Pressure-relieving devices External fires

Figure 7-7A. Pressure level relationship conditions for pressure relief valve installed on a pressure vessel (vapor phase). Single valves (or more) used for process or supplemental valves for external fire (see labeling on chart). Reprinted by permission, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1 Sizing and Selection, API RP-520, 5th Ed., July 1990, American Petroleum Institute.

When a pressure vessel is exposed to external heat or fire, supplemental pressure relieving devices are required for this excessive pressure. These devices must have capacity to limit the overpressure to not more than 21% above the maximum allowable working pressure of the vessel. (See Figures 7-7A and 7-7B.) A single relieving device may be used to handle the capacities of paragraph UG-125 of the code, provided it meets the requirements of both conditions described. 

For this condition, the API-RP-521 Code [13] (Figure 7-7A) shows an allowable 16% maximum accumulation relieving pressure above the set pressure. For external fire conditions on a vessel, the maximum allowable accumulation pressure is 21% above the set pressure [13] for both single or multiple relieving devices (Figure 7-7A). 

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