Page 102 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 75

1.4.4.2
Published Overviews of Methods for Isolation

Basically there are two categories of methods, the passive ones activated directly by the
propagating explosion and the active ones, which require a separate flame or pressure
sensor system that triggers a separately powered system to operate the isolation mech-
anism. For obvious reasons, the passive systems are generally preferable, if they func-
tion as intended and are otherwise suitable for the actual purpose.
Several authors have discussed the different technical solutions for interrupting dust
explosions in the transfer system between process equipment. Walter (1 978) concentrated
on methods for stopping or quenching explosions in ducts. The methods included auto-
matic, very rapid injection of extinguishing agent in the duct ahead of the flame front,
and various kinds of fast response mechanical valves. Scholl et al. (1979) also included
the concept of passive flame propagation interruption in ducts by providing a vented 180"
bend system (see Figure 1.82). Furthermore, they discussed the use of rotary locks for
preventing explosion transfer between process units or a process unit and a duct. Czajor
(1984) and Faber (1989a) discussed the same methods as covered by Scholl et al. and
added a few more. See also Section 9.3.7.3 in Chapter 9.

I .4.4.3
Screw Conveyors and Rotary Locks

One of the first systematic investigations described in the literature is probably that by
Wheeler (1935). Two of his screw conveyor designs are shown in Figure 1.80.

Figure 1.80 Screw conveyors designed to prevent
--
. transmission of dust explosion (From Wheeler,
1935)
The removal of part of the screw ensures that a plug of bulk powder or dust always
remains as a choke. Wheeler conducted a series of experiments in which rice meal explo-
sions in a 3.5 m3 steel vessel were vented through the choked screw conveyors and
though a safety vent at the other end of the vessel. Dust clouds were ejected at the down-
stream end of the conveyors but no flame.