Page 642 - Dust Explosions in the Process Industries

Research and Development 609

of the particles prior to combustion. In addition to being of fundamental interest, the
observed effect also has implications with respect to industrial safety.
Zuikov and Zemsky (1997) presented experimental data for the kinetics of release of
hydrogen when silicon powder makes contact with water or some aqueous solutions.
It was shown that the released hydrogen can form explosive mixtures with the air into
which it is released. Zemsky, Zuikov, and Devlicanov (1997) suggested that the for-
mation and ignition of hybrid mixtures of silicon dust, hydrogen, and air could well have
been the cause of several explosions in silicon powder production plants.

9.2.5
BLASTWAVES GENERATED BY BURNING DUST CLOUDS

A useful condensed introduction to the complex field of the properties and effects of blast
waves from explosions was given by Harmanny (1992).
One case of practicalinterest is blast waves from explosions in partly confinedgeome-
tries, for example, deliberately vented or bursting process equipment and workrooms.
The strength and shape of blast waves from dust explosionsdepend on the way in which
the dust clouds burn. For example,Wirkner-Bott,Schumann,and Stock (1992) conducted
a fairly detailed study of the nature of the “secondary explosion,” that is, the explosion
of unburned dust cloud outside the vent opening. This phenomenon was discussed fur-
ther by Schumannand Wirkner-Bott(1993).Central variables influencing blast wave gen-
eration, in addition to the type of dust and geometry of system,includethe dynamic state
of the dust cloud at the moment of ignition, the ignition point in relation to the vent, the
vent size, and the vent-cover opening pressure. Wingerden (1993) presented an inform-
ative overview of pressure and flame effects in the direct surroundings of installations
protected by dust explosion venting.
Some basic studies of shock wave emission from burning dust clouds were performed
by Gelfand et al. (1990).
edvedev, Polenov, and Gelfand (1994a) studied, experimentallyas well as theoret-
ically, the blast wave generated by sudden expansion of a dust-filled enclosure, such as
a hopper or a pipe. The same authors (199410) also studied the interaction between blast
waves and dust deposits, using a specially developed shock tube technique. The exper-
iments revealed a strong dependence of the pressure amplitude transmitted though the
dust on the duration of the compression phase of the primary air shock wave. Smirnov,
Kuksenko, and Chen Dongqing (1994) presented a new mathematical model of shock
wave propagation in dust clouds, comprising a range of different particle sizes within
the same cloud. Interparticle collision was not considered.An experimental and numer-
ical study of the supersonic flow behind a shock wave passing through a dust cloud was
performed by Boiko et al. (1994). Gelfand et al. (1994) investigated experimentallythe
attenuation of shock waves propagating through dust clouds in a 50 mm diameter shock
tube. Reasonable agreement between the experimental data and analytical and numeri-
cal predictions was found for incident shock waves of Mach < 3.
The effect of a given blast wave on humans, buildings, and process equipment is an
important area where more research is needed. Valuable reviews were given by Mercx
(1992) and L‘AbbC (1992). Britan et al. (1994a) studied the interaction of shock waves
with layers of water-based foam used in fire fighting. Induction times and time constants