Hydraulics and Pumping - Irrigation NZ
Transcript of Hydraulics and Pumping - Irrigation NZ
i|HYDRAULICS AND PUMPING
Hydraulics and Pumping
BOOK 10
ContentsHydraulics 1
Volume 1
Flow 1
Headandpressure 2
Frictionloss 3
Commonproblems 6
Reducingairaccumulation 8
Pumps 10
Whyarepumpsrequiredforirrigation? 10
Basicpumpdesignfeatures 10
Centrifugalpumpcomponents 12
Pumpstypesandtheirapplications 13
Pumpperformance 15
IntroductiontoNPSH(NetPositiveSuctionHead) 18
Pumpsinseriesandparallel 19
Pumpsetup 21
ThisbookispartofaseriesprovidingacomprehensivetrainingresourceforirrigationindustryparticipantsinNewZealand.
Itprovidesanoverviewofhydraulicandpumpingconsiderationsforirrigation.
Compiledby:BirendraKC,S.McNally,I.McIndoeandA.Curtis.
©IrrigationNewZealand2015
SupportedbySustainableFarmingFund
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Hydraulics
HYDRAULiCS
• Thetransportationofwaterthroughpipeshasanumberofcharacteristicsandpropertiesthatmustbeconsideredwhenbothdesigningandmanagingirrigation.
VolumeThestandardunitformeasuringavolumeofwaterusedforirrigationinNewZealandisacubicmetre(m3).Forsmallervolumeslitres(l)mayalsobeused.Whenconvertingbetweenunits1m3=1,000l.
• 1mmdepthofwaterspreadacrossanareaof1m2isavolumeof1litre(1l).
• 1mmdepthofwaterspreadacross1hectare(ha)is10,000lor10m3–thisisacommonlyusedterminirrigationapplication.
Flow
FLOW RATEFlowrateisameasureofthevolumeofwatertravellingpastagivenpointforagiventime.TherearethreemainmeasuresusedforirrigationinNewZealand:
1. litrespersecond l/s
2. cubicmetresperhour m3/hr
3. cubicmetrespersecond m3/sorcumecs(normallyusedforschemeflows)
Whenconvertingbetweenunits:
• 1l/s=3.6m3/hr
• 1,000l/s=1cumec
FLOW VELOCiTY Flowvelocityisthespeedatwhichwatertravelswithintheirrigationsystem.Forexample,themainlineflowvelocity.ThecommonunitusedforirrigationinNewZealandismetrespersecond(m/s).
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Head and pressure Pressureisameasureoftheforceperunitarea.Influidspressureisgeneratedbycontainingaliquid,withinapipeforexample,andthenapplyingaforcethrougheithermechanicalmeans,byapumpforexample,orfromgravity.Headisameasureofpressureandisexpressedinmetres.Itisthepressureexertedbytheequivalentheightofacolumnofwater.
Figure 1.
1 m
0.1 bar g 0.3 bar g
3 m
1 bar g
10 m
Thestandardunitsofmeasureforpressureare:
1. kilopascals kPa
2. bar bar
3. poundspersquareinch psi(imperialmeasurement)
Whenconvertingbetweenunits:
• 10metresofhead=1barofpressureor100kPaor14.5psi
Onebarofpressureistheequivalenttotheatmosphericpressureatsealevel.
Forwater,headandpressureareeffectivelythesamething.Forexample,ifaverticalcolumnofwaterhasaheadof10m,apumphastoproduce100kPaofpressuretopushthatwatertothetopofthecolumn(equalisingtheforcebeingexertedbygravity).
Forliquidsotherthanwater,headcanbedifferenttothepressuredependingonwhethertheliquidisheavierorlighterthanwater.Withinanirrigationcontextthisisusuallynotsignificantunlessotherliquids,effluentforexample,areinjectedintotheirrigationsystem.
Temperaturealsohasanimpact.The‘weight’ofthecolumnofwaterathighertemperatures(e.g.25˚C)isfractionallylighterthanatlowerones(e.g.5˚C)becauseofthedifferentdensitiesofwateratdifferenttemperatures.Withinanirrigationcontextthisisnotsignificantacrossthelikelyrangeoftemperatures.
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FLOW CHARACTERiSTiCSWatereithermovesaslaminarorturbulentflow.Thereisalsoaphasecalledtransitionalflowwherethereisamixofthetwo.Laminarflowoccursatlowvelocity,andasthewatermovesfastertheflowchangesthroughtransitionaltoturbulentflow.Thepointatwhichtheflowtypechangesisvariable.Itisdeterminedbyanumberoffactorsincluding;pipediameterandroughness,suddenchangesindirectionorobstructionssuchasvalves.Turbulentflowshaveamuchhigherfrictionlossbecauseoftherandommovementofthewatermolecules.Velocityisthereforeanimportantfactorasithasadirectrelationshiptothefrictionlosseswithinapipe.
Figure 2. Turbulent and laminar flow of water.
TURBULENT
LAMINAR
Friction lossForwatertoflowthroughpipes,itneedstobepressurisedthroughapplyingenergy.Aswatermovesagainstthewallsofthepipe,frictionoccursandthewaterlosesenergy.ThislossresultsinalossofpressurewhichisbetterknownasFriction Loss.
Anythingthatmakesitharderforwatertoflowthroughapipeincreasesfrictionloss.Excessfrictionlosscaneitherreducesystemperformanceand/orwasteenergy.Thisbecomesmorecriticalwhenpayingforelectricityorfossilfuelstodriveapumptogeneratetheflow.
Thethreemainpipedesignfactorsthatinfluencefrictionlossinpipes:
• Diameter. Foragivenflowrate,thesmallerthepipediameter,thehigherthevelocityofthewaterthroughthepipe;themorefrictionlossoccurs.Thisisthemostsignificantfactortoconsiderwithinthepipedesignprocess.
