ENGINEERING DESIGN GUIDELEVEL MEASUREMENT3DG-J33-00001, Revision 000, 2003 July 10Reason for Revision: Issued for UsePrepared by: A. PaineChecked by: D. A. HuthApproved by: A. P. DiMartino

INTRODUCTION

This design guide is intended to introduce the reader to and provide guidance for the selection of level measuring and monitoring devices. The devices described in this document are applicable to all types of plants or facilities that require level measurement.

Page 2 of 41

INTRODUCTION 1

1.0 PURPOSE 4

2.0 CODES AND STANDARDS 4

3.0 CLASSIFICATION OF LEVEL MEASURING AND MONITORING DEVICES 4

3.1 Direct Method 5

3.2 Indirect (Inferential) Method 5

4.0 MEASUREMENT CONSIDERATIONS 5

5.0 LEVEL MEASURING AND MONITORING DEVICES 6

5.1 Gauge Glasses 6

5.2 Magnetic-Type Gauges 16

5.3 Mechanical Tank (Float and Tape) Gauges 19

5.4 Tank Gauging (Electrical/Electronic) 19

5.5 Float Type Level Switches 21

5.6 Displacement 22

5.7 Differential Pressure 25

5.8 Ultrasonic 28

5.9 Capacitance 29

5.10 Conductivity 30

5.11 Bubbler (Dip Tube Systems) 32

5.12 Nuclear 33

5.13 Thermal 35

5.14 Fiber Optic Level Switches 35

5.15 Strain Gauge Weighing Systems 36

5.16 Tilt Switches 36

Page 3 of 41

5.17 Rotating Paddle 37

5.18 Pressure Sensitive Diaphragm Type Level Switch 37

5.19 Vibrating Wand Type Level Switch 37

5.20 Radar Type Level Transmitters 38

5.21 Guided Wave Radar Level Transmitters 38

5.22 Plugged Transfer Chute Switches 39

5.23 Sludge Blanket Level Detectors 39

6.0 BOILER DRUM WATER LEVEL MONITORING AND CONTROL 40

7.0 REFERENCES 40

FIGURES

FIGURE 1 Typical Work Process Flow Diagram for Level Measurement

Page 4 of 41

1.0 PURPOSE

The purpose of this design guide is to provide guidance for the selection of level measuring and monitoring devices.

This design guide is applicable for all business lines. If a portion of the guide only applies for a specific business line it will be noted in the text at that point.

2.0 CODES AND STANDARDS

American Petroleum Institute

API Recommended Practice 551, “Process Measurement Instrumentation”

API Standard Practice for the Manual Gauging of Petroleum and Petroleum Products, “Manual of Petroleum Measurement Standards Chapter 3 – Tank Gauging”

American Society of Mechanical Engineers (ASME)

ASME B31.1, "Power Piping"

ASME B31.3, "Chemical Plant and Petroleum Refinery Piping"

ASME Boiler and Pressure Vessel Code, including all mandatory addenda, Section 1, "Power Boilers"

National Electrical Manufacturers Association (NEMA)

NEMA ICS6, "Enclosures for Industrial Controls and Systems"

National Fire Protection Association (NFPA)

NFPA 70, "National Electrical Code"

Refer to the latest codes and standards and any international or local codes that may be applicable.

3.0 CLASSIFICATION OF LEVEL MEASURING AND MONITORING DEVICES

The measurement and control of liquid level is essential in a process plant, where a wide variety of liquids are handled in both batch and continuous processes. The accurate measurement of level is important for plant safety, product quality, environmental protection (for example, tank overflow to drains), and inventory control.

Almost all liquid level devices measure by way of the position or height of the liquid above a zero or lowest point, or the hydrostatic pressure or head.

Page 5 of 41

The level measurement may be expressed either in units of length or volume, or in percentage of total volume. There are two methods to measure a liquid level: the direct method and the indirect or inferential method.

3.1 Direct Method

The direct method measures the liquid height above the zero point by any of the following techniques:

1) Direct visual observation of the height by means of sight glass, level gauge or dipstick

2) A float, which is mechanically linked or electrically connected to an indicator or alarm device

3) An electrical probe in the liquid

4) Reflection of sonic waves from the liquid surface or bottom

5) Microwave (Pulse Radar)

3.2 Indirect (Inferential) Method

The indirect or inferential method of measurement uses the changing position of the liquid surface to determine level. The techniques involve:

1) The buoyant force on a float or displacer, which is partially or completely immersed in liquid

2) Hydrostatic pressure of the liquid

3) The amount of radiation passing through the liquid

4) Electric systems by which liquid level may be inferred

5) Microwave (Frequency Modulated Continuous Wave (FM-CW) Radar)

4.0 MEASUREMENT CONSIDERATIONS

The range of the level measuring instrumentation should be large enough to cover the expected or required operating range including upset conditions.

Measurement of an interface level may be straightforward when the interface is clearly defined, as is usual between petroleum oil and water. But when the interface is ill-defined, as may occur with some combinations of liquids, or if one of the liquids froths, the distance between the

Page 6 of 41

highest point of the lower level of clean upper phase and the lowest point of the upper level of clean lower phase, and, in addition, make allowance for variation of the interface level. In other words, where there is an interface band rather than an interface line, suitable allowance should be made to measure the entire height of the band plus its movement. This holds true even where the upper phase is a vapor or gas; if the liquid may froth or swell, this should be taken into account when determining the required measuring range.

A vessel may have a number of level sensors responding to the level at assorted elevations and ranges. At least one indicating instrument having an independent measuring loop should span the entire range of levels measured by other instruments. Depending on how much of the height of the vessel that may have to be utilized or checked, this indicating instrument may have to span as much as the full height of the vessel.

An external-chamber level instrument, even a gauge glass, often does not accurately reflect the actual level in the vessel. A disparity exists if the density of the liquid(s) in a chamber piped to a vessel is not equal to the density of the liquid(s) in the vessel, as often occurs when the vessel and the chamber are at different temperatures. Such errors can be significant in applications such as high-pressure steam boilers and liquefied gas storage vessels.

External-chamber instruments are also subject to inaccurate readings or time lags due to restrictions or plugging in the connecting piping.

In a vertical, flat-bottom cylindrical tank, the change in volume is linear with respect to the change in level. However, if this tank is placed in a horizontal position, the change of volume isnonlinear with respect to the change of level. Additional non-linearity is introduced if the tank has conical or dished ends rather than flat ends. Thus, it is generally more effective in this case to indicate percentage of level instead of volumetric units. However, tables are available to relate the partial volume of horizontal cylinders at any height to the total volume, and increased use of digital control systems make such corrections relatively easy to implement for measurements within the system.

Level instrument connections shall be located on a vessel so that liquids or vapors entering or exiting the vessel do not impinge on the instrument connection.

5.0 LEVEL MEASURING AND MONITORING DEVICES

5.1 Gauge Glasses

Gauge glasses are simple devices, which indicate level by sight. Various techniques are used to visually see the level of fluid in a vessel. These include glass tubes or windows, or reflexive lenses.

5.1.1 TypesSeveral types of gauge glasses are available. The selection of the proper gauge glass depends on the process conditions and the fluid type. The two basic types are Armored, and Tubular.

Page 7 of 41

5.1.1.1 Armored

Where a gauge glass is to be used, the armored type, using flat glasses is generally to be used. An exception may be made for mild operating conditions that permit the use of a tubular gauge glass.

The basic types of level gauge glasses are as follows:

A reflex gauge glass may have one or more sections. Each section has a single flat glass with prismatic vertical serrations that can show a steam/water interface and other non-staining gas-or-vapor/liquid interfaces very clearly as white and black. A reflex glass should be selected for ease of reading where an armored gauge is desired and service conditions permit, especially for light colored liquids such as propane or lighter, and for clean (boiler) steam service up to 300 psig (20 barg). Reflex Gauge type level glasses are not normally designed for thermal cycling operation and hence not preferred for use in steam/water applications with thermal cycling. They are also not recommended for steam or condensate applications for tempearture above 300 deg F (150 deg C). A transparent gauge glass is preferred in those cases.

A transparent gauge glass may have one or more sections. Each section has one flat glass window in front and one in back. A transparent gauge glass is more expensive than the reflex gauge glass, and should, therefore, not be used indiscriminately. It should be used when an armored gauge glass is desired and any of the following conditions apply:

a. The liquid is sticky and dirties the glass. A light may then be placed behind the gauge glass to improve visibility.

b. A liquid/liquid interface is to be observed.

c. The glass would be etched by the liquid, thereby reducing visibility, and a shield is used to protect the glass from the liquid (see "Mica Shields," Section 5.1.4.1.6).

