Pressure Relief System Design Fire Consideration
The main result of fire exposure is heat input, causing thermal expansion or vaporization or thermally induced decomposition, resulting in pressure rise. An additional result of fire exposure is overheating a vessel wall to high temperature in the vapor space where the wall is not cooled by liquid. In this case, the vessel wall may fail due to high temperature even though the relief devise is operating. Guidelines for calculating heat input are found in API recommended practices, NFPA 30 (for bulk storage tanks), OSHA 1910.106, and corporate engineering standards. In determining heat input from fire exposure, NFPA allows credit for application of water to a tank; however, API does not.
Pressure vessels (including heat exchangers and air coolers) in a plant handling flammable fluids are subject to potential exposure to external fire. A vessel or group of vessels which could be exposed to a pool fire must be protected by a pressure relief device. Additional protection to reduce the device relief load can be provided by insulation, water spray/deluge, or remote-controlled depressuring device (control valve).