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Edward Valves Pressure Locking and Over-Pressurization

of Double-Seated Valves V-Rep 91-1

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ABSTRACTDouble-seated valves provide benefits overvalves with single seats and create few dis-advantages. However, field experiencereveals that trapped pressure in the centercavity of these valves can be a source ofoccasional problems. Because actuatorsare normally sized based primarily ondownstream loading, trapped pressureloading on both seats may greatly exceeddesign limitations and cause the valve tobecome pressure locked. Trapped pressuremay also damage valves if over-pressuriza-tion occurs, through heating or freezing ofthe line fluid. Improvements in packing,bonnet seals, and seats have reduced themeans of pressure relief in the center cavi-ty. Fortunately, operating procedures andspecial design features may be employedto relieve trapped pressure and preventboth pressure locking and over-pressuriza-tion.

IntroductionPlug valves, ball valves, and gate valvesusually have two seats in series. There aremany sub-types of each of these valves,and there is considerable variation in thesealing effectiveness of upstream anddownstream seats in different designs. Twoseats in series are often thought of as anadvantage, and they sometimes providegenuine benefits. When a valve is opened,closed, or throttled with a high differentialpressure, upstream and downstream portsprovide two orifices in series to reduce thepressure drop across each port (Figure 1 ).When closed, two seats in series may pro-vide a sealing advantage in the event oneof the seats is damaged.While problems with double-seated valvesare not usually common or serious, thereare some rather simple characteristics ofthese valves that should be considered bysystem designers as well as valve users.Failure to attend to these characteristicsmay lead to valve malfunction or, in somecases, hazardous conditions.

Figure 1:Horizontal cross section of throttled ball

valve showing two orifices in series.

Seat Sealing EffectivenessBefore discussing potential problems withdouble-seated valves, it may be helpful toconsider the forces that influence the effec-tiveness of the individual seats. Figure 2illustrates simplified force balances onclosed floating ball and gate valves. Notethat the differential pressure acting acrossthe closure member (ball or gate) providesa sealing force on the downstream seat atthe low pressure side of the valve. Thus,these valves are sometimes referred to asdownstream sealing.

Pressure Locking and Over-Pressurization of Double-Seated ValvesE. A. BakeResearch ManagerEdward ValvesFirst Published 1991

a.

b.

Figure 2:Simplified force balance diagram onclosed valves:a. Floating ball valve

b. Wedge gate valve

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Since the seating forces due to differentialpressure may be inadequate to produceeffective tight sealing when differentialpressure is low, it is common practice toassemble floating ball valves with interfer-ence (e.g. controlled squeeze on softseats) which provides some upstream seat-ing force. In wedge gate valves, as illus-trated, stem load and wedging produce aloading on upstream seats. Gate valveswith parallel seats sometimes employsprings or internal wedges to provide sup-plemental loading for low pressure sealing.Thus, while all of these valves are normallyconsidered as downstream seating, there is

usually moderate to significant loadingwhich provides some sealing effectivenessof upstream seats as well.The sealing of two seats in series can pro-duce a pressure trapping condition which,in turn, can produce an increase in theforce required to open a valve pressurelocking. Heating or freezing of trappedfluid between the two seats may also pro-duce conditions that are hazardous topressure boundary parts of a valve.

Pressure TrappingReferring toFigure 3 , consider the sce-nario which accompanies and follows clo-sure of a double-seated valve. A flexiblewedge gate valve is shown in the illustra-tion, but other double-seated valves reactsimilarly to one degree or another.If the valve is closed with a high line pres-sure and the downstream pressure decreas-es slowly, the pressure in the valve centercavity, Pc, immediately after closure wouldbe substantially the upstream line pressure,

P1. If the valve is closed under a flowingcondition, the center cavity pressure afterclosure might be less than the upstreampressure due to Bernoulli effects or pres-sure drop across the upstream port duringclosure.The behavior of the center cavity pressureafter closure depends on many things, suchas (1) temperature, (2) type of line fluid,and (3) individual valve design characteris-tics. For example: If the line fluid is either a liquid or gas at

ambient temperature (and there is nosubsequent temperature change), the

pressure Pc would remain constant unlessthere is seat leakage.

