Confined vapour cloud explosions

Explosion a confined vapour cloud explosion (CVCE) can result from ignition of vapour within a building or equipment 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 unconfmed vapour cloud explosion (UVCE) can result from ignition of a very large vapour or gas/air cloud. 

Confined vapour cloud explosion gas or vapour burns in a confined volume and rapid expansion of the combustion products is restrained until failure of the container or building occurs. 

Confined vapour cloud explosion, 13, 214, 215 Construction materials, 59, 60 Contact dermatitis, See Dermatitis Containment, 135, 165, 220, 221 Control of Asbestos at Work Regulations, 148, 594 Control of Major Accident Hazard Regulations 1999, 5, 424, 594... 

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. 

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. 

In a deflagration the flammable mixture burns relatively slowly. Flame propagation is mainly determined by molecular diffusion and turbulent transport processes. Mixtures of hydrocarbons and air burn in the absence of turbulence, i.e. under laminar or almost laminar conditions, with flame speeds of the order of 5-30 m/s. If there is no confinement this is too slow to produce tangible overpressures and only a flash fire is produced. That is why there is always turbulence involved in a vapour cloud explosion (turbulent flame speeds 100-300 m/s), which increases the rate of combustion and hence the overpressure [12]. 

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. 

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. 

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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