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Research and Development 6 19

by calculation (280 mJ), was considerably higher than the minimum ignition energy
obtained by direct experimentationusing electric sparks (3-7 mJ).

9.3.5.7
Design of Electrical Apparatus for Areas Containing Combustible
Dusts, to Prevent Ignition by Such Apparatus

This topic was addressed only very briefly in the two preceding editions of this book.
Therefore, in this edition, an updated comprehensive discussion is given in the new
Chapter 8.

9.3.6
EVENTING EXPLOSIVE DUST CLOUDS

9.3.6.1
lnerting by Adding Inert Gas

This can be accomplishedby adding inert gases such as nitrogen or carbon dioxide to
reduce the volume percentage of oxygen in the atmosphereto a level at which the dust
cloud can no longer propagate a self-sustainedflame. Discussionof alternativetechniques
adopted in practice is given in Section 1.4.3.1 in Chapter 1.A fair amount of data exists
on the maximum permissible oxygen content in the atmosphere for inerting (see Table
A.2, Section 1.3.6in Chapter 1,and Section 7.19 in Chapter 7). However, there is room
for improving the test methods by which such data are obtained.
Glor and Schwenzfeuer (1996) determined the limiting oxygen content in the atmo-
sphere, below which a cloud of a given material suspended in that atmosphere is not
ignited, as afunction of the energy of the ignition source. It was found that the limiting
oxygen contents with electrostatic discharges or impact sparks as ignition sources are
significantly higher than the absolute limit (LOC) determined in standard tests, using a
very strong pyrotechnicalignition source. Schwenzfeuer,Glor, and Gitzi (2001)extended
these studies to cover a much wider ignition energy range than in the previous studies,
from 1 mJ to 10kJ, and a total of nine dusts.All the data could be represented by a single
equation expressing the critical concentration of oxygen for inerting the actual dust
cloud, as a function of the actual ignition energy (E)and the MIE of the actual dust in
air determined in the standard test.
Most LOC data are for atmospheric pressure and normal temperature. Data for other
conditions, in particular for elevated temperatures and pressures, are sometimesrequired,
and adequate test methods should be developed. Glor (personal communication with
M. Glor, Ciba-Geigy AG, Basle, Switzerland, 1993)determinedmaximum permissible
O2 contents for inerting clouds of coal dusts at elevated temperatures and pressures.
Wolinski and Hayashi (1993) determined standard explosibility parameters of dusts of
various rare metal alloys in air containinghalon-1301 and additionalnitrogen. Theories
for flame propagation limits (see previous discussion with reference to the third column
of Table 9.1) would be useful even in this context, and their development should be
encouraged. Krause, Weinert, and Wohrn (1993) proposed a simplified, quite approxi-
mate method for first-order estimation of the minimum oxygen concentrationfor flame