5.1.1.2 Tubular Glass

This is the lowest-pressure and lowest-cost type of gauge glass. It consists essentially of a glass tube held between special gauge valves. It should be used only for fluids that are not hazardous and that will not etch the glass (see "Mica Shields," Section 5.1.4.1.6). For safety, it must always be provided with guard rods or other protective devices (see 5.1.4.2.1 “Guards”). Furthermore, a tubular glass gauge should not be used where the gauge may experience constant or strong vibration. Tubular Gauge glasses are normally not recommended for process applications as they break easily.

Gauges of so-called "red line" type as well as clear gauges are available. The red-line gauge shows a solid red line from the bottom of the glass up to the liquid level. Its use may be considered for maximum visibility.

Page 8 of 41

5.1.2 ApplicationWith some exceptions, every liquid-containing vessel should have a level gauge to provide local measurement of level for operation and for calibration of other level instruments. The most common type of gauge is the glass-window type but others are available. Exceptions to the use of a glass-window gauge may include the following cases:

a. A closed vessel that contains a poisonous or radioactive liquid whose release by failure of a gauge glass is totally unacceptable. (Level gauge glasses are routinely used for corrosive and flammable fluids in the process industries.)

b. A vessel under pressure/temperature that exceed the design rating of the gauge.

c. A vessel requiring level measurement over a span of more than 12 feet, (3.6 meters) provided that (1) a suitable substitute indicating system is available, and (2) there is a worthwhile saving of money by using a substitute. If either of the two conditions is not satisfied, then as many level gauges as are necessary should be used.

d. An open vessel whose level can be judged by looking into the vessel and for which accurate measuring is not required or whose level can be measured by a stick. However, a gauge may be used in this case for convenience.

e. A closed vessel where the liquid would coat the glass so that the level cannot be seen behind the glass.

f. A storage tank with a floating or fixed roof, where liquid level is being measured and indicted by a mechanical tank gauge with a readout unit external to the tank.

g. A closed vessel with cryogenic (below –240OF (–150OC)) or other low boiling point liquid where the boiling point is so far below the maximum ambient design temperature that use of a large chamber (see 5.1.4.1.1 “Large Chamber”) cannot eliminate turbulent boiling of the liquid in the level gauge.

5.1.2.1 Service Rating

a. Armored gauge glasses are suitable to 10,000 psig (690 barg) and 750°F (400°C) (not concurrently), possibly higher. Manufacturer's literature should be consulted for the exact ratings of different brands and different models. Standardization by a project on a single pressure rating may be worthwhile by enabling gauge glass dimensions to be uniform throughout the project.

b. Tubular gauge glasses should not be used above 50 psig (3.5 barg) or 212°F, (100°C) although they are commercially available for higher temperatures and pressures.

5.1.2.2 Length

5.1.2.2.1 Armored

Page 9 of 41

Manufacturers generally offer different models of gauges for three pressure classes, to cover low, medium and high-pressure applications. Generally, the higher the pressure the shorter the visible length between reinforcements. Except for welding-pad-type gauge glasses, an armored gauge is made up of a number of standard-length sections, each relatively short because of strength limitations, assembled together to provide a single chamber to provide the required overall length. Each manufacturer has several different lengths for his standard sections of a given pressure rating. A given gauge should have the same size glass sections. Also, a given project should generally standardize on a limited number of section sizes for all gauges of the same pressure class for uniformity. The section length is usually chosen to be the longest offered by a given manufacturer for the model of interest. Although slightly different in design, all manufacturers adhere to nearly similar end-to-end or side-to-side dimensions. They also make similar length gauge sections, so that one manufactures complete assembly can often replace another. However, the individual components are not interchangeable.

Armored level gauges are available in various standard numbers of sections from one to a maximum that depends on the brand and model. The usual practice is to use any number of sections from one to five, as appropriate for the application, and to use more than one gauge if additional visibility is required. If more than one gauge is used to span a given distance, then adjacent gauges should be installed with their visible ranges overlapping a nominal amount, say one inch.

Welding-pad-type glasses, which are installed in a hole or slot in the vessel wall, are available in both rectangular and circular forms.

5.1.2.2.2 Tubular Glass

Tubular gauge glasses are available in any length desired up to 72 inches (1800 mm), in some cases longer. The length of glass may be specified either directly or by implication, by stating the center-to-center distance of the gauge valves that are part of the assembly.

5.1.2.2.3 Overall

For an armored gauge glass, each model and mounting arrangement has a manufacturer's standard length, or standard minimum length in the case of side/side-connected glasses. However, a side-connected end of a gauge may be fabricated to any specified length beyond a minimum, and use of a special length may be helpful, particularly for a tight installation that does not have room for connection fittings (see 5.1.2.3 "Mounting," below).

The overall length of a tubular gauge glass cannot be considered separately from the length including the gauge valves because the glass tube is held by the valves.

5.1.2.3 Mounting

5.1.2.3.1 Armored

Armored gauge glasses can be obtained with the upper connection at top, side, or back, and the lower connection at bottom, side, or back. Unless a special need exists for one or two side

Page 10 of 41

or back connections, the gauge glass should have top and bottom connections. The use of one or two side or back connections permits a shorter installation for a given size of gauge glass, and should be used when conditions are cramped, as, for example, when visibility is required to be as close as possible to a vessel connection (see discussion of overall length above). Side or back connections should not be used for fluids that tend to plug the connections.

5.1.2.3.2 Welding-Pad

An armored gauge glass mounting style that is infrequently used, the so-called welding-pad type, causes the gauge to be welded to and to become part of the vessel. This type may be useful in any of the following cases:

1) For slurries.2) To provide an accurate view of the vessel level, free of thermal errors.3) Where space is limited.

A welding-pad-type reflex gauge glass is used in single sections and should be mounted by the vessel manufacturer. As this design requires a slit in the vessel wall, it is only suitable for low-pressure applications.

5.1.2.3.3 Tubular Glass

A tubular gauge glass has limited choice of mounting arrangements. Two angle valves hold the glass tube and provide the mounting connections.

5.1.2.4 Connections

Armored gauge glasses are generally available with chamber connections of either 3/4- or 1/2-inch size, usually FNPT, but options are offered in manufacturers' catalogs. The 3/4-inch size and the FNPT connection should be used for the chamber unless there are special requirements.

Tubular gauge glasses have no chamber connection other than the ends of the glass tubing.

5.1.3 Specifications5.1.3.1 Armored

The body material armored gauge glasses shall meet the design conditions of the vessel. Generally the body material corresponds to the vessel material. The body is also available with various corrosion-resisting linings--such as rubber, lead, plastic--over a relatively low-cost base material. The manufacturer's ratings may vary depending on the material used.

Cover plates must be adequately strong but are not wetted by the process and need not be corrosion-resistant to the process. However at higher or sub ambient temperatures a similar material to the body is required both to match thermal expansion (contraction) and provide a material suitable for the environment.

Page 11 of 41

Gaskets may be of non-asbestos sheet, Graphite sheet, Teflon, or other materials to suit the service. The choice of gasket material may reduce the gauge glass rating due to high temperature, radioactivity, or other factor. Teflon should not be used in radioactive service.

5.1.3.2 Tubular Glass

Tubular glasses are available with different grades of glass but the type used shall be suitable for the design pressure and temperature of the service. The glass should be of Pyrex or other thermal-shock-resistant type.

5.1.4 Features and Options

5.1.4.1 For Armored Gauge Glasses

5.1.4.1.1 Large Chamber

This is a 2-inch-(50 mm) inside-diameter chamber, which is larger than the usual gauge chamber. This construction should be specified only if bubbling is expected within the chamber. The bubbling may occur by flashing of the liquid, or release of dissolved gas because of reduced pressure, or by the temperature within the chamber rising to the boiling point, as will occur for a very-low-boiling-point liquid in a warm environment.

5.1.4.1.2 Heat-Exchange Construction

This may use either an internal tube or an external tube through which either a coolant or a heating fluid may be passed. Another arrangement uses an electric heating element.

A heating feature may prevent freezing of the gauge fluid. Heat exchange with a coolant may prevent boiling. Do not use glass gauges for LNG. Heat-exchange construction should be used, where practical, in preference to external tracing if either winterizing or summarizing is required.

5.1.4.1.3 Non-Frost Extension

This is used for liquids whose temperature is below the freezing point of water. It consists of a thick length of essentially colorless, transparent plastic that is mounted flush against the flat glass. The thickness may be as many inches as required to provide a temperature gradient so that the back face of the plastic is at or close to the cold-liquid temperature while the front face is at or close to the ambient temperature. Thus, frost will not form on the front to obscure the glass. Temperature absorption by, and boiling of, the gauge liquid are also reduced.

