Hi,

On our plant site we have a Pressure Control valve installed on LP steam line to the drier. The valve has a history of stucking, later a strainer was added before valve to avoid foreign particles. But this time we faced a new issue that for a few minutes (6 mins) pressure abruptly dropped by 2 barg and flow rate dropped by 10 Tn/hr and against this action the valve should been opened to maintain the Pressure but the valve closed (a stopper of 50% is installed which retained 50% opening). The vale is fail-to-close so in case of loss of signal or instrument air failure the valve the valve should have been opened. But in field it was observed that the valve opening dropped from 75% to 50% opening. So, we can't conclude the reason behind this valve behavior. Now the valve is normal but suspected till we overhaul it during any SSD.

Additionally:
There is one PT installed on upstream of valve and one FT installed on downstream of the PCV.
There are three IV's in the LP steam circuit line but none of it was throttled during the running operation.
I have attached the response graph please review.

Is there any possible solution/checks from instrumentation side that we can carry out to rectify this issue?

 

I am a little confused with your statement “ The val(v)e is fail-to-close so in case of loss of signal or instrument air failure the valve the valve should have been opened.” I have heard of a valve being “fail close” or “fail open” or “air to close” or “air to open”. I ‘m not familiar with the term “fail to close”. It sounds as if you have an air to open valve with a minimum air signal selector set at 50%. So in this case a loss of signal to the valve would have it fail to 50% value. A loss of air to the valve would have it fail closed. This would mean you probably lost signal to the valve but not air. This would account for the analog out increasing but the valve apparently not responding. Our DCS system will alarm on an open output. Assuming yours does too and you didnt get receive the alarm, I would suspect a short around the I/P somewhere giving the current an alternate path home. I would look for water or corrosion in fitting or J-box.
Good luck. And let us know if you find anything.