Page 196 - Dust Explosions in the Process Industries

Case Histories 169

Figure 2.1 1 Dust accumulation on the floor in the head house of a grain silo plant in the United
States (Courtesy of C. W. Kauffman, University of Michigan).

a dust layer only 0.5 mm thick may propagate a dust flame when being entrained by the
blast wave preceding a propagating dust flame. This experience has been transformed
into a simple rule of thumb, saying that, if footprints are visible, the dust layer is unac-
ceptably thick.
During the explosion at St. Paul, the flame front and pressure wave from the primary
cross-tunnel explosion also traveled into the three tunnels under the grain storage tanks.
However, these tunnels were clean, the blast was unable to pick up sufficient dust to sus-
tain the flame propagation, and the explosion dissipated. However, the pressure wave con-
tinued down the three tunnels, sweeping away objects in its path, and finally damaging
the aeration fans before venting itself to the atmosphere at the tunnel ends.

2.4.3
TRAIN-LOADING COUNTRY GRAIN TERMINAL AT FONDA,
IOWA, JULY 15, 1980

This explosion, which occurred in the early afternoon, caused neither fatalities nor
injuries. The material loss was modest, estimated at US$0.03 million.
The probable cause of the explosion was electrical welding on a bucket elevator.
However, the ignition source was not the welding spot itself, but probably a hot spot in
the casing of the elevator boot caused by poor electrical contact between the grounding