Transcript Chapter 8 Control Loop Troubleshooting

Chapter 10
Control 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
Actuator
System
F1
F2
T1
T2
Sensor
System
Controller
T
F
TC
TT
Components and Signals of a
Typical Control Loop
Actuator System
F1
F2
T1
T2
Thermowell
3-15 psig
T
F
Air
I/P
Operator
Console
4-20 mA
Tsp
D/A
DCS
Control
Computer
Controller
Thermocouple
millivolt signal
A/D
4-20 mA
Transmitter
Sensor System
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.
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–
–
–
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
Measured
Flow Rate
Setpoint to Flow Controller
Time
Common Problems with the
Actuator System
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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
Controlled
Variable
Setpoint
Time
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 Chart
A Method for Evaluating the LongTerm Performance of a Controller
Product Composition
Upper Limit
Lower Limit
0
1
2
3
4
Time (days)
5
6
7
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 steadystate 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 bioreactor are based on distilling the
experience of experts into a set of “if-thenelse” 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 offspecification 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.