If the downstream seat leaks to thedepressurized side of the valve, thecenter cavity pressure would decrease,but, since the upstream seat often sealsless effectively than the downstream seat,it could feed the center cavity to main-tain the pressure. Precise prediction ofthis pressure after closure is impossible,but it would tend to remain near theupstream pressure.

If the line fluid is at an elevated tempera-ture, the valve would normally tend tocool and approach ambient temperatureafter closure and cessation of flow. Thefluid in the center cavity would tend tocontract, reducing Pc. The behavior willbe affected significantly by the type ofline fluid: If the fluid is a liquid (e.g. hot water),

the pressure would decrease rapidly,possibly approaching a vacuum andproducing a vapor cavity. If theupstream seat is not effective at highdifferential pressure, leakage into thecenter cavity would tend to offset thispressure decrease in time. Precise pre-diction of P

cis not possible, but the

center cavity would normally be liquidfilled after cool-down, with a pressurenear the upstream pressure P1.

If the fluid is a gas (e.g. hot air), thepressure would tend to decrease slow-ly in proportion to the absolute temper-ature. As in the case of closure with aliquid, upstream seat leakage wouldtend to offset this decrease, and thecenter cavity would be filled with gasafter cool-down at a pressure near P1.

Pressure Locking and Over-Pressurization of Double-Seated Valves

Figure 3:Horizontal cross section of closed flexi-ble wedge gate valve, showingupstream pressure P1 and center cavitypressure Pc.

P1

Pc

P=0

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If the fluid is a condensable vapor (e.g.steam), the process in the center cavityafter closure is complicated by conden-sation when the pressure decreases tothe saturation pressure of the trappedline fluid. As in the case of closure witha liquid or gas, upstream seat leakagewould tend to offset the decrease in Pcdue to cooling, but the incoming leak-age would also condense.In the vapor case, prediction of centercavity pressure after cool-down isagain impossible, but it would normal-ly be near P1 Significantly, the centercavity would be partially or completelyfull of condensate (e.g. water), evenwith a vapor in the upstream line.

To explore the pressure trapping issuefurther, consider what happens if the system(boiler, chemical process) containing aclosed double-seated valve is shut down,letting P1 go to zero. The fluid left in thecenter cavity after closure (above) would betrapped indefinitely until relieved by leak-age. If a valve in a hot line cools off furtherafter shutdown, pressure may be relievedby contraction of liquid or condensation ofvapor, but significant pressure might remaintrapped in the valve cavity in some cases;in the case of a gas, the stored energywould remain significant. This could occurwith any type of double-seated valve; asshown in Figure 4, both seats in some typesof valves can be loaded by the center cavi-ty pressure Pc, thus enhancing seat tightnessand increasing probability of long-termpressure trapping.

Since the pressure trapped within double-seated valves may not be foreseen, twoconditions might present hazards duringshutdown if not properly attended to:1. Leakage from the trapped center cavity

might complicate maintenance on near-by equipment. This could be of concernif the trapped fluid is flammable or haz-ardous in any other way.

2. Opening of the valve during shutdowncould cause an unexpected energyrelease due to relief of trapped pressureto the upstream and downstream lines.This would be of particular concern with

trapped gas.Pressure LockingPressure trapping in the center cavity ofsome valve types can lead to another prob-lem if not foreseen and attended to. In thecase of the gate valves illustrated in Figure 4 , note that the seating loads applied toboth seats would tend to increase the stemload required for opening.Gate valves and actuators are normallydesigned for opening with a differential

pressure applied as shown in Figure 2,with loading primarily on the downstreamseat. The load required for opening isobviously a function of the seat area, thedifferential pressure, and the friction coeffi-cients at sliding interfaces on seats andguides. Actuators may not be sized toopen a valve against full system pressureP1if operation is required only for compo-nent isolation during shutdown.Since the seating loads shown in Figure 4are applied to both seats, the opening

force with a trapped pressure Pc may beas much as twice that required with thesame pressure as an upstream pressure. If

Pressure Locking and Over-Pressurization of Double-Seated Valves

Figure 4:Cross sections of closed valves to showseat sealing loads with both sides ofvalve de-pressurized:a. Flexible wedge gate valveb. Split wedge gate valvec. Parallel slide valve

a.

b.

c.

