What are the layers of protection in process safety?

Layers of protection (LOPs) are independent safeguards that prevent or mitigate incidents. From innermost: process design (inherently safer design), BPCS control (first automated response), alarms and operator response, safety instrumented systems (SIS/ESD), physical protection (relief valves, rupture disks), containment (dikes, blast walls), and emergency response (fire brigade, evacuation). Each layer provides independent risk reduction. SIL determination considers existing layers - SIS provides additional risk reduction when other layers are insufficient. LOPA (Layer of Protection Analysis) quantifies risk reduction from each layer.