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Bolted Bonnets vs.Pressure SealsTo better understand the pressure seal design concept,lets contrast the body-to-bonnet sealing mechanismbetween BBVs and pressure seals. Figure 1 depicts thetypical BBV. The body flange and bonnet flange arejoined by studs and nuts, with a gasket of suitabledesign/material inserted between the flange faces to

facilitate sealing. Studs/nuts/bolts are tightened to pre-scribed torques in a pattern defined by the manufacturerto affect optimal sealing. However, as system pressureincreases, the potential for leakage through thebody/bonnet joint also increases.

Now lets look at the pressure seal joint detailed inFigure 2. Note the differences in the respectivebody/bonnet joint configurations. Most pressure sealdesigns incorporate bonnet take-up bolts to pull thebonnet up and seal against the pressure seal gasket.This in turn creates a seal between the gasket and theinner diameter (I.D.) of the valve body. A segmented

thrust ring maintains the load. The beauty of the pres-sure seal design is that as system pressure builds, sodoes the load on the bonnet and, correspondingly, thepressure seal gasket. Therefore, in pressure seal valves,as system pressure increases, the potential for leakagethrough the body/bonnet joint decreases.

This design approach has distinct advantages overBBVs in main steam, feedwater, turbine bypass, andother power plant systems requiring valves that can handle the challenges inher-ent in high-pressure and temperature applications. However, due to its reliance onsystem pressure to aid in sealing, pressure seal valves are best applied in systemswhere the minimum, consistent operating pressure is in excess of 500 psi.

IN THE TOUGH WORLD OF HIGH-PRESSURE,

HIGH-TEMPERATURE VALVE APPLICATIONS,

PRESSURE SEAL GATE,GLOBE,AND CHECK

VALVES CONTINUE TO PROVIDE A WIDERANGE OF INDUSTRIES WITH A SAFE,LEAK-

FREE, PRESSURE-CONTAINING BOUNDARY.

Relying on fairly simple design principles,pressure seal valves have proven their

capability to handle increasingly demandingfossil and combined-cycle steam isolationapplications, as designers continue to pushboiler, HRSG, and piping systempressure/temperature envelopes.

Pressure seal valves are typically availablein size ranges from 2 inches to 24 inches andASME B16.34 pressure classes from #600to #4500, although some manufacturers canaccommodate the need for larger diametersand higher ratings for special applications.

Keeping pace with advancements inmaterial technology, todays pressure sealvalves are available in carbon (A105forged and Gr. WCB cast), alloy (F22

forged and Gr. WC9 cast; F11 forged andGr. WC6 cast), austenitic stainless (F316forged and Gr. CF8M cast; for over 1000F, F316H forged and suitable austeniticcast grades with carbon content > 0.04%),as well as a number of other alloy/stain-less/special materials. Also available frommost manufacturers is the F91 forgedand/or C12A cast alloy (9 Cr.-1 Mo-modi-fied Vanadium) material used for high-tem-perature (e.g. main steam) piping systemsin the last round of combined-cycle power

plant construction and newer fossil units.The pressure seal design concept can be

traced back to the mid-1900s, when, facedwith ever increasing pressures and tempera-tures (primarily in power applications), valvemanufacturers began designing alternativesto the traditional bolted-bonnet approach tosealing the body/bonnet joint. Along with pro-viding a higher level of pressure boundarysealing integrity, many of the pressure sealvalve designs weighed significantly less thantheir bolted bonnet valve (BBV) counterparts.

Back toBASICSBack toBASICS

PRESSURE SEAL VALVES:

SIMPLE DESIGNS,DEMANDINGAPPLICATIONS

Figure 1

Figure 2

BY DONALD A.BOWERS,JR.

AS SEEN IN THE FALL: 2005 ISSUE OF...

2005 Valve Manufacturers Association. Reprinted with permission.

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2 | Valve M A G A Z I N E

But over the years, as operating pres-sures/temperatures increased, and withthe advent of peaking plants, this sametransient system pressure that aided insealing also played havoc with pressure

seal joint integrity.

