum accuracy is required. gravity-compensating dcvices are available and should be used.

9. Some of the practices mentioned herein :ire outlined in API Standard 2500: Measiiririg, Striiipliiig, aiici Test- ing Crude Oil, which covers the installation and use of automatic tank gages; for further details reference should be made to this publication.

d. Hydrostatic Head Pressure Gages

1. APPLICATIONS A N D LIMITATIONS Level indication by this means is limited to tanks or

vessels not under pressure. The height of a liquid above a pressure gage can be inferred from the pressure gage reading (hydrostatic head). provided the density of the liquid is known. However. where specific gravity changes are large. this type of level indicator will be highly in- accurate if read under one condition of calibration.

2. INSTALLATION Instruments used for reading head pressure are stand-

ard pressure instruments of relatively low range and should be installed in accordance with the recommendations outlined in Sect. 4. Pressure sage arrangements. illustrated in Fig. 3-4. include the direct hydrostatic head type, see arrangement -A: the diaphragm and the bell (trapped air) types. see arrangcment B: and the air bubbler system with either remote o r local gage. \ee arrangement C.

3. PRECAUTION Great care must be taken to prevent dirt. scale. or

sediment from entering the lead lines or tubing. as these instruments ordinarily have small (?,i in. or ?4 in.) process connections and are easily plugged.

e. Differential Pressure Level Iiitlirntors

For level indication alone. differential pressure instrumcnts arc seldom used except as transmitters. Where they arc used. however, the installation of this type of instrument is the same as the installation of the differential pressure transmitter for level transmission. see Par. 2 .4(d) .

f. ,Miscellaneous Gages

Devices otner than the aforementioned are sometimes used to detect level in certain special cases. Among the more common devices are: I . Frost plugs, shown in Fig. 2-5iA). are used to detect the level of butane and other liquids which will boil in an ordinary gage glass. With sufficient humidity in the air, frost will form on all plugs which are below the surface level of the liquid. 1. RLim’s horns. shown in Fig. 2-5tBj. are used especially in dirty, waxy, heavy black or coking oil services which are too severe for most types of float instruments. The large-size, nonclogging, curved pipe permits the product to flow up through it into the

35



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

,,PRESSURE GAGE FOR DIRECT HYDROSTATIC HEAD

VESSEL

ALEkLATE

GAGE COCK

? A

R

DIAPHRAGM BELL-TYPE

B TRAPPED AIR SYSTEM BOX-YEAD TYPE

PRESSURE GAGE(LOCAL.REMOTE .OR BOTH) ,-REGULATOR

)CORIFICE OR ALTERNATE, SIGHT FEED BUBBLER

__

c - BUBBLE TYPE r L

FIC. 2-L-ll~<Iroaiaiic. I l t ~ ~ c i I'rcasiire Gage Arrangements.

straight horizontal pipe, where it will cover a thermocouple and indicate by a temperature change that the level is at that point. These units are placed one above the other on ;i vessel at desired points where the liquid level is to be checked.

2.4 LEVEL TRiiNS3IITTERS Transmitters include pneumatic and electrical systems

which have measuring elements of the displacer, baii Hoat. differential pressure, and hydrostatic head types. Some transmitters are equipped with dual pilots, one with an adjustable throttling range for control and one with a fixed band for transmission of level indication. In all cases, the transmitter should be located so as to be visible from the control valve whenever possible; the transmission circuits should be installed as outlined in Sect. 7.

a. Dia placer Transmitters 1. TYPES AND FUNCTIONS

Displacer transmitters may be either blind or of the local-indicating type. They may be used as transmitters or as locally mounted controllers.

2. LIMITATIONS Because the displacer itself has relatively little motion,

its use should be avoided when heavy black, waxy oils

SECTION

.a A

COLUMN SHELL / 4 " S C H 80 PIPE i

FIG. 2-5-Miscellaneous Gages : Frort Plug ( A ) : Ram's IIorii ( B ) .

or (especially) coking oils are encountered. or where material will settle out on the displacer or in the chamber. When it is necessary or perhaps desirable to use a displacer in such service. a liquid purge should be used.

3. MOUNTING O F EXTERNAL CAGE DISPLACERS ON VESSELS

For external cage displacer installations, connections to vessels should be made by means of nozzles, block valves, and pipe fittings selected for the service. Trans- mitters and controllers usually are piped with gage columns in parallel as shown in Fig. 2-6 through 2-5. Occasionally, however, it is advantageous to have an additional set of taps on the vessel for independent indication of level.

4. CONNECTIONS TO VESSELS

When screwed connections are permitted. the nozzles and piping may be 1% in. with unions placed as shown in Fig. 2-6. In most process applications, however, and especially where viscous fluids are involved, level transmitters and controllers should have 2-in. flanged connections; vessel nozzles and piping from nozzles to controllers should be 2 in. with flanged connections. Drain valves (gates) % in. in size always should be provided and if a vent or vents are required or desired, they should be gate valves % in. or 95 in. in size installed as

36



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL ___

NOZZLE-SIZE TEE

CONNECTIONS

AIR S!GNAL VNIONS WhEN

CC"NECT;ONS

VOZZLE-SIZE 3 k X K V I L .'E

-4- ~ " q U T 0 V A P C ,' G G E 13C6 SR TEE TOP AND BOTTOM

CONNECTIONS

SIDE AND BOTTOM CONNECTIONS

A

3

SIDE AND SIDE CONNECTIONS

S

Y i I

I !

TOP AND SIDE CONNECTIONS

FIG. %-CiEsterna1 Cape Displacer wiih Parallel Cage Glass.

indicated in Fig. 2-6. (Some companies permit the use of %-in. gate valves in both these locations. particularly in clean services.)

5. MOUNTING OF ESTERNAL CAGE DISPLACER AND STANDPIPE

Where greater ranges of level are encountercd. e.& over 4 ft or 5 ft. a standpipe and overlapping gage glasses may be used as shown in Fig. 7-7 and 2-8. The standpipe, usually of 2-in. or 3-in. pipe, serves as a mechanical support for the instruments and as a surge chamber to prevent turbulence or foam from interfering with the operation of the transmitter. In addition, the arrangement in Fig. 2-8 permits direct calibration of zero and span of a transmitter or controller (with the vessel either in or out of service) by properly manipulat- ing the block, drain, and vent valves in such a way as to run the level of the fluid up and down in the gage column and transmitter in parallel. In cases where levels of considerable range are to be transmitted, it may be

I ~ C G 2 , ' x :''SWAGED N~PPL:

TYPICAL INSTALLATION ,'S!NG Ai ÏEGNATE - WIT- DUAL BLOCK VALVES ANS ONE SÏANOP:CE (WELDEQ -AGE. ; V l Ï + W E L ~ E D E-SC'N ;_LaOK Ï :"E ) =CR CONSTRUCTION S J ' E ~ L A P P I I I G GAGES

ivorc,s: i . Some companies require the third block valve ( a t the

nozzle) in this type of assembly. 2 . Controller may be piped nith side and bottom. side and

side. or top and side connections. as shown in Fig. 1-6. 3. Nozzle spacing on the vessel is critical on close-coupled

installations, especially where side and side connections are used. because of differential expansion of vessel and controller. Double or reverse elbow connections are sometimes used on the upper side connection to minimize trouble from this source.

FIG. Z-T-Esternal Cage Displacer with Parallel (;age or Standpipe.

preferable to use a differential pressure transmitter, see (d) following.

6 . PURGING In some installations, for example on crude oil unit

steam strippers where condensing steam can drip into hot oil in the displacer cage, it is sometimes necessary to purge the top of the displacer cage with gas. Purging installations are described in Sect. 8.

7. INTERNAL DISPLACERS Occasionally, and particularly when it is desired to

avoid steam tracing, it is preferable to mount the displacer inside the vessel rather than in an outside cage. Whcn mounted on top of a vessel, the vessel nozzle and the head casting of the instrument must be providcd with mating flanges of the type and specification required by the service.

Ample overhead clearance must be provided for removal of the float and rod. When a side mounting is required, the vessel nozzle and the torque-tube housing

37



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

PRESSURE VESSEL

$'GATE VALVE (VENT) - 2"OR LARGER NOZZLE

OR PLUG

2"OR 3' STANDPIPE AUTOMATIC GAGE C O W

OR TEES

OVERLAPPING GAGE GLASSES

GATE VALVE

:IC 2 - 7

:Vow: Controller may be piped with side and bottom or side and side connections as hhown in Fig. 1-6.

Fi(;. 2 - L S t n n d p i p e wi th Extenial Cage Dispiacer (hiiiro1lt-r :irid Miiltiple Sight Cages.

of the instrument should be provided with the proper type of mating Hanses and provision should be made t'or ~ C C C S S to the float-torque-tube connection, e.g.. a manhole.

8. INTERNAL DISPLACER GUIDES In many internal displacer installations, guides are

rcquircd. A stilling wcll for side-mounted displacers, as shown in Fig. 7-9, usually is provided for this purpose. although rod or ring guides are sometimes used. Ring guides are particularly suitable for emulsion service.

9. SIGNAL TRANSMISSION

Where the signal is transmitted to a remote controller or board-mounted instrument. the transmission should be acconiplished as outlined in Sect. 7. When the displacer instrument. mountcd in any of the foregoing ways. is of thc elcctrical type. as for alarms or protective devices, it should be piped as described herein. The electrical wiring should conform to the electrical code applicablc; sce. also, Scct. 13.

ID. Cageal Ball Float ïraiisrniîíers

This type of transmitter is most generally used in clean services for the direct operation of valves or electrical switches for alarms or pump motor controls. Where they are installcd directly on vessels, connections

PIPE,

WELDED TO SHELL

WELDEDA TO PIPE

l I I I I

I I l I

l I I

H HOL ES

BLIND

I ALTERNATE 1 L------J

I$' WIDE SLOT FOR 48': ea''

DESIGN

AND UP RANGE STILLING WELL

I"WIDE .SLOT (FULL LENGTH ) H)R UP TO 32" RANGE STILLING WELL

FIG. 2-9-Typical Stilling Weil.

should be made as described for the installation of displacer transmitters in ( a ) preceding.

c. Internal Ball Float Transmitters

1. APPLICATION This type of instrument is sometimes used for heavy

black, waxy or coking oil service, or where the liquid contains particles or materials which tend to settle out and which would eventually block the float action in an external cage type of instrument. On severe coking applications. it may be desirable to use a steam or flushing oil purge to keep the shaft free and the packing in suitable condition. The trend in such applications is toward the use of dip-tube. purge-type. or differential-pressure type level transmitters and controllers, where possible.

2. INSTALLATION Where an internal ball float is considered necessary.

either the rotary-shaft type (Fig. 2-10) or the 10-in. ilange-mounted type (Fig. 2-1 1 ) may be used. Where the Hoat will be subjected to turbulence within the vessel. shielding, guiding. or other provision should be made to climinate the effects of turbulence on the float. Pneumatic piping or electrical wiring to such transmitters should be in accordance with the recommended practices for transmission as outlined in Sect. 7.

3. SUPPLEMENTAL INDICATOR í n severe services, as outlincd in the preceding para-

graph, it is recommended that the transmitter or con-

38



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL

NOTE. , A *,, THIS TYPE CF iNSÏALLA-

: TION SHOULD NOT BE ~ USED FOR VESSELS WITH I AN INSIDE DIAMETER OF 1 LESS THAN 4'6"

PLAN VIEW FIG. 2-10-Installation o f Internai Rotary Shaft Bal1 Float

Level Iii$trunieiit in Vertic:al Versel .

troller be supplemented by another type of instrument to serve as a check indicator: e.g.. a hydrostatic head instrument. a through-vision gage column. or a set of try cocks or ram's horns as described in Par. 3.3íf-2).

(l. Differential Pressure Transmitters

There are three commonly used types of differential pressure transmitters-force balance. motion balance

I I- 4 0 " M A X

PLAN VIEW

*ANGLEOF FLOAT T~AVEL-, ,--,

\ . .\ L-q- ,-Y .\. -.'

VESSEL WALL-/-

ELEVATION VIEW *. WITH PILOT VALVE: 30° MAX '.-_'

FIG. 2-11-InsiaIIaiioii of Intcriial Floai F i ~ ~ i i ~ ~ ~ - ~ ~ c ~ ~ ~ ~ i ~ ~ ~ i - T y p . L e v d Iiirtriitiicnt.

39

(bellows and diaphragm ) . Lind mercury. In ceneral. differential pressure transmitters for level transmission will be most accurate where the mcasured fluid is of fairly constant specific gravity.

1. FORCE-BALANCE TRANSMITTERS

Applicarions of force-balance transmitters include local control. remote control. and remote recording of wide ranges of liquid level. Such instruments are commonly known as differential pressure (DP) cells. or converters.

Connections to the vessel may be made by means of pipe fittings of the material and rating recommended for the service. or by means of '/?-in. tubing and tubing fittings. The transmitter should not depend upon its own piping for support, but should be yoke-mounted or bracket-mounted. Typical installations are shown in Fig. 2-12, views (A) and (F) .

Constant head may be maintained on the external or reference leg of the transmitter when condensables are present by means of a seal pot. as shown in Fis. 2-12(B).

Temperature compeiisatioii may be accomplished automatically and a constant head maintained by the method shown in Fig. 3-13. There are numerous other methods of heating or cooling to keep the reference leg at the same temperature as the vessel liquid; however, because they are not generally used, they are not described herein.

2. MOTION-BALANCE TRASSMITTERS

Appiicaiiotis are generally the same as for force- balance transmitters. They normally provide local indication independent of the transmitter mechanism.

Connecrions to the vessel may be made in the same manner as for force-balance transmitters. However, because of the liquid volume displacement. jnstallation details as shown in Fig. 2-12(C) and (D) should be followed.

Constant l i e d may be maintained on the reference leg of the transmitter when condensable are present by means of a constant-head pot as shown in Fig. 3-12 (B) .

Tempercitirre competisution may be accomplished by the method shown in Fig. 2-1 3 with the addition of a constant-head pot.

3. MERCURY

Mercury manonieters are not extensively used for Icvel transmission. their placc having been taken by the forcc- and motion-balancc transmitters. Howevcr. where they arc used. the installation is the same as for the othcr typcs. cxccpt that the mercury manometer requires scal pots and a different and more careful wcathcrprooting trcritment.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

STANDARD TYPE OF INSTALLATION FOR FORCE-BALANCE I TRANSMITTERS

~r STANDARDTYPE OF INSTALLATION. WITH CONSTANT HEAD POT, FOR MERCURY OR

TRANSMITTERS MOTION-BALANCE 9 % UBING

TUBING(


/ T PLUG* PLUG 4 \ USED FOR CALIBRATING MA-

ANOTHER TYPE NOMETER OR TRANSMITTER AND FOR FILLINGSYSTEM

I

,WITH SEALING FLUID

- i

"*'"i

\TRANSMITTER I WHERE NECESSAR" 70 CHECKZEROANOSP4NOF TRANSMITTER WHILE IN SERVICE, THIS CHECK BYPASS

+ ARRANGEMENT i5 ,g SOMETIMES USED - SEAL POT ;NSTALLATION i O R MERCURY ANDMCTION- BALANCE TRANSMITTERS

-915 SAME OUXPOSE )

PIPE OR TUBING IF PIPE IS USED,

I I 1 MA;~F;LD 1 TRANSMITTER

C D OiFFERENTIAL PRESSURE TRANSMITTER LOCATE0 ON PLATFORM ABOVE TOP CONNECTION)

NEEDLEVALVES CHECK

PURGE GA 5

J HEAOER-

. .

VESSEL

LEVEL MEASUREMENT WITH DIFFERENTIAL PRESSURE -- INSTRUMENTS APPLYING PURGE GAS

LEVEL MEASUREMENT WITH DIFFERENTIAL PRESSURE -- INSTRUMENTS APPLYING PURGE GAS

FIG. 2-12-Typicul Iiistallaiioiis of DiKereniiai Presi'iiirv L6vt.l Instrunients.

V E S S E L

I CUP ADDED WHEN USED WITH MOTION-BALANCE TRANSMITTERS

INTERNAL PIPE

EXTERNAL PIPE

1

l I ' FIG. 2-13-Differc.riti:il Lcvc~i Arrangement to

Compensate for Teniperature.

e. Hydrostatic Head Transmitters

1. INSTALLATION Hydrostatic head may be transmitted by means of u

bubbler tube and differential pressure transmitter as shown in Fig. 2-14(.A) and (B) , or by means of a diaphragm- or bellows-actuated air pilot or differential- pressure-type transmitter mounted directly on the vessel as shown in Fig. 2-14(C). The latter.type should be mounted on a flanged nozzle at such a point that it will not be subject to blocking by sediment. (It should be pointed out that some makes of the diaphragm- or bellows-actuated pneumatic pilot are nonlinear in the lower 20 percent of their range.)

3. PRECAUTIONS Bubbler tubes must be sized to prevent pressure-drop

errors. which result from purge gas flow. They must be installed so that sediment cannot block the open ends, and must be supported, if necessary, so that turbulence or mechanical strains cannot bend or break them. In addition, for greatest accuracy the connecting leads must be leakproof.

f. Electric and Electronic Level Transniitters

Scveral types of electric and electronic level transmitters are available, some of which are actuated by floats, others by hydrostatic head or differential pressure. In all cases, the sensing device is installed in

40



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL

,--ORIFICE UNION

':;:'-i' ,) lh 7

I' '

SAME AS A EXCEPT AS

USED ON PRESSURE

VESSEL

I L Caged aiitl Internal Bail Flout Controllers

1. P N E U M A T I C AND ELECTRIC TYPES Installation is the same as outlined for the equivalent

types of transmitters in Par 2 .4íb) and ( c ) .

2. MECHANICAL TYPES These are most generally used in water service and

consist of a mechanically actuated valve connected by a shaft or lever linkage to either an external caged float or an internal float. Installation of the float mechanism is the same as that for a pneumatic or electric ball float instrument. Care must be taken to see that nothing blocks the action of the float and that the float is protected from turbulence. Furthermore. the valve and the piping must be installed and supported so that there is no strain on the valve or packing gland and no interference with linkages or levers which might prevent full travel of the float and valve.

c. Differential Pressiire Controllers TANK OR 1

VESSEL UNDER

PRESSURE

DIAPHRAGM OR BELLOWS ACTUATED PNEUMATIC PILOT. OR FLANGE- MOUNTED DIFFERENTIAL VESSEL FEESSURE TDANSMIT lEQ

SAME AS c EXCEPT AS USE0 ON PRESSURE

1. APPLICXTION These arc basically the same ils transmitters and.

without accessory devices. are 100-percent proportional band i nonadjustable ) controllers. The 100-percent proportional band unit can be used together with a valve positioner to attain a system which can be applied as an adjustable proportional band controller or an on-off controller. This unit gives a wide latitude of

FIG. 2- lkHyrlrostat ie Head and Differential Pressure ad jus tmen t.

2. INSTALLATION Levei Transmitters.

accordance with the practices outlined in preceding paragraphs. and the transmission of the signal is accom- dished as described in Sect. 7.

The installation is basically the same as for equivalent types of transmitters. see Par. 2.4(d-1. - 2 ) .

Transmitters or transducers for electronic instruments should not be located too close to hot lines. vessels. or t i . Direct Esparision Controllers

other equipment. Ambient temperatures which exceed 1. a ~ p PLICATION 140 F are likely to result in calibration dificulties and rapid deterioration of electronic components. Suscepti- bility of mechanical or electronic components to vibra-

ments should be made in the mounting.

Direct expansion controllers are used on steam gen- erators or in applications where a single boiling-point

or thermostat is different in the liquid and vapor regions. Cenerallv the expansion of the tube operates either a

tion should be ascertained and, where necessary, adjust- liquid is being vaporized so that expansion of the tube

2.5 LOC-ALLY MOUKTED CONTROLLERS

Locally mounted controllers used on all pressure vessels include the following types: displacer, caged ball float, internal ball float, differential pressure, and direct expansion. The altitude vaIve or other static-head type of locally mounted controller is used for vessels or tanks not under pressurc.

a. Displace-r Controllers

Recommended practices for the installation of displacer controllcrs are the same as for equivalent types of transmittcrs outlined in Par. 2.4( a ) .

41

control pilot or i valve directly to control the flow of Huid into the vessel.

7. INSTALLATION Care must be exercised in installation to assure free-

dom of motion of the free end of the thermostat tube. The tube musr not be painted. If mounted where it is exposed to the weather. it must be houscd or shielded to protect it from rain and snow.

Piping leads to the expansion tube should be weil insulated. Fig. 2-15 shows a typical arrangement of such a controller opcrating a direct-connected valve in the feedwater line to a boiler.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

NORMAL WATER LEVEL- w \ /

,IN SOME INSTALLATIONS, A DIRECT-CONNECTED VALVE IS MOUNTED HERE. IN OTHERS. THIS IS A PILOT VALVE

MOUNTED ON SIDE OF BOILER

& ,STEAM \

EXPANSION TUBE OR THERMOSTAT-”

TENSION RELIEF LINK

DRAIN

TED VALVE

c- FEEDWATER

=IN SOME INSTALLATIONS THIS IS A DIFFERENTIALLY LINKED VALVE POSITIONED BY BOTH BOILER WATER LEVEL AND STEAM FLOW

FI(;. 2-15-Espaiision Tube Syteni.

e. -4ltitiicle Valve (Static-Head Type) Controller

A standard piping arrangement for an altitude valve type of controller is shown in Fig. 2-16. A self-contained. self-rcgulating level controller of this type may be mounted at almost any distance from the (nonpressurized) vessel or tank provided the pilot valve is considerably lower than the lowest desired level of the liquid. Because the line fluid is the actuating medium, the altitude valve senerally is used in water service.

I-- I - -

2 .U REBIOTE OR BOARD-MOUNTED RECEIVERS

Although level is sometimes recorded by transmitting to a second pen on a flowmeter or pressure recorder, it may also be indicated, recorded. and/or controlled by individual level instruments actuated by transmitted. signals. Receivers may be either pneumatic or electric.

a. Installation

Recommended practices for the installation of remote or board-mounted receivers may be found in Sect. 4’ 5. 7, and 11. Design of the installation should be such that a high level causes the pointer or pen to move upscale or toward the outside of round charts. (Instruments which read in the reverse of normal are apt to cause confusion and be misread, particularly during upset conditions when it is most important that they be read easily, quickly. and correctly.)

b. Ranse

ments is O to 100. representing percent of maximum. Recommended scale or chart range for level instru-

2.1 LEVEL ALARMS

Basic instruments for initiating high-level or low- level alarm signals are, with the possible exception of the float size, the same as the float-type controllers discussed in Par. 2.5 ( b ) . Other types are sometimes used. e.g., pressure switches at the receiver in pneumatic transmission systems, hydrostatic head pressure-actuated switches on nonpressurized tanks, and differential pressure-actuated switches on pressurized vessels. (For a detailed discussion of alarms and protective devices. see Sect. 13.)

a. Installation of Float Alarms

The installation of float alarms is the same as for the equivalent types of transmitters covered in Par. 2.4( a ) , (b ) , and (c). A typical installation of high-level and low-level alarm switches with parallel gage glass is shown in Fig. 2-17.

II. Iiistaliation of Other Alarms

Pressure switches in pneumatic transmission circuits are installed without block valves. A sensitive pressure actuated switch or differential pressure-actuated switch mounted directly on a tank or vessel to signal high- or low-hydrostatic head should be located at a point on the tank or vessel which is not subject to blocking by sediment.

c. S i p a l Traiisiiiissioii

Installation practices are discussed in Sect. 7 and 13.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

LEVEL

ARRANGEMENT FOR ALARM SWITCHES /

!I r( 1 - ELECTRICAL ~,Hl"NOZZLE OR BOSS

CONNECTIONS ON VESSEL

+"AUTOMATIC GAGE COCKS

I l nFF+FT TYPF

1 - ELECTRICAL ~,Hl"NOZZLE OR BOSS CONNECTIONS ON VESSEL

+"AUTOMATIC GAGE COCKS OFFSET TYPE

HLI tKNWI t ARRANGEMENT

FOR ALARM C W ITC HE S !I "1%

PURGE OR TEST CONNECTION

FIG. 2-17-Arraiigenieiit of High- and Low-Level Alarm Switches with Parallel Gage Glass.

2.8 ACCESSORIES

;I. Seals and Purges

Occasionally it is necessary to use seal pots or purges in connection with liquid level instruments. The application of seals and purges is discussed in Sect. 8.

b. Gage Glass Illuminators

Where it is necessary to back-illuminate transparent gage columns. it is recommended that lighting fittings, made for the purpose and suitable for the service conditions. be purchased and installed in accordance with applicable codes and the manufacturer's recommendations.

c. Weather Protection

1. GENERAL All locally mounted instruments and lead lines

handling water or process fluids which may freeze. form hydrates. or become excessively viscous in cold weather should be heated and insulated. In addition, transmitters and locally mounted instruments other than gage glasses should be suitably protected by housings or other protective shielding to prevent improper instrument performance or excessive maintenance as caused by the effects of weather. Frost shields should be used on transparent and reflex gage glasses if the operating temperatures are below 32 F. Heated gage glasses and jacketed gage cocks (see Fig. 3-18) are available from some manufacturers and arc used in

JACKETED VALVE\

4 A'' ?IFE+

.-. \ \

Y

A

I " 2 ÏUBING

DETAIL

HEATING f TUBE

CROSS-SECTION (REFLEA GAGE)

many applications which require heating of gage glasses. They should be installed as shown in the view depicted by Fig. 2- I 8.

3. STEAM TRACING Steam tracing is commonly used for protection of

both instruments and lead lines. A correctly installed steam-tracing system must have an individual shutoff valve and a trap on each individual lead. Where the process fluid in the lines or instruments being steam- traced has a boiling point lower than the steam temperature. care must be taken to separate or insulate the steam tracer to prevent the possibility of causing the fluid to boil; see Sect. 8, Par. 8 .4(b) .

3. OTHER METHODS OF HEATING In some climates it is satisfactory to use steam con-

densate for tracing. In isolated cases, particulariy in nonhazardous areas, electrical heaters are sometimes used to heat gage glasses. instrument cases, and short lead lines.

4. WINTERIZING For complcte coverage of steam-tracing practices,

seals and purges. and winterizing in gencral, refer to Sect. 8.

43



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CDPY PROVIDED FOR . tIISTORICAL PUWPOSES ONLY

SECTION 3-TEMPERATURE

3.1 CONTENT

This section presents common practices for installation of devices for measuring temperature in refinery process services and for: 1. Indicating the temperature at the point of measurement. 2. Utilizing the temperature for local control of the process variable. 3. Transmitting the temperature to a remote location for indication or control at that point.

Included in the discussion are the more common types of measuring devices: thermowells, glass and dial thermometers. filled-system instruments, thermocouples. and resistancc thermometers. Self-acting temperature controllers are also included insofar as the temperature actuation i s concerned. The installation of automatic control equipment is discussed in Sect. 5 . Only brief mention is made of methoas for the transmission of tcmpcrriturc mcasiircmcnts. Transmission systems are discussed in Scct. 7.

3.2 ‘I“ERJl0B;ELLS

il. d;c~iic*ral

It is not usually possible to expose the temperature- sensing device to the process fluid. In spite of the thermal lag introduced. thermowells (see Fig. 3-1 through 3-7 1 arc rmploycd in temperature measurement to protcct thc thermal elcments and to permit removal of thcsc elements during plant operation. It is important to maintain good contact betwcen all temperature-\cnsing clcmcnts and their wells.

