We have a pressure control valve that is not controlling at low pressure (maximum downstream pressure achieved is below setpoint) and ok with high pressure. we concluded that the valve is small and needs resizing to cover the high flow portion (low pressure) . Hope I am correct in my analysis! my question would be: in this case, do I go for valve resizing or just a trim modification will do? Can you please advise from your experience . for resizing, do i look for the upstream flow rate value only or also the upstream pressure?
From my experience, numbers are everything. It sounds like the original valve was specified without discussing off-nominal conditions with the vendor. You will need flow and upstream and downstream pressures, because all are needed to define the correct valve coefficient, BUT, you also need both the nominal and worst-case off-nominal conditions you expect the valve to see. You may be able to keep the valve body and just replace the trim, IF the numbers pan out, but without any hard numbers and real world equipment data, there is no way to guess. First things, first, you need to gather the critical information: What is the maximum flow rate you could reasonably need to achieve at the minimum allowable pressure at your source and maximum downstream pressure required? What is the typical flow rate and source and regulated pressures? It would also help to have fluid/gas data and operating temperatures ready. Then, you can talk with your vendor to see if they have a trim for your valve body that will do the job or if you have to replace the whole thing. The first question will define the minimum necessary maximum valve coefficient (what it can do wide open). The second will help define the trim characteristic that will give you the best performance for your control system. If your typical condition would result in a linear valve trim operating near 50% open, that trim may be best. If you need finer accuracy when operating over a wide dynamic range of flow rates, an equal-percentage valve may be preferred. A quick-opening valve would be best if you tend to operate stably at a single condition near the high-end and need to quickly respond to an onset or drop of demand. Again: gather information about the specifics of your application and discuss it with an expert on your valve. Note that this is assuming other requirements have already been accounted for: required accuracy and precision of the pressure regulation, response time, etc. Depending on the specific requirements of the application, a single valve may not even be the right answer, but that is a much bigger discussion. Again, probably already dealt with, but one other side thing to verify: make sure the lines upstream and downstream of the valve are sized correctly. If they are too long and narrow, then it doesn't matter how far your valve can open, if the flow chokes in the line.
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