Beginner's Guide to Differential Pressure Level

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    Home/Articles/2009/Beginner'sGuidetoDifferentialPressureLevelTransmitters

    Beginner's Guide to Differential Pressure LevelTransmittersTheNotSoStraightforwardBasicsofThisMeasurementTechniqueFeb09,2009

    ByDavidW.Spitzer

    GIGOmeans"garbagein,garbageout."Thisphraseappliesinindustrialautomationbecauseusingfaultymeasurementscanfooleventhebestcontrolsystem.OneremedythatcanhelpavoidaGIGOscenarioistounderstandthemeasurementtechniqueanditslimitationstotheextentthatitsapplicationcanbereasonablyevaluated.DifferentialpressurelevelmeasurementisoneofthosekeymeasurementsyouneedtounderstandtoavoidthedreadedGIGO.

    Theimportanceoflevelmeasurementcannotbeoverstated.Incorrectorinappropriatemeasurementscancauselevelsinvesselstobeexcessivelyhigherorlowerthantheirmeasuredvalues.Lowlevelscancausepumpingproblemsanddamagethepump,whilehighlevelscancausevesselstooverflowandpotentiallycreatesafetyandenvironmentalproblems.Vesselsoperatingatincorrectintermediatelevelscanresultinpooroperatingconditionsandaffecttheaccountingofmaterial.

    Thelevelofaliquidinavesselcanbemeasureddirectlyorinferentially.Examplesofdirectlevelmeasurementincludefloat,magnetostrictive,retracting,capacitance,radar,ultrasonicandlaserlevelmeasurementtechnologies.Weightanddifferentialpressuretechnologymeasurelevelinferentially.Allhaveproblemsthatcanpotentiallyaffectthelevelmeasurement.

    Differentialpressurelevelmeasurementtechnologyinfersliquidlevelbymeasuringthepressuregeneratedbytheliquidinthevessel.Forexample,awaterlevelthatis1000millimetersabovethecenterlineofadifferentialpressuretransmitterdiaphragmwillgenerateapressureof1000millimetersofwatercolumn(1000mmWC)atthediaphragm.Similarly,alevelof500millimeterswillgenerate500

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    mmWC.Calibratingthisdifferentialpressuretransmitterfor0to1000mmWCwillallowittomeasurewaterlevelsof0to1000millimeters.

    Notethatthisexamplepresumesthattheliquidiswater.Liquidswithotherspecificgravitieswillgenerateotherdifferentialpressuresandcauseinaccuratemeasurements.Continuingwiththepreviousexample,thesame500millimeterlevelofanotherliquidwithaspecificgravityof1.10atoperatingconditionsintheabovevesselwillgenerate550mmWCofpressureatthetransmitter.Assuch,thedifferentialpressuretransmittercalibratedforwaterwouldmeasure50millimetershigherthantheactual500millimeterliquidlevel.Conversely,iftheliquidhasaspecificgravitythatislowerthanthatofwater,thistransmitterwillmeasurelowerthantheactuallevel.Thisexampleillustratesthatdifferentialpressuretechnologydoesnotmeasurelevel,butratherinferslevel.

    Three CalculationsAllisnotlostbecausethecalibrationofthedifferentialpressuretransmittercanbemodifiedtocompensateforadifferentspecificgravity.Thistechniqueusedtocalculatethenewcalibrationisusefulforbothstraightforwardandmorecomplexinstallations.

    Figure1showsthevesselbothat0%and100%level.Thepressuregeneratedbytheliquidattheleveltransmitterdiaphragmistheliquidheighttimesthespecificgravity.Thepressureis1.10*(0mm)whenthevesselat0%and1.10*(1000mm)whenthevesselat100%.Therefore,thetransmittershouldbecalibrated0to1100mmWCtomeasureliquidlevelsof0mmto1000mm.

    AsomewhatmorecomplexapplicationisillustratedinFigure2.Inthisapplication,forprocessreasons,weneedtotakethemeasurementfrom200mmto1000mmabovethenozzle.Inaddition,thetransmitterislocated500mmbelowthenozzle.Notethatthetechniqueofsketchingconditionsatboth0%and100%levelisthesameasperformedinFigure1.At0%level,thepressureatthetransmitteris1.10*(500+200mm),or770mmWC.At100%level,thepressureatthetransmitteris1.10*(500+1000mm)or1650mmWC.Therefore,thetransmittershouldbecalibrated770to1650mmWCtomeasureliquidlevelsof200mmto1000mmabovethenozzle.

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    Figure3illustratestheuseofadifferentialpressuretransmitterwithdiaphragmsealstosensethepressuresatthenozzlesinapressurizedvessel.Inthisapplication,thelowpressurediaphragmislocatedabovetheliquidtocompensateforthestaticpressureinthevessel.Othercomplicationsincludethedensitiesofliquidandcapillaryfillfluidand0%and100%levelsthatdonotcorrespondtothenozzlepositions.

    Usingsimilartechniquesasinthepreviousexamples,at0%level,thepressuresatthehighandlowsidesofthetransmitterare{1.10*(200mm)+(3bar)}and{1.05*(1300mm)+(3bar)}respectively.Therefore,thedifferentialpressuretransmitterwillsubtractthehighsidefromthelowsideandmeasure{1.10*(200mm)+(3bar)}minus{1.05*(1300mm)+(3bar)},or1145mmWC.

    At100%level,thepressuresatthehighandlowsidesofthetransmitterare{1.10*(1000mm)+(3bar)}and{1.05*(1300mm)+(3bar)}respectively.Similarly,thedifferentialpressuretransmittersubtractsthehighsidefromthelowsidetomeasure{1.10*(1000mm)+(3bar)}minus{1.05*(1300mm)+(3bar)},or265mmWC.Therefore,thetransmittershouldbecalibrated1145mmWCto265mmWCtomeasureliquidlevelsof200to1000millimetersabovethelowernozzle.

