Case Studies for Dangerous Dust Explosions in South Korea During Recent Years

sustainability

Article Case Studies for Dangerous Dust Explosions in South Korea during Recent Years

1, 2, 3,4,5 6, Seonggyu Pak †, Seongho Jung †, Changhyun Roh and Chankyu Kang * 1 Korea Occupational Safety and Health Agency, Major Accident Prevention Center in Chungju, 85 Daerimro, Chungju, Chungbuk 27477, Korea 2 Department of Environmental and Safety Engineering, Ajou University, Worldcupro 206, Yeongtong-gu, Suwon 16499, Korea 3 Decommissioning Technology Research Division, Korea Atomic Energy Research Institute (KAERI), 989-111 Daedukdaero, Yuseong, Daejeon 34057, Korea 4 Advanced Radiation Technology Institute (ARTI), Korea Atomic Energy Research Institute (KAERI), Jeonbuk 56212, Korea 5 Quantum Energy Chemical Engineering, University of Science and Technology, 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea 6 Ministry of Labor and Employment, Chemical Accident Prevention Division, 422 Hannuridae-ro, Sejong 30117, Korea * Correspondence: [email protected]; Tel.: +82-44-200-2556 These authors contributed equally to this work. † Received: 4 August 2019; Accepted: 31 August 2019; Published: 6 September 2019

Abstract: Despite recent extensive research and technical development to prevent and mitigate dust explosions, processes that produce and handle combustible materials in the form of powders and dusts, either as a main product or as an undesired by-product, have become a constant dust explosion threat as they become more sophisticated and complicated. This study analyzed the characteristics of 53 dust explosions that occurred in South Korea over the last 30 years, and investigated the differences of dust explosions that happened in various countries, such as Japan, the United States, the United Kingdom, and France. In addition, case studies showed the severity of dust explosions occurring in South Korea. Through the special focus on the three most recent years of dust explosions, the causes and processes of the accidents were identified. Analyses of dust explosions in South Korea show that they were mainly caused by organic matter and metal, and, unfortunately, dust explosions occurred repeatedly during grinding, mixing, and injection of powder materials into facilities. No reported accidents occurred during the production processes of wood or paper during the last three years. Taking these characteristics into account, effective ways to prevent or mitigate dust explosions at workplaces where many dust explosions occurred were suggested.

Keywords: dust explosions; facility; prevent; mitigate; organic matter; metal

1. Introduction Many industrial dust explosions occur in powder-processing equipment during crushing, grinding, conveying, storing, and other processes [1]. These operations are still widely used in industrial plants and will be applied to new ﬁelds of industry, potentially triggering many dust explosions in the future [2]. As a result, the threat of dust explosions remains a continued risk [3]. Many kinds of industrial plants use powder manufacturing facilities, which produce several types of dust known to be explosive, and cause catastrophic loss of life and property [4]. In a plant environment where ﬁnely ground dust spreads in the air, dust explosions may occur immediately, depending on the source of ignition if certain conditions (i.e., minimum explosive concentration (MEC)) are reached. The

Sustainability 2019, 11, 4888; doi:10.3390/su11184888 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 4888 2 of 13 types of dust that cause dust explosions are wood, paper products, grain and food, metal and metal products, power generation, coal mining, and textile manufacturing. Factors such as dust properties, dust particle size, and the formation of combustion by-products vary the speed and extent of flame propagation and have a complex mechanism in that they include simultaneous momentum, energy, and mass transport in reactive multi-phase systems [5,6]. In addition, it is a phenomenon involving a rapid increase in pressure and heat, accompanied by sound, which is caused by the fact that the substance burns at a very high rate [7]. Dust explosions seriously destroy human beings, properties, and the environment. Among the classification of dusts, relative dust explosion severity is classified by the Kst index, divided into four groups from non-explosive (i.e., St 0) to highly explosive (i.e., St 3) [8]. Several factors, such as dust concentration, the composition of the dust and moisture content, particle size and shape, type of dust, and turbulence in the system, affect the degree of ignition and propagation of flame in the dust cloud [9,10]. This means that substances with a high risk of dust explosion (i.e., smaller sizes and thin flats of particles, metallic particles, a dust cloud with less oxygen content) need special attention during process control. Recently, several dust explosions have caused serious damage: (i) 14 deaths and 38 injuries occurred in the Imperial Sugar refinery in the United States in 2008 [11]; (ii) an accident occurred at a maize starch plant in China in 2010, killing 21 employees and injuring 47 [12]; (iii) an HDPE (high density polyethylene) dust explosion occurred in the silo at the Yeosu Industrial Complex in South Korea, resulting in the deaths of six workers who were in the process of installing a manhole, and injuring an additional 11 workers on 14 March, 2013; (iv) an aluminum dust deflagration occurred at an auto parts factory in Jiangsu, China, on 2 August, 2014, resulting in a catastrophic 146 deaths and 114 injured [13], with the investigation concluding that the bag filters in the dust collection bags had to be cleaned by mechanical shaking at regular intervals, but the primary deflagration had begun with the shaking system, which was out of operation for a long period of time; (v) in 2015, 10 people were killed and 485 were injured by an explosion of a colored, starch-based dust at a party called “Color Play Asia” in Taiwan [14]. Among various types of dusts, there have been many studies on aluminum dust because it is the most explosive among metal dusts, and accounted for about 25% of the metal dusts from 1980 to 2005 in the United States [15–17]. The temperature of metal dust flame is more than 1000 K higher than organic dust flame, which causes an increase in pressure. The risk of dust explosion as a result of increased temperatures and pressure has contributed to metal dust receiving more attention [18]. Meanwhile, coal dust explosions were the most frequent and serious accidents in China, with 106 cases reported between 1949 and 2007 [19]. Fabiano et al. (2014) analyzed 98 incident statistics related to coal between 2000 and 2011 using data bank FACTS, managed by the Unified Industrial and Harbor Fire Department of Rotterdam–Rozenburg, NL [20]. Coal and coal dust caused 171 and 39 fatalities, respectively, and classifying the major accident scenario corresponded to explosions (56.0%), followed by fires (30.8%) and releases (13.2%). Analysis of the incidents revealed that two-thirds of them occurred in power and process plants due to human error and technical failure/malfunction of a component or equipment. To create a dust explosion, the necessary conditions for dust explosions are well represented by the explosive pentagon: (i) Fuel, (ii) oxidant, (iii) ignition source, (iv) mixing of the fuel and oxidant, and (v) confinement of the resulting mixture [21]. There must be a certain density of dispersed dust in the air so that the dust accumulates. A confined space maintains sufficient pressure for the primary dust explosion to occur [22]. The consequence of the secondary dust explosion causes disastrous and uncontrolled explosions, which occur when the blast wave from the primary explosion entrains the dust layer and propagates through the plant. The dust becomes the fuel of the emerging flame, leading to extensive explosions due to large amounts of dust and the high energy of ignition [23]. Several controversial issues have recently been solved to provide efficient preventive and mitigatory measures. Farrell et al. (2013) conducted an experiment with a large-scale test rig, consisting of a 5 m3 vented vessel connected to two 400 mm diameter pipelines, to study dust explosion propagation. This study showed that the dust explosion was able to move against a process flow at a long distance [24]. Sustainability 2019, 11, 4888 3 of 13

