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

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7-76 Combustible Dust Explosion Page 36 FM Global Property Loss Prevention Data Sheets enclosure, any fines will be easily picked up into suspension. Because more fines are continuously added to the air space in the elevator with very little are falling out, the concentration can exceed the MEC and create a dust explosion hazard. An example of this involves unground soybean extraction grouts, which are not explosible. Most of this product is very coarse material (median particle size approx. 1000 microns) but about 3% of fines are smaller than 63 microns. In one instance, after conveying this product in a bucket elevator for several minutes, the fines were liberated and an ignition and explosion occurred. 3.1.27 Size-Reduction Equipment (Grinders, Pulverizers, Hammer Mills, etc.) Size-reduction equipment presents the inherent hazard of mechanical impact in the presence of combustible dusts. The mechanical impact can create frictional and impact heating. Sparks may occur if unintended metal enters the machine. The result can either be glowing particles leaving the equipment to become an ignition source in downstream equipment or actual ignition of the dust cloud in the equipment. These machines are most frequently of substantial construction and able to withstand an internal explosion without damage. Explosion venting or other protection techniques are not needed. Maintain these devices regularly to ensure adequate lubrication is provided, proper clearances are maintained, and dust and debris don’t accumulate and become ignited or impair proper operation. 3.2 Loss History Loss history was surveyed for dust explosion losses for the period 1983 through 2006. A total of 166 losses at FM Global client locations resulted in a gross loss of US$284 million (all figures indexed to 2007 dollars). The losses by industry group are shown in Table 3. In five of the top six categories there was one event that was substantially larger than typical in terms of gross loss. This will skew any estimate of average loss based on the total number of losses. In fact, the top 2 losses account for almost 60% of the loss cost. Loss history is best used to help prioritize what materials, processes, and equipment should be the focus of protection efforts. Table 3. Losses by Industry (by Number of Losses) Industry Type No. Losses Woodworking 64 Food 26 Metals 18 Chemical/Pharmaceutical 14 Pulp/Paper 12 Mineral 11 Utility 7 Plastics 5 Rubber 5 Printing 1 Textile 1 Other 2 Total 166 Table 4 examines the loss data for the ignition source that caused the event. Friction and sparks have been commonly considered the major ignition source and the loss data shows that. Hot work is also a significant contributor. ©2008 Factory Mutual Insurance Company. All rights reserved.
Combustible Dust Explosion 7-76 FM Global Property Loss Prevention Data Sheets Page 37 Table 4. Losses by Cause (Ignition Source) Cause Type No. Losses Friction 50 Spark 38 Chemical Action 16 Hot Work 13 Burner Flame 10 Electricity 6 Static Electricity 6 Overheating 4 Hot Surface 2 Unknown/No Data 21 Total 166 Another informative approach to investigate would be the dust group involved in the loss. Table 5 shows that the wood group is by far the most significant by number of losses. Table 5. Losses by Dust Type Dust Group No. Losses Wood 70 Food 25 Chemical 17 Metal 15 Coal 14 Plastic/Rubber/Resin 13 Paper 8 Various 4 Grand Total 166 Looking at the equipment type involved, the losses in Table 6 clearly show that dust collectors are by far the leading piece of equipment to experience explosions. Obviously, a dust collector has dust that is suspended in air. Also, because a dust collector is designed to handle material produced elsewhere, the ignition source does not have to come from within the dust collector. A source picked up in the dust production area can ignite an explosion in the dust collector. Explosions in impact equipment are also frequent. The data shows 22 incidents involving grinders, sanders, pulverizers, ball and hammer mills, chippers, shredders, etc. The mechanical energy expended in these processes can translate into the generation of heat or spark energy required to ignite a dust cloud. In Table 6, the “various” category consists of 30 single-occurrence items. ©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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