Almost every major accident to occur share a common finding, there were clear warning signs that were not acted upon.

Key Performance Indicators (KPIs) are used to measure performance. Deciding upon the correct KPIs is a difficult exercise but bowtie diagrams can be used in the determination of leading and lagging KPI’s.

The figure below presents the four Tiers of the process safety indicator pyramid developed by API 754. The Tiers of the pyramid represent a continuum of leading and lagging process safety indicators. Tier 1 is the most lagging and Tier 4 is the most leading indicator.

According to the API 754, Tier 1 events are defined as loss of primary containment (LOPC) with the greatest consequence. A Tier 1 event is an unplanned or uncontrolled release of any material, including non-toxic and non-flammable materials, from a process that results in one or more of the consequences listed in API 754. These Tier 1 events can be directly linked to the consequence element of a bowtie. With Tier 2 events representing a LOPC event with lesser consequence.

The consequence element from a bow tie diagram can be classed as Tier 1 events (i.e. fires and explosion) and Tier 2 (i.e. gas relief without ignition) events which are lagging indicators.

The performance of barriers and degradation factors could be classed as Tier 3 and Tier 4 events (i.e. performance of barriers before the consequence to influence the likelihood and the mitigation of the consequence) these events are leading indicators.

By measuring, recording and – of great importance – sharing the results of these, the correct action can be taken to help prevent a major incident from occurring.

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