Page 79 - Dust Explosions in the Process Industries

52 Dust Explosions in the Process Industries

only for volatile contents above 14%. With 1 vol% methane in the air, the critical value
was 13%; for 2% methane, about 12%; and for 3% methane, about 9% volatiles.
Cardillo andAnthony (1978) determinedempirical correlationbetween the content of
combustible gas (propane) in the air and the minimum explosible concentration of
polypropylene, polyethylene, and iron. It is interesting to note that iron responded to the
propane addition in the same systematicway as the organic dusts. For no propane in the
air, the minimum explosible iron dust concentration was found to be 200 g/m3,whereas
for 1 vol% propane, it was 100 g/m3.
The influence of small fractions of methane in the air on the minimum electric spark
energy for ignitingclouds of coal dusts was investigated systematicallyby Franke (1978).
He found appreciable reductions in MIE, by factors on the order of 100, when the
methane content was increased from 0 to 3 ~01%.
Pellmont (1979) also investigatedthe influenceof combustiblegas in the airon the min-
imum ignition energy of dust clouds.A set of results, demonstratinga quite dramaticeffect
for some dusts, is given in Figure 1.60. Pellmont found that the most ignition sensitive
concentration of the various dusts decreased almost linearly with increasing content of
propane in the air. For example, for 20 ,um PVC in pure airthe most sensitive concentra-
tion was 500 g/m3,whereas with 2 vol% propane in the air,it was 250 g/m3.Figures 1.61
and 1.62 give some results presented by Foniok (1985). In agreement with the findings
of Pellmont, Foniok observed that the dust concentration most sensitive to ignition, and
at which the reported MIE values were determined,decreased systematicallywith increas-
ing combustible gas content in the air. For example, for the 31% volatile dust, for which
data are given in Figure 1.61, the most sensitive concentration was 750 g/m3 with no
methane in the air, whereas with 3.5% methane in the air, it dropped to 200 g/m3.

A PVC, 125 prn

Figure 1.60 The influence of small fractions of
propane in the air on the minimum electric spark
I I ignition energy of clouds of various organic dusts
0
1 2 3 4 at normal ambient conditions (From Pellmont,
PROPANE CONTENT IN AIR [val%l 1979).
Torrent and Fuchs (1989), probably using more incendiary electric sparks of longer
discharge times than those used by Foniok (1985),found little influence of methane con-
tent in the air on MIE for coal dusts up to 2 vol% methane. For all the coal dusts tested
but one, the MIE in pure air was 400 d.For one exceptional coal dust, containing 18%
moisture and 12% ash, the MIE dropped from 300 mJ for no methane to about 30-50 mJ
for 2% methane.