Process Controls PRINCIPLES OF CONTROL SYSTEMSPRINCIPLES OF CONTROL SYSTEMSControl systems integrate elements whose function is to maintain aprocess variable at a desired value or within a desired range ofvalues.EO 1.1 DEFINE the following process control terms:a. Control systemb. Control system inputc. Control system outputd. Open-loop systeme. Closed-loop systemf. Feedbackg. Controlled variableh. Manipulated variableIntroductionInstrumentation 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 informationrecorded helps the operator evaluate the current condition of the system and take actions if theconditions are not as expected.Requiring the operator to take all of the required corrective actions is impractical, or sometimesimpossible, especially if a large number of indications must be monitored. For this reason, mostsystems are controlled automatically once they are operating under normal conditions. Automaticcontrols 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 becontrolled. Think of a central heating system. A basic heating system operates on temperatureand disregards the other atmospheric parameters of the house. The thermostat monitors thetemperature of the house. When the temperature drops to the value selected by the occupantsof the house, the system activates to raise the temperature of the house. When the temperaturereaches the desired value, the system turns off.Automatic control systems neither replace nor relieve the operator of the responsibility formaintaining the facility. The operation of the control systems is periodically checked to verifyproper operation. If a control system fails, the operator must be able to take over and controlthe process manually. In most cases, understanding how the control system works aids theoperator in determining if the system is operating properly and which actions are required tomaintain the system in a safe condition.Rev. 0 Page 1 IC-07
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