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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 Page 1 IC-07


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