Unconfined vapour cloud explosions

Explosion (a confined vapour cloud explosion (CVCE) or a boiling liquid expanding vapour explosion (BLEVE)) can result when unvented containers of flammable chemicals burst with explosive violence as a result of the build-up of internal pressure unconfined vapour cloud explosion can result from ignition of a very large vapour or gas/air cloud. 

A Category 3 incident is an emergency of major proportions. It may have escalated from a smaller incident or arise instantaneously, e.g. due to a confined or unconfined vapour cloud explosion or a catastrophic failure of a storage vessel. The response from the emergency services will again need to be predetermined by pre-planning. Extensive evacuation of the general public may be required. 

Unconfined vapour cloud explosion a large flammable gas or vapour-air cloud burns in free space with sufficient rapidity to generate pressure waves, which propagate through the cloud and into the surrounding atmosphere. Such events are extremely rare. 

Gugan, K. (1979) Unconfined Vapour Cloud Explosions (Gulf Publishing). 

These are gas fires accompanied by an explosion, which causes mechanical damage as well as thermal damage. The nature of the explosion is different according to whether it takes place in a confined space, which is the general case of domestic gas explosions in buildings, or in an unconfined environment, when organic vapours spread in the atmosphere (UVCE = Unconfined Vapour Cloud Explosion) an example of this is the FLIXBOROUGH accident which is described below. 

Sometimes erroneously referred to as unconfined vapour cloud explosion ). Explosive oxidation of a vapour cloud in a partly confined space (not in vessels, buildings, etc) — for example, in densely packed plant areas (pipe lanes, units, etc) — which cause accelerations in flame speeds and blast generation. 

Ale, B. J. M., eind F. Bruning. 1980a. Unconfined Vapour Cloud Explosions, Letter, The Chemical Engineer, vol. 352, pp. 47-49. 

Roberts, A. F., and D. K. Pritchard. 1982. Blast Effect from Unconfined Vapour Cloud Explosions, Journal of Occupational Accidents, vol. 3, pp. 231-247. 

Zeeuwen, J.P., Van Wingerden, C.J.M., and Dawe, R.M.,"Experimental Investigation into the Blast Effect Produced by Unconfined Vapour Cloud Explosions," 4th Symposium on Loss Prevention and Safety Promotion in the Process Industries, Harrogate, Sept. 1983. 

Guidance on the hazards of unconfined vapour cloud explosions... 

The consequences of both gas and dust explosions will depend upon the environment in which the mixture (dust or gas cloud) is contained or, in the case of the gas cloud, what it engulfs. A gas explosion will be classified in relation to the environment in which the explosion takes place, for example confined gas explosions which occur within vessels, pipes, tunnels or channels, or unconfined gas explosions commonly referred to as unconfined vapour cloud explosions, which occur in process plants and other unconfined areas. 

At approximately 16 53 hours there was a massive unconfined vapour cloud explosion which caused extensive damage and started numerous fires on the site. 

If the mixture (or a dust cloud) is confined, even if only by surface irregularities or local partial obstructions, significant pressure effects can occur. Fuel-air mixtures near to stoicheiometric composition and closely confined will develop pressures of several bar within milliseconds, and material damage will be severe. Unconfined vapour explosions of large dimensions may involve higher flame velocities and significant pressure effects, as shown in the Flixborough disaster. 

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