Vogl and Radandt (2005) carried out the experiment and concluded that powder handling plants Sustainability 2019, 11, x FOR PEER REVIEW 3 of 12 should be cautious with design and operation because of the possibility of dust explosion propagation, evendust explosion with small-diameter propagation pipes, even [ 25with]. Valiulis small-diameter et al. (1999) pipes and [25] Farrell. Valiulis et al.et al. (2013) (1999) concluded and Farrell that et aal. dust (2013 explosion) concluded can that propagate a dust ifexplosion there is can little propagate to no dust if atthere all isin little the connected to no dust pipes at all [24 in ,26 the]. Recently,connected the pipes development [24,26]. Recently, of advanced the development numerical models of advanced has played numerical an important models role has in providingplayed an guidelinesimportant role on thein provi stableding and guideline optimizeds on design the stab ofle process and optimized equipment design to mitigate of process or equipment prevent dust to explosionsmitigate or [27prevent]. However, dust explosion controversials [27] subjects,. However, such controversial as explosion subject ventings, andsuch secondary as explosion explosions, venting areand still secondary unsatisfactory explosion becauses, are these still issues unsatisfactory are subject because to increased these complexity issues are and subject process to variability increased duecomplex to processity and automation. process variability due to process automation. The purpose of this studystudy isis toto provideprovide informationinformation onon dustdust explosionsexplosions that occurredoccurred inin SouthSouth Korea,Korea, and to comparecompare themthem with dust explosions in Japan, the UnitedUnited States,States, thethe UnitedUnited Kingdom, and France.France. The characteristics of dust explosion explosionss are presented through analysis of their causes over 30 yearsyears inin SouthSouth Korea.Korea. C Casease studies of catastrophic dust explosions in South Korea present thethe severityseverity and damagedamage ofof thethe accidents.accidents. Special focus on the last three years of dustdust explosionsexplosions is toto identifyidentify the the causes causes and and process process of dust of dust explosions, explosion ands, toand suggest to suggest effective effective ways to ways prevent to orprevent mitigate or themmitigate in workplaces. them in workplaces. To the best To ofthe our best knowledge, of our knowledge, this is the this first is studythe first to study introduce to introduce several cases several of dustcases accidents of dust accidents and suggest and preventive suggest preventive measures becausemeasures there because is little there awareness is little and awareness little information and little aboutinformation dust explosions about dust in explosions South Korea. in South Korea. 2. Dust Explosions in South Korea from 1984 to 2018 2. Dust Explosions in South Korea from 1984 to 2018 2.1. Dust Explosions for South Korea 2.1. Dust Explosions for South Korea From 1984 to 2018, 53 combustible dust- and flammable solid-related fire explosions were reported From 1984 to 2018, 53 combustible dust- and flammable solid-related fire explosions were in South Korea. Figure1 identifies dust explosions by type of industry and their main cause. According reported in South Korea. Figure 1 identifies dust explosions by type of industry and their main cause. to the analysis of dust explosions based on the types of industries shown in Figure1a, 30% of all dust According to the analysis of dust explosions based on the types of industries shown in Figure 1a, 30% explosions occurred in the metal industry, 25% in the chemical industry, and 21% in the food industry. of all dust explosions occurred in the metal industry, 25% in the chemical industry, and 21% in the Dust explosions in wood and paper industries were as low as 9%. As reflected in the industrial food industry. Dust explosions in wood and paper industries were as low as 9%. As reflected in the structure, the major cause of dust explosions, shown in Figure1b, was metal, which accounted for 44% industrial structure, the major cause of dust explosions, shown in Figure 1b, was metal, which of all dust explosions. Plastic and food sources each cause 19% of dust explosions. The number of accounted for 44% of all dust explosions. Plastic and food sources each cause 19% of dust explosions. accidents due to combustible metal dusts was the highest, but explosions caused by plastics accounted The number of accidents due to combustible metal dusts was the highest, but explosions caused by for about 50% of deaths and about 47% of injuries. Deaths due to weakly explosive plastic dusts were plastics accounted for about 50% of deaths and about 47% of injuries. Deaths due to weakly explosive over three times more than those of strong explosive metal dusts, presumably due to the establishment plastic dusts were over three times more than those of strong explosive metal dusts, presumably due of government safety guidelines and industrial characteristics. to the establishment of government safety guidelines and industrial characteristics.

