These vessels are often degassing drums, maintenance knock out drums, or other similar equipment. While there are practically never any relief loads from these equipment contributing to the global scenarios we typically evaluate, it is still important to ensure that the pressure in the drum does not exceed the code allowable accumulation.
The flare header can be a pressure source for these vessels. What this means is that the pressure in the flare header at the point where the vessel ties into the header needs to be documented and compared to the vessel MAWP. An easy way this can be done is by creating a pseudo-source in the network representing the vessel in order to capture the pressure at the point at which the vessel ties in. This value, along with the vessel MAWP, can then be listed in the flare report to document whether a pressure in excess of the allowable accumulation exists.