CONTROL VALVE SOURCEBOOK

PULP & PAPER

Copyright 2011 Fisher Controls International LLC All Rights Reserved.

Fisher, ENVIRO-SEAL, Whisper Trim, Cavitrol, WhisperFlo, VeeBall, ControlDisk, NotchFlo, easye and FIELDVUE are marksowned by Fisher Controls International LLC, a business of Emerson Process Management. The Emerson logo is a trademark andservice mark of Emerson Electric Co. All other marks are the property of their respective owners.

This publication may not be reproduced, stored in a retrieval system, or transmitted in whole or in part, in any form or by any means,electronic, mechanical, photocopying, recording or otherwise, without the written permission of Fisher Controls International LLC.

Printed in U.S.A., First Edition

iii

Table of Contents

Introduction v

Chapter 1 Control Valve Selection 1-1

Chapter 2 Actuator Selection 2-1

Chapter 3 Liquid Valve Sizing 3-1

Chapter 4 Cavitation & Flashing 4-1

Chapter 5 Gas Valve Sizing 5-1

Chapter 6 Control Valve Noise 6-1

Chapter 7 Steam Conditioning 7-1

Chapter 8 Process Overview 8-1

Chapter 9 Pulping 9-1

Chapter 10A Batch Digesters 10A-1

Chapter 10B Continuous Digesters 10B-1

Chapter 11 Black Liquor Evaporators/Concentrators 11-1

Chapter 12 Kraft Recovery Boiler 12-1

Chapter 13 Recausticizing & Lime Recovery 13-1

Chapter 14 Bleaching & Brightening 14-1

Chapter 15 Stock Preparation 15-1

Chapter 16 Wet End Chemistry 16-1

Chapter 17 Paper Machine 17-1

Chapter 18 Power & Recovery Boiler 18-1

iv

vPulp and Paper Control Valves

IntroductionThis sourcebooks intent is to introduce a pulpand paper mills processes, as well as the use ofcontrol valves in many of the processes found inthe mill. It is intended to help you:

Understand pulp and paper processes

Learn where control valves are typicallylocated within each process

Identify valves commonly used for specificapplications

Identify troublesome/problem valves withinthe process

The information provided will follow a standardformat of:

Description of the process

Functional drawing of the process

Fisher valves to be considered in eachprocess and their associated function

Impacts and/or considerations fortroublesome/problem valves

Valve SelectionThe information presented in this sourcebook isintended to assist in understanding the controlvalve requirements of general pulp and papermills processes.

Since every mill is different in technology andlayout, the control valve requirements andrecommendations presented by this sourcebookshould be considered as general guidelines.Under no circumstances should this informationalone be used to select a control valve withoutensuring the proper valve construction is identifiedfor the application and process conditions.

All valve considerations should be reviewed by thelocal business representative as part of any valveselection or specification activity.

Control ValvesValves described within a chapter are labeledand numbered corresponding to the identificationused in the process flow chart for that chapter.Their valve function is described, and aspecification section gives added information onprocess conditions, names of Fisher valves thatmay be considered, process impact of the valve,and any special considerations for the processand valve(s) of choice.

Process DrawingsThe process drawings within each chapter showmajor equipment items, their typical placementwithin the processing system, and process flowdirection. Utilities and pumps are not shownunless otherwise stated.

Many original equipment manufacturers (OEMs)provide equipment to the pulp and paperindustry, each with their own processes andproprietary information. Process drawings arebased on general equipment configurationsunless otherwise stated.

Problem ValvesOften there are references to valve-causedproblems or difficulties. The list of problemsinclude valve erosion from process media,stickiness caused by excessive friction (stiction),excessive play in valve to actuator linkages(typically found in rotary valves) that causesdeadband, excessive valve stem packingleakage, and valve materials that areincompatible with the flowing medium. Any one,or a combination of these difficulties, may affectprocess quality and throughput with a resultingnegative impact on mill profitability.

Many of these problems can be avoided orminimized through proper valve selection.Consideration should be given to valve style andsize, actuator capabilities, analog versus digitalinstrumentation, materials of construction, etc.Although not being all-inclusive, the informationfound in this sourcebook should facilitate thevalve selection process.

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Chapter 1

Control Valve Selection

In the past, a customer simply requested a controlvalve and the manufacturer offered the productbest-suited for the job. The choices among themanufacturers were always dependent uponobvious matters such as cost, delivery, vendorrelationships, and user preference. However,accurate control valve selection can beconsiderably more complex, especially forengineers with limited experience or those whohave not kept up with changes in the control valveindustry.

An assortment of sliding-stem and rotary valvestyles are available for many applications. Someare touted as universal valves for almost anysize and service, while others are claimed to beoptimum solutions for narrowly defined needs.Even the most knowledgeable user may wonderwhether they are really getting the most for theirmoney in the control valves they have specified.

Like most decisions, selection of a control valveinvolves a great number of variables; the everydayselection process tends to overlook a number ofthese important variables. The followingdiscussion includes categorization of availablevalve types and a set of criteria to be considered inthe selection process.

What Is A Control Valve?Process plants consist of hundreds, or eventhousands, of control loops all networked togetherto produce a product to be offered for sale. Eachof these control loops is designed to control acritical process variable such as pressure, flow,level, temperature, etc., within a required operatingrange to ensure the quality of the end-product.

These loops receive, and internally create,disturbances that detrimentally affect the processvariable. Interaction from other loops in thenetwork provides disturbances that influence theprocess variable. To reduce the effect of theseload disturbances, sensors and transmitters collectinformation regarding the process variable and itsrelationship to a desired set point. A controller thenprocesses this information and decides what mustoccur in order to get the process variable back towhere it should be after a load disturbance occurs.When all measuring, comparing, and calculatingare complete, the strategy selected by thecontroller is implemented via some type of finalcontrol element. The most common final controlelement in the process control industries is thecontrol valve.

A control valve manipulates a flowing fluid such asgas, steam, water, or chemical compounds tocompensate for the load disturbance and keep theregulated process variable as close as possible tothe desired set point.

Many people who speak of control valves areactually referring to control valve assemblies.The control valve assembly typically consists ofthe valve body, the internal trim parts, an actuatorto provide the motive power to operate the valve,and a variety of additional valve accessories,which may include positioners, transducers, supplypressure regulators, manual operators, snubbers,or limit switches.

It is best to think of a control loop as aninstrumentation chain. Like any other chain, theentire chain is only as good as its weakest link. Itis important to ensure that the control valve is notthe weakest link.

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Valve Types and CharacteristicsThe control valve regulates the rate of fluid flow asthe position of the valve plug or disk is changed byforce from the actuator. To do this, the valve must:

Contain the fluid without external leakage.

Have adequate capacity for the intendedservice.

Be capable of withstanding the erosive,corrosive, and temperature influences of theprocess.

Incorporate appropriate end connections tomate with adjacent pipelines and actuatorattachment means to permit transmission ofactuator thrust to the valve plug stem or rotaryshaft.

Many styles of control valve bodies have beendeveloped. Some can be used effectively in anumber of applications while others meet specificservice demands or conditions and are used lessfrequently. The subsequent text describes popularcontrol valve body styles utilized today.

Globe Valves

Single-Port Valve BodiesSingle-port is the most common valve body styleand is simple in construction. Single-port valvesare available in various forms, such as globe,angle, bar stock, forged, and split constructions.Generally, single-port valves are specified forapplications with stringent shutoff requirements.They use metal-to-metal seating surfaces orsoft-seating with PTFE or other compositionmaterials forming the seal.

Single-port valves can handle most servicerequirements. Because high pressure fluid isnormally loading the entire area of the port, theunbalance force created must be considered whenselecting actuators for single-port control valvebodies. Although most popular in the smallersizes, single-port valves can often be used in NPS4 to 8 with high thrust actuators.

Many modern single-seated valve bodies use cageor retainer-style construction to retain the seat ringcage, provide valve plug guiding, and provide ameans for establishing particular valve flow

Figure 1-1. Single-Ported Globe-Style ValveBody

W7027-1

characteristics. Retainer-style trim also offers easeof maintenance with flow characteristics altered bychanging the plug. Cage or retainer-stylesingle-seated valve bodies can also be easilymodified by a change of trim parts to providereduced-capacity flow, noise attenuation, orcavitation eliminating or reducing trim (see chapter 4).Figure 1-1 shows one of the more popular styles ofsingle-ported or single-seated globe valve bodies.They are widely used in process controlapplications, particularly in sizes NPS 1 throughNPS 4. Normal flow direction is most often flow-upthrough the seat ring.

Angle valves are nearly always single ported, asshown in figure 1-2. This valve has cage-style trimconstruction. Others might have screwed-in seatrings, expanded outlet connections, restricted trim,and outlet liners for reduction of erosion damage.

Bar stock valve bodies are often specified forcorrosive applications in the chemical industry(figure 1-3), but may also be requested in otherlow flow corrosive applications. They can bemachined from any metallic bar-stock material andfrom some plastics. When exotic metal alloys arerequired for corrosion resistance, a bar-stock valvebody is normally less expensive than a valve bodyproduced from a casting.

High pressure single-ported globe valves are oftenfound in power plants due to high pressure steam(figure 1-4). Variations available include

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Figure 1-2. Flanged Angle-StyleControl Valve Body

W0971

Figure 1-3. Bar Stock Valve Body

W9756

cage-guided trim, bolted body-to-bonnetconnection, and others. Flanged versions areavailable with ratings to Class 2500.

