Dates: Mar 12, 2026
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Fire exposure scenarios leading to overpressure are particularly challenging, especially where a reclosing pressure relief device provides the only means of protection. In this newsletter, Georges Melhem, Ph.D., FAIChE, shares a dynamic modeling case study of a process vessel exposed to a hydrocarbon fire.
It is widely known that if the fire duration is long enough, the process equipment will ultimately yield or fail at the reseat pressure of the reclosing pressure relief valve (PRV). The maximum allowable fire duration depends on many variables, including but not limited to the type of fire, type of fuel, size and geometry of process equipment, equipment wall thickness, equipment pressure and temperature rating, initial liquid fill level, etc. As a result, the expected time to failure (ettf) or expected time to yield (etty) value can range from a few minutes to a few hours.
Because the hazard of a long enough fire exposure scenario is already recognized, the relief systems design basis documentation should provide calculated best estimates for etty or ettf or both. A hazard is recognized under the OSHA General Duty Clause where (a) the employer has identified it, (b) it is known in the industry, or (c) it is blatantly obvious.
Reasonable estimates of etty or ettf are required for strict OSHA compliance. They are also necessary for effective risk management as well as effective emergency response and fire protection.
This is particularly important for systems that contain reactive chemicals, chemicals with high boiling points, and process equipment that are gas filled or process equipment containing liquids where the vapor space can be engulfed/impinged by fire or exposed to flame thermal radiation.
Bonus! This newsletter also includes access to a 44-minute presentation on PStv® by Georges Melhem, Ph.D., FAIChE, on best practices and compliance requirements for assessing the failure potential of pressure and storage vessels exposed to external fire. Recommended mitigation considerations will also be discussed.
Have a great and safe day.