Description of the problem:
There are two air-operated Flow Control Valves in the system allowing the controlled flow of materials. When one valve is open at 2%, another will be open at 98%, when one valve is open at 30%, another valve will be open at 70% and so on.
We noticed that the valves seem to be letting too much flow through even at the point they crack open. This continues from 0 to 2% of valve opening. Within this initial range flow drastically increases to as much as 8%. Pass the 2% mark flow rate increase becomes linear and remains that way.
The only variable that made a positive change in test performance was lowering inlet pressure. We ran the test with no inlet pressure and it was good, but that isn’t how the system is designed to run. We checked the flow through the valves with a pressure gauge on the inlet and the outlet and provided that data to the Flowserve valve.
During site visit, we attempted to run the gradient with a ton of different variables. Even at 0.5 bar though it was still oscillating hard up until 7-8% and the same on the end of the gradient. We adjusted PID parameters, inlet pressures, flow rates, etc. We check valve position via the electrical signal and physically and everything seems to be correct.
What can potentially be wrong? Apparently he problem persisted for long time, but other known good units work fine throughout the entire range. Running the system without having head pressure is not an option; all systems designed to have an inlet pressure of .5-1 Bar