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Disclaimer: R. L. Deppmann and its affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.
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April23rd2012~MondayMorningMinutes:
CoolingTowerPumpsandPipingPart3Wherecavitationbegins.
ByNormanHall
LastweektheR.L.DeppmannMondayMorningMinutedefinedNPSHR,andendedwiththeHydraulicInstitute(HI)definitionastheabsolutepressurethatwillcausethetotalheadofthepumptobereducedby3%,duetoflowblockagefromcavitation.OfimportanceisthefactthatitdoesnotsaythatNPSHRiswherecavitationbegins.
Theplacewherecavitationbeginsiscalledincipientcavitationandcanbefrom2to20%greaterthantheNPSHRreportedonthepumpcurve.Inaddition,NPSHRisatestedvalue,thetestiswithclearunaeratedwater;notdirty,highlyaerated,towerwaterandpumpshavemanufacturingtolerances.TheNPSHRcanalsovarywiththesystemfluidconditions.
Sotoreducethechanceofdamagetothepumpfromcavitation,weneedtousealargernumberthanthepumpnetpositivesuctionheadrequired.AfewyearsagoHIdefinedsomemarginstoapplytothepublishedpumpNPSHRwithdifferentsystemsandpumptypes.ThemargindependedontheapplicationandtheSUCTIONENERGYofthepumps.Forcoolingtowerapplications,therecommendationwastouseamultipliertoNPSHRof1.3forlowenergypumps,1.5forhighenergypumps,and2.0forveryhighenergypumps.
SOHOWDOWEDETERMINEWHICHMARGINTOUSE?
Thesuctionenergyofapumpdependsonanumberofvariablesbutitcanbeapproximatedbytheformula:
SuctionEnergy=DxnxSX10(6)
WhereS=SuctionSpecificSpeedor(RPMXQ(.5)/NPSHR(.75))
D=ImpellerDiametern=SpeedQ=FlowatBestEfficiencyPoint(BEP)forthemaximumimpellerNPSHisattheflowatBEPformaximumimpelleralso.Note:thatfordoublesuctionpumps,youuseoftheflowatBEP
Nowyouusetheresulttochoosethemarginusingthischart.
SuctionEnergy PumpType MarginforCoolingTowersLessthan150220 LowEnergy 1.3Between150220and250380 HighEnergy 1.5Over250380 VeryHighEnergy 2.0
NextweekweuseanexampleandcompletethisdiscussionofMarginappliedtoNPSHR.
ClickheretorequestacopyoftheXylemBellandGossettCoolingTowerPipingtechnicalbulletinTEH1075
April 23rd 2012 ~ Monday Morning Minutes: