Vapour Cloud Explosion (VCE)
- 29 Aug 2024
- 2 Minutes to read
Vapour Cloud Explosion (VCE)
- Updated on 29 Aug 2024
- 2 Minutes to read
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What is a Vapour Cloud Explosion?
A Vapour Cloud Explosion (VCE) is a hazardous event that occurs when a large quantity of flammable gas, either pressurised liquefied gas or compressed gas, is released into the atmosphere, mixes with air to form a flammable mixture, and subsequently ignites. For an event to be classified as a VCE, several conditions need to be met:
Cloud formation | A vapour cloud must form before ignition. The delay between release and ignition, typically 5-10 minutes, is critical for a VCE to occur. |
Flammable material | The released substance must be flammable and under suitable pressure or temperature conditions, such as liquefied gas under pressure, ordinary flammable liquid at high temperatures or pressures, non-liquefied flammable gases. |
Within flammable limits | A portion of the vapour cloud must be within the flammable limits of the material. |
Blast effects | The blast effects of a VCE are influenced by the speed of flame propagation through the flammable cloud. Faster flame propagation leads to higher overpressure and more significant blast effects. |
Presence of turbulence | Turbulence enhances combustion during deflagration, increasing the intensity of the explosion. |
It is common, although not always necessary, for a VCE to occur in confined or semi-confined spaces, such as within a building, between piping, or among structures. In unconfined spaces, the ignition of a flammable cloud typically results in a Flash Fire rather than a VCE. The distinction arises from the differences in the pressure and heat effects of these events (Bosch, 2005).
Mechanism of VCE
The formation of a flammable cloud can result from various sources, including Pool Evaporation, gas or vapour release, or the depressurisation and flash evaporation of a superheated liquid. When this cloud encounters an ignition source, an explosion occurs. An example of a VCE can be seen in the video below from Gexcon’s Fire and Explosion Testing.
Characteristics of VCE
The defining characteristic of a VCE is a Blast, which is the mechanical energy of the explosion transformed into a form of Overpressure Wave. This Overpressure Wave forms when the heat of combustion is partly converted into mechanical energy due to the expansion of combustion gases caused by the stoichiometry of the reaction and thermal expansion (Bjerketvedt, Bakke and Wingerden, 1992).
The resulting overpressure wave results from two competing phenomena:
Pressure increasing due to combustion.
Pressure decreasing due to the expansion of gases.
Flame propagation in a VCE can occur as either Deflagration or Detonation, leading to different types of Blast Waves. A Deflagration results in an overpressure wave with a slower pressure rise, while a Detonation creates a shock wave characterised by an immediate and intense pressure surge.
Typically, VCEs involve large volumes of flammable vapour clouds with relatively gradual energy release, resulting in deflagration as the primary mode of flame propagation. In rare instances characterised by intense turbulence, the flame propagation velocity may surpass the speed of sound, leading to a transition from deflagration to detonation. However, the occurrence of detonation remains minimal due to the lack of fuel homogeneity, which inhibits the conditions necessary for detonation to occur.
In addition to the positive impulse generated by the Blast Wave, the VCE also induces a substantial negative impulse, resulting in structural damage through suction forces.
References
Bosch, C. v. (2005). Methods for the calculation of physical effects 'Yellow book' CPR 14E. The Hague: Ministerie van Verkeer en Waterstaat.
Bjerketvedt, D., Bakke, J.R., Wingerden, K.v. (1992). Gas explosion handbook. Bergen: CMR Gexcon.
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