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:
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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
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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?
