Equipment criticality assessment. Analytical methods

This article presents the main methods used to assess the criticality of industrial equipment failures, and the tools available to help reduce the consequences of critical equipment failures. In many industrial sectors, the assessment of the criticality of equipment failures is a strategic issue. As the term "criticality" is subject to different definitions and interpretations, it will be considered in this article as a combined measure of the consequences and frequency of occurrence of equipment failures. In particular, its evaluation can be used to determine the impact of malfunctions on the safety of personnel, production stoppages, quality of service, regulatory constraints for classified installations and respect for the environment. Depending on the results obtained, it may be necessary or compulsory to use material or immaterial devices to reduce the criticality of the consequences of malfunctions. Tools have been developed to meet these needs and requirements.

This article presents the main analytical methods for exploiting reliability data extracted from experience feedback on equipment behavior. Otherwise, in the absence of experience feedback, the article [SE 4 004] proposes methods that can be used to assess criticality based on expert judgement.

The first part of the article will be devoted to a typology of methods for assessing and reducing criticality. The second part will be dedicated to brief descriptions of the main analytical tools that can be used to quantify the criticality of equipment failures. For each method, the analysis framework will be: origin, principle, steps. The following will be reviewed: PRA (preliminary risk analysis), FMECA (failure modes, effects and criticality analysis), HAZOP (HAZard and OPerability study) used for industrial risk analysis, What-If (what happens if?), fault trees, reliability diagrams-success trees blocks.

To guide the reader in making the most appropriate choice, a comparative grid of the methods' main attributes is then proposed.

In the event that the criticality of failures proves unacceptable, the third part will summarize the principles of the most commonly used methods for reducing the consequences of critical failures (safety barriers, event trees, bow-tie, MOSAR method, LOPA (Layer Of Protection Analysis) method), along with a comparison of the advantages and disadvantages of these criticality reduction methods.

The conclusion...