The explosion that occurs when a pressure vessel with a superheated flammable liquid or a liquefied flammable gas is heated and ruptures. It is the result of the rate of pressure build-up being greater than the rate of pressure relief due to venting. The pressure build-up, together with the reduced structural strength of the vessel caused by the external heating, increases the risk of explosion. At the point of rupture, the pressurized liquid discharges and immediately flashes to a vapour. The rapid rate of vapour expansion causes the explosion in the form of a blast wave. There may also be missile damage. Ignition of the expanding flammable vapour in air can result in a rapid combustion and create a fireball.