every large above ground flammable liquid storage tank should be provided with some means of relieving excessive internal pressure caused by exposure of the tank to a fire. Depending upon the strength of the tank structure, a vent sized to handle the normal venting requirements (for day-to-day operations) may be inadequate to handle the abnormal vapor efflux generated by heat from an exposure fire.
When a fire occurs near a flammable liquid storage tank, heat transmitted through the tank wall to the contents will cause the liquid to vaporize readily. In the case of petroleum based products, this vaporization may amount to roughly 30 or more cubic feet of vapor for every gallon of solvent vaporized. This generates a relatively high pressure within the confined area of the tank's vapor space. In the absence of sufficient emergency relief a tank rupture could occur, with disastrous effect on life and property and the environment.