Page 37 - Dust Explosions in the Process Industries

10 Dust Explosions in the Process Industries

1 mm LAYER OF DUST
OF BULK DENSITY

Figure 1.6 The potential hazard of even thin dust layers. A I-mm layer of a dust of bulk density
500 kg/m3 (a) generates a cloud of average concentration 100 g/m3 if dispersed in a room of 5 m
height (b).Partial dispersion up to only 7 m gives 500 g/m3 (c).
shows that the simple relationship between the bulk density of the dust layer, Pbulk; the
layer thickness, h;the height, H, of the dust cloud produced from the layer; and the dust
concentration, c, is
h
= Pbuk (1.10)

If a dust layer of thickness h on the internal wall of a cylindrical duct of diameter D
is dispersed homogeneously over the whole tube cross section, one has

4h
= Pbulk 5 (1.11)

In the case of a tube diameter of 0.2 m, typical of many dust extraction ducts in indus-
try, a layer thickness of only 0.1 mm is sufficient for generating a dust concentration of
1000 g/m3with a dust of bulk density 500 kg/m3.
In general, dispersibledust layers in process plants represent a potential hazard of exten-
sive secondarydust explosions,which must be reduced to the extent possible. Figure 1.7
illustrates how secondary explosions in workrooms can be generated if preventive pre-
cautions are inadequate.

1.I .4
IGNITION SOURCES

1.I .4.1
Background

A combustible dust cloud will not start to burn unless it becomes ignited by a source of
heat of sufficient strength. The most common ignition sources are
Smoldering or burning dust.
Open flames (welding, cutting, matches, etc.).
Hot surfaces (hot bearings, dryers, heaters, etc.).
Heat from mechanical impact.
Electrical discharges and arcs.