A large number of the process controlvalves used in fossil-fired powerplants must operate under severe-ser-

vices conditions—that is, in high-pressure and/or high-temperature applications. When speci-fying valves for such applications, extreme caremust be taken to avoid costly premature fail-ures (Fig. 1). The following discusses the stringent require-ments that valves must meet to safety operateand deliver long-term performance under severe-service conditions. Requirements are examinedfor both generic and specific applications.

Generic requirements: LiquidFor valves used in liquid service, the liquid ve-locity through the valve inlet and outlet shouldn’texceed 60 ft/s (18 m/s). By keeping the fluidvelocities at or below this level, you’ll achievegreater control over the fluid. You’llalso minimize piping and valve vi-brations, along with early erosion ofthe valve internals. The liquid velocity exiting the trimshouldn’t exceed 100 ft/s (30 m/s).The trim is defined as the parts in-ternal to the valve in which the pres-sure drop takes place. Higher veloci-ties will result in shortened trim life.In addition, you should make sure thevalve has a sufficient number of dis-

Specifying control valvesfor severe-service applications

Larry Stratton, Manager, Technical Support and AutomationDavid Minoofar, Vice President, Marketing

Control Components Inc.Rancho Santa Margarita, CA 92688

crete pressure drop stages to avoid cavitation. To minimize the potential for flow-inducedvibration and the entrapment of pipeline trashbetween the closure member and the valvetrim, specify the flow direction as flow-to-close (over-the-seat).

Generic requirements: Gas and steam For valves used in gas and steam applica-tions, the fluid velocity exiting the trim shouldresult in a velocity head that’s less than 70 psi(480 kPa). You can calculate this from the trimexit velocity (v) and the outlet fluid density(rho) using the following equation:

Pvh = rho (v2/2) Higher velocities will result in vibration,noise, erosion, and premature failure. Also, the noise measured at a location 3feet ( 1 m ) downstream from the valve and

If you use thewrong valves insevere-serviceapplications, youcould face costlyprematurefailures.

FIG. 1: Improperly specified valves can leadto premature valve failure. Being aware of themany different valve requirements can helpyou avoid the type of valve damage shownhere.

Table 1: Leakage rating classes and seat loading three feetaway f romthe p ipe( w i t h o u tthermal in-s u l a t i o n )s h o u l d n ’ texceed 85dBa. In ad-dition to be-ing an envi-r o n m e n t a lp o l l u t a n t ,no ise is asymptom ofhigh energyv i b r a t i o n ,which cans h o r t e nvalve l i feand damagedownstreampiping. Finally, toavoid ero-sion by highv e l o c i t ym o i s t u r ec o n t e n t ,make sureenough dis-crete pres-sure dropstages areprovided toe n s u r eisenthalpic

pressure drop. This, in turn, ensures smoothtemperature profiles of the fluid as it passesthrough the valve. Also, it keeps the fluid tem-perature above it’s dewpoint—i.e., the point atwhich liquids will fall out of the fluid and be-come erosive. For both liquid and gas services, make sureall velocities throughout the valve are met with-out the use of downstream orifices, mufflers,diffusers, or other fixed flow devices, since thesedevices are not effective at low-flow conditions.

Other generic requirementsIn some cases, it’s possible for the valve plugto be bounced back-and-forth at high rateswithin the cage. This can cause the plug to stick

or vibrate and, in extreme cases, lead to stembreakage. To prevent the plug from stickingor vibrating, there should be a means to as-sure equal pressurization around it. You also must be aware of the service-con-dition flow rates when specifying and design-ing control valves for severe-service appli-cations. These valves should have an inher-ent rangeability to handle both low and highflow rates. Typical rangeability is 30 to 1,although some applications require greaterthan 100 to 1. This ratio represents the maxi-mum vs the minimum flow. For example, ifa valve can handle a flow of 30 gpm. It shouldbe able to control flows down to 1 gpm. Mustremember: some valves on the market canonly control a very narrow range of flowrates. An option used to meet wide ranging flowrequirements is to install two or more valvesin parallel—a larger valve to handle thehigher flow rates, and smaller valves tohandle the lower. However, you can elimi-nate the need for multiple valves by select-ing a properly designed valve with goodrangeability and excellent control. Often, theproper valve will be a custom design ratherthan an off-the-shelf model. For ease of main-tenance, look for valve trims that are of thequick-change type, so they can be easily re-built (Fig. 2). In these types, the parts oftenare held into position by the bonnet bolting,with no internal components, such as seatrings, screwed or welded into the valve bod-ies or bonnets. Tight shutoff is another key requirement—one that can improve plant efficiency and in-crease valve life. A constantly leaking valvecauses energy from the system to be lost,increasing plant costs. Excessive leakage isa sign that there are uncontrolled velocitiesflowing through the valve. In liquid service,a high velocity leak will cut through the seat,causing wire drawing. In gas or steam appli-cations, leaking fluid will damage the seat.Even a minute leak can quickly grow into alarge one, eventually affecting the valve’sinternal parts and performance. The best way to assure that the valve pro-vides the required shutoff is to specify theamount of force for seat loading. Seat load-ing is usually expressed as force per linearinch (or mm) of seat joint circumference. As