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alone due to Charles law. Since the vol-ume of the liquid expands with heating,the volume available for gas contracts,tending to make the combination morecompletely liquid on a volume basis. Forexample, pressurized water increases involume by approximately 16% if heatedfrom 70 to 400 F, so an air bubbleoccupying 10% of the volume of the cen-ter cavity of a cool, de-pressurized valvewould not be of much help if subjectedto significant heating.

Valve stem packing leakage will prevent excessive pressure increase.Hardasbestos-base packings like those usedwith older high pressure valves oftenleaked moderately at pressures around15 times the valve pressure rating, andthis would limit the pressure increase dueto liquid thermal expansion. Newergraphitic stem packings typically sealmuch better; this is normally consideredan improvement, but it increases the riskof over-pressurization.Note: Some experienced engineers rec- ommend loosening packing gland boltswhen closed gate valves are heated.However, Edward experience shows that previously compressed graphitic pack- ings often continue to seal well on a sta- tionary stem, even with a loose gland.Thus, loose gland bolts will not ensure safety when a valve is heated.

Valve cover gasket leakage will prevent excessive pressure increase.Conventional low pressure valves withbolted bonnets and compression gasketsmay leak if internal pressure overcomes

the bolt preload and causes bolt stretch,However, pressure-seal bonnets common-ly used on higher pressure valves typical-ly seal tighter with increasing body pres-sure.

Seat leakage, upstream or downstream, will prevent excessive pressure increase.Some old design, solid wedge gatevalves had difficulty sealing simultane-ously on both the upstream and down-stream body seats because of dimension-al inaccuracies or inability to accommo-date valve body flexure. Some flexiblewedge gate valves also lack sufficientflexibility to seal well at both sides.However, some modern flexible wedgevalves and most two-piece wedge andparallel seat gate valves have sufficientfreedom to seat upstream and down-stream, particularly with high center cavi-ty pressure. Thus, seat leakage cannotbe counted upon to prevent over-pressur-ization.

Some Cases Of Field ProblemsSeveral instances have shown that haz-ardous pressures may be developed invalves with trapped liquids where tempera-ture increases of several hundred degreesFahrenheit may occur. In one power plantexperience, all internal dimensions in ahigh pressure gate valve were found tohave exhibited a permanent strain (yield)of over one percent after it had undergonea boiler startup, while closed, with watertrapped between the seats (from a priorhydrostatic piping test). Since the valveitself had passed a hydrostatic test withoutdamage at, 15 times its 100 F pressure

rating after manufacture, it has been esti-mated that the pressure was 2 to 3 timesthe valve rating when it was heated in thestart-up incident,The valve described above had failed toopen due to pressure locking (see above),and the damage due to over-pressurizationwas not recognized until the valve wasinspected to investigate the failure to open.There was no catastrophe, but the pressurehad exceeded the valve rating by so muchthat it is obvious that the safety factor wassignificantly reduced. This valve was sal-vaged and returned to service, but exten-sive rework was necessary to repair thedamage that had occurred due to over-pressurization.In another power plant experience, minorexternal damage was observed in a gatevalve during plant construction. The valvewas outside, and it was closed while.water-filled after hydrostatic testing. Anunexpected cold weather condition pro-duced an ambient temperature below thefreezing point of water. Internal inspectionof the valve after this incident revealedsevere yield of pressure boundary parts,not unlike the conditions observed in thevalve that had been overpressurized(above). This valve was also salvaged andreturned to service, but extensive reworkwas required.

Requirements Of Codes And StandardsA number of codes and standards addresspossibilities of over-pressurization from asafety standpoint. For example pipe couldbe damaged by heating of trapped fluid

Pressure Locking and Over-Pressurization of Double-Seated Valves

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between two closed valves or in an isolat-ed dead-end run. The Power Piping code,ASME B31.1, requires piping design to

withstand the increased pressure or a pro-vision for pressure relief where expansionof a fluid may increase the pressure (para-graph 101.4.2). Since valves are recog-nized as piping components, this coderequirement may be interpreted to requirethe piping designer to consider possiblethermal expansion in double-seated valves.With specific reference to valves,ASME/ANSI B16.34 (Valves Flanged,Threaded, and Welding End ) referencesdouble-seated valves and states that it isthe responsibility of the purchaser to pro-vide means to assure that the allowablepressure is not exceeded due to thermalexpansion (paragraph 2.3.3). Thesemeans may involve design, installation, oroperating procedures. For example, partialopening of a double-seated valve duringstart-up is cited as a procedural method toaddress the problem.Since codes primarily cover pressureboundary safety, not functional reliability,the possibility of failure of a valve to opendue to pressure locking is not addressed inany known binding standard. Since thecapacity of an actuator to open a valvemight be exceeded by pressure lockingwithout exceeding a safe pressure bound-ary rating, system designers and valveusers should consider this point carefully.

