Page 90 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 63

in a silo without disintegrating during the fall, has a higher probability of igniting the
dust cloud at the bottom of the silo than during the fall.
Jaeger (1 989) conducted a comprehensive laboratory-scale investigation on formation
of smoldering nests and their ability to ignite dust clouds. He found that only materials
of flammability class larger than 3 (see Section A. 1.2.9 in the Appendix) could generate
smoldering nests. Under the experimental conditions adopted, he found that a minimum
smoldering nest surface area of about 75 cm2 and a minimum surface temperature of
900°C were required to ignite dust clouds of minimum ignition temperatures 1600°C.
Zockoll(l989) studied the incendivity of smoldering nests of milk powder and con-
cluded that such nests would not necessarily ignite clouds of milk powder in air. One
condition for ignition by a moving smoldering nest was that the hottest parts of the sur-
face of the nest were at least 1200°C. However, if the nest were at rest and a milk powder
dust cloud settled on it, inflammation of the cloud occurred even at nest surface tem-
peratures of about 850°C.
Zockoll suggested that, in the case of milk powder, the minimum size of the smoldering
nest required for igniting a dust cloud is so large that carbon monoxide generation in the
plant would be adequate to detect formation of the smoldering nests before the nests reach
hazardous sizes.
Alfert, Eckhoff, and Fuhre (1989) studied the ignition of dust clouds by falling smol-
dering nests in a 22 m tall silo of diameter 3.7 m. They found that nests of low mechan-
ical strength disintegrated during the fall and generated a large fire ball that ignited the
dust cloud. Such mechanically weak nests cannot be transported any significant distance
in, for example, pneumatic transport pipes before disintegrating. They further found
that mechanically stable nests ignited the dust cloud either some time after having come
to rest at the silo bottom or when broken during the impact with the silo bottom. However,
as soon as the nest had come to rest at the silo bottom, it could also be covered with dust
before ignition of the dust cloud got under way.
Infrared radiation detection and subsequent extinction of smoldering nests and their
fragments during pneumatic transport, such as in dust extraction ducts, has proven an
effective means of preventing fire and explosions in downstream equipment; for exam-
ple, dust filters. One such system, described by Kleinschmidt (1 983), is illustrated in
Figure 1.69. Normally, the transport velocity in the duct is known, and this allows effective

TINCTION SYSTEM

Figure 1.69 An automatic system for detection and extinction of smoldering nests and their frag-
ments, applied to a multiduct dust filter system (From Kleinschmidt, 1983).