InternationalsiteforSpiraxSarco Tel:(800)5750394Fax:(803)[email protected]://www.SpiraxSarco.com/us/This tutorial briefly describes the basiccomponentsofdifferenttypesoflinearandrotaryaction control valves available for use in steamandwatersystems.Use the quick links below to take you to the mainsectionsofthistutorial:ContactUsTheprintableversionofthispagehasnowbeenreplacedbyTheSteamandCondensateLoopBookViewthecompletecollectionofSteamEngineeringTutorialsYouarehere: Home Resources SteamEngineeringTutorialsControlHardware:Electric/PneumaticActuation ControlValvesControlValvesIntroductiontoElectric/PneumaticControlsBlock 6 of The Steam and Condensate Loop considers the practical aspects of control, putting the basiccontroltheorydiscussedinBlock5intopractice.Abasiccontrolsystemwouldnormallyconsistofthefollowingcomponents:Controlvalves.Actuators.Controllers.Sensors.Allofthesetermsaregenericandeachcanincludemanyvariationsandcharacteristics.Withtheadvanceoftechnology,thedividinglinebetweenindividualitemsofequipmentandtheirdefinitionsarebecominglessclear.Forexample,thepositioner,whichtraditionallyadjustedthevalvetoaparticularpositionwithinitsrangeoftravel,cannow:Takeinputdirectlyfromasensorandprovideacontrolfunction.Interfacewithacomputertoalterthecontrolfunctions,andperformdiagnosticroutines.Modifythevalvemovementstoalterthecharacteristicsofthecontrolvalve.Interfacewithplantdigitalcommunicationsystems.However,forthesakeofclarityatthispoint,eachitemofequipmentwillbeconsideredseparately.ControlValvesWhilstawidevarietyofvalvetypesexist,thisdocumentwillconcentrateonthosewhicharemostwidelyused in the automatic control of steam and other industrial fluids. These include valve types which havelinearandrotaryspindlemovement.Lineartypesincludeglobevalvesandslidevalves.Rotarytypesincludeballvalves,butterflyvalves,plugvalvesandtheirvariants.Thefirstchoicetobemadeisbetweentwoportandthreeportvalves.Twoportvalves'throttle'(restrict)thefluidpassingthroughthem.Threeportvalvescanbeusedto'mix'or'divert'liquidpassingthroughthem.TwoportvalvesGlobevalvesGlobevalvesarefrequentlyusedforcontrolapplicationsbecauseoftheirsuitabilityforthrottlingflowandtheeasewithwhichtheycanbegivenaspecific'characteristic',relatingvalveopeningtoflow.TwotypicalglobevalvetypesareshowninFigure6.1.1.Anactuatorcoupledtothevalvespindlewouldprovidevalvemovement.ControlHardware:Electric/PneumaticActuationControlValvesControlValveCapacityControlValveSizingforWaterSystemsControlValveSizingforSteamSystemsControlValveCharacteristicsControlValveActuatorsandPositionersControllersandSensorsRelatedContentControlValvesBrowsetherangeofcontrolvalvesfromhere.PneumaticActuatorsBrowsetherangeofelectricactuatorsfromhere.ElectricActuatorsBrowsetherangeofelectricactuatorsfromhere.TheSteamandCondensateLoopBookAcomprehensivebestpracticeguidetosavingenergyandoptimisingplantperformance,thisbookcoversallaspectsofsteamandcondensatesystems.FeatureHome AboutUs Products&Services Industries&Applications Training Resources ContactFig.6.1.1TwodifferentlyshapedglobevalvesThemajorconstituentpartsofglobevalvesare:Thebody.Thebonnet.Thevalveseatandvalveplug,ortrim.Thevalvespindle(whichconnectstotheactuator).Thesealingarrangementbetweenthevalvestemandthebonnet.Figure 6.1.2 is a diagrammatic representation of a single seat twoport globe valve. In this case the fluidflowispushingagainstthevalveplugandtendingtokeeptheplugoffthevalveseat.Fig.6.1.2Flowthroughasingleseat,twoportglobevalveThedifferenceinpressureupstream(P1)anddownstream(P2)ofthevalve,againstwhichthevalvemustclose,isknownasthedifferentialpressure(DP).Themaximumdifferentialpressureagainstwhichavalvecanclosewilldependuponthesizeandtypeofvalveandtheactuatoroperatingit.Inbroadterms,theforcerequiredfromtheactuatormaybedeterminedusingEquation6.1.1.Equation6.1.1Where:A =Valveseatingarea(m)DP =Differentialpressure(kPa)F =Closingforcerequired(kN)OrderyourcopytodayInasteamsystem,themaximumdifferentialpressureisusuallyassumedtobethesameastheupstreamabsolute pressure. This allows for possible vacuum conditions downstream of the valve when the