Vapor Cloud Explosions (VCE)
The most dangerous and destructive explosions in the chemical process industries are vapor cloud explosions (VCE).
The Flixborough, England, disaster in 1974 demonstrated the potential hazards of flammable vapor releases. In this incident, tons of liquid cyclohexane heated above its atmospheric-pressure boiling point escaped through a damaged expansion bellows. Delayed ignition at a furnace caused a violent explosion that demolished the existing plant and damaged many of the surrounding buildings and residences; 28 fatalities resulted, including 26 in the nearby control building.
These explosions occur by a sequence of steps:
(1) Sudden release of a large quantity
of flammable vapor. Typically this occurs when a vessel, containing a superheated
and pressurized liquid, ruptures.
(2) Dispersion of the vapor throughout the plant site while mixing with air.
(3) Ignition of the resulting vapor cloud.
Any process containing quantities of liquefied gases, volatile superheated liquid, or high pressure gases is a good candidate for a VCE. From a safety standpoint, the best approach is to prevent the release of material. A large cloud of combustible material is very dangerous and almost impossible to control, despite any safety systems installed to prevent ignition.
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