1,. Tlieriiiowell Insertion Leiiptii The insertion length. U. is the distance from the free

cnd o f the tempcrature-sensing element or well to, but not including. the cxternal threads or other means of attachment to a vesscl or pipe (see Fig. 3-1 ) .

c. Tlieriiiowell Immersion Length

The immersion length. R, is the distance from the free end of the temperature-sensing element or well to the point of immersion in the medium. the temperature of which is being measured; see Fig. 3-1. (Normally, this point would coincide with the inner wall of the vessel or pipe.) The immersion length required to obtain optimum accuracy and response time is a function of mechanical factors such as: type of sensing element. available space. well-to-fluid container mechanical connection design. and well strength requirement. Optimum immersion depth also depends upon heat transfer considerations as detcrmined by the physical properties of the measured Huid. flowing velocity, temperature difference betwcen fluid measured and well head. and material and mass distribution of the well and the sensing element. 4 detailed review of these factors rarely ever is justified for petroleum rcfinery distillation units. Com- mon considcrations arc :

I . The cntire heat-sensitive length of a bulb (whether a bimetallic element: gas-. liquid-. or vapor-filled bulb: or rcsistancc thermometer element) must be immersed in the heat zone to be measured. 2. For thermocouples: “Ten times the outside diameter of the protecting tube is the recommended minimum immersion; this value should be increased where space permits. With flowing liquids, six diameters immersion may be used if the pipe and the external portion of the protecting tube are well insulated.” I L 3. On small lines. where adequate immersion cannot be obtained by a thermowell inserted perpendicular to the line, the usual practice is to insert the well at a 90-deg bend in the line; a less preferred method would be to enlarge a short section of the line to accommodate the thermoweil. These practices are seldom required on lines 4 in. or larger in size. Typical installations 3re shown in Fig. 3-5.

CI. Thermowell Materials

The materials selected must be suitable for the temperature and corrosion environment encountered. For general services, up to 1,200 F, the minimum quality material usually specified is Type 304 or Type 316 stainless steel. Thermowells in certain corrosive services, such as dilute acids, chlorides, and heavy organic acids, require well materials suitable for the specific corrosive media.

e. Tlierniowell Construction

Typical thermoweil construction and installation details arc shown in Fig. 3-1 through 3-7. Thermowells may be screwed as shown in Fig. 3-2 and 3-6. How- ever, where frequent inspection, special materials (e.g.,

:I Figures refer to REFERENCES on p. 50.

44



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TEMPERATURE

Í

i j

I 1 u

1. Mercury in Glass “Industrial’’ Thermometers

These instruments use mercury or other liquid of low freezing point; they are mounted in metal frames



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and usually arc provided with glass covers. The ther- 3.5 FILLED-SYSTEM TE3IPER-ATURE IKSTRU- niorneter bulb is inserted in the thermowell. A desiccant sometimes is used inside the case to prevent condensation on the glass. Glass thermometers generally have u* been superseded by dial thermometers. see (c), because of the quick readability, resistance to breakage, and other merits of the dial-type construction.

3IENTS

This type of instrument consists of a temperature- sensing bulb. a capillary tube, and a pressure-sensing device. Filled systems utilize a liquid. gas. or vapor.

C . Dial '~11c~riiioiiiett~i.c;

These are the most common thermometers in industrial LISC. They are frequently of the bimetallic type with circular dials and are available in a wide range of temperature scales and styles. Dial-type thermometers which usc filled systems are also available (sec Par. 3 .4) .

$+<- THERMOWECL --I!

- TEMPERATURE 8 u L e IBULB-TYPE TEMPERATURE RECORDER OR CONTROLLER, !NIXSTRIAL THERMOMETER, DIAL THERMOMETER. ETC)

depending upon the requirements and temperatiire range of the systcm. Ambient temperature conpensation is often requircd. Available overrange protection varies with diffcrcnt types and may also influence the selection. Where practicable. an overrange of at least 50 percent is desirable. The usc of filled-system devices is limited by the len-th of capillary tubing which may be employed and by the maximum temperature to which the bulb may be exposed.

,450 I V STANDARD I'IPE

THQEAD(U8UAL 3i4" 3 R 1"DEPENDING ON BULB SIZE

& i - 8 1 " .-.___-. IPS THREADED CONNECTION

I !

2 ° F STRAIGHT TAPER- 6''--_ BEVEL FOR VIELZ"UG

MACHINED TIP

,.-VESSEL WALL

,Z"SCHEDULE l/a"THICH STEEL RETAINING RING -

L

2"WELD NECK FLANGE TO VESSEL SPECIFICATION .'

INSTALLATION NOTE :

VALVE PROVIDED TO SHEAR THERMOCOUPLE AND SHU' C-F L E A K I P I EVENT OF THERMOWELL FAILIJRF



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

TEMP E R A TURE

Special consideration shouid bc given to the installation o f narrow-span. force-balance temperature transmitters with uncornpensatcd capillary. so that c:ipillary length is held to a mininiuni. Normally, the manufacturer's standard is 5 ft: on outdoor installations. it is advisable that capillary length not exceed 10 ft .

li. Self-Acting Teiiiperatiire Controllers

Where precise control is not essential. self-acting temperature controllers are used frequently. These devices utilize thermal expansion systems and direct-operated valvcs. In operation. an increase in temperature expands the liquid in the system and thereby operates the valve. Because of the many different fluids being uscd. bulb sizes and filling fluids vary with the temperature range. As with other temperature-sensing instruments. bulbs should be protected by thermowells.

Valve operators are in bellows form. The bellows may operate either a valve (directl!~) or a pilot valve which controls line Ruid for actuating power to operate the main valve. Temperature indication in the form of ;?

dial mounted on the top of the valve and operated by the same thermal system is available from some manufacturers. Some form of temperature indication is a1- uays desirable with these self-contained devices.

As with all capillary types of thermal systems. care must be taken to protect the bulb and capillary from any damage which may cause fluid leaks or impede the flow of fluid. Also, it is desirable that the installation be made so that the valve can be removed for inspection or service without damage to the capillary.

c'. Tem pera tu re Traiisiiii t t ers e Temperature transmitters may utilize any one of sev-

eral typcs of filled systems. together with pncumatic or electronic transmitting and amplifying devices. to con- vert the measured temperature into an air or elcctrical signal. In addition to covering a wide rangc of tcm- peratures. some instruments can be obtained with the following additional features:

1. SUPPRESSED RANCE A very accurate measurement of temperaturc can be

obtained by selecting a transmitter with full range of air pressure output over only a portion of thc opcrriting temperature limits of the instrument.

2. THERMAL L I \ C COMPENS.ATION

In some instruments i t is possible to obtain a pneumatic device (derivative action directly appiicd in the measuring system ) which. when properly adjusted. will c o m s n s a t c for thermal lag with a resultant high speed of response. Some companics usc this compensation for transmitters associated with controllers but not for transmitters for recording instruments alone.

t

(1.

In all installations of filled-system temperature instruments. it is necessary to protect the bulb and capillary tubing from mechanical damage. I t is usually desirable to use armored capillary tubing and to support the tubing run between the bulb and controller or transmitter in such a manner as to protect it from accidental damage. It is essential that the capillary tubing be not cut o r opened in any manner.

3.5 THERMOCOUPLE TEMPERATURE

u. Applications Temperature instruments utilizing thermocouples are

now the most generally applied of all temperature-meas- wing devices. They are applicable for a wide range of temperatures with reasonably good accuracy. Typical installations are shown in Fi-. 3 4 .

1). General

Prern II t i o i i s

IXSTRUNEiVTS

1. MATERIALS AND RANGES In the petroleum industry the most commonly used

thermocouple materials are: Usual

Temperature Range

Thermocouple ISA (Degrees X1a:erials Symbol Fahrenheit)

Iron-consrantan . . . . . . . . . . . . . . J o to 1.700 Chromel-rilumel * . . . < . . . . . . . . K 800 to 2,000 Copper-constantan . . . . . . . . . . . , . T -300to ?O0

Because thermocouples normally are installed in thermowells. the- couple usually is selected for the temperature range and the well material is selected suitable for the measured media. Where thermocouples are installed \I i thout protective thermowells. it is common practice to use iron-constantan in reducing atmospheres and Chromel-Alumel in oxidizing atmospheres.

2 . F.\I~RIC.\TION Fabrication details for thermocouples are covered in

;i publication ,: of the Instrument Society of America ( ISA 1 .

Rcccntly. increased use has been made of metal- shcathed. mineral-insulated thermocouple assemblies. Tlicsc assemblies are made by insulating the thermocouple conductors with a high purity. densely pricked ccrrimic insulation (usually magnesium oxide) and en- closing the assembly in an outer metal sheath. These thermocouples are available with outside diameters ranging from 0.04 in. to 0.84 in. and thermocouple wire sizes from No. 36 gage LIP to No. 8 gage. Outside sheath matcrial is available in a variety of stainless steels, inconcl. monci. titanium. tantalum. platinum, or any workablc metal. Thermocouplcs are available in lengths

:.. Trademark: products of other manufacturers may also be .. .

i iSCd.

47



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API R P 550-PART I

''C TYPE THERMOCOUPLE ~ I" MALE CONDUIT FITTING.> HEAD - ! " % - .. ,UNION 1

OPTIONAL CABLE TO CONDUIT WITH RUBBER BUSHING

~ L I U ~

', vgx 3"STEEL NIPPLE

OPEN DUPLEX WIRE SCREWED WELL, 3/4" MALE THREAD FLANGED WELL, ' /2" FEMALE THREAD

l',MALE ~,, CONDUIT FITTING, HEAD - ! " X 3/4"- \UNION I.,

P I, L TYPE THERMOCOUPLE

!"x REDUCER

STEEL C O N D U I T - - - -3/2 FLEXIBLE

74'' FLEX I B L E ST EEL CONDUIT WITH "TW JACKET(SEE NOTE)

FLEXIBLE CONDUIT SCREWED WELL, "a" MALE THREAD FLANGE0 WELL, /?" FEMALE THREAD

,VVorcr Wherc "TW" jacketed Hexible steel conduit is used. it should he vented to relieve rhe pressure in case of thermocouple well failure.

FI(;. :l-~~-T1i~~rmoc.~~iiI>lc-io-C:oiitliiit Coiineetioiir.

up to -10 ft or longer. Three types of measuring junctions (sec Fig. 3-9) arc :ivailablc: 1. A is the standard construction with grounded tip. welded or silver soldered for fast response. 3. B has ;ln exposed tip for extremely fast response. 3. C has an isolatcd tip (electrically isolated from the sheath) with slower rcsponse.

3. INSTALLATION For field assembly, thermocouples may be connected

as shown in Fig. 3-10. More commonly. however, thermocouples will be purchased ready for installation.

THERMOCOUPLE

SHEATH

/ INSULATION

CROSS-SECTION

FI C.. 3-9-Me t al-Shen thed. Mi neral-Insula ted Thermocouple Assemblies.

U IRON-CONSTANTAN THERMOCOUPLE

WITH GLASS INSULATION

CHROMEL-ALUMEL THERMOCOUPLE WITH RUBBER INSULATED EXTENSION WIRE

1-Make junction at terminal block in thermocouple head. --Terminal box. 3-No. I6 gage iron-constantan (IC) duplex thermocouple ex-

&Selector switch. 5-Instrument. 6-No. i6 gage IC duplex thermocouple extension wire;

asbestos insulated. 7-No. 8 gage TC thermocouple insulated with oval '>-hole

porcelain beads. 8-No. I6 gage IC thermocouple: glass-insulated duplex or as

specified. 9-Make junction of copper or iron to Chromel and constantan

or alloy to Alumel at terminal block in thermocouple head.

1 O-No. 16 _-age duplex thermocouple extension wire: asbestos insulated.

1 I-No. 8 gage Chromel-Alumel thermocouple insulated with round --hole porcelain beads.

12-No. 16 gage copper-constantan or iron-allov duplex thermocouple extension wire: rubber or thermoplastic insulated. Calibrated for Chromel-Alumel thermocouple.

FIG. 3-10-Thermocoupie Wiring Cuniiretioiis.

tension wire: rubber or thermoplastic insulated.

These thermocouples are installed in thermowells as discussed in Par. 3.2. It is essential that a thermocouple be in contact with the well to minimize temperature lags. Metal-sheathed. mineral-insulated couples are sometimes installed as bare elements without wells on special applications as shown in Fig. 3-1 1.

c. Tuhe Temperature Measurement

A special application of thermocouples is the measurement of "skin-point" temperature of furnace tubes. Such installations require careful attention to be certain that the thermocouple is properly attached to the tube and is shielded from furnace radiation. Care must be exercised to avoid adding mass at the point of measurement which may assume a temperature different from that of the relatively cool tube wall to which it is attached. Many companies have their own standards for this ap-

48



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TEMPERATURE -

STUFFING BOX

GATE VALVE

THERMOCOUPLE METAL-SHEATHED, MINERAL- INSULATED THERMOCOUPLE

TUBE FITTING

VESSEL OR PIPE WALL

FIG. 3-1 1-Iiiatalluiioii of Thermocouple' Without %-ells.

plication. These installations have been costly and complex and not entirely reliable. The development of mineral-insulated couples gives promise of simpler installations with greater reliability.

One design for attaching this type of thermocouple to heater tubes is shown in Fig. 3-12(A). The attachment block is ,i in. by ! I I ,; in. by 3 in. in size and is made of the same metal as the heater tubes. Two holes are drilled through the block at an angle of 30 deg for the insertion of the thermocouple. The two holes permit the replacement of the thermocouple at least once without the necessity of welding on a new block. This is important with alloy tubes as the stress-relieving required after welding is both expensive and time-consuming. The thermocouple is inserted in the hole in the block and tightly peened after the block has been welded on the tube. The thermocouple is then held firmly to the tube with stainless steel bands.

Another method of installing metal-sheathed. mineral- insulated thermocouples to heater tubes is shown in Fig. 3-12(8) . In this design, the thermocouple assembly consisting of a thermocouple welded to a small curved stainless steel pad is purchased complete from the vendor. The stainless steel pad with the thermocouple attached is then welded to the heater tube and the thermocouple strapped to the heater tube with stainless steel bands.

CI. Extension Wires Thermocouple extension wires from the thermocouple

head to the remote cold junction and temperature in-

49

f DETAIL OF THERMOCOUPLE

STAINLESS STEEL BAND

A BLOCK

METAL-SHEATHED.

THERMOCOUPLE THERMOCOUPLE PAD MINERAL-INSULATED

I HEATER STAINLESS STEEL BAND

,,,-THERMOCOUPLE WELDED TO PAD

YEATER TUBE

D E T A I L OF THERMOCOUPLE PAD ASSEMBLY

B Arriiiigeiiieiit A shows thermocouple on heater tube. Arraiigeiiient B shows thermocouple on pad.

FIG. 3-12-Iiistallatiori uf Thrriiiucoicples.

strument should be installed as described in Sect. 7. Materials for thermocouple extension wires may be selected as follows: Thermocouple ISA

Iron-constantan . . . . JX Iron-constantan.

Materials Symbol Extension Wire Materiais

( KX Chromel-Alumel for. lowest error.

Chromel-A1umel ' ' WX Iron-cupronel for slightly lower cost and generally satisfactory

j

i applications. Copper-constantan . . TX Copper-constantan.

e. Temperature Iiistruments

Temperature measurements normally will be determined from board-mounted potentiometers, either of the indicating type or recorders and controllers. For convenience in checking and servicing, it is desirable that extension wires be brought to appropriate terminal strips, either outside or inside the instrument, on which the temperature point numbers are indicated.

The following points should be considered in paralleling temperature instruments: 1. The resistance of the thermocouples and extension wires may permit erroneous reading. 2. The interaction between the instruments involved may give temporarily erroneous readings.



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in view of these factors, most companies do not permit paralleling other instruments on the signal circuit of a temperature controller.

Interactions between paralleled instruments (recorders, indicators, lo_ggers) can be reduced to a minimum with the use of high-impedance amplifier instruments. Amplifiers for industrial control applications are available with impedances from 500 ohms to more than 7,000 ohms. However, only those with impedances greater than 5,000 ohms can be considered high-impedance instruments. The higher the impedance, the lesser the interaction. To avoid this interaction. parallel extension wircs inay be run to the temperature point. and connected to: I . Dual thcimiocouples in the sanie well. if permitted by plant practices o r in important service. or 2. Thermocouples in separate \velis at thc sanic location.

f. Rt4ert.iic.n Jiiiic4oiis

The reference junction. sometimes called the cold junction. usually is located in the instrument. In some instances-where especially accurate temperature meas- ureiiients are required. or where the temperature instru- nient is subjccted to varying tcinperatures-the reference junction is external. Also. when a number of very long leads ;ire required. ;I rioncompcnsating cable is uscd and ;I i’cl’crcncc junction compensation device is located :it thc terinination point o f the conventional extension wire. Siich external reference junctions nia! be buriccl to a depth where constant tempcraturc prevails o r thcy inay bc installcd in an enclosure where the tcnipcrature is thcrniostatically controlled. In any cvent. it should bc noted that the accuracy of tcniperature iiieasuremcnt is nu better than the constancy of the rcfer- encc junction temperature o r its conipcnsation in the i nstr uiiicn t.

3.6 RESIST,INCE TIIERMORIETER INSTRC- 31ESTS

A 1’ I’! ¡ C Y 1 t ion u. Resistance thermometers can providc more accurate

measurement of temperatures than is possible with

thermocouple installations. Accordingly, resistance thermometers are used in many installations where their greater accuracy is warranted, such as in the case of low-differential temperature measurement. In order to obtain the greater accuracy and sensitivity inherent in a resistance thermometer and to minimize thermal lag, it is important that optimum thermowell dimensions (for the particular resistance element) be employed in order to maintain good contact between the element and the well. For this reason. wells for resistance thermometers frequently are provided with the resistance elements as matched units.

1). Signal Transmission

Individual extension wires (usually three ) from resistance thermometer elements are frequently brought to field terminal strips, from which they are continued to board-mounted resistance thermometer instruments in multiconductor cable. Installation practices are discussed in Sect. 7.

c . Coiiiiections

Connections to resistance thermometer instruments usually will be made directly on the instrument or at terminal strips of prefabricated consoles. To obtain the advantages of resistance thermometers. care must be taken to ensure that proper extension wire resistances are achieved and that connections are clean and tight.

REFERENCES

’ I S A R P I . I -.7: Thtrrrwco~~pl t , s crnd Tlierr~~ocoirplt Esrerisiori Wiws. Instr. Soc. Am.‘ Pittsburgh, Pa. (1959); see Sect. 1.6: “Installation.”

- I t > i r i . . Sect. 1.1: “Coding of Thermocouple Wire and Ex- tension Wire.”

Ihitl.. Sect. 1.4: “Fabrication.”

50



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COW PROVIDED FOR HiSTQRitAi. PURPOSES ONLY

SECTION &PRESSURE

1.1 CONTENT This section covers the installation of the more com-

monly used instruments for measuring, indicating, recording, and transmitting pressure. Included in the discussion are such devices as pneumatic and electronic pressure transmitters, receivers for pneumatic signals, and pressure switches. Excluded from discussion are pressure instruments which, by inference, measure other variables, e.g., rate of flow. Such variables are discussed in other sections of this manual.

4.2 GENERAL

a. Precautions Hydrocarbons, or any other material which may be

hazardous to personnel in the event of leakage, should not be piped to any instruments located in a central control room. It is customary to transmit the pressures of such materials, either electrically or pneumatically. to receiving instruments. It is advisable also to transmit pressures in cascs \vhcrc long piping connections would present problems. Examples include instances where solids present in the process tluid coulú cause plugging difficulties and where differences in elevation could result in a liquid head problem. In order to eliminate the necessity for insulation and hexing, except at the process connection, it may be desirable to transmit when the fluid involved would freeze or solidify at low atmospheric temperatures.

11. Vilwatioii Many pressure instruments are susceptible to damage

or malfunctioning if mounted in locations where they are subject to vibration. Care must be exercised in the selection of locations for mounting such devices.

4.3 PIPING

a. Size Pressure piping should be designed and installed in

accordance with the piping specification for the service involved. Piping runs to instruments should be no smaller than '/Z-in. pipe, or 3k-in.-OD annealed steel or stainless steel tubing where permitted by the piping specification. Where instruments have connections smaller than %-in. pipe size. the line size should be reduced at the instrument or its adjacent manifold.

1). Cleaning

clean of cuttings and other foreign material. All pipe should be reamed after cutting and blown

c. Sliort Coii i iwtioi is

From a n opcrnting standpoint. it usually will be most satisfactory. a s well as most economical. to install a

pressure device as close to the pressure connection as possible consistent with accessibility and required visibility. This practice requires less material. eliminates liquid and vapor traps in the piping. eliminates liquid head problems, and reduces the chances of plugging. A close-connected pressure instrument is shown in Fig. 4-1.

CI. Long Connections

If a long connection is necessary. the piping should be sloped between the pressure tap and instrument to minimize the number of traps for vapors or liquids. Where high points cannot be avoided. vents should be installed: scale traps or drains. or both. should be provided at low points in piping. Where the shutoff valve is not readily accessible from the instrument location. an additional valve should be installed at the instrument.

e. Flesildity

Instrument pressure piping should be installed and supported so that the forces deyeloped from the expansion of hot piping or vessels cannot result in a piping or instrument failure. Some refiners us< thin-walled annealed steel or stainless steel tubing to provide flexibility in pressure leads.

f. Pulsation

Pressure instruments which measure pulsating pressures of reciprocating pumps. compressors. and the like should be equipped with pulsation dampeners to prevent rapid failure of the gage movements or the pressure elements. or both. Some users prefer to employ needle valves for this purpose: see Fig. 1-2 (-1 ì .

p. Pursing and Sealing

Where there is a possibility of plugging with solids or viscous liquids, or where corrosive materials are pres-

,- ' 1 GAGE IF LOCAL I I , INDICATION IS PRESSURE / ~NSTRUMENT 1 UNION TO PERMIT

, REMOVAL OF I INSTRUMENT,

I , i

/ ,' 'T'

I 'PRESSURE CONNECTION (BLOCK VALVE MUST MEET REQUIREMENTS OF PIPING SPECIFICATION)

51



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, -- - I’ .

31APHRAGM-TYPE GAGE- DROTECTOR FOR CORROSIVE SERViCE(SEE SECT 8)

8 DQESSURE CONNECTION (BLOCK VALVE MUST MEET

RECUIREMENTS OF PIPING SPECIFICAT IONS)

A B

FI(;. 4.-2-l~ipiiig for Pulsntiiig and Corrosive Services.

ent, the pressure lead may be sealed, purged, or protected by a diaphragm seal as shown in Fig. 1-2(B). This mcthod is limited primarily to pressure gage protection. Gase manufacturers list a large selection of diaphragm matcrialc which provide protection against most substanccs; furthermore. they usually supply the gagc scalcd. Thc possibility of error resulting from diaphragm strcss o r thermal expansion of seal fluid should bc rccognizcd i f it is nccessriry to use a seal protcctor w i t h ;I Irirgc-capacity prcssure element-such 2s a spiral o r helical hoiirdon. The seal diaphragm dis- placcmcnt volume should be sufficiently large to avoid the intrcduction of Lin crror. Thc use of Lipprosimately 5 ft of Hcsiblc cripillary betwccn scal and instrument will pcrrnit locritioii of thc instrument away from a high- tcmperatiirc \vssel or line. and reducc the probability of transmitting vibration to thc instrument clement. Current practice is to purge the pressure connections on vcsscls :ind lines in the catalyst system on process units of the tluid-solids type. The purge medium is introduced through ;i rcstriction orifice or needle valve at a point close to the vessel or line. The nature of the purge niediurii must be such that no hazard is introduced during normal, abnormal, or startup operations. For information on seals, purge quantities. and purging mcthods scc Scct. (i.

4.4 INDICATING GAGES

il. (:oiiiiet*tion Location ancl Sizes

Indicating pressure gages for flush mounting on instrument panels should be back-connected. Gages for îicld iiiounting should be the bottom-connected. surface- mounted typc. Standard gage connection sizes are t 4 - h and !/i-in. pipe size. However, in order to reduce the number of small connections, there is a trend toward thc exclusive use of thc Y2-in. size.

1). CIil’l””t3

Gagcs up to and including the 4 S - h size may be siipportcd by their own pipe connections unless the lines or equipment involved are subject to severe vibration. Gages subjected to vibration should be supported inde-

pendently. In some cases this can be done best by supporting the piping close to the gage. Typical examples of gage supports are shown in Fig. 4-3.

c. Safely Devices

Pressure gage cases should be provided with disk inserts or blowout backs designed to pop out when the case pressure rises to a pound or more. These are protective devices installed to prevent bursting of the glass window in front of the gage dial or case in the event of pressure element failure. Some users require this feature on all instrument cases which contain process pressure elements. Gages can be obtained also with safety glass or plastic windows as an additional safeguard. Gage supports should be designed so that the functioning of the safety disks is not prevented. Because a coat of paint almost always will prevent the functioning of the safety disks. gage cases should not be field-painted. Care must be taken to make certain the disks are un- covered when gage cases are insulated or traced for heating. Gage cutouts. with or without velocity cheeks, are available for limiting overrange.

t l . Si~>liOiiS

Siphons or “pigtail” condenser seals should be provided in connections to close-mounted gages in steam or other condensable vapor service to maintain liquid in the pressure element and to prevent overheating; see Fig. 4-4.

e. Boiirdoii Tuhe Material

It is important that bourdon tube materials be selected for the service conditions included. For example, a numbcr of failures of “standard alloy steel” tubes (AISI Type 4130 is typical) have occurred in hydrocarbon services where sulfur compounds or hydrogen were present. The frequency of these failures led several refiners to adopt the ,4ISI Type 316 stainless steel tubes for general use. The use of other alloys may be justified in specific applications.

,BACK OCGAGE ,‘(SURFACE -MOUNTING TYPE)

V P

-

u ANGLE CLAMP: DO NOT COVER SAFFTY DISK INSEi7rS

F I L 4-3-Surface-Mounted Gage Siipports.

52



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

PRESSURE - -

SECOND BLOCK VALVE DESIRABLE IF FIPST BLOCK IS NOT ACCESSIBLE - \

UNION IVWERE NECESSARY

, -t, CORRECT FOR STATIC HELID = NECESSARY ALWAYS MOUN? ABOVE PRESSURE TAP COR

\ , VACUUM SESVICE-.

4 "PRESSURE CONNECTION (BLOCK /

VALVE MUST MEET REQUIREMENTS OF PIPING SPECIFICATIONS)

A B

n n -COUPLING OR

AS SEGUIRED -- SIPHON FOR CONDENSING VAPORS

J~~~~~~~~ CONNECTION (BLOCK,' VALVE MUST MEET REQUIREMENTS OF PIPING SPECIFICATIONS) 8

C D FIG. U - R e m o t e l y Lorateci Instruments for Pressure

Piping.

4.5 ISSTRUJIENTS OTHER THAN GAGES

a. Supports

Recorders, transmitters, controllers, pressure switches. and the like normally should be supported independently of the pressure connection. The type of support depends upon the make of instrument, the location. and the user's preference. When installing pressure instruments. care should be taken to avoid the possibility of imposing stresses from the pressure piping, conduit. etc.. n hich may cause malfunction.

b. Local Indication

Pneumatic transmitters which have no mechanical indication (the so-called blind type) normally are supplemented with pressure gages connected directly to the process: see Fig. 4-1. Provision for the installation of a test gage should be made at the output air connection of the transmitter.

c. Electroiiic Iiistriimeiits

Wiring for pressure transmitters or transducers for electronic instruments should be installed in accordance with Sect. 7.

Prcssurc transmitters or transducers for electronic instruments should not be located too close to hot lines. vesssls. or other equipment. Ambicnt temperatures which escccd 140 F are likcly to result in calibration difficultics and rapid deterioration of electronic components. Susceptibility of mechanical or electronic components to vibration should be ascertained and, where necessary. adjustments should be made in the mounting.