    Notethatthestaticpressureinthevesseldoesnotaffectthecalibrationbecauseitappearsonbothsidesofthedifferentialpressuretransmitterwhereiteffectivelycancelsout.Furtheranalysisalsowillrevealthatlocatingthedifferentialpressuretransmitteratdifferentelevationsdoesnotaffectthecalibration.

    Thesesametechniquescanbeusedtodeterminethecalibrationsforinterfacelevelmeasurements.Notethatthesetechniquesinvolveapplyinghydraulicstotheinstallationandapplication.Nowheredoweusetermssuchaselevation,suppressionandspan.Theuseofthesetermscaneasilyconfuseandmisleadthepractitioner.

    What IfsWhatiftheliquiddensitychangesduringoperation?Whatifthechangeisduetochangesinthecompositionoftheliquid?Whatifthechangeisduetotemperaturechanges?Whatifthevesselisfilledwithadifferentliquidthathasadifferentspecificgravity?Theseareimportantquestionsthatshouldbeasked(andanswered)whenconsideringtheuseofdifferentialpressurelevelmeasurement

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    instruments.Repeating,differentialpressuremeasurementdoesnotmeasureliquidlevelitinfersliquidlevelsospecificgravitychangescanaffecttheperformanceofthelevelmeasurement.Inpractice,thespecificgravityofmanyliquidsisknownandrelativelystable,sothatdifferentialpressuretechniquesarecommonlyappliedtomanyliquidlevelmeasurementapplications.

    Spanning SpecificationsThedifferentialpressuretransmittershouldbeoperatedwithinitspublishedspecificationstomaintainaccuracy.Thespanofatransmitteristhedifferencebetweenthe100%and0%calibrationvalues.Differentialpressuretransmittershavespecifiedminimumandmaximumspans.Forexample,agivendifferentialpressuretransmittermaybecalibratedwithspansbetween(say)400mmWCand4000mmWC.Inaddition,thetransmitterzeromayalsoberaisedorloweredbyupto,forexample,4000mmWC.Calibrationsthatdonotmeetthetransmitterspecificationsarepotentiallysubjecttosignificanterror.Thecalibrationsintheexampleswere0to1100,770to1650,and1145to265mmWC,respectively.Eachhasaspangreaterthan400mmWCandlessthan4000mmWC.Inaddition,theirzerosarenotraisedorloweredbymorethan4000mmWC.Therefore,allofthesecalibrationsarewithinthetransmitterspecifications.

    However,thecalibratedspanspecifiedforanothertransmittermodelofthesamemanufacturemaybebetween100mmWCand1000mmWC,andallowthezerotobechangedby1000mmWC.Thistransmitterwouldnotbeapplicabletothefirstandthirdexampleswherethespanis1100mmWC,andthezeroisloweredby1145mmWC,respectively.However,itcouldbeusedinthesecondexamplewherethespanis880mmWC,andthezeroisraisedby770mmWC.Usingthislowerrangetransmitter(1000mmWC)willusuallybemoreaccuratebecauseofthesmallerabsoluteerrorsassociatedwithotherspecificationssuchastemperature,pressureandambienttemperatureaffects.Therefore,allbeingequal,it'sgenerallydesirabletousethelowerrangetransmittertoreducemeasurementerror.

    Themaximumflowrateofflowmetersisoftenspecifiedtobesignificantlyhigherthanthedesignflowratetoallowfortransientsandincreasedplantthroughputovertime.Inlevelmeasurement,thevesselsizeisfixed,sousingahigherrangedifferentialpressuretransmitterprovidesnosimilarbenefitandtypicallyresultsinadditionalmeasurementerrorthatcanbeavoidedbyusingalowerrangetransmitter.

    Usingtheavailableinformationproperlyisanotherpotentialproblem.Someyearsago,distributedcontrolsysteminputswereincorrectlyconfiguredtocorrespondtothemaximumtransmitterspans.Asidefromusingincorrectvalues,thelevelsshouldhavebeenexpressedinpercent.Usingabsolutelevelmeasurementunitssuchasinches,feet,millimetersormetersincreasesthepotentialforerrorbecauseoperatorsmustremembertheheightofeachvesseltoputthelevelmeasurementincontextwiththevessel.Thiscaneasilybecomeoverwhelmingandcauseoperatorerrorsbecauseplantsoftenhavehundredsofvessels.Forexample,avesseloperatingat2.8metersdoesnotreadilyindicateaproblemtotheoperatoreventhoughthevesseloverflowsat3.0meters.Ontheotherhand,theoperatorcaneasilydeterminethatavesseloperatingat93%levelmightwarrantattentionandthatavesseloperatingat97%mayneedimmediateattention.

    Differentialpressuremeasurementisaworkhorseofindustriallevelmeasurementthat'sbeenusedfordecadesandwillcontinuetobeusedfordecadestocome.

    DavidW.SpitzerisaprincipalinSpitzerandBoyes,LLCandaregularControlcontributor.

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    CommentsSubmittedbyChristineBravoonThu,07/03/201412:13

    I'vealwaysbeeninterestedaboutpressurevessel.Ijustcameacrossthissiteandfoundthisveryinterestingguideindifferentiatingpressureleveltransmitter.Thanksforsharingtheseusefulmeasurementtechnique.

    ReplySubmittedbyKingsleyNwabaonSun,02/01/201512:11

    Godrewardyouforsharingthispieceofknowledge...

    Reply

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