CPT Advanced Diagnostics & Troubleshooting 2 — Questions and Answers
Question 1: A centrifugal pump is cavitating. Which corrective action addresses the root cause when suction pressure is too low?
- Increase discharge valve opening
- Raise the liquid level in the suction vessel (Correct answer)
- Increase pump speed to boost flow
- Close the discharge valve partially
Correct answer: Raise the liquid level in the suction vessel
Raising the suction vessel liquid level increases the available NPSH, eliminating the low-pressure condition that causes cavitation.
Question 2: A control valve is hunting (oscillating continuously). The FIRST diagnostic step should be to:
- Replace the valve positioner
- Switch the controller to manual and observe if hunting stops (Correct answer)
- Increase the controller integral time
- Decrease the process gain setting
Correct answer: Switch the controller to manual and observe if hunting stops
Switching to manual isolates whether the oscillation originates from the controller tuning or a mechanical problem with the valve itself.
Question 3: An orifice plate flow meter reads higher than the actual flow. Which condition MOST likely causes this error?
- Orifice plate installed backwards
- High differential pressure across the plate
- Liquid density lower than calibration value (Correct answer)
- Impulse line on the high side is plugged
Correct answer: Liquid density lower than calibration value
A lower-than-calibrated fluid density causes the meter to over-read because flow is calculated assuming the calibrated density.
Question 4: During startup of a distillation column, tray flooding is suspected. Which symptom confirms flooding?
- Pressure drop across the column decreases sharply
- Bottom temperature rises while overhead temperature drops
- Differential pressure across the column increases suddenly (Correct answer)
- Reflux drum level drops to zero
Correct answer: Differential pressure across the column increases suddenly
A sudden increase in column differential pressure indicates liquid backing up on the trays, confirming flooding conditions.
Question 5: A temperature transmitter has a fixed positive offset error of +15°F across all measurements. This type of error is called:
- Span error
- Zero or bias error (Correct answer)
- Linearity error
- Hysteresis error
Correct answer: Zero or bias error
A consistent offset across the entire measurement range is a zero (bias) error, indicating the transmitter baseline needs re-zeroing.
Question 6: A heat exchanger's outlet temperature is declining over weeks despite steady inlet conditions. The MOST probable cause is:
- Fouling on the heat transfer surfaces (Correct answer)
- Decreasing shell-side flow rate
- Tube-side fluid viscosity decrease
- Increasing condensate level on shell side
Correct answer: Fouling on the heat transfer surfaces
Gradual performance decline over time with constant operating conditions is the classic signature of fouling accumulating on heat transfer surfaces.
Question 7: A pneumatic signal line to a valve actuator is leaking. What effect will this have on a fail-open (air-to-close) valve?
- Valve will close fully and stay closed
- Valve will open to a greater degree than commanded (Correct answer)
- Valve will cycle rapidly between open and closed
- Valve position will be unaffected
Correct answer: Valve will open to a greater degree than commanded
Loss of air pressure in an air-to-close actuator causes the spring to open the valve further than the controller commands.
CPT Advanced Diagnostics & Troubleshooting 2
A centrifugal pump is cavitating.
Which corrective action addresses the root cause when suction pressure is too low?