18008849-control-valve.ppt

8/10/2019 18008849-control-valve.ppt

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Power systems.

Grounding systems.

Control valves

Interfaces


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Control Valves

IC3.7.1 Control valve basics.IC3.7.2 Principle of operation.IC3.7.3 Constructional features of control valve.IC3.7.4 Details of valve body types.IC3.7.5 Flow characteristics.IC3.7.6 MaterialsIC3.7.7 Sizing of ValvesIC3.7.8 ActuatorsIC3.7.9 Sizing of actuators.

IC3.7.10 PositionersIC3.7.11 BoostersIC3.7.12 Transducers


3/98

Control Valves Basics

Most commonly used final control element . Powered up by hydraulic, pneumatic & electric action. It is expected to modulate continuously in response to

signal to keep variable steady.

Functions of control valve-

To stop /allow flow in pipeline.

To act as safety device.(e.g.PRV,SRV) To prevent back flow of fluids.(e.g. NRV,check valve)


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Control Valve PrinciplesFlow rate in a process is expressed as volume per unit time.If mass flow rate is required then ,it can be calculated fromparticular fluid density.If fluid is delivered from pipe then volumetric flow rate is:

Q = velocity of fluid x area of pipe

A control valve regulates flow in fluid flow system. In generala close relation exists pressure along pipe & flow rate ,so thatif pressure is changed then flow rate also changes. A controlvalve places a constriction in delivery system so as to producea pressure drop.

Q = k x ( P)/sgk = Proportionality constant

P= Pressure difference between inlet & outlet.


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Body Design

Sliding stem valvesGlobe valve Single port globe valveDouble port globe valve

Angled globe valve 3-Way double port valve

Rotary shaft control valves

conventional butterfly valveButterfly valve lined butterfly valve

high performance butterfly valveconventional ball valve

Ball valve V notch ball valve

Eccentric plug ball valveSpecial control valvesHigh pressure control valves.High temperature control valves.Small flow control valve.Large flow control valve.Cryogenic service control valve.


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Globe valves

VIEW DRAWING

FEATURESTight shut off capability,Leakage less than 0.05% for single port & upto

0.01% for double port valve of rated Cv.Most common,simple .

APPLICATION: Throttling purposes,general purpose valve ,applicable over widetemperature range.

TYPES : Single seat valve.Double seat valve.Three way valve for mixing/.diverting service .

ADVANTAGES :Is fast to open or close.Throttling to control any desired degree of flow.Has positive shut off.

DISADVANTAGES :These are quite heavy as compared to butterfly & ball valves.Difficult to manufacture in small sizes.


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Globe Valve Schematics

Singleport top guided

Single port split valve

Double ported globe valve

Figure 1


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Single seat globe valveSingle seated globe valve are usually top guided ,such top guidingminimizes the effect of the weight effect on valve & increases

resistance to trim vibration.Most single seated valves are of unbalanced design tough balanceddesign are made.They are used where tight shut-off is needed .Theiruse in small body sizes ,2 & below,where tight shut off is needed.theiruse in small body sizes is due to simple design & because ,for low flow

unbalanced forces are not enough large to require large actuator sizes.

Figure 1 Single port top guided

AdvantagesHigh rangeabilityProvides tight shut offReversible plug available

Used under application less than 2.

DisadvantagesRequires large actuator since it is unbalancedLow pressure recovery characteristics.


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Double seated globe valve

These are generally top & bottom guided .here practical leakagefigure approaches 0.5% of the rated Cv ,because it is nearly impossibleto close both ports simultaneously, particularly when thermal expansion.With the top & bottom guided construction , the stroke direction can bereversed by simply inverting valve body.These are popular because oftheir balanced design. The forces tending to close the valve are onlyavailable with reverse plugs -constructed so that increasing loading

pressure moves the plugs into or out of the port .Advantages High flow capacity compared to single port valves.High rangeability.Balanced design require small actuator.

Used for sizes greater than 2. Disadvantages High leakage rate.Low pressure recovery characteristics.Likely to erode by high pressure drop. Not good for high flow,low pressure drop application

Figure 2


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3-Way double ported valve

These 3 - way valve are extension of typical double ported globe valve .theseare available in two designs:A) Diverting service B) Mixing serviceA)Diverting service valve :

It has a modified double ported body ,with lower plug seat opposite tothe normal shut off position.either direct or reverse acting actuator is useddepending on fail safe condition. The body has a internal bridge to separatethe right hand & lower outlet.A diverting valve may be used for heat exchanger

bypass where heating medium enters port C.Part of the fluid leaves port U to bypass the exchanger.the remaining portion of the fluid goes to the heatexchanger through port L,to heat an independent process stream & then rejoins the bypass fluid stream from port U.

B)Mixing service :Figure 3 illustrates 3-way valve used for combining service. Here two

separate fluids enter through L & U;respectively ,are combined in adesired ratio & leave through common port C. The ratio control is achieved by proper plug position. An upward movement of the plug decreases the flow passing through port U& at the same time increasing flow through L.The plugs are placed back to back in order for the flow direction will remain underits plug.


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3-Way double ported valve

Diverting service Mixing service

Figure 3


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Globe valve design

Trim designTrim is the heart of control valve. Its

primary function is to proportion thevalve orifice area in such a manner that

prescribed relationship exists betweenflow capacity & plug lift.A secondaryfunction is to ensure tight shut off.Avalve trim consists of removableinternal parts namely plug,seats,stem,stem

plug guide ,bushing & cages.stuffing box components considered as trim arethe packing follower, spring , packingretaining ring.secondary trim parts arestem to plug attachments, seat retainingrings ,seat to body seals ,spacers etc.

Packing

Plug

Stem

Seat ring

bonnet

Figure 4


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Factor affecting trim design Temperature Pressure Flashing Fluids

Cavitation Fluid Viscosity Solid Contents in Fluid

Each valve is given flow characteristic: i.e. for a given percentage of total plugoff the seat ,flow is given percent of the full open flow. This is true only when

pressure drop across valve is constant.this is known inherent flow characteristicsthe installed flow characteristics may differ considerably at much higher pressur drop .Selection of valve trim is based upon1) Knowledge of service pressure & flow conditions.2) Manufacturers inherent flow characteristics for different trim shapes.

3) How inherent flow characteristics is altered by varying flow.Maximum flow capacity depends upon seat port sizing, body design ,&plug lift& type .Shutoff & opening capability are determined by diameter of the mean seat to

plug seal contact ,pressure differential & available actuator force,provided thestem design is adequate & allowance is made for seating force &packing frictio


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Trim design for Maximum flow

Quick opening plug flow area

Maximum flow capacity in globe valve design is achieved by using the largest possible flow orifice combined with a quick opening plug form.The orificeshould not be so large that valve body becomes a part of the total valve pressuredrop & changes flow characteristics.discharge flow area is equal to the orificearea at lifts of 1/4-seat diameter.Turning fluid past the plug nose causes flowresistance & an additional 10 - 15% lift is required to obtain maximum flow .Thus total diameter is about 35% of orifice diameter to clear the plug out of theflow capacity than globe type with equal seat orifice diameter.

