What is the primary function of a Safety Instrumented System (SIS)?

The primary function of a Safety Instrumented System (SIS) is to automatically take a process to a safe state when predetermined hazardous conditions are detected, thereby preventing or mitigating the consequences of a hazardous event. An SIS is a dedicated safety system that is independent of the basic process control system (BPCS). It consists of sensors, logic solvers, and final elements that are designed and implemented to achieve a specific safety function. The SIS is activated when the process deviates from its normal operating parameters and enters a hazardous state. For example, if a reactor temperature exceeds a safe limit, the SIS will automatically shut down the reactor or take other actions to prevent an explosion or other catastrophic event. The SIS is designed to be highly reliable and to function independently of human intervention. The design and implementation of an SIS must meet stringent safety standards, such as IEC 61508 and IEC 61511, to ensure that it will perform its intended safety function with a high degree of certainty. The safety integrity level (SIL) of the SIS reflects its level of reliability and risk reduction capability. SIS are used to prevent catastrophic events and protect people, the environment, and assets.