Page 657 - Dust Explosions in the Process Industries

624 Dust Explosions in the Process Industries

discussed by Roser et al. (1999), whereas Vogl and Radandt (2001, 2002) presented
new experimental data on minimum pipe diameters for transmission of dust explosions
from one process unit to another. It was found that dust flame transmission could occur
in pipes of surprisingly small diameters. For example, with a wheat flour of Ks, = 100
bar.m/s, the dust flame was able to propagate through a 27 mm diameter pipe of at least
12m length. Based on another extensive series of experiments,Roser et al. (2001) devel-
oped empirical correlations for predicting the time that a dust explosion in a process unit
of a given volume would need to propagate through a given distance of a duct of a given
diameter connecting the process unit to another, neighboring unit. Knowing this time is
critical in the design of effective explosion isolation systems to be installed in pipes and
ducts connecting to process units in which dust explosions can occur.
Wiemann and Faber (1996) studied experimentally the propagation of dust explosions
in a fully closed, coupled system consisting of a 1 m3vessel and a 5 m3vessel connected
by a 10m long straightpipe of internal diameter 400,200, or 100mm. Considerablepres-
sure piling effects were observed, and with maize starch, with ignition in the 5 m3vessel,
a maximum pressure of nearly 18 bar was observed in the 1 m3vessel. Kubainsky et al.
(2001)presented results from a series of full-scale experiments,which showed that a dust
explosionin a vented filter enclosure can give rise to substantial dust flame propagation
back into the upstream raw gas duct, in the direction oppositethat of the normal raw gas
flow into the filter, It was concluded, therefore, that, in such systems, the installation of
effective explosion isolation equipment in the upstream raw gas duct is required.
Various passive and active techniques for interruptingexplosions in pipelines have been
developed, but there is room for further improvement. If adequate performance can be
achieved,passive techniques are clearly more attractive than active ones. Zellweger (per-
sonal communication with J. Zellweger, Rico-Sicherheitstech, St. Gallen, Switzerland,
1992)described work to improvepassive and active isolation valves of the VENTEX type.
Closing times (from sensing of the explosion to valve is fully closed) down to 12ms were
obtained for active valves. A simplifiedVENTEX valve, operating in one direction only,
has also been developed. Passive explosion interrupters and diverters based on venting
at a 180"bend have been in use for some time. However, therc is a need for further explo-
ration of the potential and limitations of this attractive, simple principle of explosion
isolation. A new low-pressure-drop design of this kind of diverter was described by
Alfert and Fuhre (1992) and Wingerden and Alfert (1992). Glor (personal communica-
tion with M. Glor, Ciba-Geigy AG, Basle, Switzerland, 1993)reported on work on the
performanceof explosionbarriers in ducts connectedto vessels with venting or automatic
explosion suppression. Klincewicz and Kordylewski (1993) described a new design of
an active explosion diverter for interrupting explosionsin pipelines. The new design avoids
the pressure drop created in normal operation by passive divertersbut requires active txig-
gering. Moore and Siwek (1996) investigated experimentallythe performance of automatic
explosion suppression systems for interrupting explosions in pipelines and ducting
between process units. Some quantitative design guidelines were given.
Cybulskiet al. (1994b),Lebecki et al. (1998,2000, 2001), developing systemsfor inter-
rupting coal dust explosions in coal mine galleries, used solar panels for automaticdetec-
tion of the coal dust flame and simultaneous actuation of water barriers. The water was
contained in plastic bags or containers,opened by a detonating cord or charge triggered
by the flame-generated power from the solar panel.
Siwek (1996c), in a comprehensive paper covering both dust and gas explosions,
reviewed the wide range of isolation techniques available for preventing dust explosions