Linear & equal percentage

Linear & equal percentage characteristics requires a plug plug nose extending

into the seat orifice and this must also be withdrawn to get full flow.In case ofcontoured & v-port shape aids in turning flow gradually & lifts for maximumare similar or slightly greater than for quick opening plugs ; usually ,about 45%of orifice diameter for top guided plugs.


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Factors affecting maximum flow

The break point in a flow curve ,where increasing lift of plug of the seat orificedoes not increase flow ,is due to ;

1) Plug is completely lifted out & orifice is primary flow restriction;

2) Body flow resistance is becoming a appreciable part of the total valveresistance

The break point in the curve , where increasing pressure drop does not increaseflow my be due to;1) critical flow at sonic velocity after pressure drop becomes one half of

upstream for gas service.2) flashing flow chokes the body downstream of the seat joint with the vapor

bubbles in liquid service . In cavitation no further reduction in pressure can be obtained at vena contracta to increase flow.

Lift LiftEffect of orifice & body on

flow characteristics

Flashing effect

flow stagnation

Figure 5


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Selection of Trim Material

Property Consideration

Physical PropertiesTensile , Compressive , Shear ,Yield , Ductility, Hardness, Density.

Thermal Properties Creep,rupture ,Scaling(oxidation ),Hot Hardness, Cold Impact.

Environment Erosion Resistance.

Corrosion Chemical Resistance ,Electrolytic Potential With Body.

Fabrication Castability,machinability,finishing Surfaces, Method for Hard Facing.


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Application of Common Trim Materials

Application Materials

Mild Service Bronze

General Service Type 316 SS

Severe Service 17-4 PH SSType 410 SSStellite , Colmonoy

Very Erosive Service Type 440-c SSHardened Tool SteelTungsten Carbide

Very Corrosive Service NickelMonelInconelHastelloy A/BDurimetTitanium


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MATERIAL GENERAL USE GLOBE BUTTERFLY BALL PLUG DIAPHRAGM

STANDARD METALS GENERAL SERVICE A A UPTO 12" A ACAST IRON MILD CORROSION A A UPTO 12" A ADUCTILE IRON TO 650 F A A UPTO 12" A A

TO 1000 PSI A A UPTO 12" A ACOST REDUCTION A A UPTO 12" A A

BRONZE(AL BRONZE) MILD SERVICE A VANES 12" TO 4" TO 2" ACRYOGENIC SERVICE A VANES 12" TO 4" TO 2" AOXYGEN A VANES 12" TO 4" TO 2" A

ALUMINIUM LIGHT WEIGHT - - TO 4" TO 2" ACRYOGENIC SERVICE - - TO 4" TO 2" A

STEEL GENERAL USE A A A A A ALLOY STEEL HIGH TEMPERATURE A - - - -

600F-1050F A - - - -STAINLESS STEEL CRYOGENIC SERVICE A TO 12" TO 16" TO 12" A1050-1200F A TO 12" TO 16" TO 12" ACORROSIVE SERVICE A TO 12" TO 16" TO 12" A

LINING ELASTOMER EROSIVE - A - A ATIGHT SHUT OFF - A - A A

PLASTIC CORROSIVE - - - A ATFE SEVERELY CORROSIVE 1-2" 4-12" 2-8" 2-12" A

GLASS - - - - A

INCONEL CHEMICAL SERVICE 1-4" - - - -MONEL CHEMICAL SERVICE 1-4" - - - -HSATELLOY CHEMICAL SERVICE 1-4" - - - -

ALLOY 20 CHEMICAL SERVICE 1-4" - - - -TITANIUM CHEMICAL SERVICE 1-4" - - - -GLASS CHEMICAL SERVICE - - TO 4" - -

AVAILABLE BODY MATERIALS

AVAILABLE LINING MATERIAL

CORROSION RESISTANT BODY ALLLOYS

A =AVAILABLE IN ALL SIZES


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High temperature trim design

The material properties considered for high temperature application aretensile ,yield ,creep & rupture .Other factors are scaling ,galling.The design considerations include ;clearances of moving parts & fitted

parts as related to differential rate of thermal expansion of their respective parts .The yield,creep,compressive strength are lowered by high temperature .Hot hardness is necessary to prevent galling & damage of seat.

The following design changes are considered for corresponding temperatures

Above 450F - The bonnet extension requires a longer stem to keep the packing cool.

Above 600 F - Clearances must be increased .The plug & seat sealing areasmust be hard faced .

Above 750 F - All threaded seat ring must be seal welded to prevent looseningwhich will cause leakage & undercutting.

Above 900F - All guided bushing ,plug guides & posts must be hard facedtack welded .

Above 1050 F- hard faced ,integral seat joints must be used .


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Low temperature trim design The trim design for cold & cryogenic services is based on followingrequirements1) Bubble tight sealing at low temperature .

2) Minimum heat leak.3) Minimum cool down mass.4) Quick change design.5) Simple design.6) Differential thermal contraction of materialsBubble tight design ,is a must for in cryogenic service ,is obtained by using

TFE or KEL-F on the plug seal.both materials may also be used for the guide bushing & seat joint gaskets.The stems are passed through extended bonnets to prevent freezing ofatmosphere moisture.Hollow plug are used to prevent rapid heat transfer ,yet allow a large diameter

plug plug for guiding & throttling stability.The void in the plug may be

evacuated or filled with insulating material to reduce radiant or convectiveheat transfer.Valves are either installed vertically or at 45 from vertical to maintain lowconvective vapor lock in extended bonnet .The weight of parts should be kept minimum to reduce cool down of mass &

boil-off loss of the liquefied gases entering the piping system.the plugs arehollow & seats are integral with body.


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Low temperature trim design(continued) The stuffing boxes are kept deep enough to accommodate a second or partialset of packing ,installed back to back ,to prevent leakage ion cool down if avacuum should occur in line.

Figure 6


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Low temperature trim design(continued) Summary

Low Temperature Ranges Trim design requirements Cold valve service (0 to -50 F) - Extension bonnet with long stem,addition

of ethylene glycol in packing follower .Cold valve service(-50 to -150 F)- Extension bonnet with long stem to prevent freezing .

Refrigeration service -The guide bushing may be tack welded to prevent loosening from the differentialthermal contraction of bushing & bonnet.

Seat ring ,of screwed design are seal welde to prevent loosening & leakage.Cryogenic service(-150 to460F) - a double extension bonnet with length 12

for 320F nitrogen service,& longer for-450 F hydrogen service.

