Page 149 - Dust Explosions in the Process Industries

122 Dust Explosions in the Process Industries

means on the properties of the additiverather than the main product. This, not in the least,
applies to the incendivity of the dusts in terms of their minimum ignition energies.
The protective means to be used must be technically and financially feasible. For
example, there is no point in installing vents on an enclosure that is so weak that it could
not withstand the maximum pressure to be expected, even with the largest vent area that
can be provided.
Traditionalventing may sometimesbe unacceptable due to the inevitable emission of
unburned, burning, and burned dust. This is particularly so in congested urban areas and
for some special synthetic organic powders like pesticides, pharmaceuticals, and dye
stuffs. However, the further development of the quenching tube for dust and flame-free
venting of dust explosions (see Section 1.4.6.6) may alter this situation and make vent-
ing a feasible means of mitigating dust explosions even in some of these situations.
In the case of very reactive dusts, of K,, values >>300 bar ds, automatic explosion
suppression must most often be excluded because the injection of the suppressant is too
slow to produce any significant mitigating effect on the explosiondevelopment.Inerting
is feasible only if sufficient inert gas is available at an acceptable cost, whereas rein-
forcement of process equipment to an explosion shock-resistant standard may often be
both technically and financially unacceptable.
Figure 1.139 outlines a general approach to fighting the dust explosion hazard in
industry. Sometimes,the required ignitabilityand explosibilitydata for the dust(s)in ques-
tion are availablefrom earlier test work or from the literature.However, most often, spe-
cific laboratory testing is needed.

1.5.1.2
Cost Considerations

Ritter (1978) compared the cost of the various means of dust explosion prevention and
mitigationby means of cost indices. The index for proper eliminationof ignition sources
was, by definition, set equal to unity and used as a basis for cost comparisonfor all types
of plant units except milling plants. Ignition sourceeliminationincluded use of approved
electrical equipment in all areas, grounding of all conducting equipment to avoid static
electricity, avoidance of overheating by friction, safe maximum temperatures on all
heated surfaces,and avoidance or elimination of smoldering nests.A summary of fitter’s
figures is given in Table 1.11.
Table 1.1 1 Relative costs for various means of preventing and mitigating dust explosions using the
costs of eliminating ignition sources as a basis for comparison

*Ignition sources cannot be eliminated in mills, and therefore the cost of the plant itself is used as the cost
basis of index 1.O.
Source: Ritter, 1978.