I have been working with a 500 gal. stirred vessel. It has an N2 blanket for the vapor space with N2 flowing in and the pressure controlled by operating the vent valve. We have this on the control system with a PI control loop rather than using a back pressure regulator because, during different steps of the batch process the pressure is held at different values (usually 5 to 15 psig).
During one operation the vessel contains about 200 gallons of liquid. I tuned the pressure controller during a water run with 200 gallons.
We later used the vessel in a different operation with about 450 gallons of liquid, the remaining space filled with N2. As the level increased above about 400 gallons the pressure begain to oscillate. I retuned the control loop with lower gain and it became stable. However, the pressure control still should have a higher gain during the more common operation with 200 gallons.
The vessel is a V shape and designed for operations at different levels.
My management cannot understand why the digital control loop will not work as well as a simple, cheap, pressure regulator. I can't use one of them because of the necessity of allowing batch programs to change the setpoint of the pressure. Fortunately we are still in water runs, production is a couple of weeks away, at best.
What does the level have to do with the pressure control?
Anita