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PressureTankControl

Thefunctionofapressuretankisto:

(1)Reduce(on/off)cyclingofthepumpmotor.(2)Providewaterunderpressurefordeliverywhilethepumpisoff.

WithaPressureTankonlysystemthepumpwillcontinuetocycleonandoff,fillinganddrainingthepressuretank,accordingtothedemand.Onlywhenthewaterbeingusedisequaltothemaximumoutputofthepump,willthepumpruncontinuously.Itisalmostimpossibletomatcheveryzoneorwateroutlettothemaximumoutputofthepump.Thereforepressuretankonlysystemsarenotoriousforcyclingthepumptodeath.Thelargerthepressuretankthelessthepumpwillcycle.Thelargerthepressuretank,thelongerthesystemwillbeatlowpressurebeforethepumpstarts,andthelongerthesystemwillbeathighpressurebeforethepumpisshutoff.Theseextremesinpressure,constantlychangingfrom40to60PSI,canbeaggravatingforpeopletakingshowers,andmakeirregularpatternswithsprinklers.

Drawdownfromapressuretankisgoodtohavefortimeswhenthepumpisoff.Youdontwantthepumptocomeoneverytimetheicemakerfillsoratoothbrushisrinsed.However,forlongtermusesofwaterlikeshowers,sprinklers,orheatpumps,apressuretankonlysystemwillcyclethepumpexcessively.ACycleStopValveorCSVwillallowyoutouseallthewaterinthepressuretankbeforethepumpisstarted.ThentheCSVwillmaintainaconstantpressureandkeepthepumpfromcyclingforlongertermusesofwater.

SelectingaPressureTank

Whenselectingapressuretank,certaininformationmustbeknown:(1)Pressurerequired(2)pumpcapacity:

Adifferentialpressureswitchisusedtocontrolthepressureproducedbythepump,atorabovetheminimumrequiredsystempressure.Thepressureswitchturnsonthepumpatapredeterminedlowpressure(suchas40PSI),andturnsthepumpoffatapredeterminedhigherpressure(suchas60PSI).A40/60pressureswitchgivesanaveragepressureof50PSI,asthepumpcyclesonandoffbetween40and60PSI.Thevolumeordrawdownamountofthetankisexpressedasatapisopened,causingthepressuretodropfrom60PSIto40PSI.Themoredrawdownavailableinthetank,thelongerthepumprunswhenitcycleson,andthelongerthepumpisoffwhenitcyclesoff.

SubtracttheGPMbeingusedatthetime,fromtheGPMproducedbythepump,todeterminethefillrateofthetank.Dividethedrawdownavailableinthetank,bythecalculatedfillrate,togetthetimeinminutesittakestofillthetank.

Bydividingthedrawdownavailableinthetank,bytheGPMbeingusedatthetime,yougetthetimeinminutesittakestoemptythetank.

Thetankshouldbesizedtoallowthepumptorunaminimumof1minutetoreplacethedrawdownamountinthetank.I.e.15GPMpump,requires15gallonsoftankdrawdown.Thetankshouldalsobelargeenoughtoallowwatertobesuppliedfromthetaps,foraminimumof1minutebeforethepumpisrestarted.1minuteon,and1minuteoff,istheminimumrunandofftime,toallowthemotortodissipateheatproperly.Atankwithtwice

thevolumeordrawdownamount,willallowthepumptorunaminimumof2minutes,andbeoffaminimumof2minutes.2minuteson,and2minutesoff,requiresatankwith2timesthedrawdowncapacityaswhenusing1minuteon,and1minuteoff.2minutesonand2minutesoffarepreferable,asthelessnumberoftimesapumpcycles,thelongerthepumpsystemwilllast.

