Miller Magazine Issue: 155 November 2022

91 MILLER / NOVEMBER 2022 ers would need to take measures to minimize explosion propagation risks. Which processes are most vulnerable to dust explosions? According to industrial dust explosion statistics1, dust collectors, mills and dust conveying systems are the top 3 processes prone to explosions. However based on the US Grain Industry dust explosion statistics 64% of all dust ex- plosions occurred inside bucket elevators, 48% in the mill- ing processes, and 17% in the silos. What types of materials are susceptible to feed and grain dust explosions? Most materials used in the feed and grain industry are combustible. Their dust clouds are considered as ex- plosible dusts. Particle size and moisture content are major determining factors on the explosibility of such dusts. As the moisture content and particle size gets smaller, the ig- nitibility of the dust becomes much easier, becoming more explosible. To understand the ignition sensitivity and ex- plosion severity of dusts, it is important to understand their characteristics. Typical explosibility values that need to be determined via testing3 are provided in the table below. Explosion pressures expected from a grain dust explosion can reach up to 8 barg which is 10 -12 times higher than the typical mechanical resistance of most process equipment. What are some of the probable ignition sources initiating such events? According to EN1127-1, there are potentially 14 different ignition sources in an industrial process. It is crucial for plant operators to conduct ignition source assessments to ensure they are either not present or there are measures taken to control such sources. Feed and grain operations include many moving parts such as mills, elevators which can potentially generate sparks, frictional heating as well as hot surfaces. What are some of the prevention and protection measures against such explosions? There are seven industrially recognized methods for dust explosion prevention and protection. In many cases multi- ple measures are taken together in order to minimize risks to an acceptable level. Most commonly utilized methods are: 1) Control of ignition sources (prevention), 2) Combus- tible dust reduction (e.g. cleaning, dedusting, replacement of the material with coarse ones), 3) Inerting (oxygen reduc- tion), 4) Containment (pressure shock resistant process de- sign), 5) Explosion venting, 6) explosion suppression, and in addition to 4, 5 and 6; 6) Explosion isolation. Choice of the appropriate risk reduction measure should be based on explosion risk assessment and by expert advice from Picture 2 – Feed mill after a dust explosion 5 Equipment USA UK Germany (1985 – 1995) (1979 – 1988) (1965 – 1980) Count % Count % Count % Dust Collectors 156 42 55 18 73 17 Mills 35 9 51 17 56 13 Conveying systems 32 9 33 11 43 10 Silos/bunkers 27 7 19 6 86 13 Dryers 22 6 43 14 34 8 Mixers, blenders >12 >3 7 2 20 5 Unknown 84 23 95 31 114 27 Total 372 100 303 100 426 100 Table 1 – Industrial Dust Explosion Statistics per Process Equipment 1,2 ARTICLE