Page 69 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 45

the point of release and the mist may still be capable of being ignited if
appropriately mixed with air. Another typical method by which a mist
may be produced is where a flammable vapour is contained as a liquid
at pressure. The turbulence created by pressure reduction coupled with
vaporization, which has already been described, will cause the production
of some mist in addition to the vapour at the point of release. This must
also be taken into account.
There is also the containment of a flammable liquid above its flashpoint
but below its boiling point. There is little flammable vapour produced
on liquid release and therefore, jet formation is possible. Such jets, unless
contained, can travel significant distances before striking the ground or an
obstruction and where they cease to travel a pool will be formed. This pool
will be controlled in size either by ground contours or by the evaporation
rate of the material and significant hazardous areas can be produced some
distance from the source of leakage, in addition to which there is the risk
that the liquid jet may come into contact with a hot surface which will
exacerbate the situation. All of these matters must be considered.

3.1.2 Effect of sunlight on storage vessels

A further problem exists where flammable liquids are contained for
sigruficant periods and the containment is in direct sunlight. Typical of such
cases is the normal stock tank. There is considerable evidence that in such
circumstances significant solar gain can occur and the mixture of vapour
and air in the tank above the liquid can adopt a temperature significantly
above ambient, encouraging evaporation. This may cause liquids with
flashpoints above ambient which would not otherwise be considered to be
flammable materials to become so and may cause some flammable materials
to reach their boiling points. These matters also require to be taken into
account.

3.1.3 Oxygen enrichment

Finally, it must be remembered that area classification is based upon the
mixture of a flammable gas, vapour or mist with air at atmospheric pressure.
In addition, the technology is based upon mixtures of flammable materials
with air. Conditions necessary for ignition vary, particularly with increased
levels of oxygen such as which occur in oxidation processes and equipment
which is considered suitable for use in explosive atmospheres, assuming as
it does atmospheric pressure and air, may not be suitable for use in such
circumstances. Where such situations arise the area classification evaluation
must take account of them, and they must be identified as special to ensure
that those involved in the selection of equipment for such areas are aware
of the possibility that normally protected equipment may not be suitable