Chapter 10Control Loop Troubleshooting

Overall Course ObjectivesDevelop the skills necessary to function as an industrial process control engineer.SkillsTuning loopsControl loop designControl loop troubleshootingCommand of the terminologyFundamental understandingProcess dynamicsFeedback control

Objectives for Control Loop TroubleshootingBe able to implement an overall troubleshooting methodologyBe able to determine whether or not the actuator, process, sensor, and controller are functioning properly.Recall the major failure modes for each of the elements of a control loop.

Troubleshooting Loops in the CPI

Control Diagram of a Typical Control Loop

F1

TC

T1

F2

T2

TF

TT

Sensor System

Controller

Actuator System

Components and Signals of a Typical Control Loop

I/P

F1

T1

F2

T2

TF

3-15 psig

Thermocouplemillivolt signal

Air

Thermowell

4-20 mA

OperatorConsole

Transmitter

4-20 mA

DCSControl Computer

A/D

Tsp

Actuator System

Controller

Sensor System

D/A

What is Control Loop Troubleshooting?Control loop is suspected of not functioning properly.Poor overall control performanceErratic behaviorControl loop was removed from service.Identify the source of the problem.Correct the problem.Retune the controller and monitor.

Overall Approach to Troubleshooting Control LoopsCheck subsystem separately.Actuator systemControllerSensorProcessThen check performance of the entire control loopWhats been changed lately?

Checking the Actuator SystemApply block sine wave input changes to the setpoint for the flow controller.Determine the deadband of the flow control loop from a block sine wave test. Also, estimate the time constant for the flow control loop from the block sine wave test.If the time constant is less than 2 seconds and the deadband is less than 0.5%, there is no need to evaluate the actuator system further

