Page 165 - Dust Explosions in the Process Industries

138 Dust Explosions in the Process Industries

hydrocarbon gases in air is about 35 g of gas per m3 of air and gas phase combustion
is the basic flame propagation process for organic dusts, the value of 15g/m3seems unre-
alistically low. Eckhoff et al. further found that, in up to 50 wt% of noncombustibles,
the minimum explosibledust concentration increased systematicallywith increasing pro-
portions of noncombustiblesin the powder, in such a way that the minimum explosible
concentration of the combustible fraction was constant, in the range of 32-35 g/m3.A
dust containing 50 wt% noncombustibles, therefore, had a minimum explosible con-
centration of 65-70 g/m3.
Both Bartknecht (1986) and Eckhoff et al. (1988) observed that some coating pow-
ders had exceptionally low minimum electric spark ignition energies, of <3 mJ.
CENELEC (1989) issued a comprehensiveEuropean standard for electrostaticpowder
coating, where keeping the dust concentration in the spraying cabinet and dust extraction
system below the minimum explosible concentration was reintroduced as a central pre-
ventive measure. Another preventive measure was use of antistatic materials to avoid
accumulation of electrostatic charge. Mitigating measures included interlocking sys-
tems and use of noncombustible construction materials.

1.5.3.6
Aluminum and Magnesium Powder and Dust

The fire and explosion hazards associated with production and handling of aluminum
and magnesium powders has been the subject of extensive research for many years. As
for metal powders in general, the hazard increases with decreasing particle size, right
down into the range below 1 pm. Dust clouds in air of very fine aluminum and mag-
nesium powders have exceptionally low electric spark minimum ignition energies and
produce exceptionally violent explosions (see Appendix 1j. On the other hand, coarser
aluminum powders, such as particle diameters of 100 pm, present only a moderate
explosion hazard. However, if a comparativelycoarse aluminum powder contains a fine
dust fraction, even if it represents only a few percent by mass, the explosion hazard is
considerably increased. For metal dusts like aluminum, it is particularly true that keep-
ing a watch on the explosion hazard to a large extent means keeping a watch on parti-
cle size.
Beck, Foerster, and Faber (1984) discussed the prevention and mitigation of dust
explosions in aluminum grinding plants. By using wet grinding (e.g., water), the alu-
minum particles can be collected as a slurry and the dust explosion problem eliminated
altogether.Alternatively, the grinding operationitself can be dry,with the fine metal dust
collected in a liquid either immediately after the grinding point or in a separate wet col-
lector further downstream.In general, the need for measures to prevent and mitigatedust
explosions depends on the extent to which the process is dry.
Beck et al. (1984) recommended several types of measures, adapted to the nature of
the actual process. The list included interlocking systems to prevent grinding without dust
extraction or sufficientliquid (water) supply,location of fans in dust-fi-eeareas, prevention
of mechanical and electric sparks and hot surfaces, no smoking, and good housekeep-
ing (cleanliness) in the workrooms.
Reinke (1987) described the safety measures taken in a plant for the production of com-
paratively coarse atomized aluminumpowder (63-1200 pm). The fine fraction, <63 pm,
representing the most severe explosion hazard, was separated out in an air jet filter.