Balanced-Plug Cage-Style ValveBodiesThis popular valve body style, single-ported in thesense that only one seat ring is used, provides theadvantages of a balanced valve plug often

Figure 1-4. High Pressure Globe-StyleControl Valve Body

W0540

Figure 1-5. Valve Body with Cage-Style Trim,Balanced Valve Plug, and Soft Seat

W0992-4

associated only with double-ported valve bodies(figure 1-5). Cage-style trim provides valve plugguiding, seat ring retention, and flowcharacterization. In addition, a sliding pistonring-type seal between the upper portion of thevalve plug and the wall of the cage cylindervirtually eliminates leakage of the upstream highpressure fluid into the lower pressure downstreamsystem.

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Figure 1-6. High Capacity Valve Body withCage-Style Noise Abatement Trim

W0997

Downstream pressure acts upon both the top andbottom sides of the valve plug, thereby nullifyingmost of the static unbalance force. Reducedunbalance permits operation of the valve withsmaller actuators than those necessary forconventional single-ported valve bodies.

Interchangeability of trim permits the choice ofseveral flow characteristics or of noise attenuationor anticavitation components. For most availabletrim designs, the standard direction of flow is inthrough the cage openings and down through theseat ring. These are available in various materialcombinations, sizes through NPS 20, and pressureratings to Class 2500.

High Capacity, Cage-Guided ValveBodiesThis adaptation of the cage-guided bodiesmentioned above was designed for noiseapplications, such as high pressure power plants,where sonic steam velocities are oftenencountered at the outlet of conventional valvebodies (figure 1-6).The design incorporates oversized endconnections with a streamlined flow path and theease of trim maintenance inherent with cage-styleconstructions. Use of noise abatement trimreduces overall noise levels by as much as 35decibels. The design is also available in cagelessversions with a bolted seat ring, end connectionsizes through NPS 20, Class 600, and versions for

liquid service. The flow direction depends upon theintended service and trim selection, withunbalanced constructions normally flow-up andbalanced constructions normally flow-down.

Port-Guided Single-Port Valve Bodies Usually limited to 150 psi (10 bar) maximum

pressure drop.

Susceptible to velocity-induced vibration.

Typically provided with screwed in seat ringswhich might be difficult to remove after use.

Three-Way Valve Bodies Provide general converging (flow-mixing) or

diverging (flow-splitting) service. Best designs use cage-style trim for positive

valve plug guiding and ease of maintenance.

Variations include trim materials selected forhigh temperature service. Standard endconnections (flanged, screwed, butt weld, etc.) canbe specified to mate with most any piping scheme.

Actuator selection demands carefulconsideration, particularly for constructions withunbalanced valve plug.

A balanced valve plug style three-way valve bodyis shown with the cylindrical valve plug in the downposition (figure 1-7). This position opens thebottom common port to the right-hand port andshuts off the left-hand port. The construction canbe used for throttling mid-travel position control ofeither converging or diverging fluids.

Rotary Valves

Traditional Butterfly ValveStandard butterfly valves are available in sizesthrough NPS 72 for miscellaneous control valveapplications. Smaller sizes can use versions oftraditional diaphragm or piston pneumaticactuators, including the modern rotary actuatorstyles. Larger sizes might require high outputelectric or long-stroke pneumatic cylinderactuators.

Butterfly valves exhibit an approximately equalpercentage flow characteristic. They can be used

15

Figure 1-7. Three Way Valve withBalanced Valve Plug

W9045-1

Figure 1-8. High-Performance ButterflyControl Valve

W4641

for throttling service or for on-off control. Soft-seatconstructions can be obtained by utilizing a liner orby including an adjustable soft ring in the body oron the face of the disk.

Require minimum space for installation(figure 1-8).

Provide high capacity with low pressure lossthrough the valves.

Figure 1-9. Eccentric-Disk Rotary-ShaftControl Valve

W8380

Offer an economic advantage, particularly inlarger sizes and in terms of flow capacity per dollarinvestment.

Mate with standard raised-face pipelineflanges.

Depending on size, might require high outputor oversized actuators due to valve size valves orlarge operating torques from large pressure drops.

Standard liner can provide precise shutoffand quality corrosion protection with nitrile orPTFE liner.

Eccentric-Disk Control ValveEccentric disk rotary control valves are intendedfor general service applications not requiringprecision throttling control. They are frequentlyapplied in applications requiring large sizes andhigh temperatures due to their lower cost relativeto other styles of control valves. The control rangefor this style of valve is approximately one third aslarge as a ball or globe-style valves.Consequently, additional care is required in sizingand applying this style of valve to eliminate controlproblems associated with process load changes.They are well-suited for constant process loadapplications.

Provide effective throttling control.

Linear flow characteristic through 90 degreesof disk rotation (figure 1-9). Eccentric mounting of disk pulls it away from

the seal after it begins to open, minimizing sealwear.

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Figure 1-10. Fisher Control-Disk Valve with 2052 Actuator and FIELDVUE DVC6200 Digital Valve Controller

W9425 W9418

WAFER STYLE SINGLE FLANGE STYLE

Bodies are available in sizes through NPS 24compatible with standard ASME flanges.

Utilize standard pneumatic diaphragm orpiston rotary actuators.

Standard flow direction is dependent uponseal design; reverse flow results in reducedcapacity.

Control-Disk ValveThe Control-Disk valve (figure 1-10) offersexcellent throttling performance, while maintainingthe size (face-to-face) of a traditional butterflyvalve. The Control-Disk valve is first in class incontrollability, rangeability, and tight shutoff, and itis designed to meet worldwide standards.

Utilizes a contoured edge and uniquepatented disk to provide an improved control rangeof 15 - 70% of valve travel. Traditional butterflyvalves are typically limited to 25% - 50% controlrange.

Includes a tested valve sealing design,available in both metal and soft seats, to providean unmatched cycle life while still maintainingexcellent shutoff

Spring loaded shaft positions disk against theinboard bearing nearest the actuator allowing forthe disk to close in the same position in the seal,and allows for either horizontal or verticalmounting.

Complimenting actuator comes in three,compact sizes, has nested springs and a patented

Figure 1-11. Rotary-Shaft Control Valvewith V-Notch Ball

W8172-2

lever design to increase torque range within eachactuator size.

V-notch Ball Control ValveThis construction is similar to a conventional ballvalve, but with patented, contoured V-notch in theball (figure 1-11). The V-notch produces anequal-percentage flow characteristic. Thesecontrol valves provide precise rangeability, control,and tight shutoff.

Straight-through flow design produces littlepressure drop.

Bodies are suited to provide control oferosive or viscous fluids, paper stock, or otherslurries containing entrained solids or fibers.

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Figure 1-12. Sectional of Eccentric-PlugControl Valve Body

W4170-4

They utilize standard diaphragm or pistonrotary actuators.

Ball remains in contact with seal duringrotation, which produces a shearing effect as theball closes and minimizes clogging.

Bodies are available with either heavy-duty orPTFE-filled composition ball seal ring to provideexcellent rangeability in excess of 300:1.

Bodies are available in flangeless orflanged-body end connections. Both flanged andflangeless valves mate with Class 150, 300, or 600flanges or DIN flanges.

Valves are capable of energy absorbingspecial attenuating trim to provide improvedperformance for demanding applications.

Eccentric-Plug Control Valve Valve assembly combats erosion. The

rugged body and trim design handle temperaturesto 800F (427C) and shutoff pressure drops to1500 psi (103 bar). Path of eccentric plug minimizes contact with

the seat ring when opening, thus reducing seatwear and friction, prolonging seat life, andimproving throttling performance (figure 1-12). Self-centering seat ring and rugged plug

allow forward or reverse-flow with tight shutoff ineither direction. Plug, seat ring, and retainer areavailable in hardened materials, includingceramics, for selection of erosion resistance.

Designs offering a segmented V-notch ball inplace of the plug for higher capacity requirementsare available.

This style of rotary control valve is well-suited forcontrol of erosive, coking, and otherhard-to-handle fluids, providing either throttling or

on-off operation. The flanged or flangeless valvesfeature streamlined flow passages and ruggedmetal-trim components for dependable service inslurry applications.

Control Valve End ConnectionsThe three common methods of installing controlvalves in pipelines are by means of:

Screwed pipe threads

Bolted gasketed flanges

Welded end connections

Screwed Pipe ThreadsScrewed end connections, popular in small controlvalves, are typically more economical than flangedends. The threads usually specified are taperedfemale National Pipe Thread (NPT) on the valvebody. They form a metal-to-metal seal by wedgingover the mating male threads on the pipeline ends.This connection style, usually limited to valves notlarger than NPS 2, is not recommended forelevated temperature service. Valve maintenancemight be complicated by screwed end connectionsif it is necessary to take the body out of thepipeline. This is because the valve cannot beremoved without breaking a flanged joint or unionconnection to permit unscrewing the valve bodyfrom the pipeline.

