Explosions and Explosive Combustion
Deflagration versus detonation
Explosions are rapid energy releases that produce a pressure wave. A deflagration propagates through the unreacted medium below the speed of sound, with flame speeds on the order of meters per second. A detonation propagates supersonically, kilometers per second, with a shock wave. High explosives detonate; most flammable gas and dust explosions deflagrate. Understand the BLEVE (boiling liquid expanding vapor explosion): a pressurized vessel is heated until its shell fails, and if the contents are flammable the released vapor can ignite. A BLEVE is a mechanical vessel failure, not a chemical detonation.
Damage indicators
Study overpressure effects: structural deformation, directionality of blast damage, window breakage, fragmentation, and thermal patterns on surfaces. In a gas explosion the seat is usually the most damaged area, often with a crater or a V-shaped pattern; also locate the ignition source and the leak path. Dust explosions require combustible dust, dispersion, confinement, an oxidizer, and an ignition source. Open-air vapor cloud explosions need congestion or confinement to produce damaging overpressure.
TRAP: Equating a BLEVE with a "boiling liquid expanding vapor cloud," or calling any loud deflagration a detonation. A flash fire without a pressure wave is not an explosion.
RULE OF THUMB: Low-order damage (heaving, pushing, dishing) suggests deflagration or a low explosive; shattering, brisance, and shrapnel-like fragmentation suggest detonation or a high explosive.