Cause analysis of flutter in control valves
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Our facility has a DN100 control valve in the coking unit, along with an EPP4000 intelligent positioner from Yueqing No. 9 Factory. Recently, the process control valve has exhibited significant fluctuations during automatic operation; after shutting down the process, we adjusted the positioner, and manual operation on-site worked well. When adjusting via signals from the DCS, however, the valve continued to fluctuate by 2-3% around the set signal. We addressed any leaks at the connections and replaced the pressure reducing valve, as well as adjusted various parameters within the positioner in an attempt to resolve the issue, but without success. Then a new circuit board, amplifier, and position feedback sensor were installed; in other words, everything except the housing was replaced with new parts. After making these adjustments, the situation remained the same as before with the original locator circuit board – it still had problems. Finally, a Siemens single-acting intelligent locator was installed, and things returned to normal. After the DCS sent the signal, it adjusted twice before settling in place. Once the process was put into operation, the valve functioned normally. However, based on the process of resolving this issue, it is clear that there are still problems with this control valve. Please share your opinions and analyze again: what other issues might exist with the valve? Thank you!
We had a similar situation once; it was a problem with the valve selection. Under normal conditions, the pressure inside the pipeline is too high, and the valve cannot be adjusted properly. When operated manually, the valve closed, and the pressure inside the pipeline decreased, so manual operation worked fine. Later, by replacing the valve, the issue was resolved
Does it wheeze at low manual settings too? Try what opening degree DCS allows for flaring Is this a pressure control valve?
First, check whether there is any air leakage in the valve actuator. Switch the positioner to manual mode and open the valve to 50% of its position to see if it will move on its own. If it doesn’t run, it proves there is no air leakage. Also, when performing manual operations, check whether the valve is stuck; these issues can interfere with valve calibration. In offline mode, if there is fluctuation in the valve’s opening degree after calibration, it is usually due to improper calibration; this can also be addressed by manually adjusting the PID parameters inside the controller.
There are two ways to solve this: 1. Reduce the flow rate of the shut-off valves on both sides of the control valve and increase the opening degree of the control valve, and this will resolve the issue. 2. Replace the valve core
To diagnose surge, first check whether there is a problem with the positioner. Also, with such a small opening degree, should we consider replacing the valve? The method on the 5th floor is also the correct one. But it’s estimated that the manufacturing process wouldn’t allow this to be done. . . We ran into this problem before as well. Just replace the intelligent valve positioner.
Another issue that is easily overlooked is the feedback rod; its proper installation and the sensitivity of its movement also have a direct impact on the stability of the valve. This aspect is often neglected, and if the problem persists even after replacing the components inside the actuator, then this factor should be considered
It’s unreasonable to have only a 2-3% opening degree; if this is indeed the case in practice, it simply means that the wrong valve was selected.
To use this valve, as someone else mentioned earlier, reducing the flow control valve is one solution; another option is to make the PID parameters less sensitive.
If there’s still a chance, I suggest the original poster conduct a small experiment. What is the minimum operating current of this valve before and after replacing the positioner? In other words, it refers to the amount of mA at which the valve actually starts to move. It’s very helpful for future selection.
No air leakage, no sticking, and no struggling at all when offline!