Page 201 - Dust Explosions in the Process Industries

174 Dust Explosions in the Process Industries

Figure 2.1 6 The building of the fish meal factory in Norway afflicted with a dust explosion in 1975.

designed, and severe flow problems were encountered when attempting to discharge the
fish meal by means of the screw conveyors at the hopper outlets. Therefore, the use of
the silos as buffer stores had to be abandoned. But instead of feeding the output from
the hammer mills directly to the screens, the long transport loop via the large silos was
maintained, the silos being mostly empty because of the large capacity of the screws at
the hopper outlets. Nevertheless, arching problems still occurred across the hopper outlet
just above the screw conveyors, and breaking such arches became part of the regular duties
of the staff operating the plant.
Although the grinding of the fish meal in the hammer mills in the loft produced large
quantities of fine dust, no dust extraction system had been installed. As a consequence,
the interior of the three large, empty silos acted as dust collectors, and considerable
quantities of dust accumulated on the internal walls. Furthermore, appreciable amounts
of dust escaped to most other parts of the building. In addition to having a much larger
specific surface area than the main fish meal product, in periods of hot and dry weather.
as on the day of the explosion, this fine dust would become quite dry. Because of the heat
liberated by the production process itself, the temperature in the loft of the silo building
would frequently be in the range 25-30°C. On the exceptionally hot day of the explo-
sion, the temperature in the loft in the middle of the day was 45°C.
One particular feature of the screw conveyors of this plant was that the bolts fixing
the screw blades to the shaft (bolts of lengths 110-120 mm and diameters 12-16 mm)
broke fairly regularly, presumably as a result of material fatigue. Figure 2.17 shows part
of one of the screws with three bolt heads.
In spite of frequent bolt failures, the plant made no provision for trapping tramp metal, such
as broken bolts, before it reached the hammer mills. Neither were there any instructions for
controlling the screws to replace defective bolts in advance. As a consequence, the entrance