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).