Page 393 - Dust Explosions in the Process Industries

362 Dust Explosions in the Process Industries

Kauffman et al. found that, in general, for a given mass of dust layer per unit length
of tube, a uniform layer around the entire tube wall produced the most violent explo-
sions. The dusts tested included a maize starch, a mixed natural organic dust, a wheat
grain dust, and an oil shale dust. Various ranges of particle sizes and moisture contents
were investigated.
The strength of the primary explosion was varied by varying the initial pressure of the
hydrogen/oxygen mixture and the initial quantity of dust dispersed from the V-channel.
It was generally found that, for a given mass of dust per unit length of the main tube, the
maximum pressure, temperature, and flame speed of the secondaryexplosion increased
with the strength of the primary explosion. Figure 4.68 shows how the nominal mini-
mum and maximum explosible concentrations (mass of dust layer/m3tube) varied with
the strength of the primary explosion in terms of its maximum overpressure.Assuming
a bulk density of the dust layer of 0.5 g/cm3,a nominal concentration of 1000 g/m3cor-
responds to layer thicknesses of 0.15 mm if all the tube wall is covered and 1.6 mm and
11 mm for 90 mm and 12.5 mm layer widths, respectively.

MINIMUM EXPL. CONC.
R
0 90 mrn LAYER WIDTH

3 MAXIMUM EXPL.CONC
Ln
0
+ 12.5 mm LAYER WIDTH
a 750 t 0 90 mm LAYER WIDTH /'' RANGE
I
x
EXPLOS,BLE
A LAYER ON ENTIRE TUBE WALL /A/
0 z

1.0 1.1 1.2 1.3 1.4 1.5 1.6
STRENGTH OF PRIMARY EXPLOSION [barlgll

Figure 4.68 Nominal minimum and maximum explosible concentrations for secondary explosions in
a horizontal tube oflength 36 m and diameter 0.30 m, as a function of the strength of the primary explo-
sion, using mixed organic dust of <74,urnparticle size and 12% moisture (From Kauffman et ai., 1984).

Figure 4.68 shows that the explosible concentrationrange expands at both ends as the
strength of the primary explosion increases. There is a tendency of the range to shift
toward higher nominal concentrations as the dust layer becomes concentrated in a narrow
strip. The minimum explosible concentrationof 50 g/m3at an explosion strengthof 1.53
bar(g) is close to the value measured by Eckhoff and Fuhre (1975) for wheat grain dust
of 12% moisture, in an entirely different large-scale apparatus.