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

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 9
l = length of tethering cables
a, b = side dimensions of explosion vent
e) Where the tether length is less than that determined above, consider the explosion vent as a hinged
panel when performing calculations to determine the vent area and vented explosion pressure (P red). This
will account for the hindrance to the venting process created by the short tethers.
f) Where tethers are located at all four corners, this can be considered tolerable where the cable length
exceeds the minimum set in d, above. Any less and the vent area is restricted to the annular space
determined by the cable length and the dimensions of the panel, rather than on the size of the panel itself.
2.2.3.6 Do not attach sprinkler system piping to any wall, ceiling, or roof that could be displaced by the
pressure of a room or building explosion.
2.2.4 Operation and Maintenance
2.2.4.1 In buildings where fugitive dust is released despite process design and equipment upkeep, incorporate
the following actions into a fugitive-dust-control program:
a) Assign accountability and keep records.
b) Commit time and resources regularly.
c) Create a maintenance schedule and review it periodically to ensure it is adequate.
d) Pay particular attention to eliminating accumulations above floor level, such as equipment tops and
building structural members because this dust is more likely to become suspended (airborne) and create
an explosible cloud if it is disturbed.
e) Use vacuum removal wherever possible using a central, portable, or truckmounted vacuum system
as appropriate.
f) Where vacuuming is impractical, sweeping or water wash-down are other acceptable options.
g) Where compressed air blow-down is the only practical method for cleanup, use the following
precautions:
1. Perform air blown-down frequently enough to prevent hazardous accumulations of dust.
2. Limit the extent of blow-down to small areas at a time with as low a volume and pressure of air as
possible.
3. Shut down electrical equipment not suitable for Class II, Division 2 hazardous locations.
4. Prohibit open flames and hot work, and ensure no hot surfaces exist.
2.2.4.2 Any accumulations of fugitive dust present a potential for a secondary explosion and must be
eliminated in buildings without damage-limiting construction (DLC), and controlled in those with DLC (3.1.1).
a) Base dust-cleaning frequency on preventing accumulations of more than 1 ⁄16 in. (1.6 mm) thickness
over more than 5% of the room floor area.
b) Use fugitive dust exceeding the above, either in thickness or area, as a trigger to initiate cleanup.
c) Ensure the total area of accumulation does not exceed 1,000 ft 2 (93 m 2 ).
d) The thickness of 1 ⁄16 in. (1.6 mm) is based on a typical wood or agricultural dust with a bulk density
of about 36 lbs/ft 3 (580 kg/m 3 ). The thickness of dust of different density can be based on a ratio of the
bulk densities.
2.2.4.3 Inspect explosion venting devices periodically to prevent their condition or mobility from being
impaired. Possible impairments include the following:
a) Corrosion
b) Improper painting of movable parts or rupture membranes
c) Icing
d) Snow accumulation on or in front of vents
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