Page 388 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 357

between the Ks, value, as determined in agreement with the recommendation by the
International Standards Organization (1985), and the violence of the explosions in the
tubes.
The aluminum dust was comparatively coarse, having a median particle diameter on
a mass basis of 30 pm, with 10% > 100 pm and 10%> 20 pm. Nevertheless, a maxi-
mum flame speed of 2200 m/s and maximum explosionpressure of 25 bar(g) was meas-
ured in a 20 m long pipe, with enlarged diameter in the ignition zone for increasing the
initial "push" and establishinga high level of turbulenceand burning rate at an early stage.
The explosion pressures were measured by piezoelectric sensors and were those acting
normal to the tube wall, that is, normal to the direction of propagation. There are rea-
sons to believe that the 2200 ds phenomenon observed was in fact a proper detonation
(see Section 4.5).
The coal dust produced maximum flame speeds of only up to 250 m/s and maximum
explosion pressures on the order of 1bar&). The median particle size was 22 prn, with
10% > 60 pm and 10% < 5 pm (extrapolation of data). The volatile content was not
specified.
Barthecht attributedthe comparatively slow coal dust explosionsto the relatively small
tube diameter of 0.4 m. He also conducted coal dust explosion experiments in a much
larger one-end-open tube, of diameter 2.5 m and length 130 m, with ignition at the
closed end by a pocket of methane/air. With 250 g/m3dust, maximum flame speeds of
up to 500 m/s were measured.With 500 g/m3,the maximum flame speeds were 700 m/s
or more.
Bartknecht further conducted experiments where the dust was spread as a layer along
the tube floor in a quantity correspondingto 250 g/m3if dispersed homogeneously over
the whole tube cross section. When using the same ignition source (turbulent methane/air
explosionat the closed tube end) as with the predispersed clouds with which he normally
worked, he found lower flame speeds and explosion pressures than with predispersed
clouds. However, Figures 4.61 and 4.62 show that the layer-spreading technique can
indeed give very high flame speeds if only the initiatingblast is sufficientlyviolent.This
illustrates that choosing conditions of experimentation that correspond to the actual
industrial hazard is an important aspect of applied dust explosion research.
Pineau and Ronchail(l982) and Pineau (1987) described experimental research on the
propagation of wheat and wood dust explosions in ducts of diameters from 25 mm to
700 mm. They pointed out that, in any industrial installation where dust extraction or
pneumatic transport of powdered material is used, a number of ducts will be connected
to either blowers, fans, or pumps. In addition, the arrangementsmay include cyclones,
bag filters,hoppers and bins, and other process equipment, some of which may be inter-
connected by pressure balance ducts. It is therefore essential, in the case of explosible
powders and ducts, to understand the mechanisms by which dust explosions may prop-
agate in dusts. In addition to straight ducts, ducts containing bends also need to be con-
sidered, because such bends are frequent in the process industry.
In one series of experiments reported by Pineau and Ronchail(1982), straight tubes
of diameters from 250 mm to 700 mm and lengths from 12 m to 42 m were used. The
tubes were either closed at both ends, closed at one end and fully or partly open at the
other, or fully or partly open at both ends. In some experiments, the ignition point was
at a closed tube end; in others, near an open end. In one experimentit was midway down
the tube. The dust was initially distributed as a layer along the tube floor, the quantity