What are the reasons for large fluctuations in pneumatic control valves?

The PID values are not set properly, which causes oscillation; it is recommended to reset the PID parameters and adjust them gradually based on the curve to find the optimal settings. This post was last edited by Chifeng on 2009-2-13 09:23

Based on this phenomenon, it is likely that the issue is caused by improper setting of your PID parameters; generally, a valve opening degree between 30-80% is considered reasonable.

Check whether your PID parameter settings are reasonable; try to keep the valve opening at 60~80%

Generally, a valve opening degree of around 70% is optimal. The reasons for the above instability are analyzed as follows: 1. Are the PID parameter settings correct? 2. Is the pressure of the air supply for the pneumatic control valve stable? 3. Is the selection of the control valve appropriate?

The pid parameter must be properly tuned before it can be used in automatic control.

There is an issue with the PID settings, or the air supply pressure is unstable.

Can the control valve keep up with the control speed?

Your description isn’t very clear. 1. What medium? 2. What does “a few hundred” mean? Is it the traffic that is being controlled? Is the unit liters per hour? 3. When adjusted to 50% manually, do the valve and flow rate fluctuate? Possible reasons: 1. The diameter of the control valve is too large. 2. The P value in the PID setting is inappropriate; also, if a D term is used, remove it first and try using only PI control to see

For their control valves, for example, if the set flow rate is 800 kg/h, the actual value can range from a minimum of 600 to a maximum of 1000 or even higher. As for the medium, it doesn’t matter whether it’s one of several different materials; there are varying opinions regarding what the appropriate opening degree should be

First, do it manually, then automatically, so that you can identify what’s wrong with what you said.

If the PID parameter settings are incorrect, it may result in excessive overshoot of the system and a long steady-state time, leading to large fluctuations in the valve position； If it is a linear valve and the actual flow rate is very low, not reaching the minimum flow rate specified during design, the pressure difference before and after the valve is relatively small; due to the overly sensitive control performance, oscillations occur ； Under normal circumstances, the opening degree corresponding to the maximum flow rate for proportional valves should be around 90%, for linear valves it should be around 70%, and for parabolic valves it should be around 80%.

First, switch to manual mode; if the valve position at the site does not change, check whether the flow rate is still experiencing large fluctuations. If so, it is due to process-related issues, where the disturbances exceed the range that can be addressed through adjustment, and no matter how the parameters are set, it may not be possible to bring the situation under control. If the cutting is done manually and the flow rate can be maintained at a relatively constant value, it may be due to the tuning of the PID parameters; an excessively high P value or an excessively low I value can both cause oscillations in the system. The optimal position of the valve stem depends on the type of valve; the curves for quick-opening valves, linear valves, and logarithmic valves are all different. Generally, a value around 50 or 60 is preferable, with some adjustment range above and below that being ideal. There is another reason: an excessive air supply to the valve can also cause oscillations in the valve position during adjustment.

:handshake Agree with the comment above; it’s good

When switching from manual to automatic mode, be sure to adjust manually first to ensure stability before switching to automatic mode; read up on what seamless switching means!

As mentioned above, if it remains unstable when in manual mode, one should consider whether the actuator torque of the control valve is set too low, or whether the positioning accuracy of the electrical positioner is insufficient This will lead to a specific phenomenon in the pneumatic valve: surging! Just like a person breathing. The solution is to send it back to the factory for repair.

In addition to the reasons mentioned above. If the diameter of the control valve is large, and there is a pneumatic amplifier in the pneumatic circuit, air leakage in the control signal lines of the positioner or amplifier can also cause this phenomenon.

1. There are issues with the parameter settings of the PID controller: if flow regulation is the goal, remove the differential control element and use only PI control. 2. Check whether there is any vibration in the process pipelines, as mechanical valves and potentiometers can experience fluctuations due to vibrations

What everyone is saying is too technical. According to them, the valve core of the control valve can withstand a pressure of 4 bar at the time of purchase, but in actual production the pressure reaches around 8 bar, which results in unstable flow rates. They are now asking the manufacturer to replace it