Block Sine Wave Test

Chart1

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&A

Page &P

Setpoint to Flow Controller

Measured Flow Rate

Time

F16

7.25E-013.50E+023.50E+02

1.48E+003.50E+023.50E+02

2.23E+003.50E+023.50E+02

2.98E+003.50E+023.50E+02

3.73E+003.50E+023.50E+02

4.48E+003.50E+023.50E+02

5.23E+003.50E+023.50E+02

5.98E+003.50E+023.50E+02

6.73E+003.50E+023.50E+02

7.48E+003.50E+023.50E+02

8.23E+003.50E+023.50E+02

8.98E+003.50E+023.50E+02

9.73E+003.50E+023.50E+02

1.05E+013.50E+023.50E+02

1.12E+013.50E+023.50E+02

1.20E+013.50E+023.50E+02

1.27E+013.50E+023.50E+02

1.35E+013.50E+023.50E+02

1.42E+013.50E+023.50E+02

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1.65E+013.50E+023.50E+02

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1.80E+013.50E+023.50E+02

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7.35E+013.50E+023.50E+02

7.42E+013.50E+023.50E+02

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2.11E+023.50E+023.50E+02

2.11E+023.50E+023.50E+02

2.12E+023.50E+023.50E+02

2.13E+023.50E+023.50E+02

2.14E+023.50E+023.50E+02

2.14E+023.50E+023.50E+02

2.15E+023.50E+023.50E+02

2.16E+023.50E+023.50E+02

2.17E+023.50E+023.50E+02

2.17E+023.50E+023.50E+02

2.18E+023.50E+023.50E+02

2.19E+023.50E+023.50E+02

2.20E+023.50E+023.50E+02

2.20E+023.50E+023.50E+02

2.21E+023.50E+023.50E+02

2.22E+023.50E+023.50E+02

2.23E+023.50E+023.50E+02

2.23E+023.50E+023.50E+02

2.24E+023.50E+023.50E+02

2.25E+023.50E+023.50E+02

2.26E+023.50E+023.50E+02

2.26E+023.50E+023.50E+02

2.27E+023.50E+023.50E+02

2.28E+023.50E+023.50E+02

2.29E+023.50E+023.50E+02

2.29E+023.50E+023.50E+02

2.30E+023.50E+023.50E+02

2.31E+023.50E+023.50E+02

2.32E+023.50E+023.50E+02

2.32E+023.50E+023.50E+02

2.33E+023.50E+023.50E+02

2.34E+023.50E+023.50E+02

2.35E+023.50E+023.50E+02

2.35E+023.50E+023.50E+02

2.36E+023.50E+023.50E+02

2.37E+023.50E+023.50E+02

2.38E+023.50E+023.50E+02

2.38E+023.50E+023.50E+02

2.39E+023.50E+023.50E+02

2.40E+023.50E+023.50E+02

2.41E+023.50E+023.50E+02

2.41E+023.50E+023.50E+02

2.42E+023.50E+023.50E+02

2.43E+023.50E+023.50E+02

2.44E+023.50E+023.50E+02

2.44E+023.50E+023.50E+02

2.45E+023.50E+023.50E+02

2.46E+023.50E+023.50E+02

2.47E+023.50E+023.50E+02

2.47E+023.50E+023.50E+02

2.48E+023.50E+023.50E+02

2.49E+023.50E+023.50E+02

2.50E+023.50E+023.50E+02

&A

Page &P

Common Problems with the Actuator SystemExcessive valve deadbandImproperly sized control valveValve packing is tightened too muchImproperly tuned valve positioner

Check the Sensor SystemEvaluate the repeatability of the sensor during steady-state operation.Evaluate the sensor dynamics.This may require an independent measurement of the controlled variable.Or check the elements that could contribute to a slow responding sensor.

Most Common Sensor FailuresTransmitterImproperly calibratedExcessive signal filteringTemperature sensorOff calibrationImproperly located thermowellBuildup of material on the thermowellPressurePlugged line to pressure sensor

Most Common Sensor FailuresSampling system for GCPlugged line in sampling systemFlow indicatorPlugged line to differential pressure sensorLevel indicatorPlugged line to differential pressure sensor

Check the ControllerCheck the filtering on the measured value of the controlled variable.Check the cycle time for the controller.Check the tuning on the controller.

Factors that Affect the Closed-Loop Performance of a Control LoopThe type and magnitude of disturbancesPrimarily affects variability in CVCan affect nonlinear behaviorThe lag associated with the components of the feedback control loop (actuator, process, and sensor)Results in slower disturbance rejection which affects variability Precision of the feedback componentsDirectly affects variability

Testing the Entire Control LoopClosed-loop block sine wave testVariability of the controlled variable over a period of a week or more.

