Page 46 - Dust Explosions in the Process Industries

Dust Explosions:An Overview 19

e Use of inspection windows of glass or Plexiglass in pneumatic transport pipes.
0 Continuous impact of powder particles onto an insulating surface (e.g., a coated dust
deflector plate in the cyclone of a dust separator).
e Fast movement of conveyor or transmission belts made of an insulating material or a
conductive material coated with an insulating layer of high dielectric strength.
e Filling of large containers or silos made of insulating material (e.g., flexible inter-
mediate bulk containers) or metallic containers or silos coated internally with an insu-
lating layer of high dielectric strength.
Discharge along the surface of powder or dust in bulk may occur if nonconducting
powders are blown or poured into a large container or silo. This is a fifth type of elec-
trostatic discharge. When the charged particles settle in a heap in the container, very high
space charge densities may be generated and luminous discharges may propagate along
the surface of the powder heap, from its base to its top. However, theoretical callcula-
tions by Glor (1985) revealed that, under realistic industrial conditions, only very large
particles, of 1-10 mm diameter, are likely to generate spark discharges due to this
process. It further seems that very high specific electrical resistivity of the powder is also
a requirement (>loloQ-m), which probably limits this type of discharge to coarse plas-
tic powders and granulates. Because of this large size, the particles generating the dis-
charge are unlikely to cause dust explosions, and therefore a possible explosion hazard
must be associated with the simultaneouspresence of an explosible cloud of an additional,
fine dust fraction. The maximum equivalent spark energy for this type of discharge has
been estimated on the order of 10mJ, but still little is known about the exact nature and
incendivity of these discharges. Glor (1988) pointed out that the probability sf occur-
rence of such discharges increases with increasing charge-to-mass ratio in the powder
and increasing mass filling rate.
.Ligghtning-type discharge,which may in principle occur within an electrically insulating
container with no conductive connection from the interior to the ground, was the last type
of discharge mentioned by Glor (1988) and Luttgens and Glor (1989). However, as Glor
stated, there is no evidence that lightning discharges have occurred in dust clouds gen-
erated in industrial operations. Thorpe et al. (1985) investigated the hazard of electro-
static ignition of dust clouds inside storage silos in a full-scale pneumatic conveying and
staring facility. Sugar was used as test dust. They were able to draw some spark discharges
from the charged dust cloud, but these were of low energy and incapable of causing igni-
tion. In fact, these spark discharges were not able to ignite even a propane/air mixture
of minimum ignition energy less than 1mJ.
Figure 1.15 provides an overall comparison of the equivalent energy ranges of the var-
ious electrostatic discharges just discussed and typical ME ranges for gases/vapors and
dusts in air. Equivalent energy, introduced by Gibson and Lloyd (1965), is defined as the
energy of a spark discharge that has the same igniting power as the actual electrostatic
discharge.
Further details on the generation and nature of the various types of electrostatic dis-
charges are given by Glor (1988) and Luttgens and Glor (1989). Some further details con-
cerning electric sparks and their ability to ignite dust clouds are given in Chapter 5.
Appendix 2 gives some MIE values, determined by a standardized method, for vari-
ous dusts. Further information on ignition of dust clouds by electric sparks and electro-
static discharges is given in Sections 1.4.2.7, and 9.2.3.4 and 9.3.5.4 in Chapter 9.