Page 43 - Electrical Installation in Hazardous Area

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Area classification

Philosophy, objectives and procedures

The basic objective of area classification (be it for gas, vapour, mist or dust
risks) is to identify the possibility of an explosive atmosphere existing in a
given location and, more importantly, to influence the design of any plant or
facility to minimize such risks. To this end, it is a tool to be used together
with the operational requirements in the design of any plant, process or
facility, and the result will invariably be a compromise between conve-
nience of operation and security against explosion. This balance must take
account of certain situations which are generally not permissible, such as
the continuous presence of an explosive atmosphere in an unrestricted area,
and the limits which are placed on the acceptability of any situation where
an explosive atmosphere may be present. This chapter seeks to identify the
principles upon which area classification is based to achieve these objectives,
the part played in area classification by those involved in design or oper-
ation of plants and locations where explosive atmosphere may be present,
and the procedures which need to be undertaken in the identification of
hazardous areas.

2.1 Basic properties of flammable and combustible materials

The materials with which we are concerned are gases, vapour mists and
dusts. Each of these materials behaves in a different way and this affects
the classification process.

2.1.1 Flammable gases

A gas is a material whose normal state at ambient temperatures and
pressures is as a vapour. These materials cannot be liquidized at ambient
temperature by pressure alone and must also be cooled. Therefore, they
are not likely to be present in the form of liquid and, if they are, they will
rapidly vaporize once released. Area classification is a measure of the area
contaminated by their release and is not likely to be greatly influenced by
an residual effect due to their continued presence after the release ceases,
provided there is sufficient ventilation for their removal.