Bolted Gasketed FlangesFlanged end valves are easily removed from thepiping and are suitable for use through the rangeof working pressures for which most control valvesare manufactured (figure 1-13). Flanged endconnections can be used in a temperature rangefrom absolute zero to approximately 1500F(815C). They are used on all valve sizes. Themost common flanged end connections includeflat-face, raised-face, and ring-type joint.The flat face variety allows the matching flanges tobe in full-face contact with the gasket clampedbetween them. This construction is commonlyused in low pressure, cast iron, and brass valves,and minimizes flange stresses caused by initialbolting-up force.

The raised-face flange features a circularraised-face with the inside diameter the same asthe valve opening, and the outside diameter lessthan the bolt circle diameter. The raised-face is

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Figure 1-13. Popular Varieties ofBolted Flange Connections

A7098

Figure 1-14. Common Welded End Connections

A7099

finished with concentric circular grooves forprecise sealing and resistance to gasket blowout.This kind of flange is used with a variety of gasketmaterials and flange materials for pressuresthrough the 6000 psig (414 bar) pressure rangeand for temperatures through 1500F (815C).This style of flanging is normally standard on Class250 cast iron bodies and all steel and alloy steelbodies.

The ring-type joint flange is similar in looks to theraised-face flange except that a U-shaped grooveis cut in the raised-face concentric with the valveopening. The gasket consists of a metal ring witheither an elliptical or octagonal cross-section.When the flange bolts are tightened, the gasket iswedged into the groove of the mating flange and atight seal is made. The gasket is generally soft iron

or Monel, but is available in almost any metal.This makes an excellent joint at high pressuresand is used up to 15,000 psig (1034 bar),however, it is generally not used at hightemperatures. It is furnished only on steel andalloy valve bodies when specified.

Welding End ConnectionsWelding ends on control valves (figure 1-14) areleak-tight at all pressures and temperatures, andare economical in first cost. Welding end valvesare more difficult to take from the line and arelimited to weldable materials. Welding ends comein two styles:

Socket welding

Buttwelding

The socket welding ends are prepared by boring ina socket at each end of the valve with an insidediameter slightly larger than the pipe outsidediameter. The pipe slips into the socket where itbutts against a shoulder and then joins to the valvewith a fillet weld. Socket welding ends in a givensize are dimensionally the same regardless of pipeschedule. They are usually furnished in sizesthrough NPS 2.

The buttwelding ends are prepared by bevelingeach end of the valve to match a similar bevel onthe pipe. The two ends are then butted to thepipeline and joined with a full penetration weld.This type of joint is used on all valve styles and theend preparation must be different for eachschedule of pipe. These are generally furnished forcontrol valves in NPS 2-1/2 and larger. Care mustbe exercised when welding valve bodies in thepipeline to prevent excessive heat transmitted tovalve trim parts. Trims with low-temperaturecomposition materials must be removed beforewelding.

Valve Body BonnetsThe bonnet of a control valve is the part of thebody assembly through which the valve plug stemor rotary shaft moves. On globe or angle bodies, itis the pressure retaining component for one end ofthe valve body. The bonnet normally provides ameans of mounting the actuator to the body andhouses the packing box. Generally, rotary valvesdo not have bonnets. (On some rotary-shaftvalves, the packing is housed within an extensionof the valve body itself, or the packing box is aseparate component bolted between the valvebody and bonnet.)

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Figure 1-15. Typical Bonnet, Flange,and Stud Bolts

W0989

On a typical globe-style control valve body, thebonnet is made of the same material as the valvebody or is an equivalent forged material because itis a pressure-containing member subject to thesame temperature and corrosion effects as thebody. Several styles of valve body-to-bonnetconnections are illustrated. The most common isthe bolted flange type shown in figure 1-15. Abonnet with an integral flange is also illustrated infigure 1-15. Figure 1-3 illustrates a bonnet with aseparable, slip-on flange held in place with a splitring. The bonnet used on the high pressure globevalve body illustrated in figure 1-4, is screwed intothe valve body. Figure 1-8 illustrates a rotary-shaftcontrol valve in which the packing is housed withinthe valve body and a bonnet is not used. Theactuator linkage housing is not a pressure-containing part and is intended to enclose thelinkage for safety and environmental protection.

On control valve bodies with cage- or retainer-styletrim, the bonnet furnishes loading force to preventleakage between the bonnet flange and the valvebody, and also between the seat ring and thevalve body. The tightening of the body-bonnetbolting compresses a flat sheet gasket to seal thebody-bonnet joint, compresses a spiral-woundgasket on top of the cage, and compresses anadditional flat sheet gasket below the seat ring toprovide the seat ring-body seal. The bonnet alsoprovides alignment for the cage, which, in turn,

guides the valve plug to ensure proper valve plugstem alignment with the packing.

As mentioned previously, the conventional bonneton a globe-type control valve houses the packing.The packing is most often retained by a packingfollower held in place by a flange on the yoke bossarea of the bonnet (figure 1-15). An alternatepacking retention means is where the packingfollower is held in place by a screwed gland (figure1-3). This alternate is compact, thus, it is oftenused on small control valves, however, the usercannot always be sure of thread engagement.Therefore, caution should be used if adjusting thepacking compression when the control valve is inservice.

Most bolted-flange bonnets have an area on theside of the packing box which can be drilled andtapped. This opening is closed with a standardpipe plug unless one of the following conditionsexists:

It is necessary to purge the valve body andbonnet of process fluid, in which case the openingcan be used as a purge connection.

The bonnet opening is being used to detectleakage from the first set of packing or from afailed bellows seal.

Extension BonnetsExtension bonnets are used for either high or lowtemperature service to protect valve stem packingfrom extreme process temperatures. StandardPTFE valve stem packing is useful for mostapplications up to 450F (232C). However, it issusceptible to damage at low processtemperatures if frost forms on the valve stem. Thefrost crystals can cut grooves in the PTFE, thus,forming leakage paths for process fluid along thestem. Extension bonnets remove the packing boxof the bonnet far enough from the extremetemperature of the process that the packingtemperature remains within the recommendedrange.

Extension bonnets are either cast (figure 1-16) orfabricated (figure 1-17). Cast extensions offerbetter high temperature service because of greaterheat emissivity, which provides better coolingeffect. Conversely, smooth surfaces that can befabricated from stainless steel tubing are preferredfor cold service because heat influx is usually themajor concern. In either case, extension wallthickness should be minimized to cut down heattransfer. Stainless steel is usually preferable to

110

Figure 1-16. Extension Bonnet

W0667-2

Figure 1-17. Valve Body withFabricated Extension Bonnet

W1416

carbon steel because of its lower coefficient ofthermal conductivity. On cold service applications,insulation can be added around the extension toprotect further against heat influx.

Figure 1-18. ENVIRO-SEAL BellowsSeal Bonnet

W6434

Bellows Seal BonnetsBellows seal bonnets (figure 1-18) are used whenno leakage (less than 1x106 cc/sec of helium)along the stem can be tolerated. They are oftenused when the process fluid is toxic, volatile,radioactive, or highly expensive. This specialbonnet construction protects both the stem and thevalve packing from contact with the process fluid.Standard or environmental packing boxconstructions above the bellows seal unit willprevent catastrophic failure in case of rupture orfailure of the bellows.

As with other control valve pressure/ temperaturelimitations, these pressure ratings decrease withincreasing temperature. Selection of a bellowsseal design should be carefully considered, andparticular attention should be paid to properinspection and maintenance after installation. Thebellows material should be carefully considered toensure the maximum cycle life.

Two types of bellows seal designs are used forcontrol valves:

Mechanically formed as shown in figure 1-19

Welded leaf bellows as shown in figure 1-20

The welded-leaf design offers a shorter totalpackage height. Due to its method of manufactureand inherent design, service life may be limited.

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Figure 1-21. Comprehensive Packing Material Arrangementsfor Globe-Style Valve Bodies

B2565

Figure 1-19. Mechanically Formed BellowsA5954

Figure 1-20. Welded Leaf BellowsA5955

The mechanically formed bellows is taller incomparison and is produced with a morerepeatable manufacturing process.

Control Valve PackingMost control valves use packing boxes with thepacking retained and adjusted by a flange andstud bolts (figure 1-27). Several packing materialscan be used depending upon the serviceconditions expected and whether the applicationrequires compliance to environmental regulations.Brief descriptions and service condition guidelinesfor several popular materials and typical packingmaterial arrangements are shown in figure 1-21.

PTFE V-Ring Plastic material with inherent ability to

minimize friction.

Molded in V-shaped rings that are springloaded and self-adjusting in the packing box.Packing lubrication not required.

Resistant to most known chemicals exceptmolten alkali metals.

Requires extremely smooth (2 to 4micro-inches RMS) stem finish to seal properly.Will leak if stem or packing surface is damaged.

Recommended temperature limits: 40F to+450F (40C to +232C) Not suitable for nuclear service because

PTFE is easily destroyed by radiation.

112

Figure 1-22. Measurement Frequency for ValvesControlling Volatile Organic Chemicals (VOC)

B2566

Laminated and Filament Graphite

Suitable for high temperature nuclear serviceor where low chloride content is desirable (GradeGTN).

Provides leak-free operation, high thermalconductivity, and long service life, but produceshigh stem friction and resultant hysteresis.

Impervious to most hard-to-handle fluids andhigh radiation.

Suitable temperature range: Cryogenictemperatures to 1200F (649C).

Lubrication not required, but an extensionbonnet or steel yoke should be used when packingbox temperature exceeds 800F (427C).

