A properly sized reclosing pressure relief valve (PRV) can protect process equipment against a variety of overpressure scenarios. Fire exposure scenarios leading to overpressure are particularly challenging, especially where a reclosing pressure relief device provides the only means of pressure relief.
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 PRV. A reclosing PRV can only provide overtemperature protection up to a maximum allowable duration of fire exposure. We define the maximum allowable duration of fire exposure as the expected time to failure (ettf). Another useful maximum allowable duration of fire exposure criteria is based on equipment deformation or yield but not failure or expected time to yield (etty).
The maximum allowable fire duration depends on many variables including but not limited to type of fire (pool fire or flame jet), 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, expected ettf or etty values can range from few minutes to a few hours.
Because the hazard of a long enough fire exposure scenario is already recognized, 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 and fire protection and response. 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.
A reclosing pressure relief device can only be considered adequate for overtemperature protection if the calculated ettf or etty exceeds the estimated fire duration or the estimated failure time for vessel structural supports, whichever is less. Vacuum protection may also be necessary if a reclosing relief device is the only mean of overpressure and overtemperature protection.
Time to failure and consequence of failure
Fire exposure scenarios are probably the most widely used as the dominant pressure relief design basis, especially in the hydrocarbons industries. Standards such as API-521/520[1, 2] are used to determine the relief requirements by first determining the heating rate or heat absorption rate from fire exposure into the liquid vessel contents. Only heating through the wetted surface area is considered. The heating rate is then divided by the latent heat of vaporization of the vessel liquid contents to generate a rate a vapor generation. The volumetric rate of vapor generation determines the relief requirement and relief device sizing assuming all vapor flow.
Modeling system dynamics using our software Process Safety Office® SuperChems™ can be very useful and can provide insight into the proper selection of insulation thickness, actuation time of the depressuring system, water spray density requirements, etc. More importantly, dynamics are very useful to study the sensitivity of the final design to key mitigation parameters or system characteristics.
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