Page 646 - Dust Explosions in the Process Industries

Research and Development 6 7 3

Chapter 1) for venting in-house dust explosions at the top and boot of the elevator were
used in combination with flame-triggeredwater mist injection along the elevator legs.
Going and Snoeys (2002) discussed suitablemethods for mitigating metal dust explo-
sions based on experimental investigation. In particular,isolation,venting, and automatic
suppression were considered.

9.3.4
GENERATION AND PROPERTIES OF EXPLOSIVE DUST CLOUDS
IN INDUSTRY

The challenge is to characterize typical dust cloud structures in terms of spatial distri-
butions of particle size, dust concentration,turbulence, and global flow generated in typ-
ical process units, like mills, dryers, mixers, bucket elevatorsand other conveyors,silos,
filters, cyclones, and connecting ducts. These cloud structures define the initial cloud
properties at the location of the primary dust explosion and may have a major impact
on the course of development of the explosion. It is well known that the dynamic state
of a dust cloud dramatically influences both its ignition sensitivity and its combustion
rate.
Comparatively little new quantitative knowledge of practical use has been traced.
Therefore, experimental investigation of typical processes of the generation of dust
clouds and the resulting cloud structures in various types of process equipment and
operation modes should be encouraged. This is an area where close interaction of fun-
damental and applied research can be highly beneficial.The work of Hauert et al. (19941,
discussed in Section 9.2.2, constitutes an important step in this direction.
Jong et al. (19991, in their useful review of methods for determining powder “flow-
ability,” mentioned two methods for determining the dispersibility, or “dustability,” of
powders that are of interest in the present context (see also Chapter 3 and Section 7.4.2
in Chapter 7). Dahmann and Mocklinghoff (2000), in the context of industrial hygiene,
also described a method for testing the “dustability” of industrial powders.

9.3.5
PREVENT1NG 1GNITION SOURCES

9.3.5.1
Introductory Remarks

Reference is made to the first column of Table 1.9 in Chapter 1.A considerable amount
of fundamentalknowledge is available, as discussed in Section 9.2 with reference to the
second column of Table 9.1. Generally any “ignition”process comprises a range of very
complex subprocesses. Simple parameters, such as a minimum ignition energy or tem-
perature, are not true constants for a given dust but vary significantly with the geome-
try and other properties of the ignition source as well as with the state of the dust cloud.
Maurer and Glor (1996) discussed the extent to which the minimum ignition energy
of a dust, determined in a standard test, can be used as a basic criterion for selecting an