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Shutdown systems, also known as safety instrumented systems (SIS) or emergency shutdown (ESD) systems, are independent automated systems designed to bring a process to a safe state when predetermined hazard levels are detected. Their primary function is to prevent or mitigate the consequences of dangerous process upsets. These systems are crucial in industries with inherently hazardous operations, such as oil and gas, petrochemicals, and chemical processing, where equipment failures or process deviations could lead to catastrophic events like explosions, fires, or toxic releases.

The design of a shutdown system is based on risk assessment and the concept of Safety Integrity Levels (SILs). SILs quantify the risk reduction provided by a safety instrumented function, with higher SILs indicating a greater reduction in risk. A typical shutdown system comprises sensors to monitor critical process parameters (e.g., temperature, pressure, flow), a logic solver (often a safety programmable logic controller) that evaluates sensor inputs against predefined safety logic, and final elements (e.g., emergency shutdown valves, circuit breakers) that act to bring the process to a safe state.

Unlike basic process control systems (BPCS) which aim to maintain normal operating conditions, shutdown systems are designed to operate only when necessary, in response to abnormal or hazardous situations. They are intentionally designed to be independent of the BPCS to ensure that a failure in the primary control system does not compromise the safety system's ability to function. Regular testing and maintenance are vital to ensure the reliability and availability of shutdown systems, as their failure to operate when needed can have severe consequences.