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

1|HYDRAULICS AND PUMPING

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).

2 | HYDRAULICS AND PUMPING

HYDRAULICS

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.

HYDRAULICS


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.


HYDRAULICS

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.

HYDRAULICS


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.


HYDRAULICS

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.

HYDRAULICS


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.


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


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.


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.


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.


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.


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.


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


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.


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.


PUMPS

Figure 12. Pump diagram showing performance curves for different impeller diameters.

Figure 13. Pump diagram showing performance curves for different impeller speeds.


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


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


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.


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.


NOTES


NOTES


NOTES

NOTES

REFERENCESAllphotos©andcourtesyofDanBloomer,PaulReese,AndrewCurtisandAnnetteScott.

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Hydraulics and Pumping - Irrigation NZ