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...