53

d. Receivers Each receiving device used in conjunction with a

pneumatic transmission system can be classified as a pressure instrument. The ranges of such receivers depend upon the transmission system in use, the most common being 3 psig to 15 psig. These receivers may be indicators, recorders, relays, pressure switches. or controllers. Normally, the air pressure is supplied to the receivers through %-in. tubing; see Sect. 7.

e. Pilot Pressure Regulators The pilots of certain back-pressure and pressure-

reducing regulator valves can be classified as pressure instruments. These simple proportional controllers gen- eraily are mounted either on top of the diaphra, om case. or on the yoke or spring barrel of the valve. The air connection between the controller and the valve diaphragm normally is installed with tubing at the factory. The connection to the process pressure tap should be made as shown in Fig. 4-5.

f. Differential Pressure

Differential pressure may be measured with an instrument of the same type as is used for îìow measurement. Instruments for this service are available in a variety of ranges. Such devices may be of the mercury or so- called dry type. One method which uses an instrument of this type to measure the differential pressure across a catalyst bed or a fractionating tower is shown in Fig. 4-6. The process must be able to tolerate the purge gas. which is necessary to keep the long lead to the lower pressure connection free of liquid. Occasionally. very low gage pressures are measured with a differential pressure instrument by leaving the appropriate connection of the device open to the atmosphere.

Y T E S S U R E CONNECTION (BLOCK VALVE MUST MEET r REQUIREMENTS OF PIPING SPECIFICATIONS)

SWAGE TO 5' iF DRESSURE

/ - G A G E INSTRUMENT IS "BLINC"

U A 1 PRESSURE I

UNION TO PESMIT REMOVAL OF INSTRUMENT

4



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

FLOW INDICATORS, IF DESIRED

NEEDLE VALVES,OR

ORIFICES IF

so-/ DIFFERENTIAL PRESSURE ELEMENT. LOCATE AT UPPERPRESSURETAP ON VESSEL

USE LOWER CONNECTIONS

g. Draft Gages Low-pressure instruments of the slack-diaphragm

type (called draft gages) are employed for the measurement of firebox and breeching pressures in steam gen- erators or fired-process heaters. Typical pressure connections at the firebox or breeching are shown in Fig. 4-7. Provision usually is made for a plugged access opening for "rodding" out soot deposits. The piping or tubing between the pressure tap and gage generally is Vi in. or larger and should be installed so that there are no pockets for condensation. Usually: the connection at the draft gage is a short length of M-in. tubing. A three- way cock is provided to vent the gage element to atmosphere for zero checking.

WELD HALF C?-=,ING

DRILL 'WALL

CUT PIPE FLUSh

CAULK

oa NIPPLE m û --. REDUCER

2" TEE

PLUG

REA a x

A B RRICK SETTING STEEL BREECHING

FIG. 4-7-Draft Gage Coiiiicctions.

54



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

COPY PROVIDED FOR HISTME&L PURPOSES ONIY

SECTION .5-AUTOMATIC CONTROLLERS

3.1 CONTENT

This section discusses the installation and application of automatic controllers. Because installation details vary with the application. much of this section necessarily will be general in scope.

The material presented herein applies to both pneumatic and electronic controllers. It should be noted that there are some differences between pneumatic and electronic instruments. in addition to those discussed herein. which arc only differences of degree and do not change the basic requirements for a good installation. Users should install all control instruments in accordance \vith the manufacturer's recommendations and such additional requirements as are needed. in the judgment of the design engineer. to satisfy the particular conditions of the individual installation.

For a discussion of air and power supply systems. refer to Sect. 9 and 11. respectively.

3.2 FOlZJIS OF COSTROL

has been de- voted to the analysis ot' control systems and the application of the ixrious control functions ( proportional. reset. and rate action j to these systems. Such work has included simulation studies and mathematical calculations. Valuable information has been obtained from thesc complex procedures. and such methods show considerabie promise.

In general, however? the majority of control function requirements are determined on the basis of experience and judgment. For example. in order to minimize controller dead band.', controllers actuated by narrow-range transmitters would require a low gain " (wide proportional band) with automatic reset. Tower pressures and temperatures have been controlled very satisfactorily in this manner.

In recent years. considerable stud!.

a. 011-Off

On-off control is the most elementary form of control. It is suitable for alarms and protective device actuation. for automatic startup or shutdown of individual items of equipment, and for a limited number of refinery applications where intermittent regulation of the controlled variable is not objectionable.

11. Proportioiial

Control!crs which use proportional action only arc of value where on-off control is inadequate and where. with load change. a moderate offset (deviation from the set point) can be tolerated. it is to be noted that wherc a high-gain (narrow proportional band) setting is used. the offset which results from load change will not be as

a Figures refer to REFERENCES un p. 61.

great as where low-gain (wide proportional band) settings are used. Many pressure applications are satisfied by high-gain settings. and most level applications require low-gain settings.

c. Proportional Plus Reset

The proportional plus reset control action is more widely used in refinery practice than any other form. It is required to correct the deviations of the controlled variable from the set point resulting from load changes. The addition of reset to a controller can. however. cause difficulties if the controller is in intermittent operation. On most proportional plus reset action controllers. when they are not controlling. the reset action accumulates and drives the output to a limit. This limit may be beyond normal controller and valve operating values. Con- trollers on batch operations exemplify applications where this characteristic should be considered. Another example is preferential control applications wherein two or more controllers actuate the same valve: one or more of the controllers may be ;it a reset limit. Hence. on a switch of controllers. ;i process upset could be encountered.

CI. Proportioiial Plus Rate

Proportional plus rate control is of value where lags. other than dead time. cause ;i significant delay in recog- nition o f . and correction for. a change in the process variable. This form of control is also of value where other control forms would permit too large an otfset or would permit harmful or undesirable oscillations or overpeaking of the controlled variable. Fired heaters. with large process and measurement time lags. and batch operations are examples of applications where the proportional plus rate action form of control could be used to avoid the undesirable effects of some of the other types of control forms.

e.

The proportional plus reset plus rate control is of value where the accumulated deviation from the set point must be kept to a minimum. This would occur on applications having high process gain and one or more large lags. Opinion varies as to where this type of control is best applied. Some believe that this form is necessary in most temperature-control applications. However. caution is iidvised when considering the use of this form since. occasionally, one might find that, even with properly adjusted rate action. equipment performance is upsct. Also. for intermittent servicc-type applications. it would be well to consider using controller circuit designs wherein the location of the rate action unit c1iminatc.s the reset-action-inspired overpeaking of .the controlled variablc.

Proportional Plus Reset Plus Rate

5 5



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

f. Proporiioiial Plus Reset Plus Inverse Rate

This form of control differs from the preceding type only in the effect of adjusting the proportional action; it is not a new combination. The proportional plus reset plus inverse rate action is of value where :in initial change in the measured variable may be misleading, or where equipment time constants are such that a cor- rective change should not be made too quickly. For example. a sharp increase in steam demand from a boiler may cause a rise in the drum level, due to "swell" or the momentary displacement of watcr from the boiler tubes on the temporary reduction in pressure. The inverse rate action in the drum level controller could prevent dccrcasing the fcedwatcr flow at the wrong time.

5.3 -APPLICATION OF CONTROL FORRIS

a. Required Ranges of Adjustment

In selecting a controller for any application! it should be realized that the ranges of adjustment should be sufficiently broad to permit necessary settings of control actions as required. I f ;i process requires a gain setting of 1. thcn the instrumcnt selected must have a range of gain adjustment which will include 1. Accordingly, the following discussion of ranges of adjustment is included:

1. PROPORTIONAL ACTION

Some instruments. including simple pressure and temperature controllcrs. use only proportional action and normally haw gains adjustable down to 10. 5. or even 2.

Othcr instriiments-normall\: having rcset or rate action. or both. in combination with proportional action- havc g i n s adjustable down to ;it least 0.4. i f not lower. [For ccrtain cxccptions. see ( e ) . ] Although many applications of this sccond-group of instruments do not require this range of adjustment. most controllers have adjustable ranges at lcast this wide which pcrmit inter- changeability of control equipment.

2. RESET ACTION

Thcrc arc two gencral ranges of reset action-fast anù stt i t i t lard. Because spans arc not standardized between difcrent manufacturers, these terms arc necessarily general. Fast resct can be adjusted within a range of approximately 0.1 or 0.3 to 100 or more repeats per minute. whereas standard resct can be adjusted within a range of approximately 0.02 or 0.05 to 10 or less rcpeats per niinutc. It will be notcd that there is considerable overlap. Lind cxact tigurcs arc not as important as thc orders of magnitude and rangcability. It should bc mcntioncd that somc manufacturers use "minutcs pcr repeat" (sometimes shortened to "minutes" or "min") which is the rcciprocal of "rcpeats per minute." Others mercly use an arbitrary scale value which may o r may not be convcrted to actual'tirnc or rritc values.

3. RATE ACTION As with reset, sometimes there is an option as to

available ranges of rate action. However. there is no standardization among the various manufacturers on units of measurement, nor on approximate ranges. Some manufacturers supply a single range of rate action, others have various ranges available. When rate action is required. it is recommended that the range be determined by discussion with the vendor.

1). Flow

For flow applications in general. gain adjustable down to 0.5 and adjustable reset are desirable. Fast-reset range should be used with force-balance diaphragm transmitters. and standard-reset range with mercury manometer transmitters. One exception to this practice is a special form of pneumatic controller used on liquid How with a force-balance diaphragm transmitter. This type of controller is mounted directly on the diaphragm air motor of the control valve. It is provided with fixed proportional and rate actions and an adjustable very fast reset action. This controller has been found satisfactory on liquid flow and on some gas flow applications. but it is not recommended for other services.

c. Pressure

For pressure applications, gain adjustable down to at least 0.5 and standard resct are desirable.

t i . Teniperature

For tcmperature applications. gain adjustable down to at lcast 0.5 and standard reset are desirable. In addition. adjustablc rate action may also be necessary.

e. Level

Level controllers usually fall into two general groups and different gain requirements are necessary for each. The distinction between the groups is based on the ctfect of level change on other process variables. Dif- fcrentiation is probably best illustrated by the Ïollowing examples :

Example I : Consider a fractionating tower from which the bottoms product flaws to storage through a cooler, where the flow is controlled by the level in the base of the tower. Some variations in tower level are permissible. and this fact can be used to advantage by not requiring automatic reset and by setting the gain as high as practicable to keep the lcvel within acceptablc limits. In this case. level changcs will cause fluctuations in the How rate until stabilization is reached but will not cause any harmful results.

Example 2: Next, consider the same example. except that bottoms product is going as feed to another towcr

56



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

AUTOhlATIC CONTROLLERS -

or through ;1 heat exchanger which preheats feed to the tower. Now, if the control in the first example were used, the changes in flow rate of the bottoms product would cause upsets in other parts of the process. A level controller with low gain and slow reset action will change bottoms product !low rates slowly and thus reduce. if not avoid. such upsets. For this case, the gain should be adjustable down to at least 0.4.

It should be noted that most unit processes have very little surge capacity other than change in level in process vessels designed for other purposes. To take advantage of available surge capacity, the gain setting in control examples 1 and 7 should be as low as practicable.

5.4 CASCADE _AND RATIO CONTROL

a. Cascade Control

In a cascade control system. one controller. commonly called the primary or master. is actuated by the process variable which is to be regulated to a constant value. and a secondary. or slave. controller is actuated by a variable that can be used to cause changes in the first. or primary. variable. The primary controller output adjusts the set point of the secondary controller which. in turn. operates a control valve or other final control element. This is not to be confused with the system wherein a remote manual loading station is used to adjust the set point of a controller which has been mounted adjacent to its control valve to improve control by reducing the etfective time lag.

Cascade control is of value because. if properly applied and jnstalled: 1. I t can reduce the effect of time constants in the loop even when the primary loop constant is of the order of 3 to 5 or more times the secondary loop constant. 2. I t can eliminate the etfect of disturbances in the secondary variable before the disturbances can enter the process loop. 3. In some instances. it can accomplish the effects of both items 1 and 2 ; Fig. 5-1 illustrates such an application.

Cascade control should not be attempted unless it is required for any of the preceding conditions: otherwise, cascaded systems may add unnecessary complications to the control circuit as well as higher cost.

The control forms for the individual controllers can be ùctcrmiried ils described in Par. 5.2 and 5.3. Very often both the primary and secondary controllers are provided with proportional plus reset action.

Some applications or combinations of controllers may require a gain setting of the primary controller lower than is normally available from the instrumcnt. One method of overcoming such a situation is to pass the primary controller output through a ratio rclay.

Also, for protcction of the process. it may be necessary to limit the amount of set point adjustment in the

= R I M A R Y MEASUREMENT (DESIRED TRAY TEMPERATURE:

I I

Note: Setting the pressure controller to a desired value reduces the effect of the reboiler time constant. and at the same time the pressure controller corrects for hte3m pressure variations.

FIG. 5-1-Piieumaîic-Type Cascnrie Control System.

secondary controller. This can be accomplished in any of several ways: 1. A limiting device may be installed nithin the primary controller to limit its output signal. 2. ,4 separate relay may be mounted externally but must be connected in between the primary and secondary controllers to limit the set point signal. 3. Mechanical set pointer travel stops inside the sec- o n d a n controller.

To illustrate a need for such limiting. consider a cascade control system wherein a primary controller is to adjust the set point of a compressor speed controller. The speed controller should never be directed to operate the compressor at its stalling speed or at the speed where the overspeed trip mechanism will be triggered.

h. Ratio Control I n ratio control. two variables are measured and the

secondary or "controlled" variable is regulated to maintain a predetermined ratio to the primary or "wild" variable; the ratio between the two variables is usually adjustable. It is sometimes desirable to have the ratio adjusted by the output of another controller or a transmitter. Occasionally. this is done on fractionating column applications and is sometimes called "three-element control." This is not the same as the combination of instruments í which is generally similar in purpose) that is used on boiler applications and is also referred to as "three-elenicnt control." ( In boiler feedwater control. additional adjustments permit compensation for "swell" and "shrinkage" of water in il boiler drum resulting from changes in steam rate; this is not a true ratio system. )

When ratioing two Rows, at least the following two points should ~ilways be considcred: 1. Tlie ratio unit should bc "squared" in a zero wild llow; zero wild llow calls for zero controlled How. 2. Both signals should be of the same characteristics. i t is not possible to ratio a linear signal with a square root signal.

57



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

Ratio control applications requiring extremely wide ratios may prcscnt sonie problems; the required change in ratio may bc grcatcr than the ratio change available in the control mechanisni. In this casc. cithcr one or both of the How transmitter ranges may be changed to help achieve thc desired ratio relationship.

5.5 LOCATION OF CONTROLLERS a. General

Controller location requires careful study. Controllers can be mounted on a control room panel, either integral with or detached from a recording or indicating instru,- ment: near the point of measurement andior control; or directly on the control valve operator. Set points of controllers can be adjusted directly or remotely.

The nunibcr of possible instrument combinations makes it dilficult to set up definite recommendations. Neverthclcss, the need for installation standards has led to the growth of a number of necessary working practices. Some of the more important practices are discusscd in the succeeding paragraphs.

I) . Fric. tow . i lTcc . i i rig ( :oi i t rol I w Loc-n t ion

The folloiving points (not listed in order of importance should bc considercd when deciding on the location for ;i controller:

i . Convenience to operating personnel+ase of reading. c';isc' of changing sct point. approximate place in Ilow schcnic. etc.

2. Convenience to maintenance personncl-accessibility for scrvicing. frcqucncy of nccd for servicing. etc.

3. Installed cost because of location. 4. Safety ol personnel rind equipment. 5 . Vibration elfccts on equipment and performance. 6. Corrosion caused by surrounding atniosphere and

7. Weatherproofing and winterizing, where necessary. S. Instrument lap. 9. Explosionproofing, wlicre required.

the process Huid.

IO. Protection from fire. I I . .Acccssibiiity in thc went of firc. 12. Protection from mechanical damage. 13. Xriibicnt temperature. 14. Riidirition from sun or hot equipment. 15. Conipany policy with respect to types of instruments purcli;iscd and their location. 16. instrument error due to transmission systcm characteristics.

4.. Lag I . WITI-I ELECTRONIC I N S T R U M E N T S

Process. measurement. and equipment response lags arc common to all control systcms. Electronic control systcms are essentially frec from transmission lags.

2 . WITH PNEUMATIC INSTRUMENTS

Pneumatic transfer lag can be reduced by proper controller location. Transmission lags of both transmitted and controlled air signals are affected by tubing size and length, by pilot capacity, and by volume of air to be handled through the tubing. The situation is further complicated by the fact that a given air-transfer time lag which will introduce no control problems in one application will be entirely unsuitable in another. The following points are worthy of mention for, although self-evident, they are often overlooked: 1. Lag is greater with longer tubing runs. 3. Lag is greater with very small tubing sizes (because of friction) as well as with very large tubing sizes (because of volume). 3. Lag is greater when air is Howins through the tubing to a large-volume end device (e.g.. a valve motor ) .

4. Lag is smaller when air is flowing through the tubing to a small-volume end device (e.g.. a receiver bellows). 5 . Difference in lag is not significant between commonly used tubing sizes ('4 in. and .7/8 in.) of moderate lengths.

tl. (hisitlcratioiis in '9Iiiiiiiiizing Pneumatic. Lag

For general service applications where transmission lag is not ordinarily critical. the total length of tubing from the transmitter to the controller plus that from the controller to the control valve. or its positioner. should not excccd 400 ft: neither run should exceed 250 ft. Based on the user's judgment and experience. longer runs may be tolerated for these applications if control valves having small-volume diaphragm heads are used. When control valves rire more than 125 f t from their controllers, use of valve positioners or relays may be helpful in reducing pneumatic lag. Such devices, however, may be less beneficial overall since they can intro- ducc dead time, worsen frequency response of the loop, create a phase shift. and so forth.

For the applications where transmission lag can be harmful, it is suggested that tubing runs be limited to approximately one-half of the values noted herein. Also, mounting the controllers adjacent to their valves will help considerably in reducing lag. Installing high-capacity air pilots in the controllers is another method of reducing pneumatic lag.

e. Centralization of Control Stationu

One major consideration affecting controller set point and instrument location is operator convenience. This is of considerable importance because centralization of instruments results in more eflicient and safe operation of process equipment. I t is desirable to locate at a central point, usually in the control room, a sutficient number of instruments to permit control of all major process variables from this one point.

5x



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

.\UTOMATIC CONTROLLERS

f. Locally Moiiiitetl Pneumatic Coiitrollers PROCESS

Frequently, controllers are mounted locally because of the lack of justification for control room mounting and to reduce pneumatic transfer lag. When reduction of pneumatic transfer lag is the reason for local mounting, it may be desirable to have the tubing run to the control room. A number of combinations are shown in Fig. 5-2 through 5-5. The system shown in Fig. 5-5. which uses four tubes. is the most desirable; however. it cannot always be justified economically.

Note: The panel-board-mounted controller is connected by two tubes-one to the transmitter and one to the valve. Only locally mounted controllers are shown in Fig. 5-7 through 5-5.

5.6 JIOCTINTING

u. Geiierrii

Some controllers are mounted on or within recording or indicating instruments: others are installed separately.

It is beyond the scope of this section to cover the mounting of instruments on panel boards. This topic is discussed in Sect. 12.

1). Moiiiiting of Local Controllers

iMethods of mounting local controllers include flush, surface. bracket, and yoke. Also. some controllers can be mounted on the valve yoke or operator of a control valve.

The principal considerations in mounting are rigidity, accessibility for service or mainrsnance. freedom from excessive temperatures. design of the instrument. fire

?KULATION PROCESS

ME AS'Js EU E N T 9

TRANSMITTED ME4SURED VAR I A E L E

t REMOTE RECEIVE? j IND. AND/OR iEC. !

This type of controller generally is known as a l - tube system. The transmitter must be of the proper type for the particular

process variable (temperature, flow, pressure, level, gravity, etc.) and may be indicating, recording, or neither.

The controllcr contains a manual jet point: i t must be of the receiver type for the kind and range of signal from the trnns- mitter. and may be indicating, recording. or neither. The controller moiinring should be of the t!pe most suitable for the application.

The remote receiver should be of the type to suit the requirements of the application.

FI(;. . i - P - F i d d Co~itrdler w i t h Reriioie Rwc.ivc.r.

f TRANSMITTED MEASURED VARIABLE-

I REMOTE RECEIVER , IND. AND/OR REC.

i I

4 SET POINT

r--dJ REMOTE LÛADING


process variable (temperature, flow, pressure. level, gravity, etc.) and may be indicating, recording. or neither.

The controller must be of the receiver type for the kind and range of signal from both the transmitter and remote loading station, and may be indicating. recordins. or neither. The controller mounting should be of the type most suitable for the application.

The remote receiver and the remote loading station may be set up as separate items or combined into a single unit. In either case. both should be of the type to suit the requirements of the application.

FIG. .S-3-Field Cniitrnller w i t t i Rriiiote Receiver and Loading Siatioii.

PROCESS QEGULATION iIE4SUREUENT

+- CONTROLLER

, ~ ~ ~ ~ ~ ~ ~ E D , VARIABLE I SET ?OiN* LOADING REGULATOR T O I

I REMOTE RECEIVER CONTROLLER

OUTPUT i INDICATOR

This type of controller generally is known as a 3-trrúe system. The transmitter must be of the proper type for the particular

process variable ( temperature. flow, pressure, level. gravity, etc.) and may be indicating, recording, or neither.

The controller must be of the receiver type for the kind and range of signal from both the transmitter and remote loading station. and may be indicating, recording, or neither. The controller mounting should be of the type most suitable for the Lipplication.

The remote receiver, the remote loading station, and the controller output indicator may be set up as separate items but most often they are combined into a single unit. In any case, all should be of the type to suit the requirements of the application.

FIG. C F i c . l d Coiiirollw with Reniote Rrceiver. Loading Station, and Controller Output Indicator.

59



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

_. API RP 550-PAKT I

RECJLATiON PROCESS , V E A S , U ~ ~ ' . l E N T , two-position transfer switches and regulators are satis-

factory.

REMOTE CCNTCOL STATION I -4v'tdG A RECEIVER, SE;-°CSl ADJUSTER AND 1

A b T O M A T I C - M A N U A L , --ihSFER :WITCH I

This type of controller generally is known as a 4-firbe system. The transmitter must be of the proper type for the particular

process variable (temperature. How. pressure. level, gravity, etc.) and may be indicating. recording. or neither.

The controller must he of the receiver type for the kind and range of signal from both -the transmitter and remote loading station. and may be indicating or recording. although usually it is neither. The controller mounting should be of the type most siiitable f o r the nppiiciition.

The remote control station usually is installed on the in- striiment panel in n control room and consists of a receiver which mnv be either indicritin- or recording, and contains a set point adiuster and a 3-position transfer switch for nutomatic- manual type operations.

FIG. .S-.S-Fit.id Coiiiroiiw wiïh Rvinote Control Stati«ri.

protection. and the desirability of freedom from vibration and mechanical damage. Normally, the requirement for vibration-free mounting precludes mounting a controller on a control valve. but sometimes this is not important with controllers designed for this purposc.

v. RarIi or Wall Jloiiiitiiip

A number of controllers can be mounted on racks, walls. or other surfaccs. Other than adequate attachment. there is no particular problem in mounting because the surfaccs are usually rigid and free from excessive vibration.

- - i>. 4 IIISCELL,\NEOCS CONTROL REQUIRE-

MENTS

a. B y p i s Fuvilitic-s

Controller bypass facilities should be furnished with every controller used on major process variables. This enables remote manual control in the event of unsatis- factory operation or failure of automatic control. Inas- much as switches arranged for bumpless transfer are available in a number of combinations, recommended practices for their selection are: I . Thres- or four-position transfer switches and r e p - lators should be supplied for all temperature controllers, critical control applications, and all controllers used in cascade control systems. 2. For othcr applications requiring bypass facilities,

i,. Control Requirements of Trniismitters

Requirements for transmitters ordinarily are not complex. In some instances (as in level applications), the same instrument can be used either as a transmitter or as a controller. I t should be pointed out that when any instrument is used as a transmitter, the proportional band setting should be at 100 percent with no reset and ordinarily no rate action. Any change in the proportional band should be made at the receiver-controller and not at the transmitter. The use of an instrument set at 100-percent proportional band to act simultaneously as a transmitter and controller is not recommended. The applications requiring transmission and control should be provided with dual pilots or secondary controllers. Some filled-system temperature transmitters are available with rate action to compensate for measurement or transmission lags.

c. Loc-iil Indication of Measiirenieiit for Manual

Frequently, transmission of measured variables to a control room is required, either for operating guides or for controlling. The point of measurement usually is some distance from the control room, and, if a control valve is involved, it. too, is mounted some distance from the control room. In order to facilitate manual operation of a control valve handwheel, or the bypass around a control valve, it becomes quite desirable to have an indication of the controlled variable (temperature, level, flow, etc.) near the control valve manifold. Frequently, it is possible to provide a local indicator so installed that it can be read from both the transmitter and the control valve. Also, some transmitters are available with integral indicating devices. When additional indication is desired, it is usually possible to install other indicators actuated from the transmitter output signal.

Control

d. Controller Protection

Normally, it is necessary to protect controllers (and other instruments) from extreme changes in ambient temperature, against physical damage, and from accidental painting.

Frequently, an instrument is of such rugged mechanical design that external protection against physical damage is unnecessary. However, some controllers require a cover or shield to protect against painting or accidental turning of exposed dials or adjustment screws. In some locations shields are required to protect against excessive heat from either thc sun or radiating equipment. Locations should be selected which will reduce the need for protection from accidental mechanical abuse, or suitable guards should be provided. Winteriz- ing requirements are discusscù i n Scct. 8.

60



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REFERENCES T. J. Williams and V. A. Lauher, Aiitonicrric Control of

Chemical and Pcrroleiini Processes, Gulf Publishing Co.. Houston ( 196 I ). ' Process Instrnrnenrs trird Controls Hundbook, D. M. Con-

sidine íed.), McGraw-Hill Book Co.. Inc.. New York (1957). G. K. Tucker and D. M. Willis, A Simplified Technique of

Control System Engineering, Minneapolis-Honeywell Regulator Co., Brown instrument Div.. Philadelphia ( 1958).

' L. M. Zoss and B. C. Delahooke, Theory lind Applic(rrions of índiisrricil Process Control, Delmar Publishing, New York í. 196 I ) .

ASA CSS.1: Terminology for Aiitot?iaric Conrrol, Am. Std. Assoc.. New York 1963).

fi S A M A Std. RC 18-12: Mrirkiii%gs tor Adjrisrinent Meuns in Automatic Controllers, Sci. App. Makers Assoc., New York í 1960).

61



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SECTION &CONTROL VALVES -4ND POSITIONERS

6.1 CONTENT This section presents recommended practices for the

installation of regulators and control valves, including air and hydraulic valve positioners, booster valves, and other associated relays.

A number of instruments can be mounted on control valves and regulators; reference should be made to other sections of this manual for installation and piping practices covering these devices.

For additional information on control valve inani- folds. rcfcrcnce may be made to the ISA RP 4.1: Rec- oriimetrdrtl Prtrcrice for. Srniidcrrci Cot) fro1 Vdve Mai+ foicl Designs (Ctr,hori .'irre/ V d r c ~ s Only) .

6.2 GENERAL

a. -4cwss i ld i ty

All control valves should bc installed so that they are readily acccssiblc for maintenance purposes.

Thcy should be locatcd at grade iinless pressure or other dcsign conditions niakc such an arrringement impracticable. Whcn located above grade. control valves should bc iiistallcd so t h a t they arc readily accessible from ;I pcrnianent piatform or walkway with ample clcarancc5 t'or mriintcnance ogcrritions.

1,. Locatioti

Where tlicrc is :i choice of location. it is desirable to have the control valve installed near the piece of operating equipment which must be observed while on local manual control. It is also desirable to have indication of thc controllcd \ mablc visible from the control v;livc.

4.. (:It.ur:iiic.v Sufficient clearance should be provided above and

below the control valve so that the bottom flange and plug. or the topworks and the plug, may be removed with the valve body in place. Extra clearance is required where heat-radiating fins or other accessories are used.