5.1.4.1.4 Integral Expansion Loop

Page 12 of 41

To reduce gauge glass stresses caused by differential expansion of the gauge glass and the vessel or piping, the gauge should be specified to have an integral expansion loop at the top of the gauge for steam service at 750 psig (52 barg) or higher.

5.1.4.1.5 Spring Washers

The manufacturer may recommend and supply Belleville spring washers for gauge glasses in high-temperature service. These washers are used under the gauge nuts and compensate for expansion and contraction of gauge parts, thereby maintaining proper clamping force despite wide temperature swings. Belleville spring washers may also be needed for cryogenic service.

5.1.4.1.6 Mica Shield

Glass is etched and loses transparency if exposed to alkali, say with a pH greater than 8, or clean (boiler) steam over 300 psig (21 barg). For process steam the impurities may require shielding at lower pressures. For these conditions, a mica shield or equal is required to protect the glass chemically. Use of a mica shield or equal shield material prevents the use of a reflex glass.

5.1.4.1.7 Illuminator

This accessory is a section of light-transmitting, essentially colorless plastic that comes complete with an electric lamp. It efficiently provides light for viewing a transparent armored gauge in a dark location. It should be used where suitable nighttime or indoor lighting does not exist. It can be added to an existing installation. When used, it’s the electrical enclosure type suitable for the hazardous area classification, and the voltage should be specified to meet the area classification requirements.

5.1.4.1.8 Integral Isolation Valves

These apply only to welding-pad-type gauge glasses, and they permit maintenance on the gauge glass without shutting down and venting the vessel on which the gauge glass is mounted. The integral valves require a 100-percent fillet weld to the vessel, and the gauge chamber is then tack-welded to the vessel.

5.1.4.1.9 Measuring Scale

A calibrated measuring scale may be provided, as for tubular gauge glasses.

5.1.4.1.10 Glass Material

Borosilicate Glass is the most common type of optic material. It offers good resistance to most chemicals at temperature at or below 600 degF (315degC) . Tempered Borosilicate Glass is used to improve thermal shock resistance.

Page 13 of 41

Aluminosilicate Glass has a higher viscosity and a lower thermal expansion coefficient compared to borosilicate glass. Aluminosilicate glass is commonly used where process temperatures are greater than 600 degF (315 degC) but at or below 800 degF (427degC).Fused Quartz is made from crystalline silica with few impurities. The highest temperature rating, 1000 degF (538 degC) is achieved by using quartz.

5.1.4.2 For Tubular Level Gauge Glasses

5.1.4.2.1 Guards

Four guard rods should be provided as a minimum to protect the glass against external blows. A cage of slotted metal, or plastic, or safety-glass may be provided for additional protection.

5.1.4.2.2 Measuring Scale

A calibrated measuring scale may be provided, thereby converting the LG to an LI. The scale may be of engraved plastic or of various metals, either engraved or painted.

5.1.4.2.3 Level Switch

A capacitance-type level switch is available for mounting on a tubular gauge glass. One or more switches can be used on the same glass for stepwise control or alarm. However, this arrangement is not suitable for heavy-duty service and should not be used.

5.1.4.3 Fiber Optic Level Indicator

The use of fiber optics to transmit the water level gauge image to the control room is now highly successful.

This system uses a fiber optic hood mounted to a bi-color multiport type gauge glass fitted with an illuminator. The fiber optics accepts the red (vapor) and green (clear liquid) light coming through the gauge. The light from each port is focused onto the receiving end of a glass optic fiber, one for each port. The light is then transmitted through the fiber to the control room display.

The control room display focuses the light from the optic fiber to provide a drum water level indication, which is a direct reading of the boiler drum water level gauge glass.

5.1.4.4 Gauge Glass Image (Reflection)

The gauge glass is fitted with an illuminator and a reflector hood with a mirror at 45º. This reflection is then transmitted via enclosed "duct work" to another mirror at the boiler front or the control room.

Page 14 of 41

This is an inexpensive method of direct reading of the boiler drum level gauge glass but has the disadvantage that for a completely clear image of the drum level, the operator must stand in one particular viewing location.

An inherent problem with coal-fired plants is to keep the mirrors clean. Mirror alignment is also a problem. This system is not recommended due to its disadvantages.

5.1.5 Gauge ValvesGauge glasses may have ordinary line valves for isolation but the gauge valves, the so-called "gauge cocks", should be used for the special features they offer for this service: special body patterns, integral check valve, integral bleed connection, and easy disassembly for maintenance. These features should also be used, insofar as they are available and applicable, to valves for tubular gauges. It should be noted that where a vessel trim specification does not allow threaded pipe fittings, piping valves listed for that specification may have to be used instead of gauge cocks.

5.1.5.1 Body PatternAll gauge valves are of angle pattern, with two variations: straight and offset. The straight pattern has gauge and vessel connections in one plane, the offset pattern has them offset from each other. The offset pattern should be used because it permits the inside of the glasses of a top- or bottom-connected gauge to be cleaned for improved visibility without requiring the removal of the valve or gauge. The same type of valve should be used for side-connected gauges for consistency.

Valves are also available with spherical or offset union connections to allow for slight mismatch in gauge to standpipe or vessel connection, which is particularly helpful for easier installation of top-side/bottom-side connected gauges.

5.1.5.2 MaterialsAll the valve materials shall be suitable for the vessel design conditions.

The body material generally corresponds to the vessel material except that the material is usually brass or bronze for tubular gauge glasses. The stem should be manufacturer's standard.

Packing should be Grafoil or Bechtel-approved equal, except that the packing may be Bechtel-approved manufacturer's standard for services that are at a temperature not exceeding 450F and are in non-radioactive service, as defined for the project.

5.1.5.3 Service RatingThe pressure and temperature ratings of gauge valves shall satisfy the vessel design conditions. Manufacturers' literature should be consulted to determine available ratings.

Page 15 of 41

5.1.5.4 Check Valve FeatureThough level gauges are nominally satisfactory for their design conditions and are made of very-high-quality glass that resists thermal and mechanical shock, glass is a brittle material that may fracture because of improper torquing of retaining bolts, improper installation, or for other reasons. A leak may result from failure of a sealing gasket. Gauge glasses have proven to be very reliable in service but, as a precaution in case there is a failure, all gauge valves shall be provided with an integral check valve. This is a ball check that seats automatically to block flow following a venting of the gauge when the vessel is under pressure.

5.1.5.5 Seat ConstructionThere are two seats in a gauge valve and a third one is available. One seat is for manual shutoff, another for the ball check. The manual shutoff seat may be procured with either an alloy, replaceable seat; a non-replaceable alloy insert seat; or an integral seat. The replaceable seat should be specified for manual shutoff.

The manufacturer's standard construction is used for the ball-check seat.

A third available seat is for stem backseating to enable repacking the valve while the valve is in service, Backseating should not be specified (unless a standard design feature) because a closed root valve (a) renders a backseat unnecessary, and (b) provides more safety when valve packing is to be replaced, on the rare occasions when it is necessary.

For ferrous bodies, the seat and disc should be stainless steel, Stellite, or other corrosion-resistant material. For nonferrous bodies, they should be metallic, i.e., soft seats should not be used.

5.1.5.6 BonnetBonnets should be manufacturer's standard except as follows:

a. If the bonnet is threaded, it shall be pinned or seal welded to prevent accidental disassembly.

b. In high temperature or corrosive service, the bonnet should be bolted (“Outside Screw and Yoke” or OS&Y) for ease of disassembly and to isolate the stem threads from the liquid.

5.1.5.7 Connectionsa. For armored gauge glasses, the gauge valves have three connections, which should

be as follows except where special requirements may exist:

1) Gauge connection: To match the connection on the chamber.

2) Vessel connection: ¾” NPT(M) with swivel-union joint. The swivel joint facilitates field connection by allowing some accommodation to fit a pair of

Page 16 of 41

vessel connections that are not located properly. The union feature permits easy removal of the gauge glass for maintenance.

3) Bleed/drain connection: ½” NPT(F) with 2” (50mm) bar stock plug.

b. For tubular gauge glasses, the gauge valves have three connections, which should be as follows except where special requirements may exist:

1) Gauge connection: To match the connection on the chamber.

2) Vessel connection: ¾” NPT(M) with union joint. A swivel joint should not be used because it may cause the assembly to be twisted and the glass tube to fracture.

3) Bleed connection: ½” NPT(F) with 2" (50mm) bar-stock plug.

5.1.5.8 HandlesValve handles should generally be specified as handwheels, and the stem thread should be quick-closing type. If access to a gauge valve will be difficult, a lever-type handle can be specified, with a chain or chains of suitable length, and a multi-lead quick-closing type stem thread. Chains must be located so that they can be safely operated in the event of a blown gauge glass.

