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

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 7
2.2.1.8 Provide pressure resistance and vent area in accordance with calculations conducted using FM Global
DustCalc software.
2.2.1.9 In buildings where some fugitive dust is likely despite process design and equipment upkeep, arrange
building elements and equipment to reduce the likelihood and amount of dust accumulation by any or all
of the following, as appropriate:
a) Provide smooth interior walls with minimal ledges.
b) To the extent practical, provide horizontal surfaces such as girders, beams, ledges, and equipment
tops with a sloped cover having a smooth finish, to shed dust settling out of the air.
c) Slope covers at an angle of 60° from horizontal, unless a lesser slope is known to be sufficient.
d) Box in overhead structural steel that is out of the reach of normal vacuuming or sweeping operations
and that has horizontal ledges (such as Ibeams or Ushaped channels in the up or sideways position)
with a noncombustible material to eliminate pockets for dust accumulation.
2.2.2 Occupancy
2.2.2.1 Buildings can be considered as not having a combustible dust hazard if they contain combustible
dust-handling equipment that is designed and protected to control or safely vent an internal explosion, in
accordance with the pertinent sections of this data sheet.
2.2.2.2 Take any or all of the following steps, as needed, where quantities of fugitive dust could be expected
in new operations or are excessive in existing operations:
a) Survey process equipment to identify the sources of dust release.
b) Modify, repair, or replace equipment to eliminate or at least reduce dust escape.
c) Provide permanent vacuum pickup points at the locations that release dust, such as grinding, buffing,
bag dumping, open transfer points in conveying systems, and other equipment/locations where large
quantities of dust are liberated frequently. In some cases this may require construction of a ventilated hood
or containment enclosure for existing equipment.
d) Operate closed dust-handling systems under a slight negative pressure to reduce dust escape.
e) Conduct extra housekeeping in existing operations while equipment is being modified (see Section
2.2.4).
2.2.2.3 Existing buildings containing small, localized amounts (less than 5% of the building area and in no
case exceeding 1,000 ft 2 [93 m 2 ]) of fugitive combustible dust can be considered tolerable without explosion
damage-limiting construction, if the following conditions exist:
a) The fugitive dust escape and accumulation rate is very low, i.e., less than 1 ⁄16 in. (1.6 mm) for a dust
with an approximate bulk density of about 36 lb/ft 3 (580 kg/m 3 ) per three month period AND
b) The cleaning frequency is high enough to permit one scheduled cleaning to be missed without allowing
dust accumulations to reach the unacceptable level of 1 ⁄16 in. (1.6 mm) noted above.
2.2.3 Protection
2.2.3.1 Construct pressure-relieving and resistant walls in accordance with Data Sheet 1-44, Damage-Limiting
Construction.
a) Design the explosion vent relief pressure (P stat) as low as the wind-resistance design will permit. In a
low-wind area, P stat can be as low as 20 psf (0.01 bar), whereas in higher wind areas 30 to 40 psf (0.015
bar) is more typical.
b) Provide pressure resistance and vent area in accordance with calculations conducted using FM Global
DustCalc software.
2.2.3.2 Do not use explosion vents in the roof to provide explosion relief.
2.2.3.3 Where a thorough engineering study shows that explosion-venting walls alone cannot provide the
needed explosion venting area, roof vents may be used to provide a portion of the needed vent area if snow
and ice are not allowed to build up on the vent. Any of the following are acceptable methods:
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