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Preventing Grain Dust Explosions

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Preventing Grain Dust Explosions DIVISION OF AGRICULTURE RESEARCH & EXTENSION Agriculture and Natural Resources University of Arkansas System FSA1092 Preventing Grain Dust Explosions Introduction The National Safety Council Sammy Sadaka, defnes dust as solid particles derived Ph.D., P.E. Combustible dust explosion from crushing, grinding, rapid Associate Professor - hazards are prevalent in various impact and detonation of organic or Extension Engineer industries including, but not limited inorganic materials such as rocks, to, agriculture grain, food, chemicals, metal, wood or grain. Dust originates fertilizer, tobacco and pesticides. The from operations of dry and powdery Kingsly Ambrose, Ph.D. total number of reported agricultural Associate Professor - material such as conveying, trimming dust explosion incidents in the United of excess material, solids crushing Purdue University States reached 84 cases between 2009 and screening, sanding, tank and and 2018 (Fig. 1), resulting in 16 fatal- bin feeding and storing of granular John W. Magugu, Ph.D. ities and 96 injuries cases, respectively materials, among others. Professional Assistant - (Fig. 2). There were 12 dust explosion University of Arkansas incidents in 2018 alone, the highest rate in a decade. Dust explosions in grain elevators corresponded to 51 percent of all U.S. agricultural dust ex- plosion incidents, with many of these occuring in grain milling facilities. This fact sheet presents an overview on how to help prevent dust explosions in both industrial mill facilities and producer-owned facilities. Grain Dust Figure 3. Westwego Grain Dust Explosion in Louisiana, December 1977 (Courtesy of Gambit) Figure 1. Total U.S. Agricultural Dust Explosions 14 12 Grain dust is a highly combustible 10 material, and has more combustible 8 6 power than coal dust. The generation 4 and accumulation of the combustible 2 dust in grain facilities create explosion 0 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 risks and possess an immediate dan- II Number ger to human lives and grain facilities (Fig. 3). Alongside vapor cloud explo- Figure 2. Number of Reported Fatalities and Injuries from Grain Dust Explosions sions, dust explosions pose the most Arkansas Is 18 hazard within the process industry, 16 especially within grain elevators, bins, 14 Our Campus 12 silos, four bins and feed mills. The 10 8 particle size of dust generated during 6 grain handling and processing are 4 2 very small (< 500 µm), posing fagra- Visit our web site at: 0 tion and explosion risks. https://www.uaex.uada.edu 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 13Fatalities El Injured University of Arkansas, United States Department of Agriculture, and County Governments Cooperating Grain Dust Explosion Elements Most elements required for a grain dust explosion are present in a grain handling facility. Some — pri- The critical requirements of a fre are fuel, oxi- marily oxygen, grain dust and confned spaces — are dizer and ignition. These three elements are part diffcult to control. Apart from the usual fre triangle of the fve elements required for a dust explosion to of fuel (the dust), oxygen and heat, dispersion of dust occur (Fig. 4): particles in suffcient quantities and concentrations can cause rapid combustion (defagration). The pres- 1. Fuel – Tiny particles of dry grain dust from wheat, milo, oats, barley, four, corn starch and other par- ence of fammable gases, such as methane and its ticles are highly combustible. The National Fire heterogeneous mixtures within the dust mixture, Protection Association (NFPA) standard mentions increases the propensity of these gases to explode. that dust particles of less than 500 µm size can If one of the elements of the dust explosion lead to a fre and explosion. pentagon — fuel, oxygen, heat, dispersion or confnement — is missing, the possibility of an 2. Oxygen – Enough air supply with normal oxygen levels is always present in grain handling and explosion decreases. Eliminating ignition sources processing facilities. Oxygen concentration, usu- and controlling dust through improved material ally close to 21 percent, will increase the burning handling and housecleaning remain the most effective effciency of the fuel. techniques for preventing grain dust explosions. 