5.2 Magnetic-Type Gauges

Magnetic Level Gauges are a type of direct level measurement. They consist of a magnet carried by a float inside of a vertical chamber. An indicator on the outside of the vertical chamber is affected by the magnet and shows the position of the float. It is an all-metal alternative for glass-window level gauge. It can cover as large of a span as required.

5.2.1 Types5.2.1.1 Flagged

One type of magnetic-type level gauge has a vertical chamber in which rides a magnet-carrying float. There is a readout section that holds a vertical bi-colored series of magnetic flags that sequentially appear to change color as the float rises past them and change back as the float falls past them. The gauge thereby shows the level in small discrete steps. Actual level can be read in engineering units on a calibrated scale that is attached to the assembly

5.2.1.2 Follower

A second type of magnetic-type level gauge has a sealed tube that contains a metal follower. The sealed tube is attached to the vertical chamber in which rides a magnet-carrying float.

The follower is magnetically coupled to the float in the vertical chamber. Since the follower is magnetically coupled to the float, the follower position accurately indicates the liquid level.

Page 17 of 41

Use of this type should be avoided if installation will be subject to violent level changes that could decouple the follower from the float causing loss of indication.

5.2.2 ApplicationA magnetic-type level gauge is useful where a gauge glass would normally be applied except where the use of glass is not acceptable or preferred. A magnetic-type gauge can be connected to the process system by welding, resulting in a hermetically sealed system.

This gauge may also be helpful where an interface is difficult to see because the interface is between two similarly colored liquids. The difference in specific gravities should be at least 0.2. The difference in specific gravity can be lower than 0.2, however, this would then increase the float chamber size. Minimum specific gravity difference for fluid/fluid interface level is as low as 0.07.The manufacturer should be consulted if the difference is lower than 0.2.

The magnetic gauge uses a long thin float with a magnet imbedded near the top. When installed in an application where temperature increases from ambient, the decrease in fluid density will cause the float to sink deeper causing a level offset. Where indication is critical, the supplied gauge should provide a scale indicating the drop in level due to change in temperature.

5.2.2.1 Service Rating

Magnetic-type level gauges are available in pressures to 5000 psig (345barg) or temperatures to 750oF (400°C). Temperature may be upto 800oF (426°C). But this would require insulation of the float chamber. Also, there may be demagnetizing effect on magnets at elevated temperatures. This type of gauges can be used in cold service (cryogenic) with temp down to -320oF (- 195°C), but this again requires insulation of the chamber. The manufacturer's catalog should be consulted.

5.2.2.2 Mounting Styles

Several mounting styles are available, as follows:

a. With a float chamber: The float chamber is piped similarly to the way a gauge glass is piped. The upper connection may be at the top, upper side, or upper back; the lower connections at the bottom, lower side, or lower back. The readout section may be mounted on either the side of the float chamber or, with the use of an extension rod, on a separate readout chamber that is attached to the bottom or the top of the float chamber. When using the top- or bottom-mounted readout, the use of a top or bottom piping connection, respectively, for the float chamber is ruled out.

b. Without a float chamber: The readout section is mounted on the top of the vessel with the float attached and located within the vessel. A stilling well must be provided in the vessel to protect the float against turbulence.

Page 18 of 41

5.2.3 Specifications5.2.3.1 Construction

The float chamber is normally made of 2-inch (50mm) NS pipe. It has no standard length but can be made in one piece and as long as necessary. The float chamber shall be designed and constructed to ASME B31.1 Power Piping or ASME B31.3 Chemical Plant and Petroleum Refinery Piping as applicable. Float Chamber size shall be 2" minimum. Depending on specific gravity differences, chamber size may increase. Standard sizes are 2", 2-1/2", 3" and 4". Lower the specific gravity differences higher the chamber sizes.

The readout section, for level indication, is clamped to the side of either the float chamber or the readout chamber, either above or below the float chamber. The readout section is typically available only in standard lengths—6 inch (150mm), 12 inch (300mm), 18 inch (450mm), 24 inch (600mm), and 36 inches (900mm)--but two or more such lengths may be combined to provide a total usable length matching that of the float chamber.

5.2.3.2 Materials

The readout section operates by magnetic flux and must, therefore, have a non-magnetic wall. When the readout section is mounted on the side of the float chamber, the float chamber must, therefore, be non-magnetic, and is usually of 304 stainless steel. When the readout section is not on the side of the float chamber but is rather on a readout chamber, the float chamber may be of carbon steel, which is magnetic, or of 304 stainless steel; the indicator chamber must then be non-magnetic, usually of 304 stainless steel.

5.2.3.3 Connections

The float chamber connections are essentially connections for 2-inch (50mm) NS pipe and are available as any kind of pipe connections. Connections should be specified to conform to the applicable project piping specification.

5.2.4 Features and OptionsAn external magnet-actuated level switch is available for attachment to the level gauge. A calibrated measuring scale may be provided, as for glass gauges. Also, external magneto-restrictive level transmitters are available and can be attached to the gauge. A 4-20 mA output signal or smart signal is available. However, care should be taken for mounting the transmitter for high temp (>300 degF (150 deg C)) applications.

Level gauge “cock” valves used for glass gauges are not appropriate for magnetic level gauges. Therefore, piping valves consistent with the vessel trim specification are used for isolation of the magnetic type gauge from the vessel.

Page 19 of 41

5.3 Mechanical Tank (Float and Tape) Gauges

This type of level measurement instrument consists of a float-actuated tape, which drives a gauge that provides digital readout of the float travel for level indication over the entire tank range.

5.3.1 ApplicationThis type of instrument is often included with the tank specification and supplied with the tank especially if the tank is lined. The enclosed tape run enables a loop seal to be provided if the tank has an "inert gas blanket". With this system the actuation tape is run via totally enclosed guides and pulleys to the readout unit mounted at a convenient location alongside or atop the tank.

5.3.1.1 Mounting

The float for this type of instrument is maintained in vertical alignment by guide wires firmly attached to the top and bottom of the tank, thereby requiring the tank to be empty for installation and maintenance. For fixed roof tanks, a stilling well should be provided if the content of the tank is subject to turbulence. The stilling well should either be painted or corrosion-resistant to prevent rust from forming on the piping above the liquid surface. For floating roof tanks, the tape is fastened to the floating roof rather than a separate float, since the roof itself is on the top of the liquid.

5.3.2 SpecificationsThe materials of construction must be compatible with the process system and conditions.

5.3.3 Features and OptionsThe readout unit can be readily fitted with high and low limit switches for alarm and control requirements. An analog or binary output signal transmitter is also available.

5.4 Tank Gauging (Electrical/Electronic)

Tank gauging is the generic name for static quantity assessment of liquid products in bulk storage tanks. Typically tank gauging meets the international requirements for custody transfer and is used on large outside storage tanks. Tank gauging is microprocessor based, and requires the use of a portable communication or configuration device, called a portable terminal, for setup calibration or configuration changes. Tank Gauging may also incorporate tank dimensional data or fluid temperature to perform additional calculations.

5.4.1 TypesThere are several techniques to measure the mass / volume of a fluid in a tank. All of the techniques include a level measurement. The traditional technology used in tank gauging is Displacer type. Hydrostatic and Radar methods are also used. Batch mixing tanks or weigh tanks may use load cells.

Page 20 of 41

5.4.1.1 Displacer (Servo)Displacer type of tank gauge continuously measures the buoyancy of a displacer positioned at the surface of the liquid. The displacer is attached via a stainless steel cable to a drum that is mounted in the tank gauge mounted on top of the tank. The drum contains a servo, which measures the tension and length of the steel cable. It can determine the height of the displacer within 1mm.

5.4.1.2 HydrostaticHydrostatic tank gauging uses pressure sensors to determine the hydrostatic head, density, and the mass of the fluid. For improved accuracy this type of device will measure the temperature of the fluid and of the gauge in determining the density of the fluid between two pressure sensors, which are at a fixed distance apart from each other. See section 5.7 for more information on the use of Differential Pressure for level measurement.

5.4.1.3 Radar HybridRadar level sensors mounted at the top of the tank may be used in conjunction with a hydrostatic pressure transmitter at or near the bottom of the tank. The combined surface level with hydrostatic head can be used to determine the mass or density of the fluid. See sections 5.20 and 5.21 for more details on using Radar technology for level measurement.

5.4.2 ApplicationTank Gauging is most prevalently used in the Petro-chemical industries. It may be used on many various types of tanks including those with a floating roof, or pressurized with a gas-blanket. The accuracy of these systems are very important because they are often used for custody transfer. Considering the size of some bulk fluid storage tanks, even a 1mm discrepancy in level measurement translates to a very large volume.

An particular advantage of the displacer type is it can also locate the interface point between two liquids. This is important for determining the actual volume of petroleum products, which may have water or various hydrocarbons in the tank. Additionally the servo, by measuring the tension, can determine the density of a liquid through a program change with the portable terminal.

