Page 452 - Dust Explosions in the Process Industries

Ignition of Dust Clouds and Dust Deposits 4 7 9

Gubin and Dik (1986) developed a. mathematical model assuming that the oxidation
occurred as a heterogeneousreaction between oxygen from the gas phase and the parti-
cle surface. They further assumed that the spark discharge initially generated a certain
quantity of heat located within a narrow channel in the spark gap. The heat drain from
the channel to the surroundingswas assumed to occur essentially by conduction,radia-
tion and convection being neglected. The basic heat balance equation was of the same
form as that derived by previous workers. As in the case of other workers, the ignition
criterion was the establishment of self-sustainedlaminar flame propagation in the dust
cloud. It would appear that Gubin and Dik may not have been aware of the other inves-
tigations mentioned previously.

5.4
NlTlON OF DUST CLOUDS BY HEAT
FROM MECHANICAL RUBBING, GRINDING,
OR IMPACT BETWEEN SOLID BODIES

5.4.1
BACKGROUND

Whether or not metal sparks or hot spots from accidental impacts, rubbing operations,
and the like between solid bodies can initiate dust explosionshas remained a controversial
issue for a long time. Many attempts have been made at resolving the puzzle by analyzing
past accidents with the objective to identify the ignition sources.A summary with ref-
erence to the grain, feed, and flour industry is given in Table 5.6.

Table 5.6 Percentage of dust explosions in the grain, feed, and food indust-y assumed to be initi-
ated by “friction sparks“ or unknown sources

Source: Pedersen and Eckhoff, 1987.

As can be seen, “friction sparks” are claimed to play a significant part. If one further
takes into account that it is often tacitly implied that a substantial part of the “unknowns”
may have been initiated by some untraceable source, such as metal sparks and electro-
static discharges,the friction spark becomes the most suspect of all the potential ignition
sources.
The situations in which metal sparks and hot spots can be generated in an industrial
process plant fall into two main categories. The first is grinding and cutting operations,