Severity

A numerical value that ranks the risk associated with a failure mode by multiplying three independently scored factors: Severity (impact of the failure), Occurrence (likelihood of the failure happening), and Detection (likelihood of detecting the failure before it reaches the end user or causes harm). RPN ranges from 1 to 1,000, where higher values indicate greater risk priority.

A rating of the likelihood that a specific failure mode will occur, scored on a scale of 1 (extremely unlikely) to 10 (almost certain/inevitable). Occurrence is based on historical failure data, engineering judgment, or generic reliability databases. It represents the probability of the failure cause generating the failure mode during the defined assessment period.

A rating of the ability of current controls to detect the failure mode or its cause before the failure reaches the end user or causes harm, scored on a scale of 1 (almost certain detection) to 10 (no detection capability). Detection evaluates the effectiveness of existing inspection, monitoring, and testing methods. Lower detection scores indicate better detection capability.

The consequence of a failure mode on the operation, function, or status of the item and the system. Failure effects describe what happens when the failure mode occurs, traced from the local effect on the component through the system level to the end effect on the plant, safety, and environment. Effects must be described assuming no detection or compensating provisions are in place.

A measure of the impact resulting from an equipment failure, encompassing safety (personnel injury/fatality), environmental (release to atmosphere/ground/water), production (lost throughput and business interruption), and equipment (repair/replacement cost). CoF is evaluated based on the worst credible release scenario, considering fluid properties, operating conditions, detection and isolation systems, and the surrounding environment.