CentrifugalPumps:ACloserLook

TravisBellJocelynBryantSethBosch

CIVE401TeamProjectNovember19,2014

Introduction: Pumpsareusedtoovercomelossesinapipesystem.Centrifugalpumps

convertmechanicalenergytoincreasethekineticenergyofthemovingfluid.

Interestingly,thecentrifugalforcewasconsideredafictitiousforceuntilthemodern

erawhereitwasdefinedwithintherotatingreferenceplane(SaylorAcademy1).

Centrifugalpumpsaredevicesthatconverttheworkofaspinningmotorintothe

flowandheadincreaseofafluid.Centrifugalpumpscanalsobeappliedinseriesor

inparallel,inordertocreateadditionalfloworhead.Thecentrifugalpumphas

becomecommonlyusedinmanyindustriesoverthelast60years.Thesepumpscan

movealmostanymaterialifproperlydesigned.Theyaremostcommonlyused

howevertotransport:water,sewage,petroleumandpetrochemicals.Astimehas

passedthesepumpshavebeenoptimizedandthedesignhasbeenalteredtofit

variousapplications.Forexampleclosedimpellersarethemostefficientdesignbut

ifthematerialbeingworkedwithishighlyviscousimpellerscanbealteredtoan

opendesigninordertopumpthematerial.Thisissimplyoneinstanceamongst

manythatmakecentrifugalpumpssouseful.Theseversatileandefficientpumps

havebecomeanessentialtooltomodernindustryandareappliedatsomelevelto

everyindustrialprocessinthemodernworld.

Description:

Centrifugalpumpshaveaverybasicdesignthathasbeentweakedslightlyoverthe

yearstoincreaseitsefficiency.Everycentrifugalpumpismadeofthesebasicparts:

impeller,casing,andshaft.Theimpelleriscontainedwithinthebodyofthepump.It

hasbladesthatcontainwaterconnectedtoahubplate.Whentheimpelleris

rotated,thebladesimpartcentrifugalforceonthewaterparticles.Asthewateris

spunoutofthecenter,thepressureandkineticenergyincrease.Attheedge,water

isdischarged.Inordertomaintainconservationofmassoverthecontrolvolumeof

theimpeller,watermustenterthroughtheeye.Theeyehasapressurelowerthan

atmosphericpressure,whichhelpssuckmorewaterintotheimpeller.Centrifugal

pumpsmustbeprimedwithwaterbeforetheycanbeused.Ifapumpisnotprimed,

therotationofairwithinthepumpdoesnotproducesignificantnegativepressure

andcannotdrawwaterupthoughthepump.

Theimpellerisplacedinacasing,keepingthewatercontainedandfedthroughthe

exitpipe.Thecasingontheimpellerhasgreatercross‐sectionalareaaroundthe

exterioroftheimpellerasthecasingapproachestheoutlet.Thisincreaseofcross‐

sectionalareareducesexitvelocityandincreasesthestaticpressureofthewater.As

morewaterisspunoutoftheimpeller,themassflowrateincreasesfrominletto

outlet.Theincreaseincross‐sectionalareaalsoaccountsfortheincreaseinthe

massflowrate.

Impellersaretypicallyclosedforwater,butopenimpellersareusedforfluidswith

moreviscosity.Theclosedimpellerismoreefficientthantheopenimpeller.The

impellerisdrivenbyashaftedconnectedtothemotor.Thisshaftmustbeproperly

sealedtopreventleaksfromoccurringwithinthehighlypressurizedpumpsystem.

Sinceoneendoftheimpellermustbeopentoallowtheentranceofwater,theshaft

ismountedontheothersidelikeacantilever.Ballbearingsneedabearinghousing

withacoolantfluidinsomehighflowratepumps.

Centifugalpumpsarethemostcommontypeofpumpforfluidflow.Theyaccount

forapproximately80%ofallpumpsinapetroleumplant(Girdhar10).Generally,

PumpAnatomy(“CentrifugalPump”)

theyaresplitintotwocategories,verticallyandhorizontallyorientedpumps.They

arealsofrequentlyarrangedinsetstoincreasetheoverallmassflowrate.

Additionally,thebladesattachedtotheinterioroftheimpellerdrasticallyaffectthe

efficiencyofthepump.Backwardcurvedbladesarethemostefficientbecausethey

createthemosttangentialexitofthewaterfromtheblades.Theefficiencyis

hamperedbythebladesimpartingrotationalforcesonthefluid.Thestraightblades

impartanequalforceoutasrotational,whilethebackwardcurvedbladesreduce

therotationalforceonthefluid.Thisexplainswhythebackwardscurvedbladesare

themostefficient.Asatextbookexplains“ifthevanesofthewheelarestraightand

radial;butiftheyarecurved,asismorecommonlythecase,theoutwardforceis

partlyproducedthroughthemediumofcentrifugalforce,andpartlyappliedbythe

vanestothewaterasaradialcomponentoftheobliquepressure,which,in

consequenceoftheirobliquitytotheradius,theyapplytothewaterasitmoves

outwardsalongthem”(SaylorAcademy2).Thefigurebelowshowstheflowrate

comparedtothepowerneededforthreedifferenttypesofblades.