USA Regulatory Requirements forFugitive Emissions

Fugitive emissions are non-point source volatileorganic emissions that result from processequipment leaks. Equipment leaks in the UnitedStates have been estimated at over 400 millionpounds per year. Strict government regulations,developed by the US, dictate Leak Detection andRepair (LDAR) programs. Valves and pumps havebeen identified as key sources of fugitiveemissions. In the case of valves, this is the

leakage to atmosphere due to packing seal orgasket failures.

The LDAR programs require industry to monitor allvalves (control and noncontrol) at an interval thatis determined by the percentage of valves found tobe leaking above a threshold level of 500 ppmv(some cities use a 100 ppmv criteria). Thisleakage level is so slight you cannot see or hear it.The use of sophisticated portable monitoringequipment is required for detection. Detectionoccurs by sniffing the valve packing area forleakage using an Environmental ProtectionAgency (EPA) protocol. This is a costly andburdensome process for industry.

The regulations do allow for the extension of themonitoring period for up to one year if the facilitycan demonstrate an extremely low ongoingpercentage of leaking valves (less than 0.5% ofthe total valve population). The opportunity toextend the measurement frequency is shown infigure 1-22.

Packing systems designed for extremely lowleakage requirements also extend packing seal lifeand performance to support an annual monitoringobjective. The ENVIRO-SEAL packing system isone example. Its enhanced seals incorporate fourkey design principles including:

Containment of the pliable seal materialthrough an anti-extrusion component.

113

Proper alignment of the valve stem or shaftwithin the bonnet bore.

Applying a constant packing stress throughBelleville springs.

Minimizing the number of seal rings to reduceconsolidation, friction, and thermal expansion.

The traditional valve selection process meantchoosing a valve design based upon its pressureand temperature capabilities as well as its flowcharacteristics and material compatibility. Valvestem packing used in the valve was determinedprimarily by the operating temperature in thepacking box area. The available material choicesincluded PTFE for temperatures below 93C(200F) and graphite for higher temperatureapplications.

Today, choosing a valve packing system hasbecome much more complex due to the number ofconsiderations one must take into account. Forexample, emissions control requirements, such asthose imposed by the Clean Air Act within theUnited States and by other regulatory bodies,place tighter restrictions on sealing performance.Constant demands for improved process outputmean that the valve packing system must nothinder valve performance. Also, todays trendtoward extended maintenance schedules dictatesthat valve packing systems provide the requiredsealing over longer periods.

In addition, end user specifications that havebecome de facto standards, as well as standardsorganizations specifications, are used bycustomers to place stringent fugitive emissionsleakage requirements and testing guidelines onprocess control equipment vendors. EmersonProcess Management and its observance oflimiting fugitive emissions is evident by its reliablevalve sealing (packing and gasket) technologies,global emissions testing procedures, andemissions compliance approvals.

Given the wide variety of valve applications andservice conditions within industry, these variables(sealing ability, operating friction levels, operatinglife) are difficult to quantify and compare. A properunderstanding requires a clarification of tradenames.

Figure 1-23. Single PTFE V-Ring Packing

A6161-1

Single PTFE V-Ring Packing (Fig.1-23)

The single PTFE V-ring arrangement uses a coilspring between the packing and packing follower.It meets the 100 ppmv criteria, assuming that thepressure does not exceed 20.7 bar (300 psi) andthe temperature is between 18C and 93C (0Fand 200F). It offers excellent sealing performancewith the lowest operating friction.

ENVIRO-SEAL PTFE Packing (Fig. 1-24)

The ENVIRO-SEAL PTFE packing system is anadvanced packing method that utilizes a compact,live-load spring design suited to environmentalapplications up to 51.7 bar and 232C (750 psiand 450F). While it most typically is thought of asan emission-reducing packing system,ENVIRO-SEAL PTFE packing is, also, well-suitedfor non-environmental applications involving hightemperatures and pressures, yielding the benefit oflonger, ongoing service life.

ENVIRO-SEAL Duplex Packing(Fig. 1-25)

This special packing system provides thecapabilities of both PTFE and graphitecomponents to yield a low friction, low emission,fire-tested solution (API Standard 589) forapplications with process temperatures up to232C (450F).

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Figure 1-24. ENVIRO-SEAL PTFE Packing System

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Figure 1-25. ENVIRO-SEAL Duplex (PTFE andGraphite) Packing System

KALREZ Valve Stem Packing (KVSP)systems

The KVSP pressure and temperature limitsreferenced are for Fisher valve applications only.KVSP with PTFE is suited to environmental use upto 24.1 bar and 204C (350 psi and 400F) and, tosome non-environmental services up to 103 bar(1500 psi). KVSP with ZYMAXX, which is a

Figure 1-26. ENVIRO-SEAL Graphite ULF Packing System

39B4612-A

carbon fiber reinforced TFE, is suited to 260C(500F) service.

ENVIRO-SEAL Graphite Ultra LowFriction (ULF) Packing (Fig. 1-26)This packing system is designed primarily forenvironmental applications at temperatures inexcess of 232C (450F). The patented ULFpacking system incorporates thin PTFE layersinside the packing rings and thin PTFE washers oneach side of the packing rings. This strategic

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Figure 1-27. ENVIRO-SEAL GraphitePacking System for Rotary Valves

W6125-1

placement of PTFE minimizes control problems,reduces friction, promotes sealing, and extendsthe cycle life of the packing set.

HIGH-SEAL Graphite ULF Packing

Identical to the ENVIRO-SEAL graphite ULFpacking system below the packing follower, theHIGH-SEAL system utilizes heavy-duty, largediameter Belleville springs. These springs provideadditional follower travel and can be calibratedwith a load scale for a visual indication of packingload and wear.

ENVIRO-SEAL Graphite Packing forRotary Valves (Fig. 1-27)

ENVIRO-SEAL graphite packing is designed forenvironmental applications from 6C to 316C(20F to 600F) or for those applications where firesafety is a concern. It can be used with pressuresto 103 bar (1500 psi) and still satisfy the 500 ppmvEPA leakage criteria.

Graphite Ribbon Packing for RotaryValves

Graphite ribbon packing is designed fornon-environmental applications that span a widetemperature range from 198C to 538C (325Fto 1000F).

The following table provides a comparison ofvarious sliding-stem packing selections and arelative ranking of seal performance, service life,and packing friction for environmental applications.

Braided graphite filament and double PTFE arenot acceptable environmental sealing solutions.

The following applies to rotary valves. In the caseof rotary valves, single PTFE and graphite ribbonpacking arrangements do not perform well asfugitive emission sealing solutions.

The control of valve fugitive emissions and areduction in industrys cost of regulatorycompliance can be achieved through these stemsealing technologies.

While ENVIRO-SEAL packing systems have beendesigned specifically for fugitive emissionapplications, these technologies should also beconsidered for any application where sealperformance and seal life have been an ongoingconcern or maintenance cost issue.

Characterization of Cage-GuidedValve BodiesIn valve bodies with cage-guided trim, the shape ofthe flow openings or windows in the wall of thecylindrical cage determines flow characterization.As the valve plug is moved away from the seatring, the cage windows are opened to permit flowthrough the valve. Standard cages have beendesigned to produce linear, equal-percentage, andquick-opening inherent flow characteristics. Notethe differences in the shapes of the cage windowsshown in figure 1-28. The flow rate/travelrelationship provided by valves utilizing thesecages is equivalent to the linear, quick-opening,and equal-percentage curves shown for contouredvalve plugs (figure 1-29).Cage-guided trim in a control valve provides adistinct advantage over conventional valve bodyassemblies in that maintenance and replacementof internal parts is simplified. The inherent flowcharacteristic of the valve can easily be changedby installing a different cage. Interchange of cagesto provide a different inherent flow characteristicdoes not require changing the valve plug or seatring. The standard cages shown can be used witheither balanced or unbalanced trim constructions.Soft seating, when required, is available as aretained insert in the seat ring and is independentof cage or valve plug selection.

Cage interchangeability can be extended tospecialized cage designs that provide noiseattenuation or combat cavitation. These cagesfurnish a modified linear inherent flowcharacteristic, but require flow to be in a specific

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Figure 1-28. Characterized Cages for Globe-Style Valve Bodies

W0958 W0959 W0957

QUICK OPENING LINEAR EQUAL PERCENTAGE

Figure 1-29. Inherent Flow Characteristics Curves

direction through the cage openings. Therefore, itcould be necessary to reverse the valve body inthe pipeline to obtain proper flow direction.

Characterized Valve Plugs

The valve plug, the movable part of a globe-stylecontrol valve assembly, provides a variablerestriction to fluid flow. Valve plug styles are eachdesigned to:

Provide a specific flow characteristic.

Permit a specified manner of guiding oralignment with the seat ring.

Have a particular shutoff ordamage-resistance capability.

Valve plugs are designed for either two-position orthrottling control. In two-position applications, thevalve plug is positioned by the actuator at either oftwo points within the travel range of the assembly.In throttling control, the valve plug can bepositioned at any point within the travel range asdictated by the process requirements.

