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124 The same principles of independence, effectiveness and auditability which apply to protection
layers also apply to conditional modifiers. It is important to make sure that the conditional modifier,
as defined in the LOPA, is effective in its own right in preventing the consequence without relying
on the performance of another conditional modifier or protection layer. Where the performance of
a proposed conditional modifier is conditional on the performance of a protection layer or another
conditional modifier, it cannot be considered independent. Instead it should be considered part of
another protection layer or conditional modifier. The risk reduction should only be claimed once
and the LOPA team will need to decide where best to include it.
125 The use of a given conditional modifier may not be appropriate in all circumstances
depending on the type of calculation being performed. See paragraphs 25–27 of this appendix.
126 In many cases there may be uncertainty over what value to use for a given conditional
modifier because the factors which influence it cannot all be defined or characterised, eg where
the role of human behaviour is uncertain or where the underlying science is itself uncertain. Under
these circumstances a conservative approach should be taken, consistent with the application of
the precautionary principle (see paragraphs 23–24 of this appendix).
127 The presentation of conditional modifier probability ranges in guidance is problematic
because of the number of site- and situation-specific factors that need to be considered.
Experience has shown that any values cited in literature are often used without consideration of
any accompanying caveats and without due consideration of site- and situation-specific issues.
Therefore this guidance aims to describe the relevant factors to be considered rather than
proposing specific values. These can then be addressed as part of a reasoned justification to
support the probability used for a given conditional modifier.
CM 1 – Probability of calm and stable weather
128 The Buncefield explosion occurred during calm and stable weather conditions. There is
insufficient evidence currently available to say with certainty whether the weather needed to be
both calm and stable, whether only one of these conditions was required (and if so which), and
what wind speed limit should be applied to the ‘calm’ condition. The basis of this guidance is
that the development of a large vapour cloud with the kind of compositional homogeneity that
is believed to have existed at Buncefield required both low wind speed and stable atmospheric
conditions.
129 It is not certain from the available data what limiting value should be used to define a low
wind speed condition. This guidance recommends that a value of 2 m/s is used. Analysts are
cautioned against trying to differentiate between wind speeds lower than 2 m/s because of the
difficulties in obtaining reliable measurements under such conditions (see CRR133
65
). Noticeably
higher wind speeds will disperse the vapour cloud more rapidly and may make it more likely that
an ignition would lead to a fire rather than to an explosion.
130 It is also unclear at present what level of atmospheric stability is required for the development
of the kind of large vapour cloud formed at Buncefield. The release at Buncefield occurred under
inversion conditions which promote the formation of ground-hugging vapour clouds. Given the
present state of knowledge, it is recommended that the weather conditions are confined to
classes E and F on the basis that these correspond to inversion conditions and are most likely to
be associated with low wind speeds.
131 The occurrence of Pasquill classes E and F is between the hours 1600–0800 (see Table
4.1.10 in CRR133) and therefore mainly but not exclusively outside normal office hours. Note that
weather conditions associated with the Buncefield explosion are affected by seasonal variations
and should be accounted for by the analyst.