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Safety and environmental standards for fuel storage sites
Final report
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Figure 18
Schematic showing vapour flow driven by a free liquid cascade
Scoping method
Approach and assumptions
29 The scoping method described here is based on principle that production of vapour
concentrations within the flammable range at the base of the tank will bring liquids ‘in scope’. This
is a somewhat conservative, but reasonable, assumption that might be refined if more was known
about the splashing process and its effects on the near-field dispersion.
30 The method provides a means of determining whether a given filling operation in a given tank
can lead to the generation of a flammable cloud. Such a scoping method is clearly of interest in
determining the appropriate level of protection against overfilling. The volume and concentration of
flammable vapour close to the source are outputs but to predict the potential extent of the cloud
would require a dispersion model.
31 Although it may appear initially counter-intuitive, the likelihood of producing flammable vapour
for many substances increase as the amount of fresh air entrainment is reduced. Enhanced air
entrainment leads overall to greater evaporation but the vapour produced is often below the lower
flammability limit.