valvecloses.Thedifferentialpressureinaclosedwatersystemisthemaximumpumpdifferentialhead.Ifalargervalve,havingalargerorifice,isusedtopassgreatervolumesofthemedium,thentheforcethattheactuatormustdevelopinordertoclosethevalvewillalsoincrease.Whereverylargecapacitiesmustbe passed using large valves, or where very high differential pressures exist, the point will be reachedwhereitbecomesimpracticaltoprovidesufficientforcetocloseaconventionalsingleseatvalve.Insuchcircumstances,thetraditionalsolutiontothisproblemisthedoubleseattwoportvalve.Asthenameimplies,thedoubleseatvalvehastwovalveplugsonacommonspindle,withtwovalveseats.Notonlycanthevalveseatsbekeptsmaller(sincetherearetwoofthem)butalso,ascanbeseeninFigure6.1.3,theforcesarepartiallybalanced.Thismeansthatalthoughthedifferentialpressureistryingtokeepthetopvalveplugoffitsseat(aswithasingleseatvalve)itisalsotryingtopushdownandclosethelowervalveplug.Fig.6.1.3Flowthroughadoubleseat,twoportvalveHowever,apotentialproblemexistswithanydoubleseatvalve.Becauseofmanufacturingtolerancesanddiffering coefficients of expansion, few double seat valves can be guaranteed to give good shutofftightness.ShutofftightnessControl valve leakage is classified with respect to how much the valve will leak when fully closed. TheleakagerateacrossastandarddoubleseatvalveisatbestClassIII,(aleakageof0.1%offullflow)whichmaybetoomuchtomakeitsuitableforcertainapplications.Consequently,becausetheflowpathsthroughthetwoportsaredifferent,theforcesmaynotremaininbalancewhenthevalveopens.Variousinternationalstandardsexistthatformaliseleakageratesincontrolvalves.ThefollowingleakageratesaretakenfromtheBritishStandardBS5793Part4(IEC605344).Foranunbalancedstandardsingleseatvalve,theleakageratewillnormallybeClassIV,(0.01%offullflow),althoughitispossibletoobtainClassV,(1.8x105xdifferentialpressure(bar)xseatdiameter(mm).Generally,thelowertheleakageratethemorethecost.BalancedsingleseatvalvesBecause of the leakage problem associated with double seat valves, when a tight shutoff is required asingle seat valve should be specified. The forces required to shut a single seat globe valve increaseconsiderablywithvalvesize.Somevalvesaredesignedwithabalancingmechanismtoreducetheclosingforce necessary, especially on valves operating with large differential pressures. In a pistonbalancedvalve,someoftheupstreamfluidpressureistransmittedviainternalpathwaysintoaspaceabovethevalveplug, which acts as a pressure balancing chamber. The pressure contained in this chamber provides adownforceonthevalveplugasshowninFigure6.1.4,balancingtheupstreampressureandassistingthenormalforceexertedbytheactuator,toclosethevalve.Fig.6.1.4AsteamcontrolvalvewithpistonbalancingSlidevalves,spindleoperatedSlidevalvestendtocomeintwodifferentdesignswedgegatetypeandparallelslidetype.Bothtypesarewellsuitedforisolatingfluidflow,astheygiveatightshutoffand,whenopen,thepressuredropacrossthemisverysmall.Bothtypesareusedasmanuallyoperatedvalves,butifautomaticactuationisrequired,the parallel slide valve is usually chosen, whether for isolation or control. Typical valves are shown inFigure6.1.5.Fig.6.1.5Wedgegatevalveandparallelslidevalve(manualoperation)Theparallelslidevalveclosesbymeansoftwospringloadedslidingdisks(springsnotshown),whichpassacrosstheflowpathofthefluid,thefluidpressureensuringatightjointbetweenthedownstreamdiskanditsseat.Largesizeparallelslidevalvesareusedinmainsteamandfeedlinesinthepowerandprocessindustriestoisolatesectionsoftheplant.Smallboreparallelslidesarealsousedforthecontrolofancillarysteamandwaterservicesalthough,mainlyduetocost,thesetasksareoftencarriedoutusingactuatedballvalvesandpistontypevalves.RotarytypevalvesRotarytypevalves,oftencalledquarterturnvalves,includeplugvalves,ballvalvesandbutterflyvalves.Allrequirearotarymotiontoopenandclose,andcaneasilybefittedwithactuators.EccentricplugvalvesFigure6.1.6showsatypicaleccentricplugvalve.Thesevalvesarenormallyinstalledwiththeplugspindlehorizontalasshown,andtheattachedactuatorsituatedalongsidethevalve.Plugvalvesmayincludelinkagesbetweentheplugandactuatortoimprovetheleverageandclosingforce,and special positioners that modify the inherent