Page 43 - Dust Explosions in the Process Industries

7 6 Dust Explosions in the Process industries

ways, depending on the circumstances. Glor (1988) and Luttgens and Glor (1989) dis-
tinguished among six different types of electrostatic discharges:
Spark discharge.
Brush discharge.
Corona discharge.
Propagating brush discharge.
* Discharge along the surface of the powder or dust in bulk.
Lightninglike discharge.
The differentiationamong the various dischargetypes is not always straightforward,but
Glor’s classificationhas turned out to be very useful when evaluating electrostatic haz-
ards in practice in industry.
Spark discharges and propagating brush discharges are by far the most hazardous types
of the six with regard to initiating dust explosionsin industry. Spark dischargesoccur when
the charge is accumulated on an electrically conducting, nongrounded, object and dis-
charged to ground across a small air gap. The gap distance must be sufficiently short to
allow breakdown and spark channel formation at the actual voltage difference between
the charged object and ground. On the other hand, for the spark to become incendiary,the
gap distance must be sufficiently long to permit the required voltage difference to build
up before breakdown of the gap. The theoretical spark energy, neglecting external circuit
losses, equals l/2CV2,where Cis the capacitanceof the nongrounded, charged process item
with respect to ground, and V is the voltage difference. Figure 1.13illustrates a practical
situation that could lead to a dust explosion initiated by an electrostatic spark discharge.

ELECTRIC NONCONOUCTOR

ELECTRICALLY CONDUCTING
MATERIAL. GROUNOED

Figure 1.I 3 A practical situation that could lead to a dust explosion initiated by an electrostatic spark
discharge.