Chapter 5 - Specialized Reviews—Bow-Tie Analysis, Layers of Protection Analysis, and Safety Integrity Levels

Abstract

Other than Process Hazard Analysis (PHA) and Security Vulnerability Analysis (SVA) there are three other safety reviews that are gaining use within the process industries, including Bow-Tie Analysis (BTA), Layers of Protection Analysis (LOPA), and Safety Integrity Level (SIL). This chapter describes BTA, LOPA, and SIL. BTA is a type of PHA, and the bow-tie PHA methodology is an adaptation of three conventional system safety techniques: Fault Tree Analysis (FTA), causal factors charting, and Event Tree Analysis (ETA). The bow tie has become popular as a structured method to assess risk where a qualitative approach may not be possible or desirable. Bow-tie reviews are most commonly used where there is a requirement to demonstrate that hazards are being controlled, and particularly where there is a need to illustrate the direct link between the controls and elements of the management system. LOPA is a method of analyzing the likelihood (frequency) of a harmful outcome event based on an initiating event frequency and on the probability of failure of a series of independent layers of protection capable of preventing the harmful outcome. SIL is a quantitative target for measuring the level of performance needed for safety function to achieve a tolerable risk for a process hazard. It is a measure of safety system performance, in terms of the probability of failure on demand (PFD).