Page 45 - Electrical Installation in Hazardous Area

24 Electrical installations in hazardous areas

have been formed by pressures as low as 2 x 105 N/m2 gauge) in such a
way that very fine particles (droplets) of liquid are produced. (Mists can
form explosive atmospheres in mixture with air even if the liquid forming
them is considered not to have sufficient vapour pressure to form an explo-
sive atmosphere.) These particles will recombine to form a liquid much
more readily than gas or vapour, but if very finely divided the mist has a
significant existence time and the particles will remain in suspension in air
for a long period. Much less is known about the performance of mists in
air but they are normally treated in the same way as gases and vapours for
the purpose of area classification.

2. I. 4 Flammable liquids

This is something of a misnomer as liquid does not bum. Flammable liquid
indicates a liquid which, at normal ambient temperature and pressure or
the temperature and pressure at which it is handled or stored, has a vapour
pressure (partial pressure) sufficient to liberate enough vapour to form an
explosive atmosphere.

2.1.5 Combustible dusts
A combustible dust is a dust which will bum when mixed with air or
which, in layer form, will bum if ignited. It should not be confused with an
explosive. These need to be treated in a different way to gases, vapours and
mists in that, while a release is normally in cloud form, once released the
dust will not disperse as will the gas/vapour or mist but will initially form
a cloud and then settle as a dust layer. Once released only housekeeping
can affect the persistence of clouds by disturbed layers.
The important parameters for dust are different from those used for gases
and vapours and are as follows:

Cloud ignition energy, the minimum energy which is required in the form
of an arc or spark to ignite a gas cloud;
Cloud ignition temperature, the minimum temperature at which an ideal
mixture of the dust in suspension with air will ignite. This can depend
on particle size;
Layer ignition temperature, the minimum temperature at which a layer of
the dust of specific thickness will ignite and bum;
Particle size, the size of particle from which a dust is formed and which
have an effect on its ignition capability.

2.2 Basis of area classification
Area classification is based upon those situations which can occur
in practical plant and factory operation which produce an explosive