Page 341 - Dust Explosions in the Process Industries

3 10 Dust Explosions in the Process Industries

a minimum relative pressure rise Pm/Poof 1.5 as their explosion criterion, they obtained
minimum explosible dust concentrations in close agreement with those based on self-
sustained flame propagation in a long tube of 100mm diameter. Buksowicz and Wolanski
demonstrated by direct photography that, near the minimum explosible dust concentra-
tion, the dust flame is fragmented into detached zones of burning particle clusters. They
also emphasized the need for using a sufficiently energetic ignition source, when study-
ing propagation of lean limit flames.
Schlapfer (1951) measured minimum explosibleconcentrationsof various dusts in air
in a laboratory-scalevertical tube of diameter 30 mm and with a vertical distance of 0.6 m
from the ignition sourceto the open top end of the tube. Dust suspensions of known con-
centration were conveyed upward in the tube at a laminar velocity of 0.6 ds. Propagation
of dust flame at least three-fourths of the distance of 0.6 m from the ignition source to
the tube top was used as the explosion criterion. The ignition source was an electrically
ignited 0.2 mm thick and 7 mm long aluminum wire. The results in Table 4.8 were
obtained.

Table 4.8 Minimum explosible dust concentrations measured at laminar flow conditions in a 30 mm
diameter vertical tube

Dust type Minimum explosibleconcentration(g/m3)
Aluminum flakes, mean flake thickness 0.5 pm 90
Lignin, 100% finer than 120 prn 48
Phenol resin, brown, 100% finer than 120 pm* 45
Phenol resin, gray, 100% finer than 120 pm* 36

Hertzberg et al. (1987) found that the minimum explosible dust concentration for
polymethylmethacrylatewas about 80 g/m3and independent of the particle diameter up
to 100pm. For Pittsburgh seam bituminous coal, the value of about 90 g/m3was found
to apply from 2 pm to 60 pm particle diameter. However, when the particle diameter
increased toward 200 pm, a substantial increase in the minimum explosible concentra-
tions was found for both dusts. This influenceof particle size agrees with the earlierresults
of Ishihama (1961) for various particle size fractions of coals of volatile matter contents
in the range 4649%. This worker also found that the minimum explosible dust con-
centration decreased with decreasing mean particle diameter down to about 60 pm. For
a finer fraction, of mean size 29 pm, the minimum explosible concentration was only
slightly lower than for the 60 pm fraction.
However, the actual minimum explosible concentration values found by Ishihama
were only half those found by Hertzberg et al. (1987). This can in part be explained by
the higher content of volatiles in the coals used by Ishihama,but the major factor is prob-
ably the different experimental methods and explosion criteria used.
Minimum explosible dust concentrationswere determinedin a comparativetest series
among four laboratories in different countries. Three methods were used: the 20 liter
spheremethod developed by Siwek (1977; see alsoASTM, 1988),the 1m3method spec-
ified by the International Standards Organization (1985), and the Nordtest (1989) Fire
011 method. The results are shown in Table 4.9.