Page 156 - Dust Explosions in the Process Industries

Dust Explosions:An Overview 129

Table 1.I4 Appropriate means for preventing and mitigating dust explosions in a chemical process
plant: oowder mixers

Means of
explosion
prevention1
mitigation

\

Powder
mixers
With mixing tools:
High-speed
Low-speed
Without mixing tools:
Drum mixers
Tumbling mixers
Double cone mixers
Air flow mixers:
Fluidized bed mixers 0
Air mixers 0
Source: Noha, 1989.

1.5.2.2
Crushing and Milling Equipment

The justification for some of the suggestions in Table 1.12 for crushing and milling
equipment are as follows. In crushers and roll mills, the concentration of fine dust that
can produce explosions is mostly below the minimum explosible concentration, just
because of the nature of the process itself. In the case of screen mills and air jet mills,
the probability of ignition sources can be regarded as low. Inerting is most common in
the case of batch mills, whereas other mill types are often made strong enough to with-
stand an internal dust explosion.
ayerhauser (1978) considered the dust explosion protection of mills and air classi-
fiers specifically. He concluded that pressure-resistant or pressure shock-resistant con-
struction and inerting the two most suitable methods for these kinds of equipment.
Ruttmann (1989) described the systematic design of one specific plant, in which
inerting was used to protect a combined milling and mixing system against dust
explosions ~
enever possible, one should use mill types that minimize dust cloud formation and
generation of ignition sources by high-speed impact. Figure 1.141 shows a type of open
shredder that, to an increasing extent, replaces enclosed high-speed
slow motion minimizes both dust cloud formation and the prob