Override control 

In a Override control process, a selector is used. The two basic types of these override controllers(Selectors) are high selectors and low selectors. They are generally available as both electronic and pneumatic selectors. Selectors are also available in a number of different versions that will accommodate for varying amounts of input signals. Two or more inputs are placed into the selector and one output comes out depending on the selector.

High Selectors

High selectors are designed so that they filter out all but the highest value from a multiple input feed stream. The selector then sends this single highest value through to the output signal.

Low Selectors

Low selectors are designed so that they filter out all but the lowest value from a multiple input feed stream. The selector then sends this single lowest value through to the output signal.

Override control 

Override Control, also called as selective control, is a form of multivariable control in which more than one controller can manipulate the same process and the selection of controller is determined by a constraint.

In the override control configuration, there would be at least two controllers, normal and override controller. 

At normal operating conditions, the normal controller controls the process and the override controller remains inactive.

Once the predefined constraint is reached or exceeded, the override controller takes away the control of the manipulated variable (known as override variable) from normal controller through a selector. 

This override action prevents the override variable from exceeding that particular constraint.   

Specialized selector switches enable the switching of controller outputs smoothly without affecting the process. 

The High Selector Switch (HSS) is used to prevent a variable exceeding an upper limit and the Low Selector Switch (LSS) is used to stop a variable from exceeding its lower limit.

The common applications of override control are

• Protection of process equipment by limiting the range of process variables
• Protection against instrument failure
• Automatic start up and shut down

Override Control in Heat Exchanger.

Consider a heat exchanger system where the hot vapors are cooled by river water. 

The temperature of the condensate is controlled by manipulating the flow rate of outlet river water. 

In this application, the concern is about the fouling of heat exchanger. 

The fouling increases rapidly if the river water temperature rises above 50˚C. If the river water temperature reaches or exceeds above 50˚C, the override temperature controller takes over the control of river water outflow and opens the control valve. 

This limits the temperature of river water and prevents heat exchanger fouling. 

The switching of control action is done by high switch selector.