On large valves which close on air failure. it is often advantageous to use reverse-actin: topworks so that the valve may be located a minimum distance above grade or platform and still provide accessibility and prevent interference with overhead piping.

t l . Prwniitians

Control valves which handle combustible fluids should be kept away from hot pumps, lines, or equipment. This practice reduces the possibility of liquids splash- ing upon hot lincs or equipment should thc control valves be i-cniovcd and the line between thc block valvcs drained.

Siniílrirly, control valves used in process lines or fuel

lines, or both, to fired heaters should be located outside the firewall around the heater. If no firewall is provided, the control valves should be located on the sides of the heater away from the burners oc at a sufficient distance from the heater so that the control valves may be removed and the line drained without danger of a flashback. An alternate method is to pipe the drain or bleed connections a safe distance from the heater.

In order to prevent premature failure of diaphragms and electric or cicctronic components. control valves should be located so that the topworks are not adjacent to hot lines or equipment.

During startup of any new facilities, care should be taken to keep scale, welding rods. and so forth from plugging control valves. One method which is sometimes used is to remove the valve and substitute a spool piece during flushing operations.

The control valve actuators should be selected so that on failure of operating medium the valve will "fail safe." ¡.e.. lock in position or take the position (either opened or closed) which will result in the least upset to the unit.

6 . 3 CONTROL V-!LVE TYPES

Control valves can be classified according to body design. Thc selection of a valve for a particular application is primarily a function of the process requirements. and no attempt will be made herein to cover this subject. Some of the more common types of control valve bodies are discussed in the following paragaphs.

u. Two-way Valve

The globe body control valve with top- and bottom- guiding or skirt-guiding, provided with single or double seats, is the most commonly used type of control valve.

A variation of the two-way valve is the angle valve which is used primarily in coking or slurry service.

.4nother variation is the split-body valve which is available in both globe and angle patterns. In this valve the seat ring is clamped between the two body sections which makes it readily removable for replacement. The split-body valve is used a great deal in chemical plants.

I,. Tliree-Way Valve

marily used for splitting or mixing services. The three-way valve is a special type of valve pri-

c. ßiiiterfly Vrilve

Thc butterfly valve is a rotating-vane t"pc of valve used in applications whcre low-pressure drop in the fu l ly open position is essential, and where size and light wcight must be considered. It is available with grease seals. pressurized neoprcnc. or various types of syn-

62



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CONTROL VALVES AND POSITIONERS -__ ____ ~ ~ __ ~

thetic rubber seating surfaces when tight shutoff in the closed position is essential.

d. Miscellaneous Valve Body Types

There are many other types of control valves used in control service, such as gate, plug cock, slide, Saun- ders, and rubber pinch valves. These valves use the same types of actuators as the control valves mentioned herein; however, considerably more power or torque usually is required.

6.4 CONTROL VALYE ACTUATORS

There are many types of actuators for stroking control valves. The selection of a particular actuator is a function of: 1. Operating media available. 2. Thrust requirements. 3 . Length of stroke. 4. Speed of stroke. 5 . Control valvc body type.

are described in the succeeding paragraphs. Some of the more commonly used types of actuators

a. Diapl irnp

The diaphragm actuator, with air as the operating medium. is the. most commonly used type of control valve operator. Diaphragm actuators can be either the spring-opposed type. springless, or pressure-balanced type.

1,. Self-.4rt uated Regulators

The self-actuated regulator is a variation of the diaphragm actuator and normally uses the process fluid as the operating medium. For pressure applications, some self-actuated regulators use bellows instead of diaphragms for the actuator; for temperature appiica- tions. bellows with a filled system and bulb are used instead of diaphragms.

c. Piston Act riato rs

Piston or cylinder actuators are used usually where valve designs with long strokes are required. The piston or cylinder can be operated hydraulically or with air or gas.

cl. >lotor Actiiators

Motor actuators for control valves can be electrically powered. or a vane or nutating-disk type of air- or gas-driven motor can be used as the power source. A variation of this type is the electrohydraulic operator which uses a continuously running electric motor to drive a pump and supply hydraulic pressure for a self- contained piston.

Current practices for the installation of block valves vary, but normally block valves are installed before and after the control valve. and a bypass with valve is installed around the assembly.

u. Blorlc aiitl Bypass Valves

Where the greatest flexibility is to be provided for future expansion, the block valves upstream and downstream of the control valve should be line size. In situations where the control valve is two or more sizes smaller than line size. the block valves may be one size smaller than line size.

It is often necessary that bypass valves be full-line size. or not more than one size smaller. in order to have the necessary capacity for filling and emptying the unit in a reasonable length of time. This is especially true under gravity flow conditions. .Also. where a small control valve is installed in a large line. the larger bypass valve sives the necessary mechanical strength to the ma ni fo 1 d.

Tn selecting and sizing block 2nd bypass valves. the installed cost should be considered. In some cases. the installed cost of line-size \dves is less than the cost of one size smaller valves plus the swages. welding. and labor required for installation.

Bypass valves are usually globe or gate valves in sizes up to and inchdin9 4 in. For larger sizes, gate valves normally are used: in special cases plug cocks with gear operators are used.

Recommended minimum sizes for block and bypass valves are given in Table 6-1.

1). Swages ut Coiitrol Valwe

Where a screwed control valve is used, the union connections are placed at the large end of the swage with the smaller end screwed directly into the control valve. Minimum Schedule 80 swages should be used to provide adequate support with minimum restriction to the flow. However, even heavier swages may be required to meet line specifications.

Where a flanged control valve smaller than line size is used, swages are placed adjacent to the control valve flanges except where additional pipe nipples are required to permit bolt removal.

Eccentric swages are often used in place of concentric swages to allow ready draining of thc line and to prevent buildup of deposits in the pockets formed by the concentric swages. Thc use of swages at control valvcs is illustrated i n Fig. 6-1.

c. Piping Witlioiit Bloelc and Bypass Valves

Block and bypass valves sometimes are not used.

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TAULE (~1-Ri.coiiiiiiendeil Miiiiniuiii Blot-k iiiicl ISypasa Valve Sizing

(All Sizes in Inches)

Line SiLe: !,? ?i, 1 I Y2 3 3 4 6 8 10 12

Valve Valve Valve Valve Valve Valve Valve Valve Valve Valve Valve

Control Valve Size

!/z 45

1 I !i 3 3 4 6 8 IO 13

Yi? !/I ?h ?i 1 1 1% 1% . . . . . . . . . . . . . . . . . . . . . . . . . . K "5 1 1 155 195 7 2 . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 l!5 1% 3 2 2 2 . . . . . . . . . .

1!6 I!h 2 2 2 2 3 3 . . . . . . . . . . 2 3 3 2 3 3 4 4 . . . .

3 3 4 3 4 4 6 6 . . . . 1 4 6 4 6 6 8

. . . . . 6 6 8 6 8

. . . . . . 8 8 10 . . . . . . . . . . . . . . . . . 10

. . .

. . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

. . . . . .

8 8 IO 10 8 10 10 IO 12 10

12 12

Instances where these valves are not always necessary are :

ring-type joints are used. Flexibility of necessary to keep excessive stresses from

1. With control valves in steam lines to pump drives or turbine drives sparing motor drives. 2. Where it is desirable to reduce tlie sources of leakqe of hazardous Huids, such as hydrogen, phenol. or hydrofluoric acid. 3. In slurry lines where it is difficult to introduce purge fluids or when there is a possibility that deposits may build up in any passage where the flow is not continuous. 4. In clean services where the operating conditions are mild, especially when 3-in. or larser valves are used and omission of the manifold will not jeopardize the safety or operability of the unit.

piping is also being induced

in the bÖdy of the control valve. Arrangements for vent and drain valves are shown

in Fig. 6-3. Nipples for such connections are usually 36-in. o r I-in. minimum. Schedule 80 or heavier as required to meet line specifications. Such connections may be used for:

1. Drains. 2. Telltale indicators to determine absence of pressure when removing control valves.

3. Vents. 4. Bleeds.

I n nil cases whcre the block and bypass valves are not used. the control valve should be equipped with a continuously connected, side-mounted handwheel.

d. 3Ianifolcl Piping Arranpenieiits

5. Flushing. 6 . Extra pressure taps.

7. Sample connections.

The piping around control valves should be self-supporting or should be permanently supported so that when the control valve or block valve is removed, the

The manifold piping should be arranged to provide flexibility for removing control valves, particularly where

+I LI+ A & E L C L D A E

Arraiigeiiiciit A : Swages scrcwed into the control valve;

Arrangement B: Flanges to match the control valve: weld-

Arrangenient C: Extra pipe nipple used between the swage

.4rrangemeiit D: Eccentric swages often used to permit complete drainage of line and prevent buildup of deposits in concentric swage.

Arrangement E: Reducing ells may be used in place of welding ells and swages where space is limited.

union at the large end of the swage.

ing tee or ell used at the large end of the swage.

and the flange to permit easy removal of flange bolts.

FI(;. 6-1-Swages at Contrid Vulv~s .

64



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- ALTE

Notes:

1. Vent or drain connections may be placed in the line or in the swage as shown.

3. Nipples and valves are sometimes replaced by plugs or caps.

3. Various combinations of vents and drains are used success- fully depending upon the requirements of the service.

FIG. 6-9-Locatioiir of Vent and Drain Valves.

A Y O 7 C C A N BE ROTATED ! N T C 4 N *

= L A N E , KEEPING CONTRQL VALVE VEQTICAL

lines will remain in place without the necessity of providing temporary supports.

Control valve manifold piping arrangements are shown in Fig. 6-3. Possible piping arrangements for a pressure-balanced valve in fuel gas service are shown in Fig. 6-4. Piping arrangements for steam to pumps or turbines are shown in Fig. 6-5 and 6-6. Possible piping arrangements for emergency operation of control valves are shown in Fiz. 6-7.

6.6 PIPING AND WIRING TO CONTROL VALVE ACTU-ATORS

This discussion covers the installation. of piping and wiring for the valve-actuating medium as well as control signal piping or wiring to the actuator.

The following codes and standards, as well as refer-

JA LTER NATE V A L V E

T P O S I T I O N

0 E ' NOTE : FLOW SHOULD 8 E U P

UNDER P L U G FOR HIGH A P

F NOTE: FOR USE ONLY W H E R E

L O W AP IS ESSENTIAL

Arriiiigeniwit A ( ISA RP 1.1. Type I ) is preferred because manifold is compact, control valve is readily accessible for maintenance. and the assembly is easily drained.

Arrarig<:ni<:iit B is preferred because control valve is more readily acccssible.

;irranpt:tiitmt (: is often used with angle valves. Control valve is sclf-draining.

side of the control valve are not shown. nor are the supports.

Arrarigenictit D í ISA RP 4.3, Type 3 ) is preferred because the control valve is readily accessible. Bypass is self-draining.

Arrangement E results in compact manifold. but control valve may not be too accessible.

Arranptmwiit F is preferred because byp;iss is >elf-draining: however. requires greater space.

Norr: Block and bypass valves should be installed close to tees. ns shown, to minimize pockets. Drain and vents on either

FIG. 6-3-CimtroI Valve Manifold Arrangements.

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FUEL GAS TO BURNERS TO BURNERS

RESTRICTION FROM CONTROLLER

FROM AIR SET

PSV MAY BE REQUIRED IF DIAPHRAGM OR TOPWORK IS UNSUITABLE FOR MAXIMUM SUPPLY PRESSURE, OR THRUST ON STEM AT MAXIMUM PRESSURE IS EXCESSIVE. VENT PSV TOSAFE L0CATZT:ON

A B

tirriiiigeitia-iit A bhows pressure-halanced valve with isolating Arraiigciiieiit LI shows a pilot regulator which is suggested relay and rcstriction orifice for niaintaining minimum fires. where minimum tire maintained by restriction would cause Restriction oritice is sized for the manufacturer‘s recommended furnace tube failure on process flow stoppage. Isolating relay is minimum turndown capaciiy for the burners. To prevent the used to prevent gas flow into controller on diaphragm leakage control valve from closing completely. travel stops may be used o r breakage. Ratio relay may be used to permit air loading to in licii o f the restriction orifice. match fuel gas burner pressure.

FI(;. C í l ’ i I > i i i g Arraiigeiiicirts for Pressure-Balanced Valve at Process Heaters.

enccs to sections of this manual. should be followed as they apply to the installation of equipment: 1 . X F P A Birilctiii : ïo . 70: iV!cirioiial Electriccil Code. 3. .4 P I R P 300: Recoininencie:i Pscictice for- Clussifica- r i o i i of A tccis for Eiccti-icni Iiistc~ikrtiaiis in Petrolerrin Refineries.

3 . Sect. 7. Tsmsmissiori Systetns. 4. Sect. 9. Air Sicppiy Systems. 5 . Sect. 1 O. Hytlrciuiic S?..stetn.s. 6 . Scct. 1 I . Electrical Power Sl ipply .

-0

S E E F!G 6-ói-OR PIPII4G NOTE5

a. Powel. Supply

.A power s~ipply (air. gas. hydraulic fluid. electricity) at suitablc Icvel and of adequate capacity should be providcd for the valve actuator. To assure operation of the control valve under the most severe conditions. ca- pacities should be based on the most stringent requirements of the actuator.

I L Diapliragin Actuators

Diaphragm actuators may be operated directly by the controllcd air signal or through pocitioners or

@UTPUT SPARE PUMP O? TURBINE

NORMAL PUMP OR TURBINE

A 6 Arriiiigeiiient A is used where one control valve is employed Arr~iigentcnt B is used where two normal pumps have a

for normal pump, or turbine, and spare. common spare.

FIG. &%Steam Piping to Spire Pump or Turbine.

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CONTROL VALVES AND POSITIONERS ..... . - _ _ -_ __

booster relays. Booster relays may be used to provide faster movements of the control valvc if thc controller air output is not adequate ior the application.

Pneumatic piping to diaphragm-actuated control valves is shown in Fig. 6-8 and 6-9. Piping arrangements with booster relays installed are illustrated in Fig. 6-10.

C. Piston or Cylinder Actuators

On cylinder actuators. manually operated four-way valves may be provided to locally control the operation of the valve if so desired. .4 bypass valve also should be provided between the

two ends of the cylinder actuator to equalize the cylinder pressure should manual operation of the valve be required.

Arrangement A makes the control valve readily accessible; the piping is self-draining. Steam inlet may be above minimum distance requircd for plug removal.

Arraiigenieiit B may be used where the bypass can be installed at turbine or pump inlet. Comrol valve may be too high for easy access: piping is self-draining.

.4rrangenient C is often used to make control valve more accessible because the control valve can be located at the same elevation as turbine or pump steam inlet.

Arrniigenieiit D is often used €or emergency start, or stop, of pump or turbinr.

Notcs: 1. Bypass and bypass valve should not be so located as :o

form a pocket: they should be self-drainins. 1. Location of separators. strainers. and traps not shown

because company standards differ on use and placement. The control valve should be located as low as possible for easy access.

FIG. f í f í ( : i ) i i ~ r ~ l Valve Ari.urigrnienta for Simm to Piimps or Turbiiie~.

PRESSURE ? I LOT

S E A L HEAVIER T H A N L I N E 4

LIQUID

LIOUID

FIG. &;-Piping at Regulator Valves or Prea‘ure Pilots.

FROM AIR SET

POSITIONER

1 3 FROM

CONTROLLER OUTPUT

C D Arrangement A shows nominal control valve piping with

gage €or reading diaphragm pressure. Arrangement B shows control valve with reversing relay.

The reversing relay is used to reverse controller output to valve. where two valves with different action are connected to one controller, o r is uscd to provide the same action when on automatic o r maniial control.

Arrangement C shows control valve with positioner. Arrangement D shows control valve with positioner and

reversing relay. The reversing relay is the same as used in B with standard control valve; in addition. it allows the positioner to be bypassed. A reverse-acting positioner may be used but it may not be bypassed.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

PNEUMATIC LEAD TO EblERGENCY SWITCH

ELEiTR!C LEADS TO EMERGENCY SWiTCS

FROM

PNEUMATIC VALVE VALVE

A B

Arrangement A shows control valve with emergency solenoid actuator.

Arrangement B shows control valve with emergency pneumatic actuator.

FIG. 6-Y-Coiitrol Valve Piping with Eiiiergeiicy Control Action.

VENTS

OPEN EYPASSbrF FOR MANUAL LEVER OR HANDWHEEL- BYPASS OPERATION I

MANUAL VALVE FOR OPERATED LOCAL 4-WAY

CONTROLLED HYDRAULIC

PILOT-RELAY OR MAY BE PNEUMATIC OR SOLENOID ACTUATED 4-WAY VALVE

HYDRAULIC SUPPLY

OUTPUT

HYDRAUUC ' UID RETURN B

Arrangenient ,-\ shows the cylinder actuator with provision

Arrangernent B shows the cylinder or piston actuator with- for local manual hydraulic operation.

out local hydraulic operation. .\rranztwiwit \how\ control v:iive with booster relay. Block

valves and hypahh v;iIvc> 'ire for hooster wrvicing.

FIG. 6-1O-lhwtrr I < ~ l a y Piping. FI(;. Ci l t -H~drauiic (:yIiiidcr or l'iston ..\cruator.

"ISTON C I .CYLINDEP ACT U ATC P AIR SUPPLY

'1. 4-WAY VALVE '.i

'QOM CONTROLLER

SUTPUT

REVERSING VA LV E POSIÏIONER

FROM AIR SET

PISTON OR CYLINDER ACTUATOR

C O N T R O L ~ E R FQOM AIRSE' OUTPUT

=i5TON OR VENTED ZYLINDER LOADING ACTUATOR REGULATOR

I

OPEN BYPASS VALVE FOR MANUAL LEVER OR HANDWHEEL OPERATION

FROM d-, C O ;LqpuLE R

EXHAUST

CYLINDER PISTON OR $-' ACTUATOR 41R SUPPLY

POSITIONER PILOT RELAY

B D Arr:iiigeinmt A shows the actuator used with four-way valve

Arrangement B shows the actuator with constant pressure

Arrangement C shows the actuator with reversing relay. in two-position service. Arrangement D shows the actuator with pilot relay for

loading on one side. throttling service.

FI(;. 6- l l -Air Cylintlw or P i s t o n Actiiiitors.

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CONTROL VALVES AND POSITIONERS _ _ _ ~ -

CONDUIT CONNECTION ELECTROPNEUMATIC ' r F S O M LtQ SE-

p-- - , - - - - - - - - -A CONDUIT CONNECTSON FOR SIGNAL LEADS FROM CONTROLLER

TRANSDUCER

E-ECTriOPNEUMATIC <AL'.'E =OSIT!ONER

I C N S L I T CONNECTION =.:a S:ZNAIL LEADS ==OM CINTROLLER SIGNAL LEADS FOR

- , A I R GAGE HYDRAULIC FROM AIR SET ACTUATOR

*- --- ---- -* CONDUIT CONNECTION FOR

c ON T ROLLER

k 1- A B C

Arrangement A shows the electropneumatic transducer Arrangement B shows the electropneumatic valve positioner

Arrangement C shows the electrohydraulic valve actuator. mounted at pneumatic control valve. on vaive.

Note: Provide seals in electrical leads as required by electrical specifications for area in which actuator is installed.

FI(;. (>-13-ElrctroI>tieumatic and Electrohvdraulic Actuators.

Special considerations for hydraulic cylinders are :

1. If the hydraulic manifold is rigidly piped, it should be connected to the hydraulic fluid supply and the return headers by Hexible metallic hose. 2. To assure a continuous jüpply of liydrziulic Huid to positioner pilots. it is advisable to provide an oil filter. or strainer. and spare suitably valved and piped so that either unit may be removed and cleaned nithout shutting off the supply to the pilot. 3. Vent valves should be provided at high points in the hydraulic fluid system.

4. The use of automatic. fluid-trapping valves to lock the hydraulic fluid in the cylinders to prevent valve movements on failure of the hydraulic system may or may not be required. depending upon nhether the valve will move i f the hydraulic oil pressure is lost. Fluid- trapping valves should be considered for large valves.

Pneumatic piping to cylinder or piston actuators is shown in Fig. 6-1 1. Piping arrangements for hydraulic actuators are shown in Fig. 6-13.

Some of the electrohydraulic actuators have self- contained reservoirs. pumps, and power cylinders. Such units must be mounted in an upright position to permit proper functioning of the hydraulic system. Because pump motors are operating continuously. these units should have adequate ventilation to prevent overheating. Typical piping arrangements for electropneumatic and electrohydraulic actuators are illustrated in Fig. 6-1 3.

d. Motor Actuators

Electric motor-driven actuators should be mounted so that the motor is above the gear box; this arrangement prevents gear oil from saturating the motor windings.

Air motors of the air turbine or nutating-disk typc normally require an aspirator-type lubricator in the air supply line. Pneumatic piping to air motor-actuated valves is shown in Fig. 6-14.

p- FR0h4 CONTROLLER OUTPUT

I I AUXILIARY 1

:i:_': RELAY 4

I

- .-.-A c.: y '

" A

FQCM CCNT90LLiR OUTPUT

LUBRICATOR\J a B

Arrangement A shows air motor actuator with position

Arrangement B shows air motor actuator with auxiliary transmitter.

equipment to close valve on emergency. FIG. 6 - l k A i r Moior Ac-tilators.

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C.l CONTENT Suggested practices for the installation of pneumatic,

electrical. and hydraulic measurement and control signal systems are outlined in this section.

Transmitting or receiving instruments, controllers, regulators. control valves, valve positioners, and the installation of these devices are not covered in this section. except as they may affect or be affected by the type or characteristics, or both, of the transmitting system or medium.

Radio transmission is not discussed herein. Trans- mission and control lines supplied and installed as part of the control panel are described in Sect. 12.

7.2 GESER.4L a. Bask

The installation methods described are those generally used in the United States and are based on the requirements of applicable codes.

The approach is based on the assumption that transmitters. receivers. and other devices are correctly installed.

1,. Deaigii aiitl (biistruction Considerations

The correct installation of measurement and control signal transmission systems becomes increasingly important as oil refining units prow larger and more complex. Personnel safety. unit performance, and the length of runs may be jeopardized if fast. reliable signals are lacking during either normal or emergency operation. Careful design and construction are necessary to obtain satisfactory transmission systems at acceptable costs. Consideration must be given to such items as: 1. Installation of leads so as to reduce possibility of damage by fire or mechanical shock.

. 2. Minimum use of piping carrying viscous, unstable, corrosive. toxic. or freezing fluids, slurries, and crystals. 3. Exclusion of hazardous or noxious fluids from control rooms; exclusion of high voltages (above 150 volts) from control rooms. 4. Reliability of air and power supplies. 5. Provisions for manual control, testing, and ready access to instruments.

c. Loc*ation m i t i Routing of Wireways or Tiillirig Runs

In choosing locations and routing for wireways or tubing runs. facts to be considered are: 1. Overhead conduit must be routed to rcduce accidental mechanical abuse and possible daniage t o wircs or tubes from tire or o\wheating. Conduit should not be located closc to hot lines. 2 . Routing overhead runs along pipe racks may simplify support problems.

3. Underground runs often may be shorter and better protected against fire or mechanical damage than over- herid runs. However. they should be avoided in locations where Hooding with hydrocarbons or corrosive liquids is probable. The following considerations also

a. Underground ducts must be protected against accidental excavation or crushing by passage of heavy equipment over them. Frequently, this protection is accomplished by embedding the ducts in a concrete envelope. Where ducts pass beneath roadways. suitable reinforcing should be provided; for easy identification the concrete is colored. usually red. Parkway-type installations are often protected by a concrete slab.

b. Special attention must be given to the location of pull points in underground duct systems: replacement of damaged wires or tubes may be difficult and extremely costly. Intermediate pull points may be eliminated in bundled tubing rind multiconductor cables by inclusion of sufficient spares. I€ underground routing is expected. the instrument program should be firmly established as early in the project as practicable to avoid costly changes or additions.

Fig. 7- i through 7-6 illustrate comparative pneumatic and electronic arrangements for a typical installation.

d. Signal Types Signal transmission systems in refineries usually are

electric or pneumatic; hydraulic systems have only a limited use for precise operation of large valves or dampers.

apply:

c.

7.3 PNEUMATIC SYSTEMS

a. Staiitlard Pneumatic Traiisiiiissioii Raiipes

Industry practice has been to abide by either of TWO principal transmission system pressure ranges-3 p i g to 15 psis or 3 psig to 27 psig-recognized by the In- strument Society of America (ISA) and the Scientific Apparatus Makers of America (SAMA). Refiners USIL- ally arrange to supply air at a pressure of 20 psig for instruments having 3 psig to 15 psis transmitted air range. This is recommended for new installations. However, 35 psig air pressure is used with springless, diaphragm- type operators which stroke single-seated valves, and 50 psig or 100 psig air pressurr frequently is used for actuatinz piston operators furnished with large valves or dampers. To cnsurc operation in emcrgencics. one re- finer specifies that large actuators shall dcvelop design thrust at SO percent of nominal supply pressurc and shall be mechanically capable of continued operation without damagc at i 20 percent of nomind. For rccom- mendations on air supply and instrument panel board piping. see Sect. c) and 12. respectively.

70



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13. Tuhing

Pneumatic transmission lines are usually %-in. or %-in. OD. The %-in. size is preferred. Tubing is supplied in copper, aluminum, and plastic, either in single strands or in bundles.

1. COPPER TUBING Copper tubing is more widely used than either alu-

minum or plastic tubing. Copper tubing is supplied annealed or half-hard.

2. ALUMINUM TUBING The use of aluminum tubing has become common.

In some locations. where sulfur attack may be expected because of atmospheric conditions. aluminum has shown better corrosion resistance than copper. However, it has not worked well in seacoast locations nor in some petrochemical plants. Also, it seems to be somewhat less resistant to vibration than copper. When installing aluminum tubing care must be taken to provide sufficient insulation between the aluminum tubing and other metals to prevent electrolytic corrosion. Currents caused by arc-welding in the vicinity of the tubing have been found to leave small pinholes in the tubing. Spray or rebound from “guniting” also has been found to produce small pits in the tubing surface; therefore. in areas where considerable guniting is probable. some permanent form of protection should be provided. Aluminum tubing must be protected from any contact with magnesia insulation.

3. PLASTIC TUBING Plastic tubes are light, corrosion-resistant, and have

sufficient resilience to prevent damage from occasional freezing; however, because there is some tendency for the tubing to ”cold flow” and develop joint leaks. care must be used in the selection of fittings. Also. because some plastics burn and most plastics lose strength when the temperature exceeds 200 F, the possibility of overheating or fire damage limits application in outdoor processing areas. unless the tubing is suitably protected; see (d ) .

c. Tuhe Fittings

Many types of tube fittings are used. .Where plastic tubing is used because of atmospheric corrosion. it is good practice to apply a colored plastic coating (acrylic) to the fittings. The color helps to make coating damage visible.

Brass fittings are used with copper tubing and anodized aluminum fittings are used with aluminum tubing; the anodizing prevents mechanical seizure between the tube and fitting.

ti .

The problem of supporting small lines and protecting them against mechanical damage or overheating is diffi-

Support ant1 Protection of Tubing

71

cult and costly to solve. There is no agreement as to the best method of installation. The types of installation in common use are: 1. Support on racks (see Fig. 7-7). 2. Support on expanded metal troughs (see Fig. 7-7). 3. Prefabricated multiple tube bundles (see Fig. 7-8 and 7-9).

A number of manufacturers offer prefabricated supports and attachments for both individual tubes and tube bundles.

1. RACK SYSTEMS Rack systems have the advantage that any tube can

be individually traced, repaired, or replaced. They have the disadvantage of highest installed cost, because they must be assembled by field labor. Also, they provide

,-STRUCTURAL ANGLE CLAMP

-INVERTED CHÄNNËL

CLAMP

TYPE 1 TYPE 2

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STRUCTURAL ANGLES

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END VIEW SIDE VIEW TYPE 3

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TYPE5 V

TYPE 4 Types 1 and 2 may be used for one or more lines with short

spans. These types are suitable for either indoor or outdoor applications.

