Figure 1 shows a typical control scheme with two controllers driving the same control valve via high select logic. The controller PIC-1 is the main pressure controller and maintains the vessel pressure of 6 barg by modulation of the control valve. However, if the suction to the compressors falls below 2 barg for any reason, the suction pressure controller PIC-2 opens up the valve to increase the pressure. So in normal operation, the control valve is controlled by PIC-1 while PIC-2 only takes over if the suction pressure drops too low. Suppose that for the current gas flow rates the control valve is 30% to give the 6 barg vessel pressure. Again suppose that the compressor suction is 3 barg i.e. above the setpoint of PIC-2. That means that the output of PIC-2 will be decreasing as the controller attempts to close the valve and reduce the pressure. When this output drops below 30%, it has no effect on the valve because the high select logic selects the output of PIC-1 and that controller has full control of the valve. All well and good, except if the output of PIC-2 carries on dropping all the way to 0%. If for any reason the compressor suction pressure falls away below the setpoint of PIC-2, the control output now has to wind from 0% to 30% before it can start opening the valve to increase the pressure. By this time, the pressure can easily have decreased even further and possibly have tripped the compressors on low suction pressure.

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