Page 659 - Dust Explosions in the Process Industries

626 Dust Explosions in the Process Industries

In spite of extensive research and development, dust explosion venting remains a
complex and in part controversial subject.The key issue in explosion vent arrangement
design is vent area sizing, and a thorough understanding of flame propagation processes
in dust clouds is essential for proper appreciation of the challenges encountered in
designing optimal venting arrangements for industry.
Useful reviews of various aspects of dust explosion venting in practice were given by
Scholl (1992), Lunn (1992b, 1992c), Siwek (1996b), and Grosskopf (2002). Siwek
(1996d, 1998): covering gas explosion venting as well, reviewed material that has in part
been the basis of the most recent draft edition of the German VDI 3673 guideline. This
guideline,in turn, has been a major input to the dust part of the venting guideline issued
by the European Union technical committee TC 305.
The basic understanding of flame propagation processes inside and outside vented
enclosures is still unsatisfactory. This implies that neither the processes by which dust
clouds in vented enclosures are generated nor the way in which the clouds bum are suf-
ficiently well understood. Consequently,adequate venting theories do not exist, and one
must rely on experiments. However, during the last two decades, the need for a diferen-
tiated approach to assessment of vent area requirements, in view of the different turbu-
lence levels, degrees of dust dispersion,and concentration distributions of dust clouds that
occur in practice, has gradually become accepted. Eckhoff (1993b) discussed this prob-
lem with particular reference to venting of large silo cells. Deng et al. (1993b) conducted
vented maize starch explosions in a 95 m3vertical experimentalsilo of LID =3. The vent
was located in the silo roof. The vent area required for keeping the maximum explosion
pressure below a predeterminedvalue was considerablysmallerthan that predicted by the
1979VDI venting code. Hoechst, Leuckel, and Eibl(l993) conducted systematic maize
starch/air explosion experiments in a top-vented silo of volume 50 m3 and LID =4. Both
dust concentration and initial turbulence (prior to ignition) were monitored. Both the dust
injectionprocess and the ignitionpoint location were varied. Flame propagation and pres-
sure buildup during the vented explosion were measured. Comparativeexperiments with
methane/& in the silo were also performed.Hauert (1996),Arnold et al. (2001), and Hauert
et al. (2001) presented the results of a series of large-scale wheat and maize dust explo-
sion experiments in a vertical 12 m3top vented silo of LID = 3. They found that horizon-
tal pneumatic dust injectiontangentially at the silo top gave considerably lower maximum
pressures for a given vent size, than both pneumatic dust injection vertically downward,
and dust injection by the traditional VDI pressurized-nozzle technique. Both turbulence
intensities and dust concentrations in the experimental dust clouds were measured.
A similar trend was found by Zockoll et al. (ZOOl),who performed controlled vented
dust explosion experimentsin a hammer mill grinding plant, as used in the animal feed
industries. Their overall conclusion was that, in a real plant, the vent areas required to
ensure given maximum reduced explosion pressures were substantiallysmaller, typically
by a factor 3-4, than those specified by the VDI-3673 guideline. This is not surprising
in view of the extensive discussion of these effects given in Chapter 6. Holbrow, Lunn,
and Tyldesley (2002) reported results from a comprehensive experimental program on
venting of dust explosions in full-scalebucket elevators. Both a single-leg elevator (total
height about 25 m) and a twin-leg elevator (total height about 18 m) were used in the
experiments.A number of vent openings were distributed along the length of the eleva-
tors. The dusts used were a milk powder and three different types of corn flour. On the
basis of the results obtained recommendations were given for design of vent arrange-
ments for bucket elevators in industrial practice.