Regarding the fluctuation issue of the flow control valve
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I am a process engineer. In our workshop, during operation the flow rate is regulated to 4000 NM3/H. In cases where our DCS system does not function properly, the flow rate gradually decreases by about 50 NM3/h over a period of 20–30 minutes, after which it suddenly starts rising back to the normal level. I suspect it is caused by a leak in the instrument air behind the locator. I would like to ask the professionals in this field: how does such a situation occur?
Firstly, your flow rate is in standard cubic feet; after conversion, the actual value is quite small. Will this have an impact on the process?； Furthermore, if you want to determine whether the process fluctuations are caused by surging in the control valve, you can check on-site whether the valve opens as expected. The locator itself has precision limitations; check how many valve positions your adjustment knob allows. Process parameters are inherently variable values during production; to determine whether they are normal, one needs to look at the trends
““The DCS system is not functioning”; has it frozen? It should be clear after checking for leaks in the locator....
The dead zone is set too large, there are issues with flow control, pipe pressure fluctuates, the valve actuator is faulty, or... Air leakage from the control valve can be seen on site, and when there is leakage, the amount of leakage does not gradually decrease before suddenly increasing again. It is recommended to check the locator, perform tuning, and use the five-point test method to verify the linearity of the valve. If there are no issues, refer to the control methods and process specifications.
It’s very simple! Adjust it to manual mode (check on-site) to see if the flow meter still has such large fluctuations It’s been proven that the valve is fine; the issue might lie in whether there is air or something else in the process pipeline. And if there is no test valve, several process conditions can be tried to see whether the valve’s operation still corresponds. What’s the use of guessing while sitting in front of a computer?
Such fluctuations have a significant impact on the operating conditions of our subsequent system, which is why we pay close attention to them.
I checked a few days ago; there was indeed leakage after the positioner was installed. After fixing the leak, the frequency of fluctuations dropped from once every 20–30 minutes to once every 8 hours. No other gaps have been found at present.
There are indeed pressure fluctuations in the main pipe, but I suspect they are caused by fluctuations in our control valves. The traffic level rises and falls automatically without any changes being made to our DCS. I checked a few days ago; there was indeed leakage after the positioner was installed. After fixing the leak, the frequency of fluctuations dropped from once every 20–30 minutes to once every 8 hours. No other gaps have been found at present.
All these tasks have been carried out, and indeed the valve positions at the site move in unison when the DCS is operating. Moreover, this trend of flow fluctuations occurs on its own, without any changes in our DCS.