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

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7-76 Combustible Dust Explosion Page 12 FM Global Property Loss Prevention Data Sheets 2.3.3.1.6.1 Where venting devices are heavier than light-weight membranes, calculate the effect on the vent area using FM Global DustCalc software. 2.3.3.1.7 Install vent ducts that redirect the combustion products from the vent to a safe area in accordance with the following: a) Route the vent to a safe outdoor area. b) Permit no bends in the duct. c) Limit the length to diameter ratio (L/D*) of the duct to 1. d) Ensure the vent duct is at least as strong as the P red design of the vessel. e) Permit no closures on the discharge end of the duct that obstruct the free venting of the discharged material. f) When the above conditions cannot be met, quantify the effect on the vent area using FM Global DustCalc software (3.1.8). * calculate the equivalent diameter (D eff) of a non-circular duct as follows: D eff = √ 4A d π where A d is the cross sectional area of the duct (m 2 or ft 2 ) 2.3.3.1.8 Where a wire mesh screen or other obstruction is to be provided between an explosion vent and the free atmosphere, adjust the effective area of the explosion vent for the reduction in venting efficiency as follows: a) If the screen or obstruction is less than 15% of the explosion vent area, no adjustment is required. b) If the screen or obstruction is between 15% and 40% of the explosion vent area, calculate the effective explosion venting area using: A v,eff = A v,actual X 115-% blockage 100 c) If the screen or obstruction exceeds 40% of the explosion vent area, blockage is excessive. Replace the screen or obstruction with something that causes a smaller blockage. 2.3.3.1.9 Where explosion venting devices swing out of the way rather than rupture, use gravity or mechanical devices to ensure they cannot reclose and create vacuum conditions that can collapse/implode the protected equipment (3.1.9). 2.3.3.1.9.1 Where explosion vents can reclose and there are no other openings that could draw in air, provide vacuum breakers. (See German engineering standard VDI 3673 for sizing information.) 2.3.3.1.10 To prevent the free release of explosion vent panels where they could become missile hazards, provide tethers in accordance with 2.2.3.5. 2.3.3.1.11 Do not locate objects subject to fire or pressure damage in the path of explosion vents (3.1.10.1 and 3.1.10.2). 2.3.3.1.12 For new installations, provide a distance of at least two explosion vent diameters between an explosion vent outlet (face of explosion vent or vent duct) and any large, fixed, flat obstruction (e.g., a wall). For explosion vent outlets having cross sections other than circular, use the hydraulic diameter as calculated below (3.1.11). Hydraulic diameter = 4A/p, where A = cross-sectional area of vent p = perimeter of the cross-section ©2008 Factory Mutual Insurance Company. All rights reserved.
Combustible Dust Explosion 7-76 FM Global Property Loss Prevention Data Sheets Page 13 2.3.3.1.13 Where a weather protection cover (‘‘rain hat’’) is provided over the end of an explosion vent duct, estimate the effect on the venting efficiency as follows: a) If there is a distance of at least one explosion vent diameter between the end of the duct and the weather cover, there is no effect on efficiency. b) If the distance is between 1 ⁄4 and 1 explosion vent diameter, consider the weather cover as a 90° bend in the vented flow when calculating the effect of the vent duct. c) A distance of less than 1 ⁄4 explosion vent diameter away from the vent duct is unacceptable unless it is designed to fly off in the event of an explosion. For non-circular ducts, use the hydraulic diameter for these calculations (2.3.3.1.12). 2.3.3.1.14 Do not provide explosion venting if the dust or its combustion products are poisonous, radioactive, corrosive to nearby equipment or structures, or for any other reason should not be released from an otherwise closed system. Use alternatives to venting, such as explosion hazard elimination, explosion suppression, explosion containment, or high vacuum operation. 2.3.3.1.15 Where a vessel requiring explosion venting contains significant obstructions to the free flow of gases, provide several distributed vents at different places on the vessel rather than one large vent of the same area (3.1.12). 2.3.3.1.16 Where a vessel contains both suspended combustible dust and ignitable liquid vapors or flammable gases (a hybrid mixture), the reactivity of the mixture will require larger vent areas than the combustible dust on its own. a) The presence of a flammable vapor or gas can be ignored if the concentration is at or below 5% of its Lower Explosive Limit (LEL). b) Where the gas concentration exceeds 5% of its LEL, determine the reactivity of the mixture by testing. 2.3.3.1.17 The required explosion venting area for a vessel with a dust explosion hazard operating at pressures exceeding 0.1 barg (1.5 psig) requires special consideration and needs to be carefully analyzed. FM Global DustCalc software can handle initial pressure up to 4 barg (58 psig) for full-volume explosions. a) Set the vent-relief pressure, P stat above the normal maximum operating pressure by at least 0.1 barg (1.5 psig) (3.1.13). b) Obtain vent sizing criteria from experts familiar with high initial pressure venting. 2.3.3.1.18 Recoil forces from the venting of an explosion can dislodge even large vessels that are not properly anchored. Implement one of the following to control recoil forces: a) Provide equal-sized vents at opposite sides of the vessel. b) Calculate the magnitude and duration of the dynamic recoil force (or an equivalent static force) from a vent, and provide anchorage to resist those forces (3.1.10.3). 2.3.3.2 Isolation Avoid multiple pieces of interconnected equipment containing a dust explosion hazard. Properly protected equipment can fail when an explosion propagates from another piece of equipment. Isolation systems can prevent that condition (3.1.14). 2.3.3.2.1 Provide explosion isolation on all connections of vessels (or vessel groups) designed to contain the explosion pressure (explosion-resistant design). 2.3.3.2.2 Provide explosion isolation on all connections between vessels (or vessel groups) individually protected by explosion venting (or other mitigation method) but subject to unacceptable property damage or business interruption by a propagating event. 2.3.3.3 Isolation Systems This section includes acceptable forms of explosion isolation and the features needed to ensure the systems serve as effective flame barriers. ©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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