The CCPS books, Layer of Protection Analysis – Simplified Process Risk Assessment (2001), and Guidelines for Safe and Reliable Instrumented Protective Systems Center for Chemical Process Safety (2007) identify required attributes of independent protective layers. In more than 15 years experience working with LOPA, we have frequently observed confusion about these attributes among LOPA analysts, protection layer implementers, and operations personnel.
Therefore, this paper discusses the characteristics of independent protection layers, gives examples to avoid confusion and misapplication of the characteristics, gives an easily remembered phrase to screen candidate IPLs, and provides sample tools to confirm IPL compliance with the characteristics. The paper discusses the importance of a Safety Requirements Specification for SIFs (Safety Instrumented Functions) and proposes analogous documentation for other IPLs. The paper gives guidance to determine when protection layers or scenarios require more detailed analysis such as event tree analysis or fault tree analysis.
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