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1CENTRIFUGALPUMPSINTHECHEMICALINDUSTRYAbstract:Acentrifugalpumpconvertstheinputpowertokineticenergyintheliquidbyacceleratingtheliquidbyarevolvingdevice-animpeller.Themostcommontypeisthevolutepump.Fluidentersthepumpthroughtheeyeoftheimpellerwhichrotatesathighspeed.Thefluidisacceleratedradiallyoutwardfromthepumpchasing.Avacuumiscreatedattheimpellerseyethatcontinuouslydrawsmorefluidintothepump.Thisarticlestressesonaseriesofcentrifugalpumps，F(xiàn)romabriefintroductiontotheprinciples.Keywords:centrifugalpump,Introduction,Workingprinciple,Cavitation，MechanismofCavitation，SolutionandRemedies1.IntroductionPump,deviceusedtoraise,transfer,orcompressliquidsandgases.Fourgeneralclassesofpumpsforliquidsaredescribedbelow.Inallofthem,stepsaretakentopreventcavitation(theformationofavacuum),whichwouldreducetheflowanddamagethestructureofthepump.Pumpsusedforgasesandvaporsareusuallyknownascompressors.Thestudyoffluidsinmotioniscalledfluiddynamics.Waterpump,deviceformovingwaterfromonelocationtoanother,usingtubesorothermachinery.Waterpumpsoperateunderpressuresrangingfromafractionofapoundtomorethan10,000poundspersquareinch.Everydayexamplesofwaterpumpsrangefromsmallelectricpumpsthatcirculateandaeratewaterinaquariumsandfountainstosumppumpsthatremovewaterfrombeneaththefoundationsofhomes.Onetypeofmodernpumpsusedtomovewateristhecentrifugalpump.Earlyversionofthecentrifugalpump,thescrewpump,consistsofacorkscrew-shapedmechanisminapipethat,whenrotated,pullswaterupward.Screwpumpsareoftenusedinwaste-watertreatmentplantsbecausetheycanmovelargeamountsofwaterwithoutbecomingcloggedwithdebris.IntheancientMiddleEasttheneedforirrigationoffarmlandwasastronginducementtodevelopawaterpump.Earlypumpsinthisregionweresimpledevicesforliftingbucketsofwaterfromasourcetoacontaineroratrench.GreekmathematicianandinventorArchimedesisthoughttohavedevisedthefirstscrewpumpinthethirdcenturyBC.LaterGreekinventorCtesibiusdevelopthefirstliftpump.Duringthelate17thandearly18thCenturiesAD,BritishengineerThomasSavery,FrenchphysicistDenisPapin,AndBritishblacksmithandinventorThomasNewcomencontributedtothedevelopmentofawaterpumpthatusedsteamtopowerthepumppiston.Thesteam-poweredwaterpumpsfirstwideusewasinpumpingwateroutofmines.Modern-dayexamplesofcentrifugalpumpsarethoseusedattheGrandCouleeDamontheColumbiaRiver.Thispumpsystemhasthepotentialtoirrigateoveronemillionacresofland.Alsoknownasrotarypumps,centrifugalpumpshavearotatingimpeller,alsoknownasablade,thatisimmersedintheliquid.Liquidentersthepumpneartheaxisoftheimpeller,andtherotatingimpellersweepstheliquidouttowardtheendsoftheimpellerbladesathighpressure.Theimpelleralsogivestheliquidarelativelyhighvelocitythatcanbeconvertedintopressureinastationarypartofthepump,knownasthediffuser.Inhigh-pressurepumps,anumberofimpellermaybeusedinseries,andthediffusersfollowingeachimpellermaycontainguidevanestograduallyreducetheliquidvelocity.Forlower-pressurepumps,thediffuserisgenerallyaspiralpassage,knownasavolute,withitscross-sectionalareaincreasinggraduallytoreducethevelocityefficiently.Theimpellermustbeprimedbeforeitcanbeginoperation,thatis,theimpellermustbesurroundedbyliquidwhenthepumpisstarted.Thiscanbedonebyplacingacheckvalveinthesuctionline,whichholdstheliquidinthepumpwhentheimpellerisnotrotating.Ifthisvalveleaks,thepumpmayneedtobeprimedbytheintroductionofliquidfromanoutsidesourcesuchasthedischargereservoir.Acentrifugalpumpgenerallyhasavalveinthedischargelinetocontroltheflowand2pressure.Forlowflowsandhighpressures,theactionoftheimpellerislargelyradial.Forhigherflowsandlowerdischargepressure,thedirectionoftheflowwithinthepumpismorenearlyparalleltotheaxisoftheshaft,andthepumpissaidtohaveanaxialflow.Theimpellerinthiscaseactsasapropeller.Thetransitionfromonesetoffloeconditionstotheotherisgradual,andforintermediatecondition,thedeviceiscalledamixed-flowpump.2.TheCentrifugalPumpThecentrifugalpumpisbyfarthemostwidelyusedtypeinthechemicalandpetroleumindustries.Itwillpumpliquidswithverywiderangingpropertiesandsuspensionswithahighsolidscontentincluding,forexample,cementslurries,andmaybeconstructedfromaverywiderangofcorrosionresistantmaterials.Thewholepumpcasingmaybeconstructedfromplasticsuchaspolypropyleneoritmaybefittedwithacorrosion-resistantlining.Becauseitoperatesathighspeed,itmaybedirectlycoupledtoanelectricmotoranditwillgiveahighflowrateforitssize.Inthistypeofpump,thefluidisfedtothecentreofarotatingimpellerandisthrownoutwardbycentrifugalaction.Asaresultofthehighspeedofrotationtheliquidacquiresahighkineticenergyandthepressuredifferencebetweenthesuctionanddeliverysidesarisesfromtheconversionofkineticenergyintopressureenergy.Theimpellerconsistsofaseriesofcurvedvanessoshapedthattheflowwithinthepumpisassmoothaspossible.Thegreaterthenumberofvanesontheimpeller,thegreateristhecontroloverthedirectionoftheliquidandhencethesmallerarethelossesduetoturbulenceandcirculationbetweenthevanes.Intheopenimpeller,thevanesarefixedtoacentralhub,whereasintheclosedtypethevanesareheldbetweentwosupportingplatesandleakageacrosstheimpellerisreduced.Aswillbeseenlater,theangleofthetipsofthebladesverylargelydeterminestheoperatingcharacteristicsofthepump.Theliquidentersthecasingofthepump，normallyinanaxialdirection，andispickedupbythevanesoftheimpellerInthesimpletypeofcentrifugalpump，theliquiddischargesintoavolute，achamberofgraduallyincreasingcrosssectionwithatangentialoutletAvolutetypeofpumpisshowninFig.(a)Intheturbinepump-Fig(b)theliquidflowsfromthemovingvanesoftheimpellerthroughaseriesoffixedvanesformingadiffusionringThisgivesamoregradualchangeindirectiontothefluidandmoreefficientconversionofkineticenergyintopressureenergythanisobtainedwiththevolutetypeTheangleoftheleadingedgeofthefixedvanesshouldbesuchthatthefluidisreceivedwithoutshockTheliquidsflowsalongthesurfaceoftheimpellervanewithacertainvelocitywhilstthetipofthevaneismovingrelativetothecasingofthepumpThedirectionofmotionoftheliquidrelativetothepumpcasing-andtherequiredangleofthefixedvanesisfoundbycompoundingthesetwovelocitiesInFigc，3c.vuisthevelocityoftheliquidrelativetothevaneandtuisthetangentialvelocityofthetipofthevane；compoundingthesetwovelocitiesgivestheresultantvelocity2uoftheliquidItisapparent，therefore，thattherequiredvaneangleinthediffuserisdependentonthethroughput，thespeedofrotation，andtheangleoftheimpellerbladesThepumpwillthereforeoperateatmaximumefficiencyonlyoveranarrowrangeofconditionsVirtualheadofacentrifugalpumpThemaximumpressureisdevelopedwhenthewholeoftheexcesskineticenergyofthefluidisconvertedintopressureenergy.Asindicatedbelowtheheadisproportionaltothesquareoftheradiusandtothespeed，andisoftheorderof60mforasinglestagecentrifugalpump；forhigherpressures，multistagepumpsmustbeusedConsidertheliquidwhichisrotatingatadistanceofbetweenrandr+drfromthecentreofthepump(Figd)dThemassofthiselementoffluiddmisgivenby2rdrd，whereisthedensityofthefluidand6isthewidthoftheelementoffluid。IfthefluidistravelingwithavelocityuandatanangletothetangentialdirectionTheangularmomentumofthismassoffluid=dM(urcos)Thetorqueactingonthefluiddisequaltotherateofchangeofangularmomentumwithtime，asitgoesthroughthepumpD=dM/t(urcos)=2rbdr/t(urcos)(2.1)Thevolumetricrateofflowofliquidthroughthepump：Q=2rb/t(2.2)4Dr=Qd(urcos)(2.3)Thetotaltorqueactingontheliquidinthepumpisthereforeobtainedintegratingdbetweenthelimitsdenotedbysuffix1andsuffix2，wheresuffix1referstotheconditionsattheinlettothepumpandsuffix2referstotheconditionatthedischargeThus，=Q(2u2rcos2-1u1rcos1)TheadvantagesanddisadvantagesofthecentrifugalpumpThemainadvantagesare：(1)Itissimpleinconstructionandcan，therefore，bemadeinawiderangeofmaterials(2)Thereisacompleteabsenceofvalves(3)Itoperatesathighspeed(upto100Hz)and，therefore，canbecoupleddirectlytoanelectricmotor.Ingeneral，thehigherthespeedthesmallerthepumpandmotorforagivenduty(4)Itgivesasteadydelivery(5)Maintenancecostsarelowerthanforanyothertypeofpump(6)Nodamageisdonetothepumpifthedeliverylinebecomesblocked，provideditisnotruninthisconditionforaprolongedperiod(7)ItismuchsmallerthanotherpumpsofequalcapacityItcan，therefore，bemadeintoasealedunitwiththedrivingmotorandimmersedinthesuctiontank(8)LiquidscontaininghighproportionsofsuspendedsolidsarereadilyhandledThemaindisadvantagesare：(1)ThesinglestagepumpwillnotdevelopahighpressureMultistagepumpswilldevelopgreaterheadsbattheyareverymuchmoreexpensiveandcannotreadilybemadeincorrosionresistantmaterialbecauseoftheirgreatercomplexityItisgenerallybettertouseveryhighspeedsinordertoreducethenumberofstagesrequired(2)Itoperatesatahighefficiencyoveronlyalimitedrangeofconditions；thisappliesespeciallytoturbinepumps(3)Itisnotusuallyself-priming.(4)Ifanon-returnvalveisnotincorporatedinthedeliveryorsuctionline,theliquidwillrunbackintothesuctiontankassoonasthepumpstops(5)Veryviscousliquidscannothehandledefficiently3.C