• Surface roughness. Theroughertheinsidewallofthepipe,themorefrictionloss.Thisisacharacteristicofthepipeselectedsoisanimportantconsideration.
• Length. Thelongerthepipe,themorefrictionlossoccurs.Wherelongpiperunscannotbeavoided,thedesignprocessneedstoconsiderdiameterandroughnessaspartofthepipeselectionprocess.
Othercharacteristicsthataffectfrictionlossinpipesforirrigationsystems:
• Fluid viscosity. Thickerfluidsexperiencehigherfrictionandthismaybeafactorforpumpingeffluentbutisnotanissueforfreshwaterirrigation.
• Pipe material. Somematerialscreatemorefrictionlossesthanothersduetotheroughnessandthejointingmethods.
• Pipe age.Mostpipes,especiallymetalpipe,becomerougherwithage(theycorrode)andthisresultsinmorefrictionloss.
• Fittings. Allfittingssuchaselbows,tees,reducersandvalvescreatefrictionlosses.
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Todeterminefrictionloss,allpipeandfittingsmanufacturersprovidefrictionlosschartsfortheirproductsmeasuredunderstandardconditions.
Tocalculatepipefrictionloss:
1. Determinethepipetype.
2. Determinetheflowrateorvelocity(somechartsuseboth).
3. Determinethelengthofpipe.
4. Findthefrictionvalueonthemanufacturers’chartsforthepipetypeandflow(usuallyexpressedaspressurelossper100m).
5. Multiplythefrictionvaluebythelengthofpipe.
Forexample,a32mm(OD)lowdensitypolyethylenepipewithaflowrateof1l/shasafrictionlossvalueof61kPa/100mlength.
Thereforea350msectionofpipewillhaveafrictionlossof61x3.5=213.5kPa.
FRiCTiON LOSS EQUATiONS Thefollowingequationsareusedtogeneratemanufacturers’frictionlosstables.Wheretablesarenotavailabletheycanbeusedtocalculatedfrictionlossfromfirstprinciples.
Figure 4. Friction in pipes Adapted from www.wermac.org/steam/steam_part9.html
h1 h2
Flow velocity (v)
Length (L)
Point 1 Point 2
Pipe diameter (D)
hf
Inapipewithinsidediameter(D),thefrictionloss(hƒ)betweenpoint1and2forafluidflowingwithvelocity(v)canbeestimatedusingtheDarcy-Weisbachequation:
hƒ=ƒ( LD )( v2
2g )Whereƒisthefrictionfactorofthepipeandgisaccelerationduetogravity.
Thisequationisrecommendedforfrictionlosscalculationsinlaminar,transitionalandturbulentflowconditions.
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Ifflowisfullyturbulent,theHazen-William’sequationisrecommendedinstead:
hƒ=Kx( 100C )1.852
x Q1.852
D4.866 x
L100
Wherehƒ=headloss, K=unitcoefficient(2.38×108forSIunit),C=coefficientofretardationanddependsonpipematerial,Q =flowrate,D=insidediameterandL=pipelength.
Howeverforturbulentflowinsmoothpipes(e.g.PVC,PE,ABS),theWatters-Kellerequationisbetter:
f=0.32×R-0.25
WherefistheDarcy-Weisbachfrictionfactor.
FRiCTiON LOSS THROUGH FiTTiNGSAllfittingsthatwaterpassesthroughhavefrictionlossesassociatedwiththem.Generallyfrictionlossinstandardfittings(corners,teesandreducers)isquitesmall,butformorespecialisedfittings(automaticvalves,pressurereducingvalves,backflowpreventersandfilters)itcanbequitelarge.
Thefrictionlossthroughpipefittingsneedstobedeterminedwhenworkingoutthetotalpressurerequirementsforasystem.
Frictionlossinfittingsandvalvesisbestdoneusingmanufacturer’stables.
Howeverwheretablesdon’texisttheycanbecalculatedbyusingthefollowingformula:
hƒ= k x v2
2xg
Wherekisresistancecoefficientforthefittings.
TOTAL DYNAMiC, STATiC AND DiSCHARGE PRESSUREPumpingsystemsconsistofthreemaincomponents;eachwithanumberofpartsmakingupthetotalpressurerequirements.
1. Suction or supply characteristicsa. Suctionpipefrictionlossb. Suctionliftc. Suctionentranceloss
2. Pump and delivery a. Pumpdischargepressureb. Frictionlossesthroughpipesandfittingsc. Elevationchanges
3. Discharge pressure requirements a. Multipleemitters.
Thesumofthepressurerequirements(alloftheabove)iscalledthetotaldynamicpressure(ortotaldynamichead).
Staticpressure(orhead)isthepressureneededtoovercomeallofthepressurestothepointofirrigatordischarge,includingtheelevationchanges,suctionanddistributionlosses.
Thepumpdischargepressureisthepressureneededatthepumpdischargepointtoovercomeelevationchanges,pressurelossesinthesystemanddelivertherequiredoperatingpressureoftheirrigationsystem.
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Thefollowingdiagramdisplaysthedifferentcomponents–staticsuctionhead,totalstaticheadandstaticdischargehead.
Figure 5. Static suction head, total static head and static discharge head.
Totalstatichead
Staticsuction
head
Staticdischarge
head Staticsuction
lift
Staticdischarge
head
Totalstatichead
Theabovepressurecalculationsareusedinthedesignprocesstocalculatepumpsizesandspecifications,alongsidepipesizesandclassestobeused.Eachmodelofpumpandclassofpipehasamaximumandoptimumoperatingrangesoknowingthesystemhydraulicpressureandflowrequirementsiscrucialtoensurecorrectoperation.
