Page 45 - Dust Explosions in the Process Industries

18 Dust Explosions in the Process Industries

edges and needle tips. For this reason, such discharges occur at much lower field
strengthsthan the brush discharges, and the discharge energiesthereforeare much lower.
Consequently,the possibility of igniting dust clouds by corona discharges can be ruled
out.
Propagating brush discharges can, however, initiate dust explosions.Such discharges,
which normally have much higher energies than ordinary brush discharges, occur if a
double layer of charges of opposite polarity is generated across a thin sheet (4 mm
thickness) of a nonconducting material (Glor, 1988).The reason for the high discharge
energy is that the opposite charges allow the nonconductor surfacesto accumulatemuch
higher charge densities than if the sheet had been charged on only one of the faces.
Glor pointed out that, in principle, close contact of one face of the sheet with ground
is not necessary for obtaining a charged double layer. However, in practice, ground on
one side is the most common configuration. An example is illustrated in Figure 1.14.
Powder is transported pneumatically in a steel pipe with an internal electrically insu-
lating plastic coating. Due to the rubbing of the powder against the plastic, a charge
accumulates on the internal face of the plastic coating. The high mobility of the elec-
trons in the steel causes buildup of a corresponding charge of opposite polarity on the
outer face of coating in contact with the steel. If a short passage between the two
oppositely charged faces of plastic coating is provided, either via a perforation of the
coating, due to electrical breakdown, or at the pipe exit, a propagating brush discharge
can result.

r
POSSIBLE SPARK GAP VIA HOLE

Figure 1 .I 4 Illustration of practical configuration of pneumatic powder transport that can lead to
dust explosions initiated by propagating brush discharges.

Luttgens (1985) and Luttgens and Glor (1989) discussed a dust explosion in the
Federal Republic of Germany that was initiated by a propagatingbrush discharge.Acrylic
powder was transported pneumatically in a 50-mm diameter plastic pipe outdoors, and
the groundedelectrically conductingshield on the outer surfaceof the pipe was provided
by rainwater and snow.
Glor (1988) identified five typical situations that may lead to propagating brush dis-
charges during transport and handling of powders:
High-velocity pneumatic transport of powder through an electrically insulating pipe
or a conductive pipe with an insulating internal coating.