Type 3 shows raceway for a large number of lines. Only one bank of lines is permitted. Raceway may be trussed with diagonal bars if load conditions require. This type is suitable for both indoor and outdoor use. including tunnels.

Typt? 4 shows raceway carried on angle brackets. Covers are optionai.

Type 5 illustrates a two-tier raceway for a greater number of lines than in Type 4. Design and application are the same as in Type 4.

FIG. I-I-Methotla o f Siipportiiig Instrunieiit Tiiliiig.



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API RP 550-PART I

z MULTIPLE TUBE BUNDLES

THIS ASSEMBLY IS FREOUENTLY ENCLOSED IN A JUNCTION BOX

TUBE FITTING

TUBE FITTING

MULTIPLE TUBES

FIC. 7-8-Dvtail Showing Ways to Break Out Leads from Armored Multiple Tubes.

the least protection against mechanical damage to tubes unless provided with covers. Some companies assemble sections of vertical racks on the ground before erection. The use of prefabricated support bars permits reductions in cngineering and field costs.

2. TROUGH SYSTEMS Trough systems, like rack systems. have the advantage

that any tube can be individually traced, repaired, or replaced. In addition, they provide somewhat greater protection against mechanical damage to the tubes than do rack systems. Installed cost is generally somewhat lower because less field fabrication work is required.

3. BUNDLED TUBING Several manufacturers offer bundled tubing in a vari-

ety of materials and protective coatings: Copper or uluminrim tubing is furnished in parallel

or spirally wrapped layers, one tube in each layer being color-coded for identification. The tube nests are wrapped with plastic-impregnated cloth tape. or an extruded plastic coating is applied. Interlocking galvanized steel, aluminum, or other metallic armor can be obtained which will give protection equal to that of "parkway-cable" electrical wire which is used extensively for direct burial.

Plusric tubing is available either with an extruded

plastic sheath or with armor sheathing where needed. Where fire damage is a consideration, the tube bundle may be covered with pipe insulation. Bundled plastic tubing also is available with a heat-resistant asbestos jacket over the extruded sheath and covered with a plastic protective jacket. This material can be used underground and in conduit.

Field installation of bundled tubing can be made very simple if thorough engineering is done and cable termination points are carefully selected. ,Copper or aluminum tube bundles have high-tensile strength and may be bent on short radii. Bundles may be run in expanded metal troughs, on steel messenger cable. or supported by suspension from pipe stanchions. Bundles may also be attached to building walls or structural members with pipe clamps or special fasteners.

Junction details are shown in Fig. 7-8 and 7-9. Ter- mination may be made easily with standard bulkhead tube fittings. Taps also can be made readily.

4. PRECAUTIONS

bundles are:

identified in any given length.

Some precautions to be observed in installing tube

Tubes must be color coded or their ends permanently

Spares must be determined and installed initially. Tubing system must be laid out carefully with all re-

quirements stated because the bundles must be ordered precisely.

5. PLASTIC JACKETING Plastic jacketing of metal tubes appears advantageous

in preventing corrosion; single tubes as weil as bundled tubes are available with these protective jackets.

6. PROVISION OF SPARE TUBES AND RACK SPACE In bundled tube systems it is good practice to initially

provide spare tubes, at least 10 percent or not less than two tubes per bundle. Also, spare rack space should be provided initially in all systems. Space for a 25-percent increase in leads is suggested.

e. Connections to Instrument Equipment

Inasmuch as air supply details are covered in Sect. 9, panel board details in Sect. 12, and other special items elsewhere, only two precautions will be discussed.

1. TRANSMITTERS Some refiners install a valve or cock in the signal

connection from each transmitter to permit blocking off the signal line when testing it for leaks. This valve is located between the transmitter and the tee connected to the output air pressure gage, so that the gage may be used to indicate line leakage.

2 . RECEIVING DEVICES It is good practice to install a shutoff valve in the air

lead to each transducer or other receiving device to

72



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LJ

SECT ION A-A

THIS ASSEMBLY IS FREQUENTLY EWLOSED IN A JUNCTION BOX

FIG. 7-9-Detail for Bundled Tubing.

DETAIL A

permit taking the device out of service without disturbing other instrumentation or control.

7.4 ELECTRICAL SYSTEMS

a. Background

It is essential that those responsible for electrical design and installation in refinery areas be thoroughly familiar with the National Electrical Code (NEC) and API RP 500 definitions, and with the nature of explosionproof enclosures. Recommended installation methods are described in API RP 540: Recommended Prac- tice for Electrical Installations in Petroleum Refineries.

The specification for equipment, enclosures, wiring, and installation methods should fully conform to the requirements of the latest edition of the NEC, the National Electrical Safety Code (NBS Handbook H 3 0 ) , and the regulations of authorities having jurisdiction at the job site.

b. Plant Areas Where Atmosphere May Contain

It is recommended that designs of electrical instalia- tions, where there may be some hazard from the pres- ence of volatile flammable liquids, gases, or vapors in explosive or ignitible quantities in the atmosphere, be

Flammable Vapors

based on the requirements of Art. 500, NFPA Bulletin No. 70: National Electrical Code, and API RP 500: Recommended Practice ,for Classification of Areas for Electrical Installations in Petroleum Refineries. The latter is intended as a guide in classifying potentially hazardous refinery areas and in determining their extent.

1. TYPES OF AREAS

These publications recognize three types of areas : Division I areas: The criterion for these locations is

that they are likely to be hazardous. Division 2 areas: The criterion for these locations

is that they are likely to be hazardous only under abnormal conditions, such as the failure or rupture of equipment.

Nonhazardous areas: These are locations which cannot be classified as Division 1 or 2.

2. INSTALLATIONS FOR DIVISION 1

Division 1 areas require the use of explosionproof equipment which is designed so that operation or failure of any portion of the electrical system, even though causing vapor ignition within the housings, will not release flame or hot gases so as to ignite the surrounding atmosphere.

73



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API RP 550-PART I

3. INSTALLATIONS FOR DIVISION 2 Division 2 areas require the use of equipment ar-

ranged so that full operation of the electrical system (including arcing devices such as open contact switches, relays, etc.) may occur without providing a source of ignition under normal conditions. Complete protection is not provided against electrical breakdowns, inasmuch as these occur very rarely and the equipment usually is de-energized promptly in such cases.

c. Intrinsic Safety

Intrinsically safe equipment and associated wiring are covered in Art. 500 of the NEC. Such approved equipment and construction may be used in any hazardous location for which they are approved: certain other provisions of Art. 500 and 510 will not apply to such installations. Intrinsically safe equipment and wiring are incapable of releasing sufficient electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture. Abnor- mal conditions include accidental damage to any part of the equipment. wiring or insulation. or other failure of electrical components, application of overvoltage, damage during adjustment and maintenance, and similar conditions.

1. APPARATUS A N D CIRCUITS The British have developed intrinsically safe appa-

ratus and circuits based on the principles set forth in the preceding paragraph.

2. APPLICATIONS Although intrinsically safe circuits are limited in ap-

plication, they have been used in England and Canada for some time. However. their use is just beginning to be accepted in the United States.

3. INSTALLATION Because there are no accepted standards of design

nor methods of test for intrinsically safe equipment. it is necessary to install each circuit so that any faults which may develop in the circuit. either within or outside the classified area. cannot create a spark of sufficient energy to ignite a combustible mixture within the classified area.

d. Support and Arrangeinent of Conduit Systems

The following methods are suggested for the support and arrangement of conduit systems, recommended in the absence of electrical specifications covering the entire plant: 1, Rigid conduit should be supported at least once every 7 ft; other types should be supported at closer intervals. 2. Conduit should never be supported from piping, because the piping may have to be removed for inspection or replaced.

3. Provision should be made for thermal expansion or movement of supports, such as swaying of towers in high winds. 4. Conduit should be fastened with pipe clamps or U-bolts. not tack-welded. Substantial steel hangers should be provided for groups of conduits where it is not practicable to clamp directly onto building walls or structurai members. 5. The distance between pull points in a conduit system should not exceed 250 ft for straight runs. Sixty feet should be deducted from straight-run length for each 90 deg of bend. or 243 ft per deg of bend on other than 90-deg bends. Pull fittings should be used to avoid more than 270-deg total bends between pull points. Where manufacturer's ells. or plastic or synthetic rubber wire insulations are used. it may be necessary to reduce the distance between pull points. 6. Drains and seal fittings must be provided in accordance with the NEC. 7 . Where fire damage is possible, flame barriers (often of asbestos sheeting) should be provided. Insulation of conduit should be avoided unless possible electrical overheating of wire insulation has been checked. For thermocouple or other low-current signal wires, however, jacketing conduit with suitable pipe insulation (85 percent magnesia, fiberglas, etc.) will help to protect against flame. S. Junction conduit fittinip should be installed near furnaces or other locations where wire insulation damage seems probable.

e. Conduit Materials

Conduit materials should be as specified in ASA Stanclarcl C80. I : Specification for Rigid Steel Conduit, Zinc Coated.'"

f. Wire Insulation

Because refinery conditions vary considerably, no one type of insulation has been found to be suitable for a majority of installations. When selecting insulation, the possibility of temperature extremes or of chemical or electrolytic action should be considered. The NEC gives temperature and current limits as well as corrections ?or most grades of wire. The following types of insulation are used: I . Nutrrral rubber, bruitl-covered: Type R code rubber. Immersion in petroleum liquids will cause rapid deterioration of rubber-base materials. 2 . Neoprene: Neoprenes are affected by aromatic hydrocarbons and have high coefficients of friction. 3. Polyvinyl plastic: Type TW is quite popular for general use. This wire should be carefully selected heavy-duty grade rather than building grade. 4. Poldvethylene plastic: Polyethylene will burn readily unless compounded with a flame retardant. 5. Enamel und felted usbestos or spun glass: Frequently

'' Figures refer to REFERENCES on p. 78.

74



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specified in hot locations. Such insulation deteriorates rapidly on Hexing. especially after being heated. Also. it is subject to moisture deterioration. Often it is preferable to splice to a more impervious material for leads extending beyond the hot location. 6. Type MI: Metal-jacketed, magnesium oxide insulated wire. ï h i s material has excellent heat resistance, except that in hot locations the method of sealing terminations or splices will limit the maximum allowable temperature. The magnesium oxide, which is very hygro- scopic, must be kept dry. Cut ends of the cable must be sealed immediately to prevent absorption of moisture. This may be done by immediately installing the terminai fittings, properly packed with sealing compound, or by temporarily sealing the ends by suitable waterproofing. 7. Weatherproof braid over rubber: Used by many refiners for general service; others prefer the superior resistance to hydrocarbons (other than aromatics) of polyvinyl plastic. 8. Lead sheathing: Used for underground wiring; however. lead is attacked by caustic solutions. hence, Type TW often is more suitable.

1 ; 6 "M iN FOR DRAINAGE-

g. Signal Transmission Circuit Wiring

Electrical signals from measuring and analyzing devices differ in their requirements for shielding. insulation. twisting of leads. coaxial cable. or exclusion of measuring and power leads from the same conduit. Manufacturer's requirements should be checked carefully before specifying circuit arrangement or materials.

1. SPARE LEADS Usually. spare leads are installed initially; often one

spare pair is provided for a minimum of four leads of each type and for each multiple of eight leads. 2. THERMOCOUPLE WIRING

Sparking hazard: Thermocouple wiring usually is considered as offering negligible sparking hazard. In- dustry experience has shown that failure of the potentiometer measuring system, in a manner which would permit a sparking hazard. is unlikely in normal service.

Conduit and fittings: Thermocouple extension wire usually is run in conduit and vaporproof fittings are installed (see Fig. 7-10). When attaching conduit to thermocouple wells, some sort of flexibility is necessary. As a rule, this is arranged with a loop of flexible conduit, although where the extension wire insulation is weatherproof and chances of mechanical damage are slight, some companies run the wire in the open for the last foot or so. The possibility of well breakage must be provided for in situations where the sheath is connected to the thermocouple head. Usually, the conduit is sealed off and a vent hole is left on the thermocouple side of the seal. Seal fittings are used to isolate the flexible conduit entrances and, also, to seal off conduit entrances to control rooms (see Fig. 7-10). Seals also should be provided where the conduit enters field instruments.

i c

-LADDER AND SAFETY GASKET

30"LCNGTH CF S T D

-LADDER AND SAFETY GASKET

30"LCNGTH CF S T D

PERFORATED PIPE CAP

I 8

INSTRUMENT

FIG. 7-10-Conduit Arrangement for Thermocouple Extension Wires.

Protection in control room: Thermocouple leads are often run in sheet metal wireways inside control rooms. Exterior conduits terminate in a junction box and leads are carried in wireways to panels. Thin-wall conduit and flexible conduit (Greenfield) are acceptable between wireways and instruments in nonhazardous areas, or in Class I, Division 2 areas if properly sealed and grounded.

Extension wire: Thermocouple extension wire is usually No. 16 Awg minimum, solid duplex. Multicon- ductor 20-gage cable with plastic jacketing is frequently used. For sizes and types of thermocouple and fully compensated extension wires, see Sect. 1.1, 1.2, and 1.7 of ISA R P 1.1-.7: Thermocouples and Thermo- couple Extension Wires.3 For the correct combinations of thermocouple and extension wires needed for accurate measurements, refer to Sect. 3.

Junctions: Junctions in thermocouple extension wire should be made with labeled, insulated terminal strips in conduit fittings or junction boxes (see Fig. 7-1 1 ) . A system arrangement for use where several thermocouples are involved is shown in Fig. 7-12. Watertight outdoor junction boxes and gasketed sheet metal indoor junction boxes are preferred. Some companies avoid the use of junctions (other than splices ) in therniocouple leads, particularly those going to controllers. These companies believe that temperature differences at junctions can cause errors in measurement.

Connections to iristruments: Thermocouple wires to controllers usually are not connected to any other

75



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TERMINAL SLOCK NOS. 1. 4. 7,

NO.l TERMINAL SCRE FOR EACH BLOCK

SC HE MAT IC FOR

THERMOCOUPLE JUNCTION BOX

I @-@I I

TERMINAL BLOCK Notes: I . Each junction box should have a nameplate mounted on

the outside of the cover, or door, marked “Thermocouple Junction Box A, or B, or C,” etc.

3. The terminal blocks should be numbered so that No. 1 block is in the tipper left corner of each junction box, No. 2 block directly below it, etc.

3. The top pair of terminal screws on each block should bs numbered 1, with pair No. 2 directly below it, etc. 4. A pair OE terminals might be referred to as in the follow-

ing example: “A-5-8” indicates junction box A, terminal block No. 5. terminal No. 8.

5. The tagging strip should show the number of each thermocouple; the service descriptions for each thermocouple should be inscribed on a heavy card secured in a frame attached to the inside of the box cover.

FI(;. 7-1 I-Coniiectioiis i n Thermoroitple Junction Boxes.

instrument. It is considered good practice, however, to connect at least one thermocouple from each recorder to an indicator to permit checking instrument readings. Many companies connect all recording (but not controlling) couples through the indicator to the recorder.

Cold junction compensation: Cold junction compensation should be accomplished in accordance with the recommendations in Sect. 3.

3. RESISTANCE THERMOMETER WIRING Circuit classification: Resistance thermometer circuits

usually cannot be classed as intrinsically safe, therefore wiring methods must conform to the classification of the area as specified in Art. 500 of the NEC.

Conriections: Resistance thermometers usually are connected in a manner similar to thermocouples, except that extension wires are copper. Where bridge-rneasur- ing circuits are used, three wires usually are run to each thermometer bulb to place the bridge arm junction at the elemen t.

4. ANALYZER SIGNALS For analytical equipment. it is universal practice to

not run measurement leads in the same cable or conduit

with power leads. As with other transmission circuit conduits between units in which component failure might allow hazardous process fluids to enter the conduit system. analyzer signal and control conduits should be doubly-sealed with an adequate vent between.

5. OTHER CONTROL WIRING

All other signal wiring should meet NEC requirements, particularly those of Art. 500 which apply to the area classification wherein wiring is located. A suggested electronic transmitter installation 2 is shown in Fig. 7-1 3. For general guidance on the proper handling of refinery wiring installations, see API R P 540.

For Division 1 areas rigid conduit, installed with explosionproof fittings and flexible connectors, is required. All arcing devices must be completely enclosed in explosionproof housings and isolated from the conduit with sealed fittings.

For Division 2 areas rigid conduit, installed with vaporproof fittings, generally is used outside control rooms, except that arcing devices are enclosed and sealed as €or Division 1 areas. in all cases, the conduits should be sealed at both ends and vented between seals.

Sheet metal wireways of substantial construction are employed occasionally for multiple runs. particularly inside control roomst where acceptable to local code- enforcing authority.

Intrinsically safe instrument leads may be installed in a similar fashion to thermocouple leads provided manufacturer’s requirements pertaining to static or inductive pickup are met.

I t is well to remember that line fluid may be released while checking or servicing flow, pressure, and similar electrical transmitters; therefore when installing these instruments particular care should be taken with e1.ec- trical wiring.

6. MOISTURE PROTECTION ‘

insulation to prevent entrance of moisture. Care must be taken to seal connections, wiring, and

7.3 HYDRAULIC TRANSMISSION SYSTEMS FOR CONTROL

The principal advantage of hydraulic transmission is that equipment requiring considerable force to actuate it can be quickly and very accurately operated.

The principal disadvantage is the relative bulk of the measuring, controlling. and storage equipment. and of hydraulic medium supply systems.

a. Types

There are two types of hydraulic control systems- local and central. Requirements especially for the central system are covered in Sect. 10. Connections to actuators are described in Sect. 6.

76



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THERMOCOUPLES

I I I I I I LOCATED ON

ONE COMMON BOX MAY BE USED IF A.B.AND D ARE ON THE SAME OPERATING PLATFORM

JUNCTION BOXES .(A.B.C,D,E.AND F)

LOCATED ON GROUND

3NE COMMON BOX MAY BE USED FOR FAND CWHEN LOCATED

IS KOT EXCESSIVE ON THE GROUND IF DISTANCE

LOCATED IN

i TEMP CONT. NO TRC-5 a Thermocouple

No. 1 2 3 4 5 6 7 8

:E Esnrnple, B-2-6: B denotes terminal 2 denotes terminal 6 denotes terminal Notes:

Service No. 4 cracking coil, oil to heater No. 4 cracking coil, oil from heater control No. 4 cracking coil, vapor from No. 1 tower No. 4 cracking coil, vapor from No. 2 tower No. 4 cracking coil, vapor from No. 3 tower No. 4 cracking coil, tar from No. 1 rower No. 4 cracking coil. tar from cooler No. 4 cracking coil. oil from heater

junction box. block number. number.

Instrument No. Terminal Location

TI-2 TR-8 TRC-5 TI-2 TR-8 TI-2 TR-8 TI-2 TR-8 TI-? TR-8 TI-2 TR-8 TI-2

B-1-1 D-1-1 B-1-2 B-1-3 B-2-4 B- 1-5

:> B-2-6 A-1-1

c-1-1 F-1-1 c- 1-2 C- 1-3 c-1-4 F- 1-5 C-1-6 c-2- 1

E-1-1 E-1-2 E- 1-3 E- 1-4 E-1-5 E-1-6 E-2-1 E-2-2

i . At temperature controller locations. two thermocouples are often placed in the same thermowell with one connected to the indicator.

2 . Junction boxes A, B, C, D, E. and F are required only in the vicinity of towers, heaters. reactors, or major vessels, o r where the total conduit run exceeds 500 ft.

3. All junction boxes should be mounted so that the top of each box is approximately 5 f t O in. above grade or platform. There should be a Yi-in. drain in the bottom of each box.

FIG. 7-12-Thermocouple Wiring Arrangement.

1. LOCAL SYSTEM The local system is usually a “packaged operator”

for a valve, damper, or regulator, which receives a signai from an electric or pneumatic system and positions the controlled element. It consists of a pump, usually elec- tncaily driven. a fluid storage chamber, a pilot-operated regulator, an actuating cylinder, and sometimes a pressure tank. 3,. CENTRAL SYSTEM

-MUST BE EXPLOSION PROOF EQUIPMENT IN CLASS I AREA

- PREFERABLY RIGID CONDUIT, BUT VAPOR TIGHT FITTINGS

AREA

The central system supplies several valve actuators with fluid under pressure from a common tank often maintained at pressures within a range of 200 psig to 500 psig. Fluid is returned to atmospheric storage.

b. Pipe Material Seamless or fusion-welded pipe should be used. S u p

ports should be in accordance with good piping practice.

MAY BE USED IN DIVISION 2

DRAIN TO RECEIVERS I N CONTROL ROOM NEAR CONTROL VALVE

INDICATOR IS USUALLY LOCATED

FIG. ’I-13-Iiistallatioii of Electronic Transmitter.

77



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1 . LOCAL CONTROLLER Some refiners specify %-in. Schedule 80 pipe for a

local controller. Other refiners prefer !h-in.-OD tubing. For fast valve positioning, larger tubing or higher pressures should be used.

2. CENTRAL SYSTEM For the central system, a %-in. minimum iron pipe

size. Schedule 80 wall thickness is often specified. Fit- tings should be of forged steel. either socket-welded or screwed and back-welded. Connections are usually 94 in.; valves are often of the bolted bonnet integral-gate type. Piping usually is pickled for scale removal after installation.

í .6 INSPECTION AND TEST

This section is based on the assumption that the user has satisfied himself that all measuring and control equipment is suitable for his service, adequately fastened in place, properly connected, fully capable of operation, and properly calibrated.

II. Piieuiiiati<* Traiisiiiission Systems

Pneumatic transmission systems should meet the requirements of ISA RP 7.1: Pneumntic Control Circuit Pressirre Test.

1). Electric-ai Traiisinissioii Systems

Electrical transmission systems should meet the following specifications: i . ”Merger“ wire-to-wire insulation resistance of transmission leads and wire-to-ground insulation. Instrument must be disconnccted. Any readings below 1 megohm line-to-line or 1 megohm line-to-ground may indicate faulty insulation.

2 . Check all shielded circuits, and all circuits whose connected instruments may be sensitive to inductive pickup, by putting 60-cycle alternating current (or other normal frequency) on all other circuits in the same conduit with circuit being checked to see whether any shift in instrument reading occurs.

c. Hyclraulic Transmission Systems

Hydraulic transmission systems should be flushed clean and hydraulically tested to 150 percent of maximum operating pressure but to not less than 100 psig.

REFERENCES

A S A CSO. I : AriiericLrii Sttiridtrrrl Spi,cificrrtiori for Rigid Steel Coritlirir. Ziric Corited. Am. Std. Assoc., New York (1959). ‘ ISA RP I2 series, Instr. Soc. Am.. Pittsburgh? Pa.:

LI. ISA R P 12.1: Electrictrl Jrisfriiiiierits i n Hninrdoirs

b. ISA R P 12.4: 1ii.rtriri~ient Piirgiii,q f o r Redirction of

c. Companion Recommended Practices due for publi-

Atriiospliercs ( 1960).

Htirtirdoiis Aren Cltissificcrriorr ( 1960).

cation include:

Desigriotion Sirbject RP 12.7 Intrinsic Safety RP 12.3 Explosion-Proof RP 17.5 Sealing and Immersion KP 17.6 Wiring Practices

ISA R P I .I -.7: Tlicriiiocorrples niid Tlirrriiocorrple Esten-

ti. Sect. 1 . 1 : “Coding of Thermocouple Wire and Ex-

h. Sect. 1.2: “Coding of Insulated Duplex Thermo-

c. Sect. 1.7: “Temperature-EMF Tables for Therrno-

‘ ISA HP 7.1: Priciiriidc Coiirrol Circirit Pressirre Test,

simi Wires. Instr. Soc. Am., Pittsburgh. Pa. (. 1959).

tension Wire.’’

couple Extension Wires.”

couples.’’

Insti. Soc. Am.. Pittsburgh, Pa. (1956).

78



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

CDPY BOVii153 i d3 HISTMICA1 PURPOSES ONLY

SECTION 8-SEALS, PURGES, AND WINTERIZING

8.1 CONTENT

This section presents guides for sealing, purging, and winterizing measuring instruments and their connecting lines from the process to ensure reliable instrument performance. Measuring instruments include such types as flow, level. and pressure.

Seals and purges are the means of preventing the measured material from entering the instrument or instrument lines and causing improper operation or damage to the instrument through vaporization, condensation. viscosity effects. corrosion or sedimentation.

Winterizing refers to the methods used to ensure proper performance of instrument systems at low- ambient temperatures.

Special cases of analyzer winterizing are covered in Part II of this manual.

8.2 SEALS

a. General

In the past. the use of seals has been a troublesome feature in refinery instrumentation. especially with flowmetering equipment. The use of force-balance and other types of negligible-displacement transmitters, which are close-coupled to the point of measurement. eliminates much of the need for seals. Where sealing is necessary, these transmitters permit sealing without chambers and complicated piping arrangements.

DIAPHRAGM SEAL PIPING SEAL DIP€ FILLED WITH SEALING LIOUID TOTEE. SEALING LIQUID TO BE HEAVIER THAR LINE

SEAL FLVID

31APHRAGM

i'

SEAL CHAMBERS

7 /TYPE e CHAMBER FIG 8-4 E. Is-

h , -

y ? SEAL LIQUID HEAVIER SEAL THAN L,F LIGHTER THAN

LIQUID MEASURED LIQUID MEASURED

8 SEE SECTION 4 FOR PIPING DETAILS AND GAGE SUPPORTS

FIG. 8-1-Seals for Pressure Gages.

I). Diaphragm Seals

One method of sealing is to use a flexible diaphragm or bellows to hold a seal liquid in the instrument and to mechanically separate the sealing medium from the measured material. This method. shown in Fig. 8-1, primarily is limited to pressure gage protection (refer to Sect. 4).

e. Liquid Seals

A common method of sealing instruments makes use of a liquid that is immiscible with the measured process fluid. The immiscible liquid should have a density differ- ing from the density of the measured process fluid. With this method, instruments of negligible displacement, which may range from minimum to maximum without appreciable movement of liquid in the connecting lines, may be readily sealed in the piping. Instruments with appreciable displacement require seal chambers and special piping arrangements for control of the seal level in order to prevent hydrostatic errors. Typical seal installations are shown in Fig. 8-1 through 8-3.

CI. Seal Chainhers

Through general practice a few types and sizes of seal chambers. or condensate pots. have been developed which meet most sealing problem requirements. Details for two of the most popular types of seals used are shown in Fig. 8-4. Seal chambers are available from manufacturers or may be made in the field. The materials and fabrication should comply with ISA R P 3.1: Flowmeter Installations, Seal and Condensate Cham bers.

e. Sealing Liquids

Water or ethylene glycol and water mixtures have the requisites for most sealing operations in the petroleum

S E A L T O T E E \ u SEAL TO TEE BOTH SIDES

SEESECTION I FOR PIPING DE lA ILS

TYPE A - FLOW

SEE SECTION 2 FOR PIPING DETAILS

TYPE - L E V E L

FIG. ö-2-Se;ils for Force-Balance Iiisiruiiieiits.

79



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

ONE VALVE BYPASS OPTIONAL

I*

-BYPASS OPTIONAL-

TO METER MANIFOLD TO METER MANIFOLD

TYPE A -LIQUID FLOW SEAL HEAVIER THAN LINE uauio

TYPE B -LIQUID FLOW SEAL LIGHTER THAN LINE LIOUiD

LINES SELF-DRAINING TO ORIFICE TAPS

a

TYPE C - G A S FLOW

*USE OF SIDE TAPS MAKES VAPOR TRAPOF TOP OF CHAMBER ORSEDIMENT TRAP OF BOTTOM

Y I TO INSTRUMENT

TYPE D - LEVEL

FIG. 8-3-Seals for Mercury Meters.