Common problems
WATER HAMMERWaterhammeriscausedbyrapidchangesinwatervelocityinthepipeline.Forexample,thewatermovingalongapipecollidesagainsttheclosedend,creatinganexcesspressurespike(waterhammer)whichmovesbackalongthepipe.Thisoverpressurewaveisproportionaltothevelocitychangeofthewater;ifthechangeinflowvelocityisfastthepressurewavecanbeverylarge.
Waterhammertypicallyoccursinapipelinewhenavalveisrapidlyopenedorclosedorwhenthepumpsuddenlystops.Thiscancreatesignificantpressurespikesinexcessoftheratedpressurecapacityofapipeorfitting.Inthecaseofsteelorfibrereinforcedcementpipes,waterhammerpressuresaremoreseriousbecauseoftheinelasticnatureofthepipematerial.PVCandglassfibrehaveanadvantageoverotherpipetypesduetotheirelasticitywhichcanabsorbthepressurewaveasitmovesalongthepipe.
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Figure 6. Water hammer.
Valve closed,water still
Valve open,moving water
Valve closesWATER HAMMER
Reducingwaterhammercanbeachievedby:
• Controllingandslowingvalveandpumpoperationssothatchangesinvelocityaremadegradually.
• Minimisingvelocitiesbyusinglargediametermainlinepiping–1.5m/sisanacceptablemaximumvelocityinmostinstances.
• Well-designeddistributionsystems–includingpipes,valvesandairvalvesthatmanagechangeinflowsituationoraccommodatewaterhammertowithinthetolerancesofthesystem.
• Theuseofflowcontroland/orsurgeanticipationvalves.
• Carefuldesignofpumpingstation’scontrolvalves,particularlyforlargesystems.
AiR ACCUMULATiONAiraccumulationfrequentlyoccursinirrigationsystems.Commonexamplesofthisinclude:
• Whilstthepipelineisfilling,thewaterbeingaddeddoesnotdisplacealltheairinthepipeline.
• Whenthelevelofawatersourcedropsandthesuctionorvacuumactionofthepumppullsairintothepiping.
• Infaultyinstallationsorleaksinpumpsuctionhoses(thepumpsucksinair).
• Aircanbedissolvedintowaterandisthenreleasedwhenpressureand/ortemperatureconditionschange.Aswatercancontainmoreairathighpressure,anypressuredropwillallowittoescape.Thissituationoftenoccurswithundulatingpipelines,andespeciallyatthedownstreamsideofpressure-reducingvalves,partiallyclosedgatevalves,orsimilarobstructionsthatcauseapressuredrop.
Allofthesesituationscauseairpocketstoaccumulateinthetopofpipelines,especiallyathighpointsinanundulatingnetworkoratfittingswheretheycauseachangeindiameter.
Underchangingflowvelocitysituations,theseairpocketscanbemobilisedleadingtounwantedeffectsontheirrigationsystem.Airdischargefromemittersorevenairvalvescantriggerpressureinstabilityand/orwater-hammer.
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HYDRAULICS
Airpocketscanalsocreatemajorissuesin‘flat’pipingsystemswithlittleslope,orinlow-velocitysystemswheretheflowofwaterisnotcapableof‘clearing’theair.Thepresenceofaircansignificantlydecreasethecrosssectionoftheflowpathinthepipewhichincreasesthepump’senergyconsumption;thepumphavingtoworkhardertopushwaterpasttheairpocket.
Apipelineideallydesignedforairoutletmust:
• haveuniformascendingslopeintheflowdirection,and
• bewithoutanyformofobstruction.
Fewpipelinesofanylengthandnogravitylineofanykindcanmeettheseidealrequirements.Peaksandcontinuousslopechangesarenormallyunavoidable.
Reducing air accumulation
LOCATiON OF PiPELiNEDuringthepipelinedesignprocessitisadvisabletokeepthefollowingdesignprinciplesfrontofmindsothatairaccumulationsareminimised:
• Attempttopositionthepipelineatleastsixtoninemetresbelowthehydraulicgradient.
• Avoidlongdistances(>500m)ofhorizontalpipelines.
• Maintainslopesofatleast1:500.
• Avoidextremevelocities,bothhighandlow.
Conditionswhichnecessitatetheuseofspecialisedairreleaseequipmentshould,asfaraspracticable,beavoided.Ifspecialisedairreleaseequipmentisrequired,uncomplicatedorsimpleautomatedoptionsthateliminatetheneedforconstantsupervisionandmaintenanceshouldbechosen.
Airvalvessizedtoremoveslowlyaccumulatingair(i.e.fromleaksordissolvedair)arenotgenerallybigenoughtoremovetheairattheraterequiredforlinefilling.Theaircompressedtohighpressureswithinapipelineduringuncontrolledrapidlinefillingcanleadtocatastrophicanddangerouspipelinefailure.Airvalveselectionisaspecialistareaandmanufacturers’specificationsshouldbeconsultedtoaccommodatebothslowandrapidairremoval.
Cautionshouldbeusedwheretruesiphonconditionsarise.Whilesiphonsofuptoapproximately4.5metresabovethesteepesthydraulicgradientcanphysicallybeaccommodatedbysomepipematerials,thenegativepressurecreatedcancollapsepoorlyinstalledpipesand/orrendernormalautomaticairandanti-vacuumvalvesuseless.
PLACEMENT OF AiR VALVESAirvalvesshouldbeinstalledatthefollowingpositionsonapipeline:
• Peaks Multi-purposeairvalvesarerequiredatallpositionswherepeaksarise.Peaksarecreatedwhereverthepipelinehasareversalofslopewithrespecttothehydraulicgradient.Thesecreatelow-pressurezoneswhereaircanaccumulate.Airvalvesshould,attheveryleast,besitedattheendsofsuchapipesection,andpossibly,dependingonlength,inbetween.Itisimportanttonotethat,whileairaccumulationswilloccuratpeaks,theprecisepositionoftheairaccumulationisactuallysituatedatthepointwhereapeakwithregardtothehydraulicgradientcanbeidentified.