Closed-Loop Block Sine Wave Test

Chart1

350.0681350

350.0409350

350.0594350

349.945350

349.9884350

350.0893350

350.0633350

350.0175350

350.0045350

350.0757350

350.0081350

350.0125350

350.0889350

350.2316350

349.9383350

349.9233350

349.939350

349.9952350

349.9074350

349.8946350

350.1343350.1

350.0955350.1

350.0084350.1

349.9802350.1

350.0807350.1

349.9055350.1

350.0476350.1

350.0629350.1

349.8077350.1

349.9222350.1

349.8083350.1

349.9476350.1

350.0918350.1

349.9279350.1

349.8339350.1

350.1561350.1

349.9784350.1

349.8446350.1

349.9376350.1

349.757350.1

349.8901350

349.8534350

349.9452350

349.98350

349.9608350

349.9243350

350.0786350

350.0465350

350.001350

349.8404350

349.9857350

350.0891350

349.87350

350.0323350

349.856350

349.9857350

350.0505350

350.0213350

350.0022350

350.0683350

350.1343349.9

349.8893349.9

350.0427349.9

349.9895349.9

349.9885349.9

349.8347349.9

350.0004349.9

350.1152349.9

349.8545349.9

350.0398349.9

349.8488349.9

350.1078349.9

350.0184349.9

350.0991349.9

349.9845349.9

350.0973349.9

350.0974349.9

349.85349.9

350.0825349.9

350.0799349.9

350.0848350

349.9594350

349.9576350

349.935350

349.9919350

349.936350

349.9648350

349.9675350

350.1309350

350.1758350

349.9207350

350.0857350

349.9283350

349.8937350

349.9462350

350.0766350

350.0422350

349.8591350

349.8648350

349.8694350

349.9778350

350.0239350

349.8773350

349.8833350

350.0929350

349.8612350

350.1766350

350.078350

349.9336350

349.9995350

349.9487350

350.0143350

349.9005350

349.9639350

350.1099350

349.9993350

349.8196350

350.0702350

349.9776350

349.9209350

349.9873350.2

349.9647350.2

350.1203350.2

350.1822350.2

349.8557350.2

350.0559350.2

350.312350.2

350.1181350.2

350.1946350.2

350.1893350.2

350.335350.2

350.2851350.2

350.2507350.2

350.3789350.2

350.2317350.2

350.3282350.2

350.3551350.2

350.3029350.2

350.3157350.2

350.1262350.2

350.2095350

350.1639350

350.0829350

350.3145350

350.0868350

349.9049350

349.7893350

349.9795350

350.036350

349.808350

349.8515350

350.1134350

349.8751350

349.9756350

349.8861350

349.738350

349.8967350

349.9777350

350.0251350

350.1332350

350.094349.8

350.1033349.8

350.0383349.8

349.946349.8

349.8515349.8

349.9131349.8

350.0475349.8

349.9511349.8

349.8811349.8

349.9879349.8

349.6516349.8

349.682349.8

349.8803349.8

349.6339349.8

349.8425349.8

349.6048349.8

349.6215349.8

349.789349.8

349.8717349.8

349.7547349.8

349.8828350

349.7801350

349.9453350

349.9373350

349.927350

350.1255350

350.033350

349.9339350

350.1643350

350.1378350

350.0027350

350.1258350

350.1193350

350.0683350

350.086350

350.0323350

350.0536350

350.0135350

350.0745350

349.9519350

349.9705350

350.0073350

350.052350

350.0575350

349.87350

349.9363350

350.0869350

350.0706350

350.0203350

350.057350

349.9132350

350.1364350

350.1216350

349.9583350

349.8737350

349.9291350

350.1692350

349.9318350

350.111350

349.9733350

350.1918350.4

349.9318350.4

350.192350.4

350.1547350.4

350.0687350.4

350.2222350.4

350.3632350.4

350.4567350.4

350.4742350.4

350.5585350.4

350.5911350.4

350.4523350.4

350.7635350.4

350.5999350.4

350.6136350.4

350.6276350.4

350.4339350.4

350.3604350.4

350.3898350.4

350.3132350.4

350.2939350

350.3541350

350.188350

350.1362350

350.0426350

349.9696350

349.8913350

349.9947350