Figure 1.1. Analysis of dust explosionsexplosions in SouthSouth Korea.Korea. (a) Occurrence of dust explosions by typetype ofof industry;industry; ((bb)) CauseCause ofof dustdust explosions.explosions.

2.2. Recent Dust Explosions Table 1 presents examples of dust explosions from May 2015 to May 2018. Dust explosions caused by organic matters and metal caused significant property loss as well as human loss. Analysis of major accident processes reveals that accidents occurred repeatedly during grinding, mixing, and Sustainability 2019, 11, 4888 4 of 13

2.2. Recent Dust Explosions Table1 presents examples of dust explosions from May 2015 to May 2018. Dust explosions caused by organic matters and metal caused signiﬁcant property loss as well as human loss. Analysis of major accident processes reveals that accidents occurred repeatedly during grinding, mixing, and injection of powder materials into facilities, while no accidents occurred in the production process of wood or paper during the last three years.

Table 1. Dust explosion in South Korea (May 2015 to May 2018).

Year/Month Equipment Hazard Material Accident Process 2018/5 Rocket propulsion plant Ammonium perchlorate Grinding 2018/5 Dust collector Zirconium oxide Grinding 2018/4 Dissolution Tungsten oxide alloy Injection of metal powder 2018/4 Alloy Manufacturing Co–Cr alloy Manufacture of alloy powder 2017/7 Silo Polypropylene PP pellet transfer 2016/1 Mixer Propanoic acid Mixing process 2015/10 Silo Terephthalic acid Welding and cutting of manhole 2015/9 Chemical Manufacturing Aluminum oxide mixture Mixing process 2015/3 Reactor Terephthalic acid Injection of Terephthalic acid into reactor 2015/5 Melting furnace Waste Aluminum Injection of waste aluminum into furnace

3. Case Studies There are many severe dust explosions in South Korea, and these accidents occur not only in chemical plants but also in general storage facilities. Through three case studies, the problems and characteristics of those dust explosions, which caused many casualties and property loss, can be identiﬁed.

3.1. HDPE (High Density Polyethylene) Explosions On 14 March, 2013, an explosion occurred in the HDPE silo at Yeosu Industrial Complex, killing six workers during a manhole installation and injuring eleven workers. In addition to human loss, it caused property damage to nearby factories and destroyed three silos at the factory. The accident investigation revealed that HDPE powder was attached to the inner wall of the silo body, which was filled with air, and the bag filter. During the manhole installation work, some powder dust fell off the lower side and created a deposit due to the vibration caused by tools. The primary explosion happened when the flammable material inside the silo was ignited by the welding sparks during the manhole installation. Heat and shock waves generated in the first silo, as shown in Figure2, and spread to neighboring silos through the piping connected to the outlet of the HDPE powder bag filter, resultingSustainability in the 2019 secondary, 11, x FOR PEER explosion. REVIEW 5 of 12

FigureFigure 2. (a )2. The (a) siloThe wheresilo where the dust the dust explosion explosion ﬁrst first began; began; (b) Image(b) Image of silo of silo bottom bottom due due to sudden to sudden impact. impact.

3.2. PP (Polypropylene) Silo Explosions An explosion occurred inside the polypropylene silo at the Yeosu Industrial Complex, causing a fire in the polypropylene stored inside the silo, resulting in the burning of one aluminum silo on 10 July, 2017. Fortunately, there was no human injury due to the shift time. A fire and explosion, as shown in Figure 3, caused damage to the top of the silo (about 12 m). It is possible that the unreacted components, such as propylene and ethylene impregnated in the polypropylene powders, were desorbed and accumulated in the silo during the synthesis process because the polypropylene was synthesized in a spherical shape around the catalyst. In fact, there was a danger that the flammable gas concentration in the storage silo could increase with time. The flammable gas was measured to 3764 ppm after the fire suppression. Although the gas concentration meter for measuring propylene concentration was present in the process, the calibration of the gas sensor was set without multiplying the gas correction value for methane, and it was mistakenly assumed that it did not exceed the lower explosion limit (LEL) at that time. As the pellet product was conveyed by air pressure, friction between the pipe and the pellet occurred. As a result, friction static electricity was generated and polypropylene dust floating in the silo was charged. Under normal conditions, it was transported to the silo, and even if charged, the residual fine dust was discharged through an air purification device. However, it was presumed that dust with static electricity was introduced into the storage silo, the lower part of which was open at the time of the accident, and the discharged pellet product acted as an ignition source for the flammable gas stagnation space. Estimation of the oxidant suggested that air blowers were used to transport the product to the polypropylene storage silo, and pellets were discharged to the bottom of the accident storage silo, so that air was continuously being introduced through the upper vent pipe.

Figure 3. (a) Image of fire after silo explosion; (b) Image of upper platform deviating from silo. Sustainability 2019, 11, x FOR PEER REVIEW 5 of 12

Sustainability 2019, 11, 4888 5 of 13

Details of the accident are as follows: Welding was carried out at six different positions in the silo where the fire was ignited. In general, welding is known to be a very powerful ignition source that ignites all types of explosive mixtures. Aluminum alloy, which the silo was made of, has a thermal conductivity three to five times higher than that of iron alloy. Polyethylene dust attached to the inside of the silo and the bag filter acted as a combustible material and was ignited by welding sparks, resulting in fire. Generally, combustible dust can cause an explosion if the particle size is less than 420 µm. At this time, the polyethylene dust collected inside the silo had an average size of about 60 µm, which is a condition that easily causes an explosion. It is assumed that an explosive environment had been created because of the accumulation of dust and flammable gas. To summarize the progress of the explosion in the silo: (i) The occurrence of fire during welding, (ii) the diffusion on the wall, and (iii) an explosion in the upper position. During the work after the initial gas measurement, continuous ventilationFigure and2. (a measurement) The silo where of the gas dust must explosion be carried first out. began; Unfortunately, (b) Image of thesilo organization’sbottom due to sudden permission to workimpact without. ensuring that no explosive mixtures were present in the silo caused the dust explosion.

