Page 223 - Dust Explosions in the Process Industries

Case Histories 195

deformed bidirectionally, as if an explosion in the central part had expanded violently
in both directions. This evidence was considered a strong indication of the explosion
having been initiated inside the steel tube surrounding the screw.
The blast and flame from this primary explosion, in turn, generated and ignited a
larger dust cloud in the main space inside the mixer; and finally the main bulk of the
powder in the mixer was thrown into suspension and ignited when the mixer ruptured,
giving rise to a major explosion in the workrooms.
Subsequent investigations at the Chr. Michelsen Institute, Bergen, Norway, revealed
that clouds in air of the fine aluminum flake powder was both extremely sensitive to igni-
tion and exploded extremely violently. The minimum electric spark ignition energy was
on the order of 1 mJ, and the maximum rate of pressure rise in the Hartmann bomb, 2600
bark. Both values are extreme. The thickness of the aluminum flakes was about 0.1 pm,
which corresponds to a specific surface area of about 7.5 m2/g (see Section 1.1.1.3 in
Chapter 1).
The investigation further disclosed that the design of the nitrogen inerting system of
the mixer was inadequate. First, the nitrogen flow was insufficient to enable reduction
of the average oxygen concentrationto the specified maximum level of 10 vol% within
the time allocated. Second, even if the flow had been adequate, both the nitrogen inlet
and the oxygen concentration probe were located in the upper part of the vessel, which
rendered the measured oxygen concentration unreliable as an indicator of the general
oxygen level in the mixer. It is highly probable that the oxygen concentration in the lower
part of the mixer, and in particular in the space inside the tube surrounding the screw,
was considerably higher than the measured value. This explains why a dust explosion
could occur in spite of the use of a nitrogen inerting system.
The final central concern of the investigators was identification of the probable igni-
tion source. In the reports from 1973, it was concluded that the primary explosion in
the tube surrounding the screw was probably initiated by an electrostatic discharge.
Mowever, this conclusion was not qualified in any detail. In more recent years, the
knowledge about various kinds of electrostatic discharges has increased considerably
(see Section 1.1.4.6).It now seems highly probable that the ignition source in the 1973
Gullaug explosion was a propagatingbrush discharge,brought about by the high charge
density that could be accumulated on the internal rubber lining of the steel tube sur-
rounding the screw, because of the grounded electrically conducting backing provided
by the steel tube itself. The discharge could then have occurred through a hole in the
lining (see Figure 1.14).

2.12.2
ATOMIZED ALUMINUM POWDER PRODUCTION PLANT
AT AWGLESEY, UNITED KINGDOM, IN 1983

This accident was discussed in detail by Lunn (1984), and the following brief summary
is based on Lunn’s account.
The explosionoccurredon a Saturdayevening in July 1983.Only three employeeswere
working on the site at the time of the explosion. Two of these were injured whereas the
third escaped unhurt. The plant was substantiallydamaged. Figure 2.33 shows the basic
layout of the plant.