An oscillating control valve may appear to be the source of control instability and repair efforts are usually focused only there. When this fails to solve the issue, further investigation often proves the valve behavior was merely the symptom of some other condition. This article discusses troubleshooting techniques to help plant personnel get past the obvious and discover the true cause of control problems.
“That new control valve is acting up again!” Similar words have been uttered by thousands of control room operators all over the world. The plant is not running well, and operators are quick to identify the culprit—a recently installed, misbehaving control valve. It might be cycling, it might be squealing, it might sound like it has rocks going through it, but it is definitely the cause.
Or is it? When troubleshooting control issues, it is important to keep an open mind and look beyond the obvious. It is human nature to blame the “last thing changed” for any new problem that occurs. While erratic control valve behavior might be the apparent source of concern, the true cause is usually located elsewhere.
Thorough Investigations Find the True Problems.
The following application examples illustrate this point.
Screaming Control Valve. A high-pressure spray valve was squealing after a few months of service. The valve was pulled, checked, and appeared to be functioning normally. When returned to service, the squealing resumed, and the plant demanded the “defective valve” be replaced.
The vendor was called to investigate. A little checking indicated the valve was being cycled by the control system between 0% and 10% open at a rate of 250,000 times a year. The very high cycle rate at such low flows and high-pressure drop was creating the problem. Adjustment of the loop tuning and applying a little backpressure on the valve stopped the cycling and eliminated the squeals.
Jumpy Valve Response. A boiler feedwater pump recycle valve was sticking in the seat at startup. When the valve would first come off the seat, it would jump open, creating control upsets due to uncontrolled flow.
The valve vendor was called to diagnose the valve. Diagnostics were run and the air supply pressure was found to be set well above specification and four times higher than was required for adequate seating. When the valve was pulled for inspection, the technicians discovered damage on the seat and seat rings due to the excessive actuator force, which caused the valve plug to hang. Those components were replaced, the air supply pressure lowered, and the valve was returned to service where it performed as expected.
Post time: Feb-18-2022