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CHEMICAL ENGINEERING
velocity, and the pressure build-up is slow. Whether detonation or deflagration occurs
in a gas-air mixture depends on a number of factors; including the concentration of the
mixture and the source of ignition. Unless confined or ignited by a high-intensity source
(a detonator) most materials will not detonate. However, the pressure wave (blast wave)
caused by a deflagration can still cause considerable damage.
Certain materials, for example, acetylene, can decompose explosively in the absence
of oxygen; such materials are particularly hazardous.
Confined vapour cloud explosion (CVCE)
A relatively small amount of flammable material, a few kilograms, can lead to an explosion
when released into the confined space of a building.

Unconfined vapour cloud explosions (UCVCE)
This type of explosion results from the release of a considerable quantity of flammable
gas, or vapour, into the atmosphere, and its subsequent ignition. Such an explosion can
cause extensive damage, such as occurred at Flixborough, HMSO (1975). Unconfined
vapour explosions are discussed by Munday (1976) and Gugan (1979).

Boiling liquid expanding vapour explosions (BLEVE)
Boiling liquid expanding vapour explosions occur when there is a sudden release of
vapour, containing liquid droplets, due to the failure of a storage vessel exposed to fire. A
serious incident involving the failure of a LPG (Liquified Petroleum Gas) storage sphere
occurred at Feyzin, France, in 1966, when the tank was heated by an external fire fuelled
by a leak from the tank; see Lees (1996) and Marshall (1987).

Dust explosions

Finely divided combustible solids, if intimately mixed with air, can explode. Several
disastrous explosions have occurred in grain silos.
Dust explosions usually occur in two stages: a primary explosion which disturbs
deposited dust; followed by the second, severe, explosion of the dust thrown into
the atmosphere. Any finely divided combustible solid is a potential explosion hazard.
Particular care must be taken in the design of dryers, conveyors, cyclones, and storage
hoppers for polymers and other combustible products or intermediates. The extensive
literature on the hazard and control of dust explosions should be consulted before
designing powder handling systems: Field (1982), Cross and Farrer (1982), Barton (2001),
and Eckhoff (2003).

9.3.4. Sources of ignition

Though precautions are normally taken to eliminate sources of ignition on chemical plants,
it is best to work on the principle that a leak of flammable material will ultimately find
an ignition source.