PowerConsumptionvs.OutflowForDifferentBladeGeometries(“CentrifugalPump”)

History:Thecentrifugalpumpisoneofthe

mostefficientandversatilepumps

everdesignedandthusitisthe

mostcommonpumpdesigninuse

today.Thesepumpsbasicallyactin

theoppositewayawaterwheel

operates.Insteadofusingmoving

fluidtocreateshaftpowershaft

powerisutilizedincreating

movementinafluid.Italian

renaissanceengineerFrancescodi

GiorgioMartinifirstconsidered

thisconcept.Martinidesigneda

mud‐liftingmachinein1475based

offthisprinciple(Segrest,

Michelle).However,thefirst

exampleofatruecentrifugalpumpappearedin1687.FrenchinventorDenisPapin

createdasinglestagecentrifugalpumpwithstraightvanesinordertoassistwith

localdrainagework,healsousedtheconcepttohelpwithventilationinacoalmine.

Aboutacenturylaterin1781,thecentrifugalpumpbecamemuchmorecommonas

thesteamenginewasadaptedtoproducerotarymotion(Segrest,Michelle).This

adaptationmadethecentrifugalpumpmuchmoreuseful.Asaresult,centrifugal

pumpsbegantobeinstalledinmunicipalwatersystemsinlargecitieslikeLondon.

Thecurvedvanewaslaterintroducedtocentrifugalpumpsin1851makingthe

designsignificantlymoreefficient.ThisdiscoverywasmadebyBritishinventor

JohnAppold(SaylorAcademy).Appolddiscoveredthatalargeportionofenergyin

straightvanepumpswaswastedbycreatingpurelyrotationalmovementinthe

fluidasopposedtoforcingfluiddirectlyoutwardthroughthepumpspressurized

DenisPapin(IsaacPapinEtMoi….)

outlet.Overthenextcentury,thedesignwasalteredagaintoproducethemultistage

centrifugalpump(multipleimpellers).Thesemultistagepumpsforthemostpartact

thesameasmultiplepumpsineitherseriesorparallel.Theonlydifferenceisthe

impellersareallcontainedinsideasinglehousing.Multistagepumpscancreate

higherheadorhigheroutput,butforthemostpartsinglestagepumpsarestill

preferredforthemajorityofapplications.Thecentrifugalpumphasbecomethe

mostpopularandversatilepumpdesigninthelast60yearswiththedevelopment

ofpowerfulhigh‐speedelectricmotors,whichareusedtodrivetheimpeller.Today

thesepumpsareusedinalmostallconditionsandhavenumerousapplications.

Equations:

Belowarelistedvariousbasicequationsusedtodesignandoptimizesystemsusing

centrifugalpumps.(“CentrifugalPump”)

FlowandimpellerVelocitiesatInletandOutlet(“CentrifugalPump”)

V1=FluidVelocity@InletU1=ImpellerVelocity@InletV2=FluidVelocity@OutletU2=ImpellerVelocity@Outlet

Theworkrequiredtoincreasethefluidvelocitybetweenthepumpinletandoutlet

canbeexpressedby:

Itisknownthattangentialvelocityattheinletisnonexistentsoitcanbesaid

0.Thus:

Riseinenergyheadthroughthepumpcanbefoundusing:

AgainitcanbeassumedthatVθ1=0:

Ifimpellerbladeangleattheoutletisunknownitcanbefoundusing:

SpecificSpeedofthepump,whichrelatesthepumpheadanddischargeforagiven

rotationalspeedofthepump,canbefoundusing:

.

.

FinallyPowercanbefoundincentrifugalpumpsusingthebelowequationsifH(ft)

andQ(cfs):

or

AdvantagesofCentrifugalPumps:

Therearemanyadvantagesofcentrifugalpumps,makingthemextremelypopular

inthemodernworld,mostbeingaresultofthesimplicityatwhichtheyoperate.The

simplicityallowsforcentrifugalpumpstogenerallybesmallinsizeandeasyto

maintain,whichpermitsthefacilitytobeeconomicalwithspaceandinexpensive

upkeep.Essentially,theyarealow‐riskpieceofequipment,asthereisnodangerof

damagetothepumpifthedischargevalveisclosedwhilestarting,whichisnotthe

casewithmanyotherpumptypes.Anadditionalattributethatmakestheuseof

centrifugalpumpsanattractiveoptionisthattheyareabletodealwithlarge

volumesandmediumtolowheadandmediumtolowviscousfluids.