349.9707350

349.8641350

350.015350

349.879350

349.8609350

349.9176350

349.9101350

349.9918350

349.855350

349.9958350

350.1595350

350.0175350

350.0437349.6

350.0359349.6

349.8968349.6

349.9728349.6

349.8414349.6

349.7852349.6

349.6775349.6

349.6803349.6

349.607349.6

349.5895349.6

349.4994349.6

349.4349349.6

349.6507349.6

349.4123349.6

349.6163349.6

349.4839349.6

349.5827349.6

349.6848349.6

349.7514349.6

349.4637349.6

349.7435350

349.7264350

349.88350

349.91350

349.8657350

349.8719350

349.9204350

349.8661350

350.1737350

350.277350

350.0934350

350.1247350

349.8458350

350.1448350

350.1722350

350.0343350

350.0514350

350.0179350

349.8457350

350.0266350

349.9732350

349.9806350

350.1447350

349.7524350

349.9879350

349.9682350

349.8603350

349.9248350

349.9493350

349.9749350

349.9978350

349.9689350

349.9542350

350.1267350

350.1032350

349.9245350

350.151350

350.1867350

350.1392350

350.0087350

350.168350

350.0132350

350.0825350

350.024350

349.9023350

350.1226350

349.9966350

349.9678350

349.9686350

349.7466350

349.9118350

349.9564350

350.0111350

Setpoint

ControlledVariable

Time

F16

7.25E-013.50E+023.50E+02

1.48E+003.50E+023.50E+02

2.23E+003.50E+023.50E+02

2.98E+003.50E+023.50E+02

3.73E+003.50E+023.50E+02

4.48E+003.50E+023.50E+02

5.23E+003.50E+023.50E+02

5.98E+003.50E+023.50E+02

6.73E+003.50E+023.50E+02

7.48E+003.50E+023.50E+02

8.23E+003.50E+023.50E+02

8.98E+003.50E+023.50E+02

9.73E+003.50E+023.50E+02

1.05E+013.50E+023.50E+02

1.12E+013.50E+023.50E+02

1.20E+013.50E+023.50E+02

1.27E+013.50E+023.50E+02

1.35E+013.50E+023.50E+02

1.42E+013.50E+023.50E+02

1.50E+013.50E+023.50E+02

1.57E+013.50E+023.50E+02

1.65E+013.50E+023.50E+02

1.72E+013.50E+023.50E+02

1.80E+013.50E+023.50E+02

1.87E+013.50E+023.50E+02

1.95E+013.50E+023.50E+02

2.02E+013.50E+023.50E+02

2.10E+013.50E+023.50E+02

2.17E+013.50E+023.50E+02

2.25E+013.50E+023.50E+02

2.32E+013.50E+023.50E+02

2.40E+013.50E+023.50E+02

2.47E+013.50E+023.50E+02

2.55E+013.50E+023.50E+02

2.62E+013.50E+023.50E+02

2.70E+013.50E+023.50E+02

2.77E+013.50E+023.50E+02

2.85E+013.50E+023.50E+02

2.92E+013.50E+023.50E+02

3.00E+013.50E+023.50E+02

3.07E+013.50E+023.50E+02

3.15E+013.50E+023.50E+02

3.22E+013.50E+023.50E+02

3.30E+013.50E+023.50E+02

3.37E+013.50E+023.50E+02

3.45E+013.50E+023.50E+02

3.52E+013.50E+023.50E+02

3.60E+013.50E+023.50E+02

3.67E+013.50E+023.50E+02

3.75E+013.50E+023.50E+02

3.82E+013.50E+023.50E+02

3.90E+013.50E+023.50E+02

3.97E+013.50E+023.50E+02

4.05E+013.50E+023.50E+02

4.12E+013.50E+023.50E+02

4.20E+013.50E+023.50E+02

4.27E+013.50E+023.50E+02

4.35E+013.50E+023.50E+02

4.42E+013.50E+023.50E+02

4.50E+013.50E+023.50E+02

4.57E+013.50E+023.50E+02

4.65E+013.50E+023.50E+02

4.72E+013.50E+023.50E+02

4.80E+013.50E+023.50E+02

4.87E+013.50E+023.50E+02

4.95E+013.50E+023.50E+02

5.02E+013.50E+023.50E+02

5.10E+013.50E+023.50E+02

5.17E+013.50E+023.50E+02

5.25E+013.50E+023.50E+02

5.32E+013.50E+023.50E+02

5.40E+013.50E+023.50E+02

5.47E+013.50E+023.50E+02

5.55E+013.50E+023.50E+02

5.62E+013.50E+023.50E+02

5.70E+013.50E+023.50E+02

5.77E+013.50E+023.50E+02

5.85E+013.50E+023.50E+02

5.92E+013.50E+023.50E+02

6.00E+013.50E+023.50E+02

6.07E+013.50E+023.50E+02

6.15E+013.50E+023.50E+02

6.22E+013.50E+023.50E+02

6.30E+013.50E+023.50E+02

6.37E+013.50E+023.50E+02

6.45E+013.50E+023.50E+02

6.52E+013.50E+023.50E+02

