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Safety and environmental standards for fuel storage sites
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33 The zones within Table 7 are provided as a conservative basis. The zones may be adjusted on
a case-by-case basis, due to site-specific factors such as:
Site topography. The Buncefield site is reasonably level other than higher ground to the south.
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This appears to have affected the spread of the cloud such that it extended 250 m to the
north and 150 m to the south. Therefore if a site is not level, distances shorter than Table 7
may be appropriate for the ‘uphill’ direction. Similarly, if a site has a significant slope, then it
would be appropriate to consider distances longer than Table 7 in the ‘downhill’ direction.
Significant sources of ignition within Zone A. If there are ‘continuous’ sources of ignition closer
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to the tank than 250 m located in a position that could be contacted by the cloud, then it is
very likely that the cloud will ignite before it reaches 250 m. This would mean that the distance
to the edge of Zone A is less than 250 m and CM2 (Probability of ignition) is likely to be 1.
Examples of ‘continuous’ sources of ignition are boilers, fired heaters and surfaces that are
hot enough to ignite the cloud. Typically, automotive, internal combustion engines are not a
reliable source of ignition. However, an automotive starter motor is a known ignition source.
Duration and rate of transfer into the tank. The quantity of petrol that overflowed Tank 912 at
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Buncefield from initial overflow to ignition was approximately 300 tonnes. If the transfer rate or
overflow duration is estimated to be significantly different to that at Buncefield, then this may
affect the formation and size of the cloud. An estimate of cloud generation could be made
based on modelling such as the ‘HSL entrainment calculator’ and a 2 m cloud height (for
further information see Appendix 1).
34 Other factors that should be considered when estimating the consequence to people are:
Hazards resulting from blast over-pressure can be from direct and indirect sources. For
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example, indirect sources of fatal harm resulting from an explosion can be missiles, building
collapse or severe structural damage (as occurred at Buncefield).
People on and off site within the relevant hazard zones should be considered as being at risk.
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People within on-site buildings such as control rooms or offices that fall within the hazard
zones as described above should be considered at risk unless the buildings are sufficiently
blast-rated.
The base case should be ‘normal night time occupancy’ – see CM1 ‘Probability of calm and
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stable weather’. However, a sensitivity analysis should consider abnormally high occupancy
levels, eg road tanker drivers, visitors, contractors and office staff who may be present
should the calm and stable conditions occur during normal office hours (see paragraph 131).
Additionally, sensitive populations just beyond the 250 m, eg a school or old people’s home,
should also be considered.
Environmental consequences
35 This guidance also covers the environmental risks associated with a storage tank overflow.
The consequences may be direct (pollution of an aquifer if the overflowing gasoline penetrates
the bund floor) or indirect (pollution arising from firefighting efforts). The consequence will need
to be determined on a case-by-case basis after consideration of the site-specific pathways to
environmental receptors, the condition of secondary and tertiary containment arrangements,
the location and type of specific receptors, and any upgrades planned to meet Containment
Policy requirements (
COMAH CA Policy on Containment of Bulk Hazardous Liquids at COMAH
Establishments
).
Risk tolerance criteria
General
36 Risk tolerance criteria can be defined for human risk and for environmental risk on the basis
of existing guidance. In addition, dutyholders may also have risk tolerance criteria for reputation
risk and business financial risk. However, there is no national framework for such criteria and
decisions on the criteria themselves and whether to use such criteria in addition to those
presented here lie with the dutyholder. No specific guidance is given in this report to evaluating