Page 69 - Dust Explosions in the Process Industries

42 Dust Explosions in the Process Industries

Sweiss and Sinclair (1985) investigatedthe influence of particle size of the dust on the
limiting oxygen concentration in the gas for flame propagation through dust clouds.
Natural and synthetic organic dusts were studied. The results from experiments with
narrow size fractions indicated that the limiting oxygen concentration decreased with
decreasing particle size down to 100 pm. Below 100 pm, the limiting oxygen concen-
tration was practically independentof particle size. However, addition of only 5% by mass
of a fine dust (=60pm) to a coarsemain dust (200-1000 pm) reduced the limiting oxygen
concentrationby at least 60% of the differencebetween the values of the coarse dust only
and the fine dust only.
Wiemann (1984) found that, for brown coal, dust particle size had a comparatively
small influence on the limiting oxygen concentration for inerting. Therefore, at an ini-
tial temperature of 50°C and nitrogen as inert gas, the values were 11.8 vol% for a
median particle size of 19 pm and 12.4 vol% for 52 pm.
The results in Figure 1.44, produced by Walther and Schacke (1986), show that the
maximum permissible oxygen concentrationfor inerting clouds of a polymer powder was
independent of the initial pressure over the range 1-4 bar (abs). For oxygen concentra-
tions above this limit, the data in Figure 1.44 can be represented by the simple approx-
imate relationship:

P,,,[bar(g)] = 0.35 %[bar(abs)] (vol%O,) (1.12)

where Po is the initial pressure.

4 bar

Figure 1.44 The influence of oxygen content in
m3c nn thn rn~virnr~mnvnlncinn nrncciirn fnnp 2 nnh,.
mer powder for various initial pressures in a 7 m3
0 10 20 30 closed IS0 vessel (From Walther and Schacke,
OXYGEN CONTENT IN GAS [vo~.Yo1 7986).

Figure 1.45illustrates the influence of the oxygen content of the gas on the minimum
ignition temperature of a dust cloud. For <74 pm Pittsburgh coal dust, there is a sys-
tematic increase from 500°C in pure oxygen via 600°C in air to 730°C in 10 vol%
oxygen.
The influence of the oxygen content in the gas on the minimum electric spark ignition
energy of dust clouds is illustrated by the data in Figure 1.46 for a subatmospheric