Liquefied gases service - The guide bushing may be either welded or

(-259 to -450F) the operating clearances for moving partsmay be increased by 50% to prevent bindin

from differential thermal contraction.Hollow plug extension is used .seatring areof screwed design & are seal welded to

prevent loosening & leakage.

l d


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Plug design

Plug is moving component of the valve which throttles the flow by positioningitself within the seat orifice & shuts off flow by contacting the seat. Plug ismoved against the dynamic fluid forces by stem force transmitted from the

actuator.throttling may be done by V-port plug or contoured plug.The plugs are shaped to throttle the flow with a given characteristics such asquick opening,linear , equal percentage .

Figure 7


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Low Flow Plug Designs

Figure 8


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Regular Flow Plug Designs

Figure 9


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Regular Flow Plug Designs

Figure 10


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Designing a plug to a selected flow characteristics

Theoretically Cv of a perfect orifice having one square inch are is 38.1

this means that an orifice with a efficiency of 100% would pass 38.1gpm with pressure of 1 psi. No orifice has such efficiency ,& additionof valve plug further reduces the efficiency.When designing a flow characteristic for a plug, designer must attaina specific flow ,corresponding to specific lift . To do so , we need toknow the efficiency of the annular seat plug orifice area .The efficiencyvaries with the flow rate & the length of the restricted flow path .Also,the body -flow resistance is a factor accounted for high lifts & eachtype of body has different affect on resistance.One method of plug design to make a preliminary flow test using arough shaped plug is designed to determined the flow efficiency curvefrom Cv minimum to Cv maximum for that body. The plug is designedto given flow characteristics using this data ,then it is flow tested & ifnecessary reshaped slightly to follow flow curve.


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Valve stems The stem carries the load from the actuator to plug, so in effect it is columnloaded at both ends ,it will therefor bow if overloaded . The designers choiceof stem diameter is a compromise between stiffness to prevent bowing, whichcauses striction & results in shortened packing life ,& faster bearing Vs theeffect of line pressure thrusting on the cross-sectional area of the stem,isdirectly related to amount of packing friction.Usually friction prevails thelater consideration are counteracted by additional actuator force requirement.Some close coupled ,oil field designs have short stems which run very true &increase packing life .Cage guided , balanced trim reduces stem thrust sufficiently in the the high

pressure service to give a noticeable increase in packing life, because ofreduction of even minor bowing & vibration.the stem size is reduced by usingsuch trim.

Attachment to plug: stems are usually threaded & then pinned to prevent loosening .The stem may

bottom in imperfect thread run out in plug to make a rigid connection & the pin is usually an upset design to anchor tightly.this design present amaintenance problem in replacing the plug .Other designs use an elastic nut to hold the plug against a shoulder on thestem . Some design are pinned with spring pins allowing repeated replacement.The stem shoulder take care of the load ,& satisfactory life may be obtained by

monel pins stainless pins may break from stress corrosion .Pins are seal weldefor high & low temperature application


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VIEW DRAWING

Angle Globe Valve

FEATURESSingle seated valves with special body configuration to suit specific piping

& flow requirements.specifically designed for coking hydrocarbons,posses astreamlined passage to prevent accumulation of solids on body wall.

APPLICATION Hydrocarbon application ,low pressure application, may be used for abrasivecatalyst application.,self draining design of radioactive materials,Used where

turbulence ,cavitation effects are to be minimized .

ADVANTAGESHydrocarbon application.Can handle erosive material.Handles abrasive catalyst

Used where self draining is required.High rangeability,high temperature rating.

DISADVANTAGES Cannot be used for high noise application .Avoided for throttling application.


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Angled Globe Valve Schematic

Figure 11


31/98

VIEW DRAWING

Ball Valve

FEATURESOldest of all valves,applied for wide range of application .

APPLICATIONPressure control , flow control & shut off application .Can be used for corrosive fluids,cryogenic fluids.Used for high temp application, LPG application.

ADVANTAGESLow pressure dropTight shut off.Quarter turn application.

Small in size & lighter in weight.

DISADVANTAGES Cannot be used for throttling application.Avoided in quick opening application since it cause water hammer.Fluid trapped in ball may cause corrosion.


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Ball Valve Schematic

Figure 12


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VIEW DRAWING

Conventional Butterfly Valve

FEATURES

Described as damper/throttle valve.Operation carried out by pneumatic,hydraulic,electric,manual operation.

APPLICATIONLow Pressure application where leakage is relatively unimportant.

ADVANTAGESSimple .Compact & quick opening .Good controllability.Low pressure drop.Low weights & low cost.

DISADVANTAGES Seals may be damaged if velocity is usedRequire high actuating force.Limited to low pressure application.Elastomers limits temperature.


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Butterfly Valve Schematic`

Figure 13

Li d B fl V l


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2) Liner Anchored to Body This type of valve is anchored by a metal or rubber retaining ring that

projects into groove in the valve body.

Elastomer liner design (Bubble Tight Closure )Six liner design types are utilized1) Liner Bonded to Body :

This type of liner is not replaceable in the field.It is generally used in smallsize valves & on some large valves only when application is critical.

Lined Butterfly Valve

Figure 14


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3) Liner Wrapped Around Faces The liner is wrapped around the flange faces of body in order to anchor it in

place & to from a gasket.controlled compression is obtained by extension ofrubber liner past the body flange face. The grooves allow displacement ofrubber liner on makeup of the valve in the piping without causing distortion& disc binding.the design securely anchors the liner , only being surpassed bythe bonded type.4) Push in Liner

This type of liner is made from an elastomer ring that is bonded to a metal

insert. This type of construction will provide stiffness for vacuum service &will prevent extrusion by differential pressure across the disc .the liner isreadily replaceable in field .

Liner Wrapped Around Faces` Push in Liner

Figure 15

Li d B fl V l ( i d)


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Lined Butterfly Valve (continued)5) C lamped liner :

for this type of liner a split clamp holds both liner & pipe ends together .Seattightness may be adjusted by varying the split clamp bolting torque. Piping

must be moved aside to install & remove the liner . This type of liner is used infood industries special pipe hubs are welded to the pipeline which are serratedto fit raised concentric ends of the thicker liner ,thus giving positive axialalignment of valve & piping.thicker lining allows deep penetration of the vane.A lower torque is needed to seat the valve than with conventional liners.

Another type of design includes involves clamping a thin section liner bymeans of an extension flange .The relief on the outside of the liner allows forswelling .This design is operating satisfactorily with 3.55 slurry with 85 psishutoff pressure differential at 70F.the liner may be replaced withoutremoving the stem & disc.

6 ) Special liners The offset stem design of this valve allows two seating surfaces which are

interchangeable when the valve stem is centered on pipe axis. To change seatingsurface,simply disconnect the disc from actuator & rotate it 180


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O ring Stem Seal Liner

Clamped liner: Figure 16

Figure 17

Hi h P f B fl V l


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High Performance Butterfly Valve

The high performance butterfly valve are valves with double eccentric seatinggeometry .these valves are extremely versatile as they could be used to handle

fluids for -45C to 450C .using variety of sealing system varying fromsynthetic rubber-to-Teflon to metal-to-metal seating . By virtue of its doubleeccentricity the valve disc allows friction free contact between the disc &seat ring & as soon as the valve opens the disc moves away from seat ringwithout any further frictional contact with the seat. This enhances the life of theseal & reduces the valve torque considerably.new design with fish-tail disc

profile have been developed to especially encounter high pressure dropapplication. The fish tail disc changes the pressure distribution curve acrossit compared to conventional disc & move the resultant force closer to the valvethereby reducing the dynamic torque. This design is hybrid between ball valve& butterfly valve. The fish tail design valves are specifically used for throttlingapplication through 90 of rotation ,unlike conventional valves which arerestricted to 60 opening . This results in greater capacity per valve size.

