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Physical explosions

Shock wave blasts can also be produced from physical explosions, i.e., the sudden violent expansion of a fluid not connected with a chemical reaction. The strongest man-made physical explosion is surely the nuclear explosion of an atomic bomb. The resulting blast wave reaches overpressures of 170 kPa in the central zone (14 km radius for the example of a 20 Mt explosion in 5.5 km height) and gradually decaying to the outside with still 20 kPa at 60 km distance).

The most common physical explosion is a bursting or rocketing pressure vessel which may result from a fire-induced BLEVE (Boiling Liquid Expanding Vapor Cloud Explosion). The higher the liquid density, the more destructive is the BLEVE. If the liquid is flammable, a fireball will follow. Several pressure spikes are being created upon a BLEVE: from the flashing liquid, from the expanding vapor phase, and, if applicable, from the combustion. Missiles and projectiles may be emitted also from a physical explosion and may cause injuries, fatalities, or damage at considerable distances depending on the explosion energy liberated. Projectile hazard increases with average liquid temperature.

Another example of a physical explosion is the so-called rapid phase transition (RPT), a thermal vapor explosion resulting from the spontaneous phase change of a fluid getting in contact with a much hotter or colder liquid, e.g., a cryogen spilled onto water. Prerequisites of such an explosive boiling is a temperature of the “hot” fluid above the boiling point of the “cold” fluid and a certain mixing of both fluids allowing a close and sufficiently long contact. Although the energy release is small compared with a chemical explosion, fragmentation and phase change of the “cold” fluid (vapor evolution) can occur at such a high rate that shock waves may be formed. For LNG onto a water surface, overpressures with damaging potential of up to 5 kPa were observed. RPT explosions with different materials (molten metal plus water) in the metal and chemical industries were even the cause for people killed by flying melt or the blast wave. Natural examples of RPT were the catastrophic explosions of the island volcanoes Krakatoa, Indonesia, in 1883 and Surtsey, Iceland, in 1963.

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