3.2.3.2. PP (Polypropylene) Silo Explosions AnAn explosionexplosion occurredoccurred insideinside thethe polypropylenepolypropylene silo silo at at the the Yeosu Yeosu Industrial Industrial Complex, Complex, causing causing a firea fire in in the the polypropylene polypropylene stored stored inside inside the the silo, silo, resulting resulting in the in burningthe burning of one of aluminumone aluminum silo onsilo 10 o July,n 10 2017.July, 2017 Fortunately,. Fortunately, there wasthere no was human no human injury dueinjury to thedue shift to the time. shift A firetime. and A explosion,fire and explosion, as shown inas Figureshown3 ,in caused Figure damage 3, caused to thedamage top of to the the silo top (about of the 12 silo m). (about It is possible 12 m). It that is possible the unreacted that the components, unreacted suchcomponents as propylene, such andas propylene ethylene impregnatedand ethylene in impregnated the polypropylene in the p powders,olypropylene were powder desorbeds, w andere accumulateddesorbed and in accumulated the silo during in the synthesissilo during process the synthesis because process the polypropylene because the was polypropylene synthesized wa in as sphericalsynthesized shape in a around spherical the shape catalyst. around In fact, the there catalyst. was In a danger fact, there that was the flammablea danger that gas the concentration flammable ingas the concentration storage silo couldin the increasestorage silo with could time. increase The flammable with time. gas The was flammable measured togas 3764 was ppm measured after the to fire3764 suppression. ppm after the Although fire suppression. the gas concentration Although the meter gas concentration for measuring meter propylene for measuring concentration propylene was presentconcentration in the process,was present the calibrationin the process, of the the gas calibration sensor was of the set gas without sensor multiplying was set without the gas multiplying correction valuethe gas for correction methane, value and itfor was methane, mistakenly and it assumed was mistakenly that it did assumed not exceed that it the did lower not exceed explosion the lower limit (LEL)explosion at that limit time. (LEL) As at the that pellet time. product As the was pellet conveyed product by was air pressure,conveyed friction by air between pressure, the friction pipe andbetween the pellet the pipe occurred. and the As pellet a result, occur frictionred. As static a result, electricity friction was static generated electricity and polypropylenewas generated dustand floatingpolypropylene in the silo dust was floating charged. in the Under silo was normal charged conditions,. Under it normal was transported conditions, to it the wa silo,s transported and even to if charged,the silo, and the even residual if charged, fine dust the was residual discharged fine dust through was discharged an air purification through an device. air purification However, device. it was presumedHowever, thatit was dust presumed with static that electricity dust with was static introduced electricity into was the introduced storage silo, into the lowerthe storage part of silo which, the waslower open part at of the which time was of the open accident, at the andtime the of dischargedthe accident pellet, and productthe discharged acted as pellet an ignition product source acted for as thean ignition flammable source gas stagnationfor the flammable space. Estimation gas stagnation of the space oxidant. Estimation suggested of that the air oxidant blowers suggested were used that to transportair blowers the were product used to to the transport polypropylene the product storage to silo,the polypropylene and pellets were storage discharged silo, and to the pe bottomllets were of thedischarged accident to storage the bottom silo, so of that the airaccident was continuously storage silo, beingso that introduced air was continuously through the being upper introduced vent pipe. through the upper vent pipe.

Figure 3. (a) Image of ﬁre after silo explosion; (b) Image of upper platform deviating from silo. Figure 3. (a) Image of fire after silo explosion; (b) Image of upper platform deviating from silo. Sustainability 2019, 11, x FOR PEER REVIEW 6 of 12

3.3. Dust Explosion by Ammonium Perchlorate An accident occurred in a combustion chamber while charging slurry ammonium perchlorate propellant in the mixer ball into the combustion tube, causing a disastrous explosion. This accident occurred on 29 May, 2018, in the Daejeon area, as shown in Figure 4. A sudden fire occurred in the propellant, resulting in the deaths of five workers and injuries of four workers, who were working on site. The explosion occurred during the opening of the discharge valve of the mixer bowl, which was the preparation of the propellant charge. The accident investigation was carried out through analysis of the three basic elements of fire. The propellant used a binder that had such physical properties as plastic or rubber, and was intended Sustainabilityto bond the2019 components, 11, 4888 of the propellant to each other. It can also be used as a fuel when burning6 of 13. The propellant contained 86% ammonium perchlorate, which was used as a powerful oxidizer, and can possibly expand damage in case of a fire. Sources of energy that can act as ignition sources include 3.3.thermal Dust energy Explosion (i.e. by, fire Ammonium, high temperature Perchlorate surface), mechanical energy (i.e., friction, shock), and electricalAn accident energy ( occurredi.e., static in electricity, a combustion electrical chamber spark). while As charginga result of slurry the accident ammonium investigation, perchlorate it propellantwas found inthat the the mixer discharge ball into valve the was combustion not open tube, duri causingng the charging a disastrous of the explosion. combustion This tube accident, so a occurredworker used on 29a rubber May, 2018, hammer in the to Daejeon open the area, discharge as shown valve. in FigureIt was4 estimated. A sudden that ﬁre a occurredfire occurred in the in propellant,the propellant resulting due to in the the impact deaths ofof ﬁvehitting workers the wooden and injuries rod ofonfour the workers,discharge who valve were head working with the on site.rubber The hammer explosion ins occurredide the mixer during ball the containing opening ofthe the propellant. discharge Therefore, valve of the it mixerwas presumed bowl, which that was an theimpulse preparation exceeding of the the propellant propellant charge. impact sensitivity was generated in the slurry propellant by the impact, and a fire occurred in the propellant.

