Troubleshooting & Maintenance of Control Systems Flashcards

A control valve with a positioner consistently stops 5% short of the commanded position throughout its stroke. This is most indicative of:

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.

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:

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.

What does a negative loop gain indicate in a feedback control system during troubleshooting?

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.

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?

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.

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?

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.

Which document should a technician consult to determine the correct fail-safe position of a control valve before performing maintenance?

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.

A cascade control system's secondary (inner) loop is responding poorly, causing the primary loop to oscillate. What is the recommended corrective sequence?

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.