Page 440 - Dust Explosions in the Process Industries

Ignition of Dust Clouds and Dust Deposits 407

5.3.3
lNFLUENCE OF SPARK DISCHARGE DURATION ON THE MINIMUM
ELECTRIC SPARK IGNITION ENERGY FOR DUST CLOUDS

5.3.3.1
Displacement of Dust Particles by Blast Wave from Spark Discharge

The strong influence of the spark discharge duration on the minimum spark energy for
ignition of dust clouds was probably first discoveredby Boyle and Llewellyn (1950).They
were able to demonstratethat the minimum capacitor energy l/2CV2,C being the capac-
itance and V the initial capacitor voltage, capable of igniting clouds of various powders
in air decreased quite considerably when a series resistance was introduced in the dis-
charge circuit. Some results obtainedby Boyle and Llewellyn are shown in Figure 5.13.

90% < 76 pm GRANULAR ALUMIN

0.4

0.2

0.0
10 10' io3 IO4 105 106
SERIES RESISTANCE IS21

Figure 5.1 3 Resulb fromignition of dust clouds by capacitivesparks, using an additionaiseries resist-
ance in the discharge circuit (Data from Boyle and Llewellyn, 1950).

As can be seen, the minimum 'lzCV2for ignition decreased by a factor of about 10
both for granular aluminum and magnesium, when a series resistance of 104-105Q was
added to the discharge circuit. Similar trends were also found by these workers for dust
clouds of ferromanganese, zinc, silicon, and sulfur.
Boyle and Llewellyn expressed their results in terms of stored capacitor energy '/2CV2.
However, a large series resistance in the spark dischargecircuit, during discharge, absorbs
a significant fraction of the capacitor energy, so that the energy delivered in the spark
gap is considerably lower than l/2CV2.This fractionhas been determined experimentally
by various workers, as shown by Eckhoff (1975). From independent investigations, it
can be concludeld that, with the capacitances used by Boyle and Llewellyn and using a
series resistance in the range 104-107 Q, the net spark energies were only on the order
of 5-10% of the stored capacitor energy l/2CV2.
This, in turn,means that, in the experiments of Boyle and Llewellyn, an inclusion of
a series resistance of 104-105Q in the discharge circuit reduced the minimum net spark