Safety Instrumented Systems (SIS)
End users (e.g., petrochemical plants) are increasingly using safety instrumented systems (SIS) to complement their process control systems. A SIS is utilized when the risk of an accident needs to be reduced.
SIS is defined by ISA S84.01 and IEC 61508 as: SIS loop: “An SIS is a distinct, reliable system used to safeguard a process to prevent a catastrophic release of toxic, flammable, or explosive chemicals.”
SIS loop scope: “System composed of sensors, logic solvers, and final control elements for the purpose of taking a process to a safe state, when predetermined conditions are violated.”
Every element in the loop is part of the SIS, and needs to be considered when doing an analysis of the SIS. This could include pressure and temperature transmitters, a control system or stand-alone controller, control valves or other final control devices, electrical wiring, process piping, power supplies, software, etc.
The function of the SIS is to monitor the process for potentially dangerous conditions (process demands), and to take action when needed to protect the process.