6.60E+013.50E+023.50E+02

6.67E+013.50E+023.50E+02

6.75E+013.50E+023.50E+02

6.82E+013.50E+023.50E+02

6.90E+013.50E+023.50E+02

6.97E+013.50E+023.50E+02

7.05E+013.50E+023.50E+02

7.12E+013.50E+023.50E+02

7.20E+013.50E+023.50E+02

7.27E+013.50E+023.50E+02

7.35E+013.50E+023.50E+02

7.42E+013.50E+023.50E+02

7.50E+013.50E+023.50E+02

7.57E+013.50E+023.50E+02

7.65E+013.50E+023.50E+02

7.72E+013.50E+023.50E+02

7.80E+013.50E+023.50E+02

7.87E+013.50E+023.50E+02

7.95E+013.50E+023.50E+02

8.02E+013.50E+023.50E+02

8.10E+013.50E+023.50E+02

8.17E+013.50E+023.50E+02

8.25E+013.50E+023.50E+02

8.32E+013.50E+023.50E+02

8.40E+013.50E+023.50E+02

8.47E+013.50E+023.50E+02

8.55E+013.50E+023.50E+02

8.62E+013.50E+023.50E+02

8.70E+013.50E+023.50E+02

8.77E+013.50E+023.50E+02

8.85E+013.50E+023.50E+02

8.92E+013.50E+023.50E+02

9.00E+013.50E+023.50E+02

9.07E+013.50E+023.50E+02

9.15E+013.50E+023.50E+02

9.22E+013.50E+023.50E+02

9.30E+013.50E+023.50E+02

9.37E+013.50E+023.50E+02

9.45E+013.50E+023.50E+02

9.52E+013.50E+023.50E+02

9.60E+013.50E+023.50E+02

9.67E+013.50E+023.50E+02

9.75E+013.50E+023.50E+02

9.82E+013.50E+023.50E+02

9.90E+013.50E+023.50E+02

9.97E+013.50E+023.50E+02

1.00E+023.50E+023.50E+02

1.01E+023.50E+023.50E+02

1.02E+023.50E+023.50E+02

1.03E+023.50E+023.50E+02

1.03E+023.50E+023.50E+02

1.04E+023.50E+023.50E+02

1.05E+023.50E+023.50E+02

1.06E+023.50E+023.50E+02

1.06E+023.50E+023.50E+02

1.07E+023.50E+023.50E+02

1.08E+023.50E+023.50E+02

1.09E+023.50E+023.50E+02

1.09E+023.50E+023.50E+02

1.10E+023.50E+023.50E+02

1.11E+023.50E+023.50E+02

1.12E+023.50E+023.50E+02

1.12E+023.50E+023.50E+02

1.13E+023.50E+023.50E+02

1.14E+023.50E+023.50E+02

1.15E+023.50E+023.50E+02

1.15E+023.50E+023.50E+02

1.16E+023.50E+023.50E+02

1.17E+023.50E+023.50E+02

1.18E+023.50E+023.50E+02

1.18E+023.50E+023.50E+02

1.19E+023.50E+023.50E+02

1.20E+023.50E+023.50E+02

1.21E+023.50E+023.50E+02

1.21E+023.50E+023.50E+02

1.22E+023.50E+023.50E+02

1.23E+023.50E+023.50E+02

1.24E+023.50E+023.50E+02

1.24E+023.50E+023.50E+02

1.25E+023.50E+023.50E+02

1.26E+023.50E+023.50E+02

1.27E+023.50E+023.50E+02

1.27E+023.50E+023.50E+02

1.28E+023.50E+023.50E+02

1.29E+023.50E+023.50E+02

1.30E+023.50E+023.50E+02

1.30E+023.50E+023.50E+02

1.31E+023.50E+023.50E+02

1.32E+023.50E+023.50E+02

1.33E+023.50E+023.50E+02

1.33E+023.50E+023.50E+02

1.34E+023.50E+023.50E+02

1.35E+023.50E+023.50E+02

1.36E+023.50E+023.50E+02

1.36E+023.50E+023.50E+02

1.37E+023.50E+023.50E+02

1.38E+023.50E+023.50E+02

1.39E+023.50E+023.50E+02

1.39E+023.50E+023.50E+02

1.40E+023.50E+023.50E+02

1.41E+023.50E+023.50E+02

1.42E+023.50E+023.50E+02

1.42E+023.50E+023.50E+02

1.43E+023.50E+023.50E+02

1.44E+023.50E+023.50E+02

1.45E+023.50E+023.50E+02

1.45E+023.50E+023.50E+02

1.46E+023.50E+023.50E+02

1.47E+023.50E+023.50E+02

1.48E+023.50E+023.50E+02

1.48E+023.50E+023.50E+02

1.49E+023.50E+023.50E+02

1.50E+023.50E+023.50E+02

1.51E+023.50E+023.50E+02

1.51E+023.50E+023.50E+02

1.52E+023.50E+023.50E+02

1.53E+023.50E+023.50E+02

1.54E+023.50E+023.50E+02

1.54E+023.50E+023.50E+02

1.55E+023.50E+023.50E+02

1.56E+023.50E+023.50E+02

1.57E+023.50E+023.50E+02

1.57E+023.50E+023.50E+02

1.58E+023.50E+023.50E+02

1.59E+023.50E+023.50E+02

1.60E+023.50E+023.50E+02

1.60E+023.50E+023.50E+02

1.61E+023.50E+023.50E+02

1.62E+023.50E+023.50E+02

1.63E+023.50E+023.50E+02

1.63E+023.50E+023.50E+02

1.64E+023.50E+023.50E+02

