Page 35 - Dust Explosions in the Process Industries

8 Dust Explosions in the Process Industries

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Figure 1.3 The range of explosible dust concentrationsin air at normal temperature and atmospheric
pressure for a typical natural organic dust (corn starch), compared with typical range of maximum
permissible dust concentrations in the context of industrial hygiene, and a typical density of deposits
of natural organic dusts. (Note that self-sustainedflame propagation may also occur in such deposits.)

explosible limit. This is illustrated by the fact that the range of maximum permissible
dust concentrations specified in the context of industrial hygiene in working atmo-
spheres are three to four orders of magnitude lower than minimum explosible dust con-
centrations.This means that the unpleasant dust concentrationlevels that can sometimes
occur in the general working atmosphereof a factory, and calls on the attention of indus-
trial hygiene authorities, are far below the concentration levels that can propagate dust
flames.Therefore, the minimum explosible concentration corresponds to dust clouds of
high optical densities, which are unlikely to occur regularly in work rooms of factories.
A visual impression of the density of explosible dust clouds is provided in Figure 1.4,
which illustrates a cubical arrangement of cubical particles.

L

Figure 1.4 Cubical dust particles of edge x
arranged in a cubical pattern, with interparticle
center-to-center distances of L.

On average, there is one cubicalparticle of volume 2 per cube of dust cloud of volume
L3.If the particle density is pp, the dust concentration equals

c = p,(x/L)3 (1.8)