-5 > LIOUID CAPACITY THRU C MVST EXCEED INSTRUMENT DISPL4CEMENT IF SIDE TAPS ARE USED i

ENDS FORMED,CAPPED ORFLAT

CONNECTIONS ~/Z”NPT 6000-Lû HALF COUPLING OR ‘WELD P4D

TYPE A

I

TYPE B Noce: ASA B31.1: Code f o r Pressitre Piping governs ma-

terial and fabrication. Refer, also. to ISA RP 3.Z: Flowmeter ltisfaíl~ifiotis, Seal nrid Coriderisrife Clicini bers.

FIL. %Seal Chambers.

1 i U

vl w U GLYCOL ABOVE 64%CAUSES 3 + REVERSALOFCURVE

- 0 - INCREASE OF ETHYLENE

œ 2 -20 I l ) i I /

j !

V o VOLUME ETHYLENE SPECIFIC GLYCOL GRAVITY

10 1.016 2 0 1.030 30 I . O M 4 0 1.058 5 0 1.070 6 0 1.081

\ I ? 2 W !- -- -40 -

-60 I I I ! 1 IO 20 30 40 50 60 70

% VOLUME Of ETHYLENE GLYCOL

Note: Curve does not represent true freezing point of ethylene glycol and water solution. It gives recommended mixtures which assure the proper operation of a sealed instrument.

FIG. 8-5-Ethylene G l y o l and Water Solution.

industry and are used almost to the exclusion of any others. Ethylene glycol should be the inhibited type to prevent it from becoming corrosive. Characteristics of ethylene glycol and water mixtures are given in Fig. 8-5. Other sealing liquids and their properties are given in Table 8-1.

8.3 PURGES a. General

Some instrument applications are made possible by the use of purge fluids which may be liquid or gas. These fluids are introduced into the instrument lines and flow out through the instrument taps. The purge liquid serves to seal the instrument and to sweep the lines clean of the measured material which tends to enter the instrument lines. Typical purging arrangements are shown in Fig. 8-6 and 8-7.

11. Purge Fluids Pursing of instrument lines requires a suitable purge

liquid or gas at a pressure sufficiently high to ensure continuous and even flow of the purge into the instrument lines. For example, purge oil should be clean, free of a tendency to flash, noncontaminating to products, and available at a pressure greater than that of the measurement. Purge gases must be clean, dry, and compatible with process conditions. Generally, the purge is fed continuously at a controlled rate. Restriction orifices or sight flow indicators with adjustable restrictions are used to determine and limit flow. They may be combined with differential relays for controlling the flow. The point of entry of the purge into the instrument lines should be as near the instrument tap as possible in order to hold the

80



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s1 COPYRIGHT 2003; American Petroleum Institute


--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

-. - $GATE VALVE

;“GATE,

CAGE OPTIONAL FILTER OR STRAINER 0’ / KCHECKVALVE

-.*i I

i PIPE OFFSET TO PREVENT DRAIN BACK. TURN 180° IF SEAL IS HEAVIER THAN FLUID MEASURED

TO IN2

\-URGE CONTROL ORIFICE IN SCREWED UNION OR SIGHT FLOW INDICATOR OR FLOW CONTROLLER

RUMENT

TYPE A

MEASUREMENT FLOW RATE CATALYST 30 TO 120SCFH LEVEL(IN VACUUM I .OTO 1.5 SCFH

SERVICE)

TURN LINES AT A IF INSTRUMENT IS BELOW TAP

i VALVES

\PURGE UNIT, RESTRICTION ORIFICE, /’ OR DRILLED GATE VALVE

TYPE E

Type A is suitable for purging pressure and vacuum gages. Type B is suitable for catalyst measurement as well as for

level in vacuum columns or low-pressure vessels. FIG. %&--Purge Iiistallations.

TURN LINES UP IF METER IS ABOVE, 1

7GATE VALVES

TYPE A PURGE LIGHTER THAN LINE FLUID WITH SURGE POT

TURN LINES UP IF METER IS ABOVE,

) h A - R w I I

I ’

TYPE B PURGE HEAVIER THAN LINE FLUID WITH SURGE POT Note: Where the restriction (R) is an orifice plate, the fol-

Orifice Flange lowing quantities may be used for calculations:

Drilling Gas Flow Liquid Flow (Inch) (Cubic Feet per Hour) (Gallons per Hour)

‘A 1 .o 3.0 Yß 7.0 5.0 !/z 5.0 8.0

FIG. ö-7-Purges for Flowmeters.

pressure drop in the lines, as a result of flow, to a minimum.

c. Rate of Flow The rate of flow to be established in any purge in-

stallation has a range of variance limited on the low side by extremely small orifices or restrictions, which become difficult to maintain, and on the high side by excessive flow-producing instrument errors. There are too many factors involved to attempt to set high flow limits. Usually, if errors exist because of purge flow they are made apparent by momentary interruption of the purge ñow. Care should be exercised in applying purge rates to orifice flanges, as the orifice tap is bottom-drilled with either a %-in.. ?”-in.. or %-in. drill. The %-in. orifice drilling may prove restrictive for the higher purge rate.

The purge rotameter is the most convenient device to determine and establish purge flow. One company offers a standard purge rotameter with ranges of 0.38 to 3.8 gph of water and 0.2 to 2.0 std CU ft per hour of air at 10 psig. These ranges are satisfactory for purging against clean liquids; however, where the material measured tends to clog or deposit sediment. the ranges should be extended. Another company offers a rotameter range of 0.1 to 8.0 gph of water and 0.1 to 30.0 std CU ft per hour of air at 10 psis. These ranges cover most purge applications. except those used with catalyst measurements.

Restriction orifices properly sized and installed give reliable service. Fig. 8-7 suggests quantities to be used in calculating purge orifices for limiting flow through orifice flange taps.

(1. Surge Chainhers

. Where the rate of purge flow is not greater than the displacement of fluid brought about by the operation of the instrument, surge chambers, or pots. should be installed as shown in Fig. 8-7.

8.4 WINTERIZING

a. General The need for winterizing and the methods of protect-

ing instruments vary with the severity of the winters in a particular locality. To a great extent, winterizing requirements are influenced by individual plant practices.

1. LINES The need for and the degree of winterizing required

for the lines will vary with the material being measured. Lines which contain dry, nonviscous, and nonfreezing fluids. with pour points below the minimum temperatures encountered, can be installed without any protection. Lines which contain liquids that can freeze, set up, or carry traces of moisture should be protected by steam tracing or some form of heating and insulation.

82



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

SEALS. PURGES, AND WINTERIZING

2. INSTRUMENTS

insulation, or housing and heating, or sealing.

b. Steam Tracing and Heating

As steam is usually readily available in a reñnery, it is generally used as the heat medium for winterizing. Properly applied steam heating always is effective and usually is trouble-free. However, it can be a possible source of instrument error if overheating or uneven heating results from improper application. Steam tracing and heating arrangements are shown in Fig. 8-8.

Two methods of steam tracing generally are recognized. One method, called heavy tracing, refers to steam tracing in direct contact with the connecting lines or instrument. Where maximum heating effects are desired. the tracing may be cemented to the instrument or connecting lines with a thermal transfer cement. The other method, called light tracing, refers to steam tracing placed away from the equipment it protects. by spacing or by insulation, to prevent the hi-gher heat input of direct contact (see Fig. 8-9). When the instrument requires heating, it is generally recommended that it be provided with a housing which can be heated. Addi- tional details for steam tracing of lines and instruments are shown in Fig. 5-8 through 5-12.

c. Electrical Heating

The advantage of electrical heating is that heat input can be tailored to the application and can be controlled

Instruments may require protection by heating and

<EEP TRACNG FñEE OF ?OCKEÏS

AEiGHTOF "OC.<ETSTO A+B+.**',.,% @ C

kEATING STE,AM .+€ADER PRESSURE. ?SiG

= -3S5.tL.z. F 9OCKETS E X ! S T L IMIT TOTAL SUM UF

2.3

FEET FEET

-INSULATION 1 SEE FIG. 8 -9 FOR DEÏAILS

/INSULATED IOUCING

HEATING COIL MOUNTED ON @OTTOM OR /SIDE OF -0JSING ANCHOR AND Sh lEL3 TO

PRËVEYÏ ACCIDENT CONTACT A N D BURNS COIL MAY 8 E MADE IN FIELD FROM

TUBING 9ACiATING SURFACE REQUIRES COPE? TUBING. USE 3/8"0~ LARGER

VARIES WITH CLIMATE, SIZE OF~OUSING. IOUSING INSULATION.AND S Ï E P M PRESSU9E

-STEAM T 3 A P OR HAND VALVE OR USE LIQUID EXPSNSION TWERMOSTATIC VALVE FOR CONTROL OF HOUSING TEMPERATURE

TYPICAL METHOD OF STEAM TRACING AND HOUSING FOR FIELD-MOUNTED PRESSURE INSTRUMENT

FIG. 8-8-Steani Tracing aiid Heating.

83

HEAVY TRACING LIGHT TRACING WEATHERPROOFING

PIPE COVERING Ve" MAGNESIA OR EQUIVALENT

SBESTOS PAPER CER CONTACTS

I INSTRUMENT LINES

METER LINES

TRACER

STEAM TRACING AND INSULATION METHODS FOR INSTRUMENT LINES

LIGHT TRACING

TOTRAP TYPICAL FOR SEAL POTS

PROVIDE TRACER WITH DISCONNECT UNIONS FOR EASY REMOVAL OF GAGE. TRACING FOR GAGE WITH BLOWOUT

TRACER MAY CONTACT PIPING BUT NOT GAGE

WEATHERPROOFING) BLOCK ASBESTOS' ASBESTOS PAPER

TYPICAL FOR GAGES STEAM TRACING AND INSULATION METHODS

FOR PRESSURE INSTRUMENTS

Note: Insulation must not be applied in a manner which

FIG. ô-9-Steam Tracing and Insulation Methods for Iiistrument Lines and Pressure Instruments.

will obstruct blowout protection features.

thermostatically at a reasonable cost. When selecting heating elements, care should be exercised to assure that they are not potential sources of ignition. MI cable with low surface temperature is frequently used for this purpose. Reference should be made to NFPA Birllerin No. 70: National Electrical Code, Art. 500. Sect. 50 13-b-2. The increasing use of electronic control instruments should expand the use of electrical heating by providing a convenient source of electricity for heating purposes; see Fig. 8-1 3 ( A ) .

d. Hot-Water Heating

satisfactory in mild climates. Hot-water or steam-condensate heating may prove

e. Process Flow Heating

Where there is a continuous flow through a line of a material with a suitable temperature, the heat available therefrom may be utilized for instrument protcction. Fig. 8-13(B) is representative of such an installation.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

TRACER ‘/n’‘OR 3/g”TUBING PROVIDE TRACER SHUTOFF VALVE AT SOURCE I

I I l l

n a-.

I

I TOTRAP ! TOTRAP

,WEATHERPROOFING 1 /‘)/TRACER-,\\ ’ PIPE COVERING

REFLEX TRANSPARENT GAGE GAGE

-- I/d” OR 3%” 2 COPPERTUBE TRACER

I I! 11 /i METHOD OFRUNNING

II I TRACING OF TRANS- I TRACER FOR HEAVY

il li PARENT GAGE

TYPICAL GAGE GLASS TRACING

1 TRACER 1/4” OR 3/cTUBING ! PROVIDE SHUTOFF VALVE AT SOURCE,

TRACING

I LIGHT e TRACING

:TO TRAP

TYPICAL TRACING TRACING FORCED- GAGE GLASS AND LEVEL BALANCE LEVEL

Now: Refer to Fig. 8-9 for tracing and insulation methods.

FIG. Il-IO-Steam Tracing and Insulation for Level Instruments.

TRACER i/4” OR ’/~TU81NG

SEALED TRANSMITTER LINE TO SEAL TRACING ~~

TRANSMITTER OIL OR WATER SERVICE

-Ï?ACER I / q y SR?’g‘ V B I N G PROVIDE SHUTOFF VALVE 4T SOURCE

.-USE LIGHT OR HEAVY A5 REOUIRED TRACING

SEE FIG. 8-9

----REFER TO FlG.8-9 FOR INSULATING METHODS

CONTINUE TRACER TO 9

USE LIGHT TRACING __ IF HEATING IS REOUIRED.

TRACE AND INSULATE LINES TOGETHER LIGHT TRACING 1 *-

I

SEALED METER ’ STEAM METER OR OR


TRACE AND INSULATE LINES -,,-v TuGETHER TO PREVEFIT GRAVITY ERRORS FROM UNEVEN HEATIKG. C)EFER TO FIG. 8-9 FOR PISdLATiNG METHOCS K... ,’ I

. I

I .’ .;%’ OIL OR WATER METER

OR TRANSMITTER *,c..* CE.’ ‘- .IL,

Note: All tracers have shutoff valve at source and steam trap or valve at termination for condensate disposal.

FIG. 8-12-Steam Tracing and Insulation for Flowmeters.

COLD CONNECTING M.1. WIRING

STRAP IRON SUPWRTATTACHED TO DOOR AND TACK WELDED TO LINE TD SUPPORT BOX. SIMILAR SUPPORT AT BOTTOM

RELAY AND THERMOSTAT ARE OPTIONAL

A B Arrangement A shows an electrical heating installation. Arrangement B shows a process flow heating installation.

Notes: This type of protection can be used where there Is continuous 1. All tracers have shutoff valve at source and steain trap flow and line temperature ranges between 60 F and IS0 F.

Line remains bare inside the housing. Sides of housing are 2. Refer to Fig. 8-9 for tracing and insulation methods. made of block insulation or metal coated with insulating paint.

FIG. 8-11-Steani Tracing and Insulation for Force- FIG. ô-13-Eieetriral Heating und Heating by Process

or valve at termination for condensate disposai.

Balance Flow Iiiatrunienta. Flow.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

SEALS. PURGES, AND WINTERIZING

I f . Housings

Instruments of weatherproof design can, in many instances, work through ambient temperature changes and give satisfactory results without additional housing. Even so, users still have to design or buy a variety of instrument housings to protect instruments. Features which should be considered in determining the type and design of housings are:

1. WORKING SPACE AND ACCESS The free space around the instrument. inside its hous-

ing, should be adequate for routine maintenance procedure and for the removal of the instrument. Properly sized and positioned access doors are necessary. Ob- servation windows may be a desirable optional feature.

2. LINEENTRY Entry is preferably through the sides or the bottom

of the box. Entries should be located to minimize piping and fittings.

3. INSULATION The inside of a housing may be lined with foil-faced

fiberglas or suitable insulating material securely attached to the walls. Insulating paint may also be used inside or outside the box with satisfactory results.

4. MOUNTING

attached to the instrument support. Housings may be self-supporting, wall-anchored. or

5 . VISIBILITY OF INSTRUMENT Instruments may be flush-mounted on housing walls

or behind windows in the door or side of the housing.

6 . WEATHERPROOFING Housings should be rainproof and dustproof with line

entries sealed. Metal housings should be galvanized or painted. or both. The hardware and assembly bolts and screws should be corrosionproof.

Additional details for instrument housings and mountings are given in Fig. 8-14.

FULL DOORS

L

3/< THICK SHEET METAL

.EGS ANCHORED

' BOTTOM OF HOUSING CLAMPS BETWEEN FLANGES OF

DOOR WITH OR WITHOUT

PLATE WINDOW

ALTERNATE MOUNTINGS FOR ARRANGEMENT A

Arrangement A shows a typical instrument housing and mountings. Line entry point is optional; door in both front and back: window in front door. Minimum thickness for metal housing is 22 gage except for the bottom plate which is in.: sheet metal should be galvanized.

Arrangement B shows self-supporting housing; instruments are flush-mounted.

Arrangement C shows wall- or line-supported housing. Back of box is Yi-in. metal. It is bolted to the wall or line bracket and supports the instrument.

Note: Housing insulation is ?h -in. foil-faced fiberglas, celotex, or insulating paint.

FIG. ö-ILInstrumeiit Housings and Mountings.

g. Air Supply System

This system should not require winterizing if air dryers are supplied in accordance with Sect. 9 and the dryers are of adequate capacity to maintain a safe dew point.

Extensive winterizing of the air supply system and of instruments which use air may be required if dryers are not used.

85



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

SECTION 9-AIR SUPPLY SYSTEMS

9.1 CONTENT

installation of instrument air supply systems. This section presents common practices for the

9.2 GENERAL

For proper instrument operation. instrument air should be oil- and dust-free and sufficiently dry to prevent condensation of water.

a. Compressors

Compressors in instrument air systems normaiiy are two-stage compressors with an air or water cooler between stages. Compressors which use no oil in the parts exposed to the compressed air are advisable. For required capacity see Par. 9.3. The driving power for the compressor normally will be either electricity or steam; see (c) following.

I). Treatment Facilities

TVDF El (BRASS)

TYPE A LOW-PRESSURE HEADER WITH MASTER FILTER

b I I L U

HIGH-PRESSURE HEADER WITH INDIVIDUAL FiLÏER REGULATORS. I F L E S S Ï H A N ÏENCONSUMERS

AND REDUCING VALVES ON-FÄNEL

FIG. 9-2-Piping for Controllers on Back of Instrument Panel.

should be used for the standby unit. Automatic startup instrumentation should be provided for the standby compressor. The capacity of the standby unit should be sufficient for the entire instrument load. Both centrifugal and reciprocating compressors are used for this service, depending upon size, economics, and preference of the user.

d. Arrangements

To clean, dry, and prevent freezing of instrument air, it should, after compression, pass through an aftercooler and a water separator to remove the major portion of the water. If oil-free compressors are not used, the air should then pass through an adsorber (oil pre- filter) which will selectivelv remove anv oil vapors. The air should then be dried t i a dew poini of at ieast 10 F below the lowest known local ambient temperatures. The Heating, Ventilating, Air-Conditioning Guide la

may be used as a guide for ambient temperatures. Where there is a possibility of adsorbent fines entering air lines, a dust collector or filter should be used following the dryer, in addition to the individual filters shown in Fig. 9-1 and 9-2.

c. Standby Provisions

For reliability, a standby compressor powered from a different source should be provided to supply air in the event the primary source fails. If the normal air supply is derived from an electrically driven unit, a steam driver

Figure refers to REFERENCE on p. 90.

GATE VALVE- '/,"PIPING

PIPING I i / z n l / z ~ y Y ~ ~ ~ h

V4"COPPER TUBING Y L-+TO SECOND INSTRUMENT FILTER REGULATOR-USE ?'AIR CAGE

I F NOT FURNISHED IN INSTRUMENT

FIG. 9-1-Pipirig for 1 iiairunient i n Field.

A typical arrangement of a primary source of compressed air used with a plant air compressor is shown in Fig. 9-3. If a separate source of instrument air is desired, the system might be arranged as shown in Fig. 9-4. The connections as shown by broken lines could be used if, upon failure of the instrument air supply, automatic makeup from plant air .is desired. If an instrument air-cleaning and air-drying system is installed at a location remote from the plant air compressors. a positive shutoff check valve should be installed in the plant air line to prevent a backflow when the standby compressor is operating; see Fig. 9-5.

e. Safety Valve

Instrument air systems normally are designed for pressures up to 125 psig and should be protected by safety valves set for the design pressure.

f. High-Pressure Air

If small quantities of high-pressure dry air are required, booster compressors (which may require separate drying and cleaning facilities) should be supplied. Booster compressors should take suction from the air receiver. as shown in Fig. 9-3 and 9-4. The high- pressure system design should follow the same principles as the design for the regular instrument air system.

86



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

AIR SUPPLY SYSTEMS

Note: Symbols are in accordance with ISA RP 5.1.

FIG. 9-3-Instrument Air Supply System with Refinery Compressors Used as Primary Source.

g. Precautions b. Water Separator

startup or standby service? a compressor which will contaminate the air lines with oil should not be used. Once the air system becomes contaminated. it will con-

Even under emergency conditions. such as plant The water separator should have an automatic trap to discharge the disengaged liquids.

c. Oil Vapor Adsorber The oil vapor adsorber should have a capacity of at

least lb of oil vapor for each std ft per min of design capacity. Block and bypass valves should be installed to permit replacing the adsorbent.

tinue io contaminate clean air. It is essential that a good filter be supplied to remove

adsorbent fines. Otherwise, instrument troubles will develop.

9.3 CAPACITY The capacity of all components of an instrument air

system should be based on the total requirements of all connected loads, assuming all instruments operate simultaneously. Where accurate figures are not available, a figure of 1 cfm should be used for each instrument component. Allowances for air motors, piston positioners, purge and blowback requirements vary from 3 cfm to 10 cfm. At least IO-percent extra capacity should be included to allow for the capacity loss of dryers during operation. leaks in the distribution system, and future expansion.

CI. Air Dryer The dryer should be the adsorptive type and should

use silica gel, activated alumina, or the equivalent to remove water vapor. A refrigerative type may be used if the required dew point conditions can be met.

e. Permissible Pressure Drop The pressure drop throughout the entire drying and

cleaning system, which consists of an aftercooler, water separator, oil vapor adsorber, and air dryer, should not exceed 10 psi.

9.4 DRYING AND CLEANING a. Coinpressor Aftercooler

The compressor should be supplied with an aftercooler to cool the air to within 10 F of the incoming The two basic types of instrument air distribution cooling Huid. systems are the loop and the radial. In the loop

87

9.3 DISTRIRUTION SYSTEMS

a. Types of Systems



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

COOLER

-x

CHECK ' VALVE.

AIR OR WATER

SV COOLED

AIR FILTER

CHECK _L VALVE

INSTRUMENT AIR

COMPRESSORS

AIR OR WATER

-.. COOLE0

CHECK VALVE

:ROM PLANT i i R SYSTEM

WATER

SE PARATOR

AFTER- COOLE9

A UTOM AT1 C

INSTRUMENT

COMPRESSORS GLOBE (EMERGENCY) i" BLEED VALVE

I -Y I z I i: I 1 I'

DEW POINT RECORDER

(OPTIONAL) I I ----y

b

DUST COLLECTOR OR FILTER WHEN NEEDED

(SEE PAR. 9.2 b)

DRY ERS w y ' REMOVER

HIGH-PQESSURE AIR

DRYING SYSTEM --- ---- -- -----+COMPPESSORS AND

Now: Syii ibols are in accordance with ISA R P 5 .1 .

FIG. 9-4-Instrument Air Supply System Used with Separate Compressors.

FROM PLANT - AIR SYSTEM -

p-- AUTOMATIC

C I t i r.4 v y

r . 4 Y U INSTRUMENT

AIR COMPRESSORS GLOBE

1" BLEED VALVE

Nore: Symbols are in accordance with ISA RP 5.1.

FIG. 9-5-Iiiatrumcnt Air Supply System

DEW POINT RECORDER

/pic\ (OPTIONAL] 1 . 1 '~ r---_l

/%-y>&- DUST FILTER COLLECTOR WHEN NEEDED OR

(SEE PAR. 9.2b) w DRY ERS

qjJJ REMOVER

for Remote Location.

88



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

AIR SUPPLY SYSTEMS

type, each process unit receives air along the distribution main from either direction and the main describes a loop through all process units. In the radial type, each process unit receives air along the distribution main from only one direction and the main does not describe a closed loop.

b. Line Sizing

Lines in the distribution system should be sized so that the maximum pressure drop does not exceed 5 psi between the dryer outlet and the most remote consumer when all consumers are taking air at maximum rates. A minimum pipe size of ?h in. should be used for takeoffs to individual consumers except where many instruments are in close proximity and connected to one header, such as on a control panel. In this case a smaller pipe size may be used.

c. Instrument Supply Piping

Air supply piping details for instruments should be similar to Fig. 9-1 and 9-2. It should be noted that for installations in which a large number of instruments are installed, it will probably be more economical to install a single- or a dual-master filter with the reducing valve. If air consumption is less than 10 cfm to a panel. individual filters are often used (see Fig. 9-2 Type B ) . An air header is required for Types A and B.

9.6 STANDBY SYSTEM CONTROLS If a standby compressor is supplied. it should be

equipped to start automatically if the outlet pressure of the dryer falls below the desired value.

Additional safeguards against loss of instrument air, such as automatic cutback of noninstrument air users or automatic cut-in of plant air. should be considered. Typical systems are shown in Fig. 9-3 through 9-5.

a. Symbols in Fig. 9-3

PIC 1 : This is a dual-pilot instrument. The first pilot starts the standby compressor when air pressure falls below a safe minimum. A lockup device maintains it in a running condition until this device is manually reset. This system is used to prevent the instrument from attempting to control the system pressure by throttling the steam supply. Pressure control is incor- porated in the compressor through unloading valves. The second pilot is set to control at a pressure lower than that of the first pilot. It throttles the cutback valve to maintain this air pressure.

The control valve in the steam line normally will open as air pressure decreases. It will have either a 15-psi; signal (compressor shut down) or a 3-psig signal (compressor running) transmitted to it. Alarm PA 1 is set to operate when this control pressure falls below 12 psig, Le., when the compressor starts.

PA 1 :

PA 2: The control valve in the plant air system normally will close as air pressure decreases. Alarm PA 2 is set to operate when the pressure to the valve fails below 14 psig, indicating that the plant air system is being cut back.

b. Symbols in Fig. 9 4

PIC 1: This is a dual-pilot instrument. The first pilot controls the standby compressor as described in (a ) . The second pilot has a set point below the first and is adjusted to open the control valve and admit plant air if the instrument air pressure falls below the set point.

PA 1: PA 2:

Same as PA 1 described in (a). The control valve in the plant air line nor-

mally will open with decreasing air pressure. Alarm PA 2 is set to operate when the control pressure to this valve falls below 14 psig, indicating that plant air is being used as an emergency makeup.

c. Syml>ols in Fig. 9-5

PIC 1: This is the automatic startup pressure controller which opens the steam valve to start the standby compressor when the instrument air pressure falls below a safe minimum.

PA 1 : Same as PA 1 described in ( a ) .

d. Alarms

Pressure alarms PA 1 and PA 2 should consist of pressure-sensing devices, alarm lights, and either separate howlers or a common howler. The alarm lights and howler should be located in the control house.

e. Control Valves

The automatic control valve in the steam line to the driver of the standby compressor should be line size, arranged to open when an air failure occurs. In addition, a 1-in. bypass globe valve should be used to bleed sufficient steam to keep the driver warm. The automatic control valve in the plant air line, as shown in Fig. 9-3, should be line size, arranged to close when an air failure occurs. The automatic control valve in the plant air line, as shown in Fig. 9-4, should be line size, arranged to open on instrument air failure.

9.7 AIRDRYERS

The air dryers, as shown in Fig. 9-3 through 9-5, are set up for manual control of regeneration. However, most systems are designed for automatic regeneration on a strict time basis. The regeneration cycle may be based on the dew point of the dried air (Le., regeneration starts when dew point rises to some set point), if

89



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API RP 550-PART 1

desired. If automatic regeneration is provided, switching valves should be used which will not interrupt the flow of air even if stopped in some intermediate position. Regeneration normally takes place either at line pressure

loss of instrument air pressure for even a short period of time.

REFERENCE or at atmospheric pressure. If atmospheric pressure regeneration is Used, provision be made to have both dryers pressurized during switchover to prevent

Heating, Ventilating, Air-Conditioning Guide, American SOC. of Heating, Refrigerating and Air-conditioning Engineers, Inc., New York (1960).

90



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COPY PROVIDED FUR HISTORICAL PURPOSES ONLY

SECTION 1O-HYDRAULIC SYSTEMS

10.1 CONTENT

This section discusses the installation of central hydraulic pressure systems which utilize hydraulic cylinders to move slide valves, dampers, and similar types of equipment.

10.2 BASIS OF DESIGN

Most hydraulic supply systems are designed in such a way that the liquid is pumped from a vented storage drum to a pressure drum and held under pressure by a blanket of inert gas. From the pressure drum the liquid flows to the actuated devices and returns to the vented storage drum; see Fig. 10-1.