HYDRAULICS
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Figure 7.
a + b + c + d
Pump Shut-off Head
Air Valves Closed
Pump
Hydraulic Gradient – Pipe Flowing Full
ReservoirH.W.L.
L.W.L.
Air Valves Closed
d
c b
a
aa + b
a + b + c
• Slope changes that do not create peaks Aircanaccumulateatanypointwhereadescendingslopesteepens.Whileapeakisnotalwaysformed,itisadvisabletoinstallatleastasmallorificeairvalveatthispoint.Similarly,itisgoodpracticetoinstallasmallorificeairvalveatanypointwhereanascendingslopelevelsoff.
• Long ascending pipe section
• Additionalvalvesmayberequiredtoaccommodatehighairflowratesduringfillingordraining,dependingonthelengthofthesection.Theseairvalveswillmostlybethelargeorificetypespacedatapproximately400to800m.
• Long, horizontal pipe sections (slope <1:500) Iflonghorizontalpipesectionsareunavoidable,multipurposeairvalvesmustbeprovidedattheendsofsections,aswellasat400to800mintervals.
• The pipeline as a whole Oncethesittingofairvalveshasbeenundertakenforallindividualpoints,itisthenadvisabletoinvestigatethepipelineasawholetoensurethatasufficientnumberofairvalveshavebeenprovidedforinthetotaldesign.
Generallymoreairvalvesshouldbeprovidedinthefirstsectionofapipelinethaninthelastsection.Aruleofthumbbeing,airvalvesshouldgenerallybeplacedascloseas150mapartatthebeginningofapipeline,andasfarapartas800to1000moverthelastsection.
Itisgenerallygoodpracticetoensurethatasmuchairaspossibleisremovedfromhighpressuresections.Thisminimisesthepotentialforairexpansioninlow-pressurezones.
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PumpsWhy are pumps required for irrigation?Watersourcesforirrigationcanbeatahigher,thesame,orlowerelevationthantheareatobeirrigated.Waterfromahigherelevation,whendistributedthroughaclosedconduit(apipe),possessespotentialenergy.Thismayenablewatertobesuppliedforirrigationattherequiredpressurewithoutaddingexternalpower.However,forsimilarorlowerelevationsthewaterwillhaveazeroornegativepotentialenergy.Thisentailstheadditionofenergysoitcanbesuppliedattherequiredpressure.Apumpisneededtodothis.Putsimply,apumpisamachinethattransfersmechanicalenergyintopressureandvelocityenergyinflowingwater.
Therequiredflowrateanddischargepressure,combinedwiththeverticaldistancebetweenthewaterlevelandthepump,formthebasicvariablestoselectthetypeandpowerrequirementsofapump.Flowrateisameasureofthevolumepassingapointperunittime.Dischargepressuredescribesthepressureofaliquidasitleavesapump.
Anothersignificantfactortoconsiderwhenselectingapumpforirrigationisthewatersource.Ifthewatersourceisshallow,surfacewater(river,lakes,canals,ponds,sumps,reservoirs,etc.,typically1–6mlift),asubmersiblepump,short-coupledverticalturbinepump,orendsuctioncentrifugalpumparetheavailablechoices.Ifthewatersourceisdeepgroundwater,orthepumpissignificantlyabovethewatersurface(6–300mlift),adeepwellormulti-stagesubmersibleturbinepumparenecessary.
Thequalityofwaterisalsoafactortoconsiderwhenselectingapump.Abrasivessuchassandorsilt,debris,andothersolids,alongwithanycorrosivecharacteristicsofthewatersourceneedtobecarefullyconsideredinordertoselectrightpump.
Basic pump design features
CENTRiFUGAL PUMPSThemajorityofpumpsusedintheirrigationindustryfallunderthegeneralcategoryofcentrifugalpumps.Thesubcategoriesofthecentrifugalpumpinclude;ClosedCoupledElectric,HorizontalFrameMounted,VerticalMountInducer,SpecialApplicationEngine(SAE)EngineMount,GearBoxPTO,VerticalTurbineandSubmersible.
Centrifugalpumpsrelyonacentrifugalforcetoaddenergytowater.Theyhaveanimpellermountedonashaftthatisrotatedbyamotororengine.Theimpellerismountedinthepumphousing–thevolutecasing.
Theliquidentersthecentre,oreye,oftheimpellerthroughthesuctionorpumpinlet(suctionside).Duringrotation,theimpellerdispersestheliquidradiallythroughcentrifugalforcetoitsouterperipheryatahighvelocity.Astheliquidleavestheimpeller,thishighvelocitydecelerateswithinthevolutecasingwhereitsenergyisconvertedintopressurebythediffuser(theinsideshapeofthepumphousing).Theliquidthenpassesthroughthevolutecasingintothedischargenozzle.Thefollowingfigureshowsthemaincomponentsofacentrifugalpump.
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PUMPS
Figure 8. Main components of a centrifugal pump
Suction eyeImpeller
Volute casing
Vanes
Discharge
POSiTiVE DiSPLACEMENT PUMPS Pistontypereciprocatingpositivedisplacementpumpsarecommonlyusedforinjectingagri-chemicalsintoanirrigationsystem,butarerarelyusedforthemainirrigationpump.
Positivedisplacementpumpsuseareciprocatingmechanicalelementsuchasapistonorflexingdiaphragmtomovewater.Eachstrokeofthepiston(orflexofthediaphragm)pushesafixedamountoffluidafixeddistance.
Thespiralshapedrotarypumpissometimesusedfordeeperboreholes.
Themostimportantcharacteristicofpositivedisplacementpumpsistheydeliverlargepumpheads.However,thischaracteristicisalsoadisadvantage.Ifthesystemdeliveryrequirementsdecrease,thesystempressurewillincrease,whichcouldhaveseriousconsequencesincludingpipefailureorpumpdamage.Pressurereleasevalvescannegatethisrisk.