Somepressuretankcontrolledsystemsuseapressureswitchwith10PSIbetweenonandoff,insteadof20PSIbetweenonandoff.Thisreducesthepressurefluctuationsseenbythewateruser.Achievinganaveragepressureof55PSIwhenlowflowsarerequired,thepumpcyclesonat50,andoffat60PSI.Reducingthisbandwidthby50%alsoreducestheavailabledrawdownofthepressuretankby50%.Tomaintainthesamedrawdownavailablefromthetank,andthecorrectruntimeforthepump,asystemwithonly10PSIbetweenonandoff,requirestwicethesizeofpressuretankaswhenusing20PSIbetweenonandoff.Apressuretanklargeenoughtodeliver2minutesofruntimewitha40/60pressureswitch,willonlyallow1minuteofruntimewitha50/60pressureswitch.

Whenthepointsinthepressureswitchareopenandthepumpisoff,nopowerisbeingconsumed.Whenthepumpisrunning,itisusingmaximumpower,anddeliveringmaximumflow.Thepumpiseitherrunningclosetoitsbestefficiencypoint,orthepumpiscycledoff.Pumpsaremadeforcontinuousduty,andcareshouldbetakentopreventthepumpfromcyclingonandoff.Highstartingcurrentfromrapidcyclingcanincreasepowerconsumption,anddecreasepumpandmotorlife.Topreventcyclingwithapressuretanksystemthepumpmustbesizedtoexactlymatchtheamountofwaterusedforlongtermrequirements,suchasirrigationsystemsandheatpumps.

TypesofTanksandTotalTankVolume

Selectingthepressuretanktotalvolumefortypicalsystemswillconsiderthetankdrawdown,ascomparedtothepumpcapacityandrequiredpumpruntime.Totaltankvolumeisnotameasureoftankacceptancevolume,whichistypicallyconsideredtobeavailableamountofwater,ordrawdownfromthetank.Totaltankvolumeisameasureofthetotaltanksizeneededtoprovidetherequiredamountofdrawdown.Mostofatanksvolumeisfilledwithcompressedair,whichisusedasaspringtoforcewateroutofthetank.Thereforetheactualsize,ortotaltankvolumeismuchlargerthanthetankdrawdown,orusableamountofwaterinthetank.Thetotaltankvolumewillvarydependingonthetanktype.Thetwomostcommonarethecaptiveairtank,andthehydropneumaticorsteeltank.

Thecaptiveairtankhasadiaphragmorbladderthatseparatestheairchamberfromthewaterchamber.Inacaptiveairtank,theaircannotmixwiththewaterandbedrawnoutofthetankwiththewater.Thesetankshaveverylittlewastedspace,aseventhelastgallonofwaterinthetankcanbeexpressedunderpressurebytheprechargeintheairchamber.Thesetypetanksdonotrequireanyadditionalairchargingequipment,andneedverylittlemaintenance.

Hydropneumaticorsteeltankshavenomeansofseparatingtheairandwaterchambers.Thesetanksmustincorporatesomemeansofcontinuallyaddingair,tomakeupfortheairthatmixeswiththewaterandislostwithuseofthewater.Normallyableederorificeinthewellisusedinconjunctionwithanadditionalcheckvalve,schradervalve,andAirVolumeControl(orAVC).Eachtimethepumpshutsoffthewaterisdrainedbetweenthebleederorifice,whichisabout5'downinthewellonthedroppipe,totheadditionalcheckvalveatthetank.Whenthepumpisrestarted,thispipefullofairispushedintothepressuretank.AnairvolumecontrolorAVCismountedabouthalfwayupthesideofthetank.TheAVChasafloatprotrudinginsidethetankthatadjusttheairlevelbyexpellingexcessair,andmaintainsthecorrectairtowaterratio.Airinjectorsandcompressorscanalsobeusedtoaddadditionalairasneeded.Airchargingdevicesrequireperiodicmaintenance.Thistypeoftankcanbeneededforwellsthathavesulfurorothercontaminants,wheremixingairwiththewaterisadvantageous.

Whenusing10PSIbetweenonandoff,thesizeofthetankshouldbedoubled.Differencesinthetypesofpressuretanksrequirethatyoufollowthemanufacturersguidelinesforselectionandinstallation.

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