Basic Process Control - CANDU Owners Group

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Basic Process Control

Basic Control PrinciplesInflowSupplyllllllLevel(Controlled Variable)PumpOverflow

Open vs Closed Loop!Closed loop!!Automatic controlOpen Loop!!Manual controlPerson takes the place of the controller

Feedback ControlDisturbancesControllerSet PointComparatorErrorAmplifier(SP - M)ManipulatedVariableProcess (Tank)Output(ControlledVariable - Level)MeasurementSignalProcessSensor

Feedback vs. Feedforward!Feedback!!Control action after an error existsFeedforward!Reacting to the disturbance before theerror occurs

Typical ON/OFF Control SystemV1SPlllQoutQinlllL2L1P1P1 closes @ L1P1 opens @ L2InflowElectrically OperatedSolenoid valveSolenoidPower Supply

Typical ON/OFF ResponseL2SPtimeL1Periodic Time

Proportional Control

Level Control of Open TankV1SPlllQoutQinlllA/CLT20 - 100 kPa

Simple Proportional SystemInflowValvePivotllFloatlllOutflow

Open Tank ControlA/O20 - 100 kPaQinSPLICLTQout

Controller ActionA/CSPlllQoutQinlllDirect20 - 100 kPaLICLTA/OSPReverselllQoutQinlll20 - 100 kPaLICLT

Proportional ControlSetpoint Error-k e BiasMeasurementm k(SP-M) biasOutput

Proportional Band and GainProportional Band – the input change required tochange the output 100% outputgain input100%gain PB

Narrow, Wide, High & LowInWide PB – Low GainNarrow PB – High GainOut

Proportional Control ResponseCurveNew mass balanceoccurs hereLoss in VolumeOutflowInflowInput/Outputt0timet1tLevel originallyat setpointOffsetLevelt0t1timeNew levelbelow setpoint

Proportional Response withNarrower PBNew mass balanceoccurs hereLoss in VolumeOutflowInput/OutputInflowt0timet1tLevel originallyat setpointLevelOffsett0New levelbelow setpointt1time

Load ChangeResponse Versus PB, ProportionalControl OnlySPStep Disturbancetime"Wide" PBSystem ResponseOffsetSPSP"Moderate" PBOffset"Narrow" PBtime

System Response¼ Decay Response CurveAA/16A/4time

For You To Do!!Read pp. 89-105Answer Questions pp. 121-122, #1-19

Reset or Integral

Response Curve: ProportionalControl OnlySystem ResponseStep DisturbancetimeSPOffset

Additional Control Signal RestoresProcess to SetpointInitial mass balanceFinal mass balanceOutflowReset ActionInflowSetpointOffsetRemovedtime

Integral Action1edt TRSetpoint Error-Measurementk e Bias 1m ke edt bias TROutput

Units!Minutes per repeat!!!MPRThe length of time that it will take theintegrator to add an amount equal to theproportional responseRepeats per minute!!RPMThe number of times the proportionalresponse is repeated in one minute

Proportional Plus Reset, Open LoopResponseerrorControl SignaleFa}stseeRtNokeProportionalResponseR eslarmetSlow Resettime

A problemOutput initially 50%, Gain 2, reset 2 minutes per repeatA direct acting controller control is subjected to a sustainederror of 5%What is the output after 4 minutes?Proportional Response ke 2x5 10%Integral Action- in 4 minutes the control will go through 2 repeats.Integral action 2 x 10 20%Total output change is proportional integral 30%

A Couple More Things!!Reset WindupInstability because of lag

For You to Do!!Read over text pp. 89 –110Answer questions pp. 121-122, 1-24

Rate or Derivative

OutputInputProportional and Derivative – OpenLoop PressureDerivative ceases aserror stops changingProportionalActionDerivativetime

Derivative Control1edt TRSetpoint Errork e- Output BiasMeasurementTDdedt 1 de biasm ke edt kTDdt TR

Simple Flow Control Systeml ll FTA/CFC

ProcessThe open Loop Response of Proportional PlusDerivative (PD) Action to Rapidly ChangingError SignalsControlSignal%BACt0t1 t2timeProportional actionA-BRate action due to cessation of increase in eRateRate actionactionB -CA-BControl signal at end of excursionRate action due to cessation of increase in eProportionalaction B - Ct0t1 t2time

Large System Under Proportional andProportional Plus Derivative ControlControl SignalLoadDisturbanceAppliedProp. DerivativeProp. OnlySetpointLeveltime

Multiple Control Modes!!!!Virtually all controls have a proportionalresponseIntegral and derivative are added toimprove performanceMajority have proportional and integralSome, typically heat exchangers havederivative added

Open Tank Level Control WithValve In InflowA/OLICSPLTlllQiQolll

Typical Flow Control Loopl l FTA/OFICSP

Pressure Control – ConstantBleedSPPICFeedPTPressure VesselA/CPressureBleed

Split Ranged Feed and BleedPressure ControlSPPICPTA/OFeedPressure VesselA/CBleed

Representative Hot Bleed/Cold ServiceWater Heat ExchangerHot BleedColdA/CTTCooled BleedTCSP

For You To Do!!Read pp. 106-120Answer Questions pp. 122-123, #20-38

Control Signal % Process. Large System Under Proportional and Proportional Plus Derivative Control time Control Signal Setpoint Level Load Disturbance Applied Prop. Derivative Prop. Only. Multiple Control Modes! Virtually all controls have a pr

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