1.65E+023.50E+023.50E+02

1.66E+023.50E+023.50E+02

1.66E+023.50E+023.50E+02

1.67E+023.50E+023.50E+02

1.68E+023.50E+023.50E+02

1.69E+023.50E+023.50E+02

1.69E+023.50E+023.50E+02

1.70E+023.50E+023.50E+02

1.71E+023.50E+023.50E+02

1.72E+023.50E+023.50E+02

1.72E+023.51E+023.50E+02

1.73E+023.51E+023.50E+02

1.74E+023.50E+023.50E+02

1.75E+023.51E+023.50E+02

1.75E+023.51E+023.50E+02

1.76E+023.51E+023.50E+02

1.77E+023.51E+023.50E+02

1.78E+023.50E+023.50E+02

1.78E+023.50E+023.50E+02

1.79E+023.50E+023.50E+02

1.80E+023.50E+023.50E+02

1.81E+023.50E+023.50E+02

1.81E+023.50E+023.50E+02

1.82E+023.50E+023.50E+02

1.83E+023.50E+023.50E+02

1.84E+023.50E+023.50E+02

1.84E+023.50E+023.50E+02

1.85E+023.50E+023.50E+02

1.86E+023.50E+023.50E+02

1.87E+023.50E+023.50E+02

1.87E+023.50E+023.50E+02

1.88E+023.50E+023.50E+02

1.89E+023.50E+023.50E+02

1.90E+023.50E+023.50E+02

1.90E+023.50E+023.50E+02

1.91E+023.50E+023.50E+02

1.92E+023.50E+023.50E+02

1.93E+023.50E+023.50E+02

1.93E+023.50E+023.50E+02

1.94E+023.50E+023.50E+02

1.95E+023.50E+023.50E+02

1.96E+023.50E+023.50E+02

1.96E+023.50E+023.50E+02

1.97E+023.50E+023.50E+02

1.98E+023.50E+023.50E+02

1.99E+023.50E+023.50E+02

1.99E+023.50E+023.50E+02

2.00E+023.50E+023.50E+02

2.01E+023.50E+023.50E+02

2.02E+023.50E+023.50E+02

2.02E+023.50E+023.50E+02

2.03E+023.49E+023.50E+02

2.04E+023.49E+023.50E+02

2.05E+023.50E+023.50E+02

2.05E+023.49E+023.50E+02

2.06E+023.50E+023.50E+02

2.07E+023.49E+023.50E+02

2.08E+023.50E+023.50E+02

2.08E+023.50E+023.50E+02

2.09E+023.50E+023.50E+02

2.10E+023.49E+023.50E+02

2.11E+023.50E+023.50E+02

2.11E+023.50E+023.50E+02

2.12E+023.50E+023.50E+02

2.13E+023.50E+023.50E+02

2.14E+023.50E+023.50E+02

2.14E+023.50E+023.50E+02

2.15E+023.50E+023.50E+02

2.16E+023.50E+023.50E+02

2.17E+023.50E+023.50E+02

2.17E+023.50E+023.50E+02

2.18E+023.50E+023.50E+02

2.19E+023.50E+023.50E+02

2.20E+023.50E+023.50E+02

2.20E+023.50E+023.50E+02

2.21E+023.50E+023.50E+02

2.22E+023.50E+023.50E+02

2.23E+023.50E+023.50E+02

2.23E+023.50E+023.50E+02

2.24E+023.50E+023.50E+02

2.25E+023.50E+023.50E+02

2.26E+023.50E+023.50E+02

2.26E+023.50E+023.50E+02

2.27E+023.50E+023.50E+02

2.28E+023.50E+023.50E+02

2.29E+023.50E+023.50E+02

2.29E+023.50E+023.50E+02

2.30E+023.50E+023.50E+02

2.31E+023.50E+023.50E+02

2.32E+023.50E+023.50E+02

2.32E+023.50E+023.50E+02

2.33E+023.50E+023.50E+02

2.34E+023.50E+023.50E+02

2.35E+023.50E+023.50E+02

2.35E+023.50E+023.50E+02

2.36E+023.50E+023.50E+02

2.37E+023.50E+023.50E+02

2.38E+023.50E+023.50E+02

2.38E+023.50E+023.50E+02

2.39E+023.50E+023.50E+02

2.40E+023.50E+023.50E+02

2.41E+023.50E+023.50E+02

2.41E+023.50E+023.50E+02

2.42E+023.50E+023.50E+02

2.43E+023.50E+023.50E+02

2.44E+023.50E+023.50E+02

2.44E+023.50E+023.50E+02

2.45E+023.50E+023.50E+02

2.46E+023.50E+023.50E+02

2.47E+023.50E+023.50E+02

2.47E+023.50E+023.50E+02

2.48E+023.50E+023.50E+02

2.49E+023.50E+023.50E+02

2.50E+023.50E+023.50E+02

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F16

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Setpoint

ControlledVariable

Time

Closed-Loop Block Sine Wave TestClosed-loop deadbandIndication of the effect of actuator deadband, sensor noise, and resolution of A/D and D/A convertersClosed-loop settling timeIndication of the combined lags of the control loop componentsA means of determining if all the major problems with in a control loop have been corrected.