Pressure Seal GasketsOne of the primary components involvedin sealing the pressure seal valve is thegasket itself. Early pressure seal gasketswere manufactured from iron or softsteel. These gaskets were subsequentlysilver-plated to take advantage of thesofter plating materials ability to pro-vide a tighter seal. Due to the pressureapplied during the valves hydrotest, aset (or deformation of the gasket pro-file) between the bonnet and gasket wastaken. Because of the inherent bonnettake-up bolt and pressure seal joint elas-ticity, the potential for the bonnet tomove and break that set when sub-jected to system pressure increases/decreases existed, with body/bonnet jointleakage the result. This problem could beeffectively negated by utilizing the prac-tice of hot torquing the bonnet take-upbolts after system pressure and tempera-ture equalization, but it required

owner/user maintenance personnel to doso after plant startup. If this practicewas not adhered to, the potential forleakage through the body/bonnet jointexisted, which could damage the pressureseal gasket, the bonnet and/or the I.D. ofthe valve body, as well as creating com-pounding problems and inefficiencies thatthe steam leakage could have on plantoperations.

As a result, valve designers took sev-eral steps to address this problem. Figure

3 shows a combination of live-loadedbonnet take-up bolts (thus maintaining a

constant load on the gasket, minimizingthe potential for leakage) and thereplacement of the iron/soft steel, silver-plated pressure seal gasket with onemade of die-formed graphite. The gasketdesign shown in Figure 3 can be installedin pressure seal valves previously sup-plied with the traditional type gasket.Figure 4 shows another design approachaccomplishing the same end result.

The advent of graphite gaskets hasfurther solidified the dependability andperformance of the pressure seal valvein most applications and for even dailystart/stop operating cycles. Althoughmany manufacturers still recommendhot torquing, the potential for leak-age when this is not done is greatlydiminished.

The seating surfaces in pressure sealvalves, as in many power plant valves,are subjected to, comparatively speaking,very high seating loads. Seat integrity ismaintained as a function of tight machin-ing tolerances on component parts,means of providing the requisite torqueto open/close as a function of gears oractuation, and selection/ application ofproper materials for seating surfaces.Cobalt, nickel, and iron-based hardfacingalloys are utilized for optimal wear

resistance of the wedge/disc and seat ringseating surfaces. Most commonly usedare the CoCr-A (e.g., Stellite) materials.These materials are applied with a vari-ety of processes, including shielded metalarc, gas metal arc, gas tungsten arc, andplasma (transferred) arc. Many pressureseal globe valves are designed havingintegral hardfaced seats, while the gatevalve and check valves typically havehardfaced seat rings that are welded intothe valve body.

Valving TerminologyIf you have dealt with valving for anylength of time, youve probably noticedvalve manufacturers are not overly cre-ative with the terms and vernacular used

in the business. Take for example,bolted bonnet valves. The body isbolted to the bonnet to maintain systemintegrity. For pressure seal valves, sys-tem pressure aids the sealing mechanism.For stop/check valves, when the valvestem is in the closed position, flow ismechanically stopped, but when in theopen position, the disc is free to act tocheck a reversal of flow. This same prin-ciple applies to other terminology usedfor design, as well as valve types andtheir component parts.

Pressure seal gate valves basicallycome in two types:

1. Flex-wedge type gate valves (Fig-ure 5) incorporating a flexible,wedge-shaped closure elementthat, relying on the torque gener-ated by the hand-wheel or motoroperator, is driven into the seats ofthe valve, thus effecting sealing.The flex-wedge gate valve is said tobe torque seated, because itrelies on this applied torque to pro-

vide the sealing force, as well assome assistance from system pres-sure. This flexibility comes fromthe design of the wedge, wherematerial is removed either by sawcutting or other processes inherentin forming/manufacturing, concen-trically around a central hub. Theincreased flexibility allows for:a. less required torque to drive in

and to extricate the wedge fromthe seat rings

b. greater resiliency to deal withthermal expansion and in find-ing the optimal downstream

Figure 3

Figure 4

Figure 5

P R E S S U R E S E A L V A L V E S



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seating surface as a function ofupstream pressure

c. less potential for coming off theseat as a function of pipingloads and/or during piping sys-

tem movement caused by seis-mic or a variety of potentialplant operation events.

2. Parallel slide gate valves (Figure6) feature two discs retained in acage assembly, the seating surfacesof which are parallel to the valveseat rings. When the valve is

cycled, these seats slide over oneanother. The two discs are heldapart by spring(s). Parallel slidevalves are said to be positionseated, in that optimal sealing is

achieved when the disc and seatring mating surfaces are mostclosely positioned (i.e., no amountof additional turning of the hand-wheel will effect better sealing oncethe seating surfaces are mated).

Other designs are available includingtwo-piece and double-disc wedgetypes that have proven to be effective.

