形状阻力公式大全形阻公式Word文档下载推荐.docx

形状阻力公式大全形阻公式Word文档下载推荐.docx

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whereis:

hL-headloss;

v-velocity;

gn-accelerationofgravity;

Thenumberofvelocityheadslostduetoresistanceofvalvesandfittingsis:

K-resistancecoefficient;

Theheadlossduetoresistanceinvalvesandfittingsarealwaysassociatedwiththediameteronwhichvelocityoccurs.

TheresistancecoefficientKisconsideredtobeconstantforanydefinedvalvesorfittingsinallflowconditions,astheheadlossduetofrictionisminorcomparedtotheheadlossduetochangeindirectionofflow,obstructionsandsuddenorgradualchangesincrosssectionandshapeofflow.

HeadlossduetofrictioninstraightpipeisexpressedbytheDarcyequation:

f-frictionfactor;

L-length;

D-internaldiameter;

Itfollowsthat:

L-lengt;

TheratioL/Disequivalentlengthinpipediametersofstraightpipethatwillcausethesamepressuredroporheadlossasthevalvesorfittingsunderthesameflowconditions.AstheresistancecoefficientisKisconstanttheequivalentlengthL/Dwillvaryinverselywiththechangeinfrictionfactorfordifferentflowconditions.

Forgeometricallysimilarvalvesandfittings,theresistancecoefficientwouldbeconstant.Actuallytherearealwayssmallerorbiggergeometricalnonsimilarityinvalvesandfittingsofdifferentnominalsize,sotheresistancecoefficientisnotconstant.TheresistancecoefficientKforagiventypeofvalvesorfittings,tendstovarywithsizeasdoesfrictionfactorforstraightcleancommercialsteelpipeatthesameflowconditions.

Someresistancesinpipinglikesuddenorgradualcontractionsandenlargements,aswellaspipeentrancesorexistsaregeometricallysimilar.ThereforetheresistancecoefficientorequivalentlengthL/Disfortheseitemsindependentofsize.

ThevaluesforresistancecoefficientorequivalentlengthL/Darealwaysassociatedwithinternalpipediameterwheretheresistanceisoccurring.

IftheresistancecoefficientorequivalentlengthL/Dshouldbeusedfordifferentinternalpipediameterthanthediameterforwhichexistingvaluescanbefoundfollowingrelationshipcanbeused:

wheresubscript"

a"

definesKanddwiththereferencetointernalpipediameter,andsubscript"

b"

definesKanddwiththereferencetotheinternaldiameterforwhichvaluesofKcanbefoundintablesordiagrams.

ThisequationcanalsobeusedifthepipingsystemhasmorethanonesizeofvalvesandfittingstoexpresstheresistancecoefficientorequivalentlengthL/Dintermsofonesize.

ResistancecoefficientKcalculatorforvalvesandfittingscanbeused.

ResistancecoefficientKforinternaldiametersuddenandgradualcontractionandenlargement

Usingmomentum,continuityandBernoulliequationtheresistanceduetosuddenenlargementsmaybeexpressedas:

andtheresistancefactorduetosuddencontractionas:

K1-resistancecoefficient;

d1-internaldiameter（smaller）;

d2-internaldiameter（larger）;

Usingβasdiameterratio,bothequationcanbeexpressedas:

β-diameterratiod1/d2;

Inordertoexpresstheresistancecoefficientintermsoflargerpipediameter,followingrelationshouldbeused:

K1-resistancecoefficientbasedonsmallerinternaldiameter;

K2-resistancecoefficientbasedonlargerinternaldiameter;

Iftheenlargementisnotsuddenbutgradual,orifangleofgradualenlargementisdifferentfromO,GibsoncoefficientCecanbeusedfordifferentangleofdivergenceasfollows:

Inotherwords,ifangleofdivergenceisbiggerthan45O,theresistancecoefficientisequaltooneforsuddenenlargement.

ForgradualcontractiontheresistancecoefficientonthesamebasisbasedonCranetestdata,contractioncoefficientCccanbeusedfordifferentanglesofconvergence,asfollows:

Usingaboveexpressionsforenlargementandcontractioncoefficient,resistancecoefficientcanbecalculatedas:

Forgradualenlargement:

Ce-coefficientofenlargement;

θ-enlargementangle;

Forgradualcontraction:

Cc-coefficientofcontraction;

Forresistancecoefficientbasedonthelargepipediameterexpression:

shouldbeused,withaboveequations.

