Whenever multiple relief devices protecting multiple equipment items discharge into a common collection system, the impact of the collection system on the generation of built-up back pressure at the individual relief devices must be analyzed. Note that the back pressure evaluation described in “Outlet Piping” on page 1200-47 involves only the flow from one device, and only from the device outlet to the discharge line’s entrance into a large common header. The additional sources of flow introduced by the common collection system create the possibility of pressures higher than atmospheric where an individual discharge line empties into a collection header. Therefore, the analysis of the impact of a relief discharge collection system on the proper performance of the various relief devices connected to it consists of a detailed evaluation of the cause of overpressure. A “common contingency” or “simultaneous release” is one in which a single event can result in discharge to the collection system from multiple sources. This analysis should be performed whether the ultimate discharge location is the process, the atmosphere, or a flare.
In addition, common contingencies that result in large flows from single devices may require consideration on a case-by-case basis.