Page 161 - Dust Explosions in the Process Industries

734 Dust Explosions in the Process Industries

in Chapter 6 provides some evidence. Pinkwasser and Haberli (1987) described specific
designs of relief panels in the roof of large silo cells.
Bucket elevators are well-known sites of primary dust explosions. Wilcoxen (1981)
reported on an actual dust explosion in a grain storage facility in which the bucket ele-
vators were in part located outdoors and fitted with explosion vents. Due to the vents in
the elevator legs, the extent of the explosion and resulting damage was comparatively
minor. It was concluded that the design adopted had proven successful.
The French organizationfor standardization,Afnor (1986), issued a recommendation
for mitigating dust explosions in the grain, feed, and flour industry by venting. However,
in view of the fast development in the field, one may wish to revise the recommenda-
tion at some points by including recent experimental evidence.
The ignitability and explosive characteristicsof dusts influence the choice and design
of means of dust explosion prevention and mitigation. Ignitability and explosibility,in
turn, depend on basic dust chemistry, effective particle size (see Section 1.3.3), and
moisture content. Contributionselucidatingvarious relationships for grain, feed, and flour
dusts were written by Hartmann, Cooper, and Jacobson (1950); Jacobson et al. (1961);
Eckhoff (1977/1978);Eckhoff and Mathisen (1977/1978);Enright and Bullock (1983a,
1983b); Chiotti and Yoshizaki (1983); and Ambroziak (1985)

1.5.3.2
Milk Powder, Fish Meal, and Sugar

According to Beever (1984), the number of reported fires and explosions in operations
involving spray drying of milk increased during the early 1980s. Spray drying of milk
is known in particular to generate self-heatingand charring of the dried product. In milk
spray dryers, there will always be some regions containing explosible dust clouds. The
question is only whether an ignition source is also present. Self-heating and charring in
deposited dried milk powder can generate effective ignition sources, and Beever (1984)
concluded that glowing or burning powder deposits were the most likely source of igni-
tion in milk spray dryers. She estimated the minimum thicknesses for self-ignition of
deposits of various types of milk powders at 7-14 mm for 200°C ambient temperature
and 100-320 mm for 100°C.
Following an extensive explosionin a milk spray dryer in France, Pineau (1984b, 198s)
conducted a comprehensive study of the self-ignitionproperties of milk powders and their
ignitability and explosive properties in cloud form.
Fish meals constitute anotherproduct group that can give rise to dust explosions.One
example is given in Chapter 2. The self-heatingproperties of fish meals as functions of
meal chemistry,moisture content,and so forth were discussedby Dreosti (1980). Eckhoff
(1980) gave some further data for the ignitability and explosibility of fish meals.
In a series of full-scale sugar dust explosion experiments in the dust removal plant of
a disused sugar factory, Scholl(l973) demonstratedthe potential of this material to give
rise to serious dust explosions. Meek and Dallavalle (1954) tried to correlate explosi-
bility properties of various types of sugar (C18.C1,, and C,) with molecular structure and
particle size. However,possibly due to a very weak ignition source and nonhomogeneous
dust clouds, this was only partly successful.