5.4.3 SpecificationThe materials of construction must be compatible with the process system and conditions. A stilling well should be provided for displacer type and may be useful for enhancing accurarcy in some radar applications. The stilling well material should be corrosion-resistant. Rust forming in the stilling well and falling onto the displacer will adversely affect the accuracy since the buoyancy will be impacted. Radar antennae material must be resistant to corrosion, condensing vapors, and be rated to handle the pressure and temperatures for the application.

The traditional displacer technology must only be used for clean, low viscosity fluids. If the product can stick to the cable suspending the displacer, it will distort the density calculation

Page 21 of 41

and may foul the servo drum. For thick fluids such as heavy oil, radar or radar hybrid systems are usually used, and hydrostatic tank gauging may be used.

Some guided wave radar systems are able to detect interfaces, but the dielectric of the fluids must be significant enough for this type to detect an interface. Hydrostatic or Radar hybrid systems that use an array of pressure transducers, can determine the fluid density. Or if the specific gravity of the fluids are know, hydrostatic gauges may be able to calculate the interface level. These density calculations depend on the distance between the radar level instrument and or the pressure sensors. Any fluctuation due to tank flexure, or thermal movement will effect this calculation. Temperature is often measured at the pressure sensor points to compensate for thermal expansion between the pressure transducers.

5.4.4 Features and OptionsLocal indication may be provided in the unit on the top of the tank and a remote indication unit is available that can be located at ground level. Calibration can then be performed at ground level using a portable terminal. Multi-point TE option is available, which works in conjunction with level transmitter to select the representative TE.

The tank gauge can provide a 4-20 mA analog or digital output signal. The tank gauge is capable of providing programmed level alarm outputs and relay contacts for mixer or pump control.

5.5 Float Type Level Switches

Float level switches incorporate in their design a float, which follows the liquid level or the interface level between liquids of differing specific gravities. Standard floats are normally spherical or cylindrical for top-mounted designs and spherical or oblong for side-mounted designs. Spherical floats are available from 3 to 7 in (76 to 178 mm) in diameter. The small diameter floats are used in higher density materials whereas the larger floats are used for liquid-liquid interface detection, for lower density materials, or when the float must buoy up a long motion take-off assembly. The binary action is provided by one or more switching elements coupled with the float assembly. The switching elements are normally mercury filled switches, pneumatic switches, micro switches, reed switches with mechanical or magnetic coupling, tilt switches, etc.

5.5.1 ApplicationFloat type level switches are not normally considered for industrial processing applications, except in the utility services or as alarm switches. The repeatability is normally about 1 in (25 mm) for float type level switches.

Use of float type switches is not recommended in the following situations:

• The size of the float is too large or the associated support and guide tubes cannot be accommodated.

Page 22 of 41

• In some cases where immersed magnets attract pipe scale and other ferrous metal particles in the process that can interfere with proper switch operation.

• Many of the float-operated designs have moving parts exposed to the process and cannot be used in dirty or plugging services.

5.5.2 ProcessFloats level switches can operate up to 1000 psig (70 barg) and –40 to 225 deg F (-40 to 107 deg F). Special designs can go up to 200 psig (140 barg) and 500 deg F (260 deg C).

5.5.3 MaterialThe materials of construction must be compatible with the process system and conditions. Floats are available in various metal, metal-alloy and plastic material. Also, the switching element with electrical outputs and its enclosure must meet hazardous are classification requirements.

5.5.4 MountingThe External Float chamber type level switches have float in a chamber that can be mounted externally with side/bottom process connections. In this case the entire unit can be isolated for maintenance without disrupting normal plant operation. There are also direct-connected designs such as top-mounted and side-mounted switches. These designs should be considered only if the vessel can be depressurized and drained when the switch needs maintenance.

5.6 Displacement

This type of instrument utilizes a displacer (float) connected to a torque tube (spring), which in turn actuates the transmitter. The torque tube supports the dead weight of the displacer when it is out of the liquid. As the liquid level rises it displaces some of the weight of the displacer reducing the load on the torque tube and turning the output shaft. Displacers can be used to handle a range of densities, however the lower the density the less the standard output shaft turns. For special applications such as interface, or density measurement different spring rate torque tubes are available.

5.6.1 ApplicationDisplacement Level instruments are not usually used for measurement over 60 inches (1500mm), although displacers up to 120 inches (3000mm) are available. Displacers can be used with a variety of fluids from cryogenic to 750°F (400°C) and in a variety of materials. Generally the displacer is housed in an external cage except in low boiling point liquids like cryogenic fluids. Displacer level instrument may be internal also, but normally not preferred. External cage type is preferred for most services, including liquid-liquid interface, except those listed below:

• Where the displacer range (length) exceeds 5 feet use a D/P cell or other device suitable for the application.

Page 23 of 41

• For dirty, corrosive, or viscous service.

• For direct-connected level switches, use a displacer or packless ball float type. Connecting process piping to level switch should be same size as the vessel nozzle. Provide vent and drain valve connections to check the process actuation of the level switch without changing the vessel liquid level.

• In refinery and chemical applications on fractioning columns having steam stripping, use a purged D/P cell instead of a displacer. If purging is impractical use a sealed D/P cell. Performance of displacers may be affected by the condensing of steam in the top section of the displacer housing containing vapor and subsequent flashing of steam condensate in the bottom section of the displacer housing containing hot liquid. Stripping steam entering the column should be directed away from the level

connection to the column.

• On steam drums and generators, consider using a sealed D/P cell, or a "wet-leg" hook-up.

For applications where vessel penetrations are not permitted, other type instruments should be considered.

5.6.2 SpecificationStandard displacer ranges should be 14", 32”, 48” and 60” for ASME English standards and 350 mm, 500 mm, 750 mm, 1000 mm, 1200 mm and 1500 mm for DIN Metric standards. Units longer than 48"(1200 mm) while available, are to be avoided due to weight and thermal expansion problems between the vessel and the instrument. The minimum connection size/ASME rating should be 2"/ Class 150 psig flanged. Vessel nozzle connections should be 2". However if multiple displacers are used, and may be later changed out by the owner during maintenance, they may prefer to standardize on a specific Class rating for all displacers. This should be determined at onset of the Project. Displacement type level devices should be installed with a vent and/or a bottom drain oriented to allow a probing rod to check or free the displacer element.

5.6.2.1 Connection

This instrument can be flange mounted from the top of the vessel or mounted within a cage installed off the side of the vessel. For liquid level measurement the displacer is installed so that the surface to be measured remains within the length of the displacer. Therefore, the length of the displacer is critical.

An off-vessel cage can be supplied with a selection of process connection locations, as follows:

Top and bottomTop and lower side Side and side (use only when required by space considerations)Upper side and bottom

Page 24 of 41

Lower-side connection with upper-top connection may be considered for those applications requiring level measurement close to the bottom level tap location. Upper-side and lower-side (side-side) connections may be considered for those applications with space limitations.

In general, controllers and transmitters will have top-side connections and lower-bottom connections with a rotatable head flange and bottom drain. Displacement chambers should be of flanged construction for easy float removal. The case mounting should be right handed unless a left-handed orientation is required by the piping layout. Other variations may be used where required. If upper-top and lower-bottom connections are used, rotatable heads are not required.

With the off-vessel cage the required location of the transmitter head with respect to the vessel connections should be determined. If design is incomplete then a rotatable head should be specified. This allows a 360-degree horizontal plane location.

5.6.2.2 Material

Materials of construction should be compatible with the process fluid and the system design pressure and temperature. Normally the body or cage should be fabricated carbon steel with a 304 or 316 stainless steel displacer or ball and Inconel torque tube, unless process conditions or vessel construction requires higher-grade alloys.

5.6.2.3 Process

The displacer chamber and flanges shall be suitable for the vessel design pressure and temperature. Displacer chambers rated higher than ASME Class 600 made from alloy materials, operating over 700°F (371°C) or under –20°F (-29°C) are costly and should be avoided if other suitable level measurement device is available. Above 1000 psig (69 barg), solid displacers should be considered in order to avoid crushing. Displacement level instruments should have thermal extensions on services above 350°F (177°C) and below –20°F (-29°C) or as recommended by the manufacturer.

Electronic transmitters shall meet the National Electrical Code (NEC) (NFPA 70) or other International area classification requirements. The torque tube housing should be isolated from the pneumatic or electronic mechanisms by a positive sealing arrangement.