FIG. 2: This valvefeatures a valve him ofthe quick-change type,which makes it easy totake apart andreassemble.

shown in Figure 3,leakages reduced oreliminated with seatloading. Table 1 (p 100)shows four differentleakage rating classeswith their suggestedseat loading. Newvalves will meet theserequirements at the fac-tory. However, once avalve begins operatingin the field, it oftenwon’t shut off as wellas it did in the factory,when all of the partswere new.

The first rating class listed in table 1 statethat there should be 400 lbf/in (7kg.mm) of ac-tuator force for each inch of the seat circumfer-ence. For example, for a valve with a one inchseat (25.4 mm), the circumference would beabout 3.14 in. (80 mm). If you multiply the seatcircumference by the actuator force you findthat slightly more than 1,200 lb (560 kg) of forcepushing down on the plug are required to getthe proper shutoff. If you follow these guidelines for applyingforce to the seat of the valve, you’ll be assuredthat seat leakage requirements are met in fieldconditions. To ensure long shut-off life, insiston a guarantee that the suggested seat loadingwill be provided by the actuator. Choosing theproper actuator, including velocity-controlledinternals, ensures the long-term survival of thevalve (Fig. 4).

Specific requirements In addition to the generic requirements for se-vere-service control valves, specific require-ments are often associated with certain appli-cations. When specifying control valves forthese, more stringent criteria, beyond the mini-mum specifications stated above may be re-quired. For example, when determining liquidexit velocities, if the conditions of the applica-tion lead to a prediction of cavitation, lower theliquid velocity-even though the valve meets thespecified velocity requirements. Let’s look at a few of the applications that typi-cally call for more stringent selection criteria:

• Recirculation or leakoffvalves—To eliminate cavita-tion damage in recirculationvalves, which include boosterfeed pump recirculation andmain feed pump recirculationvalves, make sure the liquid ve-locity exiting the cage doesn’texceed 75 ft/s (23 m/s). To assure minimum fluid ve-locities during the pressure re-duction, specify at least 12pressure drop stages. Forstraight through globe configu-rations, the valve body materialshould be A21 7-C5. On modulating valves, provide a relay toapply full actuator thrust when the controlsignal calls for the valve to operate at lessthan 5% of the valve stroke. This protectsagainst positioner calibration drift, whichmay leave the valve closed without properactuator loading. With proper design, thevalve and actuator should provide continualoperation for a period of 24 months, withoutrequiring disassembly for maintenance.These valves require MSS-SP61 seat-leak-age ratings.• High-pressure regulation valves—Withthese valves, which include start-up and mainfeedwater regulator valves, make sure theliquid velocity exiting the cage doesn’t ex-ceed 100 ft/s (30 m/s).

FIG 3: Tight shutoff isan important require-ment for increasedvalve life. This graphshows that higher seatloading reduces oreliminates seat leakagethat can erode a valvesperformance.

FIG 4: Choosing theproper actuator andincluding velocity-controlled internals,assures the long-termsurvival of the valve.

Referencing ISA S75.05 1983 Standard for Control ValveTerminology, these valves should have a linear installedflow characteristic. This means the relationship betweenthe flow rate through the valve, and the travel of the clo-sure member as the closure member is moved from theclosed position to rated-travel, will be linear, even thoughthe pressure drop across the valve varies. The inherent rangeability of these valves must be at least100 to 1, and you should give favorable selection consider-ation to single valve bodies meeting startup and main-flowconditions. Their resolution must be less than 1.5% of fullstroke. Actuator resolution is the smallest discrete move-ment made by the actuator in response to a signal change.The valves should meet ANSI/FCI 70-2, Class IV seat leak-age requirements.• Low-pressure regulation valves— With low- pressureregulation valves, including deaerator level control and con-densate flow control valves, the liquid velocity exiting thecage shouldn’t exceed 100 ft/s (30 m/s). The inherentrangeability of the valve should be at least 100 to 1. How-ever, give favorable selection and design consideration tosingle valve-bodies meeting start-up and main-flow condi-tions. It is imperative that the actuator resolution be 1.5% offull stroke or better. These valves also require a seat leak-age rating of ANSI FCI 70-2, Class IV or better.• Start-up bypass valves—These valves include Babcock& Wilcox 202 primary superheater bypass valves, Babcock& Wilcox 207 secondary superheater bypass valves. Com-bustion Engineering boiler extraction valves and boiler ex-traction bypass valves, and Foster Wheeler P valves. Withthese, the liquid velocity exiting the cage shouldn’t exceed100 ft/s (30 m/s). But, if the nonflashing liquid is causingcavitation, lower the velocity to eliminate cavitation dam-age. For flashing liquid conditions, keep the exit velocitybelow 75 ft/s (22 m/s). These valves should control upstream pressure to within3% of setpoint pressure and require an actuator resolutionof 1.5% or better. To produce reliable seating, make surethe closure member and the seat interface are composed ofstellited material, and specify MSS-SP61 seat leakage re-quirements.

