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

7-76 Combustible Dust Explosion
Page 22 FM Global Property Loss Prevention Data Sheets
2.5.2.3 Provide explosion venting along the length of a duct on the following basis:
a) Compute the maximum distance between explosion vents (L max) as follows:
L max = 7.5 D 1/3 , D and L in meters, OR
L max = 16.5 D 1/3 , D and L in feet.
For noncircular ducts, calculate the effective diameter for the above equation by:
D eff = √ 4A d
π
where: A d = cross sectional area of the duct (m 2 or ft 2 )
b) Provide the vent area at each location at least equal to the cross sectional area of the duct.
c) Provide a full-size vent no more than 2 diameters away from the point of duct connection to a piece
of equipment.
d) Set the explosion vent relief pressures (P stat) as low as possible, with a maximum value of 0.1 barg
(1.5 psig).
e) Provide an explosion vent at all elbows and end flanges (see Figure 10).
f) When located inside, route the explosion products outside via a short duct (L/D less or equal to 1).
2.6 Silos (3.1.25)
2.6.1 Protection
Fig. 10. Example of duct explosion venting at an elbow
2.6.1.1 When a dust storage silo has a dust collector (other than a simple vent sock) on the breather vent,
provide explosion protection as follows:
a) Treat the collector/silo combination as a single unit if either of the following conditions exist:
1. The duct connecting the silo to the dust collector has an L/D ratio less than 2, OR
2. The cross-sectional area of the connecting duct is at least as large as the explosion-venting area
needed to protect the volume of the dust collector alone.
b) Provide explosion venting for the silo/collector combination based on the total volume of the two units.
c) Install part of the explosion venting on the dust collector if possible, not to exceed the cross sectional
area of the connecting pipe, although all of the explosion venting can be on the silo.
2.6.1.2 Where the dust collector/silo combination does not meet the criteria of a single unit:
a) Provide explosion venting for each component based on their individual volumes.
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