The contour of the valve plug surface next to theseat ring is instrumental in determining theinherent flow characteristic of a conventionalglobe-style control valve. As the actuator movesthe valve plug through its travel range, theunobstructed flow area changes in size and shapedepending upon the contour of the valve plug.When a constant pressure differential ismaintained across the valve, the changingrelationship between percentage of maximum flowcapacity and percentage of total travel range canbe portrayed (figure 1-29), and is designated asthe inherent flow characteristic of the valve.

Commonly specified inherent flow characteristicsinclude:

Linear Flow

A valve with an ideal linear inherent flowcharacteristic produces a flow rate directlyproportional to the amount of valve plug travelthroughout the travel range. For instance, at 50%of rated travel, flow rate is 50% of maximum flow;at 80% of rated travel, flow rate is 80% ofmaximum; etc. Change of flow rate is constantwith respect to valve plug travel. Valves with alinear characteristic are often specified for liquidlevel control and for flow control applicationsrequiring constant gain.

Equal-Percentage Flow

Ideally, for equal increments of valve plugtravel, the change in flow rate regarding travel maybe expressed as a constant percent of the flow

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Figure 1-30. Typical Construction to ProvideQuick-Opening Flow Characteristic

A7100

rate at the time of the change. The change in flowrate observed regarding travel will be relativelysmall when the valve plug is near its seat, andrelatively high when the valve plug is nearly wideopen. Therefore, a valve with an inherentequal-percentage flow characteristic providesprecise throttling control through the lower portionof the travel range and rapidly increasing capacityas the valve plug nears the wide-open position.Valves with equal-percentage flow characteristicsare used on pressure control applications, onapplications where a large percentage of thepressure drop is normally absorbed by the systemitself with only a relatively small percentageavailable at the control valve, and on applicationswhere highly varying pressure drop conditions canbe expected. In most physical systems, the inletpressure decreases as the rate of flow increases,and an equal percentage characteristic isappropriate. For this reason, equal percentageflow is the most common valve characteristic.

Quick-Opening Flow

A valve with a quick opening flowcharacteristic provides a maximum change in flowrate at low travels. The curve is essentially linearthrough the first 40 percent of valve plug travel,then flattens out noticeably to indicate littleincrease in flow rate as travel approaches thewide-open position. Control valves withquick-opening flow characteristics are often usedfor on/off applications where significant flow ratemust be established quickly as the valve begins toopen. As a result, they are often utilized in reliefvalve applications. Quick-opening valves can alsobe selected for many of the same applications forwhich linear flow characteristics arerecommended. This is because the quick-openingcharacteristic is linear up to about 70 percent ofmaximum flow rate. Linearity decreasessignificantly after flow area generated by valveplug travel equals the flow area of the port. For atypical quick-opening valve (figure 1-30), thisoccurs when valve plug travel equals one-fourth ofport diameter.

Valve Plug GuidingAccurate guiding of the valve plug is necessary forproper alignment with the seat ring and efficientcontrol of the process fluid. The common methodsused are listed below.

Cage Guiding: The outside diameter of thevalve plug is close to the inside wall surface of thecylindrical cage throughout the travel range. Sincethe bonnet, cage, and seat ring are self-aligningupon assembly, the correct valve plug and seatring alignment is assured when the valve closes(figure 1-15). Top Guiding: The valve plug is aligned by a

single guide bushing in the bonnet, valve body(figure 1-4), or by packing arrangement. Stem Guiding: The valve plug is aligned with

the seat ring by a guide bushing in the bonnet thatacts upon the valve plug stem (figure 1-3, leftview). Top-and-Bottom Guiding: The valve plug is

aligned by guide bushings in the bonnet andbottom flange.

Port Guiding: The valve plug is aligned by thevalve body port. This construction is typical forcontrol valves utilizing small-diameter valve plugswith fluted skirt projections to control low flow rates(figure 1-3, right view).

Restricted-Capacity Control ValveTrimMost control valve manufacturers can providevalves with reduced- or restricted- capacity trimparts. The reduced flow rate might be desirable forany of the following reasons:

Restricted capacity trim may make it possibleto select a valve body large enough for increasedfuture flow requirements, but with trim capacityproperly sized for present needs.

Valves can be selected for adequatestructural strength, yet retain reasonabletravel/capacity relationship.

Large bodies with restricted capacity trim canbe used to reduce inlet and outlet fluid velocities.

Purchase of expensive pipeline reducers canbe avoided.

Over-sizing errors can be corrected by use ofrestricted capacity trim parts.

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Conventional globe-style valve bodies can be fittedwith seat rings with smaller port size than normaland valve plugs sized to fit those smaller ports.Valves with cage-guided trim often achieve thereduced capacity effect by utilizing valve plug,cage, and seat ring parts from a smaller valve sizeof similar construction and adapter pieces abovethe cage and below the seat ring to mate thosesmaller parts with the valve body (figure 1-28).Because reduced capacity service is not unusual,leading manufacturers provide readily availabletrim part combinations to perform the requiredfunction. Many restricted capacity trimcombinations are designed to furnishapproximately 40% of full-size trim capacity.

General Selection CriteriaMost of the considerations that guide the selectionof valve type and brand are rather basic. However,there are some matters that may be overlooked byusers whose familiarity is mainly limited to just oneor a few valve types. Table 1-1 below provides achecklist of important criteria; each is discussed atlength following the table.

Table 1-1. Suggested General Criteria for Selecting Typeand Brand of Control Valve

Body pressure ratingHigh and low temperature limitsMaterial compatibility and durabilityInherent flow characteristic and rangeabilityMaximum pressure drop (shutoff and flowing)Noise and cavitationEnd connectionsShutoff leakageCapacity versus costNature of flowing mediaDynamic performance

Pressure RatingsBody pressure ratings ordinarily are consideredaccording to ANSI pressure classes the mostcommon ones for steel and stainless steel beingClasses 150, 300 and 600. (Source documentsare ASME/ANSI Standards B16.34, SteelValves, and ANSI B16.1, Cast Iron PipeFlanges and Flanged Fittings.) For a given bodymaterial, each NSI Class corresponds to aprescribed profile of maximum pressures thatdecrease with temperature according to thestrength of the material. Each material also has a

minimum and maximum service temperaturebased upon loss of ductility or loss of strength. Formost applications, the required pressure rating isdictated by the application. However, because allproducts are not available for all ANSI Classes, itis an important consideration for selection.

Temperature ConsiderationsRequired temperature capabilities are also aforegone conclusion, but one that is likely tonarrow valve selection possibilities. Theconsiderations include the strength or ductility ofthe body material, as well as relative thermalexpansion of various parts.

Temperature limits also may be imposed due todisintegration of soft parts at high temperatures orloss of resiliency at low temperatures. The softmaterials under consideration include variouselastomers, plastics, and PTFE. They may befound in parts such as seat rings, seal or pistonrings, packing, rotary shaft bearings and butterflyvalve liners. Typical upper temperature limits forelastomers are in the 200 - 350F range, and thegeneral limit for PTFE is 450F.

Temperature affects valve selection by excludingcertain valves that do not have high or lowtemperature options. It also may have some affecton the valves performance. For instance, goingfrom PTFE to metal seals for high temperaturesgenerally increases the shutoff leakage flow.Similarly, high temperature metal bearing sleevesin rotary valves impose more friction upon theshaft than do PTFE bearings, so that the shaftcannot withstand as high a pressure-drop load atshutoff. Selection of the valve packing is alsobased largely upon service temperature.

Material SelectionThe third criterion in table 1-1, materialcompatibility and durability, is a more complexconsideration. Variables may include corrosion bythe process fluid, erosion by abrasive material,flashing, cavitation or pressure and temperaturerequirements. The piping material usually indicatesthe body material. However, because the velocityis higher in valves, other factors must beconsidered. When these variables are included,often valve and piping materials will differ. The trimmaterials, in turn, are usually a function of thebody material, temperature range and qualities ofthe fluid. When a body material other than carbon,alloy, or stainless steel is required, use of analternate valve type, such as lined or bar stock,should be considered.

119

Flow CharacteristicThe next selection criterion, inherent flowcharacteristic, refers to the pattern in which theflow at constant pressure drop changes accordingto valve position. Typical characteristics arequick-opening, linear, and equal-percentage. Thechoice of characteristic may have a stronginfluence upon the stability or controllability of theprocess (see table 1-3), as it represents thechange of valve gain relative to travel.

Most control valves are carefully characterizedby means of contours on a plug, cage, or ballelement. Some valves are available in a variety ofcharacteristics to suit the application, while othersoffer little or no choice. To quantitatively determinethe best flow characteristic for a given application,a dynamic analysis of the control loop can beperformed. In most cases, however, this isunnecessary; reference to established rules ofthumb will suffice.

The accompanying drawing illustrates typical flowcharacteristic curves (figure 1-29). The quickopening flow characteristic provides for maximumchange in flow rate at low valve travels with a fairlylinear relationship. Additional increases in valvetravel give sharply reduced changes in flow rate,and when the valve plug nears the wide openposition, the change in flow rate approaches zero.In a control valve, the quick opening valve plug isused primarily for on-off service; but it is alsosuitable for many applications where a linear valveplug would normally be specified.

RangeabilityAnother aspect of a valves flow characteristic is itsrangeability, which is the ratio of its maximum andminimum controllable flow rates. Exceptionallywide rangeability may be required for certainapplications to handle wide load swings or acombination of start-up, normal and maximumworking conditions. Generally speaking, rotaryvalvesespecially partial ball valveshavegreater rangeability than sliding-stem varieties.

