Process Controls
PROPORTIONAL CONTROL SYSTEMS
that the actual flow rates cannot be predicted. Therefore, the system is designed to control tank
level within a narrow band in order to minimize the chance of a large demand disturbance
causing overflow or runout. A fulcrum and lever assembly is used as the proportional controller.
A float chamber is the level measuring element, and a 4-in stroke valve is the final control
element. The fulcrum point is set such that a level change of 4-in causes a full 4-in stroke of
the valve. Therefore, a 100% change in the controller output equals 4-in.
The proportional band is the input band over which the controller provides a proportional output
and is defined as follows:
Proportional band
% change in input
% change in output
x 100%
For this example, the fulcrum point is such that a full 4-in change in float height causes a full
4-in stroke of the valve.
P.B.
100% change in input
100% change in output
x 100%
Therefore:
P.B. = 100%
The controller has a proportional band of 100%, which means the input must change 100% to
cause a 100% change in the output of the controller.
If the fulcrum setting was changed so that a level change of 2 in, or 50% of the input, causes the
full 3-in stroke, or 100% of the output, the proportional band would become 50%. The
proportional band of a proportional controller is important because it determines the range of
outputs for given inputs.
Example of a Proportional Process Control System
Figure 16 illustrates a process system using a proportional temperature controller for providing
hot water.
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