Override control is used to take control of an output from one loop to allow a more important loop to manipulate the output.

The output from two or more controllers are combined in a high or low selector. The output from the selector is the highest or lowest individual controller output. The selector is shown in the diagram by the < or > symbol.

Example of override control:

The steam header must be maintained above a minimum pressure. Steam from the header is used to heat water in a heat exchanger. The temperature of the hot water is controlled by TIC-101. It is more important that the header pressure be above its minimum than that the water temperature be at its setpoint.

The setpoint of the steam header pressure controller is set at the minimum steam pressure (below the normal pressure). If the pressure falls below its setpoint, the pressure controller's output will decrease. When it is less than the output of the temperature controller the pressure controller will begin to close the valve.

The water temperature will fall below its setpoint, but it is more important that the steam header pressure be maintained.

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