Hi h P f B tt fl V l


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High Performance Butterfly Valve

Figure 18


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VIEW DRAWING

Rotary Eccentric Plug Valve

FEATURES

Rotates through 50 angle with full open position & cams into the seat. Plug is free to rotate axially along its shaft to align with seat. Available from 1- 24 in size. Applicable for temperature from -320 to 750 C. Provides tight shut off.

Requires low actuating force. Plugs are made up of hard stellite material Because of seals temperature & pressure ranges are restricted. Have high flow capacities & low pressure drops. The disc valve seals are highly reliable. High pressure recovery makes valve susceptible for cavation

More expensive than butterfly valve.


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Rotary Eccentric Plug valveSchematic

Figure 19

High pressure control valves


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High pressure control valves Modern process employ working pressure above 6000 psig (414bar).Pressure upto50000 psig are not unusual.at these high pressure the techniques & methods used toseal valve bodies are very important.Usually the valve is of two piece angle designwith seat ring clamped between the two body halves .the seat ring to body seal is made

with retained, metallic ,hollow Oring .As pressure builds O -ring tends to allow line pressure to enter the hollow correction of the O-ring .As pressure builds Oring tendsto inflate creating a tight seal between body seat & seat ring.the exterior surface of theseat is plated with soft silver to assist in forming the seal.Stem packing material: Valve stem packing tolerances become more critical for high pressure units.At high

pressure elastomer packing can be extruded through very small clearances.Packingcompound is usually TFE compound impregnated with glass to make it more resistantto extrusion.Stem material: The stem of these control valves are also made of high strength material suchas 4140 steel.The stem is short ,well guided & plated with chrome to prevent galling byhigh pressure .Valve bodies Forged diecast bodies are used as high pressure bodies . The material is usually heattreated type 4340 steel for pressure upto 50000 psig, & annealed steel SS316 for

pressures upto 10,000 psig. Forging process provides bodies free of voids & can be heattreated to high strengthValve plug : The valve plug tip is made up of furnished tungsten carbide for resistance against corrosio

& abrasion. Actuation done by piston & diaphragm actuators.


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High pressure control valves

High pressure low noise angle valve

Figure 20


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High temperature control valve

Control valves for service above 450F must be designed & specified with temperatureconditions in mind . At elevated temperatures standard materials may be inadequate

plastics,elastomers & standard gaskets are insuitable & are to be replaced by moredurable ones .metal to metal seating materials are always used.

Packing material :Semi metallic or laminated graphite packing spiral wound SS and asbestos gaskets areused.

Valve body material:Chrome -Moly steels are used for temperatures above 1000F.ASTM 4217 grade WC9 is used where there are chances of oxidation & scaling.ASTM 4217 grade C5 used for shortcoming above 1100F.ASTM A351 grade CF 8M is applied for temperature 1500F.

Trim materials

Chromeplated SS316Cobalt based alloy 6High vanadiumHigh chromium steels for additional resistance to high temperatures.

Small flow control valves


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Small flow control valves Applied in small pilot laboratories ,pilot plant ,commercial process plants .These employ special trims for extremely small flow rates is necessary .Thespecial trim parts used are normally only two parts - a reduced port seat ring

& a valve plug with a tapered flat milled on one side . These parts are machinedto very low tolerances and are usually made of a hardened stainless steel or har faced with alloy 6 to minimize erosion.Using a 3/16 inch diameter port .

A

Angle flat A determines control of small flow rates

Seat ring

Figure 21


47/98


Low flow valves are those with Cv less than 0.05.There are many applicationcalling for small ,accurate control .In low flow characteristics the selectioncriteria is quite different since there is absolutely negligible no frictional lossin valve & there is no unfavorable pump head characteristics . There is noneed of equal percentage characteristics.From design stand point ,physical

physical dimension of the required are very small indeed ,the problem

incurred is high pressure drop with erosion problems .For such low flow ratesa actuator with a short stroke is required .The actuator has a smooth throttlingaction for high pressure service .For less exacting services needles or pistons

with milled notches or slots are generally used in orifices ranging from 1/8 to 1/4 .The valve stroke is of order of 1/2 .The Cv required is calculated byconventional formulas & then next largest size trim is chosen. If the trim is too

small then a capacity increase is possible simply by increase in valve stroke orslightly modifying valve plug.



48/98


Compact valve suitable for pilot plant

Low flow valve with variable stroke

Figure 22

Large flow control valves


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Large flow control valves

The globe style valve larger than 12 ,ball valves larger than 24 ,eccentric plug valve larger than 24 ,butterfly valve 72 fall in special valve categoryas valve size increases arithmetically the shutoff pressure increasesgeometrically . Consequently then shaft strength ,bearing loads unbalance

forces & actuator forces become of greater significance. Normally maximumallowable pressure drop is reduced on larger valves to keep design & actuatorrequirement within limits even with lower pressure of the flow requirementsare awesome ,so naturally the actuator requirements are severe & long stroke ,

double acting pneumatic piston or electric actuators are specified for large flowapplication. Installation & maintenance procedures are complicated .For these type of valves the noise levels are carefully considered since noiselevel increases indirect proportion with flow volumes .For keeping noiseunder control the valve bodies cage type ,with usually long valve plug travelwith large number of flow opening.The fabricated valve body is designed forfor high pressure .Over protection equipment must be included in downstream system to ensurethat the body shell & outlet connection are not subjected to pressure in excessof rated capability.

Cryogenic & cold service control valves


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y g

When control valves are applied to operate at temperature below freezing point , special precaution are taken .In certain cases special designs arerequired .the principle problem is selection of proper materials of construction ,

particularly on moderately cold services (to -150 F). Cryogenics is the termapplied for process operating in temperature range below (-150F).In temperature range (-20 to 150F ) special impact resistant carbon steels areused for pressure containing parts,the commonly used parts are of carbon steel(grade LCB).In temperature range (-20 to 50 F)3.5% nickel steel (grade LC3) is used .

Valves are generally equipped with plain extension bonnet .In these temperaturerange the primary objective is to reduce influx of heat system to reasonably lowvalue ,& to prevent packing box .The simple extension box is installed inupright position to minimize heat transfer in operating fluid .At cryogenic temperatures , material of construction now exclude carbon steel &include austentic stainless steel .bronze, monel.A special attention is paid to design bonnet.Care is taken that no liquefied gasis trapped in bonnet space , where vaporization could generate dangerously high

pressures.in all cases the valve is completely insulated the process piping &valves in cold section are often installed in cold box.the valve in this case may have exceptionally long ,plain extension bonnet .

