Override Control
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If the temperature controller does not have control of the steam valve, and the temperature falls below its setpoint, the integral action in the controller will cause the output to continue to increase until it reaches 100%. This is known as reset windup.
The problem with reset windup is that after the header pressure reaches its sepoint and the pressure controller output increases, the valve will continue to open. The temperature controller will not begin to close the valve until the temperature rises above its setpoint. Then, as the temperature rises above its setpoint the controller will lower the output. This "reset winddown" will cause an overshoot in the temperature setpoint.
One solution: External Feedback.
If the PID algorithm uses a positive feedback loop for integration, then that positive feedback loop can use the output of the selector (the signal to the valve) as the feedback.
The external feedback (EF on the diagram) is the output of the low selector. The output of the temperature controller is the gain multiplied by the error added to the external feedback. If the error is positive (temperature below setpoint) the output of the temperature controller is higher than the output of the selector. However, when the temperature rises and equals the setpoint, the error will be zero and the controller output will equal the selector output.
As soon as the temperature exceeds the setpoint (negative error) the output of the controller will be less than the output of the selector, decreasing the output of the selector and closing the valve.
