Reasons for the back-and-forth movement of the control valve stem
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A new control valve, DN25, has been installed in the device. During operation, the valve stem moves up and down; the pressure before the valve is 1.2 MPa, and the liquid flows smoothly. Could everyone please analyze the reason for this?
Cause analysis: The spring stiffness of the control valve is insufficient; unstable and rapid changes in the output signal of the control valve can lead to oscillations in it. Also, the frequency of the selected valve is the same as the system frequency, or the pipes and base vibrate violently, causing the control valve to vibrate as well. Due to improper selection, the control valve operates at a low opening degree, resulting in severe variations in flow resistance, flow rate, and pressure. When these variations exceed the valve’s stiffness, its stability deteriorates, and oscillations may occur in severe cases. Solution: Since there are various reasons for oscillations, each case must be analyzed on its own. For those with mild vibration, the stiffness can be increased to eliminate it, such as by using control valves with high-stiffness springs or switching to a piston actuation mechanism ； Severe vibration in pipes and bases can be eliminated by adding supports to reduce vibration interference ； When the frequency of the valve is the same as that of the system, replace the control valves with different structures ； The oscillations that occur when the control valve operates at a low opening degree are caused by an inappropriate selection; specifically, it is due to an excessively high Cv value of the valve. It is necessary to select a valve with a lower Cv value, or to use stage control or mother-child valves in order to overcome the oscillations that arise when the control valve operates at a low opening degree. One more thing I’d like to ask the original poster: there is already a pinned thread on this forum dedicated to the selection of control valves and answering related questions. Did you not see it, or do you want to create your own thread? This post was last edited by zhx7540 on 2007-11-9 12:04.]
As a supplementary note: Consider the issue of auxiliary devices such as positioners/servo amplifiers； There are such questions and answers on the forum, but I can’t find the links right now...
I was in a hurry and didn’t search; please, the moderator, put it there or delete this post
It is usually due to the stiffness of the spring and issues with the actuator
The phenomenon you're referring to is obvious oscillation, and there are many reasons for this oscillation. First, the pipeline vibrates severely, causing the control valve to oscillate ； At this point, it is necessary to find ways to reduce the vibration of the pipeline. The second issue is improper selection – the spring of the control valve is not capable of overcoming the process pressure difference. In such cases, replacing the spring is the most economical solution; replacing the entire valve would be too wasteful ； Third, when the valve is in a low-open position, it is necessary to replace the valve (ensuring that the bypass valve is not open). Fourth, the locator itself is oscillating; check the locator. Fifth, there is slight air leakage from the diaphragm; check the diaphragm
On the pipeline, it remains stable when there is no flow, but it fails as soon as flow starts. The pressure before the valve is approximately 1.35 MPa, with a differential pressure of 50 kPa. Does the magnitude of this pressure difference have an impact on it? Previously, valves with a DN50 size were used, and there was no oscillation. It was changed to DN25 because of the large leakage. Due to limited process capability, the pressure difference before and after the valve cannot be determined; it is generally assumed to be 50 kPa.
You mean full open and full closed of the control valve, right? Our unit has a valve of this type, used for regulating liquid propane; the controller is located on-site, and it is a pneumatic controller. Logically, it should be adjusted to stay around a certain opening; otherwise, wouldn’t it become a two-position valve? But it doesn’t; it’s either 0 or 100. After checking, I found that resonance in the nozzle/valve assembly of the on-site pneumatic controller and in the system was causing the controller to respond slowly. Once the resonance was resolved, the problem was eliminated. I’m not sure if it will be helpful to those upstairs. Hehe, nonsense. :lol :lol
Changing the direction of the valve solved the problem.
The valve was chosen to be too large; I’ve encountered this issue before, and it was fixed by using a smaller one
It’s probably caused by a large pressure difference before and after the valve; try installing a throttling element in front of the valve. This should help resolve the issue