CCST Troubleshooting & Maintenance of Control Systems 2 — Questions and Answers
Question 1: A 4-20 mA transmitter is reading 3.8 mA with no process variable change. What is the most likely cause?
- Open circuit in the signal loop (Correct answer)
- Shorted signal wiring
- Incorrect span calibration
- Power supply voltage too high
Correct answer: Open circuit in the signal loop
A reading below 4 mA (typically 3.6-3.8 mA) indicates a broken or open circuit in the loop, which is the NAMUR standard for wire-break detection.
Question 2: During preventive maintenance on a pneumatic control valve, you notice the diaphragm casing is dented. What is the primary concern?
- The dent changes the actuator stroke length
- Reduced actuator force due to altered diaphragm area (Correct answer)
- Increased spring pre-load causing offset
- Air supply pressure will fluctuate more
Correct answer: Reduced actuator force due to altered diaphragm area
A dented diaphragm casing distorts the diaphragm, reducing its effective area and therefore the force the actuator can develop at a given pressure.
Question 3: A PID controller is hunting (oscillating continuously) around the setpoint after a process load change. Which tuning adjustment is most appropriate first?
- Increase integral time (decrease integral gain)
- Decrease proportional gain (increase proportional band) (Correct answer)
- Increase derivative time
- Decrease setpoint
Correct answer: Decrease proportional gain (increase proportional band)
Continuous hunting usually indicates excessive proportional gain; reducing it (widening the proportional band) is the first corrective tuning step.
Question 4: A thermocouple reading drifts low over several months despite stable process conditions. What maintenance action should be taken first?
- Increase controller output
- Recalibrate the controller input card
- Inspect and replace the thermocouple if oxidized or contaminated (Correct answer)
- Adjust the cold junction compensator
Correct answer: Inspect and replace the thermocouple if oxidized or contaminated
Gradual drift in thermocouple readings is most often caused by wire oxidation, decalibration from repeated thermal cycling, or contamination of the junction.
Question 5: A technician is troubleshooting a loop where the control valve fails to move even though the I/P converter output is changing. What should be checked next?
- Controller setpoint setting
- Air supply to the I/P and actuator (Correct answer)
- Transmitter zero adjustment
- Loop wiring polarity
Correct answer: Air supply to the I/P and actuator
If the I/P transducer output changes correctly but the valve does not move, the most likely cause is loss of instrument air supply to the I/P or actuator.
Question 6: Which method best verifies the calibration of a differential-pressure flow transmitter in the field without removing it from service?
- Apply known DP with a dead-weight tester
- Compare flow reading to a portable clamp-on ultrasonic flow meter (Correct answer)
- Shut down the process and perform a dry calibration
- Check transmitter supply voltage only
Correct answer: Compare flow reading to a portable clamp-on ultrasonic flow meter
Comparing the DP transmitter output against a portable ultrasonic clamp-on meter provides a live-process cross-check without interrupting flow.
Question 7: A control system's historian shows that a flow control loop output saturates at 100% every afternoon. What is the most likely root cause?
- Integral windup due to an undersized control valve (Correct answer)
- Derivative kick from setpoint changes
- Proportional band set too wide
- Transmitter signal noise
Correct answer: Integral windup due to an undersized control valve
Afternoon saturation at 100% output suggests a capacity or sizing issue where the valve is too small to meet the increased demand, causing integrator windup.
CCST Troubleshooting & Maintenance of Control Systems 2
A 4-20 mA transmitter is reading 3.8 mA with no process variable change.
What is the most likely cause?