Page 44 - Electrical Installation in Hazardous Area

Area classification 23

There are several features of primary importance in respect of gases
which determine the approach which is adopted in respect of electrical
installations with which they may come into contact. These are:
Relative density in respect of air and determines how the gas will disperse
when no other influences are present (e.g., gas released at high speed will
disperse in a manner governed by the energy of release rather than by
the effects of wind and density;
Minimum ignition energy, the minimum energy released as an arc or spark
which will ignite the most easily ignitable mixture of the gas and air;
Maximum experimental safe gap defines the burning characteristics of the
ideal gas/air mixture (which may differ from the most easily ignitable
mixture) insofar as its ability to burn through small gaps is concerned;
Ignition temperature, the minimum temperature at which the ideal gas/air
mixture will spontaneously ignite.
As soon as it is released a gas will mix with air, either by the energy of its
own rapidity of release or turbulent air movement. Where the release of a
gas is at low velocity and the air movement is small (as in badly ventilated
buildings) mixing is not as efficient and the mixture will vary from place
to place giving large and unpredictable hazardous areas.

2.1.2 Flammable vapours
Flammable vapours are very similar to gases in that they are in the vapour
phase. The difference is that they can be liquefied by pressure alone and
thus are more easily liquefied. A flammable vapour may be only the vapour
which exists because of a flammable liquid; as all such liquids have vapour
pressures (partial pressures in air) at temperatures well below their boiling
points and once the percentage of vapour in air created by the vapour
pressure exceeds the lower explosive limit, an explosive atmosphere can
exist even though the release is a liquid.
The explosive atmosphere created by a vapour release will have a signif-
icantly varying persistence time after cessation of a release. If it is released
as a vapour or as a liquid which has a boiling point significantly lower
than ambient temperature then the persistence will be low, whereas if it is
released from a liquid surface and the liquid boiling point is above ambient
temperature then it will be high. The formation of an explosive atmosphere
is caused by the mixing of the vapour with air and the criteria are as
described for gas, as are the parameters which have primary importance.

2.1.3 Flammable mists

This is an entirely different situation to that which exists in the case of
gases and vapours. A mist is created by release at high pressure (mists