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PUMPS
Centrifugal pump componentsThediagrambelowoutlinestheimportantcomponentsofabasiccentrifugalpump.
Figure 9. Important components of a basic centrifugal pump.
Suctionnozzle
Impeller
Volute
Discharge nozzle
Casing
SealShaft Oil rings
Bearings
SUCTiON NOZZLEThisistheinlettothepumpvolutecasingonthesuctionside.Itsfunctionistoprovideaconnectionforsystempipingtothepumpinlet,allowingforsmoothtransitionoftheliquidintotheeyeoftheimpellerwithoutobstruction.
iMPELLERThisisthemaincomponentofthepump.Theimpeller,thebladedrotatingassembly,canbeopen,semi-openorenclosed.Theopenimpellerhasnoshroudoneitherside,semi-closedimpellerhasashroudononesideandtheenclosedimpellerhasshroudsonbothsides.Theshroudshelptodirectandguidetheliquidflow.Theenclosedimpellerismostcommonlyusedinirrigationpumpswhilethesemi-closedimpellerisusefulforeffluentapplications.
Theimpellor’spurposeistoimpartenergytotheliquidbymeansofacentrifugalforce.Theimpellerwidthinfluencescapacityandhead;theimpellerareainfluencestheNetPositiveSuctionHead(NPSH)requiredandhydraulicefficiency;thenumberofimpellervanesinfluencetheslopeofthehead-capacitycurve.
VOLUTE CASiNGThisisthecomponentofthecentrifugalpumpthathousestheimpeller.Itsfunctionistocollectthehighvelocityliquidexitingtheimpeller,convertittopressure,andchannelittothedischargeflange.
SHAFTThisisthesolidcylindricalcomponentonwhichtheimpeller,shaftsleeve,andbearingsaremounted.It’sthebackboneofthepump.Itspurposeistoprovideameansthroughwhichpoweristransmittedtotheimpellerfromthepumpdriver.
BACK PLATEThisisaremovablecomponentthatisattachedtothepumpvolutecasingoppositethesuctionside.Dependingonthemanufactureritmayalsohouseacasingwearring.
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PUMPS
GLANDThisattachestothepumpbackplate.Itspurposeistofollowandcompressthepackinginastuffingbox,formingasealaroundtheshaftwhereitextendsthroughthestuffingbox.
BEARiNGSTheprimarypurposeofthebearingistoholdtheshaftinthebearingframeandabsorbtheradialandaxialforcesappliedtotheshaftasaresultofpumpoperation.
WEAR RiNGSThesecomponentsarereplaceablestationaryringsmadeofvariousmaterialsinavarietyofsizesandwidths.Theirpurposeistoprotectthevolutecasingfromwear,andmaintainapredeterminedrunningclearancewiththeimpellerforpressurebreakdown.Thewiderthewearring,themorethrottlingsurfaceavailabletomaintainefficiencyoverthelifeofthecomponent.Somepumpmanufacturersprovidereplaceablewearringscalledimpellerorrotatingwearringsontheimpelleraswellasthevolutecasing.Wearringsarereplaceable,sothepredeterminedrunningclearancescanbemaintainedoverthelifeofthepump,withouthavingtoreplaceexpensivemajorcomponentssuchasvoluteorimpeller.
Pumps types and their applications
SiNGLE STAGE END-SUCTiON PUMPSThispumpisthesimplestandmostcommonlyusedforirrigation.Itisthemostbasiccentrifugalpumpandalltheothertypeshavebeendevelopedfromittosatisfyspecificrequirements.
Parallelandseriesarrangementsarecommonlyfoundwiththesetypesofpumps.
ThemainreasonnottousethesetypesofpumpsisNetPositiveSuctionHead(NPSH)problems.
MULTiSTAGE PUMPSAmultistagepumpisacentrifugalpumpwithaseriesofimpellersarrangedinseriesononeshaft.Thewaterispumpedfromoneimpellertothenext,andjustasforpumpscoupledinseries,itismeantforlargerpumpheads.Eachsuccessivepumpactsasaboosterpumpaddingpressuretothewater.Asufficientnumberofstages(bowlassemblies)areconnectedtoproducethedesiredpressure.Byusingalargenumberofstages,veryhighpressurecanbeproduced.Themaximumflowrate(capacity)ofthepumpisdeterminedbythefirstimpeller.Eachsuccessiveimpelleronlyservestoincreasethepressureorhead.Atthetopofthepump,thewaterischannelledthroughadischargeheadintohorizontalpiping,whichsuppliestheirrigationsystems
SUBMERSiBLE SiNGLE STAGE END SUCTiON PUMPSAsubmersiblepumpiswherethewholepumpunitisinthewaterwiththeimpellers,bowls,andtheelectricmotorpoweringthepump,allinstalledbelowthewaterlevel.
ThesetypesofpumpshavebeenspeciallydevelopedforsituationswhereNPSHproblemsarebeingexperienced.
Thepumpisinstalledinsuchawaythattheimpelleritselfisbelowthewaterlevelandthepumpthushasapositivesuctionhead.Theyareusuallyappliedwherelargepumpdeliverieshavetobepumpedagainstsmallpumpheads.
Submersibletypesofpumps,however,alsoshowupasmultistagepumps.
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PUMPS
SUBMERSiBLE MULTiSTAGE PUMPSTherearetwocommontypesofsubmersiblemultistagepumps,namelythetypeswherethedriverisplacedabovegroundandthetypeswherethedriver–inthecaseofanelectricmotor–isbelowwaterlevel.Anexampleoftheformeristheverticalturbinethatwassopopularforboreholes,whichintheearlydayswasknownasa‘bucketpump’.Thedriverisplacedabovegroundandtheimpellers,whicharearrangedinseries,aredrivenbyashaft.