valve characteristic to a more useful equal percentagecharacteristic(valvecharacteristicsarediscussedinTutorial6.5).Fig.6.1.6Sideviewofaneccentricplugvalve(showninapartiallyopenposition)BallvalvesFigure6.1.7showsaballvalveconsistingofasphericalballlocatedbetweentwosealingringsinasimplebodyform.Theballhasaholeallowingfluidtopassthrough.Whenalignedwiththepipeends,thisgiveseitherfullboreornearlyfullboreflowwithverylittlepressuredrop.Rotatingtheballthrough90opensandclosestheflowpassage.Ballvalvesdesignedspecificallyforcontrolpurposeswillhavecharacterizedballsorseats,togiveapredictableflowpattern.Fig.6.1.7Ballvalve(showninafullyopenposition)Ballvalvesareaneconomicmeansofprovidingcontrolwithtightshutoffformanyfluidsincludingsteamattemperatures up to 250C (38 bar g, saturated steam). Above this temperature, special seat materials ormetaltometalseatingsarenecessary,whichcanbeexpensive.Ballvalvesareeasilyactuatedandoftenused for remote isolation and control. For critical control applications, segmented balls and balls withspeciallyshapedholesareavailabletoprovidedifferentflowcharacteristics.ButterflyvalvesFigure6.1.8isasimpleschematicdiagramofabutterflyvalve,whichconsistsofadiscrotatingintrunnionbearings.Intheopenpositionthediscisparalleltothepipewall,allowingfullflowthroughthevalve.Intheclosedpositionitisrotatedagainstaseat,andperpendiculartothepipewall.Fig.6.1.8Butterflyvalve(showninitsopenposition)Traditionally,butterflyvalveswerelimitedtolowpressuresandtemperatures,duetotheinherentlimitationsof the soft seats used. Currently, valves with higher temperature seats or high quality and speciallymachinedmetaltometalseatsareavailabletoovercomethesedrawbacks.Standardbutterflyvalvesarenowusedinsimplecontrolapplications,particularlyinlargersizesandwherelimitedturndownisrequired.Specialbutterflyvalvesareavailableformoredemandingduties.A fluid flowing through a butterfly valve creates a low pressure drop, in that the valve presents littleresistancetoflowwhenopen.Ingeneralhowever,theirdifferentialpressurelimitsarelowerthanthoseforglobe valves. Ball valves are similar except that, due to their different sealing arrangements, they canoperateagainsthigherdifferentialpressuresthanequivalentbutterflyvalves.OptionsTherearealwaysanumberofoptionstoconsiderwhenchoosingacontrolvalve.Forglobevalves,theseincludeachoiceofspindleglandpackingmaterialandglandpackingconfigurations,whicharedesignedtomakethevalvesuitableforuseonhighertemperaturesorfordifferentfluids.SomeexamplesofthesecanbeseeninthesimpleschematicdiagramsinFigure6.1.9.Itisworthnotingthatcertaintypesofglandpackingproduceagreaterfrictionwiththevalvespindlethanothers.Forexample,thetraditionalstuffingboxtypeofpackingwillcreategreaterfrictionthanthePTFEspringloadedchevrontypeorbellowssealedtype.Greaterfrictionrequiresahigheractuatorforceandwillhaveanincreasedpropensityforhaphazardmovement.Springloadedpackingreadjustsitselfasitwears.Thisreducestheneedforregularmanualmaintenance.Bellows sealed valves are the most expensive of these three types, but provide minimal friction with thebeststemsealingmechanism.AscanbeseeninFigure6.1.9,bellowssealedvalvesusuallyhaveanothersetoftraditionalpackingatthetopofthevalvespindlehousing.Thiswillactasafinaldefenceagainstanychanceofleakingthroughthespindletoatmosphere.Fig.6.1.9AlternativeglandpackingsValvesalsohavedifferentwaysofguidingthevalvepluginsidethebody.Onecommonguidancemethod,asdepictedinFigure6.1.10,isthe'doubleguided'method,wherethespindleisguidedatboththetopandthebottomofitslength.Anothertypeisthe'guidedplug'methodwheretheplugmaybeguidedbyacageoraframe.Somevalvescanemployperforatedplugs,whichcombineplugguidanceandnoisereduction.Fig.6.1.10GuidingarrangementsSummaryoftwoportvalvesusedforautomaticcontrolByfarthemostwidelyusedvalvetypefortheautomaticcontrolofsteamprocessesandapplicationsistheglobevalve.Itisrelativelyeasytoactuate,itisversatile,andhasinherentcharacteristicswellsuitedtotheautomaticcontrolneedsofsteam.Itshouldalsobesaidthattwoportautomaticcontrolvalvesarealsousedwithinliquidsystems,suchaslow, medium and high temperature hot water systems, and