单级离心泵设计翻译原文Word文件下载.docx
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单级离心泵设计翻译原文Word文件下载.docx
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centrifugalpump,Introduction,Workingprinciple,Cavitation,MechanismofCavitation,SolutionandRemedies
1.Introduction
Pump,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,AndBritishblacksmithandinventorThomasNewcomencontributedtothedevelopmentofawaterpumpthatusedsteamtopowerthepump’piston.Thesteam-poweredwaterpump’sfirstwideusewasinpumpingwateroutofmines.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.Acentrifugalpumpgenerallyhasavalveinthedischargelinetocontroltheflowandpressure.Forlowflowsandhighpressures,theactionoftheimpellerislargelyradial.Forhigherflowsandlowerdischargepressure,thedirectionoftheflowwithinthepumpismorenearlyparalleltotheaxisoftheshaft,andthepumpissaidtohaveanaxialflow.Theimpellerinthiscaseactsasapropeller.Thetransitionfromonesetoffloeconditionstotheotherisgradual,andforintermediatecondition,thedeviceiscalledamixed-flowpump.
2.TheCentrifugalPump
Thecentrifugalpumpisbyfarthemostwidelyusedtypeinthechemicalandpetroleumindustries.Itwillpumpliquidswithverywiderangingpropertiesandsuspensionswithahighsolidscontentincluding,forexample,cementslurries,andmaybeconstructedfromaverywiderangofcorrosionresistantmaterials.Thewholepumpcasingmaybeconstructedfromplasticsuchaspolypropyleneoritmaybefittedwithacorrosion-resistantlining.Becauseitoperatesathighspeed,itmaybedirectlycoupledtoanelectricmotoranditwillgiveahighflowrateforitssize.
Inthistypeofpump,thefluidisfedtothecentreofarotatingimpellerandisthrownoutwardbycentrifugalaction.Asaresultofthehighspeedofrotationtheliquidacquiresahighkineticenergyandthepressuredifferencebetweenthesuctionanddeliverysidesarisesfromtheconversionofkineticenergyintopressureenergy.
Theimpellerconsistsofaseriesofcurvedvanessoshapedthattheflowwithinthepumpisassmoothaspossible.Thegreaterthenumberofvanesontheimpeller,thegreateristhecontroloverthedirectionoftheliquidandhencethesmallerarethelossesduetoturbulenceandcirculationbetweenthevanes.Intheopenimpeller,thevanesarefixedtoacentralhub,whereasintheclosedtypethevanesareheldbetweentwosupportingplatesandleakageacrosstheimpellerisreduced.Aswillbeseenlater,theangleofthetipsofthebladesverylargelydeterminestheoperatingcharacteristicsofthepump.
Theliquidentersthecasingofthepump,normallyinanaxialdirection,andispickedupbythevanesoftheimpeller.Inthesimpletypeofcentrifugalpump,theliquiddischargesintoavolute,achamberofgraduallyincreasingcross—sectionwithatangentialoutlet.AvolutetypeofpumpisshowninFig.(a).Intheturbinepump[-Fig.(b)]theliquidflowsfromthemovingvanesoftheimpellerthroughaseriesoffixedvanesformingadiffusionring.
Thisgivesamoregradualchangeindirectiontothefluidandmoreefficientconversionofkineticenergyintopressureenergythanisobtainedwiththevolutetype.Theangleoftheleadingedgeofthefixedvanesshouldbesuchthatthefluidisreceivedwithoutshock.Theliquidsflowsalongthesurfaceoftheimpellervanewithacertainvelocitywhilstthetipofthevaneismovingrelativetothecasingofthepump.Thedirectionofmotionoftheliquidrelativetothepumpcasing--andtherequiredangleofthefixedvanes—isfoundbycompoundingthesetwovelocities.InFig.c,
c.
isthevelocityoftheliquidrelativetothevaneand
isthetangentialvelocityofthetipofthevane;
compoundingthesetwovelocitiesgivestheresultantvelocity
oftheliquid.Itisapparent,therefore,thattherequiredvaneangleinthediffuserisdependentonthethroughput,thespeedofrotation,andtheangleoftheimpellerblades.Thepumpwillthereforeoperateatmaximumefficiencyonlyoveranarrowrangeofconditions.
Virtualheadofacentrifugalpump
Themaximumpressureisdevelopedwhenthewholeoftheexcesskineticenergyofthefluidisconvertedintopressureenergy.Asindicatedbelow.theheadisproportionaltothesquareoftheradiusandtothespeed,andisoftheorderof60mforasingle—stagecentrifugalpump;
forhigherpressures,multistagepumpsmustbeused.Considertheliquidwhichisrotatingatadistanceofbetweenrandr+drfromthecentreofthepump(Fig.d).
d
Themassofthiselementoffluiddmisgivenby2πrdrdρ,whereρisthedensityofthefluidand6isthewidthoftheelementoffluid。
Ifthefluidistravelingwithavelocityuandatanangleθtothetangentialdirection.Theangularmomentumofthismassoffluid
=dM(urcosθ)
Thetorqueactingonthefluiddτisequaltotherateofchangeofangularmomentumwithtime,asitgoesthroughthepump
Dτ=dMα/αt(urcosθ)=2πrbρdrα/αt(urcosθ)
Thevolumetricrateofflowofliquidthroughthepump:
Q=2πrbα/αt
Dr=Qρd(urcosθ)
Thetotaltorqueactingontheliquidinthepumpisthereforeobtainedintegratingdτbetweenthelimitsdenotedbysuffix1andsuffix2,wheresuffix1referstotheconditionsattheinlettothepumpandsuffix2referstotheconditionatthedischarge.
Thus,τ=Qρ(
cos
-
)
Theadvantagesanddisadvantagesofthecentrifugalpump
Themainadvantagesare:
(1)Itissimpleinconstructionandcan,therefore,bemadeinawiderangeofmaterials
(2)Thereisacompleteabsenceofvalves.
(3)Itoperatesathighspeed(upto100Hz)and,therefore,canbecoupleddirectlyto
anelectricmotor.Ingeneral,thehigherthespeedthesmallerthepumpandmotorforagivenduty.
(4)Itgivesasteadydelivery.
(5)Maintenancecostsarelowerthanforanyothertypeofpump.
(6)Nodamageisdonetothepumpifthedeliverylinebecomesblocked,provideditisnotruninthisconditionforaprolongedperiod.
(7)Itismuchsmallerthanotherpumpsofequalcapacity.Itcan,therefore,bemadeintoasealedunitwiththedrivingmotorandimmersedinthesuctiontank.
(8)Liquidscontaininghighproportionsofsuspendedsolidsarereadilyhandled.
Themaindisadvantagesare:
(1)Thesingle—stagepumpwillnotdevelopahighpressure.Multistagepumpswilldevelopgreaterhead
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