Page 189 - Dust Explosions in the Process Industries

162 Dust Explosions in the Process Industries

Figure 2.3 Provisional lightweight wall acting as vent during the wheat grain dust explosion in
Stavanger'in June 1970 (Courtesy of Egil Eriksson).

2.3.2
WHEAT GRAIN DUST, NEW PART OF STAVANGER PORT SILO,
OCTOBER 1988

The explosion was described by Olsen (personal communication from 0. Olsen, 1989).
Because of effective mitigation by explosion suppression and venting, both the extent
of and damage caused by the explosion were minor. There were neither fatalities nor
injuries. The incident deserves attention, however, because the chain of events leading
to explosion initiation was identified and the incident illustrates that proper measures for
explosion mitigation are effective.
The explosion occurred in a bucket elevator head immediately after termination of
transfer of Norwegian wheat grain between two silo cells. At the moment of explosion,
the transport system was free of grain. In this new part of Stavanger Port Silo, the bucket
elevator legs are cylindrical and mounted outdoors, along the wall of the head house. A
number of vents are located along the length of the legs. The vent covers on the eleva-
tor legs involved were blown out, which undoubtedly contributed to reducing the extent
of the explosion. There was no significant material damage, either by pressure or by heat.
Figure 2.4 illustrates the head of the bucket elevator in which the explosion occurred.
Because of a slight offset, the steel cover plate for the felt dust seal for the pulley shaft
touched the shaft and became heated by friction during operation of the elevator. The
hot steel plate, in turn, ignited the felt seal, from which one or more glowing fragments
dropped into the wheat grain dust deposit on the inclined surface below and initiated
smoldering combustion in the deposit. Figure 2.5 shows the burned, charred felt seal when
investigated just after the explosion. Just after the elevator had stopped, presumably
still enough dust was in the air to be ignited by the smoldering dust and be able to prop-
agate a flame. Alternatively, some of the smoldering dust may have slid down the inclined
surface and become dispersed into an explosible dust cloud. Just after the explosion, some
smoldering dust was still on the inclined plate below the elevator pulley.