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

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 19
2.3.4.2 To minimize dust-cloud ignition due to static electricity accumulation and discharge, implement the
following:
a) Apply all applicable recommendations in Data Sheet 5-8, Static Electricity, to minimize the probability
of ignition. Where plastic ducts are used to transfer material, they do not need bonding or grounding
because the insulating properties of plastic do not normally allow large capacitive charges to discharge
in a hazardous spark. The insulating properties also prevent metal grounding wires touching the duct from
draining charges away from the overall duct surface.
b) If materials are very easily ignitable (Minimum Ignition Energy less than 10 mJ), operate any equipment
with an explosion hazard under an inert environment (see 2.3.2.4) (3.1.19).
2.3.4.3 Provide magnetic separators upstream of all size-reduction equipment e.g., grinders, pulverizers,
hammer mills, or other equipment involving mechanical impact with the process material (3.1.20).
2.3.4.3.1 Separators other than magnetic (e.g., air separators, grates, coarse screens) may be used if
nonferrous metal or other objects (e.g., rocks) could enter the product stream and cause an ignition hazard.
2.3.4.4 Fans and blowers in the fugitive dust-air stream can become an ignition source. Install these devices
as recommended below:
2.3.4.4.1 In negative pressure systems, locate the fan on the discharge side (i.e., clean side) of the dust
collector.
2.3.4.4.2 In positive pressure systems, locate the blower upstream of the dust injection point.
2.3.4.4.3 Where, for design reasons, the fan must be located in the dirty air stream, and the dust/air stream
concentration is higher than the 25% of the MEC:
a) install fans and blowers of Type A or B spark-resistant construction per AMCA 99-0401-86,
Classifications for Spark Resistant Construction
b) comply with the appropriate criteria in 2.5, Connecting Ducts, below.
2.3.4.4.4 Ordinary fans and blowers may be used in a dust-air stream of unlimited concentration if the dust
has been shown by test to be hardtoignite.
2.3.4.4.5 Systems handling wood dust can use ordinary fans upstream of bagtype dust collectors if a cyclone
collector (primary dust collector) is upstream of the fan.
2.3.4.4.6 Ordinary fans and blowers may be used if a high-speed abort gate or FM Approved sparkextinguishing
system is provided between the fan and any important or valuable downstream equipment.
2.3.4.5 Prevent materials subject to spontaneous heating from becoming an explosion initiation source by
implementing the following:
a) Prevent accumulation in ductwork by maintaining sufficient transport velocity.
b) Prevent accumulation in equipment by performing frequent cleaning.
c) Do not allow moisture to contact such material; however, automatic sprinklers or spark-extinguishing
systems can be used in ducts, when needed.
d) Clean collectors handling residues subject to spontaneous heating daily or as needed to prevent heating
and hazardous accumulations.
2.3.4.6 Avoid using mechanical drives with high rpm or power as they can cause dust ignition due to heat
generated by friction or sparks.
2.3.4.6.1 Use the following guidelines to determine the potential for hazardous conditions based on the
tangential velocity (v) of the rotating component:
a) When v < 1 m/s (3.3 ft/s): there is no danger for ignition.
b) When 1 < v < 10 m/s, (3.3 < v < 33 ft/s): judge each case separately considering the product and material
specific characteristics such as MIE and particle size (for each, the lower value will tend to be more
susceptible to ignition).
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