We're going to need a lot more information (and some better grammar) before we can help you.
You have a pressure transmitter. Fine. Is it measuring the air pressure of a sealed vessel, or the pressure of the water in a tank (e.g., level) ?
You have a flow transmitter. OK. Is it measuring the infeed or outflow rate?
What control mechanisms are you trying to tie with (maybe) a PID loop:
- a flow control valve on the infeed side?
- a flow control valve on the effluent side?
- a gas inlet valve to increase pressure?
- a vent valve to relieve pressure?
You (we) can't control something unless you (we) know what it is you're trying to control.
How does the pressure affect the flow rate, and vice versa?
I assume also that you mean
... we would want to maintain that [setpoint] unless flow exceeded "X".... Then we would need to back it down.
What do you mean by "back it down" ? The control valve position (assuming that you only have one object of control), or the pressure setpoint?
Backing down the pressure setpoint might be the easiest to implement: a separate flow control loop (PID) would try to achieve X flow rate. If the flow is less than X, the loop's CV hit's 100%, which means the pressure loop is 100% of its setpoint ("60 PSI"). As the flow exceeds X, the CV backs down, and so the pressure setpoint is some percentage of "60 PSI".
Easy to implement; probably be a bear to tune. And without knowing all of the above details of your system, this seat-of-the-pants kludge is the best that I (at least) can come up with.
The
real PID experts will have better ideas (and a lot more questions).
Cheers,