Page 87 - Dust Explosions in the Process Industries

60 Dust Explosions in the Process Industries

-
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-P 0PEA FLOUR
2
3 DUST CLOUD &. BROWN COAL
v) 4 FATTY COAL
VI
2 5-
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z
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5
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w 300 - DUST DEPOSIT BROWN COAL
2 FATTY COAL
a
4 200 -
W
c
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Figure 1.67 Comparison of the influence of oxygen content in the gas on the oxidizability of dust
clouds and dust deposits (From Walther, 1989).
flour or corn starch, the permeability of the inert gas may be too low for efficient inert-
ing of large bulk volumes.
Further details concerning the extinction of powder and dust fires are given by Palmer
(1973a) and Verein deutscher Ingenieure(1986).The use of inert gas for extinctionof smol-
dering fires in silos was specificallydiscussedby Dinglinger (1981) and Zockoll and Nobis
(1981). Chapter 2 gives some examples of extinction of smoldering fires in practice.
Some synthetic organic chemicals, in particular cyclic compounds, can decompose
exothermally and become ignited by a hot surface, a smolderingnest, frictional heat, or
another ignition source. Such decompositionrequires no oxygen, and therefore inerting
has no effect.Zwahlen (1989) gave an excellentaccount of this specialproblem. He pointed
out that this type of exothermic decomposition can be avoided only by eliminating all
potential ignition sources. However, by taking other processing routes, one can elimi-
nate or reduce the problem. Zwahlen suggested the following possibilities:
Process the hazardous powder in a wet state, as a slurry or suspension.
If wet processing is impossible, avoid processes involving internal moving mechan-
ical parts that can give rise to ignition.
If this is not possible, strictly control to prevent foreign bodies from entering the
process. Furthermore,use detectors to observe early temperature and pressurerise and
provide sprinkler systems.Adiabatic exothermal decompositionof bulk powder at con-
stant volume, due to the very high powder concentration, can generate much higher
pressures than a dust explosion in air.