Page 204 - Dust Explosions in the Process Industries

Case Histories 777

wooden floor of the loft, is low, even a modest dust explosion is hazardous under such
circumstances. The housekeeping was very poor. There was neither any dust extraction
system nor any routine for frequent and regular removal of the considerable amounts of
fine dust accumulated throughout the plant and building. This dust certainly was the main
source of the extensive secondary explosion and fire sweeping through the entire loft.
Because of the dry weather, the dust moisture content was probably low and the dust easy
to entrain and disperse. The process design was inadequate, in that the large silos below
the hammer mills did not serve their purpose and merely acted as large potential dust
explosion bombs.
Also prevention of potential ignition sources was inadequate. In the grinding plant,
no provision was made for removing foreign metal objects before they entered the
hammer mills. Allowing tramp metal into the hammer mills at all created a considerable
risk of potential ignition sources being introduced. Furthermore, the procedure for remov-
ing broken conveyor screw bolts from the hammer mills by opening the door in the mill
chute was indeed questionable.
Hence, the three key ingredients needed for generating a serious dust explosion were
present: large enclosures that were empty apart from explosible dust clouds, large quan-
tities (of dust throughout the entire building, and an ignition source.

2.6
NG GAS EXPLOSION IN A SILO PLANT
GER, NORWAY, IN NOVEMBER 1985

This accident, described by Braaten (1985), was not primarily a dust explosion but an
explosion of combustible gases released from a solid organic material during self-heat-
ing in a silo cell. At first glance, such an event may seem out of place in the context of
dust explosions. However, smoldering combustion is most often related to powders and
dusts; therefore, the initial smoldering gas explosion, in most cases, entrains combustible
dust and the explosion can easily develop into a normal dust explosion.
The cause of events was in accordance with Figure 1.9 in Chapter 1. The explosion
occurred in a fairly modern reinforced concrete silo complex used to store various feed-
stuffs. Pellets of Canadian rape seed flour had been stored in one of the silos for some
time, when it was discovered that the material in the bottom part of the silo had become
packed to a solid mass and could not be discharged through the silo exit. Some time
later, one week before the explosion, flames were observed in the silo. The fire brigade
was called and covered the pellets in the silo with foam from above. Various unsuc-
cessful attempts were then made at discharging the pellets mass at the silo bottom.
During this phase there was considerable development of smoke, which mixed with
the air, not only in the silo cell in question but also in the silo loft above the cells. It
is probable that the smoke contained combustible gases, such as 60, and that the
strong explosion, which occurred just after the top of the pellets had been covered with
foam once more, was mainly a gas explosion. However, any dust deposits in the loft
may also have been involved. The entire roof of the building was blown up, and debris
was thrown into the surrounding area (see Figure 2.19). Because the explosion occurred
in the middle of the night (3:OO A.M.), just after the fire brigade had left, nobody was
killed or hurt.