The setup is a compressor which is equipped with a bypass valve with a 1 second stroke time and a slide capacity valve which has a 70 second stroke time.
The slide requires to be the main and fine control, whereas the bypass is quick acting and therefore dominant.

The idea is the bypass shall act quickly to change the pressure and the slide shall catch up, also when no gas is being drawn the slide requires to close down and the bypass open to recirculate.

I am using an AB compact logics and have looked at ratio control, however this does not look like it shall react quick enough, any ideas?

 

If you can tolerate the bypass valve being open in normal service, then use it as the primary pressure control. Take the bypass valve position into a second ZC (position controller) and use the output of this to operate the slide valve. If the slide still allows some material to flow when fully closed, the pressure will rise and open the bypass valve. If the bypass is usually closed and there to open on loss of load, you might be able to use a selector - use separate controllers for the slide valve and bypass valve, tuned to suit the final control element, and feed both outputs to a high or low selector depending on action. You may need a signal inverter to drive one of the valves. On similar installations I have dealt with, the major problems came when trying to set up the starting control logic...

Bruce

 

After further investigation, the bypass under normal operating needs to be closed and can only reduce the pressure in the system. I have had a thought that if as follows using two pids:
PID1
PV = Output Pressure
SP = Operator Input
CV = Bypass Valve (4-20Ma)

PID2
PV = Bypass Feedback (4-20Ma)
SP = 4Ma = 0%
CV = Position Controller = Capacity Slide (digital open close signal)

Thus the pressure rises, the Bypass opens, let say 20%, thus closing the Slide Valve which reduces pressure, thus closing the bypass again.

What do people think?

 

Since the bypass valve is normally closed and will act to open on high pressure, you can set it to have a set point say 5 or 10 % (1 or 2 barg for a 20 barg system) above that of the slide valve controller. If the load drops in a hurry causing a large pressure rise, the bypass will open and
limit pressure. The slide valve will also act to reduce the flow through the compressor, but slowly. As the slide valve unloads the compressor, the bypass valve will shut off again.

Bruce