The flow rate of the pneumatic butterfly valve fluctuates occasionally
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The 10-inch pneumatic butterfly valve with fail-safe shutdown function experienced fluctuations twice in December 2018, and further fluctuations occurred in February 2019; the nature of these fluctuations was similar in both cases – the valve would close on its own, resulting in a decrease in flow rate, and once the controller was used to intervene manually, the flow rate returned to normal. The steady-state opening is 12%. Dear sea friends, could you help analyze what the problem is?
Take a look; does your valve positioner have a self-check function? If it is enabled, it might be due to the self-check feature.
There are two reasons for the fluctuations: one is that the spring force of the valve is insufficient, or it is due to issues with the actuator. The second reason is changes in process parameters, such as insufficient pressure, which cause the valve position to change automatically
The process requires stability; there should be no significant changes in pressure, and the flow rate must not suddenly drop by half or more without any warning. The actuator opening feedback is fine; the trend shows that the valve opening feedback is indeed decreasing, which matches the flow rate. At this time, the controller’s output value does not match the valve opening, but it matches at other times. I suspect there’s a problem with the pneumatic actuator or the output of the positioner; I’m not very familiar with this area, so it’s hard to say exactly where the issue lies.
I’m not sure if the self-check I understand is the one you’re referring to; shouldn’t a self-check be carried out periodically or upon manual command? This sudden fluctuation occurred without any warning, and the interval between such occurrences is quite long
Is there a steam trap? It might be a problem with the steam trap
By self-check, he meant the self-tuning of the locator
After manual operation of the controller, the flow rate returned to normal. What does it refer to? Which controller? It sounds like there is no issue with the actuator opening feedback; the pneumatic actuator and the actuator itself are functioning properly most of the time. What is the control logic for this pneumatic valve? What are its inputs and outputs? Is it controlled by a DCS? Is it possible that there is a problem with the signal output from the controller? In other words, is there a problem with the DCS control logic? The two instances when problems occurred were actually due to other factors that caused the DCS to reduce the valve opening?
The 10-inch valve is only used at 12% opening; the CV value for this valve was chosen too high, so by reducing the inner diameter the valve’s opening was increased to 45%