Figure 4. ((a)) Front image of the place where the accident occurred;occurred; (b)) Image ofof accident equipment.equipment.

4. DiscussionThe accident investigation was carried out through analysis of the three basic elements of fire. The propellant used a binder that had such physical properties as plastic or rubber, and was intended to4.1. bond Dust the Explosion components for Other of theCountries propellant to each other. It can also be used as a fuel when burning. The propellant contained 86% ammonium perchlorate, which was used as a powerful oxidizer, and can possiblyFigure 5 expand shows damage the results in case of of 87 a casesfire. Sources of dust of explosions energy that in can Japan act as from ignition 1987 sources to 2003 include [27]. thermalApproximately energy 51% (i.e., of fire, dust high explosions temperature were surface),caused by mechanical metal dust, energyfollowed (i.e., by friction,17% of the shock), accidents and electricalbeing caused energy by wood. (i.e., static Dust electricity, explosion electricals occurred spark). in 16% As and a result8% of ofthe the accidents accident caused investigation, by plastic it wasand food, found respectively. that the discharge According valve to wasthe Japan not open Institute during for theOccupational charging of Safety the combustion and Health tube,(JNIOSH), so a workermetal, wood, used a and rubber plastic hammer dust account to open thefor about discharge 85% valve. of the Itdust was explosions. estimated that a fire occurred in the propellant due to the impact of hitting the wooden rod on the discharge valve head with the rubber hammer inside the mixer ball containing the propellant. Therefore, it was presumed that an impulse exceeding the propellant impact sensitivity was generated in the slurry propellant by the impact, and a fire occurred in the propellant.

4. Discussion

4.1. Dust Explosion for Other Countries Figure5 shows the results of 87 cases of dust explosions in Japan from 1987 to 2003 [ 27]. Approximately 51% of dust explosions were caused by metal dust, followed by 17% of the accidents being caused by wood. Dust explosions occurred in 16% and 8% of the accidents caused by plastic and food, respectively. According to the Japan Institute for Occupational Safety and Health (JNIOSH), metal, wood, and plastic dust account for about 85% of the dust explosions. SustainabilitySustainability 20192019,,11 11,, 4888x FOR PEER REVIEW 77 of 1312 Sustainability 2019, 11, x FOR PEER REVIEW 7 of 12

Figure 5. Cause of dust explosions in Japan from 1987 to 2003. Figure 5. Cause of dust explosions in Japan from 1987 to 2003.2003. Figure 6a presents combustible dust fires and explosions from 1980 to 2005 in the United States. Figure6 6aa presentspresents combustiblecombustible dustdust firesfires andand explosionsexplosions fromfrom 19801980 toto 20052005 inin thethe UnitedUnited States.States. TheyThey identifiedidentified 119 119 deaths deaths in in 78 78 of of the the 281 281 dust dust explosions explosions [28 ].[28 An]. An average average of 5 of deaths, 5 deaths, 29 injuries, 29 injuries, and Theyand 10identified dust explosions 119 deaths occurred in 78 of annuallythe 281 dust, according explosions to CSB[28]. (ChemicalAn average Safety of 5 deaths, Board) 29 statistical injuries, 10and dust 10 explosions dust explosions occurred occurred annually, annually according, according to CSB (Chemicalto CSB (Chemical Safety Board) Safety statistical Board) statistical analysis. Althoughanalysis. Althoug no propertyh no property damages damages have been have reported been reported by governments by governments or private or entities,private entities, FM Global FM analysis.Global Insurance Althoug hCompany no property reported damages over have 1 million been dollars reported of propertyby governments losses from or private 22 dust entities, explosions FM InsuranceGlobal Insurance Company Company reported reported over 1 millionover 1 million dollars dollars of property of property losses losses from 22from dust 22 explosions dust explosions from 1983from to 1983 2003 to [ 292003]. According [29]. According to the to distribution the distribution of combustible of combustible dust explosions, dust explosion foods, products, food products, wood, fromwood 1983, and to metals 2003 [account29]. According for more to than the distribution65% of explosions of combustible, and plastic dust account explosions fors, 14% food. More products, than andwood metals, and metals account account for more for than more 65% than of explosions,65% of explosions and plastic, and accountsplastic account for 14%.s for More 14% than. More 73% than of the73% accidents of the accidents were caused were by caused combustible by combustible dusts. The CSB dusts. also The reported CSB also that report equipmented that malfunctions equipment 73%malfunctions of the accidents caused by were dust caused collectors by were combustible the most dusts. common The accidents CSB also in report all industries.ed that equipment Figure 6b causedmalfunctions by dust caused collectors by dust were collectors the most were common the most accidents common in all accidents industries. in all Figure industries.6b classifies Figure the 6b dustclassifies generated the dust in generat industriesed in in industr the Unitedies in States the United for two States years, for with two moreyears, than with 60% more in than combining 60% in classifiescombining the food dust and generat wooded products in industr amongies in the176 Unitedcases [ 30States]. In 2016,for two a totalyears, of with 31 accidents more than occurred. 60% in foodcombining and wood food products and wood among products 176 casesamong [30 176]. In cases 2016, [30 a total]. In of2016, 31 accidentsa total of occurred.31 accidents With occurred. respect toWith combustible respect to materials,combustible wood materials, and food wood products and food accounted products for accounted more than for 60%,more and than metal 60%, andand Withmetal respect and coal to accountedcombustible for materials, 9%. Analy woodsis of andthe dustfood explosion productss accounted from equipment for more showed than 60% that, 19and% coalmetal accounted and coal accounted for 9%. Analysis for 9%. of A thenaly dustsis of explosions the dust explosion from equipments from equipment showed that showed 19% occurredthat 19% inoccurred dust collectors, in dust collector 16% in storages, 16% in silos storage/hoppers, silos and/hopper 10%s, in and elevators 10% in/ conveyors.elevators/conveyor In 2017,s. 145 In cases2017, occurred145 cases in were dust reported, collector s and, 16 % the in numberstorage ofsilo dusts/hopper explosionss, and 10increased,% in elevators but blowout/conveyor panelss. In 2017, were were145 case reported,s were and reported, the number and the of number dust explosions of dust explosions increased, increased, but blowout but panels blowout were panels installed were ininstalled equipment in equipment to make smaller to make explosions smaller and explosion reduces disastrous and reduce explosions. disastrous The explosion classifications. The of installedclassification in equipmentof combustible to make materials smaller that causedexplosion thes accidentand reduces was similardisastrous to that explosion of 2016,s. withThe combustibleclassification materials of combustible that caused materials the accidentsthat caused was the similar accident tos that was of similar 2016, with to that wood of 2016, and food with productswood and accounting food products for more accountin than 55%,g for and more metal than accidents 55%, and by 8%.metal However, accidents the by type 8% of. However, equipment the in woodtype of and equipment food products in the accidentsaccountin wasg for slightly more thandifferent 55%, from and 2016,metal withaccidents 17% in by dust 8% .collector However,s, 14% the thetype accidents of equipment was slightly in the accidents different from was 2016,slightly with different 17% in dustfrom collectors,2016, with 14% 17% in in storage dust collector silos/hoppers,s, 14% andin storage 8% in elevatorssilos/hopper/conveyors.s, and 8% From in elevator Figures6/conveyb, it canor bes. From seen that Figure the 6 ratiob, it can of dust be seen explosions that the from ratio inof storagedust explosion silos/hoppers froms ,food and products8% in elevator has increaseds/convey,or buts. Fromdust explosion Figure 6b,s fromit can metal be seen have that decreased the ratio, food products has increased, but dust explosions from metal have decreased, compared to Figure6a. ofcompared dust explosion to Figures from 6a. food Detailed products statistics has increased information, but isdust provided explosion in sSupplementary from metal have Information decreased, Detailed statistics information is provided in Supplementary Information (SI.1 and SI.2). compared(SI.1 and SI. to2 ).Figure 6a. Detailed statistics information is provided in Supplementary Information (SI.1 and SI.2).

