Safety Instrumented System (SIS) is a system composed of sensors, logic solver and final control elements designed and installed to protect personnel, equipment and environment by taking the process to a safe state. Safety Instrument System shall perform any action to prevent or reduce the risk of abnormal process condition which may be hazardous.
Safety Instrumented System (SIS) is the next layer of protection following plant design, Basic Process Control System (BPCS) and alarm/operator intervention.
As mentioned earlier, SIS consists of the following devices:
Sensor could be of switch or transmitter type. Transmitter measures process variable while switch will actuate if process variable reaches preset value. Both provide information to logic solver hence it can be detected whether process is in normal or abnormal condition.
Logic solver is typically a controller. It receives signal from sensor as an input, performs pre-implemented logic and program, then provides output to final control element. Logic solver consists of but not limited to input module, output module, processor, power supply, communication module.
Final control element is part or device which performs the action as required by logic solver. It can be form of actuated valve or output contact.
SIS may include electrical, pneumatic, and hydraulic systems.
Term Emergency Shutdown System (ESD) has the same meaning and has recently used. However abbreviation of ESD has other means of Electro-Static Discharge in electrical engineering subject. Therefore term SIS is currently applied in instrumentation subject.