Cryogenic & cold service control valves (continued)


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y g ( )

cold box valve plug & seat arrangement

A large diameter bonnet selection is fabricated from stainless steel & broughtout through the cold box.This permits removal of trim,with seat ring & plug,without disturbing valve body.

Figure 23



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y g ( )

The figure below illustrates special cryogenic valve that has vacuum insulating jacket enclosing the entire body & extension body .SS expansion bellows are provided on jacket to eliminate piping strain caused by differential expansion.

Radiation losses are minimized by electropolished exterior & aluminium foil barrier.In this type of valve weight is kept minimum .

Vacuum jacketed cryogenic valve for service on liquid He,hydrogen etc

Figure 24



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y g ( )

Rotary valve ,also are equipped with a extension bonnet , are often specifiedfor cryogenic service where tight shut off is required .figure below showsspecial design ,with an offset vane & spherical seating arrangement.

Offset vane cryogenic butterfly valve with elastomer lip seal for tight shut off

Figure 25

Controlvalvef lowcharacter istics:


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Control valve f low character istics:

The valve plugs for control valves described in earlier chapters are available in avariety of geometric shapes, each with a characteristic relationship betweenfractional valve lift & the relative flow through the valve. This relationship is

denoted as the flow characteristics.The proper selection of the flow characteristicsis an important part of control valve application.If the valve gain or sensitivity is defined as Kc= Change in flow/Change in lift,then it is evident that slope of flow characteristic curve is the valve gain.(Ref.Fig.1)

In a control system valve gain (Kc) & process gain (Kp) are singular variableswhich must compensate each other if control loop has to remain stable. The basicconcept of choosing a characteristics lies in gain matching of the valve & the

process.

The quick opening characteristics provides large changes in flow for smallchanges in lift, therefore it usually has too high a valve gain for use in modulating

control.

It is limited to on-off service such as sequential operation in either batch or semi-continues processes.The bulk of control applications use valves with linear, equal %,or modified flowcharacteristics. The modified characteristics generally fall between the linear &equal% characteristics shown in fig1.


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The linear characteristics provides a change in flow which is linear with valve lift &thusWith signal to the valve. The linear characteristics would seem intuitively to be mostdesirable characteristic for control, since it provides constant valve gain throughout thestroke at constant pressure drop. However inclusion of the valve into a system ,withassociated piping equipment, & control loop, leads to considerations which generallymake equal % the most widely applied characteristic. The equal % characteristics

produces a change in flow, with change in lift, that is a constant % of the flow before thechange was made.

Putting in simple terms,Assign equal % characteristics to a control valve if, 1)Process is fast2)High rangeability is desired3)When system dynamics are not well known.4)Control valve is required in an application like heat exchangers where an increase in

product

rate requires much greater increase in heating /cooling medium.5)The major portion of control system pressure drop is not available through the controlvalve.

Assign Linear valve characteristic to a control valve if,1)Process is slow2)Where more than 40 % of the system pressure drop occurs across the valve3)When major process changes are a result of load changes


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Flow characteristics

Figure 25


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Control valve leakage classification


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FLUIDS C S C I SS BRONZMON HASTE- HASTEL- DURIMET TITAN-Cobase S304/302 EL LLOY A LLOY C 20 ium Al 416 4

SWEET NATURAL GAS & OIL A A A A A A A A A ASOUR NATURAL GAS & OILCARBONDIOXIDE ,WET C C A A B A A A A A ACARBONDIOXIDE , DRY A A A A A A A A A A APRODUCED WATER A A A A A A A A A A BSEA WATER B B B B A A A A A A C CSODIUM HYPO CHLORIDE C C C C B-C B-C C A B A IWELL STREAMS & SANDHIGH LIQUID PRESSURE

A : RECOMMENDED , B: MINOR - MODERATE EFFECT , C : UNSATISFACTORY , I L :

MATERIAL COMPATIBILITY DATA SHEET

Material Compatibility Data

Control valve sizing:


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Control valve sizing: Factors to be considered

Flow application data:

Maximum & minimum flow rates,upstream & downstream pressure.Stream temperature.Fluid data :

Name & properties of fluid, Phase of fluid ,density of fluid ,viscosity offluid,vapor pressure.

Piping influences: Presence of reducers or other disturbances at valve .System influences : Control dynamics,Economic factors .Safety.Style of valve: Capacity ,RangeabilityCorrosion or erosion.Special requirements.Sizing:Manufacturers sizing coefficient.

Si i b i


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Sizing basics

Sizing employs principle of conservation of energy. Daniel Bernoullidiscovered that as the liquid flows through the orifice ,the square if fluidvelocity is directly proportional to pressure difference across the orifice& inversely proportional to specific gravity of the fluid ,therefore greater the

pressure differential pressure greater the velocity.greater the density lowerthe velocity.logically the liquid flow rate is calculated by multiplying thefluid velocity by area of flow .There exists energy losses due to friction &turbulence .

Now the basic liquid sizing equation can be written as follows:

Q = Cv ( P/G) whereQ = capacity of gallons per minute.Cv = valve sizing coefficient .

P=pressure differential in psi. G =specific gravity of fluid.Cv is equal to number of US gallons of water flowing at 60F through thevalve in one minute when the pressure difference of one pound per squareinch. Cv provides both style & size ,also provides an index for comparing

liquid capacities of valves under standard test of condition.


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f ff f


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We will have to consider two important factors, which affect calculations of Cvfor liquids

1) Piping geometry factor (Fp): Ideally we had considered same size of piping asthat of a control valve, but in practice there are always reducers/expandersupstream/downstream of control valves & you have to correct for this changefrom ideal condition.

2) Viscosity factor (F ): When flow is turbulent there is no problem & correctionfactor is not required. But the moment the viscosity becomes low & flow startsgetting laminar, we will have to apply correction to the Cv using viscositycorrection factor.

The calculation sheet enclosed gives details of these factors & also elaboratesmethods of calculations for these factors & their use in calculating corrected Cv.

Above we have seen the fundamentals of control valve sizing for liquids, whichare non-cavitating & non-flashing.We will now turn our attention to two important phenomena, namely Cavitation &Flashing. These phenomena are of significant interest in any comprehensivediscussion of control valves since their occurrence will affect the valve sizing

procedures, may introduce noise & vibration & also may limit the life expectancyof the valve components & immediate downstream piping.

Whatiscavi tation:


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What is cavi tation: Cavitation is a two-stage phenomenon, the first step of which is the formation of voidsor cavities within the liquid system. The second stage is the collapse or implosion ofthese cavities back into an all-liquid state.H ow cavitation takes place: For cavitation to take place, requirement in the form of nucleating agents is mandatory.These tiny nuclei which which will contain either dissolved gases or vapors willenlarge into finite cavities within the liquid.In short when pressure of liquid at the outlet of control valve goes below critical

pressure the nuclei discussed above tend to form cavities & when it is recovered back,these cavities try to implode back into liquid & temporary gaseous phase is eliminated.