• Start-up valves to the turbine—These valves, which in-clude Babcock & Wilcox 201 secondary superheater pres-sure control valves, Babcock & Wilcox 401 sliding pres-sure control valves, Babcock & Wilcox 501 secondary su-perheater stop bypass valves, and Combustion Engineer-ing boiler extraction and boiler extraction bypass valves,should have a linear-installed flow characteristic. Their inherent rangeability must be at least 100 to 1, andyou should give favorable selection and design consider-ation to single valve bodies meeting start up and main-flowconditions. To eliminate the need for block valves, thesevalves must meet MSS-SP-61 seat-leakage requirements.• Foster Wheeler “W” pressure reducing valves—Withthese, the liquid velocity exiting the cage shouldn’t exceed100 ft/s (30 m/s). But, if the nonflashing liquid is causingcavitation, this should be lowered to preclude cavitation.For flashing liquid conditions, keep the exit velocity below75 ft/s (22 m/s). These valves should also have a linear installed-flow char-acteristic and their inherent rangeability must be at least50 to 1. Give favorable selection and design considerationto single valve bodies meeting start-up and main-flow con-ditions. The actuator resolution for these valves should be1.5% or better, and they need to meet ANSI/FCI 70-2, ClassIV seat leakage requirements.• Reheat bypass valves—These need to meet ANSI/FCI70-2, Class IV seat leakage ratings.• High-pressure turbine bypass—To minimize the poten-tial tor flow induced vibration and the entrapment of pipe-line trash between the closure member and the valve trim,it’s best if the flow direction be flow-to-close (over-the-seat). To eliminate the need for block valves, these valvesmust meet MSS-SP6I seat-leakage requirements.• Attemperator spray valves—These must meet ANSI/FCI70-2, Class IV seat-leakage ratings.• Auxiliary steam—The inherent rangeability of these valvesmust be at least 50 to 1. The first 10% of the closure mem-ber travel should result in less than 3% of the total valveflow capacity. In addition, auxiliary steam valves must meetANSI/FCI 70-2, Class IV seat-leakage ratings.• Soot-blower flow control—These take high pressure steam andblow soot off of the boiler tubes. To minimize the potential for

flow-induced vibration and the entrapment of pipeline trashbetween the closure member and the valve trim, the flowdirection must be flow-to-close (over-the-seat). It is essential that these valves be capable of controllingdownstream pressure to within +/- 70 kPa (10 psi). Also,they must meet ANSI/FCI 70-2, Class IV seat leakage rat-ings.

Custom can cost less in the long run Particular care should be taken to choose the propervalves tor severe-service. Often, off-the-shelf models won’twork and a custom-design velocity control valve will beneeded. Although the custom valve may cost more up frontthan a conventional valve, it can be the most cost-effectivechoice due to the increased maintenance and operating costsassociated with misapplied valves.

About the authors Larry Stratton is the manager of technical support andautomation for Control Components, Inc. His background

includes all phases of valve and actuator design, includingstandardization and automation to factory and field servicesupport and product improvement. Stratton has a bach-elors degree in Engineering from California State Univer-sity, Fullerton. David Minoofar is vice president, marketing for ControlComponents, Inc. During his nineteen years of experience,he has served in roles from engineer in research and devel-opment to international marketing. Minoofar holds a BSMEfrom UCLA and an MSME from the University of South-ern California.

The authors, Larry Stratton andDavid Minoofar, will be available toanswer questions about this article -they can be reached at (949) 858-1877 during normal business hours.

22591 Avenida EmpressaRancho Santa Margarita, CA 92688 U.S.A

Tel: 949-858-1877Fax: 949-858-1878