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

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 21
b) Regardless of the type of bags used, provide a reliable grounding connection for the bag cages (metal
wire supports) (3.1.23).
2.5 Connecting Ducts (3.1.24)
Ducts connecting pieces of a process or collection system can provide a route for spreading an initial
explosion and may contain enough dust to propagate an explosion of their own. This section does not apply
to clean air ducts downstream of air-material separators or pneumatic transfer of process materials at rates
well in excess of the MEC (dense phase transfer).
2.5.1 Occupancy
2.5.1.1 Control the dust concentration in a fugitive-dustcollection system to prevent a continuous, explosible
atmosphere from developing in the duct using the following methods:
a) When the dust-generation rate is variable, keep the dust below an average concentration of 25% of
the minimum explosible concentration (MEC). Limit peak dust-emission rates above 100% of the MEC for
only a few seconds at any time.
b) When the dust-generation rate is steady without significant peaks, keep the dust concentration to no
more than 90% of the MEC.
2.5.1.2 Where the concentration in a duct regularly exceeds the MEC, provide protection for the additional
in-duct explosion hazard (2.5.2.3).
2.5.1.3 In ducts transporting combustible dusts, maintain an air velocity that exceeds the settling velocity
for the material being transported.
Note: The velocity for typical industrial dusts (e.g., saw dust) can be from 3500 to 4000 ft/min (1070 to 1220
m/min). Where the settling velocity of a material is unknown, consult Data sheet 7-78, Industrial Exhaust
Systems, for generic transport velocity recommendations.
2.5.2 Protection
2.5.2.1 On ducts containing combustible dust in concentrations that are always less than the MEC and that
carry dust at velocities where dust settling out is unlikely, provide explosion-isolation devices as follows:
a) On connections to important pieces of equipment
b) On duct running back to buildings that contain expensive equipment or important processes that could
be damaged, that might contain fugitive dust.
If the value of MEC is not available, an estimate of 30 grams/m 3 (0.03 oz/ft 3 ) can be used.
2.5.2.2 Arrange ducts containing combustible dust in concentrations that always or frequently exceed the
MEC, or that carry dust at velocities where dust settling out might be expected, as follows:
a) Route the ducts outside.
b) Provide an explosion-isolation device at each point of connection between the duct and a piece of
equipment.
c) Protect the duct from an explosion propagating in the explosible mixture by one of the following:
1. Provide venting for the duct in accordance with 2.5.2.3.
2. Design the duct to fail at pressures as low as practical but not more than 0.3 barg (4.4 psig).
2.5.2.2.1 Where the duct containing dust above the MEC or conveyed at less than the dust-settling velocity
must be located indoors, take the following precautions:
a) Provide an explosion-isolation device at each point of connection between the duct and a piece of
equipment, AND
b) design the duct to contain the explosion (shock resistant), OR
c) provide venting for the duct in accordance with 2.5.2.3 but route the vents outdoors.
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