Exterior fire is a potential cause of overpressure for any equipment item that has a liquid inventory and that may be exposed to an external fire. When heated by the flames, the liquid boils, generating vapor that must be relieved. In equipment without liquid, expansion of the contained gas or vapor due to fire-heating does generate a small relief requirement. However, API 520 notes that a pressure relief device may not significantly reduce the potential for rupture of a vapor-filled vessel exposed to fire. In the absence of evaporative cooling afforded by a liquid inventory, the vessel wall temperature quickly reaches failure levels. Therefore, relief devices are not usually installed on vapor-filled equipment solely to address the Exterior Fire contingency. In some instances, however, depressuring systems may be installed to vent vapor filled vessels, minimizing the stress of internal pressure on a fire-heated liquid-less vessel.
In general, the relief flow rate required in a Fire contingency is equal to the vapor generation rate. This can be calculated from a heat flux, an area through which the heat flows, and the liquid’s enthalpy of vaporization at the relieving conditions. The particulars are a function of the type of equipment involved and of the particular standard to which adherence is adopted.
All of these methods include the effects of insulation, drainage, and firefighting equipment in the calculation of the heat flux. Thermal insulation limits the heat absorption from fire exposure as long as it remains intact. It is important to provide effective weather protection for the insulation so that it will not be removed by high-velocity fire hose streams or adjust the calculation to consider the quality of the installation. The differences among the methods involve precisely how these effects are included, and how the heat transfer area is calculated.