Formation of the vapour cloud
25 The Third Progress Report described extensive tests undertaken to model the behaviour of fuel escaping from Tank 912 during overfilling. Tank 912 was fitted with a deflector plate, installed to direct water from sprinklers on the tank’s top to its sides to provide cooling in the event of fire. The tests demonstrated that the deflector plate channelled some of the escaped fuel onto the tank wall, but the rest ran over the top of the plate, fragmenting into droplets that cascaded through the air. Most of the fuel running down the wall hit a wind girder (a structural stiffening ring) and detached from the tank wall, creating a second cascade of droplets.
26 These conditions would promote the evaporation of the lighter components of petrol, eg butanes, pentanes and hexanes. The free-fall of droplets leads to entrainment of air and mixing between the air and fuel vapour, and the formation of a rich fuel/air mixture. Cooling of the surrounding air, already saturated with water vapour by the evaporation, would cause some of the water content to precipitate as an ice mist, which is consistent with the cloud of mist visible on
Formation of the vapour cloud25 The Third Progress Report described extensive tests undertaken to model the behaviour of fuel escaping from Tank 912 during overfilling. Tank 912 was fitted with a deflector plate, installed to direct water from sprinklers on the tank’s top to its sides to provide cooling in the event of fire. The tests demonstrated that the deflector plate channelled some of the escaped fuel onto the tank wall, but the rest ran over the top of the plate, fragmenting into droplets that cascaded through the air. Most of the fuel running down the wall hit a wind girder (a structural stiffening ring) and detached from the tank wall, creating a second cascade of droplets.26 These conditions would promote the evaporation of the lighter components of petrol, eg butanes, pentanes and hexanes. The free-fall of droplets leads to entrainment of air and mixing between the air and fuel vapour, and the formation of a rich fuel/air mixture. Cooling of the surrounding air, already saturated with water vapour by the evaporation, would cause some of the water content to precipitate as an ice mist, which is consistent with the cloud of mist visible on
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