Hazard Knowledge

Accidental releases of flammable liquids or gases often result in the formation of a cloud of vapour that is denser than the ambient conditions. As the cloud disperses from the source it will dilute, mixing with air. In areas, potentially some distance from the source, the cloud will be at concentrations within its flammable range. If it then encounters an ignition source a vapour cloud fire (VCF) may result; in this context, VCF is taken to mean either a flash fire or a fireball. VCFs are important because:

• They pose an intrinsic hazard, in the form of thermal radiation, so that overpressures are not important (assuming no or limited confinement/congestion)
• There is a possibility of escalation; it is highly likely that secondary fires may be started because of the flash fire/fireball and there is a high probability that following a VCF there will be a steady fire, typically either a pool fire or jet fire (or a combination of the two).

Deflagration to detonation transition – Flash fires will travel through a vapour cloud at subsonic speeds. However, where obstacles produce confinement or partial confinement and increase the speed sufficiently, an area of the vapour cloud can reach supersonic speeds, producing significant overpressure. This pressure can squash the vapour in front and cause its heat to rise. If that heat exceeds the auto-ignition temperature of vapour, the flame front can transition to detonation and explosion.

This phenomenon has been observed in a number of industrial accidents such as those at Flixborough and Buncefield.