Page 203 - Dust Explosions in the Process Industries

176 Dust Explosions in the Process Industries

Figure 2.1 8 View of hammer mill no. 1, in which the fish meal explosion started, the door in the
mill chute, and the manhole in the floor. The manhole acted as vent for the explosion in silo no. 1.

It seems highly probable that the ignition took place in hammer mill no. 1 and the
ignition process was closely related to the presence of tramp metal in the mill. It seems
unlikely that sparks struck between two steel objects would be able to ignite clouds
of the fish meal in the mill. However, a metal object can be heated, even to glowing,
by repeated impact or friction and thus act as a hot surface for direct initiation of dust
explosion in a cloud. It is not unlikely that this latter process was in operation in the
actual case, because after the explosion it was discovered that a 14 x 7 mm strip of
steel was wedged into one of the 3 mm slots of the bottom screen plate of mill no. 1.
In view of the high rotation speed of the mill (25 reds), such an object could easily
have been heated to appreciable temperatures by repeatedly being struck by one of the
mill hammers.
This accident shows that normal fish meals can, under unfavorable circumstances, give
rise to quite severe dust explosions, even though the explosions produced by such mate-
rials in standard laboratory tests are relatively weak compared with those produced by
many other dusts. Because the pressure needed to blow up very weak structures, like the