Safety Instrumented System (SIS)

A safety instrumented system consists of sensors, logic solvers, and final control elements that are all separate from the basic process control system elements, and the logic solver directs the final control elements to the state required to provide a safe state if the input indicates an abnormal condition.

Safety instrumented systems are used to monitor the status of plant values and parameters within operational limits and when a risk situation occurs, they should trigger an alarm and also put the plant in a safe state or shutdown state.

Safety instrumented systems are control systems that detect inherently dangerous conditions and bring the process to a safe state.

A Safety Instrumented System (SIS) protects the environment from harm caused by specified hazardous conditions. When applied to a specific situation SIS is generally viewed as a separate control system that operates independently of any other controls or individuals.

A safety instrumented system is typically designed in 3 parts that define it.

They always require three parts

Sensor subsystem

To capture online data from the process

Logic Solver Subsystem

Evaluating data and making decisions about when and how to act

Actuator sub-system

Taking necessary action at the plant

Specific examples of safety systems

Emergency shutdown system
Safety shutdown system
Safety interlock system
Fire and gas systems

Alternative names found in service

Trip and Alarm System
Emergency Shutdown System
Safety Shutdown System
Safety Interlock System
Safety Related Control System