Which Valve to Use?So why choose one over the other? Par-

allel slide valves rely almost entirely onupstream pressure to affect a seal on thedownstream seat. Because of this, atlow-pressure conditions, leakage throughthe seat may occur. In addition, the slid-ing action of the seating surfaces is moreprone to wear, and if particulate istrapped between the seating surfaces,damage is more likely to occur. Due tothe width and orientation of the seats,parallel slide valves are more difficult tomaintain than their flex-wedge counter-

parts. But before you replace all your

parallel slide valves with flex-wedgegates, read on.

Because the flex-wedge gate valverequires torque to energize the sealingsurfaces (and then un-wedge itself),

an actuator capable of providing com-paratively higher torques (to parallelslide valves) is required, usually at ahigher cost. Then theres the thermalbinding issue.

At operating temperatures approxi-mating 800 F and higher, wedge-typegate valves have the potential to bind ina variety of modes (e.g. when exposedto high operating temperatures, closed,then allowed to cool; or as is commonin startup, beginning at ambient tem-peratures, exposed to a rapid thermaltransient, then opened). This phenome-non is dependent on a wide variety ofdesign and operating conditions, butcan be mitigated by incorporating oper-ating procedures that verify thermalequalization between the upstream anddownstream bores (delta T of 200 F orless). Bypasses (Figure 7) connectingthe valve upstream and downstreambores can facilitate this thermal equal-ization. Care must be taken to verifyupstream vs. downstream thermal

equalization at the valve itself, notaway from the valve on connecting pipe.The new generation combined-cycleplants with their comparatively (withfossil plants) fast startup proceduresare particularly susceptible to thermalbinding. However, the most effectiveway to guard against thermal binding isto choose the parallel slide design.

If you recall, in the parallel slidedesign (Figure 6), the two discs are heldapart in the cage by a spring(s). This

spring(s) allows the discs more thanenough travel to exceed the effects ofthermal expansion.

Thermal binding is not endemic to

pressure seal valves. However, becausepressure seals are routinely used in high-temperature service, take special carewhen addressing this issue.

Other Operating ConcernsIn addition, users of pressure seal valvesmust address two related operating con-cerns: center cavity over pressurization(CCOP) and pressure locking. Like ther-mal binding, these phenomena can resultin an inability to stroke the valve. Notethat thermal binding, CCOP, and pres-sure locking are three distinct concerns,the potential of which must be carefullyevaluated and addressed in the design/procurement phase of the project. Para-graph 2.3.3 of ASME B16.34 places theresponsibility on the owner to determinethe potential for, and provide a means toprotect against, CCOP and pressurelocking.

The closure element of double-seatedvalves (wedge gates, parallel slidegates, ball valves, etc.) may becomelocked in place by either a buildup ofpressure in the center cavity (CCOP) oran increase in the differential pressureupstream, downstream, or both of theseats in a closed valve as a function of

decreased line pressure (pressure lock-

ing). In the case of CCOP (Figure 8),fluid trapped in the center cavity atambient temperatures will expand whenheat is introduced (e.g., during startup).This will cause the fluid to expand and,depending on the fluid type and temper-ature, could reach a pressure whereinsufficient torque is available (manu-ally or actuated) to overcome the pres-sure and open the valve.

Pressure locking (Figure 9) occurs indouble-seated valves where the line pres-

sure drops (as a function of plant opera-

Figure 6

Figure 7

Figure 8




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4 | Valve M A G A Z I N E

tion or accident) on either the upstream,downstream, or both sides of the valveseats, creating a sufficient differentialpressure to preclude opening the valve.

As in thermal binding, there are sev-eral methods to guard against CCOPand pressure locking. These include thefollowing:

1. A pressure relief hole drilledthrough the pressure side of thebody or wedge/disc half into thevalves center cavity, thus relievingoverpressure to that pressure side.This effectively makes the valveunidirectional in its sealing capa-bility (Figure 10).

2. A pressure equalizing pipe, drilledand tapped from the center cavity

to the valves pressure sidebore (Figure 11). When a bypassvalve is included, bi-directional

sealing is maintained. Justremember that when the bypassvalve is closed, center cavity pres-sure is not being relieved.

3. A pressure relief valve that is con-

nected to a pipe drilled andtapped into the valves center cav-ity. This method maintains thevalves bi-directional sealingcapability (Figure 12).

4. A drain valve that is connected toa pipe drilled and tapped into thevalves center cavity (Figure 13).Remember that when the drainvalve is closed, center cavity pres-sure is not being relieved.

5. Bypass with one or more bypassvalves and an equalizing pipejoining the center cavity of the

valve with the bypass pipe.Depending on the number andconfiguration of the bypassvalves, bi-directional sealing maybe maintained (Figure 7).

