Page 120 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 93

Ejection of strong flame jets from the vent opening.
Emission of blast waves from the vent opening.
Reaction forces on the equipment, induced by the venting process.
Emission of solid objects (vent panels and parts that can be tom off by the venting process).
Emission of toxic combustion products.
In general, flame ejection is more hazardous the larger the vent is and the lower the static
opening pressure of the vent cover. This is because, with a large vent and a weak cover,
efficient venting starts at an early stage in the combustion process inside the enclosure.
Therefore, in the early stages of venting, large clouds of unburned dust are pushed out
through the vent and subsequently ignited when the flame passes through the vent. The result-
ing, secondary fireball outside the vent opening can present a substantial hazard. If, on the
other hand, the enclosure is strong, allowing the use of a small vent and a high Prd, only
the combustion products are vented and the flame outside the vent is considerably smaller.
If a dust explosion is vented indoors, the blast waves and flame jet may generate seri-
ous secondary explosions in the workrooms (see Section 1.1.3). Some methods for pre-
venting this are discussed in the following section.

1.4.6.5
Vent Ducts
One traditional solution to the flame jet problem is the use of vent ducts. This implies
that a duct of cross-sectional area at least equal to the vent area is mounted between the
vent and a place where a strong flame jet presents no hazard. The principle is illustrated
in Figure 1.106.
Vent ducts generally increase the flow resistance and, therefore, the pressure drop to the
atmosphere. Consequently, adding a vent duct increases the maximum explosion pressure