5.6.3 Features and OptionsThe signal output from a displacement level transmitter may be pneumatic or electronic 4-20 mA. Generally, output signal transmission should be electronic for remote indication or control, while local loops may use pneumatic equipment. Electronic transmitters should be solid state, two wire “Smart’ type, providing an isolated 4-20 mA signal while operating from a remote power supply of 24-48 Vdc. Pneumatic transmitters shall have an output range of 3 to 15 psig (0.2 to 1 barg), should be enclosed with a weatherproof case and should be furnished with an air supply filter/regulator set. The pneumatic version has a higher temperature rating than the electronic one. An interposing pressure transmitter is required if the pneumatic version is used and a 4-20 mA output is needed.

Page 25 of 41

5.7 Differential Pressure

The hydrostatic pressure of fluids increases linearly with height. A differential pressure transmitter will indicate the pressure exerted by the fluid and thus, the level may be inferred. The density of the fluid determines the relationship between pressure and level. If the

density of the fluid changes, this must be compensated in order have an accurate level measurement.

5.7.1 ApplicationConsider D/P cell for level measurement application in the following services:• Where displacer range (length) would exceed 4 feet.• On steam drums and generators.• In refinery and chemical applications on fractioning columns having steam stripping, use

a purged D/P cell instead of a displacer. If purging is impractical, use a sealed D/P cell.• The use of D/P cells mounted with a diaphragm seal on flange is recommended on

vessels containing corrosive or highly viscous material, low temperature applications, fluids with particulates, applications involving turbulence or agitation, or applications where the sensor flange is close to grade.

5.7.1.1 Open Tanks

On open tanks, a direct connected pressure instrument can utilize the process fluid head pressure to provide level measurement over the entire range of the tank. For clean liquids or slurries a pressure or differential pressure (D/P) transmitter can be used. With the D/P transmitter one side must be left open to atmosphere and fitted with a "Bug Screen" to prevent contamination.

5.7.1.2 Pressurized or Closed Vessels

On closed vessels, compensation must be made for the vessel internal pressure. This is accomplished by using a D/P transmitter with one side connected to the lower tank nozzle and the other to the upper tank nozzle. In the presence of a condensing vapor, a condensate pot can be directly connected to the tank upper nozzle so as to provide a wet reference leg. In this application the transmitter working range (span) may be lowered or suppressed. This provides a negative differential so that the output signal increases with level increase.

5.7.1.3 Access is Restricted to the Top Only

Applications that involve level measurement in wells or forebays with access only from the top can use a submersible pressure transmitter as an alternative method to the bubbler (see Section 5.11). The transmitter is lowered to the bottom of a stilling well for protection and is connected to the surface via a two conductor cable that provides an isolated 4-20 mA signal. Standard wetted materials are polyurethane cable strengthened with Kevlar and a titanium

body for corrosion resistance.

Page 26 of 41

5.7.2 Specification5.7.2.1 Connections

5.7.2.1.1 Impulse Tube

The most common method of connecting a pressure level transmitter to the process is with 3/8” or ½” (see 5.7.2.2 “Materials”) 316 stainless tubing. This is called impulse tubing or sensing line. The transmitter should have instrument valves or a manifold at the transmitter connection to provide a means for isolating, testing, and servicing the instrument.

In D/P level transmitter installations the sensing line going to the upper tank connection becomes a "reference leg" of constant pressure head. Care must be taken in applying reference legs when the atmosphere may alternate from condensing to non-condensing.

5.7.2.1.2 Flanged

On many tanks, a flange connection is provided for the level measurement instrument. Most pressure transmitters may be specified with an integral flanged connection. With pressure and differential pressure type level transmitters not mounted onto the tank nozzle, the actual mounting location must be considered when calibrating for the tank level measurement required.

For installations that use a flanged level transmitter, the use of a spacer (“bleed ring”) (not required on an open tank) located between the isolation valve and the transmitter provides a means for running zero and span checks. Threaded connections are provided in the spacer for draining and for connecting test instrumentation. This spacer is also used in slurry service as a flushing ring for cleaning the diaphragm. Generally, material would be minimum carbon steel for water applications and stainless steel for flammable or hazardous fluids.

In slurry service extended diaphragms are often used in order to have the sensing face level with the vessel wall.

5.7.2.1.3 Remote Pressure Sensing Diaphragm

Transmitters may be equipped with remote pressure sensing diaphragms; these are also called remote seals or capillary-diaphragms. They are available as a variation of the flange mounted transmitter, with an extension to bring the diaphragm flush with the tank wall.

Page 27 of 41

Reasons for using a remote seal include the following:

• The fluid is dirty, corrosive, or viscous.• A sanitary application.• When the process temperature is outside of the normal operating limits of the

transmitter and cannot be brought into those limits with impulse piping.

Remote seals are a thin diaphragm mounted on a flange. The space behind the diaphragm is filled with oil or other fluid, which is compatible with the process temperature. The pressure exerted on this oil by the diaphragm is sensed at the transmitter via very small diameter tubing, called capillary tubing. Remote seals are sensitive to ambient temperature fluctuation. They can be easily damaged during installation and maintenance, and they can effect the response time of the pressure measurement. However, they do provide an option for D/P level measurement in some tough applications.

For dirty, corrosive, or viscous service, a purged D/P cell may be used but it is more susceptible to blockage and effects from the purge fluid pressure.

Designs using diaphragm-capillary seals should include installation instructions which provide for installation after the process vessel, piping, etc., hydrostatic pressure testing, acid washing, flushing, etc. is complete to prevent possible damage to instrument diaphragms from overpressure, corrosion, etc. Use of an instrument tee with pancake type diaphragms is recommended to enable venting and draining process fluid.

5.7.2.2 Materials

The materials of construction must be compatible with the process and system conditions. Wetted parts should consist of a carbon steel body and a 316 stainless steel sensing element, as a minimum, unless the application requires otherwise. Process connections (other than diaphragm seals) should be 1/2 inch NPT for refinery and chemical applications, and 3/8-inch tube or 1/4 inch NPT for power applications. Differential pressure transmitters should be able to withstand overrange pressure equal to the meter body rating. Level transmitters will generally not be specified to have local receiver indicators unless the vessel is not equipped with level glasses.

5.7.3 Features and OptionsPneumatic transmitters shall have an output range of 3 to 15 psig, (0.2 – 1barg) should be enclosed with a weatherproof case and should be specified to be furnished with an air supply filter/regulator/pressure gauge set. Electronic instruments should have an output signal compatible with the instrument loop, but generally 4-20 mA DC. Differential pressure level transmitters should be solid state electronic, two wire ‘Smart’ type, providing an isolated 4-20 mA signal while operating from a remote power supply of 24-48 V DC. Generally, output signal transmission should be electronic for remote indication or control, while local loops may use pneumatic equipment. Electronic transmitters shall meet the National Electrical Code (NEC) (NFPA 70) or equal International area classification requirements.

Page 28 of 41

5.8 Ultrasonic

5.8.1This type of level measurement operates on the principle of ultrasonic ranging.

Ultrasonic signals are pulsed from a transducer towards the liquid whose level is to be measured. Signal echoes are received after being reflected from the liquid surface. The time interval between signal and echo is proportional to the distance from the transducer and the liquid.

5.8.2The transducer is mounted to a suitable tank top connection and the transmitter is mounted at a remote location within the limitation of the system. Ambient temperature compensation should be specified for outdoor tanks and any vessel whose operating temperature will vary.

5.8.3This type of level measurement system has a limited measurement range and cannot tolerate any tank internal interferences.

5.8.4 In order to ensure proper operation and avoid measurement interference from vessel walls, connections, internal piping, etc, ultrasonic instrument orientation should be carefully considered and coordinated with the manufacturer.

5.8.5The materials of construction must be compatible with the process and system conditions.

5.8.6Ultrasonic devices can be useful for measuring solids level in bins or silos. However, the measurement can be severely affected by interference from the dusty atmosphere and background noise that can be experienced during filling operations. Ultrasonic level devices are also affected by presence of vapours.

5.8.7An ultrasonic level measuring device will also operate poorly if "sloshing" occurs in the tank (i.e., if the liquid surface does not maintain an even level). If an ultrasonic device is planned to be used in these circumstances it should be installed in a stilling well in the tank; however, special attention must be given to the proper sizing and selection of the stilling well to ensure that the ultrasonic signal is not interfered with.5.8.8

Ultrasonic level measuring devices are very useful in measuring slurry levels in tanks, sumps, and distribution boxes.

5.8.9 Ultrasonic Level Switch

5.8.9.1This type of instrument mounts the ultrasonic sensor (transducer) at the point of control.

Page 29 of 41

5.8.9.2The level sensor has a slot or gap through which the process can flow. This slot can be either horizontal or vertical and vary in form to suit the process fluid.

5.8.9.3A remote mounted control unit sends an electronic signal to the sensor which converts it into a high frequency ultrasonic signal.

5.8.9.4When the sensor slot or gap is filled with process fluid, this ultrasonic signal is transmitted across to the receiver side and reconverted to an electrical signal.