VERTiCAL TURBiNE PUMPSInthesepumpsadrivemotororengineislocatedabovetheground,butthepumpitselfisinstalledbelowtheground,belowthewaterlevel.Itistypicallyusedforirrigationapplicationswherethewaterlevelisbelowthepracticallimitsofacentrifugalpump.Theterm‘verticalpump’isusedbecausethegeneraldirectionofwatermovementisverticallyupwardthroughthevariousstages.
SUBMERSiBLE BORE HOLE PUMPSSubmersibleturbinepumpsusedinirrigationareanevolutionoftheverticalturbinepumpdesign.Verticalturbinepumpshaveamotorontopofthewell.Thesubmersiblepumphasthemotorattachedtothepumpinthewell.
Likeverticalturbinepumps,submersiblesareusedinirrigationapplicationwherethewaterlevelisbelowthepracticallimitsofacentrifugalpump.Asthenameimplies,theentireunitofthepumpissubmergedunderwaterandeverythingelectrical,fromthecabletomotorissealedfrommoisture.Thereareawiderangeofsizesanddesigns.
Asubmersiblepumpconsistsofaseriesofimpellersanddiffusersstackedtogether,withawater-proofelectricmotormountedbelowthepumpend.Thisallowstheinstallationofsubmersiblesatdepthsunreachablebycentrifugalorjetpumps.Theuseofmultiplestagesallowstheproductionofadequatehead(pressure)fortheirrigationsystemoperationtobedeliveredatthesurface.
PUMPiNG FROM BORES – PUMPiNG LEVEL CONSiDERATiONSAwellissimplyaholeinawater-bearinglayeroftheearth.Thelayeriscalledanaquifer.Thecreationofaholeintheaquifercreatesastoragespacethatallowsthecollectionofwaterwithinit.Theamountofwaterandtheratethatwaterflowsintothewellisdeterminedbytheaquifer.Aloose,porousgravel-typeaquiferwillproducemorewaterthananaquifercomposedofasolidormoretightlycompactedmaterialsuchasclayorgranite.
Insideawelltherearetwowaterlevelscalled‘standingwaterlevel’and‘pumpingwaterlevel’.Thestandingwaterlevelisastaticwaterlevelwhenthewellisnotbeingpumped.Pumpingthewellcausesthestandingwaterleveltodropdown.Thisdropiscalleddrawdown.Thepumpinglevelisthedepthatwhichthewellstopsdrawingdown.Figure10illustratesthis.Figure 10. Static and pumping level.
Depth to water
Land surface
Static water level
Cone ofdepression
Pumping level
Drawdown
Radius ofinfluence
15|HYDRAULICS AND PUMPING
PUMPS
Pump performance
PUMP CURVESForapumptoadequatelydothejobrequired,itmustbeabletodeliverthecorrectamountofwater,anditmustalsoproducethecorrecthead(pressure).Thepumpcurveforaparticularmakeandmodelwillhelpdeterminewhetherornotthatpumpwillperformcorrectlyintheirrigationsystem.
Theperformanceofapumpisindicatedonthepumpcurve.Atypicalpumpcurvewillindicatetotaldynamichead,energy(kW),efficiency,andNetPositiveSuctionHeadRequired(NPSHR),allplottedinrelationtothecapacityrangeofthepump.
Figure 11.
H (m)
50
40
70
Head
60
50
40
20
10
2
4
6
8
10
0
30
30
20
10
0
10
0 0
2
4
6
8
0 10 20 30 40 50 60 70 Q [m3/h]
P (kW)2NPSH(m)
h (%)
Efficiency
NPSH
Power consumption
Theyaxis(vertical)showstotalheadwhilethexaxis(horizontal)showsflowcapacity.Forexample,topumpagainstatotalheadof42myoucouldpumpatarateofabout70m3/hrwithaNPSHrequiredofthreemetresandatanefficiencyofabout78percent(seeFigure11above).Usingtheabovedata,theenergy(kW)wouldbe10kW.
Fromthis,itcanbeseenthatthepumphead(H)isamaximumwherethepumpdelivery(Q)equals0.However,HdropsasQincreasesandtheHQlineisacurvedlinecharacteristicofthatparticularpump.
WhereQ=O,Hisdirectlyequaltotheperipheralvelocityoftheimpeller,i.e.thefasterthepumpspeedand/orthelargerthediameteroftheimpeller,thelargertheperipheralvelocityoftheimpellerandthelargerthepumpheadthepumpcangenerateatnoflow.
Therelativevelocityattheoutletoftheimpelleris,however,alsoafunctionoftheangleofoutletofthevanesoftheimpeller.Thedesignoftheimpeller,andespeciallythatofthevanes,thusdeterminestheslopeoftheHQline.
16 | HYDRAULICS AND PUMPING
PUMPS
Theactualpumpheadatnoflow(pumpdelivery)iscalledthecut-offrange.Itisparticularlyimportantforthedeterminationofpipeclasses.Eachpumpthatisavailableinthetradehasitsowndistinctivepumpcurve.Thispumpcurveisdeterminedbytestingthepump,orinafewexceptionalcasesbydeterminingittheoretically.Apumpinoperationwillalwaysoperatesomewhereonitspumpcurve.Thispointonthepumpcurvewherethepumpoperatesiscalledthedutypoint.
PUMP AFFiNiTY LAWSTheremaybeinstancesinwhichthepumpoperatingspeedisdifferentfromthatshownonpumpcurve,ortheimpellerdiameterisdifferent.Thiswillimpactonthespeedoftherimoftheimpeller,whichinturnchangestheperformanceofapump.