What is F lashing: Flashing is similar to cavitation, only difference being that in cavitation the pressurerecovery is full but in flashing outlet pressure remains below critical pressure of thefluid.Fig.1 shows the process of cavitation & flashing graphically.In short if cavitation has to take place following criteria to be fulfilled,1) The fluid at both inlet & outlet to be in an all liquid state 2) The liquid must be in subcooled state at the inlet .3) The valve outlet pressure must be either at or above the vapor pressur e of theliquid. If flashing has to take place, following criteria to be fulfilled,1)The fluid at inlet must be in all liquid condition , whi le some vapor must be presentat the valve outlet. 2)The fluid at the inlet may be in either a satur ated or a subcooled conditi on 3)The valve outlet pressure must be either at or below the vapor pressur e of the liquid

Cavitation evidences :


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1) Noise: In a control valve the evidence of cavitation is usually a hissing sound .Ascavitation intensity increases due to increasing pressure differentials, the sound levelalso increases.2)Vibration: more noise due to cavitation, more vibrations emanating from control

valve.3) Material damage: due to cavitation, there is serious damage to the valve internals

Cavitation contr ol:

1)Generally control valves with high recovery of pressure drop are more prone tocavitation than low recovery valves. Hence globe valves are less prone to cavitationthan butterfly/ ball valves.2) Use hard trim to avoid material damage to the control valve trim. Stelliting of trim isa standard procedure to delay effects of cavitation.3) Pressure balancing of trim is to be done to improve throttling stability.4) 90 degree bends in flow path create a series of velocity head losses reducing pressuregradually.

5) Pressure drop may be divided across a series of orifices.6) A combination of 4) & 5) above having multiple small differential pressures ratherthan one larger differential pressure to keep the liquid above its vapor pressure so thatcavitation does not occur.7) Flow may be jetted against flow & swirled to create a massive turbulence & internalfriction to dissipate the energy as heat.

Cavitation & flashing phenomenon the control valve sizing also gets affected & the


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g p g gdeviations from standard formula for control valve sizing for liquids is discussed

below,Cavitation & Flashing both produce a decrease in ability of the valve to convert pressure drop across it into a mass flow rate. Referring to basic equation for liquidsizing, it can be observed that the flow rate is proportional to the square root of

pressure drop & that the constant of this proportionality is liquid flow coefficient Cv.It has been observed that if pressure drops of above 5~10 psi are considered whilecarrying out the valve capacity experiment, then it is observed that above 5~10 psirange, the flow of fluid through control valve tends to decrease rather than followingstandard flow & square rooted P relationship. This indicates incipient cavitation ofthe main flow stream.

Cavitation index:

A dimensionless ratio, experimentally determined from plot of q versus squareroot ofP at fixed values of inlet pressure & valve opening is used to describe the point of

initial departure from a proportional relationship..This ratio is called cavitation index& is given as below,

Kc= P1-P2/ P1-Pv = P/P1-PvAfter cavitation has has begun, further decrease in in valve outlet pressure (increased pressure drops) results in increased vaporization, increased cavitation intensity &further decreases in the apparent liquid flow coefficient. It is observed that withsufficient pressure drop the flow becomes FULLY CHOKED., so that increasing

pressure drop results in no increase in flow rate. Increasing the pressure drop afterchoked flow has been reached will result in increased amounts of cavitation damageuntil the valve outlet pressure is decreased to to the value that will permit flashing.


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Valve recovery coefficient:

An additional experimental coefficient is determined by flow test to approximatethe point above which no increase in flow rate is achieved for an increase in

pressure drop. This coefficient is called valve recovery coefficient

Km= Pm /P 1-Pvc

From the definition, it is clear that Km represents the fraction of the difference between inlet pressure & choked flow vena contracta pressure that may be taken

as pressure drop across the valve, before choked flow occurs.

The procedure for sizing for choked flow condition is given in accompanying procedure for Cv calculations.

Si ing for t rb lent & nonca itating liq ids


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Sizing for turbulent & noncavitating liquids

Formulae to be used:

Q = N 1FpCv ( P/G f )

Q= volumetric flowrate N1=1.00 (US) =0.00865(SI)Fp= piping correction factor Cv=control valve coefficientP= differential pressure

G f =specific gravity

Wf = N6 F LP Cv ( P )

N6 =63.3 (US)=2.73(SI)FLP=combined pressure &

piping lossCv=control valve coefficientP =differential pressure=specific weight

Calculation for piping correction factor (Fp)


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Calculation for piping correction factor (Fp)& Calculation for correction factor ( FLP)

Fp = [ K/N 2 (Cd) 2 ] -1/2 FLP = [ 1/ (F L )2 Ki/N 2 (Cd) 2 ] -1/2

N2= 890 (US)

=0.00214(SI) K=sum of coefficient heads

=K 1 +K 2+K B1+K B2K 1=0.5[1-(d/D) 2 ]2

K 2 = 1.0[1-(d/D)2

]2 K B1 = K B2 = 1-(d/D)

Ki= K 1 + K B1

Control valve sizing for choked flow


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Control valve sizing for choked flow

Formulae for Cv calculation:

Q = N 1FLPCv (P-Pvc)/G f

Wf=N 6FLPCv (P-Pvc)/G f

where : Pvc= Ff*Pv

Ff =critical liquid pressure ratio factor.=0.9996-0.28 (Pv/Pc)

Pv=vapor pressure of liquid.Pc=thermodynamic critical pressure.FLP =combined pressure & piping factor.

Sizing of gas application


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Sizing of gas application

Formulae

W=N 6FpCvY (xp 1 1 )Q=N 7FpCvp 1Y x/GgT 1Z

Expansion Factor Y= 1-x/3F k X t

Sp.ht.ratio factor F k = k/1.40

Manufacturers factor xT = (C 1)2/1600

Sizing of steam(dry & saturated) application


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Sizing of steam(dry & saturated) application

Formulaefor X< X

TP

W=NFpCvp 1(3-X/X TP ) (X) N=1 (US)=0.152(IS)

for X> X TP (choked flow)

W=NFpCvp 1 (XTP ) N=2 (US)=0.304(IS)

Fp= Piping correction factor

X=Ratio of differential pressure to absolute pressure .Xt=Ultimate value of X used ti establish expansionfactor,Y.