SPC ChartA Method for Evaluating the Long-Term Performance of a Controller

Chart1

4-1

4.51

4.50

4-1

40

-2-1

-11

2-0.5

-40

2-1

1-0.5

-50.5

4-0.5

-41

41

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Upper Limit

Lower Limit

Time (days)

Product Composition

F1A

0.00E+004.00E+00-1.00E+00

5.00E-014.50E+001.00E+00

1.00E+004.50E+000.00E+00

1.50E+004.00E+00-1.00E+00

2.00E+004.00E+000.00E+00

2.50E+00-2.00E+00-1.00E+00

3.00E+00-1.00E+001.00E+00

3.50E+002.00E+00-5.00E-01

4.00E+00-4.00E+000.00E+00

4.50E+002.00E+00-1.00E+00

5.00E+001.00E+00-5.00E-01

5.50E+00-5.00E+005.00E-01

6.00E+004.00E+00-5.00E-01

6.50E+00-4.00E+001.00E+00

7.00E+004.00E+001.00E+00

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Long-Term Measurement of VariabilityDirect measure of control performance in terms that relate to economic objectivesTakes longer to develop than closed-loop block sine wave test

Troubleshooting ExampleSymptom- The variability in the impurity level in the overhead product of a distillation column is greater than the specified limit.Step 1 Check the actuator systemBy applying a series of block sine wave tests, it was determined that the deadband and time constant of the flow control loop were 0.3% and 1.5 second which indicates that the actuator system is functioning properly.

Troubleshooting Example (cont)Step 2 Check the controllerThe filtering on the product analyzer reading was found to be excessiveThe controller was retunedThe control performance was improved but at times it was still not meeting the product variability specifications

Troubleshooting Example (cont)Check the product analyzerThe repeatability was determined by observing steady-state periods and was found to be well within the product variability specifications.The cycle time of the controller was found to be appropriate.Excessive transport delay in the sample system was identified and a new sample pump installedThe composition controller was retuned and control performance met specifications.

Troubleshooting ExerciseStudents pair up into groups of two.One student represents the process and the other, who is acting as the control engineer, performs the troubleshooting.The process student must choose a loop fault and the control engineer requests the results of certain tests from the process.After the engineer identifies the problem and fixes it, the students switch roles and repeat the exercise.

Overview of CPI TroubleshootingIn order to ensure that a control loop is functioning properly, the control engineer must have a thorough knowledge of the proper design and operation (Table 2.3) of the various components that comprise the control loop.

Troubleshooting in the Bio-Tech Industries

Overall ApproachFor the CPI, troubleshooting usually involves evaluation of one control loop at a time.For the bio-tech industries, it usually involves evaluating the operation of a bio-reactor.For the bio-tech industries, poor operation of a bio-reactor can involve poorly performing control loops or poorly performing sensors. Therefore, troubleshooting is a global problem.

Expert SystemsExpert systems for troubleshooting a bio-reactor are based on distilling the experience of experts into a set of if-then-else rules that guide the operator to the root problem(s).Expert systems can identify batches that can be returned to a normal operating window. Otherwise, the batch can result in off-specification products that are useless.

Actuator SystemsBlock sine wave tests can be used to determine the deadband and time constant for the actuator system.See Table 2.3 for desired performance levels.

Sensor SystemsCoriolis flow meters- require periodic calibration.Ion-specific electrodes (DO, pH and Redox)- require regular replacement and proper location is important.DO- membrane should be replaced regularly.pH- calibration drift a problem requiring calibrationRedox- regular calibration a problem

Sensor SystemsTurbidity sensor- cell can accumulated in the measurement cell.Mass spec- highly reliable due to regular calibration with air samples.HPLC- use guard columns to reduce fouling of the HPLC column.FIA- malfunctioning valves a major problem.

Overview of Bio-Tech TroubleshootingExpert systems are used to guide the troubleshooting activity.Troubleshooting bio-reactors is a global problem requiring a complete understanding of the entire system.Effective troubleshooting of bio-reactors can greatly reduce the frequency of bad batches and is, therefore, economically important.