10.3 PUMPS

Two pumps should be supplied. one with an electric motor drive and one usually with a steam drive. Each pump should be sized to supply the normal requirements of the actuated devices and should be capable of supplying 200 percent of the anticipated leakage of all pilots plus pistons, or 4 gpm per pilot-whichever is greater. The inert gas pressure will supply the power to circulate the abnormally high quantities required under emergency conditions. Pumps should be designed for the normal operating pressure required [see Par. 10.5 ( b ) ] and suitable for the liquid used. over the ambient temperature range anticipated.

.)

DRUM I

A = bllNIMLhl DISTANCE DERMITTE3 B i PQESSURE IESSEL 3ESIGN CONSiDESATiLJNS

FIG. 10-l-Hydraulic Supply System.

10.4 DRUMS

a. Pressure Drum

A pressure storage drum should be supplied in accordance with Unfired Pressure Vessels, Sect. VIII, of the ASME Boiler and Pressure Vessel Code as well as other regulations which may be required by the governing authority at the place of instaliation. The vessel usually is designed for a maximum allowable working pressure of 110 percent of the normal operating pressure [see Par. 10.5(b)], or normal operating pressure plus 25 psig, whichever is greater. In installations where normal operating pressure is below 250 psig, a maximum allowable working pressure of normal operating pressure plus 15 psig sometimes is used to obtain a substantial decrease in cost.

1. PRESSURE STORAGE DRUM The pressure storage drum should have a total vol-

ume, above the takeoff line to the cylinders, equal to the output of one pump for 5 min plus 10 times the total volume of all hydraulic cylinders.

2. LOW-LEVEL ALARM The low-level alarm should be at the level where the

volume of liquid above the takeoff line to the cylinders is equal to the output of one pump for 5 min plus twice the total volume of all hydraulic cylinders.

3. HIGH-LEVEL ALARM The high-level alarm should have a minimum setting

at the level where the volume of liquid above the low- level alarm is equal to twice the volume of all the hydraulic cylinders.

4. REQUIREMENTS The conditions in the three preceding paragraphs

combined with that of Par. 10.5(b) should ensure that sufficient liquid and energy wili be stored to meet emergency conditions which will occur when both pressurizing pumps are out of operation. These quantities normally should be sufficient to move the actuated devices two full strokes (from one end to the other and back) at the emergency speeds necessary to meet process re- quiremen ts.

b. Storage Drum

A vented storage drum should be supplied equal in size to the pressure drum and designed for a minimum of 15 psig.

10.5 PRESSURE

Pressures referred to in this section are those at the top of the pressure drum. All regulators, safety valves, and alarms located at any other elevation should have

91



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their set points adjusted to compensate for the effects of elevation. Typical pressure relationships for a hydraulic system are shown in Table 10-1.

a. Minimum Pressure The minimum pressure in the pressure drums should

not be less than that required to operate any actuated device against its maximum load, plus the static head of liquid between the highest actuated device and the drum, plus the pressure loss caused by friction in the pipelines when ali actuated devices are operating at their emergency speeds. Experience has shown that this pressure usually will not exceed 165 psig.

1). Normal Operating Pressure The normal operating pressure should be at least one

and a half times the minimum operating pressure. This normal operating pressure should allow sufficient energy from gas expansion during emergencies to operate the hydraulic cylinders [see Par. 10.4( a -4 )] . This pressure usually will not exceed 250 psig.

c. Inert Gas Pressure is maintained by nitrogen trapped in the

pressure drum and compressed by the hydraulic liquid

TAULE 1O-i-T.vpiral Pressurc Rrlutiuiialiips for u Iiydraulic

Pressure (Pounds per Square Inch

Gage) 3 17-

303-

290-

265-

240-

215-

145-

15-

e

Maximum pressure fire

System

accumulation resulting from

Safety valve set point [Par. 10.6(b)]

Pump relief valve discharging a t rated capacity [Par. 10.6(a)]

ci4oait)iiitn dlmvrihle i i ~ d i i t z ~ ~ prc?ssirre

Pump relief valve set point [Par. 10.6ía)I High-pressure alarm set point [Par. 10.8í b ) ]

[Par. 10.4ía)l

Norind oprrdi ig pressirre [Par. 10.5 í b ) ] Pressure drum pressure regulator set point

[Par. lO.iO(a)]

Steam pump pressure regulator set point [Par. 10.10(,b)]

Low-pressure alarm set point [Par. 10.8ía)I

Minimirin pressitre [Par. 10.5(aì]

Minimum design pressure for storage drum [Par. 10.4( b ) ]

being pumped into the drum by the pressurizing pumps. Other inert gas may be used provided it does not adversely affect the hydraulic liquid. Air is not used because of its corrosive effect on the hydraulic system components and because of the hazardous conditions which would exist if air came into contact with combustible hydraulic fluids.

The quantity of inert gas is usually such that at normal operating pressures the liquid level should be below the levei of the high-level alarm, and above a point half-way between the levels of the high- and low-level alarms.

All additions or withdrawals of inert gas necessary to maintain this condition must take place when the pressure in the pressure drum is the normal operating pressure.

10.6 SAFETY AND RELIEF VALVES

The pressure drum should be protected from overpressure in accordance with Unfired Pressure Vessels, Subsection A, ?General Requirements.?

a. Pump Relief Valves

A relief valve should be supplied for each pump with a capacity equal to that of the pump. These relief valves should discharge into the pump suction piping. They should be set to open when the pressure in the top of the drum reaches the maximum allowable working pressure.

h. Safety Valve

A safety valve should be supplied for the pressure drum to provide overpressure protection in case of fire. This valve should be installed on the top of the pressure drum in accordance with API RP 520: Design and Inst«llation of Pressure-Relieving Systems (see Part I) and all applicable codes.

10.7 LEVEL ALARMS

u. Storage Drum Low-Level Alarm

A low-level alarm (LA-1, Fig. 10-1) should be installed on the vented storage drum, located and set to signal when the level falls to one-third of the drum capacity.

h. Pressure Drum Low-Level Alarm

A low-level alarm (LA-2, Fig. 10-1) should be installed on the pressure drum, located and set to signal when the level falls to the minimum liquid volume point [see Par. 10.4(a-2) J.

c. Pressure Drum High-Level Alarm

A high-level alarm (LA-3, Fig. 10-1) should be installed on the pressure drum, located and set to signal



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

HYDRAULIC SYSTEMS

when the level rises to the maximum liquid volume point [see Par. 10.4(a-3)].

10.8 PRESSURE ALARMS

a. Pressure Drum Low-Pressure Alarm A low-pressure alarm should be installed on the

pressure drum and set to signal when the pressure falls to within 10 percent of the minimum pressure [see Par. 10.5(a)].

1). Pressure Drum Higli-Pressure Alarm

A high-pressure alarm should be supplied on the pressure drum and set to signal when the pressure rises to the maximum allowable working pressure [see Par. 10.4(a)].

10.9 PRESSURE GAGES AND GAGE GLASSES

Pressure gages and gage glasses should be installed as shown in Fig. 10-1. For a detailed discussion on level and pressure instruments see Sect. 2 and 4. respec- îively.

10.10 PRESSURE REGULATORS

a. Pressure Drum Pressure ReguIator A self-acting pressure regulator (PCV-2, Fig. 10-1 )

should be supplied for bypassing the liquid from the pressure drum to the vented storage drum. This regulator should be set to maintain the normal operating pressure at the top of the drum when the constant-speed electric pump is operating. The capacity of this regulator should be equal to that of the electric pump.

11. Steam Pump Pressure Regulator

A self-acting pressure regulator (PCV-1, Fig, 10-1 ) usually is installed for throttling the steam to the steam pump. This regulator should be set to maintain a pressure 10 percent below the normal operating pressure at the top of the drum. The pressure tap should be taken directly from the drum vapor space.

10.11 STRAINERS

a. Locations

Dual strainers should be supplied in the bypass h e from the pressure drum to the storage drum and in the line from the pressure drum to the cylinders, as shown in Fig. 10-1.

11. Type

Strainers should be 100 mesh or finer and have a maximum pressure drop of 1 psi at twice the capacity of one pump [see Par. 10.31.

10.12 PIPING

a. Sizing

Piping sizes should be determined by the requirements of the cyiinders under emergency conditions.

b. Header Connections

Connections between hydraulic devices and headers should have car-sealed open block valves at the headers.

10.13 FLUID

a. Types

There are many types of hydraulic fluids used. These range from hydrocarbons. such as special lubricating oils. to synthetic compounds and mixtures of water and ethylene glycol.

b. Temperate Climates

For locations where ambient temperatures below 32 F are not anticipated, the hydraulic liquid may be water with a water soluble inhibitor added to prevent rust and oxide formations.

c. Cold Climates

For locations where ambient temperatures may fall to 32 F and below, the hydraulic liquid may be a mixture of inhibited ethylene glycol and water. The percent by weight of the inhibitors in ethylene glycol and the quality of the chemicals on which the percentages are based (in parenthesis) are shown in the following tabulation :

Quality of Inhibitor

Inhibitor by Weight (Percent) Percent

Diethyiethanolamine . , . . . . . _ . . . . . . 3.0 (100) Phosphoric acid . . . . . , . . . , . . . , . . . . ( 8 5 ) Sodium mercaptobenzothiazole . . . . . 0.4 í 50) Ethylene glycol (100)

1. ETHYLENE GLYCOL The inhibited ethylene glycol should be mixed with

water, in the proportions given in Fig. 10-2, to ensure protection from the minimum expected ambient temperature. The percentages of inhibitors in the ethylene glycol should remain constant irrespective of the amount of water in the total mixture.

Note: Under no circumstances should the percent by volume of inhibited ethylene glycol in the total mixture exceed 60 percent because the freezing point of the mixture will increuse rapidly if the concentration of ethylene glycol becomes greater than this amount. I t is recommended that the concentration of inhibited ethylene glycol in the total mixture should not exceed 56 percent by volume.

1.2

95.4 , . . . . . . . . . . . . . . . . .



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

API RP 550-PART I

2. PROPERTIES W

W J

k Z z- W J

a 8 W E

O1 1 -40 -30 -20 -10 O +IO S Z O +30

MINIMUM EXPECTED AMBIENT TEMPERATURE. F

The properties of the mixture, 56 percent by volume and with a pH of 8.8 to 9.0, will be approximately:

(Degrees Fahrenheit) (Centipoises) Temperature Viscosity

1 O0 2 O 22

-20 55 -40 175

The specific gravity will be approximately 1.073 at 68 F.

3. LOW-TEMPERATURE SERVICE

In extreme low-temperature service ( - 10 F to -40 F) some separation of inhibitor may occur. If



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

. WY PRWIDED FOR AL WRWSES ONLY

SECTION 11-ELECTRICAL POWER SUPPLY \

11.1 CONTENT

This section is presented as a guide to those responsible for furnishing electrical power supplies to re- fìnery instrument and control installations. Safe, reliable, weli-designed instrumentation is dependent upon a power supply having these same characteristics. Recom- mendations include what to install as well as how to install p y e r supplies. Wiring methods (covered in Sect. 7 and 12) will not be discussed in this section. For additional information and for guidance in the proper handling of refinery electrical installations in general, reference should be made to API RP 540: Recom- mended Practice for Electrical Installations in Petro- leum Refineries.

11.2 GENERAL The particular type of instrument power supply

needed is dependent upon many variables: such as: 1. Type and size of processing unit. 2. Sources and reliability of plant power supply. 3. Type and size of instrument system.

The demands on modern instrument systems often necessitate the installation of an emergency power supply to ensure safe. continued operation. When a plant power failure occurs, an emergency source is needed to permit normal shutdown of process equipment. Considering all safety factors, the total cost of an emergency power supply should be compared to the accumulated savinss afforded by maintaining full or partial production during normal power failure. Infrequent power failures of short duration which are not likely to cause damage, complete shutdowns, or long delays to full onstream operations may not justify the expense of emergency service. For example, catalytic reformer units provided with all-electric drives and electronic instrumentation have been designed with “fail-safe” controls which have eliminated the need for emergency power. A power dip (a failure for a period of a few cycles to several seconds) poses special problems. ’ Some alarm relays, solenoid valves, and equipment safety devices may be affected and cause process upsets of serious consequence, even though plant drives, lighting, electronic recorders, and some controllers are unaffected. These problems will be discussed in ( a ) through (c ) following. Designs should provide for periodic testing of emergency power systems without upsetting plant operations; see Par. 11.8(c).

a. Type and Size of Processixi9 Unit

When considering the possibilities of power failures, process and utility flow diagrams should be thoroughly studied, keeping the following points in mind: 1. Safety aspects.

95

2. Methods and sequence of events in handling process feed and products. 3. Effect upon equipment and piping. 4. Effect upon other refining units and plant facilities.

It is good practice to determine the sequence of events as well as the risk to personnel and plant, assuming both power dips and extended power failures to instrument systems. Thought should be given to the effects of shutting down equipment on other related refinery units or facilities. The possibility of fire or other hazards deserves full consideration. Difficulty of starting or stopping equipment, the maintenance involved, and the disposal of feedstocks or off-specification. products are among the many factors to be considered.

b. Plant Power Considerations

The plant power supply system should be studied with particular emphasis on the probability and frequency of power dips and interruptions, and the effects of such occurrences on plant operations. It is customary to provide energy from two or more separate sources where service interruption could cause serious problems.

1. PURCHASED POWER (DUAL SERVICE) A reliable power supply from a public utility usually

includes two or more supply feeders from separate sources operating in parallel; see Fig. 11-1. Their individual reliability. routing, exposure. and type of switching (circuit protection. response of automatic relaying. bus ties, etc.) are of prime importance. If. for example. two or more supply feeders. continuously connected to the refinery bus. are relayed so that a faulty feeder is promptly disconnected at the refinery, the voltage dip during the switching operation generally will be of very short duration. Troubles from such a dip usually can be overcome by incorporating a time delay in the instrument. safety, or interlock devices. This assumes that small voltage and frequency variations are not con-

Note: Supply feeders operate in parallel. Each supply feeder circuit breaker is relayed to trip on failure of its feeder. Failure of one supply feeder can be sustained with no power interruption to the refinery bus.

FIG. Zl- l -Pi ir~hu~rt l Power, PUrdlctl-Ft-ctl<.r Olieraiioii.



--`,,,``,,```,```,,``,`,`,,-`-`,,`,,`,`,,`---

sidered problems. A different dual-feed arrangement with load transfer from a normal supply feeder to an emergency feeder at the refinery bus presents a more serious problem, see Fig. 11-2. Frequently, the switching time from the normal to the emergency feeder is long enough to require an emergency instrument power

2. GENERATED POWER Any degree of reliability may be built into systems

where the plant generates all or part of its own electricity. The time delay when starting standby generating equipment may be an important consideration in supplying energy to the instrument circuits. Regulation of voltage and stabilization of frequency usually are more difficult with plant-generated power than with purchased power.

supply.

c.

The characteristics of an instrument system also dictate the reliability needed from the power supply. If a process unit is operated with fully automatic electrical instrumentation or an elaborate alarm system with safety features, or both, the power supply usually will have to be very reliable. Conversely, a small processing unit, which uses pneumatic or semiautomatic electrical controls with fail-safe features, may operate acceptably on a single circuit. The use of electrical instrumentation with fast response and fine control calls for power supplies of uninterrupted stability. A discussion of these points follows.

Most instruments, controls, alarms, and safety circuits operate from a single-phase, 1 15-volt. alternating-current í a-c ) supply. Motor-operated valves usually operate on single- or three-phase voltages between 115 and 550 volts. Particular attention must be given to the recommendations of the instrument manufacturer. Some requirements may involve special voltage or current

Type and Size of Instrument System

NORMAL EMERGENCY SUPPLY FEEDER SUPPL” FEEDER

T T NORMALLY

CLOSE0 f NORMALLY OPEN

Note: On failure of normal supply feeder. relays operate to open the normal supply feeder breaker and close the emergency siipply feeder breaker; thus power to the load is restored lifter ;i momentary interruption.

Fit;. 1 1 -2-i’iirchuwd Ptww. Si tigle-Ft-tdt!r Opt-ra t io ti

with Eniergenc:y SI u lid by Feeder.

100 80 60 40 20

FIG. ll-3-Effect of Load and Power Factor on Output

LOA0 POWER FACTOR IN PERCENT LAGGING

Voltage at 115-Volt Input.

limitations. Other requirements stipulate rigid specifications on frequency and harmonic distortion of wave form. Electronic instruments frequently require voltage stabilizers with suitable filtered outputs. With some equipment, all components of a control loop must receive energy “in-phase.” A common suppiy source will take care of this requirement; see Par. 11.3(c). It is important to note the wide variation in output from voltage regulators when applied frequency, power factor, and loading vary; see Fig. 11-3 and 1 1 3 .

In the past, refinery units were controlled principally by pneumatic systems often actuated, in the case of temperature, by slide wire potentiometers. Power dips and short power failures had little effect on these systems. The same outages on automatic electronic controls with all the safety interlocks and emergency trip circuits on heaters, compressors, and so forth must be

i601

O 40 80 I20 i60 200

”\, = cycles per second. INPUT VOLTS

FI(;. ll&Effevt of Frvqiiriicy mi Chitpiit Voltage.

96



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considered more carefully. For example, if the normal movement of a control valve is through only a fraction of its range, deposits from the process fluid may build up on the stem or plug. A voltage drop may cause this valve to stick in an off-normal position. The application of rate action in controls can cause wide momentary swings in control valve position (following smaii power disturbances) to the extent of actuating the interlock or trip devices.

Instrument circuits which are critically affected by power dips should have an alternate power supply arrangement to overcome the diíñculty.

Note: In certain cases where air-operated control valves are affected, solenoid valves can be installed in the controlled air line. Closing on power failure. these solenoid valves trap diaphragm air and hold a valve position through the dip or short power failure. thus minimizing an upset condition. Return of the solenoid valves to normal can be immediate or can be delayed to permit the electrical equipment to warm up.

11.3 POWER SUPPLY CIRCUIT ARRANGE- MENTS

In the United States today, instrument electric power generally is supplied at 1 15 volts, single-phase, 60-cycle alternating current (ac) . Wattage requirements are low enough to permit the use of small conventional lighting transformers. They may be connected across one phase of the process unit’s secondary three-phase power supply (220 to 550 volts). The transformer is located near or in the control house with the secondary power supply (low-voltage side) connected to an instrument panel board. A simple arrangement of a power supply circuit is shown in Fig. 11-5.

a. Isolation of Instrument Circuits

Many potential control and metering problems can be avoided by carefully isolating instrument supply and signal circuits. Isolation of instrument circuits is helpful in reducing the effects on instrumentation of power system load changes, switching transients or human switching errors, short circuits, unintentional grounds, stray ground currents or “ground loops,’’ and inductive and capacitive pickups. First consider supply circuits. Because only instrument loads are to be supplied from the 480-volt circuit breaker at the unit bus, as shown in Fig. 11-5, the effects of load changes, switching transients, shorts, or grounds originating in noninstrument circuits are minimized. Also, human mistakes should less frequently affect controls or metering. Use of an isolating type of voltage stabilizer may help to minimize stray ;round currents through ground loops.

Also, thought should be given to the proper isolation of signal circuits, as outlined in Sect. 7, and to the proper grounding of instrument cases, conduit, and the like. This will help to minimize unwanted inductive or capacitive pickup, as weil as to avoid metering errors

(SEE FIG.11-8AND 11-9) l - T T

I 1 1 ’ IF A 3-POLE BREAKER IS FURNISHED IN LOAD CENTER, 3 R D POLE SHOULD NOT

\ \ BE USED TO FEED OTHER LOADS

INSTRUMENT POWER FEEDER - NO OTHER LOADS FED FROM THIS CIRCUIT-

460-12OV TRANSFORMER

2-5% TAPS BELOW RATED ~ DRY-TYPE COMPOUND-FILLED -

PANEL BOARD, PRIMARY VOLTAGE

Clocks and clock drives. Miscellaneous relays. Solenoid valves. Analyzers (such as infrared

types) with matched or buiit-in voltage regulators.

Instruments not voltage-sensitive.

+ 1 Potentiometer-type temperature

indicators, electronic recorders, and controllers.

Analyzers requiring stable voltage (paramagnetic, flaming filament, thermoconductivity, pH, electrical conductivity, etc.).

FIG. 1 1 - h P o w e r Supply.

because of varying ground potential in separated plant areas.

b. Grounding Provisions

Every petroleum processing unit should have a good grounding system, consisting of multiple grounds (rods, water line ground connections, etc.), ali interconnected with large stranded cable. It is customary to bring cable stubs up to several convenient points for connections. Neutrals and other grounds from circuit breaker panels, transformer cases, and the like should be directly connected to these ground connection points with wire adequately sized to carry fault currents. It is not good practice to loop a ground wire from, say, a circuit breaker panel to a transformer and then to the ground connection point.

One side of each instrument supply circuit should be grounded for safety reasons and to ensure reliable performance of certain devices. It is best to make this ground connection at the instrument panel. Transform- ers can then have fuliy insulated leads, which facilitate testing and which free the circuit connections for inspection. The steelwork of each instrument panel board should have its own “private” ground wire to the process unit ground system connection point; it should not be included with breaker panels, transformers, or similar equipment. Where the panel board is nonmetallic, each instrument housing mounted on it should be wired to the panel board ground (or nearest panel steelwork as a minimum).

97



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It should be noted that many voltage stabilizers have no electrical connection between primary and secondary circuits. This factor should be field checked by continuity testing in doubtful cases. Secondary circuit grounding, as shown in Fig. 11-5, nearly always will be desirable. Conversely, where the alternate stabilizer circuit is used (also shown in Fig. 11-5), some instruments may require the installation of special isolating transformers.

c. Single-Power Source Usually, in a process area it is best to supply all parts

of the instrument system from a single source to preserve voltage and power factor relationships. Sometimes this is essential. Where power-consuming devices are used at the point of measurement, the instrument power supply circuit should be extended rather than make use of a nearby lighting circuit.

(1. -4larrn Circuits and Interlocks

Interconnected wiring frequently occurs in control room alarm circuits. A common bell or horn (with a common silencing button) may serve ail of these circuits. A branch circuit from the instrument breaker panel should be reserved exclusively for alarms as shown in Fig. 11-5. A typical alarm arrangement is shown in Fig. 1 1-6. Circuit variations sometimes include 3-pole disconnect switches with the third pole breaking the horn circuit as shown in Fig. 11-7. In cases where “bouncing liquid levels” or other repetitive upsets cause repeated alarms, a separate horn disconnect switch may be desirable to avoid making other alarms ineffective, or the horn may be provided with a 3-sec or 4-sec time delay.

Interlock circuits should be designed to prevent feed- back or parasitic supply to. or from. interconnected power and control circuits. In addition. the design should be such that voltage ”dips” rind momentary outages do not cause unwanted control or alarm action. Experience must be combined with good judgment in the selection of relay and other device contact configura-

MOTOR.3RIVEFI FLASHER

ALARM ACTUATING CONTACT OPEN DURIffi NORMAL UNIT OPERATION

FIC. 1 I -CiAlurrn Coniieciion.

HORN

SUPPLY

___-__i PANEL BOARD

+ TO INDIVIDUAL ALARM UNIT

FIG. 11-í-Multiple Wiring of Alarm Units.

tions. Where atmospheric and environmental conditions permit. it will usually be advantageous for such contacts to remain in the closed position during normal operating periods, with abnormal process conditions causing the contacts to open. Fail-safe circuitry is inherent with this practice; however. there will be cases where it is more practical to use contacts which close when the abnormal condition occurs. The corrosive effects of atmospheric contamination, along with such factors as heat and vibration, must be evaluated in making this important selection. If mercury switches are used. possible effects of vibration (slopping mercury around) should be considered.

e. Disconnect Switches

The arrangement for disconnect switches on panels is discussed in Sect. 12.

Where locally mounted instruments are to be supplied, the electrical circuit function may be considered in determining the number of disconnecting switches to be provided. Where a local transmitter‘receives power through a separate circuit from the main instrument panel, field disconnect switches are seldom needed and should be considered as possible sources of trouble. (Even in remote locations, it is well to consider running a pair of wires for “sound power” telephone communi- cation rather than insert an undesirable disconnect switch into a control loop.) Where electrical power is used only for chart drives, one disconnect switch may serve as many as four instruments, as discussed in Sect. 12, see Par. 12.8(d). Disconnect switch enclosures for field use should be explosionproof in Class I areas. Con- sideration also should be given to other environmental factors, such as humidity, rainfall, dust, and sait air. The choice of 2-pole and 3-pole types is discussed in Par. 12.8 (d ) .

11.4 POWER SUPPLY TO LOADS a. Regulated Voltage Necessary

The supply voltage to some instruments must be regulated if accurate, dependable performance is required.



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For example, the life of electronic tubes, capacitors, and the like may be seriously shortened by overvoltage. Electronic equipment usually is designed to tolerate the voltage and frequency fluctuations which may be expected from purchased electrical power. However, refinery instrumentation designers should not overlook the probability that plant voltage fluctuations may be more severe than a public utility power company would tolerate-the utility company must attempt to minimize lamp flicker. Plants have been troubled with undependable operation of some types of analyzers and voltage-sensitive electrical equipment, particularly those plants generating their own electricity. The system shown in Fig. 11-5 represents good practice.

h. Regulated Voltage Unnecessary

In general, clocks, chart drives, relays, contactors, and solenoid valves will be unaffected by wide voltage vana- tions. It is usually simple to avoid coil burnouts by co- ordinating specifications for transformer ratio and tap connections with device voltage ratings. I t is economical to supply such equipment independently of the main voltage stabilizer. Devices needing regulation will work better if their stabilized supply is not affected by the intermittent changes in loading which solenoid or relay action might entail. The performance of many potentiometer instruments is not seriously affected by minor voltage variations.

Some instruments, such as infrared analyzers, require close voltage regulation obtained through matched or built-in voltage stabilizers. Such instruments should not be connected to regulated branch circuits. The harmonic content in the output of the main voltage stabilizer may adversely affect their operation. Refer to Part II of this manual.

c. Characteristics of Saturalde Reactor and Ca- pacitor (Resonant) Voltage Stabilizers

Stabilizers of this type closely regulate output voltage with input variations of i 15 percent voltage. In doing so, the output wave form is distorted. Third-harmonic content may vary from 3 to 35 percent and smaller percentages of fifth or seventh harmonics may appear. Wave shape is not critical in most instruments. Some instruments which employ a-c signal circuits require expensive stabilizers equipped with harmonic filters. The cutput voltage of resonant stabilizers, as shown in Fig. 11-3 and 11-4, varies with load. power factor, and frequency. The following guides may be used for better results: 1. Preferably, supply stabilized voltage to those instruments (or parts of instruments) presenting a steady load. Avoid loads which are frequently switched on and off. 2. If the power factor is low, oversize the stabilizer, see Fig. 11-3. 3. Use a frequency-compensated regulator where the frequency is likely to vary more than 1 percent. This

should be preferred practice on refinery-generated power systems (note that a noncompensating 60-cycle stabilizer cannot work well on 50-cycle or other fre- quencies). 4. Install voltage stabilizers in well-ventilated indoor locations; they have high operating temperatures and develop strong external magnetic fields. Stabilizers should be physically separated from instruments which have high-gain audio-frequency circuits. 5. The resonant type of voltage stabilizer is current limiting; therefore, its power supply circuit breaker will not trip if the secondary is shorted (after an initial rise to twice normal, output current will drop to between 1 and 1 Y2 times full load). Although this limits damage to shorted instruments, the resulting output voltage drop will affect other devices. This effect should be considered when providing protection on the output side.

d. Other Voltage Stabilizer Characteristics

Other voltage regulators, free of harmonic distortion in their output, may require a few cycles to reach sta- biiity after a change of input voltage.

11.5

a. Two Nornial Circuits

Frequently, two power sources are available which have sufficient capacity, reliability, and voltage and phase-angle stability to permit connecting them both to the instrument supply circuits most of the time. They can be provided with reverse power-relaying protection arranged to trip a faulty feeder as shown in Fig. 11-8.