A t t


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Actuators

Pneumatic/Diaphragm actuators

Piston actuators

Electric actuators

Positioners

Boosters

Pneumatic Actuators


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Probably 90% of the actuators in the process are pneumatic.The reason fortheir wide application is use of compressed air which is very good source of

power for actuators .The energy stored in compressed air provides a largereservoir of readily available power to meet the needs of the actuators. Thereservoir formed by the air receiver & the distribution system will supply

power during short periods of power interruptions.Upto a point high pressureair is more effective than low pressure air as far as actuators are concerned .Taking economic factors into consideration ,the optimum system pressure is

between 60 to 120 psig.If cylinder actuated valves are used in plants economy

dictates that the pressure be at least 80 psig.The quality of compressed is a important factor to be considered for safety ,reliability & maintenance cost .Suitable air quality standard should be adoptedfor the purpose.The air should be dried to a dew point at least 10F below theambient temperature.The compressor should be non lubricated so there is nooil present .Liquid ring avoid both oil & particulate problems.Many plants are

expected to run 100% of the time without failure ;yet many pumps in lesscritical service are spared.Other compressed gases may be used in place of air , for instance , natural gasis often used on outdoor installation.other gas which is used is dry nitrogenwhich is to be used should be oil free.Great care is to be taken to avoid thehazard if pilots are located in closed areas where air can be displaced by

nitrogen.Even control rooms can be hazardous if ventilation fails.

Pneumatic Actuators l ll d d h ll l fl bl d h


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Also called diaphragm actuators.usually employ a flexible diaphragm, placed between two stampings or cast casing& at least one section ismade pressure tight .the actuator generally has a range spring opposingthe force generated within actuator.The control air signal is connected to

pressure air tight chamber & an increase or decrease in pressure result inforce which is used to overcome the pressure -drop forces within valve

body, forces of actuator spring ,& hysterisis forces in valve body.the sole purpose of actuator is to move the valve in response to error correctingsignal.The size of the actuator depends on the pressure drop in the valve.Types of pneumatic actuators:Direct acting - Air tight chamber is above diaphragm.Reverse acting - Air tight chamber is below chamber .Diaphragm materials:

Neoprene diaphragm with fiberglass.Cotton /nylon-for ambient conditions.Silicones,viton,polyacrylics with Dacron for high temperatures.Fiber glass fabric.Application Widely used for proportional control.Advantages: Good adaptability.available in wide no.of sizes,least expensive in market.Disadvantages: Employ large diaphragm,large casting,imposes stresses on valve,not fast.

Pneumatic / Diaphragm Actuators


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Pneumatic / Diaphragm Actuators

Figure 26

Piston Actuators/ Electro Hydraulic Actuators


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y

The usual form of of a hydraulic actuator is a double acting cylinder .Hydraulic fluid from a external source is admitted to one side of the piston

& exhausted from the other side of the 4 way pilot valve /jet type arrangement.This pilot valve is governed by a control signal & a position feedback fromstem.An actuator of this type has the potential to exert enormous forces & todrive the valve at high speed , since the remote motor & pump furnishing the

power can be as large as is necessary to match performance needs . Hydraulicfluid is virtually incompressible so the actuator is extremely stiff .It can be

made fail safe by by installing a trip valve & an accumulator on the hydraulicfluid line to store energy for use when the supply source fails .alternativelyan accessory lock up relay will hold the valve in last position.Hydraulic actuators form an expensive system.There must be an unusualrequirement for high performance valve actuation to justify this expenditure.The hydraulic actuators are likely to be found where there are a number of large

& heavy dampers, large ball valves ,large butterfly valve with high torque.If these devices are to be moved at high speed ,or are used in control systemloop requiring superior performance , there is greater incentive to considerhydraulic power .The selection of right hydraulic fluid is an important point to be consideredfor high temperature application.The system also requires filters,relief valves &other accessories.

Piston actuators


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These are available from most manufacturers.These are used along with positioners .the piston cylinder used is made of cast/wrought iron withstanding

high pressure than stamped or cast casing.Due to high pressure rating theactuators are able to provide more force for smaller diameters.It is consideredas effective means of coping pressure drops,when used along with actuators.Application Proportioning or positioning control valves.Advantages:

Provide high thrust.Good frequency response.High reliability Exact positioning relative to control signal.Relatively fast response Safe in electrically hazardous location.Disadvantages: Requires high pressure air supply. More expensive than spring & diaphragm type.Cumbersome to achieve fail safe condition

Piston Actuator


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Piston Actuator

Figure 27


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Electric Actuator


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Electric Actuator Advantages:

Extremely fast response.Develops maximum power for actuation.Ensures accurate positioning of valve.

DisadvantagesExpensiveMore difficult to maintain

Figure 29

Selection of Actuators


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Selection of Actuators

Selection criteria : 1) Availability of powering source viz. Hydraulic, pneumatic,electric

mechanical.2) Thrust requirement for the maximum pressure operation & tightness of

shutoff required to overcome friction versus the size available for each typeof actuator.

3) The availability of the actuator to hold the plug in a fixed position(stiffness)through surges of line pressure .this is achieved by a high rate mechanical

spring ,low case air volume,hydraulic fluid loading .the rate of force changeof the actuator with stroke should be at least twice the rate of change ofline pressure unbalances in within the valve acting on the plug & stem.

4) The required actuating action upon failure of powering medium ; valve failsopen; valve fail close; close; hold position.

5) Adequate frequency response to satisfy process dynamic and /or safeemergency full open or closing times .

6) Temperature limits of elastomer diaphragm material. Neoprene(-20 to 180F); silicone(-30 to 300F); Viton(-10 to 350 F)metal bellows (> 350F)

7) Actuator cost escalting in this orderAir diaphragm type,air piston, air rotary, electric & electrohydraulic type.

Actuator Sizing


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Actuator Sizing

When one selects an actuator following points should be weighed

carefully

Torque required .Speed of response required .duty cycle limits.Energy Costs.Adjustibility for travel ,split ranging ,or other loops needs.Price of installation cost.Maintenance cost.Reliability .Space requirement & weight.

Actuator Sizing


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Static ForcesThe static forces are those forces that exist with the valve under pressure butwith no fluid flow .An ideal actuator should be able to overcome all the forcesassociated with the valve .It should be able to move the valve mechanism tospecified position with specified tolerance ,despite the varying forces exerted

b the flowing fluid .In other words it should have power , stiffness & goodfrequency response qualities suitable for application.The static imbalance is a major force .On single ported ,unbalanced globevalves this force is measured by the area of the seat multiplied by thedifferential pressure.Even on balanced valve there is difference in theopposing areas.The double port valve has unequal seat areas .To account forthese forces the areas of the seat & pistons must be known.The direction offorce depends on direction of flowing fluid .Another static forces is the stemforce .It is measured b the area of the stem multiplied by the pressure in thevalve body.

To meet the leakage tolerance the plug valve must be seated with appropriateforce.This force varies from about 20 lb per lineal inch of seat circumferencefor class II leakage rate to about 80 lbs per lineal inch for the larger size valvesClass V valves require a seating force that increases with shut off pressuredifferential across seat. Soft seated valves require fairly high seat loads to gettight shut off. TFE requires a forces of 30 lbs plus a force of 22lb/in for each

100 psi differential pressure drop across it.max drop of TFE is 400 psi.

