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DeterminingSystemDemand
Thesystemdemandisafunctionofwaterusageandlocation,expressedasgallon(s)perminute(gpm)andpound(s)persquareinchgauge(psig),respectively.Usageorflow(gpm)canbedeterminedusingoneofseveralmethods(refertoTableIV.1.1fortypicaldemands):
1)Pressurerequired,asstatedinpoundspersquareinchorPSI,isthepressureneededforthefixturesinthehouseorirrigationsystemtofunctionproperly.Themostcommonpressurerangeis40/60,whichstartsthepumpat40PSI,andstopsthepumpat60PSI.Thisgivesanaverageoperatingpressureof50PSI.
2)Pumpcapacity,asstatedingallonsperminuteorGPM,isthevolumeofwaterthatthepumpcanproduceafterliftingfromthedepthofthepumpinglevel,andproducingtheaveragepressureasstatedabove,plusallfrictionlossesinthepipingsystem.
Thepressurerequiredformostfixturesinahouse,orforsprinklersoutsidethehouse,isabout50PSI.Usingthechartormethodsbelow,youcanfigurethemaximumgallonsperdayrequired,andthemaximumgallonsperminute(GPM)neededtosupplypeakdemands.
Accordingtothechartbelow,ahousewith3bathroomsneedsapeakdemandof14GPM.Havingawellthatproduces14GPMormoremakesthisaneasyproject.Simplydesignapumpsystemthatcanproduceatleast14GPMatthedepthandpressurerequired.Thepumpmustbeabletoproduce14GPMduringpeakdemands.Thepumpcontrolsystemmustalsobeabletosafelysupplyanyflowratelessthan14GPMasrequired,whenflowsneededarelessthanpeakdemands.
Apumpsystemshouldbedesignedthatcanprovidethepeakdemandrequired.Ifthewellcannotsupplythevolumeneededforpeakdemand,astoragetanksystemshouldbedesigned.SeeLowYieldWellsandStorageTanks.
Thefixturemethoddeterminesthesystemdemandbytotalingthenumberoffixturesinthehome,includingoutsidehosebibs,andmultiplyingthisnumberby1gallonperminute(gpm).Forexample,10fixturesx1gpm=10gpm.
Thepeakdemandmethoddeterminessystemdemandconsideringthatmorethanonefixturewillbeinuseunderpeakdemand.Thenumberoffixturesbeingusedatthesametimeisdeterminedandmultipliedby3gpm.Forexample,4fixturesx3gpm=12gpm.
Analternatemethoddeterminessystemdemandbycalculatingthenumberofbathrooms(halfbathsareconsideredas1)andmultiplyingby4gpm.Forahomewith2bathrooms,multiply3x4gpm=12gpm.Usethelargestsystemdemanddeterminedbytheabovemethods.
Thepumpcapacityshouldbeselectedaccordingtothesystemdemand.Ifapumpexists,thecapacitymustbedetermined.Thewellcapacityshouldbedocumentedwhenthewellhasbeendeclaredreadyforuseandwilloftenbereferredtoingallonsperhour(gph).Ifthewellcapacityisunknown,itshouldbedeterminedbymeasuringthewellwaterlevel.Thewaterlevelmustbeloweredviapumping,measured,thenallowedtorecovertothestaticwaterlevel.Arecordofthetimerequiredtoreturntostaticwaterlevelalongwiththewellpipesizecanbeusedtocalculatethewellcapacity(recovery).Analternatemethodofdeterminingifthewellcapacityissufficientforsystemdemandistodrawwaterfromthe
