Posted by David g on November 08, 2019 at 15:12:23 from (209.181.208.169):
I was working at the natural gas interconnect on erratic flow readings.
This pipeline can handle up to a billion cubic feet per day at about 800 PSI.
We replaced the meters and the valve this summer, but we were not allowed to run the flow up and test. Well they ran the flow up last week and the readings were erratic, so they called me. The control system is tuned to respond with what is determined to be an equal response to what changes in the process, this keeps it in balance. On these, we typically control the flow and "override" or limit to prevent over pressure. We got lucky and they let us crank up the flow to test. I found that it would become erratic when the valve opened more than 25%, and would settle back down when it closed.
Well, it took me back to my schooling. A ball valve opening is not linear, the biggest changes happen when the valve nears the 50% mark and diminish towards the ends. I detuned the response from the controls and it settled right down and worked correctly.
There were two contributing factors to this. First and foremost the insides "trim" of the new valve were not the same size as the old one, which caused much more response to the process when we changed it a certain percentage than the old one. The second factor was that we did not get to test over the full range of control before handing it off to operations. It is not tested unless it is fully tested.
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