Pumpaffinitylawsindicatetherelationshipsbetweenflowrates,pressure,power,pumpspeed,andimpellerdiametertopredictperformanceunderdifferentconditions.Aslongasthespeedanddiameterdonotdiffergreatlyfromthoseshownonthepumpcurve,thepredictedperformanceisrelativelyaccurate.Usingacomputerspreadsheet,itisfairlysimpletorepeatthepumpaffinitycalculationsforarangeofhead/flowcombinationsandderivesufficientdatatodrawanewpumpcurve.
Pumpaffinitylawsarefoundedontheassumptionthatpumpefficiencydoesnotchangeasthespeedanddiameterchangesaresmall(lessthan10%).Accordingtothepumpaffinitylawstheflowratevariesdirectlywithspeed,thehead(pressure)varieswiththesquareofthespeedandthepowerrequiredvarieswiththecubeofthespeed.
Q 1
Q2 =
N1
N2
H 1
H2 =
(N1)2
(N2)2
P1
P2 =
(N1)3
(N2)3
Likewise,accordingtothepumpaffinitylawstheflowratevariesdirectlywithdiameter,thehead(pressure)varieswiththesquareofthediameterandthepowerrequiredvarieswiththecubeofthediameter.
Q 1
Q2 =
D 1
D2
H 1
H2 =
(D 1)2
(D2)2
P1
P2 =
(D 1)3
(D2)3
Anexamplecanbeseenofthepumpcurveforanactualcentrifugalpump,aspublishedbythemanufacturer.Thefollowinginformationappearsonit:
• HQcurvesforthefullimpeller.
• Forpumpsthatcanhavealternativeimpellorsinstalled,thediagrammayalsoshowtheHQcurvesforanumberofselectedsmallerdiameters.
• Forpumpsthatcanbespeedcontrolled,thediagrammayshowtheHQcurvesatalternativespeeds.
• Pumpefficiency.
• NPSH.
• Power(kW)requiredforthedifferentimpellerdiameters.
17|HYDRAULICS AND PUMPING
PUMPS
Figure 12. Pump diagram showing performance curves for different impeller diameters.
Figure 13. Pump diagram showing performance curves for different impeller speeds.
18 | HYDRAULICS AND PUMPING
PUMPS
Introduction to NPSH (Net Positive Suction Head)Thepressureinsideapumpvariesfromtheinlet(thesuctionside)totheoutlet(thedischargeside).ThedifferencebetweeninletpressureandthelowestpressurelevelinsidethepumpiscalledNetPositiveSuctionHead(NPSH).NPSHisthereforeanexpressionofthepressurelossthattakesplaceinsidethefirstpartofthepumphousing.
Iftheinletpressureistoolow,NPSHwillcausethelowestpressureinsidethepumptodecreasebelowthevapourpressureofthepumpedliquid.Bubblesorcavitiesareformedinliquidwhichiscalledcavitation.Theimplodingorcollapsingofthesebubblestriggersintenseshockwavesinsidethepump,causingnoise,inefficiency,damagetothevoluteandultimatelyleadingtobreakdowns.
NetPositiveSuctionHeadRequired(NPSHR)istheabsolutepressurethatmustbepresentattheeyeoftheimpellerforanyparticularpumpinordertopreventcavitation.
NPSHRvariesaccordingtothedesignofeachpumpandtheheadandflowrate.NPSHRcanbereadfrompumpcurveforanyparticularpump.DifferentmanufacturersprovidethedataforNPSHRindifferentformswhichneedtobereadcarefullywhenselectingapump.
Figure15atleftshowsatmosphericpressure(P),thestaticsuctionhead(H)andthefrictionlossinthepipe(Hƒ).
NetPositiveSuctionHeadAvailable(NPSHA)attheeyeoftheimpellerisequaltotheatmosphericpressure(P)availableatthesourceofthewater,minusverticaldistancetotheeyeoftheimpeller,fromthewaterlevelcalledthestaticsuctionhead(H),thefrictionlossinthepipe(Hf),andthevapourpressure(Hvp)ofthewaterbeingpumpedatthetemperaturewhilepumpingasinthefollowingequation:
NPSHA=P–H–Hƒ–Hvp
Thedesignofthepumpsite,suctionpipingandtheverticalsuctionliftmustbesuchthatNPSHAexceedsNPSHRforalloperatingconditions.
Figure 14.
Fluid vapour pressure
Discharge pressure
A
C
D
B
Suction pressure
Bubbles form
Bubbles collapse
Pressure
A
C
D
B
Figure 15. Atmospheric pressure (P), the static suction head (H), the friction loss in the pipe (Hƒ).
HP Hƒ
19|HYDRAULICS AND PUMPING
PUMPS
SOLUTiONS TO REDUCiNG AND/OR AVOiDiNG CAViTATiONCavitationcanbeavoidedbyadoptingthefollowingmeasurestoincreasetheNPSHA:
• Lessenthedistancebetweenthepumpintakeandthewatersurface.
• Decreasethepumpcapacitytolowertheflowrateintheimpeller.Thismeasurewillincreasethepressureattheimpellereyeandlessentheamountofpressurelosttofrictionintheintakepipe.
• Increasethediameteroftheintakepipeandremoveanyvalves,elbows,andotherfixtures.
• ChooseapumpwithalowNPSHR.
Pumps in series and parallelFigure16belowillustratesthedifferencebetweenpumpsinparallelandpumpsseries.
Figure 16. Pump in parallel and series.
OUT
IN
OUT
IN
Series Operation Parallel Operation
PUMPS iN SERiESTwoormorepumpsareconnectedinseriesinordertoachievehigherpressures.Typicalapplicationforseriespumpsincludes:
• Iftotaldynamicheadrequirementisgreaterthanthecapabilitiesofreadilyavailablepumps.
• Afieldinwhichcertainirrigatedblocksareatahigherelevationoragreaterdistancefromthepumpthanotherblocks,andrequireahigherpumpinghead.
• Anirrigationdesignwhichcallsforhigherpressureatcertaintimes,suchasforgerminationorfrostcontrol.
