What is the difference between an SIS and a DCS?
A distributed control system (DCS) controls processes by facilitating the monitoring of process values such as temperature, pressure, and flow and the manipulation of final elements such as valves and actuators. When a process value exceeds a preset acceptable level, the DCS alerts operators by issuing an alarm.
However, if the operator's action is not succeeded to handle the process under control, an SIS automatically moves a process to safe state for prevention a chance of accident.
A fire & gas (F&G) system is another kind of SIS that mitigates the consequences of an accident.