Figure 6. (a) Distribution of types of dust explosions in the United States (%) from 1980 to 2005; (b) Figure 6. (a) Distribution of types of dust explosions in the United States (%) from 1980 to 2005; (b) Distribution of types of dust occurring in the United States (%) from 2016 to 2017. FigureDistribution 6. (a) ofDistribution types of dust of typeoccurrs ofing dust in the explosions United States in the (%) United from States 2016 to (%) 2017. from 1980 to 2005; (b) Distribution of types of dust occurring in the United States (%) from 2016 to 2017. Sustainability 2019, 11, 4888 8 of 13

SustainabilitySustainability 2019 2019, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 88 of of 12 12 Figure7 showed the analysis of 303 dust explosions in the United Kingdom from 1979 to 1988 [ 31]. FigureFigure 7 7 showed showed the the analysis analysis of of 303 303 dust dust explosions explosions in in the the U Unitednited Kingdom Kingdom from from 1979 1979 to to 1988 1988 Sixty-nine cases by paper/wood, 55 cases by metal dusts, and 43 cases by food products were reported, [31[31].] .Sixty Sixty-nine-nine cases cases by by paper/wood, paper/wood, 55 55 cases cases by by me metaltal dusts, dusts, and and 43 43 cases cases by by food food products products were were as shown in Figure7a. Those materials caused more than 50% of dust explosions. In addition, 227 cases reported,reported, as as shown shown in in Figure Figure 77a.a. Those Those materials materials caused caused more more than than 50% 50% of of d dustust explosions. explosions. In In (74% of 303 cases) were caused by combustible dusts. Analysis of 303 dust explosions with respect to addition,addition, 227 227 cases cases (74%(74% of of 303 303 cases) cases) were were caused caused byby combustible combustible dusts. dusts. Analysis Analysis of of 303 303 dust dust equipment, as shown Figure7b, revealed that 53 cases of accidents occurred in a mill /grinder, 47 cases explosionsexplosions withwith respect respect to to equipment, equipment, as as shown shown Figure Figure 77b,b, revealed revealed that that 53 53 cases cases of of accidents accidents of accidents in a ﬁlter, and 43 cases of accidents in a dryer. Meanwhile, 19 cases of dust explosions in a occurredoccurred in in aa mill/grinder,mill/grinder, 47 47 cases cases of of accidents accidents in in aa filterfilter, , andand 43 43 cases cases of of accidents accidents in in aa dryer.dryer. silo/hopper accounted for 6.3% of total accidents. Meanwhile,Meanwhile, 19 19 cases cases of of dust dust explosions explosions in in a a silo/hopper silo/hopper account accounteded for for 6.3% 6.3% of of total total accidents. accidents.

