Page 68 - Electrical Installation in Hazardous Area

44 Electrical installations in hazardous areas

examples of extents of hazardous areas based upon that philosophy, and
its execution and additional information given in this chapter. Account is
also taken of the possibility of using the mathematical formulae given in
Chapter 4 and these formulae are used in producing many of the examples
given in this chapter.

3.1 Containment of flammable materials

Flammable materials may be contained in a variety of ways in a process
plant; the most obvious way being as a gas or vapour usually at some
elevated pressures which are necessary either to transport it or for some
other process reason. Such circumstances are the most easily dealt with
as the construction of an explosive atmosphere is by mixing of the gas or
vapour with air and the definition of explosive limits is by volume mixture.
Therefore, it is merely a question of determination of the release rate and
the mechanism by which the gas or vapour is mixed with air which, in
freely ventilated areas, is usually by virtue of its own motion or by wind
effects. There are several other ways in which a flammable material may
exist and this makes the area classification exercise much more complicated.

3.1.1 Effects of storage conditions

The flammable material, may for instance, be contained at high pressure
and, therefore what at normal ambient pressure would be a vapour is actu-
ally stored as a liquid. When such a liquid is released, the pressure falls
and vaporization takes place. This of course is typically the way in which a
refrigerator works and a study of this identifies the necessity of heat input
to effect vaporization (latent heat of vaporization). As the liquid vaporizes
there will be a loss of temperature until the temperature of the liquid falls
to its boiling point. At this point, vaporization will cease until external heat
is added. In such circumstances there will be an immediate evaporation of a
percentage of the liquid followed by further, slower evaporation when the
residual liquid reaches a heat source. This percentage needs to be defined;
a possible calculative method is given in Chapter 4.
The flammable material may be contained at a pressure below its boiling
point and this may be thought to indicate that it will not produce much
vapour on initial release. This is not wholly true as the release orifice may
be such as to support the formation of mists which are finely divided liquid
particles and perform in a similar manner to vapours as far as formation of
explosive atmospheres are concerned. With small orifices it can be shown
that significant mist formation is possible at gauge pressures as low as
2 x 105 N/m2 while with larger orifices this is less likely although even in
these latter circumstances some part of the liquid release may be released
as a mist. In addition, the liquid may even be below its flashpoint at