5.8.9.5The control unit amplifies this signal and the switch relay is energized.

5.8.9.6The materials of construction must be compatible with the process and system conditions.

5.9 Capacitance

5.9.1This type of level measurement instrument consists of a capacitance probe and a transmitter.

5.9.2The capacitance probe is mounted direct into the vessel using a 1" NPT connection. This can also be mounted into a flange if required.

5.9.3The capacitance to current transmitter can be mounted directly onto the sensing probe or mounted remotely, within the limitations of the system. The transmitter provides a 4-20 mADC output for use to alarm, control or provide level measurement indication.

5.9.4Capacitance type level measurement should not be used on vessels with irregular or varying dimensions.

5.9.5This level measurement system can be supplied with optional alarms and level indication.

5.9.6There is a variety of capacitance systems and, despite suppliers’ claims; they are susceptible to material buildup malfunctions. Therefore, if the process fluid could cause deposits to bridge between the probe and the probe tank connection, other level measurement systems should be considered.

Page 30 of 41

5.9.7When used to measure the continuous level of certain fluids the capacitance level probes tend to become coated. Under these circumstances a coating compensation circuit should be specified and furnished by the probe supplier.

5.9.8 Radio Frequency (RF)

5.9.8.1The Radio Frequency (RF) level sensor uses low voltage to transmit a radio frequency oscillating signal from the probe to ground. When the material level reaches the probe, the control contacts are activated. They can withstand high temperature and pressure conditions up to 5000 psi (345 bar) and 1000°F.(538°C) The sensor could be used to measure the level of liquids, powders, granular solids, and slurries. The RF sensors are also applicable for continuous level measurement. They are commonly used in fossil power plants to detect fly ash hopper level.

5.9.8.2If RF sensors are used under conditions where it is possible for a highly conductive build-up to form on the sensing probe there may be problems with accuracy and repeatability.

5.9.9 Capacitance Probe Level Switch

5.9.9.1This is similar to the capacitance type level transmitter, but for switch applications a capacitance sensing point type probe for on/off actuation is used.

5.9.9.2The point type sensing element is unaffected by the vessel shape.

5.9.9.3The control unit containing the control relay can be mounted directly to the probe housing or remotely mounted in a suitable location.

5.9.9.4The sensing probe can be either flange mounted or use a one inch threaded connection.

5.9.9.5This type of point sensing probe can also be mounted in the horizontal plane, in the side of a vessel.

5.10 Conductivity

5.10.1

Page 31 of 41

Conductivity (Electric Type) level gauges are routinely used on steam drums operated at pressures up to 3000 psi (207 bar) for monitoring drum level. The system typically consists of three components: a) a water column containing the conductivity probes mounted directly to the vessel, b) control unit, and c) display units installed locally, at the drum elevation, and remotely, in the control room. Conductivity level gauges are also suitable for feedwater heaters, flash tanks, condensers, deaerators and turbine drains.

5.10.2Caution is advised when using this type of level gauge in chemical processes due to the potential for sparking.

5.10.3This type of instrument utilizes a water column connected to the vessel with a vertical array of "conductivity" electrodes installed within the water column.

5.10.4Differences in the electrical conductivity of steam and water are the means of water level detection.

5.10.5The water column is grounded so that the conductivity (resistance) measurement of the steam and water in contact with the electrode tip is made between the whole surface of the tip and the grounded water column.

5.10.6The electrodes are connected to a locally mounted electronic module that reads the signals to determine which electrodes see steam or water. The result is indicated by a red and greenLED display on the module case.

5.10.7The "gauge glass" error due to contrasting densities also applies to this type of instrument, although it is claimed that due to the heavy wall thickness of the water column and the electrode protective enclosure, this system is more accurate than a normal gauge glass. Fouling of the probes may occur in sour applications, resulting in measurement error.

5.10.8 Conductivity Probe Type Level Switch

5.10.8.1This type of instrument utilizes the conductivity of the vessel contents to complete the electrical circuit and actuate the switch or relay.

5.10.8.2These instruments can have multiple probes to provide the separate electrical circuits for multiple on/off control or alarm functions.

5.10.8.3

Page 32 of 41

The control relays are normally provided in a separate remotely mounted enclosure from the probe.

5.10.8.4These probes are usually installed from the top of the vessel using either a flange or threaded connection. The size of the connection is dependent upon the number of probes installed in one housing.

5.10.8.5This type of instrument is susceptible to material build-up malfunctions, and this should be considered in choosing the method of level switching to be used.

5.10.8.6 Single point conductivity probes are used to measure the level in drain pots and provide the smallest volumetric area. Like all these probes the circuit is made when the liquid reaches the probe, so to provide fail-safe design a secondary circuit is required to confirm the probe is working correctly. For isolation provide full port gate or ball valve to ensure all fluid clears the switch on decreasing level.

5.11 Bubbler (Dip Tube Systems)

5.11.1On open tank applications, a pneumatic back-pressure method of level measurement

is used.

5.11.2In this application a pipe (dip tube) is installed in the vessel and a constant supply of air is fed into the pipe through a constant flow regulator. This supply air pressure must be higher than the head of liquid to be measured.

5.11.3The pressure in the pipe will build up until air escapes out of the bottom of the pipe, or against a sealing diaphragm. The pressure in the pipe is equal to the head of the liquid.

5.11.4The pressure is connected to a transmitter or switch to provide level indication or alarms as required. As the level changes the pressure head will change.

5.11.5Dip tube systems used in a vessel under pressure require the use of a differential pressure type measuring instrument with regulated gas purge for both the dip tube connection and the equalizing connections to the instrument. This method of measuring level should be avoided whenever possible and cannot be used on vessels containing toxic, flammable, or hazardous fluids.

5.11.6

Page 33 of 41

All dip tubes should be installed with a tee for unplugging without disconnecting instrument piping and sufficient headroom should be provided above the vessel or equipment for removal of the dip tube.

5.11.7When local indication is required, a metal bellows type pressure gauge with a 0-100 scale should be used as the local indicator. The gas supply should use a rotameter to control the air purge flow rate. The rotameter should have a range of 0-1 ft3/min (450 cc/sec) and be equipped with an integral needle valve. Differential regulators should be used only in services that require precise measurement.

5.11.8The materials of construction must be compatible with the process and system conditions.

5.11.9A constant dP type flow regulator should be included on the instrument data sheet, unless the flow regulator is not needed, e.g., for local indication of non-critical level (see 5.11.7 above).

5.11.10 An air bubbler system should not be used in a power plant condensate or feedwater system. Great care is taken to eliminate any dissolved air from this "ultra-pure" water. The use of an

air bubbler level measuring system would defeat the purpose of maintaining oxygen free water.

5.11.11Although the air bubbler type level measurement has successfully been used for many years in countless applications, its use for new applications is discouraged, unless no alternative level measurement method is suitable or available. This is due to the many components and utility systems that must function correctly for the system to work.

5.11.12The size of dip tube should be such that pressure drop across it is negligible. The bottom end of the dip tube should be located far enough above the tank bottom so that sediment or sludge will not plug it. Also, its tip should be notched with a slot or "V" to ensure the formation of a uniform and continuous flow of small bubbles.

5.12 Nuclear

5.12.1This type of level measurement system utilizes the attenuation of beamed electromagnetic energy (radiation). Any variation of the energy received by a detector is indicative of a change in the process level. The beam is angled up (or down) through the liquid so that absorption is proportional to change in height of the fluid or solid

5.12.2

Page 34 of 41

This type of instrument is nonintrusive and is therefore not effected by the internal process conditions. Nuclear level instruments can operate in very harsh operating environments. Because it is looking at a change in absorption, initial absorption through thick or lined walls can be filtered out in the dry condition.

5.12.3The gamma ray emitting source is encapsulated in a stainless steel, lead filled enclosure installed external to the vessel. The source housing has a shutter that is opened and the gamma beam detected by a detector installed external to, and on the opposite side of, the vessel.

5.12.4The detector is a low voltage ionization chamber that is monitored by a remotely mounted amplifier/transmitter that contains all the required electronic circuitry and can also be fitted with a level indicator.

5.12.5The standard output available is 4-20 mA dc. Alarm actuation is available as an option.

5.12.6The size of the source is dependent upon the size of the vessel, the vessel material and wall thickness, and the process material.

5.12.7With a point source the output signal is non-linear, but with a strip source, extending down the side of a vessel, a linear control range can be obtained.

5.12.8For measurement of level in coke drums in a delayed coker unit, neutron backscatter point sensors have been the main devices used in the past 15 years. This type of measurement uses at least three point sensors located at critical level elevations to detect residuum and foam levels. The nuclear radiation source material and the sensor are located in the same housing for each point sensor, so a separate “detector” does not have to be mounted on the vessel. These sensors require specific mounting stud bolts to be welded to the exterior of the coke drums, and operator access must be provided to perform regular calibration checks.

