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

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7-76 Combustible Dust Explosion Page 8 FM Global Property Loss Prevention Data Sheets a) Position the explosion vents at a minimum 60° angle, either on a roof pitched at that angle or as a projection above the roof line (see Figure 2). For vents projected above the roof line, the effective vent relief area to be used in vent sizing calculations is the smallest cross sectional area the combustion gases would have to flow through. b) Provide heat tracing along the perimeter and across the surface area of the explosion vent. 1. Leave heat tracing on permanently, or automatically actuate the system whenever the outside temperature drops to 0°C (32°F) or lower. 2. Ensure the wiring for the heat trace cabling incorporates enough slack to allow the explosion vent to deploy as intended. 3. Use FM Approved heat-tracing equipment. c) Provide explosion venting panels without insulation to allow building heat to melt away snow or ice. Expect condensation under the explosion vent and take measures to ensure condensation does not cause problems. 2.2.3.4 Where explosion venting devices swing out of the way rather than rupture, use gravity or mechanical devices to ensure they cannot reclose. (3.1.9) 2.2.3.5 To prevent uncontrolled release of explosion vent panels where they could become missile hazards, take the following steps: a) Provide tethering cables to limit the vent movement. b) Attach tethering cables to no more than two corners, making sure the tethered corners are adjacent. c) To prevent the tethered panels from swinging back into the vent opening after the explosion, make connections at the side or bottom of the panel rather than the top. This allows the panels to completely swing out of the way of the vent opening, leaving an unobstructed path for the combustion gases to flow along during the explosion, and for fresh air to flow into the protected enclosure after the explosion. d) Set minimum tether length to that determined by the following equation: l ≥ a x b 2 x (a+b) where Fig. 2. Schematic of rooftop explosion vents that project above the roof line ©2008 Factory Mutual Insurance Company. All rights reserved.
Combustible Dust Explosion 7-76 FM Global Property Loss Prevention Data Sheets Page 9 l = length of tethering cables a, b = side dimensions of explosion vent e) Where the tether length is less than that determined above, consider the explosion vent as a hinged panel when performing calculations to determine the vent area and vented explosion pressure (P red). This will account for the hindrance to the venting process created by the short tethers. f) Where tethers are located at all four corners, this can be considered tolerable where the cable length exceeds the minimum set in d, above. Any less and the vent area is restricted to the annular space determined by the cable length and the dimensions of the panel, rather than on the size of the panel itself. 2.2.3.6 Do not attach sprinkler system piping to any wall, ceiling, or roof that could be displaced by the pressure of a room or building explosion. 2.2.4 Operation and Maintenance 2.2.4.1 In buildings where fugitive dust is released despite process design and equipment upkeep, incorporate the following actions into a fugitive-dust-control program: a) Assign accountability and keep records. b) Commit time and resources regularly. c) Create a maintenance schedule and review it periodically to ensure it is adequate. d) Pay particular attention to eliminating accumulations above floor level, such as equipment tops and building structural members because this dust is more likely to become suspended (airborne) and create an explosible cloud if it is disturbed. e) Use vacuum removal wherever possible using a central, portable, or truckmounted vacuum system as appropriate. f) Where vacuuming is impractical, sweeping or water wash-down are other acceptable options. g) Where compressed air blow-down is the only practical method for cleanup, use the following precautions: 1. Perform air blown-down frequently enough to prevent hazardous accumulations of dust. 2. Limit the extent of blow-down to small areas at a time with as low a volume and pressure of air as possible. 3. Shut down electrical equipment not suitable for Class II, Division 2 hazardous locations. 4. Prohibit open flames and hot work, and ensure no hot surfaces exist. 2.2.4.2 Any accumulations of fugitive dust present a potential for a secondary explosion and must be eliminated in buildings without damage-limiting construction (DLC), and controlled in those with DLC (3.1.1). a) Base dust-cleaning frequency on preventing accumulations of more than 1 ⁄16 in. (1.6 mm) thickness over more than 5% of the room floor area. b) Use fugitive dust exceeding the above, either in thickness or area, as a trigger to initiate cleanup. c) Ensure the total area of accumulation does not exceed 1,000 ft 2 (93 m 2 ). d) The thickness of 1 ⁄16 in. (1.6 mm) is based on a typical wood or agricultural dust with a bulk density of about 36 lbs/ft 3 (580 kg/m 3 ). The thickness of dust of different density can be based on a ratio of the bulk densities. 2.2.4.3 Inspect explosion venting devices periodically to prevent their condition or mobility from being impaired. Possible impairments include the following: a) Corrosion b) Improper painting of movable parts or rupture membranes c) Icing d) Snow accumulation on or in front of vents ©2008 Factory Mutual Insurance Company. All rights reserved.
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DS 7-76 Prevention and Mitigation of Combustible Dust ... - FM Global

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