Use of PositionersA positioner is an instrument that helps improvecontrol by accurately positioning a control valveactuator in response to a control signal. They areuseful in many applications and are required withcertain actuator styles in order to match actuatorand instrument pressure signals, or to provide

operating stability. To a certain extent, a valve withone inherent flow characteristic can also be madeto perform as though it had a differentcharacteristic by utilizing a nonlinear (i.e.,characterized) positioner-actuator combination.The limitation of this approach lies in thepositioners frequency response and phase lagcompared to the characteristic frequency of theprocess. Although it is common practice to utilize apositioner on every valve application, eachapplication should be reviewed carefully. Thereare certain examples of high gain processeswhere a positioner can hinder valve performance.

Pressure DropThe maximum pressure drop a valve can tolerateat shutoff, or when partially or fully open, is animportant selection criteria. Sliding-stem valvesare generally superior in both regards because ofthe rugged nature of their moving parts. Manyrotary valves are limited to pressure drops wellbelow the body pressure rating, especially underflowing conditions, due to dynamic stresses thathigh velocity flow imposes on the disk or ballsegment.

Noise and CavitationNoise and cavitation are two considerations thatoften are grouped together because both resultfrom high pressure drops and large flow rates.They are treated by special modifications tostandard valves. Chapter four discusses thecavitation phenomenon and its impact andtreatment, while chapter six discusses noisegeneration and abatement.

End ConnectionsThe three common methods of installing controlvalves in pipelines are by means of screwed pipethreads, bolted flanges, and welded endconnections. At some point in the selectionprocess, the valves end connections must beconsidered with the question simply being whetherthe desired connection style is available in thevalve being considered.

In some situations, this matter can limit theselection rather narrowly. For instance, if a pipingspecification calls for welded connections only, thechoice usually is limited to sliding-stem valves.

Screwed end connections, popular in small controlvalves, offer more economy than flanged ends.

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The threads usually specified are tapered femaleNPT on the valve body. They form ametal-to-metal seal by wedging over the matingmale threads on the pipeline ends. Thisconnection style is usually limited to valves notlarger than NPS 2, and is not recommended forelevated temperature service.

Valve maintenance might be complicated byscrewed end connections if it is necessary to takethe body out of the pipeline. Screwed connectionsrequire breaking a flanged joint or unionconnection to permit unscrewing the valve bodyfrom the pipeline.

Flanged end valves are easily removed from thepiping and are suitable for use through the rangeof working pressures that most control valves aremanufactured (figure 1-13).Flanged end connections can be utilized in atemperature range from absolute zero (273F) toapproximately 1500F (815C). They are utilizedon all valve sizes. The most common flanged endconnections include flat face, raised face, and ringtype joint.Welded ends on control valves are leak-tight at allpressures and temperatures and are economicalin initial cost (figure 1-14). Welded end valves aremore difficult to remove from the line and arelimited to weldable materials. Welded ends comein two styles, socket weld and buttweld.

Shutoff CapabilitySome consideration must be given to a valvesshutoff capability, which is usually rated in terms ofclasses specified in ANSI/FCI70-2 (table 1-4). Inservice, shutoff leakage depends upon manyfactors, including but not limited to, pressure drop,temperature, and the condition of the sealingsurfaces. Because shutoff ratings are based uponstandard test conditions that can be different fromservice conditions, service leakage cannot bepredicted accurately. However, the shutoff classprovides a good basis for comparison amongvalves of similar configuration. It is not uncommonfor valve users to overestimate the shutoff classrequired.

Because tight shutoff valves generally cost moreboth in initial cost, as well as in later maintenanceexpense, serious consideration is warranted. Tightshutoff is particularly critical in high pressurevalves, considering that leakage in theseapplications can lead to the ultimate destruction of

the trim. Special precautions in seat materialselection, seat preparation and seat load arenecessary to ensure success.

Flow CapacityFinally, the criterion of capacity or size can be anoverriding constraint on selection. For extremelylarge lines, sliding-stem valves are moreexpensive than rotary types. On the other hand,for extremely small flows, a suitable rotary valvemay not be available. If future plans call forsignificantly larger flow, then a sliding-stem valvewith replaceable restricted trim may be theanswer. The trim can be changed to full size trimto accommodate higher flow rates at less cost thanreplacing the entire valve body assembly.

Rotary style products generally have much highermaximum capacity than sliding-stem valves for agiven body size. This fact makes rotary productsattractive in applications where the pressure dropavailable is rather small. However, it is of little orno advantage in high pressure drop applicationssuch as pressure regulation or letdown.

ConclusionFor most general applications, it makes senseboth economically, as well as technically, to usesliding-stem valves for lower flow ranges, ballvalves for intermediate capacities, and highperformance butterfly valves for the very largestrequired flows. However, there are numerousother factors in selecting control valves, andgeneral selection principles are not always thebest choice.

Selecting a control valve is more of and art than ascience. Process conditions, physical fluidphenomena, customer preference, customerexperience, supplier experience, among numerousother criteria must be considered in order to obtainthe best possible solution. Many applications arebeyond that of general service, and as chapter 4will present, there are of number of selectioncriteria that must be considered when dealing withthese sometimes severe flows.

Special considerations may require out-of-the-ordinary valve solutions; there are valve designsand special trims available to handle high noiseapplications, flashing, cavitation, high pressure,high temperature and combinations of theseconditions.

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After going through all the criteria for a givenapplication, the selection process may point toseveral types of valves. From there on, selectionbecomes a matter of price versus capability,coupled with the inevitable personal and

institutional preferences. As no single control valvepackage is cost-effective over the full range ofapplications, it is important to keep an open mindto alternative choices.

Table 1-2. Major Categories and Subcategories of Control Valves with Typical General Characteristics

Valve Style MainCharacteristics

Typical SizeRange,inches

TypicalStandard Body

Materials

Typical StandardEnd Connection

TypicalPressureRatings

Relative FlowCapacity

RelativeShutoff

Capability

RegularSliding-stem

Heavy DutyVersatile 1 to 24

Carbon SteelCast IronStainless

ANSI FlangedWeldedScrewed

To ANSI 2500 Moderate Excellent

Bar Stock Machined from BarStock to 3 Variety of AlloysFlangelessScrewed To ANSI 600 Low Excellent

EconomySliding-stem

Light DutyInexpensive to 2

BronzeCast Iron

Carbon SteelScrewed To ANSI 125 Moderate Good

Thru-BoreBall On-Of f Service 1 to 24

Carbon SteelStainless Flangeless To ANSI 900 High Excellent

Partial Ball Characterized forThrottling 1 to 24Carbon Steel

StainlessFlangeless

Flanged To ANSI 600 High Excellent

Eccentric Plug Erosion Resistance 1 to 8 Carbon SteelStainless Flanged To ANSI 600 Moderate Excellent

Swing-ThruButterfly No Seal 2 to 96

Carbon SteelCast IronStainless

FlangelessLuggedWelded

To ANSI 2500 High Poor

Lined Butterfly Elastomer orTFE Liner 2 to 96Carbon Steel

Cast IronStainless

FlangelessLugged To ANSI 300 High Good

HighPerformance

ButterflyOffset Disk

General Service 2 to 72Carbon Steel

StainlessFlangeless

Lugged To ANSI 600 High Excellent

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Table 1-3. Control Valve Characteristic Recommendations

Liquid Level Systems

Control Valve Pressure Drop Best InherentCharacteristic

Constant P LinearDecreasing P with increasing load, P at maximum load > 20% of minimum load P LinearDecreasing P with increasing load, P at maximum load < 20% of minimum load P Equal-percentageIncreasing P with increasing load, P at maximum load < 200% of minimum load P LinearIncreasing P with increasing load, P at maximum load > 200% of minimum load P Quick Opening

Pressure Control Systems

Application Best InherentCharacteristic

Liquid Process Equal-PercentageGas Process, Large Volume (Process has a receiver, Distribution System or Transmission Line Exceeding 100 ft. ofNominal Pipe Volume), Decreasing P with Increasing Load, P at Maximum Load > 20% of Minimum Load P LinearGas Process, Large Volume, Decreasing P with Increasing Load, P at Maximum Load < 20% of Minimum Load P Equal-PercentageGas Process, Small Volume, Less than 10 ft. of Pipe between Control Valve and Load Valve Equal-Percentage

Flow Control Processes

Application Best Inherent Characteristic

Flow Measurement Signal toController

Location of Control Valve in Relationto Measuring Element Wide Range of Flow Set Point

Small Range of Flow butLarge P Change at Valve

with Increasing LoadProportional to Flow In Series Linear Equal-Percentage

In Bypass* Linear Equal-PercentageProportional to Flow Squared In Series Linear Equal-Percentage

In Bypass* Equal-Percentage Equal-Percentage*When control valve closes, flow rate increases in measuring element.

