Pneumatic control valve
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A control valve is experiencing significant fluctuations; the problem persists even when using manual operation. Could it be an issue with the valve positioner? Is there still a problem with the diaphragm of the pneumatic valve? How to check or adjust it? I seek advice from experts.
Maybe the bolts are too tight, causing high resistance； Or the gain of the positioner amplifier is too high; the gain can be reduced.
Could you describe the fault symptoms in more detail? What is the fluctuation method?
The control valve experiences significant fluctuations; try using manual control to see if the fluctuations persist, and it’s also possible that the flow coefficient of the valve was not selected appropriately.
1 First, determine whether the output of the valve positioner is stable. Disconnect the interface between the valve positioner and the valve diaphragm head, and use your hand to check whether the output signal from the positioner is stable. If the signal fluctuates greatly, then check whether the air supply is stable and whether the DCS signal is stable, before determining whether there is a problem with the valve positioner. 2 If the diaphragm leaks, it will not cause fluctuations; it is merely a problem with the valve not being in the correct position.
I encounter this phenomenon frequently; the situations vary among different manufacturers. First, determine whether it is related to the fluid. Isolate the control valve; if the fluctuations persist, it indicates that the issue is not related to the medium or flow rate. If things become normal after isolating the valve, then it’s necessary to check whether the valve chosen is appropriate. If the problem lies with the valve itself, it is most likely a problem with the valve actuator. This area is quite complex, and different manufacturers use different designs. I’ve encountered cases of fluctuations at low opening degrees with FISH valves, as well as continuous up-and-down fluctuations with valves made by Wuxi Industrial Equipment. In some cases, the problem is the actuator itself, while in other cases it’s due to an overly tight connection between the actuator and the control valve’s linkage. It’s difficult to determine the exact cause, so one has to investigate step by step. Additionally, tangled connections or damaged diaphragms could also cause this situation, so one needs to analyze it carefully! If the problem is solved, please share it with everyone so that we can all exchange our experiences!