Page 591 - Dust Explosions in the Process Industries

558 Dust Explosions in the Process Industries

accidental bursting of sacks and bags). The dust particles start to settle out of suspen-
sion as soon as the cloud generation process terminates, and typical total lifetimes of
explosive clouds outside process equipmentis on the order of fractions of a minute. One
exceptionis long-durationminor dust leaks from, say, flanges in pneumatic transportlines,
but the dust cloud volumes produced in such cases normally are quite small.

8.3.1.2
Migration of Dust Particles through Narrow Holes and Gaps in Enclosure Walls

Because dust particles are so much bigger than gas molecules,they do not travel through
narrow holes and slots on the order of 1 mm diameter and smaller in the same way as
gas molecules. In principle, dust particles may be carried through narrow passages by
the airflow generated by a moderate pressure difference across the passage. However,
dust particles may easily adhere to the area around the passage entrance and the passage
walls and eventually block the passage.
Furthermore, dust particles that have been able to pass through narrow holes or gaps
in this way settle out of suspension as soon as they have passed the hole or gap and the
air velocities dropped to negligible values. Therefore, dust particles entering enclosures
in this way do not remain suspended in the air and eventuallyform explosivedust clouds
but settle out as layers on the internal surfaces of the enclosure. It seems difficultto envis-
age that any foreseeable mechanical process inside typical electrical apparatus enclosures
could redisperse such dust layers into explosive dust clouds within the enclosure.
The NFPA (1997) containsa paragraph that, in an excellentway, using practicalterms,
clarifies the basic difference between gases and dusts with regard to their abilities to
migrate through narrow passages. In a slightly modified form, this paragraph says:

Walls are much more important in separating hazardous and non-hazardous zones in the case of
combustible dusts than in the case of combustible gases. Only completely non-perforated solid walls
make satisfactory barriers in the case of gases, whereas closed doors, light-weight partitions, and even
partial partitions could make satisfactory barriers between hazardous and non-hazardous zones in the
case of dusts.

For the context of electrical apparatuses, this paragraph may be slightly rephrased by
replacing the first word, walls, by enclosures.

8.3.2
THERMAL HAZARDS ASSOCIATED WITH ACCUMULATION
OF DUST LAYERS INSIDE ELECTRICALAPPARATUS ENCLOSURES

8.3.2.1
Combustible, Electrically Nonconductive Dusts

If the surface temperature of a heat-producing component inside an enclosure reaches the
ignition temperatureof the dust layer under the prevailing conditions,the layer startsto burn
or glow. Then the temperatureof the componenton which the dust layer rests may become
considerably higher than had the dust been noncombustible, and the component may fail
and give rise to problems that may or may not be related to fire and explosion hazards.