3. Ignition source – Overheated bearings within Development and Occurrence of elevator leg boots, heads or conveyors; elevator Grain Dust Explosions leg belts rubbing against leg sidewall casings; Combustible grain dust is composed of fne par- electrical shorts, electrical arcs from a non-ex- ticles that pose an explosion hazard when suspended plosion proof electrical devices; static electricity; in the air under certain conditions. Several dust phosphine pellets or tablets exploding in a wet explosion incidents have resulted from employers’ or aeration duct; cigarette lighters or lit cigarettes; employees’ lack of awareness. It remains critical to cutting torches or weldings; metal sparks from determine if the facility has dust hazards and, if so, to grinders; metal to metal sparks; dropped tools take immediate action. and lightning are some of the potential ignition sources located in the grain elevator or feed mill In most cases, dust explosions often occur at facility. grain transfer points – in bucket elevators or enclosed conveyors (Fig. 5), where small dust particles separate – Grain dust must be sus- 4. Dispersion of dust from kernels due to tumbling, agitation and impacts, pended in the air to create an explosion. Layers of as fast-fowing grain hit the bucket elevator cups or dust in a confned space can ignite and lead to a change direction in drag or belt conveyors. Reducing minor explosion. This minor explosion may unset- the dust generation in these conveyors through tle the dust and create a secondary explosion of appropriate design substantially reduces this risk. greater magnitude. The air turbulence of the dust cloud within a grain dust matrix will increase the chances of an explosion. head pulley 5. Confinement – A vertical elevator leg casing or housing; an enclosed drag conveyor; basement tunnel; dust bin; aeration duct; downspout; bin f ,j..,C. ....... --..-- Drive reducer deck gallery or silo are some examples of confned --~ -- - Driving strap spaces within a grain handling facility. Within confned enclosures like buildings, rooms, vessels or process equipment, an explosion may result in a pressure rise that can damage the facilities. Figure 5. A schematic drawing of a Bucket Elevator System Figure 4. The Essential Elements for Grain Dust Explosion Mechanical or electrical devices can become a common grains including wheat four, corn starch potential ignition source. The ignition sources are usu- and rye dust are 410-430°C, 410-450°C and 410- ally either sparks or a hot surface. In some cases, igni- 500°C, respectively. tion of the dust cloud occurs directly; in other cases, fre or an open fame may take place frst, acting as 4. Minimum Ignition Energy (MIE) of Dust – The the ignition source for the dust cloud. Energy require- Minimum Ignition Energy of a dust cloud is less ments for igniting the dust clouds are quite small, and than 15 MJ. most ignition sources that occur during grain han- dling are capable of supplying suffcient amounts of 5. Maximum Explosion Pressure and Rate of – The maximum explosion this energy. Pressure Rise pressure and the rate of pressure rise depend For illustration, an initial explosion in the pro- on the dust type, dust particle size and dust cessing equipment or in an area where fugitive dust cloud concentration. Decreasing the particle size has accumulated may eject additional accumulated increases the explosion pressure, and to a more dust into the air. This dust, if ignited, may cause a signifcant degree, increases the rate of pressure secondary explosion, which is more likely fatal and rise. For most organic dust, the explosion pressure destructive. and rate of pressure rise tend to plateau at a particle size of 10 to 40 microns. Explanation of Grain Dust Terminologies Preventing Grain Dust Explosions For a grain dust explosion to occur, all the ele- Flame propagation within the dust cloud matrix is ments in the dust explosion pentagon must be present mainly a function of the nature of dust, dust particle (Fig. 4). Thus, if a single factor is removed, a dust size and the byproducts formed. The following termi- explosion can be prevented. The following approaches nologies must be understood: can reduce the formation of dust explosion pentagon: 1. Minimum Explosion Concentration (MEC) of 1. Modify the process to reduce dust handling Dust – Dust concentrations within certain limits effectively – Minimize hazardous materials, the of 0.050-0.100 kg/m3 (lowest concentration) and substitute less hazardous materials, and moderate 2.0-3.0 kg/m3 (highest concentration) are enough hazardous materials handling methods. for a dust explosion to occur, provided an ignition – Work source is present. Fine dust particles tend to com- 2. Prevent suspension of combustible dust with smaller piles of dust, remove dust and thor- bust faster than larger dust particles. ough clean dust “hot spots” within facilities. 2. Minimum Depths of Dust for Explosion (MED) 3. Remove the ignition sources entirely or at least – The minimum explosion depth (MED) varies minimize their presence – Eliminate ignition between 0.001 in and 0.021 in, depending on the sources traceable to workers’ actions, including depth of the enclosed space, as shown in Table 1. smoking, open fames, open light bulbs, weld- On the other hand, the maximum explosion depth ing, cutting, and grinding through training and (OED) for dust varies between 0.01 in and 0.20 in. enforcement. – Table 1. Relationships between the height of the enclosed space 4. Provide appropriate equipment maintenance and the minimum and optimum grain dust explosion depths. Read the equipment operator’s manuals, become familiar with them and follow the instructions for Height of enclosed Minimum depth of The optimum depth space (ft) dust for the explosion of dust for the regular equipment maintenance.
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