BLEVE is an acronym for boiling liquid expanding vapor explosion, which is a type of explosion that occurs when the internal pressure of a tank exceeds what its relief valve can safely vent into the atmosphere and a rupture forms. Extreme heat over time is generally the cause of a BLEVE.
If the tank is ruptured, a source of ignition must be present to cause an explosion. BLEVE tank explosions are rare and remote events, since tanks are designed with multiple safety and structural mechanisms to prevent such occurrences. The relief valve is designed to vent the tank when its internal pressure reaches a certain limit and close when the pressure drops again. This occurs at a flow rate that is appropriate for normal circumstances.