Page 159 - Dust Explosions in the Process Industries

132 Dust Explosions in the Process Industries

gas is feasible in batch mixers, whereas continuous mixers may preferably be designed
to withstand the pressure rise caused by a possible dust explosion.
Radandt (1969) discussed the dust explosion protection of mixing silos and contain-
ers and concluded that the choice of means to prevent and mitigate dust explosions
depends on the specific equipment and process design.

1.5.2.5
Powder and Dust Conveyors and Dust Removal Equipment

As Table 1.15 for conveyors and dust removal equipment shows, Noha recommended
the use of explosion-resistantconstruction whenever fast-moving mechanical elements
constitute part of the system. This, for example, applies to bucket elevators, which can
preferably be equipped with cylindrical, pressure-resistant legs, vented to a safe place
via the elevator top. In some cases, elevator legs can be mounted along the wall outside
the building and vented directly to the atmosphere.
Some materials collected in filters may form consolidated plugs in the dust dis-
charge hopper at the filter bottom. This may give rise to frictional heating and self-
ignition, which can result in dust explosions. Provided that the main enclosure as well
as the filter bag supports are properly grounded, incendiary electrostatic discharges
would not normally be expected in filters. (In the case of hybrid mixtures, the situ-
ation may be different.) However, if the equipment upstream of the filter, such as mills
and spray dryers, can generate ignition sources that may be conveyed to the filter,
the filter must be protected against possible explosions. By adopting a cylindrical or
conical body shape, the use of pressure shock resistant filter enclosures is a feasible
possibility.
According to Noha (1989), industrial vacuum cleaners operate mostly at dust con-
centrations below the lower explosible limit. The fan is normally located on the clean
side of the filter and therefore constitutes no ignition hazard. However, the possibility
of internaldust explosions in industrialvacuum cleaners cannot be fully excluded in gen-
erally; therefore, such equipment is often designed to be explosion- (shock-)resistant.
Various aspects of preventing and mitigating dust explosions in pneumatic and other sys-
tems for conveying powders and dusts were discussed by Palmer (1973b) and Eckhoff
(1982ab). Pinkwasser (1985) described the extinction of smoldering powder nests in a
dust cloud during pneumatic transport, and Gopfert (1981) discussed means of dust
explosion prevention and mitigation in continuous conveying equipment in general.
Palmer (1975) paid specific attention to dust explosions in dust collecting plant includ-
ing cyclones and filters.

1.5.2.6
ConcI us i on
Noha (1989) emphasized that Tables 1.12-1.15 should be regarded as a starting point
for discussion rather than as a final answer. The solution ultimately adopted must be
the result of detailed analysis of the relevant factors in each specific case. General
guidelines are useful as a point of departure, but the end result will always be tailor-
made.