Prevention Of Pressure Lock AndHazardous Center Cavity PressureThere is, at this time, no known singleideal means of preventing pressure lock

or hazardous center cavity pressures indouble-seated valves. Nevertheless, asnoted at the outset, there are few serious

problems. Most piping designers and valveusers address the problem in accordancewith the standards mentioned above toavoid safety hazards, and INPO SOER84-7 provides good guidelines on avoid-ance of pressure lock to nuclear powerplant operators.While the methods for avoiding problemsare not new to many people in the valveand piping industries, the fact that prob-lems are still encountered occasionally sug-gests that these solutions are not knownwidely enough. The specific functionalrequirements of a valve should be consid-ered carefully in selecting the bestapproach. Several of the measures in com-mon use and their advantages and disad-vantages will be addressed: Operating procedures

As suggested in ASME/ANSI B16.34,special plant operating procedures, suchas partially opening doubleseated valvesbefore starting a heat-up of a piping sys-tem, may be quite effective in preventinghazardous over-pressurization.This approach requires (1) identificationof valves that may be most hazardous,with trapped liquid, (2) cracking valvesaway from fully seated positions withhandwheels or actuators before start-up,and (3) resealing of valves that mustremain closed after heating. This mayproduce unacceptable leakage in somecomponent isolation valves, and the leak-age during start-up may cause seat dam-

age due to erosion. There might still bepressure locking problems during shut-down with both sides of valves depres-

surized. However, no special valve fea-tures are required, and the valve retainsbidirectional shutoff capabilities.

Hole in upstream side of gate or ballA hole can be drilled in the upstream

side of a flexible wedge, two-piecewedge, or parallel seat valve gate asshown in Figure 5 ; a similar change canbe effected in the ball in a floating ballvalve.This change effectively bypasses theupstream seat, assuring that the centercavity pressure is the same as theupstream pressure. Since these valvesare primarily downstream sealing, theonly loss in normal unidirectional appli-cations is the extra insurance that wouldbe provided by the upstream seat in theevent of downstream seat damage.

Pressure Locking and Over-Pressurization of Double-Seated Valves

Figure 5:Illustration of hole in upstream gate of

gate valve.

HOLE

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While this change may be the simplestone for correction of a field problem orfor use of a standard valve out of stock,

one disadvantage is that the gate orgate assembly cannot be reversed;reversing a gate assembly is one com-mon method of extending life of partsafter extensive valve maintenance, butplacing the gate with the hole on thedownstream side would cause leakage.As in the case of following changes withinternal and external equalizers, the holein the upstream part of the gate makesthe valve unidirectional to some degree.High leakage would be experiencedwith many valves if the, differentialpressure is ever reversed, although theamount of leakage may be limited by the

orifice size used for the hole. Somewedge gate valves seal reasonably wellat their upstream seats with low and

moderate pressures, so a valve with ahole might be suitable for some reversedpressure conditions. High reversed pres-sures would probably cause significantleakage in all gate valves and mightover-stress gates not designed for highdifferential on the upstream side. If thischange is to be made to correct a prob-lem with an installed valve, the manufac-turers recommendations should beobtained.

Hole in seat insert or body bridge wall at upstream side (internal equalizer)

A hole in the valve body or seat insert,as shown in Figure 6, bypasses theupstream seat in the same manner as a

hole in the upstream side of a gate. Asimilar change could probably be effect-ed in a ball valve.As compared to the hole in the gate, thischange is preferable from, a long termmaintenance standpoint, because thegate or gate assembly could be reversedfor extended life. The body drillingwould be difficult in an installed valve,but it is easily accomplished duringmanufacturer of a new valve. The onlyconcern is to be sure that the valve ismarked and installed in the properdirection.This feature might be difficult to modifyin a valve in line if there were ever asystem change that would require seal-ing in the reversed direction.