Figure 7. (a) Distribution of types of dust explosions occurring in the United Kingdom; (b) Distribution FigureFigure 7. 7. (a(a) ) DistributionDistribution of of type typess ofof dust dust explosions explosions occurr occurringing inin thethe UUnitednited Kingdom Kingdom; ; (b (b) ) of dust explosion by equipment. DistributionDistribution of of dust dust explosion explosion by by equipment. equipment. Figure8 shows 37 cases of dust explosions from 2000 to 2018 in France. Wood products and plastic FigureFigure 8 8 shows shows 37 37 cases cases of of dust dust explosions explosions from from 2000 2000 to to 2018 2018 in in France. France. Wood Wood products products and and plastic plastic dusts accounted for half of the dust explosions, followed by metal and food products at 19% each [32]. dustsdusts accounted accounted for for half half of of the the dust dust explosion explosions,s, followed followed by by metal metal and and food food products products at at 19% 19% e eachach More than 50% of dust explosions occurred in silos/hoppers and ﬁlters/dust collectors. About 90% or [3[322].] .More More than than 50% 50% of of dust dust explosions explosions occurred occurred in in silo siloss/hopper/hopperss and and filter filterss/dust/dust collector collectorss. .About About more of accidents were caused by combustible dusts. Compared with the previous Japanese accident 90%90% or or more more of of accidents accidents were were caused caused by by combustible combustible dusts. dusts. Compared Compared with with the the previous previous Japanese Japanese analysis, this shows a similar characteristic for dust explosions. accidentaccident analysis, analysis, this this shows shows a a similar similar characteristic characteristic for for dust dust explosions. explosions.

Figure 8. Dust explosions in France over 18 years. (a) Distribution of types of dust involved in FigureFigure 8. 8. DDustust explosions explosions in in France France over over 18 18 years years. . (a(a) ) Distribution Distribution of of type typess ofof dust dust involved involved in in explosion; (b) Distribution of dust explosion by equipment. explosion;explosion; ( b(b) )Distribution Distribution of of dust dust explosion explosion by by equipment. equipment. 4.2. Characteristics of Dust Explosions between South Korean and Other Countries 4.2.4.2. Characteristics Characteristics of of Dust Dust Explosions Explosions between between South South Korean Korean and and Other Other Countries Countries Accident analysis of dust explosions in South Korea and other countries found that the causes and typesAAccidentccident of industry analysis analysis diof offf dust erdust from explosions explosions country in toin South country.South Korea Korea With and and the other other development countries countries offound found the heavythat that the the chemical cause causess industries,andand type typess of dustof industr industr explosionsyy differ differ were from from mainly country country caused to to country. country. by metal, With With plastic, the the development development and food. The of of the mainthe heavy heavy cause chemical chemical of dust explosionsindustries,industries, dust indust Japan explosions explosions exhibited were were a similarmainly mainly patterncaused caused toby by Southmetal, metal, Korea. plastic, plastic, It and and is estimated food. food. The Thethat main main the cause cause geographic of of dust dust proximityexplosionsexplosions and i nin Japan industrialJapan exhibited exhibited structure a a similar similar are similar pattern pattern to thoseto to South South of South Korea. Korea. Korea. It It is is estimated However,estimated specialthatthat the the care geographic geographic must be takenproximityproximity for dust and and explosions industrial industrial fromstructure structure metals, are are since similar similar the explosivenessto to those those of of South South of metallic Korea. Korea. dustHowever, However, can also special special be an care importantcare must must causebebe taken taken of dust for for dust explosions. dust explosions explosions On the fromfrom other metals, metals, hand, since since the sources the the explosiveness explosiveness of dust explosions of of metallic metallic in the dust dust United can can also States also be be and an an importantimportant cause cause of of dust dust explosions. explosions. On On the the other other hand, hand, the the sources sources of of dust dust explosions explosions in in the the United United StatesStates and and thethe UUnitednited Kingdom Kingdom werewere food, food, wood/paper, wood/paper, and and metallic metallic materials materials, , althoughalthough they they Sustainability 2019, 11, 4888 9 of 13 the United Kingdom were food, wood/paper, and metallic materials, although they occurred with different frequency. The main sources of dust explosions in France were wood, plastic, and metal. As the recent dust explosions in silos were mentioned in the case studies, many dust explosions were caused by equipment, such as silos/hoppers, filters/dust collectors, and mills/grinders in other countries. The workplace should strive to prevent dust explosions and train workers before use to raise safety awareness about the risk of dust explosions.

