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
Examples of Override Control
1. Protection of a boiler
system
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.
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| Override Control for Boiler Protection |
2. Protection of a heat
exchanger
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| 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.
1 Fouling is the
deposition of unwanted material on the heat transfer surfaces.
Also read other control schemes
Also read other control schemes
Well explained article on override control. This is very helpful to instrumentation professionals.
ReplyDeleteThank you. Keep following.
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