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Area classification practice for dusts 151

required for a dust layer. It should be noted that ignition energy is not
mentioned. This is not because dusts do not have an ignition energy (see
Table 6.2) but because the methods of protection of electrical apparatus in
areas made hazardous by dusts rely on adequate exclusion of dust from
the electrical parts within the apparatus. Therefore, ignition energy is not a
necessary piece of information in general.
Information on both of these values for a variety of dusts is given in
Table 6.2. It should be noted that the layer ignition figures are based upon
a layer thickness of 5mm, as thicker layers are not normally acceptable
because they could clearly be in contravention of Section 31 of the Factories
Acts' and it is required that housekeeping normally maintains layers at
smaller thicknesses. If, in a particular case, this is not possible then the
figures in Table 6.2 are not appropriate and tests must be carried out to
determine the ignition temperature at the relevant thickness. In addition,
particle size and distribution in dusts, together with moisture content varies
and with it both ignition energy and temperature. Table 6.2 is based upon
particular particle size make-up of dusts, which is in some cases identified.
Once again, if the dust in a particular case, while being included in Table 6.2,
has a significantly different constitution then the figures in Table 6.2 may
not be accurate and it may be necessary to determine its particular ignition
temperature. The figures in Table 6.2, however, are a good yardstick as they
are based on relatively fine dusts and may be used with a fair degree of
confidence in most practical situations.
Having stated that ignition energy is not a necessary item of information
for dust releases it should not be overlooked that many plants contain
both flammable gases/vapours and combustible dusts and protection is
necessary against the ignition of both. Also, some dusts may decompose
to give off flammable vapours. Where flammable gases/vapours do occur
ignition of these may well ignite the dust cloud or layer and protection
against both risks must be provided. (It is worthy of note that, although
the manufacture of explosives is specifically excluded by the majority of the
Standards and Codes to which this book refers, both flammable vapours and
combustible dusts are present in the early stages of explosives production
and the approaches in this book may need to be implemented to prevent the
ignition of such vapours and dusts which could otherwise act as triggers
for detonation of the explosives.)

6.7.7 The ignition of dust clouds

When dust is mixed with air to the degree necessary to form an explosive
atmosphere, the mixture is very dense and clearly visible. Dust clouds of
any size which exhibit this possibility will be rare because of the environ-
mental problems which they may create. The ratio of dust to air is a variable
quantity depending upon particle size, etc., and no specific ideal mixture
is given, unlike the situation in respect of gases and vapours. It is also not
possible to ensure an even concentration of dust throughout the cloud.