Flame acceleration, vapor cloud explosions

Vapor Cloud Explosion (VCE) Explosive oxidation of a vapor cloud in a non-confined space (not in vessels, buildings, etc.). The flame speed may accelerate to high velocities and produce significant blast overpressure. Vapor cloud explosions in plant areas with dense equipment layouts may show acceleration in flame speed and intensification of blast. 

Experiments in tubes are not directly applicable to vapor cloud explosions. An overview of research in tubes is, however, included for historical reasons. An understanding of flame-acceleration mechanisms evolved from these experiments because this mechanism is very effective in tubes. 

Kjaldman, L., and R. Huhtanen. 1985. Simulation of flame acceleration in unconfined vapor cloud explosions. Research Report No. 357. Technical Research Centre of Finland. 

Vapor cloud explosion The explosion resulting from the ignition of a cloud of flammable vapor, gas, or mist in which flame speeds accelerate to sufficiently high velocities to produce significant overpressure. 

If the release forms a vapor cloud that premixes with air before ignition occurs, and turbulence is developed (for example, by the flame front propagating through a process structure), the flame speed can accelerate sufficiently to cause a blast. This event is referred to as a vapor cloud explosion. In addition to blast effects, radiant heat and flame contact effects may also occur. Flashback to the source may cause a pool and/or jet fire. 

The TNT equivalency method also uses an overpressure curve that applies to point source detonations of TNT. Vapor cloud explosions (VCEs) are explosions that occur because of the release of flammable vapor over a large volume and are most commonly deflagrations. In addition, the method is unable to consider the effects of flame speed acceleration resulting from confinement. As a result, the overpressure curve for TNT tends to overpredict the overpressure near the VCE and to underpredict at distances away from the VCE. 

Unconfined vapor cloud explosion explosive oxidation of a flammable vapor cloud in a nonconfined space (e.g., not in vessels or buildings) the flame speed may accelerate to high velocities and can produce significant blast overpressures, particularly in densely packed plant areas. 

These explosions may occur in unconfined areas, although some degree of congestion is still required. The overpressure is created by the rapid and accelerating combustion of the gas and air mixture. The speed of the flame front can reach over 2,000 meters per second, (6,000 ft/s) creating a shock wave as it pushes the air ahead of it. Vapor cloud explosions can only occur in relatively large gas clouds. 

Research on water explosion inhibiting systems is providing an avenue of future protection possibilities against vapor cloud explosions. British Gas experimentation on the mitigation of explosions by water sprays, shows that flame speeds of an explosion may be reduced by this method. The British Gas research indicates that small droplet spray systems can act to reduce the rate of flame speed acceleration and therefore the consequential damage that could be produced. Normal water deluge systems appear to produce too large a droplet size to be effective in explosion flame speed retardation and may increase the air turbulence in the areas. 

Finally, there must be a flame acceleration mechanism, such as congested areas, within the flammable portion of the vapor cloud. The overpressures produced by a vapor cloud explosion are determined by the speed of flame propagation through the cloud. Objects in the flame pathway (such as congested areas of piping, process equipment, etc.) enhance vapor and flame turbulence. This turbulence results in a much faster flame speed which, in turn, can produce significant overpressures. Confinement that limits flame expansion, such as solid decks in tnulti-lcvc process structures, also increases flame speed. Without flame acceleration, a large fireball or flash fire can result, but not an explosion. 

Vapor cloud explosions in partially confined areas with obstacles exhibit a further increase of the maximum explosion pressure compared to unconfined explosions due to additional turbulent flame acceleration at obstacles. In experiments we found an increase up to a factor of four and a scaling behavior similar to that of the unconfined case. 

FM Global. Property Loss Prevention Data Sheet 7—42, Evaluating vapor cloud explosions using a flame acceleration method. Norwood, MA FM Global 2012. 

The speed of the flame propagation must accelerate as the vapor cloud burns. This acceleration can be due to turbulence, as discussed in the section on confined explosions. Without this acceleration, only a flash fire will result. 

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