Thursday, 18 January 2018

OVERRIDE CONTROL

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,
1. Protection of process equipment by limiting the range of process variables
2. Protection against instrument failure
3. Automatic start up and shut down

Examples of Override Control

1. Protection of a boiler system
Override Control for Boiler Protection
Override Control for Boiler Protection
Consider a boiler system where the outlet steam pressure is to be controlled. It is usually done by controlling the outflow rate of steam through a pressure control loop. Here, the constraint is that the boiler water level should not fall below a lower limit. The lower limit is fixed in order to keep the heating coil immersed in water so that it is not burnt out. The main aim is to control the steam pressure and at the same time the safety of the heating coil of boiler is more important. As long as the heating coil is under the water, the pressure control loop controls the process. Once the water level drops below the specified limit, the level controller overrides the pressure controller and closes the outlet steam control valve. The switching of control action is done by low switch selector.

2. Protection of a heat exchanger      
Override Control for Heat Exchanger Protection
Override Control for Heat Exchanger Protection 
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 fouling1 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.



2 comments:

  1. Well explained article on override control. This is very helpful to instrumentation professionals.

    ReplyDelete

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