CCST Troubleshooting & Maintenance of Control Systems 4 — Questions and Answers
Question 1: A control valve with a positioner consistently stops 5% short of the commanded position throughout its stroke. This is most indicative of:
- Valve seat erosion
- Positioner feedback linkage slippage or incorrect cam (Correct answer)
- Air supply pressure fluctuation
- Excessive derivative action in the controller
Correct answer: Positioner feedback linkage slippage or incorrect cam
A consistent gain error across the full stroke (always 5% short) points to a mechanical error in the positioner feedback linkage or an incorrectly adjusted cam.
Question 2: During loop checkout, a technician applies 4 mA to a controller input and the display reads 1.2% instead of 0%. The most appropriate correction is:
- Adjust controller output manually
- Perform a zero trim on the transmitter or input card (Correct answer)
- Replace the controller input card
- Increase the process variable damping filter
Correct answer: Perform a zero trim on the transmitter or input card
A small offset at the live zero (4 mA) with no corresponding process variable indicates a zero calibration error that should be corrected with a zero trim.
Question 3: What does a negative loop gain indicate in a feedback control system during troubleshooting?
- The process is self-regulating
- The controller action (direct/reverse) is set incorrectly (Correct answer)
- The integral time is too long
- The measurement lag is excessive
Correct answer: The controller action (direct/reverse) is set incorrectly
Negative loop gain (output moves in the wrong direction relative to error) means the controller action (direct vs. reverse acting) is set opposite to what the process requires.
Question 4: A technician is calibrating a RTD temperature transmitter. The measured resistance at ice point is 2 ohms higher than expected. What is the likely cause?
- Incorrect RTD type selected in transmitter configuration
- Lead wire resistance not compensated (2-wire RTD used without compensation) (Correct answer)
- Ice bath temperature is above 0°C
- Transmitter span set too wide
Correct answer: Lead wire resistance not compensated (2-wire RTD used without compensation)
In a 2-wire RTD connection, lead wire resistance adds directly to the measured resistance; a 3- or 4-wire configuration must be used or lead resistance must be compensated.
Question 5: An analog control loop passes loop current through a junction box where several connections are made. Intermittent signal drops to 0 mA are observed. What is the first thing to inspect?
- Transmitter housing for moisture ingress
- Junction box terminal connections for corrosion or loose terminations (Correct answer)
- Controller analog input filter settings
- Grounding of the transmitter shield at both ends
Correct answer: Junction box terminal connections for corrosion or loose terminations
Intermittent drops to 0 mA suggest a momentary open circuit, and the highest-probability location in a loop is corroded or loose terminals inside the junction box.
Question 6: Which document should a technician consult to determine the correct fail-safe position of a control valve before performing maintenance?
- P&ID (Piping and Instrumentation Diagram) (Correct answer)
- Equipment location drawing
- Cable tray layout
- Instrument index only
Correct answer: P&ID (Piping and Instrumentation Diagram)
The P&ID specifies valve failure positions (fail-open, fail-closed, fail-in-place) using standard symbology and is the primary reference for safe valve maintenance.
Question 7: A cascade control system's secondary (inner) loop is responding poorly, causing the primary loop to oscillate. What is the recommended corrective sequence?
- Retune the primary (outer) loop first
- Break the cascade and tune the secondary loop independently, then re-engage (Correct answer)
- Increase the primary loop's setpoint
- Reduce the secondary loop's proportional gain only
Correct answer: Break the cascade and tune the secondary loop independently, then re-engage
Best practice is to tune inner-loop controllers independently in manual mode before engaging the cascade so that the outer loop sees a well-behaved inner loop.
CCST Troubleshooting & Maintenance of Control Systems 4
A control valve with a positioner consistently stops 5% short of the commanded position throughout its stroke.
This is most indicative of: