CCST Troubleshooting & Maintenance of Control Systems 3 — Questions and Answers
Question 1: When troubleshooting a smart HART transmitter, which tool is most effective for reading device diagnostics without interrupting the 4-20 mA signal?
- Standard digital multimeter
- HART communicator connected in parallel with the loop (Correct answer)
- Oscilloscope across the shunt resistor
- Megohmmeter for insulation resistance
Correct answer: HART communicator connected in parallel with the loop
A HART communicator superimposes FSK digital communication on the existing 4-20 mA signal, allowing diagnostics to be read without breaking the loop.
Question 2: After replacing a control valve packing, the valve stem requires excessive actuator force to move. What is the most likely cause?
- Wrong valve trim material selected
- Packing gland over-tightened (Correct answer)
- Air supply pressure set too low
- Incorrect positioner zero adjustment
Correct answer: Packing gland over-tightened
Over-tightening the packing gland compresses the packing material excessively, dramatically increasing stem friction and the force needed to stroke the valve.
Question 3: A technician finds that a PLC digital output card is not energizing a solenoid valve. The PLC ladder logic shows the output coil is energized. What should be checked next?
- Rewrite the PLC program
- Check output card fuse and field wiring voltage (Correct answer)
- Replace the solenoid coil immediately
- Verify the PLC processor scan time
Correct answer: Check output card fuse and field wiring voltage
When PLC logic shows an output ON but the field device does not respond, the next step is checking for a blown output card fuse or missing field power.
Question 4: A temperature control loop exhibits a slow, sinusoidal oscillation with a period much longer than the process time constant. Which controller parameter is most suspect?
- Proportional gain too low
- Integral time set too short (reset rate too fast) (Correct answer)
- Derivative time too long
- Setpoint too high
Correct answer: Integral time set too short (reset rate too fast)
Long-period sinusoidal oscillation is a classic symptom of integral instability caused by an integral time that is set too short (too aggressive reset).
Question 5: A pressure transmitter consistently reads 2 psi high across its entire range. This error is best described as:
- Span error
- Zero offset error (Correct answer)
- Hysteresis
- Non-linearity
Correct answer: Zero offset error
A constant error that shifts the entire output curve up or down uniformly across the range is a zero (bias) offset error, corrected by adjusting the zero.
Question 6: Which safety practice must be followed before opening a hydraulic actuator supply line for maintenance?
- Increase hydraulic system pressure to flush residuals
- Isolate and bleed pressure from the hydraulic system, then lock out the energy source (Correct answer)
- Replace the hydraulic fluid with a lower-viscosity grade first
- Verify the controller is in manual mode only
Correct answer: Isolate and bleed pressure from the hydraulic system, then lock out the energy source
Stored hydraulic energy must be fully released and locked out/tagged out before opening any hydraulic line to prevent injection injuries or unexpected actuator movement.
Question 7: A flow transmitter using a vortex flowmeter produces erratic output at low flow rates. What is the most likely cause?
- Incorrect 4-20 mA loop resistance
- The flow rate is below the meter's minimum Reynolds number requirement (Correct answer)
- Incorrect pipe schedule entered in the transmitter configuration
- Faulty thermowell nearby causing EMI
Correct answer: The flow rate is below the meter's minimum Reynolds number requirement
Vortex flowmeters require a minimum Reynolds number (typically >10,000) to produce stable vortex shedding; below this threshold, output becomes erratic or zero.
CCST Troubleshooting & Maintenance of Control Systems 3
When troubleshooting a smart HART transmitter, which tool is most effective for reading device diagnostics without interrupting the 4-20 mA signal?