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Someimportantparametercheck-pointsforselectingacontrolvalveareasbelow:

1.Valvesize,CvThevalvesizeandCvaredeterminedbydoingCvcalculations.TheformulaeforcalculatingCvisdifferentforliquidsandgas/vapors.ThisisaddressedbyISAStandard75.01.01.Also,asathumbruleavalvesizeisselectedfromhalfthelinesize.Thisisdonetoavoidbendingstressesresultingfromwidelydifferentpipe&valvesizesandalsotoavoidturbulence/swageeffectsresultingfromthedifferenceinsizes.AftertheCviscalculatedanappropriatevalvesizetomeettherequiredCvisdecided.ForperformingCvcalculationvariousprocessdatalikeFlowrate,Inletpressure,pressuredrop,fluiddensity/Molecularweight,compressibility&specificheatsratio(bothforcompressiblefluids),vaporpressure&criticalpressure(forliquids),etc.arerequired.

2.ValveRatingThevalve'srating(ratingclassesaredefinedinANSIStandardB16.34)dependsonthematerialbeingused,themaximumdesignpressureencounteredinthelineandtemperature.Usually,pipingengineersdefinepipingclasses.Valvematerial&ratingnormallyfollowthepipingclassspecifications.

3.EndConnectionOnceagainthevalve'sendconnectionshouldmatchwiththepipingendconnections.SomecommonendconnectionsusedareFlange(raisedfaceflangeiscommonly

used),Welded(Socketweld,buttweld),RTJ(Ringtypejoint),etc.

4.ValveBody/trimmaterialBodymaterialusedisnormallyinaccordancewiththepipingclassspecifications.Trimselectionisdonebasedonthetypeoffluidbeinghandled,temperature/pressure,possibilityoferosion/corrosion,etc.

5.TypeofvalveForcontrolapplicationsthetypeofvalvespreferredareasbelow,topbeingmostpreferred:1.Globetype2.Balltype3.Butterflytype

Butterflyvalvesarecheapandcanbeusedforon-offcontrol.

Thereareseveralotherdetailsalsotolookforwhenselectingavalve,butthesearethemajorcriteria.

Asforthevalvecharacteristics-thereare2commonlyusedcharacteristics1.Linear2.Equalpercentage.

Whatyoumentionedabout10%opening=10%flowmightbeapprox.applicableonlyforLineartrim.ButbecarefultoreferCvratherthanflow,becausetheflowratedependsonboththevalveCv(capacity)andthepressuredrop.Equalpercentageisthecommonlypreferredcharacteristic.

OnceyoudotheCvcalculationandselectanappropriatevalvesize,thevalvewouldhaveenoughcapacityandcontrollabilitytocatertotherequiredflowrateintheprocess.

Thepositioneriscalibratedsothatthe4-20mAsignalismappedwith0-100%valvetravel.Fore.g.,a12mAcontrolsignal(mid-value)wouldproduce50%travelinaproperlycalibratedpositioner&valveassembly.

Additionally,youcanalsorefertothevalve'sCvcharacteristiccurvetoknow

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howthevalveCvvariesasthevalveopens.Usuallyvalvemanufacturerwillbeabletogiveyoutheinformationsoyoucanknowatwhat%openingyouneedtomaintainthevalveinordertopasstherequiredflow.ForthisyouneedtodoaCvcalculationwiththeprocessdataparametersthatImentioned.

CVisaconstantrelatingtheflowthroughacontrolvalvetothepressuredropacrossit.Thebasicrelationshipis:

Q=CVsqrt(DP/SG)whereSGisthespecificgravity.

Theformaldefinitionreferstoafully-openvalve,butthetermCVisusuallyusedthroughoutthevalvestrokerange.

Thebasicstepsinvolveinvalveselectionareto:1.Findtherelevantprocessfluiddataanddesignconditions.

2.Usingtheaboveequation,estimatetheCVrequiredtoachievethedesignflowattheassociatedpressuredrop.Notethattheflowacrossthecontrolvalvewillnotnecessarilybethedesignpressuredrop-thiscouldoccuratlowflows.

3.Frommanufacturer'sdata,selectavalvehavingtherequiredCVatabout80%ofstroke-soyouhavesomemargin.

SDVstandforshutdownvalveandBDVstandsforblowdownvalveandasthiernamesimplythatisexactlywhattheydo.

OnaoffshoreplatformsafetyisalwayslookedatfirstsotheremustbeawaytoisolatetheincomingrisersfromtheplatformorFPSOduringaemergency.Theseareyourshutdownvalves.

Soasanexampleyouwillinstallashutdownvalveonamainincominglinefrom

anotherplatformorontherisersonaFPSOorplatformtoisolateyourprocessplantfromanyotherincominglinesinordertoisolateaemergencybyshuttingdowntheprocessandisolateitfromanyotherincomingsources.ItiscalledashutdownvalvesinceitwillonlyactiveviatheESDsysteminasituationwhereyouwantacompleteshutdownofthewholeplantpartofwhichwillbetheshuttingoffofthemainincominglines.

Theblowdownvalveisalsousedduringanemergencyshutdownofaplantdownbutit'sjobistode-pressurizetheplantthatis,asyouhaverightlystated,whytheflarewillincreasetenfoldduringaemergencyshutdownofaplantsincethatishowtheBDV'sde-pressurisestheplant.ItisprettyscarytobeoutsideduringaemergencyblowdownofaoffshoreplatformbutitisjusttheSDV'sandBDV'sdoingthierjobs.