Troubleshooting & Maintenance of Control Systems Flashcards

A control valve is cycling rapidly (chattering) even though the controller output signal is stable. What is the most likely cause?

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.

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?

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.

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?

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.

When checking a grounding system in an instrumentation panel, what resistance value between instrument common and earth ground generally indicates an acceptable installation?

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.

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?

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.

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?

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.

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?

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.