Actuator Sizing


84/98

Static forces of rotary motion valves manifest themselves in a entirelydifferent manner .First Off ,the forces are measured in torque units .Ball & other symmetric valves have no unbalanced forces .Ball valve

plugs are often designed so that closure member is always in contactwith the seat.These valves have extremely high seat frictional forcesat all opening .The other rotary valve incur sealing forces only at smallangles of opening .The torque required to cope up these forces are

breakaway or breakout torque.

Actuator Sizing


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Fluid force -Quick opening valve , flow to open

Dynamic forces Dynamic forces are those forces created by fluid by the fluid flow throughthe valve.For some valves these forces are very complex .

In flow to close direction ,the stem force on this valve increases slightly asthe plug moves off the seat , up to about 2.5 % of seat diameter.then the forcedecreases linearly to about 25% of its closed value.The double ported ,Quick opening ,with one port flow to open & one port flow to close ,has afluid force it fairly balanced .

Figure 30


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Fluid force -Quick opening Flow to close

Fluid force -Quick opening Double port valve

Figure 31

Figure 32


87/98

Fluid force countered plug FTO & FTCFigure 32


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For Butterfly Valve- The fluid forces acting on butterfly valves differ widely in design of the disk& orientation of the stem .Consider first aligned disk with centered stem.

When the valve is closed there is no torque due to differential pressure .Butthe thrust caused by the differential pressure on disk creates a substantialfriction on bearings. This adds to friction caused by packing & seal .As thevalve opens the vane ,the actuator must oppose the torque that tend to closethe valve the torque reaches the maximum value when valve is 60 to 80 open.As dynamic torque increases , thrust on the bearings is affected by lifting

forces on the disk,changing torque requirement of the actuator.The torque is proportional to pressure drop & cube of diameter of disk.

The other factors affecting the torque curve for this type of valve are :The quality of bearingsLubricity of fluid

Fitting adjacent to valveFree dischargeChoked flowCompressible fluids

Torque curve for a butterfly valveFigure 33


89/98

q yg


90/98

When the valve is in motion there are certain factors that assume importancerelative to valve stability On balanced cage guided valves balancing piston


91/98

relative to valve stability .On balanced cage guided valves balancing pistonis connected to the body pressure by holes through the plug leading to

balancing chamber.When plug moves moves ,fluid must flow through these passages .This

pressure .If the plug moves fast & holes are small ,there are will be substantial pressure drop across the passages .this pressure difference in effect changesthe shape of the force curve & can create excessive negative fluid force &instability.On the other hand , rapid valve movement caused by fluid forces on actuatorin a manner to increase the effective spring rate ,thus increasing stability .Movement of diaphragm or piston in the actuator causes air to flow in or outof actuator case through small opening .This is a snubber & acts as anadditional spring force .If the valve time constant is ,at frequencies greaterthan ( ) -1 radians/sec .The effective air spring rate is

Kp = k p (Ad)2 / V

Wherek = ratio of specific heats (sir=1,40)

p = average air pressure ,psiaAd= Area of diaphragm,sq.inchesV =Volume ,cu.inches

Friction is dominant at natural frequency . Above natural frequency inertiaserves to increase the stiffness still further .At these frequencies less then

l f i h i i ff di i i h idl l h l i


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natural frequencies the air spring effect diminishes rapidly unless the valve isequipped with positioner.Because of its high gain the positioner enables anactuator to resist fluid forces that act at lower frequencies. As the frequenciesapproaches ( ) -1 radians/sec stiffening effect diminishes to that of a valvewithout positioner.For choked flow there may be little or no pressure recovery.In that case the

P used in equationKh = (F + P 2 As)/ ( P Ap )

Is to be replaced by P e

P e= F L(P1-FFPV)For gas or vapor the effective pressure drop P isP e=Yx P 1

And for choked flowP e=0.44 F k XT P1

In order to cope up with the problem caused by negative fluid gradient , the

valve manufacturer adopts certain design procedures to overcome the lack ofcomplete information.One method is to list for each valve style & stem travel a factor to define the

slope of the negative gradient the factor is called KnKn = (dF / dH)/(psi) -inches

The spring rate must be greater than ( P Kn).

The operating pressure is taken as P .

Summary of actuator sizing


93/98

To be able to properly size a control actuator ,one must have access

to all the necessary parameters ;Valve Body Force or Torque Characteristics .Packing & Seal Friction Factors.Actuator Force Characteristics.Spring Rates Available.

It is also necessary to know the limits of the various components likeSeat ForceSpring LoadSpring Adjuster TravelActuator Casing Pressure Limits..

Su a y o actuato s g


94/98

Boosters


95/98

Volume boosters:A volume booster can be used to increase the speed of operation of diaphragmcontrol valve .A volume booster being used in conjunction with controller &

pneumatic valve.the controller applies its output signal to booster instead ofto the control valve .Only about 1 cubic inch of air is requiredto position the pilot in the booster , thus the volume of air moved through theconnecting tubing is small .The air that operates the valve comes through the

pilot in booster .The pilot has large capacity the stroking time is substantially

reduced .

35 psig

exhaust

controller booster

20 psig

Volume booster in control valve loop Figure 34

Boosters


96/98

Pressure boosters :

They are also volume boosters ,however their main function is to

increase the pressure from controller to above 20 psig. In certain valveapplication.A booster may be preferred over positioner because of its lower cost. Alsoa booster does not close the loop around the valve. It also enhances stabilityin fast response systems.

35 psig

exhaust

controller 2:1

20 psig

Pressure booster in control valve loop

A-T-O

6 - 30 psig

Figure 35

Transducers


97/98

On -off relay s:Applicable where the maximum output of controller is insufficient to operate

a diaphragm valve.

35 psig

exhaust

controller

20 psig

A-T-C

6 - 30 psig

ON_OFF

The figure shows on-off pneumatic relay .the relay applies 35 or 0 psig to diaphragm

valve to open or close valve. The 35 psig allows the single seated valve to close againsthigh pressure line than would be possible with 20psig output from controller.

The pneumatic relay is used a throttling controller where the relay is used in emergencyservice,as shown above.in this application the relay is normally positioned such thatthe exhaust port is closed & path through the relay & diaphragm valve is kept open.In emergency signal is applied to relay to close controller signal & open exhaust .

Throttling

Figure 35

Transducers


98/98

A-T-C

6 30 psig

35 psig

exhaust

Solenoid valvesOn-Off control

Used in combination with diaphragm valve. It is used to supply or exhaust airfrom diaphragm control valve to achieve on-off control.Depending on size thearrangement may be considered less expensive & faster response.Throttling controlThe solenoid valve is used with throttling control valve in emergency service .The solenoid valve is positioned such that exhaust port is closed & path

between controller & diaphragm is open .In emergency the solenoid coil isactivated & controlled is blocked & exhaust port is opened & valve is closed .

A-T-C

35 psig

exhaust20 psig

Throttling SOV

SOV

Documents

18008849-control-valve.ppt