A basic problem with integral controllers is that integral action continues as long as an error exists. Assume a proportional-plus-integral controller is used to maintain the level in the gas-liquid separator vessel in Figure 300-4. If a valve is closed upstream of the vessel, the level drops below the setpoint. The controller then closes the control valve in the outlet line to maintain the level setpoint. With no inlet flow, the control valve closes completely and the vessel level is still less than the setpoint.
A pneumatic control valve will typically be fully closed at a controller output of 15 psig. Since the measured vessel level is less than the setpoint, the integral action of the controller continues to increase the controller output to the air supply pressure (typically 20-30 psig). The action of the integral controller trying to exceed the normal range of the controller output is called wind-up.
If the upstream valve is opened and flow is restored, the vessel level will rise above the setpoint. The response of the controller to this high level will be delayed by the wind-up. When the controller does respond, the output goes to the opposite limit. In this case, the control valve will fully open and the vessel level will drop sharply. The controller may oscillate through several cycles, stroking the control valve from stop to stop on each cycle, before the oscillations cease and control is restored.
Such oscillations overwork the control valve and, depending on the fluid and pressures involved, can cause mechanical damage and seriously disrupt the process downstream on the valve. An anti-wind-up feature may be included on controllers that are frequently subject to this type of disturbance. This limits the controller output range and thus prevents wind-up. When the process returns to normal, the controller lag is eliminated and the oscillations are no worse than those in a proportional controller.