Safety Controls, Alarms, and Interlocks As IPLs

Layers of Protection Analysis (LOPA) involves the analysis of the sequence of events leading to an identified scenario. This analysis yields an understanding of the role that automation plays in detecting and responding to each scenario. Consequently, LOPA facilitates the identification of safety controls, alarms, and interlocks (SCAI), which are process safety safeguards implemented with instrumentation and controls, used to achieve or maintain a safe state for a process, and required to provide risk reduction with respect to a specific hazardous event (ISA 84.91.01-2012).

The CCPS book, Guidelines for Independent Protection Layer and Initiating Events divides SCAI into four types of systems: control, alarms, interlocks, and SIS. This paper discusses the design, operation, maintenance, and testing requirements for SCAI to be considered as independent protection layers (IPL).  It also provides guidance on claiming multiple layers of protection in basic process control and safety instrumented systems.