Typical Instrumentation and Control Systems


#1

When revising the instrumentation and control scheme, it is necessary to look at the guide rules used
in the design, developed from the process flow sheet and to:

  • Identify loops needed for steady plant operation;
  • Identify the key process variables that need to be controlled to achieve the specified process
    efficiency;
  • Identify and include those additional control loops required for safe operation;
  • Decide upon and show those ancillary instruments needed by operators for monitoring plant
    operations, for troubleshooting and plant development;
  • Decide on the location of sample points;
  • Decide on the alarms and interlocks needed.

Other criteria to utilize are the basic rules of process control:

  • There can be only a single control valve on any given stream between unit operations;
  • A level controller is needed wherever a vapour-liquid or liquid-liquid interface is maintained;
  • Pressure control is more responsive when the pressure controller actuates a control valve on a
    vapour stream;
  • Two operations cannot be controlled at different pressures unless there is a valve or other
    restriction (a compressor or a pump) between them;
  • Temperature control is usually achieved by controlling the flow of a utility stream (such as
    steam or cooling water).