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Table 1-4. Control Valve Leakage StandardsANSI

B16.104-1976Maximum Leakage Test Medium Pressure and Temperature

Class II 0.5% valve capacity at full travel Air Service P or 50 psid (3.4 bar differential),whichever is lower, at 50 or 125F (10 to 52C)

Class III 0.1% valve capacity at full travel Air Service P or 50 psid (3.4 bar differential),whichever is lower, at 50 or 125F (10 to 52C)

Class IV 0.01% valve capacity at full travel Air Service P or 50 psid (3.4 bar differential),whichever is lower, at 50 or 125F (10 to 52C)

Class V 5 x 10- 4

mL/min/psid/inch port dia. (5x 10- 12 m3/sec/bar/mm port dia) Water Service P at 50 or 125F (10 to 52C)

Class VI Nominal PortDiameterBubbles per

Minute mL per MinuteTest

Medium Pressure and Temperature

In1

1-1/22

2-1/23468

mm2538516476102152203

12346112745

0.150.300.450.600.901.704.006.75

AirService P or 50 psid (3.4 bar

differential), whichever is lower, at 50or 125F (10 to 52C)

Copyright 1976 Fluid Controls Institute, Inc. Reprinted with permission.

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www.Fisher.com

Chapter 2

Actuator Selection

The actuator is the distinguishing element thatdifferentiates control valves from other types ofvalves. The first actuated valves were designed inthe late 19th century. Today, they would be betterdescribed as regulators since they operateddirectly from the process fluid. These automaticvalves were the mainstay of industry through theearly 1930s.

It was at this time that the first pneumaticcontrollers were used. Development of valvecontrollers and the adaptation of standardizedcontrol signals stimulated design of the first, true,control valve actuators.

The control valve industry has evolved to fill avariety of needs and desires. Actuators areavailable with an array of designs, power sourcesand capabilities. Proper selection involves processknowledge, valve knowledge, and actuatorknowledge.

A control valve can perform its function only aswell as the actuator can handle the static anddynamic loads placed on it by the valve.Therefore, proper selection and sizing are veryimportant. Since the actuator can represent asignificant portion of the total control valve price,careful selection of actuator and accessory optionscan lead to significant dollar savings.

The range of actuator types and sizes on themarket today is so great that it seems the selectionprocess might be highly complex. With a few rulesin mind and knowledge of fundamental needs, theselection process can be simple.

The following parameters are key as they quicklynarrow the actuator choices:

Power source availability

Fail-safe requirements

Torque or thrust requirements

Control functions

Power Source AvailabilityThe power source available at the location of avalve can often point directly to what type ofactuator to choose. Typically, valve actuators arepowered either by compressed air or by electricity.However, in some cases water pressure, hydraulicfluid, or even pipeline pressure can be used.

Since most plants have both electricity andcompressed air readily available, the selectiondepends upon the ease and cost of furnishingeither power source to the actuator location.Reliability and maintenance requirements of thepower system must also be considered.Consideration should also be given to providingbackup operating power to critical plant loops.

Fail-safe RequirementsThe overall reliability of power sources is quitehigh. However, many loops demand specific valveaction should the power source ever fail. Desiredaction upon a signal failure may be required forsafety reasons or for protection of equipment.

Fail-safe systems store energy, eithermechanically in springs, pneumatically in volumetanks, or in hydraulic accumulators. When powerfails, the fail-safe systems are triggered to drivethe valves to the required position and to thenmaintain this position until returned to normaloperation. In many cases, the process pressure isused to ensure or enhance this action.

22

Actuator designs are available with a choice offailure mode between failing open, failing closed,or holding in the last position. Many actuatorsystems incorporate failure modes at no extracost. For example, spring-and-diaphragmactuators are inherently fail open or closed, whileelectric operators typically hold their last position.

Torque or Thrust RequirementsAn actuator must have sufficient thrust or torquefor the prescribed application. In some cases thisrequirement can dictate actuator type as well aspower supply requirements.

For instance, large valves requiring a high thrustmay be limited to only electric or electro-hydraulicactuators due to a lack of pneumatic actuators withsufficient thrust capability. Conversely,electro-hydraulic actuators would be a poor choicefor valves with very low thrust requirements.

The matching of actuator capability with valvebody requirements is best left to the control valvemanufacturer as there are considerabledifferences in frictional and fluid forces from valveto valve.

Control FunctionsKnowledge of the required actuator functions willmost clearly define the options available forselection. These functions include the actuatorsignal (pneumatic, electric, etc.), signal range,ambient temperatures, vibration levels, operatingspeed, frequency, and quality of control that isrequired.

Signal types are typically grouped as such:

Two-position (on-off)

Analog (throttling)

Digital

Two-position electric, electro-pneumatic, orpneumatic switches control on-off actuators. Thisis the simplest type of automatic control and theleast restrictive in terms of selection.

Throttling actuators have considerably higherdemands put on them from both a compatibilityand performance standpoint. A throttling actuatorreceives its input from an electronic or pneumaticinstrument that measures the controlled processvariable. The actuator must then move the finalcontrol element in response to the instrumentsignal in an accurate and timely fashion to ensureeffective control. The two primary additionalrequirements for throttling actuators include:

Compatibility with instrument signal

Better static and dynamic performance toensure loop stability

Compatibility with instrument signals is inherent inmany actuator types, or it can be obtained withadd-on equipment. But, the high-performancecharacteristics required of a good throttlingactuator cannot be bolted on; instead, lowhysteresis and minimal deadband must bedesigned into actuators.

Stroking speed, vibration, and temperatureresistance must also be considered if critical to theapplication. For example, on liquid loopsfast-stroking speeds can be detrimental due to thepossibility of water hammer.

Vibration or mounting position can be a potentialproblem. The actuator weight, combined with theweight of the valve, may necessitate bracing.

It is essential to determine the ambienttemperature and humidity that the actuator willexperience. Many actuators contain eitherelastomeric or electronic components that can besubject to degradation by high humidity ortemperature.

EconomicsEvaluation of economics in actuator selection is acombination of the following:

Cost

Maintenance

Reliability

A simple actuator, such as aspring-and-diaphragm, has few moving parts andis easy to service. Its initial cost is low, andmaintenance personnel understand and arecomfortable working with them.

23

An actuator made specifically for a control valveeliminates the chance for a costly performancemismatch. An actuator manufactured by the valvevendor and shipped with the valve will eliminateseparate mounting charges and ensure easiercoordination of spare parts procurement.Interchangeable parts among varied actuators arealso important to minimize spare-parts inventory.

Actuator DesignsThere are many types of actuators on the market,most of which fall into five general categories:

Spring-and-diaphragm

Pneumatic piston

Rack and Pinion

Electric motor

Electro-hydraulic

Each actuator design has weaknesses, strongpoints and optimum uses. Most actuator designsare available for either sliding stem or rotary valvebodies. They differ only by linkages or motiontranslators; the basic power sources are identical.

Most rotary actuators employ linkages, gears, orcrank arms to convert direct linear motion of adiaphragm or piston into the 90-degree outputrotation required by rotary valves. The mostimportant consideration for control valve actuatorsis the requirement for a design that limits theamount of lost motion between internal linkageand valve coupling.

Rotary actuators are now available that employtilting pistons or diaphragms. These designseliminate most linkage points (and resultant lostmotion) and provide a safe, accurate and enclosedpackage.

When considering an actuator design, it is alsonecessary to consider the method by which it iscoupled to the drive shaft of the control valve.Slotted connectors mated to milled shaft flats aregenerally not satisfactory if any degree ofperformance is required. Pinned connections, ifsolidly constructed, are suitable for nominal torqueapplications. A splined connector that mates to a

splined shaft end and then is rigidly clamped to theshaft eliminates lost motion, is easy todisassemble, and is capable of high torque.

Sliding stem actuators are rigidly fixed to valvestems by threaded and clamped connections.Because they dont have any linkage points, andtheir connections are rigid, they exhibit no lostmotion and have excellent inherent controlcharacteristics.

Spring-and-Diaphragm ActuatorsThe most popular and widely used control valveactuator is the pneumatic spring-and-diaphragmstyle. These actuators are extremely simple andoffer low cost and high reliability. They normallyoperate over the standard signal ranges of 3 to 15psi or 6 to 30 psi, and therefore, are often suitablefor throttling service using instrument signalsdirectly.

Many spring-and-diaphragm designs offer eitheradjustable springs and/or wide spring selections toallow the actuator to be tailored to the particularapplication. Because they have few moving partsthat may contribute to failure, they are extremelyreliable. Should they ever fail, maintenance isextremely simple. Improved designs now includemechanisms to control the release of springcompression, eliminating possible personnel injuryduring actuator disassembly.

Use of a positioner or booster with aspring-and-diaphragm actuator can improvecontrol, but when improperly applied, can result inpoor control. Follow the simple guidelinesavailable for positioner applications and look for:

Rugged, vibration-resistant construction

Calibration ease

Simple, positive feedback linkages

The overwhelming advantage of thespring-and-diaphragm actuator is the inherentprovision for fail-safe action. As air is loaded onthe actuator casing, the diaphragm moves thevalve and compresses the spring. The storedenergy in the spring acts to move the valve back toits original position as air is released from thecasing. Should there be a loss of signal pressureto the instrument or the actuator, the spring canmove the valve to its initial (fail-safe) position.

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DIAPHRAGM

DIAPHRAGM CASING

DIAPHRAGMPLATE

ACTUATOR SPRING

ACTUATOR STEM

SPRING SEAT

SPRING ADJUSTOR

STEM CONNECTOR

YOKE

TRAVEL INDICATOR DISK

INDICATOR SCALE

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LOWER DIAPHRAGM CASING

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Figure 2-1. Spring-and-diaphragm actuators offer an excellent first choice for most control valves. They are inexpensive, simple and have built-in, fail-safe action. Pictured above are cutaways of the popular

Fisher 667 (left) and Fisher 657 (right) actuators.

