The basic process control system (PCS) comprises “the control equipment installed to perform the normal regulatory functions for the process, e.g. PID control loops”. The purpose of a process control system is, of course, to control the process, primarily for economic reasons. The PCS is often used together with other plant safety systems.
The control system may provide steady state or change of state (start-up, shutdown, batch) control functions. The latter may be implemented by automatic sequences or procedurally under manual control. Control systems should be implemented to provide stable controlof the process under all expected normal and upset circumstances, including start-up and shutdown.
In a typical control operation, the PCS receives process variable measurements from the process and for each of the controlled variables continually manipulates an output to a final control element (typically a valve).
In some cases the PCS controls variables that are related to safety (or a definition of “safety critical” system,. For example, it may control a level that, if not controlled, could overflow a vessel causing a release of hazardous material.
However, in these cases the safety and interlock system should activate to prevent the level from overflowing. When safety interlock systems are used, the value of the PCS is to prevent a process shutdown or interruption by preventing the level from becoming so high as to cause a process shutdown.
In addition to control function, an important function of the PCS is to provide information to the process operator and to others. Much of this information is for long term record keeping and process analysis purposes, however, some of the information is needed by the operator to make decision about the operation of the process. Often this information can be safety related. Information such as quantities of materials in storage, temperatures, pressures, etc can be critical to safe operation.
The criticality of the information can continue even after the operation of the shutdown system. For example, after a fuel trip to a furnace the temperatures in the furnace must be monitored to ensure that there is no fire in the furnace.
Part of the information capability of the PCS is implemented in the form of alarms. The majority of alarms implemented on a typical PCS are related to the operation of the process. These alarms prevent economic loss, e.g. they can alert the operator to situations that, if not corrected, can be expected to affect product quality or production rates, or that may cause the activation of the emergency shutdown system.
Other, more critical, alarms are sometimes implemented in the PCS which alert the operator to conditions that, if not corrected, could cause personnel or property danger.