Typical Instrumentation and Control Systems

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
  • 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).