4.3. Effective Protective/Mitigatory Measures of Dust Explosions Protective/mitigatory measures of dust explosions play a significant role in minimizing human and property losses. As of today, explosion isolation, partial inerting, explosion venting, explosion suppression, and preventing secondary explosions outside process equipment are well-known preventive measures. Explosion isolation is an important prevention technique to prevent spreading from a primary explosion to other processes. Lunn et al. (1996) conducted a coal dust explosion experiment in a system of linked vessels connected by a pipe [33]. Holbrow et al. (1999) conducted a similar experiment and presented a quantitative guide for design and protective methods, such as explosion containment and explosion venting in interconnected process systems [34]. Taveau (2017) reported that large-scale testing required verifying explosion isolation systems and needed to place the explosion isolation device in the correct position [35]. Partial inerting decreases the oxygen concentration in the atmosphere, rather than the complete removal of oxygen, thereby reducing both the ignition sensitivity and the combustion rate of the dust cloud, as well as substantially increasing the minimum ignition energy (MIE) [36]. Several studies have been conducted experimentally on the effect of oxygen content of MIE on various types of dust. Ackroyd et al. (2011) measured the MIE of organic powders at reduced oxygen concentration and correlated it with the oxygen concentration of the atmosphere [37]. Choi et al. (2015) reported that MIE increased in all the powders used in the experiment as the N2 in the air increased, though it varied depending on the type of powder [38]. Explosion venting is the most efficient and widely used method to mitigate the effects of dust explosions, and many studies have been conducted on it. Tascon et al. (2016) carried out dust explosion experiments. Four vessels with different geometries in a silo were used to analyze the effect of the length/diameter (L/D) ratio, and it was recommended to increase the vent area in elongated vessels when L/D is greater than 1 [39]. Tascon (2017) analyzed the effectiveness of inertia in three different venting systems and presented practical recommendations for vent area design in silos [40]. Explosions in industrial plants usually occur mainly in tanks, reactors, and technical systems, and then extend to the entire system before emerging outside. Active explosion suppression for mitigation of dust explosion is complex and expensive, and is used only when a simple and inexpensive method does not apply. An effective way to minimize the loss of these explosions was to suppress the explosion in the plant and localize the losses. Moore (1987) developed automatic explosion suppression systems and designed early detection of developing explosions. Dust explosions usually last from several tens to several hundred milliseconds in a tank that has a volume of several cubic meters. The extinguishing agent was sprayed at a high speed of about 100 m/s to suppress the explosion for several milliseconds from the beginning to prevent dust explosions [41]. Recently, active explosion suppression using extinguishing powders or water in occupational safety zones is becoming more widespread [42,43]. Based on computational fluid dynamics (CFD), Morgan (2000) confirmed the applicability of numerical models of complex explosion suppression processes and designed novel suppressant injection nozzles [44]. The remaining issues of all efforts to fight the hazards of dust explosions are preventing secondary explosions outside process equipment and are closely related to the impact of the accidents. Cybulski et al. (1993) reported that relatively weak secondary dust explosions in short and narrow tunnels in grain elevators were extinguished by an appropriate design and triggered by water barriers [45]. Eckhoff (2003) claimed that extensive secondary explosions could be removed by freeing dust from outside process equipment [10]. Sustainability 2019, 11, 4888 10 of 13

Although many studies are in progress, eﬃcient design of venting arrangements is one of the hardest tasks to prevent propagation of dust explosions in complex coupled-process systems.

5. Conclusions Since dust explosions cause not only human and property loss, but also secondary dust explosions, this study investigated various cases of dust explosions that occurred in South Korea and several other countries. It was found that not only the cause of dust, but also the industrial structure changed as time passed. Therefore, these situations should be identified, and appropriate preventive or mitigating methods should be presented. In this study, 53 combustible dust- and flammable solid-related fire explosions from 1984 to 2018 in South Korea were reported, with the causes of the dust explosions being similar to those in Japan. For instance, metal dust accounted for 44%, and plastic and food causes accounted for 19%, which was considered to reflect industrial characteristics. There were 176 cases of accidents in the United States between 2016 and 2017, and more than 60% were caused by food and wood products. In 2016, a total of 31 accidents occurred. Analysis of the dust explosion equipment showed that 30% occurred in dust collectors, 12% in storage silos, and 6% in grinding equipment. In 2017, 145 cases were reported, and the number of dust explosions increased, but blowout panels were installed in equipment to make a smaller explosion and reduce disastrous explosion. The equipment that caused the accidents was identified as 18% being dust collectors, 14% storage silos, and 8% elevators/conveyors. There were 303 dust explosions in the United Kingdom from 1979 to 1988. Among them, 53 cases of dust explosions occurred in mills/grinders, 47 cases of accidents in filters, 43 cases in dryers, and 19 cases occurred in silos/hoppers, which are increasingly used in South Korea. Dust explosions from 2000 to 2018 in France showed that wood products and plastic dusts accounted for half of the dust explosions, followed by metal and food products at 19% each. Dust explosions during the last three years were caused by organic matters and metal, and unfortunately, dust explosions have occurred repeatedly during grinding, mixing, and injection of powder materials into facilities. There were no accidents reported during the production process of wood or paper during the last three years. Dust explosions represent catastrophic accidents, but can be mitigated easily if the mechanism of the dust explosion is identified. Therefore, understanding of basic knowledge contributes to reducing the risk of dust explosions. In addition, prevention measures for dust explosions should be presented, based on recent accident characteristics and the complexity of the process. Taking these facts into account, the following are measures to prevent dust explosions in chemical plants where dust explosions mainly occur:

(i) Good housekeeping is necessary to prevent accumulation and scattering of dust on the floor of the building; (ii) Dust generating facilities should be improved by means of installment of the lid or a sealed structure so that dust is not scattered to the outside; (iii) A metal separator must be installed at the crusher inlet to prevent sparks; (iv) All dust generation facilities should be connected to the damping system, and if there is a risk of heat accumulation due to internal fixation, a thermometer should be installed; (v) If the gas used to calibrate the sensor differs from the actual measurement of the gas concentration, the manufacturer provides the correction value, and the user must apply the correction value to set the sensor sensitivity and alarm; (vi) Silos used in the production and storage of various resin products must be evaluated and monitored for the occurrence of gas concentrations at a normal time, due to process characteristics; (vii) Oxygen concentrations must be lower than the explosion minimum concentration through the inclusion of an inert gas, such as nitrogen. If a change in oxygen concentration is observed, it is considered to be a dangerous situation, and the operation should be immediately stopped until the problem is resolved; Sustainability 2019, 11, 4888 11 of 13

(viii) High-speed operation valves, explosion pressure vents, and explosion suppression devices should be installed to protect against dust explosions.

Supplementary Materials: The following are available online at http://www.mdpi.com/2071-1050/11/18/4888/s1, Title: Case Studies for Dangerous Dust Explosions in South Korea during Recent Years. Author Contributions: Conceptualization, C.K., S.J.; methodology, S.P., C.R.; formal analysis, S.J., C.R.; writing—original draft preparation, C.K., S.J. Funding: This research received no external funding. Acknowledgments: This paper cites data from the Korea Occupational Safety and Health Agency (KOSHA) and the Korea Ministry of Employment and Labor. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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