PT Notes
Multiple Failures in PHA
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Hazard scenarios may be initiated by single or multiple failures. Other scenario events, such as the responses of safeguards, are also subject to multiple failure, and enablers may combine with initiating events or other scenario events to produce a type of multiple failure. Some practitioners focus on the identification of scenarios with single failure initiating events and do not address multiple failures. If multiple failures are not addressed in PHA, important scenarios may be missed, and the risks of scenarios may be underestimated.
Sometimes multiple failures are referred to as "double jeopardy", etc. and also as double contingency, etc. Failures that occur some time prior to another failure are usually considered to be latent events and can be treated as enablers. They include equipment that may have been taken out of service or left in a disabled state, e.g. a disabled alarm.
An example of an initiating event multiple failure is the level controller on one fractionation column failing at the same time as the level controller on another column causing a higher-than-expected load of liquids in the overhead system that may not be designed to handle both concurrent failures. An example of a safeguard multiple failure is the failure of dual relief valves on a vessel at the same time resulting in over-pressurization failure of the vessel. Of course, at issue is the credibility of such multiple failures, that is, their likelihood of occurrence.
It can be argued that actions taken to protect against single failures will also protect against multiple failures because they help to protect against the individual contributors to the multiple failures. It can then be argued that it is sufficient to address single failures and that multiple failures need not be addressed. Certainly, actions taken to prevent single failures that contribute to multiple failures will help to prevent the multiple failures. However, that is not the whole story.
Multiple failures may occur as a result of dependency between failures. Usually, the causes of such dependencies will not be eliminated by implementing measures to reduce the likelihood of the individual failures. For example, if the cause of the dependent failure of both column level controllers is a set point error by the control systems engineer, improving their reliability will do nothing to address this dependent failure. The same is true for the dual relief valves if the cause of their dependent failure is plugging of their inlets.
Furthermore, multiple failure scenarios may have more severe consequences than scenarios involving any one of their contributors. They may merit additional safeguards beyond those taken to protect against the single failures. Also, protective actions against single failure events may not have been taken as they may have been deemed unnecessary for the lesser consequences involved. For example, the scenario with the initiating event of the failure of both column level controllers is more serious than a scenario with the failure of either one alone. Similarly, the scenario with the failure of both relief valves is more serious than a scenario with the failure of either one alone.
Thus, PHA studies should consider credible multiple failures. Both independent multiple failures and dependent failures, including common cause failures should be addressed. Generally, independent multiple failures are less likely than most single failures but they may still be credible. Dependent multiple failures can be as likely as single failures since they reduce to a single failure. It is essential that PHA teams understand the meaning and importance of multiple failures.
For more information, you can contact Primatech by clicking here or consult the article:
Treatment of Multiple Failures in Process Hazard Analysis, by Paul Baybutt, Process Safety Progress, Vol. 32, Issue 4, pages 361–364, December 2013.
The article is available at: http://onlinelibrary.wiley.com/doi/10.1002/prs.11599/abstract
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