6. Proprietary bypass valves thatchange sealing direction as doessystem pressure. Bi-directionalsealing is maintained (Figure 14).

Pressure Seal Globe ValvesPressure seal globe valves are utilized in

applications where some degree of flow

control (or throttling) may be required(e.g., in plant startup or shutdownmodes). They are well suited for isolationapplications in power plants (e.g., mainsteam isolation, feedwater heater isola-tion, boiler/economizer isolation, etc.).Pressure seal globes may be supplied inthe same material types, actuation vari-eties (manual, gear operated, motoroperated, pneumatic, electro-hydraulic,etc.), trim combinations and materials,and ASME pressure classes as their gatevalve counterparts. They can be suppliedin a stem vertical (Figure 15) or inclined(Y-pattern) orientation (Figure 16), as a

function of required flow (Cv). Pressureseal globes may be supplied with the discmechanically affixed to the stem so that,when in the open position, flow is free tooccur. However, they may also be sup-plied with the stem freely floating in thedisc pocket (see inset, Figure 16). In thisorientation (stop/check), when the valveis in the open position, the valve disc willclose when a reversal of flow occurs,thus providing a check valve function inaddition to the basic stop (or isolation)

function.



Figure 10

Figure 11

Figure 12

Figure 9

Figure 13

Figure 14

Figure 15


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The profile of the disc in globe valvesmay be modified to more finely controlthe amount of flow at operating condi-

tions (Figure 17). This is particularlyuseful in throttling applications wherethe system is dependent on flow controlto optimize performance. Scales maybe affixed to the valve topworks tomeasure the valve stroke, which can becorrelated to flow curves to accuratelycontrol the actual flow through thevalve. Where tighter control such asmodulating is requiredand whereconsistent throttling at less than 20%open is expectedcontrol valves, which

incorporate valve operating systemsdesigned for the application, are recom-mended. It is common to include anequalizing pipe to the non-pressure sideto help balance the valve, increase lift,and control turbulence.

Pressure seal globe valves are notsubject to CCOP, pressure locking, orthermal binding; however, the effects ofhigh temperature (e.g., thermal expan-sion) must be evaluated on componentparts (stem, seats, etc.), especially when

the valve is to be actuated.

Pressure Seal Check ValvesThe primary responsibility of pressureseal check valves is to seal against a sys-tem flow reversal, thus protecting pipingand components (pumps, instruments,

etc.) not designed to handle that condi-tion. They are supplied in the same mate-rials, pressure classes, and orientations(vertical and inclined) as pressure sealglobe valves. Selection of check valves istypically based on a number of variables,including system flow characteristics(e.g., Cv, velocity), media (e.g., type andsize/concentration of particulate), andplant operating characteristics.

Pressure seal check valves may besupplied in configurations, as follows:

1. Swing Check (Figure 18). Pres-sure seal swing check valves arecommonly used in combinationwith gate-type isolation valves forreverse flow protection. Their rela-tively higher Cv (vs. piston checks),simple operation, and relative easeof maintenance make them popu-lar among piping system designers.

2. Tilting Disc Check (Figure 19).Due to a shorter moment between

the hinge pin and centerline of the

disc (vs. swing checks), tilting disccheck valves can react quicker toreversals in flow, thus providing ahigher margin of safety to theupstream equipment and media

hammer potential. Note the dif-ferences in cost and Cv betweenthese two common types of check

valves before purchasing.3. Piston Check (Figure 20). Pres-

sure seal piston check valves arefrequently used in conjunction withglobe-type isolation valves forreverse flow protection. Thesevalves are sometimes supplied withsprings to aid in closing and/orequalizing pipes to aid overall per-

formance.4. Specialty Check Valves. Although

similar to their ancestors inconcept, todays pressure sealvalves have evolved to address awider range of applications whiledelivering higher performancelevels. Valve manufacturerscontinue their work in tweakingthe various design elementscontributing to pressure seal valveperformance in order to provide

greater value to the end user, thusassuring the key role that thesevalves will continue to play inpower plant operation. VM

DONALD A.BOWERS, JR., a 25-year veteran of the

valve industry, is director of sales and product

manager for power products at Velan Valve

(www.velan.com) in Montreal, Quebec.Bowers

chairs both ASMEs Sub-Group Procedure and

Performance Qualification of ASME Subcommit-

tee IX, as well as the National Boards NBIC Sub-

committee on Overpressure Protection.Reach him

at [email protected] or 802-864-3350.

Figure 17

Figure 18

Figure 19

Figure 20

Figure 16


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