Page 553 - Dust Explosions in the Process Industries

520 Dust Explosions in the Process Industries

two factors tend to cancel each other, and this may be the reason for the surprisingly good
agreement obtained in some cases between MEC values from the traditional small-scale
lab test and large-scale experiments. For example, Jacobson et al. (1961) found that var-
ious grain dusts and starches all had MECs on the order of 50 g/m3in the small lab-scale
test, which compares favorably with the value 60 g/m3found for a typical wheat grain dust
containing 10% moisture in industrial-scale experiments by Eckhoff and Fuhre (1975).
However, such good agreementbetween the small-scaletest and large-scale conditions
would not be expected to be the general rule. For this reason, considerableefforts have
been made in severalcountriesduring the 1980sto develop an improved test for the MEC.
In the United States, Hertzberg et al. (1979) at the Bureau of Mines first developed an
8 liter explosionvessel in which transient dust clouds of quite homogeneous concentration
distributions could be generated. An important conclusion from these studies was that
determinationof true MEC values requires a strong ignition source.Therefore,Cashdollar
and Hertzberg (1985) subsequentlydeveloped a 20 liter explosionvessel that would yield
meaningful results even with quite strong ignition sources.
A cross section of the 20 liter vessel is shown in Figure 7.43. A photograph of the
opened vessel, showing one of the light attenuation probes for measuring the dust con-
centration development in the transient dust cloud, is given in Figure 7.44.

PGH-53
1012
Dispersion
air
Figure 7.43 Cross section of US.Bureau of Mines'2O liter explosion vessel for determination of the
minimum explosible concentration and other parameters of explosible dust clouds (From Cashdollar
and Hertzberg, 7985).