Explosions in Electrical Control Boxes as a Potential “Nested Bang-Box” Mechanism for Severe Vapour Cloud Explosions

The ignition source for Buncefield, the United Kingdom’s most severe recent vapour cloud explosion (VCE) was potentially electrical control boxes situated inside a pump house immersed in the vapour cloud. There are other reports of confined or bang box ignition sources for other VCEs, such as Port Hudson and Jaipur where it is proposed these ignition sources were responsible for transition to detonation (DDT). There has, however, been relatively little previous research into this type of ignition mechanism and its effect on the explosion severity. Commercially available electrical control boxes measuring 600 mm high, 400 mm wide and 250 mm deep were used to explore the pressure development, venting processes and flame characteristics of stoichiometric propane/air explosions using cling film, aluminium foil and the supplied doors as vent coverings. In this work the boxes were empty of their usual contents in order to establish a baseline for the effect of the internal congestion of the boxes. It was found that, in these empty-box tests the overpressure was dominated by the bursting pressure of the ventcovering and the external explosion, although clearly presenting significant ignition source to a potential surrounding flammable cloud, it produced no significant overpressure. The door produced a flat petal shaped flame that differed drastically from the rolling vortex bubble flame shape traditionally associated with vented explosions.