Page 85 - Dust Explosions in the Process Industries

58 Dust Explosions in the Process Industries

Several authors have written survey papers on the prevention of ignition sources in
process plants. Kiihnen (1978a) discussed the important question of whether preventing
ignition sources can be relied on as the only means of protection against dust explosions.
His conclusion was that this may be possible in certain cases but not in general. Adequate
knowledge about the ignition sensitivity of the dust, both in cloud and layer form, under
the actual process conditions, and proper understanding of the process, are definite pre-
conditions. Schafer (1978) concluded that relying on preventing ignition sources is
impossible if the minimum electric spark ignition energy of the dust is in the region of
vapors and gases (40 mJ). However, for dusts of higher MIE, he specified several types
of process plants that he considered could be satisfactorilyprotected against dust explo-
sions solely by eliminating ignition sources.
In a more recent survey, Scholl(l989) concluded that the increasedknowledge about
ignition of dust layers and clouds permits the use of prevention of ignition sources as
the sole means of protection against dust explosions, provided adequate ignition sensi-
tivity tests have shown that the required ignition potential, as identified in standardized
ignition sensitivity tests, is unlikely to occur in the process of concern. Scholl distin-
guished between organizationaland operational ignition sources. The first group, which
can largely be prevented by enforcing adequate working routines, includes
* Smoking.
Openflames.
Open light (bulbs).
Welding (gas or electric).
Cutting (gas or rotating disc).
Grinding.
The second group arises within the process itself and includes
Openflames.
Hot surfaces.
Self-heating and smoldering nests.
Exothermic decomposition.
Heat from mechanical impact between solid bodies (metal sparks or hot spots).
Exothermic decomposition of dust via mechanical impact.
Electric sparks and arcs and electrostatic discharges.

1.4.2.2
Self-Heating, Smoldering, and Burning of Large Dust Deposits

The tendency to self-heating in powder and dust deposits depends on the properties of
the material. Therefore, the potential for self-heating should be known or assessed for
any material before admitting it to storage silos or other parts of the plant where condi-
tions are favorable for self-heating and subsequent further temperature rise up to smol-
dering and burning.
Possible means of preventing self-heating include
Control of temperature,moisture content, and other important powder and dust prop-
erties before admitting powder or dust to, for example, storage silos.