The Azbil control valve is surging; please help analyze the cause.
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In the process reaction, a 1.0 MPa steam flow transmitter showed fluctuations; after examining the curves and the on-site transmitter, it was determined that the problem lay not with the flow transmitter. The control valve for that flow rate was checked again, and it was found that the valve was experiencing slight up-and-down fluctuations. The process engineers were contacted to switch to manual operation, but the valve still showed such fluctuations; no leaks were detected during the inspection. It is preliminarily assumed that there may be a problem with the actuator. The process was switched to bypass mode, and a new actuator was installed. After proper calibration, the system started operating normally. However, after three or four hours of stable operation, the same fluctuation problems reappeared. The signals coming from the safety barrier were found to be quite stable. Later, it was suspected that the issue might be intermittent instability in the output signal of the safety gate, so a new safety gate (from Bürkert) was installed. It has now been operating steadily for ten hours – does this indicate that the problem indeed lay with the safety gate? What other possible causes could lead to this phenomenon?
High stem resistance in control valves can also cause surging in such valves.
What is the pressure difference between the upstream and downstream valves, and what is the current valve position? Vibration caused by noise cannot be ruled out either
The reason is that the valve size chosen is too large, resulting in a low differential pressure
Is there an amplifier in the valve’s air circuit? If so, the adjustment screw can be loosened slightly to reduce the sensitivity. If a problem with the safety barrier is suspected, a signal generator can be used to send a signal to see if the valve still operates.
If the pressure difference exceeds the specified value, it will cause fluctuations in the control valve; it would be better to switch back to the bypass and test the control valve, while observing on-site to see if surging still occurs
The normal opening degree of the valve is 55%, so there should be no issue with the valve size.
At first, I didn’t look at your graph. When operating in manual mode, did you go to the site to check whether the actual valve position changed? If it did change, that indicates a problem with the valve; if it didn’t change, then the issue lies with the flow meter. In your graph, the flow rate should be proportional to the valve position, but in your case it’s inverse – which is clearly incorrect. This phenomenon is caused by fluctuations in flow rate leading to fluctuations in the valve position. It seems the problem with this curve isn’t related to the valve; it’s likely a problem with the flow meter. Is it an orifice plate type flow meter or a vortex flow meter? Also, verify through the process whether there is water hammer in the pipeline
Switch the locator to the field mode, manually set the valve position to 50%, and check whether the valve will close toward the faulty position
I don’t know whether your locator has HART functionality; you can check the changes in the input and output values of the locator to determine if there is a problem with the signal. I’ve encountered similar issues as well; fluctuations in the process reaction valves caused pressure swings. After checking everything, no problems were found. Eventually, a handwheel (this control valve is equipped with one) was used to fix the opening degree of the valve, but the fluctuations continued. In the end, the production team was asked to investigate the cause, but they couldn’t find any solution.