Dust explosions usually occur in
industry inside process vessels and
units such as mills, grinders, and
dryers—i.e., inside equipment where
the conditions of the explosion pentagon
are satisfied. Such occurrences are often called primary explosions, especially
if they result in secondary explosions
external to the process unit (as
described in the next section). The
reason for the majority of dust explosions
being initiated in this manner can
be understood by examining Figure 5.
Here, the range of explosible dust concentrations
in air at normal temperature
and pressure for a natural organic dust (e.g., cornstarch) is compared
with the typical range of maximum
permissible dust concentrations that
are relevant in the context of industrial
hygiene, and with a typical density of
deposits or layers of natural organic
dusts. Clearly, the range of explosible
concentrations is orders of magnitude
greater than the concentrations permitted
in areas inhabited by workers