shutdown sequence

A process plant shutdown system is a functional safety countermeasure crucial in any hazardous process plant such as oil and gas production plants and oil refineries. The concept also applies to non-process facilities such as nuclear plants. These systems are used to protect people, assets, and the environment when process conditions get out of the safe design envelope the equipment was designed for.
As the name suggests, these systems are not intended for controlling the process itself but rather for protection. Process control is performed by means of an independent process control systems (PCS) and should not be relied upon to execute critical safety actions.
Although functionally separate, process control and shutdown systems are usually interfaced under one system, called an integrated control and safety system (ICSS). Shutdown systems typically use equipment that is SIL 2 certified as a minimum, whereas control systems can start with SIL 1. SIL applies to both hardware and software requirements such as cards, processors redundancy and voting functions.

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