Page 673 - Dust Explosions in the Process Industries

640 Dust Explosions in the Process Industries

A marked increase of the explosive range was found in the 30 liter bomb when increas-
ing the ignition energy from 1 to 10 kJ.This effect was practically absent in the 1 m3
vessel in the ignition energy range 4-20 kJ.Tian, He, and Zhang (1994), using a 20 liter
explosionbomb, found that the minimum explosive concentrationsof coal dusts decreased
by a factor of 2 or more when the ignition energy was increased from 2.5 to 10 kJ.Xu
et al. (1994) also arrived at the conclusion that the apparent MEC determined in a 20 liter
bomb depends markedly on the ignition energy. It was found that 10 kJ would be too
high to yield realistic results.
All this suggests that, ideally, limiting conditions for flame propagation should be
determined in apparatuses of sufficientvolume to prevent a significant influence of even
quite strong ignition sources on the main phase of flame propagation. In Europe, the stan-
dards organization CEN adopted the standard (ISO) 1 m3bomb for this kind of tests.
Zhou, Zhang, and Yu (1994b) proposed an alternativeprocedure for determining the
MEC in closed-bomb explosion experiments. Their experimental evidence indicated
that, at MEC, the time interval from ignition to the pressure peak has its highest value.
They suggested that this criterion be used instead of some arbitrary pressure rise crite-
rion of explosion.
Chawla, Amyotte, and Pegg (1996) compared two different procedures (ASTM and
IEC) to determine the MEC in closed 20 liter bomb tests. Because of an excessiveigni-
tion energy of 10 kJ,the IEC procedure gave unrealistically low MEC values, whereas
theASTM procedure, using 2.5 kJignition energy, produced MEC values in good agree-
ment with those found in experiments at a larger scale.
Sapko et al. (2000) found that tests in a 20 liter closed bomb gave quite good predic-
tions of the MEC values actually found in large-scalemine gallery dust explosion exper-
iments with dusts of bituminous coals, gilsonite, oil shales, and sulphide ores. The
ignition source in the 20 liter tests was a 2.5 or 5 kJ pyrotechnical ignitor.

9.4.5
MISCELLANEOUS

Tian et al. (1994), using a 20 liter explosion bomb, found that, when using a 2.5 kJigni-
tor, and adding 2 vol% methane to the air, the minimum explosive dust concentration
dropped by at least a factor of 2 compared with the values for dust in air. This “hybrid”
effect had been studied previously by several other workers (see Chapter 1).
Hertzberg et al. (1992b) determined a range of dust explosibilityparameters for nine
dusts of solid explosives when dispersed as clouds in air in a closed bomb. In the low-
concentration range (5400 g/m3),the dusts behaved as dusts of normal carbonaceous and
plastic materials. At higher concentrations, they became more hazardous, starting to
exhibit genuine explosives properties.
Cashdollar (1994) measured the explosibility parameters of clouds in air of a range
of metal dusts, silicon, boron, and carbon, using the ASTM 20 liter bomb. The mea-
sured parameters were the minimum explosive dust concentration (both 2.5 and 5 kJ
ignition energy), P,,, and (dP/dt),,. In addition, nominal stoichiometric dust concen-
trations and nominal adiabatic flame temperatures were calulated.
Wang and Zhang (1994) determined the minimum ignition energy, minimum explo-
sive concentration, and maximum explosion pressure for clouds of TNT dusts in air.