The override control concept is a technique by which process variables are kept with in certain limits, usually for protective purposes. Override control maintains the process in operation but within and under safer conditions.
To illustrate override control, consider the simple process shown in Figure
A hot, saturated liquid enters a process surge tank, and then it is then pumped into the process. Normally, the tank operates at the level shown, but if the level gets too low the liquid will not have enough net positive suction head (NPSH), and the pump will start to cavitate. Override control can provide protection; a scheme for this is shown in Figure below.
The tank level is now controlled. The variable-speed pump will, of course, pump more liquid as the energy input to it increases. It follows that the flow controller must, therefore, be a reverse-acting controller (output increases as input decreases), and the level controller must be a direct-acting (output increases as input increases) controller. The output of each controller is connected to a low-level selector relay, and its output goes to the pump.
Under normal operating conditions, the actual level is above the set point of the level controller, and the level controller will attempt to speed up the pump. Normally, the output of the flow controller will be less, and the low-level selector relay will select the flow controller output to manipulate pump speed. If the flow of hot, saturated liquid slows down and the level drops, the level controller will try to slow down the pump by reducing its output. When the output of the level controller drops below the output of the flow controller, the low-level selector relay will select the output of the level controller to control the pump.
That is how the level controller “overrides” the flow controller.