This is gradually changing from the use of neutron backscatter to the gamma ray emitters and detectors. Although the gamma ray devices require much more substantial mounting hardware on the coke drum because of the very heavy weights of the units, the Cesium material has a much shorter half-life than the Americium-Beryllium (Am-Be) used in the neutron backscatter sensors. There are no nuclear remediations or waste recovery companies, which will accept the Am-Be material except the nuclear level gaugemanufacturers themselves, who re-use the same material in new sensors. Furthermore, the cost of the gamma ray devices is much less than the neutron backscatter sensors.

5.12.9

Page 35 of 41

Nuclear transmitters should only be used when required due to an extreme plugging problem or when vessel nozzles need to be minimized. Constant Specific Gravity is required for nuclear instruments. Application of nuclear instruments tends to depend on many factors and require special engineering in areas such as source placement, source types, etc. Access to the vessel may need to be restricted. Maintenance personnel require specilized training and certification. A license for the nuclear instrument is always required.

5.13 Thermal

5.13.1A level sensing switch is commercially available that operates on the principal that the thermal conductivity of air/vapor/gas is significantly different than that of water/liquid.

5.13.2The measuring sensor of the switch consists of three probes:

• One probe is a low powered heating element.

• One probe, located close to the heating element, consists of a platinum resistance temperature detector (RTD) and is called the "active" probe.

• One probe, located further away from the heating element, also consists of a RTD and is called the reference probe.

5.13.3The device is installed at the highest desired liquid level in the vessel. As long as the liquid level is below this level the sensor will be surrounded by air that has a relatively low thermal conductivity. When the liquid level increases above the level of the probes the switch will be activated.

5.13.4Level transmitters are available, that consist of several (up to eight) thermal conductivity switches, which generate a 4-20 mADC analog signal which is stepped proportionately up or down as the individual switches operate.

5.14 Fiber Optic Level Switches

5.14.1This device contains an LED that sends infrared light through the fiber optical cables to the sensing element. With no liquid present, most of the light continues on into a photodetector. When rising liquid immerses the optical sensing element, the intensity of the light that

reaches the photodetector is greatly diminished. This change actuates a switch. Caution is advised due to fiber optic cable length usually limited to 500 feet and cable pulling requirements

Page 36 of 41

5.14.2Fiber optic switches can be used with most liquids. However, their use with any liquid that will crystallize on the probe or leave behind a solid residue is not generally recommended.

5.15 Strain Gauge Weighing Systems

5.15.1When other devices do not provide accurate level measurement of the contents of a vessel, consideration should be given to the use of an electronic strain gauge weighing system. Strain gauge weight cells are mounted on the support structure of the vessel, and they measure the amount of deformation caused by the weight of the product inside.

5.15.2The advantage of this system is that no direct contact with the contents of the vessel is required.

5.15.3The disadvantages of this system are:

• The devices/installation are expensive.

• The true level depends on density (or mass) of the measured product.

• Strain gauges are temperature sensitive.

• Piping load must be isolated from the bin.

• The system must be calibrated so that the sensors ignore the weight of the bin.

5.16 Tilt Switches

5.16.1Tilt switches are mounted by suspending it over the material whose level needs to be measured using a cable, rope, chain, or wire. The normally closed circuit is opened when the base of the switch is tilted 15degrees or more from its vertical position in any direction, which occurs as the material level rises and deflects the switch. Tilt switches are widely used as dry material high level detectors in bins, vessels, silos and material handling system transfer chutes.

5.16.2In situations where the electrical wire may be damaged by falling material, the wire should be protected by pipe shield, wide enough as to allow the tilt switch to move freely to its alarm position.

Page 37 of 41

5.17 Rotating Paddle

5.17.1When a paddle is placed in a tank, its power requirement when rotating in air is low. Power consumption increases as the liquid level rises and starts to cover the paddle. Dry level switches are used to eliminate bin overflows problems. This type of instrument normally works on the use of a synchronous motor driving a paddle at slow speed. On material buildup the paddle stops stalling the motor that in turn actuates switches.

5.17.2When selecting mounting location, the proper location should allow the measured material toflow freely both to and away from the paddle and shaft. The mounting location should be determined by a Material Handling specialist in the Mechanical Engineering discipline due to the complexities of bin designs and the physics of solid material accumulation in the bins.

5.17.3Caution should be used when using this device to measure the level of solid material in bins or silos that have mass flow since the shearing action can damage this type of device.

5.18 Pressure Sensitive Diaphragm Type Level Switch

5.18.1Pressure sensitive diaphragm type level detection devices can be used to detect the presence or absence of bulk solids. These devices can be used to measure the level of solid material within a wide range of particle sizes, e.g. from powder to lump size, at atmospheric pressure. The units can be used at the top and bottom of blending bins, in screw conveyors, scale hoppers, etc.

5.18.2The main advantage of this device is that all parts, including the diaphragm can be serviced without removing the mounting flange.

5.19 Vibrating Wand Type Level Switch

5.19.1These switches are used for point level detection on liquids, slurries, powders and granules.

5.19.2The principle of operation is a vibrating paddle or wand which has its vibration amplitude dampened by contact with a liquid, slurry or solid. The change in signal amplitude is sensed and operates a relay output.

5.19.3The vibration mitigates prevents build-up on the wand or paddle.

Page 38 of 41

5.19.4This type of switch is very useful for detecting levels for cement, metallic ore concentrates, grains, plastic pellets, carbon granules, dust in dust collector bins, etc.

Care should be exercised when using this type of switch on pellets and granules etc. as the material must be of a size that will not wedge in the wand.

5.20 Radar Type Level Transmitters

5.20.1The principle of radar level measurement is based on the reflection of microwaves on surfaces with sufficient reflective index. The measurement is largely insensitive to disturbances within the vessel; e.g. steam, condensate, foam, dust etc. are passed without influence. 5.20.2Radar level devices must be used in enclosed metal tanks because of radio interference danger from external sources.

5.20.3There is no radiation hazard. The power concentration is a fraction of the leakage normally associated with a microwave oven.

5.20.4The price of radar level devices is about the same as ultrasonic level devices. Also, low power high frequecy devices are available eliminating the need for FCC licensing.

5.21 Guided Wave Radar Level Transmitters

5.21.1This type of level measurement system is based upon the technology of Time Domain Reflectometry (TDR). TDR uses pulses of electromagnetic energy, which are transmitted down a probe. When the pulse reaches the liquid surface that has a higher dielectric constant then the air/vapor in which it is traveling, the pulse is reflected back to its source. A high-speed timing circuit precisely measures the transit time and provides a measure of the liquid level.

5.21.2A single probe style is available for liquids with dielectric constants higher than 10. It can be supplied with a rigid probe in lengths up to 20 feet (6 meters) or a flexible probe can be provided for depths up to 50 feet (15.24 meters). For dielectrics as low as 2.0, a twin rod probe is available.

5.21.3

Page 39 of 41

This type of level measurement device is designed to provide reliable measurement to the very top of the vessel or chamber without the possibility of erroneous signals.

5.21.4This type of device is being marketed as an ideal replacement for torque tube transmitters, which makes it an extremely attractive alternative for feedwater heater or condenser hotwell level monitoring, either in a retrofit or new application. Or for other applications with similar dielectrics. Temperatures up to 750°F (400°C) and 2000 psi (138 bar) are available.

5.21.5This type of device is also available in a combined single chamber with a magnetic level indicator creating a redundant level measurement system.

5.21.6This instrument is a two wire loop-powered ‘Smart’ type, providing an isolated 4-20 mA signal while operating from a remote power supply of 11 to 36 Vdc.

5.22 Plugged Transfer Chute Switches

5.22.1A plugged chute switch is used to alarm (and interlock the drive) in the event that a transfer chute in a solids material handling system plugs. The type of switch used depends on the type of material handled and the geometry of the chute and transfer point. No one type of switch is “best” or works for all cases.

5.22.2For large material (+¾” to 6”) (19 mm to 150mm) nuclear type level switches have proved very successful although expensive. Conductance type switches and tilt type switches have worked in applications where the chute geometry prevents the switch from being buried after actuation.

5.22.3Capacitance switches with flat or curved plates are available for flush installation in rectangular or circular chutework.

5.23 Sludge Blanket Level Detectors

5.23.1Originally developed for water and waste treatment facilities these devices have been successfully used for thickener control in the mining industry.

5.23.2These instruments use various techniques for detecting the sludge/liquid interface in a clarifier, thickeners, etc. These units have the capability to automatically rise out of the way of rakes or plows as they pass the sensor.