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PT Notes

Use of Risk Matrices in Process Safety

PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

Risk matrices are used in process safety to rate and rank risks of hazardous events to help with decision making on risk reduction for processes. In particular, they have become a key aspect of performing process hazard analysis (PHA). Risk matrices appear to be simple and useful tools for risk management. However, pitfalls exist for unwary users. There are mathematical inconsistencies and flaws in their theoretical framework which can produce illogical decision requirements that are not recognized as such by practitioners. Considerable care is required in designing and using risk matrices to avoid generating invalid risk rankings, even by experienced users.

One of the most significant flaws is the potential for risk ranking reversals wherein an incorrect assignment of a higher risk event to a lower risk level and a lower risk event to a higher risk level may occur. In order to avoid risk ranking reversals, practitioners must be aware of the risk model that underlies the matrix and carefully map severity and likelihood levels into risk rankings.

Guidance for avoiding risk ranking reversals is provided in the article:

Designing Risk Matrices to Avoid Risk Ranking Reversal Errors, Process Safety Progress, Volume 35, Issue 1, pages 41-46, March 2016.

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