What Makes an Element Safety Critical

A safety critical element is any part of a facility — equipment, structure, system or software — whose failure could cause or contribute substantially to a major accident, or whose purpose is to prevent or limit the effect of one.

Two families

• Those that could cause — primary containment, structures, lifting equipment over live plant, marine systems.
• Those that protect — detection, shutdown, isolation, relief, blowdown, fire protection, escape and evacuation systems.

Typical SCEs on a process facility

• Pressure containment: vessels, pipework, flanges, valves.
• Relief and blowdown systems, flare.
• Emergency shutdown system and emergency shutdown valves.
• Fire and gas detection; fire water systems, deluge and passive fire protection.
• Structural integrity, including supports and blast walls.
• Hazardous area electrical equipment and ignition control.
• Emergency power, emergency lighting, alarm and communication systems.
• Escape routes, muster areas, temporary refuge, evacuation systems.
• Ventilation of enclosed spaces, and HVAC isolation on gas detection.

Deriving the SCE list from the major accident scenarios keeps it honest. A list assembled by asking departments what they consider important becomes everything, and therefore nothing.