Figure 2-2. Spring-and-diaphragm actuatorscan be supplied with a top-mounted handwheel.

The handwheel allows manual operation and alsoacts as a travel stop or means of emergency

operation.

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Actuators are available for either fail-open orfail-closed action. The only drawback to thespring-and-diaphragm actuator is a relativelylimited output capability. Much of the thrustcreated by the diaphragm is taken up by the springand thus does not result in output to the valve.

Therefore, the spring-and-diaphragm actuator isused infrequently for high force requirements. It isnot economical to build and use very largespring-and-diaphragm actuators because the size,weight and cost grow exponentially with eachincrease in output force capability.

Piston ActuatorsPiston actuators are generally more compact andprovide higher torque or force outputs thanspring-and-diaphragm actuators. Fisher pistonstyles normally work with supply pressuresbetween 50 and 150 psi and can be equipped withspring returns (however, this construction haslimited application).Piston actuators used for throttling service must befurnished with double-acting positioners thatsimultaneously load and unload opposite sides ofthe piston. The pressure differential created acrossthe piston causes travel toward the lower pressureside. The positioner senses the motion, and whenthe required position is reached, the positionerequalizes the pressure on both sides of the piston.

The pneumatic piston actuator is an excellentchoice when a compact unit is required to producehigh torque or force. It is also easily adapted to

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Figure 2-3. The Fisher 2052 spring-and-diaphragm actuator has many features to provideprecise control. The splined actuator connectionfeatures a clamped lever and single-joint linkage

to help eliminate lost motion.

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Figure 2-4. Double-acting piston actuators suchas the Fisher 1061 rotary actuator are a goodchoice when thrust requirements exceed thecapability of spring-and-diaphragm actuators.

Piston actuators require a higher supply pressure,but have benefits such as high stiffness and small

size. The 1061 actuator is typically used forthrottling service.

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services where high ambient temperatures are aconcern.

The main disadvantages of piston actuators arethe high supply pressures required for positionerswhen used in throttling service and the lack offail-safe systems.

Figure 2-5. Spring fail-safe is present in thispiston design. The Fisher 585C is an example ofa spring-bias piston actuator. Process pressurecan aid fail-safe action, or the actuator can be

configured for full spring-fail closure.

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Figure 2-6. Since the requirements for accuracyand minimal lost motion are unnecessary for

on-off service, cost savings can be achieved by simplifying the actuator design. The Fisher

1066SR incorporates spring-return capability.

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There are two types of spring-return pistonactuators available. The variations are subtle, butsignificant. It is possible to add a spring to a piston

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actuator and operate it much like a spring-and-diaphragm. These designs use a single-actingpositioner that loads the piston chamber to movethe actuator and compress the spring. As air isunloaded, the spring forces the piston back. Thesedesigns use large, high output springs that arecapable of overcoming the fluid forces in the valve.

The alternative design uses a much smaller springand relies on valve fluid forces to help provide thefail-safe action. In normal operation they act like adouble action piston. In a fail-safe situation thespring initiates movement and is helped byunbalance forces on the valve plug. Theseactuators can be sized and set up to provide fullspring closure action without process assistance.

An alternative to springs is a pneumatic tripsystem which often proves to be complex indesign, difficult to maintain and costly. While a tripsystem is completely safe, any fail-saferequirement consideration should be given first tospring-and-diaphragm operators if they arefeasible.

Special care should be given during the selectionof throttling piston actuators to specify a designthat has minimal hysteresis and deadband. As thenumber of linkage points in the actuator increases,so does the deadband. As the number of slidingparts increases, so does the hysteresis. Anactuator with high hysteresis and deadband canbe quite suitable for on-off service; however,caution is necessary when attempting to adapt thisactuator to throttling service by merely bolting on apositioner.

The cost of a spring-and-diaphragm actuator isgenerally less than a comparable piston actuator.Part of this cost saving is a result of the ability touse instrument output air directly, therebyeliminating the need for a positioner. The inherentprovision for fail-safe action in the spring-and-diaphragm actuator is also a consideration.

Rack and Pinion Actuators

Rack and pinion actuators may come in adouble-acting design, or spring return, and are acompact and economical solution for rotary shaftvalves. They provide high torque outputs and aretypically used for on-off applications with highcycle life. They may also be used in processeswhere higher variability is not a concern.

Figure 2-7. The FieldQ actuator is a quarterturn pneumatic rack and pinion actuator. It comeswith an integrated module combining the solenoid

and switchbox into a low profile, compactpackage.

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Electric ActuatorsElectric actuators can be applied successfully inmany situations. Most electric operators consist ofmotors and gear trains and are available in a widerange of torque outputs, travels, and capabilities.They are suited for remote mounting where noother power source is available, for use wherethere are specialized thrust or stiffnessrequirements, or when highly precise control isrequired.

Electric operators are economical versuspneumatic actuators for applications in small sizeranges only. Larger units operate slowly and weighconsiderably more than pneumatic equivalents.Available fail action is typically lock in last position.

One key consideration in choosing an electricactuator is its capability for continuous closed-loopcontrol. In applications where frequent changesare made in control-valve position, the electricactuator must have a suitable duty cycle.

High performance electric actuators usingcontinuous rated DC motors and ball screw outputdevices are capable of precise control and 100%duty cycles.

Compared to other actuator designs, the electricactuator generally provides the highest output

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available within a given package size. Additionally,electric actuators are stiff, that is, resistant to valveforces. This makes them an excellent choice forgood throttling control of large, high-pressurevalves.

Actuator SizingThe last step in the selection process is todetermine the required actuator size.Fundamentally, the process of sizing is to matchas closely as possible the actuator capabilities tothe valve requirements.

In practice, the mating of actuator and valverequires the consideration of many factors. Valveforces must be evaluated at the critical positions ofvalve travel (usually open and closed) andcompared to actuator output. Valve forcecalculation varies considerably between valvestyles and manufacturers. In most cases it isnecessary to consider a complex summation offorces including:

Static fluid forces

Dynamic fluid forces and force gradients

Friction of seals, bearings, and packing

Seat loading

Although actuator sizing is not difficult, the greatvariety of designs on the market and the readyavailability of vendor expertise (normally at nocost) make detailed knowledge of the proceduresunnecessary.

Actuator Spring for Globe ValvesThe force required to operate a globe valveincludes:

A. Force to overcome static unbalance of thevalve plug

B. Force to provide a seat load

C. Force to overcome packing friction

D. Additional forces required for certain specificapplications or constructions

Total force required = A + B + C + D

A. Unbalance ForceThe unbalance force is that resulting from fluidpressure at shutoff, and in the most general sensecan be expressed as:

Unbalance force = net pressure differential X netunbalance area

Frequent practice is to take the maximumupstream gauge pressure as the net pressuredifferential unless the process design alwaysensures a back pressure at the maximum inletpressure. Net unbalance area is the port area on asingle seated flow up design. Unbalance area mayhave to take into account the stem area dependingon configuration. For balanced valves there is stilla small unbalance area. This data can be obtainedfrom the manufacturer. Typical port areas forbalanced valves flow up and unbalanced valves ina flow down configuration are listed in table 2-1.

Table 2-1. Typical Unbalance Areas of Control Valves

Port Diameter,Inches

Unbalance AreaSingle-SeatedUnbalancedValves, In2

Unbalance AreaBalanced Valves,

In2

1/4 0.049 3/8 0.110 1/2 0.196 3/4 0.441 1 0.785

1 5/16 1.35 0.041 7/8 2.76 0.062

2 5/16 4.20 0.273 7/16 9.28 0.1184 3/8 15.03 0.154

7 38.48 0.818 50.24 0.86

B. Force to Provide Seat LoadSeat load, usually expressed in pounds per linealinch or port circumference, is determined byshutoff requirements. Use the guidelines in table2-2 to determine the seat load required to meetthe factory acceptance tests for ANSI/FCI 70-2and IEC 534-4 leak Classes II through VI.

Because of differences in the severity of serviceconditions, do not construe these leakclassifications and corresponding leakage rates asindicators of field performance. To prolong seat lifeand shutoff capabilities, use a higher thanrecommended seat load. If tight shutoff is not aprime consideration, use a lower leak class.

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Table 2-2. Recommended Seat Load Per Leak Class forControl Valves

Class I As required by customerspecification, no factory leak testrequired

Class II 20 pounds per lineal inch of portcircumference

Class III 40 pounds per lineal inch of portcircumference

Class IV Standard (Lower) Seat only40pounds per lineal inch of portcircumference (up through a43/8 inch diameter port)Standard (Lower) Seat only80pounds per lineal inch of portcircumference (larger than 43/8inch diameter port)

Class V Metal Seatdetermine poundsper lineal inch of portcircumference from figure 2-9

C. Packing FrictionPacking friction is determined by stem size,packing type, and the amount of compressive loadplaced on the packing by the process or thebolting. Packing friction is not 100% repeatable inits friction characteristics. Newer live loadedpacking designs can have significant friction forcesespecially if graphite packing is used. Table 2-3lists typical packing friction values.

D. Additional ForcesAdditional forces