Sulzer ABS PC 441 - Overflow Flow Calculation

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3.3 Overowowcalculation
Thereareseveralmethodsthatcanbeusedtomeasureandcalculateoverowow:
1. Useaconventionalowmeter.
Advantage InmostcasesforstandardPLC-systemsthiswillincreasetheaccuracyonthemeasurement.
Drawbacks Expensiveandcansu󰀨erfromdetritusdryingontothesensorcausingmeasurementissues,whenthepitis
operatinginnormalconditions.Thesensorhastobecleanedregularlytoensurecorrectmeasurements.
2. Usethesamesensorthatisusedforthelevelmeasurementinthepitandaweirandstarttheowmeas-
urement on analog set point.
Advantage Theinvestmentcostislowandthesensorwillnotneedtobecleanedregularly.
Drawbacks Thesystemmusthaveaverygoodresolutionontheinputtobeabletomeasuretheoverowcorrectlyand
averyaccurate0-pointotherwisethemeasurementwillbeinaccurate.
3. Usethesamelevelsensorthatisusedforthelevelmeasuringinthepitandaweir,andusealevelswitch
tostarttheoverowmeasurement.
Advantage TheInvestmentcostislowandthesensornotrequireregularcleaning.Theaccuracyofthe0-pointdoes
nota󰀨ectthemeasurementsduetotheswitchbeingusedasa0-point.
 Drawbacks: Theanaloginputneedstohaveaverygoodresolutiontobeabletomeasurethesignal.ThePC441hasno
problemwiththis,e.g.forasensorwitharangeof10m,thePC441hastheresolutionof<0.7mm.
The third method is preferred and used in the PC 441
Adigitaloverowswitch,likeSulzerMD131connectedtoadigitalinputindicatesifanoverowisoccurring
independentofwhatthelevelsignalshows.ThePC441locksthisactuallevelandthePC441startscalculat-
ingtheoverowlevel/owfromthisvalue.
Thismeansthatthelevelismeasuredwithaveryhighaccuracyfromthecorrect0-point.Ifanexactow
measurement is needed a weir or channel should be used.
Figure 3-9
Overflow
Analog pit level sensor
Inflow
OverowswitchMD131
ThePC441programhasallthefunctionsrequiredforcalculatingowinweirsandchannels.Theoverowis
measuredseparatelyforeachpumppit.Numberofoverows,overowtime,overowlevelandtheoware
logged.
Thelevelssensorisusedastheactuallevelsignalwhentheswitchisactivateditsetsthe0-pointfortheow
measurement.IfnolevelswitchorMD131isconnectedtothePC441the0-pointfortheoverowcanbeset
in“Settings / Pump pit / Calc. overow / Overow detect“manually.Overowwillberegisteredwhenthelevel
exceedspresetoverowlevelonthelevelsensor.
NOTE! Thissetpointhasnofunctionifadigitalinput(overowswitch)issetforoverowindicationinthepumppit.
Adelaycanbesettopreventdisturbancesorwavestriggeringtheswitch.Afterthisdelaytheowmeasure-
mentstartsandthetimeoftheoverowisrecorded.Acounterkeepstrackofhowmanytimesthepithas
overowed.Theoverowtimeisonlytriggedwhenthelevelishigherthanthestored(set)0-point.Ifaoat
sensorisusedforapumppit,whichhasnolevelsensor,theoverowtimecountsallthetimetheoatisactive.
Theoverowalarmwillstopaftertheoatgoesbacktonormalandthestopdelaytoavoiderrorsinthecounter
andtocompensateforthestartdelay.
NOTE! Overowalarm and counterisonlyactivatedifalarmisenabled.