| Summary: | Medium scale vapour cloud explosion experiments demonstrated that the severity of a vapour cloud explosion (VCE) can be reduced substantially by initiating counter fires within a vapour cloud as soon as a developing flame can be detected within the vapour cloud. For instance, experiments with two simultaneous ignitions at opposite sides of the used symmetrical test rig resulted in explosion pressures which were reduced by a factor of three to five compared to single point ignition at one side.
A later re-analysis of the test findings drew the attention to a flame acceleration mechanism, which had been known before, but its contribution to the severity of a VCE had not been fully appreciated. The generally accepted theory is that flame acceleration occurs as the flame passes obstacles. In addition to that, the contribution of the expanding combustion gas has been found to play a paramount role as well.
A big part of the strong reduction of the explosion severity with two-point ignition could be related to the fact that the two developing flames were acting against each other, creating a kind of expansion barrier against each other. This prevented the expanding combustion gas to push into the rig, which otherwise would have added to the flame speed.
In the past, walls in a congested area had been assumed to increase the severity of a VCE. The test findings suggest, however, that barriers could be implemented in a way such that they slow down the acceleration of a flame front.
It is expected that such barriers can be effective in particular when they are installed perpendicular to the anticipated direction of flame front acceleration. Process equipment is typically installed in rows 8 to 15 m wide, often extending over a length of more than 50 m, with access corridors at both sides. Horizontal equipment such as drums and heat exchangers are typically arranged perpendicular to the longitudinal direction.
For such an arrangement, a flame can be assumed to accelerate preferentially in the longitudinal direction, perpendicular to the obstacles in its way, and to speed up as it passes gaps between and below equipment, pushed through the gaps by the expanding gas behind. By closing such gaps or by installing barriers, the longitudinal hot gas expansion could be partly suppressed and its contribution to the flame speed would be reduced substantially. In addition, the barriers would force the hot gas to vent towards the uncongested space aside and above the equipment and they would force the flame front to “take a detour” around the barriers. Together, these effects would reduce the burning rate.
Hence, properly arranged barriers are expected to subdivide larger congested domains into smaller explosion cells, where less severe explosions occur. Because of the time needed for the flames to proceed from one cell to another, the pressure waves from these cells would be spread out over time, resulting in a lower overall explosion pressure, but continuing over a longer time period.
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