12 Yasir.docx

Tanveer 44 METHODSFORINACCURATEFLOWMEASUREMENTSOFGASMETER Aflowmeterisaninstrumentusedtomeasurelinear,nonlinear,massorvolumetricflow rateofaliquidoragas.Thebubblemetertechniqueisanabsolutemethodofmeasuring thevolumeflowrateofgases. Itrequiresaccuratestandardsofmassandtime. Itisfoundthaterrorsmaybekeptbelow1%forflowsbetween0.1and1000Cm3sec. Theerrormayreduceto¼%forflowsbetween0.1and100Cm3sec. 1.FirstStepstoChoosetheRightFlowMeter Thefirststepinflowsensorselectionistodetermineiftheflowrateinformationshould becontinuousortotalized,andwhetherthisinformationisneededlocallyorremotely.If remotely,shouldthetransmissionbeanalog,digital,orshared? 2.Fluidandflowcharacteristics Thefluidanditsgivenanditspressure,temperature,allowablepressuredrop,density(or specificgravity),conductivity,viscosity(Newtonianornot?)andvaporpressureat maximumoperatingtemperaturearelisted,togetherwithanindicationofhowthese propertiesmightvaryorinteract. 3.Pressure&TemperatureRanges Expectedminimumandmaximumpressureandtemperaturevaluesshouldbegivenin additiontothenormaloperatingvalueswhenselectingflowmeters. 4.PipingandInstallationArea Concerningthepipingandtheareawheretheflowmetersaretobelocated,consider:For thepiping,itsdirection(avoiddownwardflowinliquidapplications),size,material, schedule,flangepressurerating,accessibility,upordownstreamturns,valves,regulators, andavailablestraightpiperunlengths. 5.FlowratesandAccuracy Thenextstepistodeterminetherequiredmeterrangebyidentifyingminimumand maximumflows(massorvolumetric)thatwillbemeasured.Afterthat,therequiredflow measurementaccuracyisdetermined.Typically,accuracyisspecifiedinpercentageof actualreading(AR),inpercentageofcalibratedspan(CS),orinpercentageoffullscale (FS)units.

6.MassorVolumetricUnits Whenthevelocity(volumetricflow)ofincompressibleliquidsismeasured,thepresenceof suspendedbubbleswillcauseerror;therefore,airandgasmustberemovedbeforethe fluidreachesthemeter.Inothervelocitysensors,pipelinescancauseproblems

(ultrasonic),orthemetermaystopfunctioningiftheReynoldsnumberistoolow(in vortexsheddingmeters,RD>20,000isrequired).