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

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 23
b) Provide explosion venting for the connecting duct itself in accordance with 2.5.2.3.
2.6.1.3 Ensure the explosion venting area provided for a silo does not exceed its cross- sectional area (A x-sect).
Any explosion venting in excess of this value will not help to decrease the explosion pressure.
2.6.1.3.1 If calculations show the explosion venting area required is more than the cross-sectional area, use
an explosion-suppression system or reinforce the silo so it can withstand the value of P red corresponding
to A v = A x-sect.
2.6.2 Equipment and Processes
2.6.2.1 Do not use air cannons to break bridges in silos handling combustible materials if there are particles
smaller than approximately 500 microns within the material.
2.7 Bucket Elevators (3.1.26)
2.7.1 Construction and Location
2.7.1.1 Enclosed bucket elevators handling combustible dusts present an explosion hazard and need to
be installed in accordance with one of the following:
a) Locate bucket elevators outdoors.
b) Locate indoor bucket elevators adjacent to an exterior wall so explosion venting can be directed to
the outside via short ducts.
c) Provide the indoor bucket elevator with either explosion suppression or explosion venting through
FM Approved quench pipes.
2.7.2 Protection
2.7.2.1 For enclosed bucket elevators handling dusts with K st from 100 to 200, provide explosion venting
based on the following design:
a) Locate explosion vents no more than 20 ft (6.1 m) apart along the height of the bucket elevator.
b) Size each explosion vent at least as large as two thirds of the cross sectional area of the elevator leg
enclosure.
c) Vent the head section (i.e., top) of the bucket elevator leg based on a ratio of 1 ft 2 of venting for every
20 ft 3 of head section volume (1 m 2 per 6.1 m 3 ).
d) Provide explosion venting for the up and downside leg casings.
e) Set the explosion vent relief pressure to less than 1 psi (0.07 barg) and construct vents of lightweight
material.
Note: Because this recommendation is based on fullscale tests in a bucket elevator, use it only for this
application.
2.7.2.2 Where the elevator head or boot feeds into equipment or areas that have an explosion hazard (e.g.,
a dusty conveyor gallery), provide an explosion-blocking system using explosion-suppression devices or
other types of physical barriers, such as a product ‘‘choke’’ or air lock between the head or boot and adjacent
areas or equipment.
2.7.3 Ignition Source Control
2.7.3.1 To help prevent bucket-elevator explosions from occurring:
a) Provide belt driven elevators with a mechanical or electromechanical device to cut the power to the
drive motor and sound an alarm if the belt slows downs more than 20%.
b) Do not locate or expose bearings within the elevator casing.
c) Provide belt-alignment interlocks to shut down the elevator if the belt misaligns.
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