Page 125 - Dust Explosions in the Process Industries

98 Dust Explosions in the Process Industries

through the comparatively large wall area of the quenching tube. The wall is designed
to yield a low-pressure drop but high retention efficiency for dust particles and efficient
cooling of combustion gases. This means that flame ejection from the vent is effectively
prevented and the blast effects significantly reduced.
Furthermore, burning lumps of powder and other smaller objects that could be ejected
through an open vent are retained inside the quenching tube. However, any toxic gaseous
combustion products, such as carbon monoxide, escape to the atmosphere.
The increase of the maximum explosion pressure in the vented enclosure due to the
flow resistance through the quenching tube wall is mostly moderate and can normally
be compensated for by a moderate increase in the vent area.
Section 9.3.7.5 in Chapter 9 gives references to more recent work on this promising
principle of dust explosion protection. Bucket elevator legs and silos in congested areas,
where normal venting is prohibited, are likely areas of application. The very high tem-
peratures of burning light metal dust clouds (magnesium, aluminum, silicon) place heavy
demands on the design of the quenching tube wall, but there is no a priori reason for not
assuming that even this problem will be solved. Whether the remaining problem of pos-
sible toxic gas emission can be tolerated, depends on the actual circumstances, and must
be considered specifically in each particular case.
Figure 1.114 shows a commercial prototype of a quenching tube. Figure 1.115 shows
venting of a 5.8 m3 bag filter unit without and with the quenching tube. The white smoke
in the lower picture is mostly condensed water vapor.

Figure 1.1 14 Commercial prototype of a
quenching tube (Courtesy of Rembe GmbH,
Brilon, Germany).

1.4.6.7
Reaction Forces and Blast Effects

Experience has shown that the reaction forces from dust explosion venting can increase
significantly both the material damage and the extent of the explosion. Equipment can
tilt and ducts be torn off, and secondary dust clouds can be formed and ignited. Whenever
installing a vent, it is therefore important to assess whether the equipment to be vented
can withstand the reaction forces from the venting, should an explosion occur. A very
simple, static consideration says that the maximum reaction force equals the maximum