1139 / 1145
Further study confirmed that it was reasonable (indeed conservative) to assume that
the idealized hard sphere was surrounded by a shell of gas of no less than 20% LFL
at a distance of 5m from the edge of the ‘hard’ sphere. This was then used to improve
the performance of (particularly open path) gas detection systems.
This approach – the assumption of a diffuse cloud of dilute gas surrounding the core
hazard - was incorporated into some F&G Mapping software, but must be applied
with care. When applied in an external environment with lower set points than the
specified 20%LEL, these dispersion principals can become meaningless, and will
typically result in an inadequate system which can result in significant potential for
undetected gas clouds providing the conditions for explosion overpressures well in
excess of 150mBar. Engineering judgment must always be applied when using this
model to ensure this does not happen.
For all of the factors discussed above, we can conclude there is far more to account
for than simply mapping the area, and the adage of claiming “the area has been
mapped and can therefore be considered compliant and safe” has significant dangers
associated.