Typically, Safety Instrumented Systems consist of three elements: A Sensor, a Logic Solver and a Final Control Element
Sensors:
Field sensors are used to collect information necessary to determine if an emergency situation exists.
The purpose of these sensors is to measure process parameters (e.g. temperature, pressure, flow, etc.) used to determine if the equipment or process is in a safe state. Sensor types range from simple pneumatic or electrical switches to Smart transmitters with on-board diagnostics.
These sensors are dedicated to the Safety Instrumented System SIS.
Logic Solver:
The purpose of this component of Safety Instrumented Systems SIS is to determine what action is to be taken based on the information gathered.
Highly reliable logic solvers are used which provide both fail-safe and fault-tolerant operation. It is typically a controller that reads signals from the sensors and executes pre-programmed actions to prevent a hazard by providing output to final control elements.
Final Control Element:
It implements the action determined by the logic system. This final control element is typically a pneumatically actuated On-Off valve operated by solenoid valves.
It is imperative that all three elements of the SIS system function as designed in order to safely isolate the process plant in the event of an emergency.