Explain the concept of layers of protection and provide two examples of independent protection layers (IPLs).

• A. IPLs are barriers that operate independently to prevent or mitigate events; examples include a Safety Instrumented System (SIS) and a pressure relief device (PRD).
• B. IPLs are only physical barriers like fences and walls.
• C. IPLs refer to regulatory audits performed by authorities.
• D. SIS and PRD are not considered IPLs.

Answer: (A)

Layers of protection are multiple, independent barriers that prevent or greatly reduce the consequences of a process upset. Each layer should operate independently from the others so that a single failure can’t disable several safeguards at once. An example of an independent protection layer is a Safety Instrumented System, which monitors critical process variables and can automatically take action—such as shutting down a process or altering a valve position—to bring the process to a safe state when dangerous conditions are detected, all while being distinct from the normal control system. Another example is a pressure relief device, like a relief valve or rupture disk, that activates to limit overpressure and vent or release fluid, functioning independently of the primary control system. This independence is what makes them IPLs—they have separate sensing, actuation, and often power sources, reducing the chance that a single failure compromises both the primary controls and the protective layer. IPLs aren’t limited to physical barriers like fences; they include active and passive safety devices. Regulatory audits, on the other hand, are oversight activities, not protective layers themselves.