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CM 2 – Probability of ignition of a large flammable cloud
132 This conditional modifier represents the probability that the ignition of the vapour cloud from
a storage tank overflow is delayed until it is sufficiently large to cause a widespread impact.
Alternative outcomes are an earlier ignition that causes a localised flash fire, or safe dispersal of
the cloud without ignition.
133 As a general rule, as the size and duration of a Buncefield-type release increases the
probability of ignition will increase, eventually tending towards 1.0. For shorter duration large
releases, some available data has been quoted in LOPA studies by operators based on Lees’
Loss Prevention in the Process Industries
66
suggesting a probability of ignition of 0.3 although this
value is based on offshore blowouts and is not directly applicable to Buncefield-type events.
134 The bulk of available literature on ignition probabilities is pre-Buncefield and is based on
scenarios and circumstances that differ significantly from the Buncefield incident. This can in many
cases make their adoption for Buncefield-type scenarios inappropriate. Therefore, a number of
factors need to be taken into consideration when determining the probability of ignition for gasoline
and other in scope substances. These include, but are not necessarily limited to the following:
Size and duration of release – which may require an estimate of how long an overflow might
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persist before it is discovered, how big the cloud can get and how long it might take to
disperse. In the absence of better information, the size and duration of release should be
based on the Buncefield incident.
Site topography, which can lead to a flammable cloud drifting either towards or away from an
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ignition source.
The potential ignition sources present that could come into contact with the flammable cloud
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such as a vehicle, a pump house or a generator. This assessment should include any off-site
sources within the potential flammable cloud.
Immediate ignition is likely to produce a flash fire, delayed ignition may produce a flash fire or
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explosion.
135 The significance of area classification in preventing ignition should be considered carefully.
While area classification will limit the likelihood of ignition of a flammable cloud in the zoned
areas, it will not stop it completely (eg see section 1.6.4.1 of Ig
nition probability review, model
development and look-up correlations
67
and section 8.1.3 of
A risk-based approach to hazardous
area classification
68
), and the type of release being considered in this report is outside the scope
of conventional area classification practice. ‘Classified’ hazardous areas are defined by the
probability of flammable or explosive atmospheres being present in ‘normal’ operations or when
releases smaller than those at Buncefield occur due to equipment failure. Most major hazard
releases would go beyond the ‘classified’ hazardous areas.
136 Even if a dutyholder chooses as a matter of policy to purchase Zone 2 minimum electrical
equipment throughout their facility, this may not apply to every type of equipment (for example, street-
lighting). Also, normal site layout practice may allow uncertified electrical equipment (such as electrical
switchgear and generators), ‘continuous’ sources of ignition such as boilers or fired heaters, and hot
surfaces, to be present close to Zone 2 boundaries, increasing the chance of ignition.
137 It is also possible that the operation of emergency response equipment (including switchgear
and vehicles) may act as an ignition source. The operation of such equipment may be initiated
directly or indirectly by the tank overflow and therefore cannot be assumed to be independent of
the overflow event.
138 Where a more detailed estimate of ignition probabilities is required further information is given
in the HSE’s research report CRR203
69
and the Energy Institute’s
Ignition probability review,
model development and look-up correlations
. The assessment should take into account the
spread of the cloud over the facility and its environs and should identify all credible sources of
ignition within the area.