Chapter 10 Control Loop Troubleshooting. Overall Course Objectives Develop the skills necessary to function as an industrial process control engineer.

Chapter 10Control Loop Troubleshooting

Overall Course Objectives

• Develop the skills necessary to function as an industrial process control engineer.– Skills

• Tuning loops

• Control loop design

• Control loop troubleshooting

• Command of the terminology

– Fundamental understanding• Process dynamics

• Feedback control

Objectives for Control Loop Troubleshooting

• Be able to implement an overall troubleshooting methodology

• Be 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

Controller

F1

T1

TF

F2

T2

TC

ActuatorSystem

TT

SensorSystem

Components and Signals of a Typical Control Loop

TF

F2

T2

Thermocouplemillivolt signal

Transmitter4-20 mADCS

ControlComputer

3-15 psig

4-20 mA

OperatorConsole

Tsp

I/PAir

F1

T1

Thermowell

Actuator System

Controller Sensor System

D/A

A/D

What is Control Loop Troubleshooting?

• Control loop is suspected of not functioning properly.– Poor overall control performance– Erratic behavior– Control loop was removed from service.

• Identify the source of the problem.• Correct the problem.• Retune the controller and monitor.

Overall Approach to Troubleshooting Control Loops

• Check subsystem separately.– Actuator system– Controller– Sensor– Process

• Then check performance of the entire control loop

• What’s been changed lately?

Checking the Actuator System• Apply 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

Time

Setpoint to Flow Controller

Measured Flow Rate

Common Problems with the Actuator System

• Excessive valve deadband

• Improperly sized control valve

• Valve packing is tightened too much

• Improperly tuned valve positioner

Check the Sensor System

• Evaluate 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 Failures

• Transmitter– Improperly calibrated– Excessive signal filtering

• Temperature sensor– Off calibration– Improperly located thermowell– Buildup of material on the thermowell

• Pressure– Plugged line to pressure sensor

Most Common Sensor Failures

• Sampling system for GC– Plugged line in sampling system

• Flow indicator– Plugged line to differential pressure sensor

• Level indicator– Plugged line to differential pressure sensor

Check the Controller

• Check 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 Loop

• The type and magnitude of disturbances– Primarily affects variability in CV– Can affect nonlinear behavior

• The 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 components– Directly affects variability

Testing the Entire Control Loop

• Closed-loop block sine wave test

• Variability of the controlled variable over a period of a week or more.

Closed-Loop Block Sine Wave Test

Time

Setpoint

ControlledVariable

Closed-Loop Block Sine Wave Test

• Closed-loop deadband– Indication of the effect of actuator deadband,

sensor noise, and resolution of A/D and D/A converters

• Closed-loop settling time– Indication of the combined lags of the control

loop components

• A 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

0 1 2 3 4 5 6 7Time (days)

Pro

duct

Com

posi

tion

Upper Limit

Lower Limit

Long-Term Measurement of Variability

• Direct measure of control performance in terms that relate to economic objectives

• Takes longer to develop than closed-loop block sine wave test

Troubleshooting Example

• Symptom- 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 system– By 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 controller– The filtering on the product analyzer reading

was found to be excessive– The controller was retuned– The control performance was improved but at

times it was still not meeting the product variability specifications

Troubleshooting Example (cont)

• Check the product analyzer– The 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 installed

– The composition controller was retuned and control performance met specifications.

Troubleshooting Exercise

• Students 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 Troubleshooting

• In 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 Approach• For 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 Systems

• Expert 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 Systems

• Block 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 Systems

• Coriolis 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

calibration– Redox- regular calibration a problem

Sensor Systems

• Turbidity 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 Troubleshooting

• Expert 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.