Page 148 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 12 1

SELECTING APPROPRIATE MEANS FOR PREVENTING
AND MITIGATING DUST EXPLOSIONS

1.5.1
BASK PHILOSOPHY, COST ESTIMATION, AND RISK ANALYSIS

1.5.1 .I
The Optimal Solution, or Striking the Balance

The extensive menu of means of preventing and mitigating dust explosions, summarized
in Table 1.9, is discussed in Section 1.4.
Noha (1989) emphasized that the concepts of “primary” and “secondary” means of
protection against dust explosions, used in the Federal Republic of Germany in the past,
can be misleading, by indicating that mitigation is of secondary importance as compared
to prevention. The rational approach is to seek an optimal combination of means of both
categories for each specific application. In doing so, Noha suggested the need to break
down the problem and evaluate specifically

@ The efficacy of the protective means.
@ The technical feasibility.
@ The environmental acceptability.
@ The financial acceptability.
Figure 1.138 illustrates the situation.

Figure 1 .I 38 The appropriate set of means for
preventing and mitigating dust explosions de-
pends on type of dust, type of process, and the
boundary conditions in terms of plant layout,
type of building(s1, environmental constraints,
financial constraints, and so forth.
One pitfall related to assessing the efficacy of the protective means is the selection of
the dust sample on which the assessment is to be based. Noha (1989) mentioned as an
example a comparatively coarse polypropylene powder to which 4%of fine calcium
stearate had been added to increase flowability. Such additives have large specific sur-
face areas and correspondingly low minimum ignition energies. But, as long as they are
homogeneously mixed with the polypropylene, the small fraction of additive has little
influence on the ignitability and explosibility of the polypropylene powder as a whole.
However, if segregation OCCLU-s,the fine, reactive additive may accumulate in certain areas
of the process, for example, in a filter. This can create a much more hazardous situation
than would have been anticipated on the basis of the properties of the polypropylene
powder. Ilnsuch cases, it may be wise to base the assessmentof the efficacy ofthe protective