Strange phenomena of control valves during DCS commissioning
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During the DCS integration testing in our company, the pipelines were empty with no pressure, and the switching status of the pneumatic control valves was normal. During the subsequent joint testing, liquid was present in the pipelines and pressure was applied; it was found that some control valves could not be opened to 100%, sometimes reaching 50% opening and other times only 30%. What fault points do I need to check when this problem occurs?
1. Is it possible that the large pressure difference before and after the valve prevents it from opening fully? In such cases, switch the valve to manual mode and check whether it can be opened and closed completely; if it still cannot be opened, increase the pressure of the air supply and try again. 2. Could it be that the medium flowing into the valve is dirty, containing impurities such as weld slag, which causes the valve to get stuck?
OP, your description isn’t very clear. First, let’s clarify what it means when it’s not possible to operate it – is it that the DCS cannot input a value of 100, or is the valve opening less than 100?
When the concept cannot be activated, the DCS sets the output to 100%, the on-site control valve opens to 50%; then the DCS sets the output to 0%, causing the on-site valve to close. After that, the DCS sets the output to 100% again, and the on-site valve opens to 30%. The entire process is continuous.
This post was last edited by mywlyx on 2009-12-29 09:43. What do you mean by the pressure difference before and after the valve? ? ? Gas source pressure and liquid pressure in the pipeline? ? The liquid pressure should be between 0.2 and 0.3 MPA
Try increasing the air source pressure, or the valve may not be the right choice.
Still, re-check the valve positioner. This is the most likely possibility.
The pressure of the air supply does not reach the specified normal value for pneumatic diaphragm control valves; check the air supply pressure to see if it is sufficient. If not, increase the capacity of the air compressor； If the air supply pipeline provides sufficient air but the intake volume is too low, clean the air filter.
People from the valve factory came today. Since it’s still in the debugging stage, we can’t make any changes on our own. They came and found that the connections were loose; tightening them fixed the problem (actually, they had loosened them themselves last time):curse:
Then why is debugging normal in the absence of a pressure difference?
Then why is debugging normal in the absence of a pressure difference?