DS 7-76 Prevention and Mitigation of Combustible Dust ... - FM Global

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 29
3.1.8 Effect of Explosion Vent Ducts (2.3.3.1.7)
The effect of the vent duct must be anticipated in the sizing of explosion vents. Placing a duct at the discharge
of an explosion vent can significantly affect the pressure inside the enclosure. Depending on the length and
diameter of the vent duct, the value of P red can be increased by as much as an order of magnitude (10 times),
which is usually enough to completely destroy the protected vessel.
Two factors are most important when considering the effect of vent ducts on the venting process: (a)
combustion of dust within the duct during the explosion, and (b) the inertia of the air mass inside the duct
before the explosion.
During the early part of the venting process, unburned dust is ejected into the duct ahead of the flame front
from the vessel. When the flame front moves into the duct, dust starts to burn within the duct and generate
additional combustion products. Those combustion products expand in ALL directions, thus slowing down
or even reversing the flow out of the vessel and the pressure builds up within the vessel.
Inertia of the air within the duct also increases explosion pressure within the protected vessel. When the
explosion vent opens and combustion gases first start flowing into the duct, those gases must push all of
the air out of the duct. During the time required to eject the air, the pressure continues to grow within the vessel
because the combustion gases are obstructed from reaching the open atmosphere. In a long duct, that mass
of air can delay the venting of combustion gases enough to significantly increase the pressure in the vessel.
Friction effects from gas flowing through the duct generally have little effect on the explosion, except possibly
from bends or other geometric discontinuities, compared to the previously noted effects of in-duct combustion
and air inertia.
Because P red sharply rises as vent duct length increases, vessels with a low strength generally cannot be
protected by venting through ducts if the L/D of the vent duct exceeds a value of 1 to 2 unless the vent size
is proportionately increased.
Although fitting an explosion vent duct having a cross-section smaller than the explosion vent will increase
the value of P red (i.e., create a worse situation), using an explosion vent duct with a cross section larger than
the explosion vent area will not lower the P red.
FM Global’s DustCalc software readily calculates the effects caused by vent ducts.
3.1.9 Re-Closing Explosion Vents (2.2.3.4 and 2.3.3.1.9)
After the combustion of dust inside of a room or vessel has ended and gases are no longer being produced
to generate pressure, explosion vents that can reclose by gravity will do so. The very hot combustion gases
trapped within the room or vessel begin to cool and, unless fresh air moves into the enclosure fast enough
(e.g., via an appropriately-sized vacuum breaker), this will produce a vacuum. This vacuum can result in
vessel damage or equipment implosion. In rooms or buildings, vacuum damage is less severe, but is still
possible.
3.1.10 Pressure and Fireball Effects from an Explosion (2.3.3.1.11)
A dust explosion results in short duration pressure and fireball effects outside of the enclosure. There are
limited studies that allow these effects to be estimated, and suggested equations are shown below:
Estimate the pressure effects close to, and in a direction normal to, the vent using the following equations:
Step 1: Calculate the maximum overpressure (P blast,max) generated by the explosion outside of the vented
enclosure using the following equation:
0.1 0.18
Pblast,max (barg) = 0.2 Av V Pred
Where
A v is the explosion vent area, m 2
V is the volume of vented enclosure, m 3
P red is the reduced pressure in the enclosure during venting, barg
Step 2: Calculate the perpendicular distance (R max) from the vent opening to P blast,max using the following
equation:
R max (meters) = 0.25 L f,max
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