外文翻译--化工工业离心泵.doc
1CENTRIFUGALPUMPSINTHECHEMICALINDUSTRYAbstract:Acentrifugalpumpconvertstheinputpowertokineticenergyintheliquidbyacceleratingtheliquidbyarevolvingdevice-animpeller.Themostcommontypeisthevolutepump.Fluidentersthepumpthroughtheeyeoftheimpellerwhichrotatesathighspeed.Thefluidisacceleratedradiallyoutwardfromthepumpchasing.Avacuumiscreatedattheimpellerseyethatcontinuouslydrawsmorefluidintothepump.Thisarticlestressesonaseriesofcentrifugalpumps,Fromabriefintroductiontotheprinciples.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.Cavitationincentrifugalpump(1)ThetermcavitationcomesfromtheLatinwordcavus,whichmeansahollowspaceoracavity.WebstersDictionarydefinesthewordcavitationastherapidformationandcollapseofcavitiesinaflowingliquidinregionsofverylowpressure.Inanydiscussiononcentrifugalpumpsvarioustermslikevaporpockets,gaspockets,holes,bubbles,etc.areusedinplaceofthetermcavities.Theseareoneandthesamethingandneednotbeconfused.Thetermbubbleshallbeusedhereafterinthediscussion.Inthecontextofcentrifugalpumps,thetermcavitationimpliesadynamicprocessofformationofbubblesinsidetheliquid,theirgrowthandsubsequentcollapseastheliquidflowsthroughthepump.Generally,thebubblesthatforminsidetheliquidareoftwotypes:VaporbubblesorGasbubbles.1.Vaporbubblesareformedduetothevaporisationofaprocessliquidthatisbeingpumped.ThecavitationconditioninducedbyformationandcollapseofvaporbubblesiscommonlyreferredtoasVaporousCavitation.2.Gasbubblesareformedduetothepresenceofdissolvedgasesintheliquidthatisbeingpumped