CPT Advanced Diagnostics & Troubleshooting 3 — Questions and Answers
Question 1: A level transmitter using a differential pressure cell reads low level when the actual level is high. Which fault is MOST likely?
- High-side impulse line is plugged (Correct answer)
- Low-side impulse line is plugged
- Transmitter span is set too high
- Process fluid density has increased
Correct answer: High-side impulse line is plugged
A plugged high-side impulse line prevents high pressure from reaching the transmitter, causing it to report a lower level than actual.
Question 2: A reciprocating compressor shows increased vibration on every other stroke. The MOST likely cause is:
- Worn main bearings
- A faulty suction or discharge valve on one cylinder (Correct answer)
- Misalignment of the drive coupling
- High discharge temperature
Correct answer: A faulty suction or discharge valve on one cylinder
A faulty valve on one cylinder causes unequal loading between cylinders, producing vibration that repeats every other stroke.
Question 3: When performing a loop check on a 4-20 mA transmitter, a technician measures 4 mA at 0% and 19.5 mA at 100%. This indicates:
- Zero error only
- Span error only (Correct answer)
- Both zero and span errors
- A wiring fault in the loop
Correct answer: Span error only
The zero (4 mA at 0%) is correct but the span is short (19.5 mA instead of 20 mA at 100%), indicating a span calibration error only.
Question 4: A process vessel pressure is rising unexpectedly with no increase in feed rate or temperature. Which cause should be investigated FIRST?
- Blocked vent or relief path (Correct answer)
- Leak in the vessel wall
- Incorrect pressure transmitter calibration
- Condensate accumulation in the vessel
Correct answer: Blocked vent or relief path
An unexpected pressure rise with constant inputs most urgently points to a blocked vent or plugged relief valve preventing normal pressure release.
Question 5: During troubleshooting, a technician uses a step test by making a step change to the controller output in manual mode. The primary purpose is to:
- Verify the final control element moves as expected and measure process response (Correct answer)
- Reset integral windup in the controller
- Determine the transmitter calibration accuracy
- Check for electrical noise in the signal loop
Correct answer: Verify the final control element moves as expected and measure process response
A step test confirms valve stroke response and reveals the open-loop process gain, time constant, and dead time needed for tuning diagnostics.
Question 6: A pump discharge pressure is normal but flow is lower than expected. The MOST likely cause is:
- Worn or damaged impeller
- Partially closed discharge valve (Correct answer)
- Air entrainment in the suction line
- High suction pressure
Correct answer: Partially closed discharge valve
A partially closed discharge valve restricts flow while maintaining or elevating discharge pressure, matching the observed symptoms.
Question 7: A thermocouple reads erratically and the signal drops to a very negative value intermittently. The MOST probable fault is:
- Open circuit or broken thermocouple wire (Correct answer)
- Reversed thermocouple polarity
- High ambient temperature at the transmitter
- Wrong thermocouple type selected in transmitter
Correct answer: Open circuit or broken thermocouple wire
An open circuit in a thermocouple causes the input to float, often driving the transmitter to a downscale (very negative) burnout indication.
CPT Advanced Diagnostics & Troubleshooting 3
A level transmitter using a differential pressure cell reads low level when the actual level is high.
Which fault is MOST likely?