What causes fluctuations in the control valve positioner? What’s the solution?

First, check whether the air supply pressure is stable

1. Similar to electrical converters, when the instrument air is not clean, the nozzle/diaphragm structure can experience fluctuations in the output of the actuator due to dirt on the nozzles/diaphragms. In the case of valves controlled by slide valves, aging or wear of the \"O\"-ring seals on these slide valves can also cause output fluctuations, which in turn leads to fluctuations in the control valve. Treatment: For nozzles/valves that are too dirty, it is sufficient to clean them only. However, if the \"O\"-ring seals of the slide valves are aged or worn out, those \"O\"-ring seals must be replaced. 2. Loose feedback in the positioner can also cause fluctuations in the positioner’s output. When the feedback is loose, changes in the valve position of the control valve cannot provide negative feedback to the positioner; as a result, the signal and the output do not correspond to each other. This leads to delayed action of the control valve, and the requirements imposed by the signal conflict with this delay, thereby causing fluctuations in the valve’s operation. Treatment: For those with loose connections, the loose parts need to be tightened. 3. The exhaust passage of the locator is blocked; when the locator operates, its amplifier or spool valve may need to be vented. If the exhaust passage of the locator is not unobstructed, fluctuations will occur in the operation of the control valve during regulation. In the actuator section, in addition to the reasons for signal fluctuations mentioned above, there are also two reasons for fluctuations within the actuator itself. Commonly, poor sealing of the actuator’s air chamber leads to air leakage and cross-ventilation, causing fluctuations. In the case of diaphragm actuators, if the diaphragm is damaged, air will leak from it when there is pressure in the air chamber; as a result, the regulator must continuously adjust its output to supply instrument air to the air chamber, which can lead to fluctuations in the control valve. For cylinder-piston actuators, if the piston cannot seal properly and air leaks into the gas chamber, the actuator’s controller must continuously adjust its output, which also leads to fluctuations. Therefore, for those with air leakage or cross-ventilation, the relevant components need to be replaced to eliminate such issues. Another reason is that the effective output force of the actuator is insufficient; under certain operating conditions, it is unable to overcome the unbalanced forces acting on the valve core as well as the friction caused by the packing. This results in fluctuations in the performance of the control valve. Such issues can generally be resolved by increasing the air supply pressure or the spring preload, or by slightly loosening the packing to reduce friction.

The second floor is correct; it can be made more concise with a few additions. Surge in control valves may be caused by the packing being too tight, or by poor movement of the actuator (from the perspective of the valve)； Either the pressure reducing valve is blocked, or the air supply pressure is insufficient (pressure reducing valve setting) ； After ruling out these two possibilities, the issue lies with the locator. Besides the reasons mentioned above, the problem could also be caused by an improperly calibrated electronic locator, or by improper installation that does not follow the standard procedures. All of these can lead to the issue you are experiencing……

The original poster can discuss such issues on the Instrumentation version; although instrumentation and automatic control are closely related, they are separated in the forum……

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