Page 194 - Dust Explosions in the Process Industries

Case Histories 167

The explosion probably started in a dust cloud in a silo cell used for the receipt and
delivery of grain. The probable ignition source was an electric arc between the electric
wires of the lower-level indicator in the silo. Repeated filling and discharge of grain had
pulled the level indicator from the wall and the electric arc occurred between the bare
wires that had been pulled out of their conduit.
Severe structural damage occurred to almost all the silos in the head house and mod-
erate damage to most of the head house structure. Most of the head house silo roofs were
blown up, destroying the spout floor and the top of the cleaner floor. Rupture of the silos
around the edge of the head house caused failures in the outside wall. The casings of all
bucket elevators, steel as well as concrete, had opened up in many places. A silo com-
plex comprising 18 cells suffered severe explosion damage to the gangway connecting
it to the head house, to the gallery, to the far end of the tunnel, and to a small group of
silos centered around an air shaft approximately one-third of the way along the gallery.
At the location of the air shaft, the gallery wall and roof had been completely destroyed.
Beyond this point the explosion damage to the gallery was still significant but not as
severe. The exterior concrete silo walls had been extensively shattered, in many places
leaving only the reinforcing rods. Figure 2.9 presents a detailed view down the air shaft
made after the grain was removed. As can be seen, the concrete fragments were quite
small and much concrete had been removed from the steel reinforcement. Concrete frag-
ments from this area of the plant had been thrown about a hundred meters into the adja-
cent railway yard.

Figure 2.9 Air shaft along the damaged walls of reinforced concrete silo cells of the grain terminal
at St. Joseph, Missouri, 1980 (Courtesy of C. W Kauffman, University of Michigan).