Page 135 - Dust Explosions in the Process Industries

7 08 Dust Explosions in the Process Industries

Figure 1.1 26 Application of automatic dust explo-
sion suppression to bucket elevators (Courtesy of
T. Pinkwasser, Biihler, Switzerland). For a much
clearer picture, see Color Plate 4.

problems caused by these chemicals became a major issue, Bartknecht (1978) showed
that powder suppressants, such as NH4HzP04, in general, were much more effective for
suppressing dust explosions than halons. Therefore, powder suppressants have been
used to suppress dust explosions for many years. But powders differ in their suppres-
sive power, and efforts have been made to identify the most effective ones.
Figure 1.127 shows that the addition of only 30 weight% of NH4H2P04 powder is
required to prevent flame propagation in dust clouds in air of Pittsburgh bituminous
coal, whereas with CaC03 dust (limestone), 70 weight% is needed.
Similar systematic investigations were undertaken by Szkred (1983). He used a coal
dust of 38% volatiles, 7% moisture, and 38 pm mean particle size as fuel and found that

Figure 1.1 27 The influence of the chemistry of
inorganic powder suppression on mass per-
centage required for suppressing coal dust
0 10 20 30 40 50 60 70 80 explosions: 400 1 ignition source and 20 liter
CONTENT OF SUPPRESSANT IN MIXTURE closed explosion vessel (From Hertzberg et a/.,
WITH COAL DUST Iwt. %1 1984).