LOAD SUPPLY FROM TWO SOURCES

1). Spare Circuit

Where only a spare (less reliable) power source is available for emergency use and a momentary power dip is not objectionable, an automatic transfer switch may be used to transfer instruments from the normal to the alternate source of power when the normal source fails. The switch automatically transfers the load back to the normal circuit when voltage is restored. A typical transfer scheme is shown in Fig. 11-9. The switch may be arranged to transfer load at 70 percent of normal voltage and restore normal service at 90 percent of normal voltage (after a short time delay to prevent chattering).

NORMAL FEEDERS FROM SEPARATE SOURCES

DIRECTIONAL RELAYING (OR ECUIVALENT )

1 TO INSTRUMENT

PANEL BOARD (SEE FIG.11-5)

FIG. 1 l - l l - T w o Normal Circuits.



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NORMAL SOURCE

.‘sspARE“

TO INSTRUMENT PANEL BOARD (SEE FIG. 11-5)

FIG. 11-9-NormaI Circuit and Spare Circuit Used for Emergencies.

11.6 SUPPLYING ELECTRICITY DURING POWER DIPS

Assuming power dips are of short duration, one frequently used method of maintaining instrument power supply is to normally furnish power from a small generator which is electric-motor-driven through a flywheel. During a power dip, the flywheel inertia is sufficient to maintain voltage levels. However, the flywheel should not be expected to maintain voltage levels for a period exceeding approximately 15 sec.

11.7 SUPPLYING EMERGENCY POWER TO INSTRUMENTS _ A N D -ASSOCIATED LOADS

When a potential power plant interruption involves enough risk to warrant providing an emergency power source. it is customary to make the source capacity big enough to supply power to some gage, control room, and area lights. Occasionally, electric motor valve operators and safety protective circuit loads are supplied from the emergency sources. Unless a spare emergency circuit of sufficient capacity and reliability can be provided, these requirements nearly always involve an emergency power generator driven by one or a combination of the following: steam or gas turbine, internal-combustion engine, or battery and motor.

a. Emergency Generator Characteristics

It is best to arrange the generator so it can pick up load rapidly. It should be of simple design with as few accessories as practicable and should require a minimum of maintenance. The package type of generator unit with a direct-connected exciter and built-in voltage regulator is often furnished. An enlarged terminal box may contain the control cabinet with voltmeter and am- meter. Unless a Class I enclosure or outdoor installation is necessary, the generator units are provided with dripproof protection. For simplicity a “static” (rectifier type) voltage regulator is often used. Operating voltages may be 120/208, 240, or 480 volts, single- or three- phase, 60-cycle ac. The higher voltages are used where valve motor operators are included in the system. Gen- erator drivers generally are direct connected.

b. Steam Turbine Drives

Simple steam turbines usually are arranged for atmospheric exhaust, and inlet steam at less than 250 psig, and 550 F (total temperature). A constant speed governor is furnished, together with an overspeed trip (set as high as permissible). A typical starting scheme is shown in Fig. 11-10. The turbine should be designed to start without warmup. Where a slight delay in load pickup cannot be tolerated, either continuous turbine operation or the methods described in Par. 1 1.8 (a) may be considered.

c. Gas Turhine Drives

Small, rapid-starting, gas-turbine-driven emergency sets have been developed, primarily for military use. Usually, they are designed to use diesel or jet fuel, and require a battery and a motor or other means to start them. Gas turbine drives can pick up load in 15 sec to 40 sec. Refinery experience with the gas turbine drive is very limited at the present time, probably because available ratings are 50 kw and above. More detailed information is available from several manufacturers.

d. Gasoline (and Diesel) En,’ mines

Gasoline engines are usually 4-cycle, with built-in centrifugal flyball governors. Engines which will run on natural gas or propane fuels also are available. The latter fuels may be cleaner, and will not lose volatility in standby use as gasoline may. High-tension magneto- ignition systems are commonly furnished. Battery and starting motor may be provided. The gasoline tank should be elevated, or a half-gallon tank containing starting fuel should be provided above the engine unit. Cold climates require additional precautions in order to keep the engine warm and ensure a prompt.fue1 supply. In the sizes usually applicable for plant emergency use, gasoline engines are cheaper and often have better starting characteristics than diesel engines. Fuel costs and efficiencies have little bearing when considering standby service. With the engine-driven standby, there will be a significant time lapse before the driver can pick up the load.

COMBINATION PRESSURE 3-WAY SOLENOID VALVE REDUCING VALVE

AND FILTER \

AIR I SUPPLY

FIG. ll-10-Emergency Control for Steam Standby.

1 O0



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11.8 COMBINATION DRIVES FOR POWER DIPS AND INTERRUPTIONS

a. Motor-Generator-Engine Combination

Where both voltage dips and power interruptions are experienced and a reliable instrument power supply is required, it is practical to provide a motor-flywheel generator as mentioned in Par. 1 1.6. It should be connected through a centrifugal or magnetic clutch to one of the engines (or directly to one of the turbines) described in Par. 11.7. This combination should be able to hold voltage within 2 5 percent and frequency within & 2 cycles. A typical installation might include the following equipment:

A 115-volt, 60-cycle a-c generator of suit-

A synchronous electric motor. A gas, gasoline, or diesel engine with cen-

able rating with heavy flywheel.

trifugal clutch.

The electric motor, powered from the refinery supply, would continuously drive the generator and flywheel. The generator would feed 1 15-volt power continuously to the electronic instrumentation and safety interlock equipment. This might amount to approximately 20 percent of rated load. On power failure of more than ?A sec, the electric motor would be disconnected from its refinery power supply and simultaneously the engine drive would start automatically. Its centrifugal pawl clutch would engage the generator drive shaft after com- ing up to speed. The inertia of the flywheel will keep driving the generator during this period. Because the engine can start and speed up unloaded, it would pick up the rated load in approximately 3 sec. Experience indicates that the generator voltage wouíd drop slightly as the load is applied but not enough to affect the equipment.

Undervoltage time delay relays of i /z sec should be provided in the refinery main power supply to the electric motor drive and to the emergency lights to preclude their dropping out during power dips. A longer power failure will cause loss of power to the motor and switch the emergency lighting from the refinery power supply to the generator. Provision also should be made for switching to the refinery power supply if the motor-generator-engine set becomes inoperative. This provision rarely is expected to be used; however, it could cause a dropout during the switchover.

Startup of the engine should be checked periodically to ensure reliable operation. The engine is started automatically by a phase failure relay in the starter circuit; it can also be manually started. A simple sketch of this installation is shown in Fig. 11-1 1.

101

REFINERY POWER SUPPLY

%-

I l

TO EMERGENCY LIGHTING 7

TO INSTRUMENTATION AND ALARM SYSTEM

b

I - i

A-C GENERATOR ENGINE WITH PHASE SYNCHRONOUS MOTOR F4iLURE RELAY IN

STARTER CIRCUIT

FIG. 1 1-11-Instrument and Emergency Power Supply.

h. Motor-Generator-Battery Combination

An alternate system, which operates continuously, consists of an a-c motor, a-c generator, and a direct- current (d-c) unit (motor or generator) ail on the same shaft. Storage batteries with a control cabinet complete the needed equipment. Normally the a-c motor drives the a-c generator and d-c unit. The d-c unit, which acts as a generator, maintains the batteries in a fully charged condition. Upon power failure, the d-c unit acts as a motor to drive the a-c generator. Such a system has a potential weakness which should be guarded against. Battery capacity limits emergency operation time. An alarm should warn the operator of the condition in sufficient time to take appropriate action. The running time allowed by this system, however, is adequate. for most power failure conditions. A simple sketch of this system is shown in Fig. 11-12.

NORMAL TO INSTRUMENT AND REFINERY - EMERGENCY LIGHTING

POWER SUPPLY PANEL BOARD l- 4

SYNCHRONOUS A-C CENERATOR 0 - t UNIT WlTH MOTOR BATTERIES

FIG. 11-12-Continuously Operating Emergency Power Supply with Motor, Generator, and D-C Unit-Battery Com- bination.



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API RP 550-PART I

c. Testing Emergency Power Supplies type which may be called upon to start cold from a non- operating condition be frequently tested. This should include a complete operating cycle. The complexity and reliability demanded of the system determines the frequency of testing, operation once a week being CUS- tomary in some plants.

Emergency power supplies for instrumentation should be listed on the maintenance schedules for checking during unit turnarounds or as the need dictates. It is of prime importance that emergency power supplies of the

102



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SECTION 12-INSTRUMENT PANELS

12.1 CONTENT This section presents common practices for the con-

struction and installation of various types of instrument panels. It is current industry practice to purchase most major instrument panels as completely wired and piped assemblies from a panel fabricator. However. the information contained in this section is applicable to both the prefabricated panel and the field fabricated panel.

12.2 GENERAL The purpose of any instrument panel is to aid the

unit operating personnel in maintaining efficient and safe performance of the unit from a central location. There- fore, the instruments generally found on the panel board include remote recorders or controllers, or both, as well as those indicators which are important for the control of the unit. Ali controllers and significant recorders should be located in the most convenient and accessible location. An attempt usually is made to have the panel layout follow the actual physical arrangement of the unit as closely as possible. A system which will enable an operator to quickly identify any particular instrument is desirable and should be considered in the panel layout. Nameplates. color codes. or symbols frequently are used. Spare panel space. about 10 percent. usually is provided in the panel layout for future expansion.

a. Panel Clearance Clearance from face of panel to control room wall be-

hind the panel in most cases is from 5 f t to 6 ft. This distance allows the subassemblies with auxiliary equip ment to be mounted on the wall. Where special equipment, such as for data loggers, is to be mounted behind the panel, this distance may have to be increased.

11. Instrument Arrangement

1. MOUNTING HEIGHTS Normally, a limitation is placed on the maximum and

minimum heights for mounting instruments on the panel. These heights vary with different users but generally are based on visibility and accessibility.

2. DENSITY The density of instruments varies with the type of

panel. complexity of the process. and preference of the user. From a cross-section of users, average panel density for the various types of panels based on recording instruments per running foot is as follows:

Conventional , , , , , , , . . . J I to 1.5 large case instruments.

\ 3 to 5 miniature instruments. Graphic panels . . . . i .5 to 2.3 miniature instruments. Semigraphic

panels . . . . . , . . . . 3 to 5 miniature instruments. Consoles . . . . . . . . . 3 to 5 miniature instruments.

12.3 CONVENTIONAL PANEL

A conventional panel is defìned as a panel with miniature or large case instruments, or both, mounted in horizontal and vertical rows. Large case instruments are mounted either two or three instruments high. Minia- ture instruments usually are mounted a maximum of four or five instruments high. The distance between these rows of instruments depends upon the type of instrument and accessibility for maintenance and ad: justment. Such items as individual alarm lights and mis- celianeous indicators normally are mounted above the top row of instruments. Alarm cabinets for annun- ciator systems sometimes are considered large case instruments and are mounted accordingly. In other arrangements they are mounted above the top row of instruments. Typical layouts for the conventional panel are shown in Fig. 12-1 and 12-2.

12.4 CONSOLE-TYPE CONTROL CENTER A console is any free-standing cabinet-type enclosure,

usually incorporating a desk or sloping area on the front of the cabinet. It is usually lower and more compact than the conventional panel. It can be designed to include a flow plan, with the instruments mounted in the sloping area. or with many other arrangements.

Consoles are becoming more popular because of the ever increasing amount of data being presented to the operator. Most console panels are fabricated as cubicles with all wiring and piping completely enclosed. Two typical consoles are shown in Fig. 12-3.

U

FIG. 12-1-Conventioiiai Panel wiïli Large Case Instrumeiits.

103



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a c3 o (b O00 CIOU c300

FIG. 12-2-Coiiventional Panel with Grouped Xiniature Instruments.

12.5 GRAPHIC PANEL A graphic panel normally includes a simplified flow

plan of the unit as a visual aid to the unit operators. The layout of the flow plan should present as much information as possible but with a minimum amount of label or nameplate reading. Locally mounted recorders and controllers generally are omitted from the graphic presentation.

a. Types of Graphic Panels

1. FULL GRAPHIC A full graphic panel is defined as a panel with minia-

ture instruments mounted in a simplified flow plan of the unit depicted on the face of the panel. End or “wing” panels are usually added for specialized instruments. Also, those instruments which do not lend them- selves to incorporation in the flow plan layout usually are mounted on the end panels or below the graphic representation. Instruments such as the large case electronic temperature instruments, analyzers, and occasionally those instruments provided for utility or data records are included in this category. A full graphic panel is illustrated in Fig. 12-4.

2. SEMIGRAPHIC The semigraphic panel combines the compactness of

a conventional panel (using miniature instruments) with the flow plan feature of the graphic panel. Such a panel has a simplified flow plan of the unit located above grouped instruments. A typical semigraphic panel is shown in Fig. 12-5.

b. Panel Arrangement and Layout 1. MAJOR EQUIPMENT

The flow plan layout usually includes towers, drums, furnaces, and other major equipment. However, an attempt should be made to eliminate all but the equip-

ment necessary to present a complete picture of the process and its associated control system. For example:

a. Minor knockout drums or pumpout pumps usually are not shown.

b. A turbine-driven pump with steam to the turbine regulated by a panel-mounted instrument generally is shown.

c. A heat exchanger influencing the process control system should be shown.

Actual scale size of equipment and vessels usually is ignored, but vessel shapes are represented as closely as feasible.

2. EQUIPMENT INTERNALS Internals of equipment, such as trays, baffles, or pip-

ing, usualiy are not shown in the flow plan layout unless their inclusion will improve understanding of the process. For example:

a.

b.

c.

d.

e.

The trays in a debutanizer or stabilizer generally are not shown. The trays and baffles in a crude oil still with many sidestream and pumparound circuits are shown. The tubes in a furnace may be shown if there are important temperature-indicating points on cross- overs or if it is desired to show complex crossover lines. The tubes in a simple reboiler furnace generally are not shown. The shell and tube sides of heat exchangers usually are indicated for clarity.

3. PROCESS PIPING Process piping which is connected directly to the

panel instrumentation or which will improve understanding of the process normally is shown in the flow plan layout. Arrowheads generally are shown on lines to indicate flow directions. Lines are continuous wherever possible. Where two lines cross, the horizontal line normally is continuous and the vertical line is broken, except that instrument lines break if they cross process lines. Crossing of the same color lines is avoided. Lines originating or terminating off the panel usually have nameplate designations, such as “GAS OIL TO TANK-

4. INSTRUMENTS In a console panel the instruments are usually

mounted conveniently around a miniaturized process flow plan of the unit. An effort is made to mount the instruments so that they are visible and accessible with a minimum amount of travel on the part of the operator. Many consoles are U-shaped to accomplish this purpose.

In a full graphic panel the instruments can be mounted to one side of the process line, in the process line, or inside of the vessel symbol provided this would not interfere with the graphic display of the vessel internals. When instruments are mounted in the vessel

AGE.”

104



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I os COPYRIGHT 2003; American Petroleum Institute


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FIG. 1 2 4 F u l l Graphic Panel (Front).

FIG. 1 2 - 5 - I ~ y o u t o f Seniigraphic Panel.

106



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INSTRUMENT PANELS - -.

symbol. the symbol usually is larger than the instrument case. Generally, an effort is made to maintain horizontal and vertical alignment of instruments on the panel.

In a semigraphic panel an attempt is always made to mount the instruments below the section of the flow plan to which they pertain. Generally, they are identified with the flow plan by use of symbols which incorporate both a color code and a shape, Le., red squares. green tri- angles. and the like.

5. FLOW PLAN SYMBOLS

Equipment symbols have been fabricated from many materials including sheet aluminum, brass, and plastic. Process piping and instrument lines have been fabricated of aluminum, brass. or plastic strips of rectangular or half-oval cross-section. Ordinarily, narrower width strips are used for instrument measurement, transmission. and control lines. The symbols are attached to the panel by use of adhesive material or studs. or a combination of both. If adhesives are used exclusively, the symbol material should have a coefficient of thermal expansion similar to that of the panel material. An alternate method is to have the flow plan painted on the face of the panel. This method generally is considered inferior to the other methods because it is less resistant to damage. Usual practice is to have spare symbols, lines, and paint furnished for panel repair or modifica- tion. Some typical flow plan symbols are shown in Fig. 12-6.

U CONTROL CONTROL CENTRIFUGAL RECIPRO-

VALVE VALVE PUMP CATING (3 WAY) PUMP

V A a LOGGED TEMPERATURE PRESSURE ANALYZER POINT LOGGER AND POINT

INDICATOR

COLOR OF INDICATOR RECORDER INDA:coR RECORDER CONNECTING

PIPING HEAT EXCHANGERS TEMPERATURE INSTRUMENTS

T - DISPLACE -

METER ROTAMETER ORIFICE MENT VENTURI

FIG. 124-Typical (;raphic Panel Syrnlmlr.

6. COLOR CODE A color code is used in flow plans to distinguish be-

tween process piping, instrument lines, wiring, etc. A typical color code is as follows:

a. 6 .

C.

d.

e . f . g. h.

i. k . 1. m. n.

I .

O .

Panel background-light gray or light green. Large equipment symbols (towers, vessels, etc.) - d a r k gray or dark green. Small equipment symbols (pumps, exchangers. control valves, etc.)-black. Instrument measurement and transmission lines- black. Instrument control lines-silver. Water -dark blue. Steam-light blue. Process air-silver. Hydrocarbon vapor and gas-light green. Hydrocarbon liquid-dark green. Tower bottoms-dark brown. Reflux and top pumparound-light yellow. Other pumparound-gold. Fuel gas-light brown. Fuel oil-red.

c. Nameplates Nameplates normally are provided for instruments,

vessels, equipment, and process line terminations. These nameplates are mounted either within or adjacent to the item covered. The most common material for nameplates is a laminated bicolor plastic. In engraving this type of plate the top layer is cut through which allows the letter to show in the second color (see Fig. 12-7). Other methods of lettering include the printed legend with a protective overlay or a back-engraved legend in clear plastic with a painted back contrasting with the lettering.

12.6 STRUCTURAL Panels are fabricated from a number of materials or

combinations of materials. The main requirements for

68 FRC 30 I

BEVEL EDGE TO SHOW WHITE LAMINATION

U Notes: I . Typical full-size nameplate. 3. Material for nameplate: %-in. thick bakelite. laminated

white core. contrasting surface, dull finish.

FIG. 12-î-L:iiiii11~ied Plastic Niiiiieplate.

107



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API RP 550-PART I

selection are rigidity, safety, smoothness of surface, and durability. O n panels using flow plan layouts, butt joints normally are used in order to have as even and smooth a surface as possible for the flow plan. Individual panel sections usually are removable without disturbing other panel sections. Filler panels sometimes are inserted above the main panel. Clocks, utility gages, and alarm indicators can be mounted on the filler panels if desired.

a. Panel Materials

Panels have been fabricated from steel. formica, ma- sonite, and such combinations as formica overlay on a metal or fireproof plywood panel. In fabrication, care should be taken to have the faces of panels flat and smooth.

b. Panel Framework

Panel frameworks are of several types, such as box frame, cubicle, and self-supporting, and usually are of steel construction. The main purpose of this framework is to support the instruments, certain auxiliary equipment (such as instrument power switches), and the interconnecting piping and wiring on the panel. It is also used to support or hold the panel in position. In some instances. it is necessary to add stiffeners to the panel itself in order to keep the face of the panel flat and relatively free from vibration. Typical framework construction is illustrated in Fig. 12-8 and 12-9.

c. Subassemblies

Such auxiliary items as pressure switches (for alarms) and miscellaneous pneumatic devices normally are fabricated as subassemblies. They are mounted directly on the panel framework, or are wall-mounted behind the panel board. Because the trend is toward wall-mounting, subassemblies normally are completed with their own wireways, pneumatic bulkheads, and supporting framework. A typical subassembly is shown in Fig. 12- 10.

d. Panel Base

Panels have been placed directly on the Boor of the control house but more often they are mounted on some type of base. This base is normally a flush, recessed, or extended curb. The curb construction usually is of concrete or steel, or both. A typical extended curb with a steel channel and concrete construction is shown in Fig. 12-11.

e. Panel Erection Panels normally are bolted to the base and, if re-

quired, suitably braced to the wall behind the panel. The extent of bracing will depend upon the self-supporting characteristics of the panel. In many cases, 3 drop-

ceiling will provide any additional support necessary and will add to the appearance of the panel. On open-end panels it is customary to provide a tumback or other means to enclose the end panels.

f. Panel Tolerances

Common practice permits a tolerance in the width of any individual panel, usually rt ?4 in. However, this individual panel tolerance should be compensating so that the overall tolerance for a panel group, for erection in any one plane, does not exceed a given amount. This overd tolerance normally is limited to I 54 in. A tolerance in panel height of I ?h in. is usually permitted. These tolerances will affect the location or tolerance in mounting holes in the panel and panel base.

g. Instrument Nameplates

Identifying instrument nameplates are desirable on both the front and back of the panel. The nameplate on the back of the panel and/or instrument should include power switch identification.

12.5 PAINTING

a. Face of Panel

Formica panels normally do not require any painting. Care should be taken with steel panels to have the face of the panel flat, smooth, and free of mill scale and foreign materials before painting. When the face of the panel is painted, the number of primer and intermediate coats will vary depending upon the panel material. How- ever, the final coat normally is a satin finish. Usual practice is to have additional paint supplied for panel repair.

b. Back of Panel

The backs of panels and interior surfaces of a console or cubicle normally are painted with a light color for visibility. The structural members of the panel framework and subassemblies generally are painted before erection and are not necessarily a light color. As a rule, the backs of panels and most structural members require a primer coat and a finish coat only.

12.8 ELECTRICAL INSTALLATIONS

Electrical installations should be in accordance with the latest edition of National Electrical Code and local codes. In classifying areas, the latest edition of APZ RP 500: Recommended Practice for Classification of Areas for Electrical Installations in Petroleum Refineries should be followed. The discussion herein is based on a Class I, Group D, Division 2 control house or area; see Art. 500 of the NFPA Birlletin No. 70: National Elec- trical Code.

108



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... .



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INSTRUMENT PANELS

FIG. 12-10-Subassemblv (Pressure Switches).

a. Electrical Supply

The most common type of electrical supply to an instrument panel is 120-volt, 2-wire, grounded, single- phase alternating current. Some refineries use an un- grounded electrical supply.

I). Wiring

Practice in connection with electrical wiring is varied. The methods used employ sheet metal wireways. rigid or flexible conduit. or combinations of these. The minimum conduit size normally used is ?h in. In all cases. the union or connector should be located at the instrument for safety and maintenance reasons. General practice is to provide spare knockouts. or conduit fittings.

: SLOTTED HOLES I FOR FON. BOLTS

DETAIL OF BASE CHANNEL

8" 11.5 LB [ INSTRUMENTPANEL

ANCHOR BOLT / / I NUTA AND WASHER

MACHINE BOLT WELDED - .u\- ' -F ILL UNDERSIDE

OF BASE CHANNEL WITH GROUT

TYPE I

CHICKEN WIRE BOTH SIDES FOR

,REINFORCING (ANCHOR BoLTS.TO

24" CENTERS)

SECURE CHANNEL TO FLOOR.TO BE ON

TYPE 2 * [ =CHANNEL

FIG. 12-1 1-liiatrument Patiel Board Bases.

111

to allow €or future additions to the panel. For cubicle or enclosed panels, the wiring generally is bundled together or run in wireways. The enclosure is considered sufficient protection. Power supply wiring should be run separately from the electrical transmission or thermocouple wiring.

e. Wiring Test

Wiring and electrical equipment should be thoroughly tested for proper installation and operation. Insulation resistance. measured from line-to-line and line-to- ground. should not be less than one megohm with the instrument disconnected. Also, transmission circuits . should be tested as described in Sect. 7, Par. 7.6(b). Chart drives should be run for at least one hour to check their operation. Alarm systems, push buttons, pressure switches, and other simple equipment should be tested under simulated operating conditions. Sequencing systems should be given operational tests by simulating field conditions with relays and switches. This operational check is superior to a point-to-point or continuity check but places the responsibility on the purchaser to provide the fabricator with sufficient information to make such a test.

d. Disconnect Switches

Disconnect switches on power supply to instruments normally are 2-pole and are arranged to disconnect both leads. Alarm unit disconnect switches usually are 3-pole and are arranged to break the connection to the howler. It is common practice to have one disconnect switch - serve as many as four instruments when only chart-drive power is involved. For electronic instruments and potentiometers where other than chart-drive power is required, one disconnect switch is used per instrument. For cabinet-type annunciators with multiple-alarm units, some users require more than one power disconnect switch €or servicing requirements. Each disconnect switch should be clearly labeled to identify the particular instruments or alarm units served by that switch.

e. Terminal Blocks

Terminal blocks usually are provided on panels and subassemblies for power supply wiring, alarm system wiring, and electrical transmission lines. Normally, no terminal blocks are permitted for thermocouple extension wires, nor for some types of analyzers (pH, etc.). It is preferred that these be run directly to the receiving instrument. The terminal blocks should be clearly identified with engraved or embossed numbers. Normally. - terminal blocks are o€ the enclosed type. They can. however, be the open type if they are contained in an enclosed panel, such as a console or cubicle, or in a wireway.



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12.9 PIPING Practice regarding instrument air supply to panels is

varied. Some refiners prefer a separate pressure-reducing regulator-filter for each instrument. Others use a header system with a master pressure-reducing regulator and separate filter to supply the header, with each instrument having a separate takeoff from the common header.

a. Supply Header

In cases where each instrument has its own pressure- reducing regulator-filter, the header is usually of gai- vanized or black iron pipe. For the reduced-pressure header system, the header is normally of brass or copper. To eliminate scale and rust formation encountered in the use of steel pipe, the piping downstream of the filters should be of brass. copper, or aluminum. The header should be properly sized for the capacity of air required. with some allowance for future expansion. Individual takeoffs for the instruments are normally brazed or threaded to the top of the header. Generally, at least 10 percent spare valved takeoffs are provided for the future addition of instruments. For the reduced- pressure header system, adequately sized dual filters and pressure-reducing regulators are provided for reliability (see Fig. 12-12). These regulators usually incorporate downstream overpressure relief features. Take- off connections are normally a minimum of ?4 in. For long panels. the header usually is joined between panel sections with a flanged or union type of connection to facilitate field installation. It is also normal practice to furnish 3 valved drain connection on the bottom of the header farthest from the air supply source.

1). Interconnecting Piping

Interconnecting piping between instruments and from instruments to bulkhead is normally !A -in.-OD copper tubing. The tubing usually is supported by clamping it to panel structural members and is made up with a minimum of joints and arranged for easy access.

FIG. 1 2 - 1 2 D u a l Pressure-Reducing Regulator and Filter Set.

c. Bulkhead Connections

Control and transmission lines and interconnecting lines between panels and subassemblies normally are brought to bulkhead fittings, usually located at the top of the panel; see Fig. 12-9. The simplest form of bulkhead consists of a steel plate, mounted vertically, with suitable fittings on either side to join the tubing from the field to the tubing from the panel instruments. In a normal prefabricated panel, the connections from, or to, the field instrument are the only ones required to be made by the purchaser during installation of the panel. Each bulkhead connection should be clearly labeled with the designation of the particular instrument o r connection it serves. I t is common practice to include a takeoff connection in the piping from the bulkhead connection to the panel instruments for testing or for future connection to a logging system or to other instruments. It is also usual to include spare bulkhead connections for the future addition of instruments.

d. Testing

Testing of the air supply header and signal. tubing normally is accomplished with air. Each joint should be tested with a soap-and-water solution and should be absolutely tight. Instruments should be tested in the manner prescribed by their manufacturer.

112



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Documents

API RP 550 Part 1 (1965) - Manual on Installation of Refinery Instruments and Control Systems. Part I- P