A new look at safety metrics in a world of evolving operational practices

On‐going changes to chemical industries' operating practices (including testing/maintenance), which aim to proof test safety instrumented function (SIF) components on various schedules rather than test the entire SIF loop at once and to increase proof test intervals for some SIF components to as much as 10 years, will continue to affect achievable safety. Safety metrics currently used may no longer provide all the information needed to ensure the preservation of the current levels of safety and aid in further optimizing safety. This paper considers the limitations of the existing safety metrics PFDavg and PFH, presents another metric to provide additional safety information, analyzes more comprehensive models of SIF safety that account for many parameters relevant to changing industrial practices including δ (rate of SIF demand), and demonstrates the importance to safety calculations of correctly modeling repair strategies when an SIF fails. Significantly, it identifies the ratio of “rate of SIF restoration to OK” to “rate of SIF demand” as an important additional variable impacting PFDavg. This ratio also defines a much wider range of applicability for the PFDavg metric than currently recognized in safety standards. This means that realistic evaluation of SIF safety requires that δ is known at a facility, not merely assumed.