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

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 11
2.3.2.4.3 The metal powders titanium, magnesium, zirconium, uranium, and thorium, will ignite in pure carbon
dioxide. Use argon, helium, or nitrogen (except for titanium, which can be ignited in pure nitrogen) for inerting
processes handling these materials.
2.3.2.5 As an alternative to an inert atmosphere for handling freshly manufactured light metal powder
(aluminum, magnesium, titanium, zirconium) take the following steps:
a) Use wet collectors (water wash) rather than dry type.
b) Remove the accumulated sludge and dispose of it in a safe manner before shutting down the collection
system.
c) Continue collector ventilation at all times until the metal/water sludge is removed from the collector.
Aluminum particularly has a very low MIE (as low as 0.1 mJ) making it easily ignitable and metal powders
in general can have high K st and be very difficult to provide adequate explosion venting. Aluminum powder
when damp with water can form small amounts of hydrogen so it needs to be kept wet at all times.
2.3.3 Explosion-Hazard Mitigation
2.3.3.1 Venting
2.3.3.1.1 Provide a vent area in accordance with calculations conducted using FM Global DustCalc software
(3.1.4).
2.3.3.1.2 Where indoor equipment with an explosion hazard cannot be relocated outside, provide explosion
venting in one of the following ways:
a) Locate the vessel next to an exterior wall and vent the explosion to the outdoors via a short vent duct.
b) Vent the explosion to the surrounding area through an FM Approved explosion quench pipe (3.1.5).
1. A vent area increase (compared to an open or unobstructed vent) will be needed to accommodate
reduced venting efficiency caused by the quench pipe.
2. Use the venting efficiency for FM Approved quench pipes as listed in the Approval Guide, a publication
of FM Approvals.
Where the above is not practical, implement other explosion hazard mitigation methods described in 2.3.1.2.b.
2.3.3.1.3 For calculations of vent area where equipment design strength data is not available, use the
following values of P red (maximum allowable pressure) for normally constructed equipment with an
assumption that some vessel deformation may occur in a safely vented explosion:
a) Weak rectangular vessels (e.g., bag-type dust collector): 0.2 barg (2.9 psig)
b) Cylindrical vessels (e.g., cyclone) or strong (reinforced) rectangular vessels: 0.3 barg (4.4 psig) (3.1.6)
2.3.3.1.3.1 For vessels where deformation is not acceptable, obtain the design strength of the equipment
or assume 1 ⁄2 the values given above for P red.
2.3.3.1.4 Set the explosion vent relief pressure (P stat) as low as possible in accordance with the following
criteria:
a) Not exceeding 0.05 barg (0.7 psig) when vessel operates below this pressure, OR
b) At least 0.1 barg (1.4 psig) below the assumed P red for higher operating pressure
2.3.3.1.5 For calculations of vent area where equipment design strength data is available, set the value of
P red according to the following criteria:
a) Where vessel deformation is acceptable, use a value equal to twice the design strength.
b) Where vessel deformation is to be prevented, use a value equal to the design strength (3.1.6).
2.3.3.1.6 Construct explosion vents of material that is as light in weight (mass per unit area) as possible to
minimize the vent area required. Explosion vents that are rupture membranes (e.g., prefabricated rupture
disks, aluminum foil, etc.) have virtually no inertia, and require no adjustment to the required explosion vent
area (3.1.7).
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