CCST Troubleshooting & Maintenance of Control Systems 5 — Questions and Answers
Question 1: A control valve is cycling rapidly (chattering) even though the controller output signal is stable. What is the most likely cause?
- Controller derivative time too long
- Excessive dead band or stick-slip (stiction) in the valve/positioner (Correct answer)
- Too much integral action in the controller
- Process dead time is too short
Correct answer: Excessive dead band or stick-slip (stiction) in the valve/positioner
Valve chattering with a stable controller output is caused by stiction (static friction) combined with positioner hunting, not by the controller tuning.
Question 2: During a safety instrumented system (SIS) proof test, a pressure switch fails to actuate at its set point. After verifying the set point is correct, what is the next diagnostic step?
- Increase the process pressure beyond the set point
- Inspect the switch for diaphragm fouling, damaged spring, or process buildup on sensing element (Correct answer)
- Adjust SIS logic solver bypass
- Replace the entire transmitter assembly
Correct answer: Inspect the switch for diaphragm fouling, damaged spring, or process buildup on sensing element
Physical inspection for diaphragm fouling, corrosion, or process buildup on the sensing element is the next step when a correctly set pressure switch fails to actuate.
Question 3: A technician observes that a control system's trend shows the process variable lags behind the controller output by an unusually long time after recent piping modifications. What likely changed?
- The transmitter damping filter was increased
- Process dead time increased due to longer pipe run or larger vessel volume (Correct answer)
- Controller scan time was reduced
- Instrument air header pressure dropped
Correct answer: Process dead time increased due to longer pipe run or larger vessel volume
Piping modifications that lengthen the distance between the control element and the measurement point directly increase transportation dead time in the loop.
Question 4: When checking a grounding system in an instrumentation panel, what resistance value between instrument common and earth ground generally indicates an acceptable installation?
- Less than 1 ohm (Correct answer)
- Less than 25 ohms
- 50-100 ohms
- Greater than 100 ohms
Correct answer: Less than 1 ohm
Instrument grounding systems should exhibit less than 1 ohm resistance to earth ground to ensure effective noise rejection and personnel safety.
Question 5: A flow control loop exhibits excellent setpoint tracking but very poor disturbance rejection. Which tuning change would most improve disturbance rejection without significantly affecting setpoint response?
- Increase proportional band
- Decrease integral time (increase reset rate)
- Enable setpoint weighting less than 1.0 (two-degree-of-freedom PID) (Correct answer)
- Increase derivative filter coefficient
Correct answer: Enable setpoint weighting less than 1.0 (two-degree-of-freedom PID)
Two-degree-of-freedom PID allows separate tuning for setpoint tracking and disturbance rejection by applying a setpoint weight to the proportional term.
Question 6: After maintenance on an orifice plate flow meter, the calculated flow reads 10% high even though upstream pressure and temperature are correct. What is the most likely cause?
- Orifice plate installed backwards (Correct answer)
- Upstream straight-run piping is too short
- Differential pressure transmitter zeroed incorrectly
- Process fluid viscosity changed
Correct answer: Orifice plate installed backwards
Installing an orifice plate backwards (bevel facing upstream) causes a different discharge coefficient, typically resulting in a significant positive flow reading error.
Question 7: A DCS analog input card is suspected of failure. Which field measurement would most quickly confirm or rule out the card as the problem source?
- Measure loop current with a clamp meter at the transmitter terminals
- Disconnect the field signal and inject a known 4-20 mA signal directly at the card terminals while monitoring the DCS reading (Correct answer)
- Check transmitter supply voltage at the cabinet
- Verify process variable against a lab sample
Correct answer: Disconnect the field signal and inject a known 4-20 mA signal directly at the card terminals while monitoring the DCS reading
Injecting a known reference current directly at the card input terminals isolates the card from field wiring and the transmitter, conclusively testing the card's accuracy.
CCST Troubleshooting & Maintenance of Control Systems 5
A control valve is cycling rapidly (chattering) even though the controller output signal is stable.
What is the most likely cause?