Page 554 - Dust Explosions in the Process Industries

Assessment of lgnitability 52 1

Figure 7.44 Photograph of opened 20 liter U.S. Bureau of Mines explosion vessel, showing one of
the light attenuation probes for measuring dust concentration (Courtesy of K. L. Cashdollar, U.S. Bureau
of Mines, Pittsburgh).

Favorable agreement was obtained between minimum explosible concentrations found
for coal dust in large-scale mine experiments and in the 20 liter vessel (Cashdollar et al.,
1987). The ignition source used in the 20 liter sphere was then a strong chemical igni-
tor of calorific energy about 2500 J. The criterion of explosion was that the explosion
pressure in the closed vessel should rise to at least twice the absolute initial pressure. For
atmospheric initial pressure this means at least 1 bar(g). In addition, the maximum rate
of pressure rise should exceed 5 bark.

7.13.2.2
German and Swiss Closed Bombs

Both the 1 m3 IS0 vessel developed by Bartknecht and the 20 liter Siwek vessel are dis-
cussed in Chapter 4 and further details are given in Sections 7.16 and 7.17. With the same
ignition source and explosion criterion as used by Cashdollar and Hertzberg, the Siwek
sphere should yield comparable results. If, however, the 10 kJ ignitor prescribed for the
Siwek sphere to determine P,,, and K,, values is used, too low minimum explosible con-
centration values would be expected for some dusts.
The 1 m3 IS0 vessel would be expected to yield the most reliable assessment of
the minimum explosible concentration. Because of the large volume of the dust cloud,
even a very strong ignition source of 10 kJ would not interfere with the main phase
of dust cloud propagation. However, just because of its large size, the 1 m3 test is not
very suitable for routine testing, and smaller, laboratory-bench-scale methods are
needed.