Design PID for controlling water pressure

Hello to all,

in attachment, you can find one problem example of maintaining water pressure in pipes.
I need to hear your advices and opinions on this.

Thank you and looking forward to your comments...

A lot depends on the valve actuator.

I normally use hydraulic valves where if I set the control signal to 0 they will close. Many big process valves will just stay where they are because there is a motor that moves the control spool. If this is the case you may not want to use the integrator gain because the control output from just the proportional gain gets integrated by the valve spool position.

If the valve acts like an integrator then even a small error will cause the valve to open slowly until the desired flow is achieved. If the error is bigger the valve will just open more so the error is reduced faster. When the error is reduced to 0 the output to the actuator motor will be reduced to 0 so the change in flow should be 0 and the current pressure will be maintain.

The point is that you may not need an integrator because the system is essentially K(p)/s. I am sure the plant gain will vary as a function of pressure p.

The sun is out, time for a long two hour walk.

It is control valve with motor and gearbox, so valve is closing until it reach end limit position or until close control signal is present. When valve is open more, pressure is bigger in the pipeline. Real problem is that there is PI controller already implemented, but never properly adjusted. If operator switch to "auto", pressure suddenly start to increase (valve is openning, but there is no position indicator) and he must switch back to manual very quickly in order not to cause damage.
I'll try to switch off integral part and reduce gain to see how controller will work.
I agree that the plant gain will vary with the pressure, that's why I wanted to determine Kp in operational point and to adjust PI in such way so the operator can manually open valve until pressure is good and then just switch to auto, so controller can keep pressure around +- delta...