Process Controls
PRINCIPLES OF CONTROL SYSTEMS
PRINCIPLES OF CONTROL SYSTEMS
Control systems integrate elements whose function is to maintain a
process variable at a desired value or within a desired range of
values.
EO 1.1
DEFINE the following process control terms:
a.
Control system
b.
Control system input
c.
Control system output
d.
Open-loop system
e.
Closed-loop system
f.
Feedback
g.
Controlled variable
h.
Manipulated variable
Introduction
Instrumentation provides the various indications used to operate a nuclear facility. In some cases,
operators record these indications for use in day-to-day operation of the facility. The information
recorded helps the operator evaluate the current condition of the system and take actions if the
conditions are not as expected.
Requiring the operator to take all of the required corrective actions is impractical, or sometimes
impossible, especially if a large number of indications must be monitored. For this reason, most
systems are controlled automatically once they are operating under normal conditions. Automatic
controls greatly reduce the burden on the operator and make his or her job manageable.
Process variables requiring control in a system include, but are not limited to, flow, level,
temperature, and pressure. Some systems do not require all of their process variables to be
controlled. Think of a central heating system. A basic heating system operates on temperature
and disregards the other atmospheric parameters of the house. The thermostat monitors the
temperature of the house. When the temperature drops to the value selected by the occupants
of the house, the system activates to raise the temperature of the house. When the temperature
reaches the desired value, the system turns off.
Automatic control systems neither replace nor relieve the operator of the responsibility for
maintaining the facility. The operation of the control systems is periodically checked to verify
proper operation. If a control system fails, the operator must be able to take over and control
the process manually. In most cases, understanding how the control system works aids the
operator in determining if the system is operating properly and which actions are required to
maintain the system in a safe condition.
Rev. 0
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IC-07
