Page 651 - Dust Explosions in the Process Industries

6 18 Dust Explosions in the Process Industries

atmospheres can be generated. Glor and Schwenzfeuer (2001) and Schewenzfeuer and
Glor (2001a) discussed further evidence facilitating the assessment of the hazard of
electrostatic discharges igniting explosive dust clouds at large, and particular attention
was paid to brush and cone discharges in powder storage systems including silos and
FIBCs.

9.3.5.5
Ignition of Dust Clouds by Glowing or Burning Particles
A brief review is given in Section 1.1.4.5 in Chapter 1. Some research work up to 1990
is discussed in Section 5.4 in Chapter 5.
Jansson et al. (1998) emphasized the dust explosion hazard presented in some indus-
tries, such as the wood working industries, by burning and glowing particles. An exam-
ple of commercially available equipment to prevent ignition in industrial plants by this
category of potential ignition sources was given by Kleinschmidt (personal communi-
cation with H.-P. Kleinschmidt,Fagus-GreCon Greten GmbH. and Co., Alfeld-Hannover,
Germany, 1992),who presented a system for detection and extinctionof “sparks” in terms
of flying burning or glowing particles. Jansson (personal communicationwith L. Jansson,
Firefly AB, Huddinge, Sweden, 1993) presented an alternative system that offers an
adjustable lower particle temperature limit of detection, down to 150°C.A multizone
checkpoint system prevents false alarms and indicates the size of the hot object (single
particle, several particles, or extensive flame). Depending on the detection temperature
and the nature of the industrial process, detection of a hot object may give rise to either
activation of an extinction system, closedown of the plant, or simply adjustmentof plant
running conditions to prevent further hot object generation. Other “spark” detection and
extinguishing systems have also been described, such as by T & B Electronic (1994).
The interior of a turbo mill, a cross beater mill, a pin mill, a pinned disk mill, and the
like can become an effective ignition sourcewhen aforeign body enters the mill together
with the material to be milled and causes a dust explosion.This problem was addressed
by Barth et al. (1995), with particular reference to small laboratory-scalemills. Such mills
are normally naturally vented and sufficiently strong to withstand the modest overpres-
sures to be expected, should a dust explosion occur inside the mill. The problem is to
eliminate the dust flame hazards associated with the venting of the system. Barth et al.
proposed several solutions to solve this problem.

9.3.5.6
Miscellaneous Ignition Sources

Proust (1996b) investigated experimentally the extent to which a beam of laser light is
able to ignite an explosive dust cloud. He found no generally valid correlation between
the ease with which a dust cloud could be ignited directly by a laser beam and the min-
imum electrical spark ignition energy of the same dust cloud. He also found that the
probability of a given laser beam igniting a given dust cloud increased markedly if igni-
tion occurred indirectly, via a small solid target inside the dust cloud that was heated
by the laser beam. Zevenbergen et al. (1996) tried to determine the actual minimum
amount of energy that had to be transferred to an explosive dust cloud from a laser beam
to cause ignition. For clouds of lycopodium in air, the laser energy required, obtained