DisadvantagesofCentrifugalPumps:

Althoughtheconceptsatworkforacentrifugalpumpareextremelyrudimentary,

thereareseveralproblemsthatcoincidewiththeuseofcentrifugalpumps.Mostof

theseissueshowever,canbepreventedwiththoroughmaintenanceandupkeep,

alongwithproperusage.Forexample,centrifugalpumpsmustbeprimedpriorto

beinginitiated.Ifthepumpisnotfilledwithaliquidbeforebeingstarted,gasesand

vaporscanenterthesystemandimpairthepumpsabilitytooperateproperly.

Similarly,aproblemthatariseswithcentrifugalpumpsistheoccurrenceof

cavitation.Cavitationinapumpwilloccurfirst,attheinlet–wherethesuction

happens.Thiscanbecausedwhenthenetpositivesuctionhead,ortheheadvalue

requiredtopreventcavitationfromoccurring,istoolow.Cavitationcanleadto

reducedefficiencyalongwithdamagetothepump.

Wearandcorrosionoftheimpellercanensueifsuspendedsolidsarepresentinthe

fluid.Thepropertiesofsaidfluidcanalsocausedamagetothepumpiftheyarenot

ideal;forexample,fluidswithahighviscosity.

Lastly,variablesofthepumpmustbecloselymonitoredinordertoquicklydetect

defects.Iftheflowistoolow,thepumpcanoverheat,reducingefficiency.Itisalso

possibleforwatertoleakaroundtheimpeller.

NetPositiveSuctionHead(NPSH):

Aspreviouslymentioned,cavitationcanoccurifthenetpositivesuctionheador

NPSHistoolow.TheNPSHisameasureoftheheadvaluetakenasthedifferenceof

thesuctionheadattheimpellerandtheliquidvaporhead.Ifthisquantityis

maintainedatalargevalue,cavitationshouldnotoccur.However,ifthisnumberis

notmaintained,cavitationbecomesalargerisk.

SuctionHead:

Where:

hs=suctionheadclosetotheimpeller

ps=staticpressureinthefluidclosetotheimpeller

γ=specificweightofthefluid

Vs=velocityoffluid

g=gravityconstant

LiquidVaporHead:

Where:

hv=vaporhead

pv=vaporpressure

NetPositiveSuctionHead(NPSH): or

Conclusion:

Centrifugalpumpsarealowcost,simpledesignthatisreliableandapplicabletoa

broadrangeofscenarios.Thesepumpsaremostoftenusedinbuildingstopumpthe

generalwatersupplyandthefiresuppressionwater.Centrifugalpumpsarealso

usedinpetroleumrefineriestotransporttheoilinitsdifferentrefinedstages.The

centrifugalpumps’lowriskofdamage,evenifnotoperatedproperly,makesthem

idealfortheseapplications.Inventedinthe17thCenturyandrefinedovertheyears,

centrifugalpumpsaregovernedbyafairlysimplesetofequationsthatallowfor

fairlyrudimentarydesign.Majorrefinementsincludedtheintroductionofbackward

sweptblades,whichreducethetangentialvelocityappliedtothewaterand

thereforeincreasetheefficiencyofthepump.Thesepumpscanbeusedforavariety

ofliquids,includingmoreviscousfluidsthatotherpumpscangenerallynothandle.

ClosedcasingsarebestforNewtonianfluids,whileopencasingsarebestforthe

moreviscousfluids.Multistagecentrifugalpumpsareafurtherdesignoptionthat

canbedesignedtocreateeitherremarkablyhighdischargeorhighheadbasedon

theapplication.Asmodernelectricmotorsimprovefurtherthecentrifugalpump

onlybecomesmoreeffectiveinawiderspectrumofapplications.Thesimplicityand

practicalityofcentrifugalpumpshaveproveneffectiveeversincetheirconception

andtheywilllikelycontinuetobeusedforyearstocome.

WorksCited:

"CentrifugalPump."LearnEngineering.org.LearnEngineering,n.d.Web.05Oct.2014.Evans,Joe.“ABriefIntroductiontoCentrifugalPumps.”PacificLiquidandAirSystems,n.d.

Web.17Oct2014.Girdhar,Paresh,andOctoMoniz.PracticalCentrifugalPumps:Design,Operationand

Maintenance.Oxford:Newnes,2005.Print."IsaacPapinEtMoi…."RevueDeWebExpoDenisPapin.N.p.,n.d.Web.30Sept. 2014.“NPSH–NetPositiveSuctionHead.”TheEngineeringToolbox,n.d.Web.17Oct2014.SaylorAcademy.CentrifugalPumps.N.p.:Xlibris,2010.Web.1Oct.2014.Segrest,Michelle."TheHistoryofPumps:ThroughtheYears."Pumps&Systems. PumpsandSystems,22Dec.2011.Web.01Oct.2014.“UtilitiesmanVolume01–Manualforelectric,plumbing,waterandotherutilities.”n.p.,n.d.

Web.17Oct2014.