thermal oil systems. Liquid systems carry aninherentneedtobebalancedwithregardtomassflows.Inmanyinstances,systemsaredesignedwheretwoportvalvescanbeusedwithoutdestroyingthebalanceofdistributionnetworks.However,whentwoportvalvescannotbeusedonaliquidsystem,threeportvalvesareinstalled,whichinherentlymaintainabalanceacrossthedistributionsystem,byactinginadivertingormixingfashion.ThreeportvalvesThreeport valves can be used for either mixing or diverting service depending upon the plug and seatarrangementinsidethevalve.AsimpledefinitionofeachfunctionisshowninFigure6.1.11.Fig6.1.12Pistonvalve(shownasadivertingvalve)Fig.6.1.11ThreeportvalvedefinitionTherearethreebasictypesofthreeportvalve:Pistonvalvetype.Globeplugtype.Rotatingshoetype.PistonvalvesThis type of valve has a hollow piston, (Figure6.1.12), which is moved up and down by theactuator,coveringandcorrespondinglyuncoveringthetwoportsAandB.PortAandportBhavethesameoverallfluidtransitareaand,atanytime,thecumulative crosssectional area of both is alwaysequal.Forinstance,ifportAis30%open,portBis70% open, and vice versa. This type of valve isinherently balanced and is powered by a selfacting control system. Note: The portingconfigurationmaydifferbetweenmanufacturers.Globetypethreeportvalves(alsocalled'liftandlay')Here, the actuator pushes a disc or pair of valveplugs between two seats (Figure 6.1.13),increasing or decreasing the flow through ports AandBinacorrespondingmanner.Fig.6.1.13GlobetypethreeportvalvesNote:Alinearcharacteristicisachievedbyprofilingtheplugskirt(seeFigure6.1.14).Fig.6.1.14PlugskirtmodifiedtogivealinearcharacteristicRotatingshoethreeportvalveThis type of valve employs a rotating shoe, which shuttles across the port faces. The schematicarrangementinFigure6.1.15illustratesamixingapplicationwithapproximately80%flowingthroughportAand20%throughportB,100%toexitthroughportAB.Fig.6.1.15RotatingshoeonamixingapplicationUsingthreeportvalvesNot all types can be used for both mixing and diverting service. Figure 6.1.16 shows the incorrectapplicationofaglobevalvemanufacturedasamixingvalvebutusedasadivertingvalve.Fig.6.1.16ThreeportmixingvalveusedincorrectlyasadivertingvalveThe flow entering the valve through port AB can leave from either of the two outlet ports A or B, or aproportionmayleavefromeach.WithportAopenandportBclosed,thedifferentialpressureofthesystemwillbeappliedtoonesideoftheplug.WhenportAisclosed,portBisopen,anddifferentialpressurewillbeappliedacrosstheothersideoftheplug.Atsomeintermediateplugposition,thedifferentialpressurewillreverse.Thisreversalofpressurecancausetheplugtomoveoutofposition,givingpoorcontrolandpossiblenoiseastheplug'chatters'againstitsseat.Toovercomethisproblemonaplugtypevalvedesignedfordiverting,adifferentseatconfigurationisused,asshowninFig.6.1.17.Here,thedifferentialpressureisequallyappliedtothesamesidesofbothvalveplugsatalltimes.Fig.6.1.17PlugtypedivertingvalveInclosedcircuits,itispossibletousemixingvalvesordivertingvalves,dependinguponthesystemdesign,asdepictedinFigures6.1.18and6.1.19.InFigure6.1.18,thevalveisdesignedasamixingvalveasithastwoinletsandoneoutlet.However,whenplacedinthereturnpipeworkfromtheload,itactuallyperformsadivertingfunction,asitdivertshotwaterawayfromtheheatexchanger.Fig.6.1.18MixingValveinstalledonthereturnpipeworkConsider the mixing valve used in Figure 6.1.18, when the heat exchanger is calling for maximum heat,perhapsatstartup,portAwillbefullyopen,andportBfullyclosed.ThewholeofthewaterpassingfromtheboilerispassedthroughtheheatexchangerandpassesthroughthevalveviaportsABandA.Whentheheatloadissatisfied,portAwillbefullyclosedandportBfullyopen,andthewholeofthewaterpassingfromtheboilerbypassestheloadandpassesthroughthevalveviaportsABandB.Inthissense,thewaterisbeingdivertedfromtheheatexchangerinrelationtotherequirementsoftheheatload.Thesameeffectcanbeachievedbyinstallingadivertingvalveintheflowpipework,asdepictedbyFigure6.1.19.TheprintableversionofthispagehasnowbeenreplacedbyTheSteamandCondensateLoopBookViewthecompletecollectionofSteamEngineeringTutorialsContactUsFig.6.1.19DivertingvalveinstalledontheflowpipeworkWhatdoIdonow?