Pipe connecting center cavity to upstream valve nozzle (external equalizer) without or with valveThe external equalizer pipe, shown inFigure 7a, also bypasses the upstream

seat. The directional nature of the valveshould be obvious to one viewing thevalve. It can be incorporated in a newvalve, or with more difficulty to aninstalled valve. This feature would be rel-atively easy to modify in line if there is asystem change requiring reversed seattightness.As shown inFigure 7b , a valve (typicallya small globe valve) can be installed inthe external equalizer and be left nor-mally open (locked open if necessary) to

Pressure Locking and Over-Pressurization of Double-Seated Valves

Figure 7:Illustration of pipe between gate valvecenter cavity and upstream nozzle(external equalizer).a. Without valve

b. With valve

Figure 6:Illustration of hole in upstream seatring or bridge wall in gate valve

(internal equalizer).

HOLEIN BRIDGE

WALL

HOLE INSEAT RING

a.

b.

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provide the upstream seat bypassingfunction. Closure of the small valvewould permit reversed seat tightness for

special purposes (e.g. hydrostatic testingof downstream piping); obviously proce-dures would have to be enforced toassure opening of the valve before nor-mal operation.

Connect pressure relief valve to centercavity and connect with piping to a safedischarge point.A relief valve may be used, as shown inFigure 8 , to prevent hazardous over-pres-surization of double-seated valve bodies.Codes and standards applicable to over-pressure protection devices have to bechecked and complied with. The setpoint has to be enough above the high-est normal operating pressure to preventweepage and inadvertent operation.Since this approach requires an activecomponent (the relief valve), it is not rec-ommended unless the preceding passiveapproaches are not acceptable.Since the valve center cavity could trap apressure up to the relief valve set point,pressure locking could still interfere withvalve opening with reduced upstreampressures. Thus the relief valve wouldoffer pressure boundary safety, but itwould not ensure operational reliability.

Summary Double-seated valves have established his-tories of safe and successful operation in amultitude of applications in general indus-trial service, as well as in power plants,refineries, and petrochemical facilities. Thetwo seats provide benefits, but some rela-tively simple technical issues should beconsidered to prevent problems due totrapped fluid between the seats. Piping sys-tem designers, valve users, and valve man-ufacturers should cooperate to establish thebest measures to assure reliability, safety,and sealing effectiveness if a potentialproblem is identified. Since individualvalve applications involve specific function-al requirements, a combination of the mea-sures cited above (operating proceduresand design features) may be necessary toprovide best performance.

Footnotes1. Trunnion mounted ball valves, as

commonly used in pipeline services,employ trunnions to support the maindifferential pressure loading, and theyare normally upstream sealing. Thesevalves are outside the scope of thisarticle.

2. Institute Of Nuclear Power Operations(INPO), Significant OperatingExperience Report (84-7, PressureLocking and Thermal Binding of GateValves, December 14, 1984.

Pressure Locking and Over-Pressurization of Double-Seated Valves

Figure 8:Illustration of relief valve connected togate valve center cavity.

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Flowserve Corporation has established industry leadership in the design and manufacture of its products. When properly selected, this Flowserve product is designed to perform its intended function safely during its useful life. However,the purchaser or user of Flowserve products should be aware that Flowserve products might be used in numerous applications under a wide variety of industrial service conditions. Although Flowserve can (and often does) provide general

guidelines, it cannot provide specific data and warnings for all possible applications. The purchaser/user must therefore assume the ultimate responsibility for the proper sizing and selection, installation, operation, and maintenance ofFlowserve products. The purchaser/user should read and understand the Installation Operation Maintenance (IOM) instructions included with the product, and train its employees and contractors in the safe use of Flowserve products inconnection with the specific application.

While the information and specifications contained in this literature are believed to be accurate, they are supplied for informative purposes only and should not be considered certified or as a guarantee of satisfactory results by reliancethereon. Nothing contained herein is to be construed as a warranty or guarantee, express or implied, regarding any matter with respect to this product. Because Flowserve is continually improving and upgrading its product design, thespecifications, dimensions and information contained herein are subject to change without notice. Should any question arise concerning these provisions, the purchaser/user should contact Flowserve Corporation at any one of its world-wide operations or offices.

2003 Flowserve Corporation, Irving, Texas, USA. Flowserve and Edward Valves are registered trademarks of Flowserve Corporation. V-Rep 91-1 3/03 Printed in USA

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