Inthecaseofseriespumps,theresultantheadcapacitycurveisarrivedatbyverticaladditionoftheindividualhead-capacitycurve.
Figure 17. Two pumps in series.
Head, h
Flow rate, q
system curve
single pump
1
2
3
TWO PUMPS IN SERIES
20 | HYDRAULICS AND PUMPING
PUMPS
PUMPS iN PARALLELTwoormorepumpsareconnectedinparallelinordertoproducehigherflowrates.Typicalsituationwhereparallelpumpsareused:
• Twoormorewatersourcesaretobefedintoacommonsystem.
• Anirrigationsystemsupplyingdifferent-sizedblocksordifferentcrops,requiringwidelydifferentflowrates.
• Anirrigationsystemwhichisbeinginstalledinincrements,requiringhigherflowrateasnewstationsareinstalled.
Inthecaseofparallelpumps,theresultantheadcapacitycurveisarrivedatbyhorizontaladditionoftheindividualhead-capacitycurve.
Figure 18. Two pumps in parallel.
Head, h
Flow rate, q
system curve
single pump
TWO PUMPS IN PARALLEL
1 2
3
Energy sourcePumpsrequireenergytooperate.Dependingonlocation,someenergysourcesaremorereadilyavailablethanothers,andsomeenergysourceswillbemorecosteffectivetousethanothers.Consequently,theavailabilityofaparticularenergysourceisamajorfactortoconsiderwhenselectingapumpanditsdriverforanirrigationapplication.Theavailableenergysourcewilldictatewhetherthepumpwillbedrivenbyanelectricmotor,dieselengineoralternativeenergysource.
ELECTRiC MOTOR DRiVEThemostcommonly-useddriverforirrigationpumpsistheelectricmotor.Electricmotorefficienciesaregood,insomecasesexceeding95%.Electricmotorsgenerallyrequireverylittlemaintenance.Theyarequietandcompact,makingthemsuitableforawidevarietyofapplications.Withpropercareandmaintenance,electricmotorscanprovidemanyyearsoftrouble-freeservice.
SiNGLE- AND THREE-PHASE MOTORSSingle-phasemotorsarerarelyusedinapplicationsrequiringmorethan10kW.Single-phasemotorsarenotpracticalinlargersizes,buttherearesomespecialtymanufacturersthatproducehigherhorsepowermotorsforsingle-phasepoweroperation.Thesemotorsaregenerallyplacedinserviceinareaswhereitwouldbeexpensivetobringinthree-phasepowerwhensingle-phasepoweralreadyexistsatthesite.
21|HYDRAULICS AND PUMPING
PUMPS
Three-phaseinductionmotorsarepopularforavarietyofreasons;thereadyavailabilityofthree-phaseelectricpoweratstandardisedvoltages;theruggedconstructionofthesquirrelcagemotor;thesimplicityofoperationmaintenanceandservice;therelativelylostcost;theavailabilityofawidevarietyoffeaturessuchastypeofenclosures,speeds,mountings,andtorquecharacteristics.
ENGiNE DRiVEEnginesarealsooftenusedtopowerirrigationpumps.Althoughelectricmotorshavetheadvantageofbeingquiet,efficientandeconomicaltooperateandmaintain,enginesofferportabilityandlowinstallationcosts.
Enginescanbeinstalledvirtuallyanywhere.Theyareeasilymoved,havetheflexibilityofvaryingspeed,areavailableinavarietyofhorsepowerranges,wateroraircooledandtheydonotrequirecostlyinstallationofelectricpowerlinesoron-goinglinescharges.Enginesareavailableintwobasictypes:dieselandgasoline.Dieselenginesoperateefficientlybetween1600–2300rpmandgasolinebetween2000–3600rpm.Theselectionofthetypeofengineusedinanirrigationsystemismostlydeterminedbysourceandexpenseoffuelsupply.
Pump setup
PUMP EFFiCiENCiES AND CHOOSiNG A PUMP Theefficiencyofanymachineissimplyhowwellitcanconvertoneformofenergytoanother.Pumpefficiencyistheratioofliquidoutputhorsepowertotheinputhorsepower,measuredasapercentage.It’stheconversionofmechanicalenergytohydraulicenergy.
Efficiencyplaysasignificantroleinthepumpselectionprocess.Thepumpwiththehighestpumpefficiencyatthedesireddutypointshouldbeselected,wherethedesireddutypointmustbeasclosetothespecificpump’speakefficiencyaspossible.Thiswillmeanthatthepumpwillgivefewertechnicalproblems.Powerrequiredcanbereadfromthepumpcurve.Itisalsoimportanttodeterminethepowerratingofthedriver.
Thereisnoonerecipefortheselectionofthemostsuitablepump,buteconomicshastobethedecisivefactor.Importantlyanydecisionmustweigh-upcapital,operation,maintenanceandreplacementcosts.Thefollowinggeneralselectionguidelinesapplyforpumps:
• Normal installations: Generalsinglestageend-suctioncentrifugalpumps.
• Large suction heads: Submersiblepumps.
• Large pump deliveries:generalsinglestageend-suctioncentrifugalpumpscoupledinparallel.
• Large pump heads: Multi-stagecentrifugalpumps,generalend-suctionsingle-stagepumpscoupledinseries,positivedisplacementpumps.
• Large pump deliveries and small pump heads: Axial-flowpumps.
Foreachapplication,therewillbeanumberofsuitabletypes,makesandmodels,butoverthelongterm,themosteconomicalpumpmustbechosen.
22 | HYDRAULICS AND PUMPING
NOTES
23|HYDRAULICS AND PUMPING
NOTES
24 | HYDRAULICS AND PUMPING
NOTES
NOTES
REFERENCESAllphotos©andcourtesyofDanBloomer,PaulReese,AndrewCurtisandAnnetteScott.
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