Page 154 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 727

on preventing ignition sources gives the greatest benefit. Scenarios C and D focus on keep-
ing the dust cloud nonexplosive and using mitigating measures, respectively. Other sce-
narios can easily be envisaged.
However, experience suggests that some scenarios, depending on the type of powder
or dust and plant, are more credible than others. For example, it can be argued that a plant
producing or handling fine aluminum flake is well represented by Scenario C in Figure
1.140,because inerting by nitrogen would probably reduce the dust explosion risk from
high to acceptably low.
Inn spite of the objections that can be raised against the use of the various hazard analy-
sis methods, several authors have suggested that risk analysis and other methods be
adopted specifically for reducing the dust explosion hazard in the process industry,
including grain, feed, and flour storage plants. These include Beck (1974, 1985);
Sorgdrager (1985); Kameyama et al. (1982); Lai, Shenoi, and Fan (1985); and Piotrowski,
Mrzewinski, and Proskurmicki (1988).

1.5.2
SELECTION SCHEME SUGGESTED BY NOHA FORTHE CHEMICAL
PROCESS INDUSTRY

1.5.2.1
General Background

Noha (1989) restricted his analysis to four groups of process equipment:
0 Crushing and milling equipment.
0 Dryers.
0 Mixers.
0 Conveyors and dust removal equipment.
Informative descriptions and illustrations of the various kinds of process equipment
typical of powder producing and handling plant are provided by Perry and Green (1984).
Before deciding how a given plant should be protected against dust explosions, it is
necessary to evaluate the extent to which protection is really necessary. This requires
knowledge of ignitability and explosibility parameters of representative dust samples as
well as information about the plant design, layout, and operation. It may be necessary
to investigate the possibility of generation of hybrid mixtures (dust + explosible gas or
vapor).
Tables 1.12, 1.13, 1.14, and 1.15 give Noha’s suggestions for selecting appropriate
means for preventing and mitigating dust explosions in four categories of process equip-
ment in the chemical process industry. The symbol X indicates the most appropriatemeans
of protection, whereas (X) implies that the use of the means indicated is possible, but
that it is not implemented very often in practice.
Noha emphasized that a given plant item, for example, a specific dryer, should not be
considered in isolation. It is always necessary also to consider the entire plant or the part
of it likely to become involved in a dust explosion in the system. Identification of prob-
able ignition sources and ignition points is an important part of the analysis.