Page 387 - Dust Explosions in the Process Industries

356 Dust Explosions in the Process Industries

He pointed out that the initiating explosion causes events analogous to those observed
in shock tubes. The initial thin turbulent dust flame entrains deposited dust and devel-
ops into the more extensive main explosion, which may in turn lead either to detona-
tionlike phenomena, including strong shock waves, or to oscillating flames, depending
on various circumstances.
Bartknecht (1971) used an external dust dispersion system by which he avoided the
use of a primary explosion for initiating dust entrainmentand flame propagation.He gen-
erated a dust cloud of the most explosibleconcentration along the whole tube length by
simultaneously injecting dust from a number of equally spaced external pressurized
reservoirs. (This is essentially the same dust dispersion method as specified in the 1 m3
test approved by the International Standards Organization, 1985.) The dust cloud was
ignited by a strong chemical ignitor or a pocket of exploding methane/air as soon as it
had been generated. On the one hand, Bartknecht’s experiments were clean and well
defined. On the other hand, they differed from conditions often met in mines and other
industry, where the dust is initially deposited as layers that are dispersed by the air blast
preceding the flame as the explosion propagates. There may be situations, however,
where Bartknecht’s dispersion method corresponds to reality, for example, in pneumatic
transport of explosible dust concentrations.
Figure 4.64 gives some of Bartknecht’s results from experiments in 0.40 m diameter
horizontal one-end-openpipes of various lengths.As can be seen, there is close correlation

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COAL METHYL ORGANIC ALUMINUM
CELLULOSE PIGMENT

Figure 4.64 Dust explosions in 0.40 m diameter,horizontal, one-end-openpipes of various lengths,
with maximum flame speeds and maximum explosion pressures as functions of tube length and K,,
value of dusts; * denotes enlarged pipe diameter in the ignition zone at the closed end (From
Bartknecht, 1971).