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

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 25
c) Develop a maintenance and inspection program to guarantee the dust suppressant spray system is
functioning properly whenever grain is being handled.
d) Install an interlock that will shut down grain handling on suppressant spray system malfunction where
this system is being relied upon as the sole means of explosion protection.
2.10 Dust Fire Hazards
In addition to the explosion hazard addressed by this document, combustible dusts can present a fire hazard
that must be controlled. The following data sheets cover protection and prevention of dust fires in various
equipment or processes:
a) Inside spray dryers: Data Sheet 6-9, Industrial Ovens and Dryers
b) Wood processing and wood working facilities: Data Sheet 7-10, Wood Processing and Woodworking
Facilities
c) Dust collectors: Data Sheet 7-73, Dust Collectors and Collection Systems
d) Grain handling facilities: Data Sheet 7-75, Grain Storage and Milling
e) Ducts: Data Sheet 7-78, Industrial Exhaust Systems
f) Wood dust storage: Data Sheet 8-27, Storage of Wood Chips
3.0 SUPPORT FOR RECOMMENDATIONS
3.1 Comments and Technical Support
3.1.1 Is There a Housekeeping Problem? (2.2.4.2)
The fundamental rule is that any dust accumulations outside of equipment present a potential for a secondary
explosion and should be removed and the source of the release eliminated. Provide DLC for areas that are
subject to such uncontrolled releases that can’t be resolved.
As a rule of thumb, 1 ⁄16 in. (0.0625 in., 1.6 mm) of dust is a cause for cleanup. (This assumes wood dust
with approximate bulk density of about 36 lb/ft 3 [580 kg/m 3 ]). This is about the thickness of a US quarter.
Where dusts of different bulk density are used, the thickness of the equivalent mass of 1 ⁄16 in. (1.6 mm) wood
dust would be inversely proportional to their bulk densities. For example, for a dust with a bulk density of
about 60 lb/ft 3 (950 kg/m 3 ), a layer of about 0.04 in. (1 mm) thick would be equivalent.
Equivalent thickness = 1.6 mm x (580 kg/m 3 /950 kg/m 3 ) or 1 mm
Any dust accumulations in a small room (200 ft 2 , 19 m 2 ) would present a severe secondary explosion hazard.
In a larger room, for example 20,000 ft 2 (1860 m 2 ), dust covering an area of 200 ft 2 (19 m 2 ) would be a
less-severe event.
For practical purposes, some dust accumulations in small parts of large areas without DLC can be considered
tolerable with frequent cleanup and actions to remove the source of the dust. Consider 5% of the area with
1 ⁄16 in. (1.6 mm) dust capable of being thrown into suspension as the limit of ‘‘tolerable.’’ In any building of
ordinary construction, any area of dust accumulations exceeding about 1,000 ft 2 (93 m 2 ) is considered
unacceptable.
Any dust that is elevated above floor level in joists, beams, equipment tops, etc., should be considered as
being capable of being thrown into suspension. Dust that has agglomerated by age, heat, moisture, etc.,
should not be considered as capable of being thrown into suspension but may suggest an ongoing
housekeeping problem or source of uncontrolled release. Dust on the floor could be suspended but is less
of a risk than elevated accumulations.
Pay close attention to dust adhering to walls since this is easily dislodged. Also consider other projections,
such as light fixtures, that can provide surfaces for dust accumulation.
The available surface area for dust deposits on bar joists or steel beams can be roughly estimated at 5%
of floor area. However, some steel beams might have an equivalent surface area as high as 10% of floor area,
such as when spans between columns are longer than average for a given geographical area, or when a
large building elevation difference requires a stronger roof structure due to anticipated snow drift loadings.
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