Equationsforgradualenlargementandcontractioncanbeusedforresistancecoefficientcalculationforreducedborestraight-throughvalveslikeballvalvesandgatevalves.Thetotalresistancecoefficientforthistypeofballandgatevalvesisthesummationofresistancecoefficientforgradualcontractionandgradualenlargement.

YoucancalculateresistancecoefficientusingresistancecoefficientKandequivalentlengthl/dcalculator.

FlowcoefficientCv,pressuredrop,controlvalveflowrate

Selectingthecorrectvalvesizeforagivenapplicationrequiresknowledgeofprocessconditionsthatthevalvewillactuallyseein.Intheindustryofcontrolvalvesitispracticetouseflowcoefficientandflowcharacteristics.

IntheUKandintheUSAcoefficientCvisusedanditisdefinedasflowrateofwateringpmat60OFthatcreatespressuredropof1psiacrossthevalve.Basicequationforvalvesizingforliquidis:

Cv-flowcoefficient[gpm];

q-flowrate[gpm];

Δp-pressuredrop[bar];

S-specificgravity（relativedensity）[-];

ToaidinestablishinguniformmeasurementofliquidflowcoefficientsCv,standardizedtestingfacilitybyFluidControlInstitute（FCI）areusedbymanufacturers.Theeffectofviscosityoffluidsotherthanwatershouldbeconsideredwhenselectingthevalve,asincreasedviscosityoffluidisreducingthevalvecapacity.

AnothercoefficientKvisusedinsomecountries,particularlyinEuropeandisdefinedasflowrateofwaterinm3/hthatcreatespressuredropof1kg/cm2acrossthevalve（1kg/cm2isequalto0.980665bar）.

Controlvalvesizingisbasedonthecalculationofflowcoefficientforgivenpressuredropandflowrate.LiquidflowcapacityofavalveinmetricunitscanbeconvertedtoCvas:

qm-flowrate[l/m];

ρ-density[kg/m3];

Also,liquidflowcapacityofavalvecanbeconvertedtoKvas:

Kv-flowcharacteristic[m3/h];

qh-flowrate[m3/h];

AboveequationsareusedinflowcoefficientCv,pressuredropandcontrolvalveflowratecalculator.

Flashingandcavitation,vaporpressureatvalvevenacontracta

Flashingorcavitationinsideavalvecanhaveasignificantinfluenceonvalvecapacity.Flashingandcavitationcanreducetheflowthroughvalveinmanyliquids.Also,damagecanbemadetothevalveaswellastothepipingsystem.Theeffectisrepresentedbythechangefromliquidtovaporstateoffluid,resultinginthevelocityincreasedownstreamfromthevalve.

Asliquidpassesthroughtherestrictionareainsidethevalveflowstreamiscontracted.Thesmallestcrosssectionareaofstreamisjustdownstreamoftheactualphysicalrestrictionatapointcalledvenacontracta.Atthatpointthevelocityisatitsmaximumandpressureattheminimum.

Asthefluidexitsthevalve,awayfromvenacontracta,velocitydecreaseandpressureincrease,sothecriticalpointforflashingandcavitationisatthepointwherethepressureissmallestwhichisinvenacontracta.Ifpressureatvenacontractadropsbellowsthevaporpressureofthefluid,duetoincreasedvelocityatthispoint,bubbleswillformintheflowstream.

Ifpressuredownstreamofthevenacontractaincreaseabovethevaporpressure,bubbleswillcollapseorimplodeproducingcavitation.Cavitationreleasesenergyandproducesanoise.Ifcavitationoccursclosetosolidsurfaces,theenergyreleasedgraduallywearsthematerialleavingtheroughsurface.Cavitationcanalsodamagethedownstreampipeline,ifatthatplacethepressurerisesabovethevaporpressureandbubblescollapse.

Chockedflowvalvepressuredropandcavitationinhighpressurerecoveryvalve

Formationofbubblesinthevalveresultingofflashingandcavitationeffectreduc