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Explosion characteristics of micron-size conveyor rubber dust
Journal of Loss Prevention in the Process Industries 45 (2017) 173e181
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Explosion characteristics of micron-size conveyor rubber dust Yunhao Li a, Feifei Liu a, Qingwu Zhang a, *, Yuan Yu a, **, Chi-Min Shu b, Juncheng Jiang a a Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control, College of Safety Science and Engineering, Nanjing Tech University, Mail Box 13, No. 200 North Zhongshan Rd., Nanjing, 210009, China b Doctoral Program, Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2016 Received in revised form 19 December 2016 Accepted 19 December 2016 Available online 21 December 2016
Explosion characteristics of micron-size conveyor rubber dust were determined employing a normal 20 L stainless steel orbicular chamber and two types of modified 1.2-L Hartmann tubes. Maximum explosion pressure (Pmax), maximum rate of pressure rise (dP/dt)max, and minimum explosion concentration (MEC) were tested in the spherical chamber. The results indicated that the explosion severity of conveyor rubber powders is weaker than some common dust, such as coal, magnesium, and aluminum dust. However, conveyor rubber dust is a kind of combustible dust. Furthermore, the explosion severity increased as the particle size decreased from 120 to 48 mm. The MEC decreased from 90 to 30 g/m3 on decreasing particle size from 120 to 48 mm. Moreover, the MEC of conveyor rubber dust was 30, 50 and 90 g/m3 for the particle size of 48, 75, and 120 mm. The minimum ignition energy (MIE) was measured by two types of modified 1.2liter Hartmann tubes. The results indicated that particle size and dust concentration had influences on MIE. However, the MIE of conveyor rubber dust was much higher than 10 mJ. Accordingly, conveyor rubber dust cannot be ignited by collision, friction, and attrition. The experimental data presented could be useful for process plants that manufacturing conveyor rubber to evaluate explosibility of their conveyor rubber powders and propose/design adequate safety measures. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Micron-size conveyor rubber dust Hartmann tube Maximum rate of pressure rise Explosion severity Particle size Minimum ignition energy
1. Introduction Synthetic rubber is widely used in the production of tires, rubber hoses, and conveyor belts, etc. The European Union, United States, and China are dominant consumers for synthetic rubber, and the demand for synthetic rubber has been growing swiftly. According to statistics, the total production of tyres, which are manufactured in about 90 plants in the EU, was ca. 355 million in 2014, corresponding to 24% of the total world production. Meanwhile, synthetic rubber is a main production material of tyres (Torretta et al., 2015). In addition, there are numerous manufacturing and processing enterprises for other rubber products. Much rubber dust is generated in the process of manufacturing and processing of tyres as well as other rubber products. Combustible dusts can be found in various industries, and dust explosions present substantial threats to people, assets, and the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Y. Yu). http://dx.doi.org/10.1016/j.jlp.2016.12.005 0950-4230/© 2016 Elsevier Ltd. All rights reserved.
environment (Yuan et al., 2015), such as the catastrophic aluminum-alloy dust explosion which killed 146 persons in China (Li et al., 2016a). Table 1 lists several serious disasters caused by dust explosion (Yuan et al., 2015). Accordingly, it is necessary to explore the explosion severity and ignition sensitivity parameters for the conveyor rubber dust. Conveyor rubber is essentially a kind of polymer. There exists some information on explosion characteristics of polymer powders (Marmo and Cavallero, 2008; Amyotte et al., 2012; Gao et al., 2015; Addai et al., 2015, 2016). Amyotte et al. (2012) conducted experiments to determine the explosion severity and ignition sensitivity parameters for fibrous wood and polyethylene. Gao et al. (2015) studied the pressure characteristics of vented 100 and 800 nm polymethyl methacrylate (PMMA) dust explosions with different venting diameters. Addai et al. (2015) tested the explosion behavior of corn starch, and investigated the effects of methane and acetone on the explosion behavior of corn starch. Addai et al. (2016) determined the minimum ignition energy (MIE) of starch, wheat flour, protein, polypropylene, and dextrin. Marmo and Cavallero (2008) tested the MIE of fibrous nylon dust cloud. For micron-size dust, Soundararajan et al. (1996) experimentally
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Table 1 Accidents caused by dust explosion (Yuan et al., 2015). Date Country Material Equipment involved
Types of industries
1993 US 1996 US
PR PR
1998 1999 1999 2002 2003 2003 2004 2007 2007 2009
US US US US US US US CHN CHN CHN
PR PR PR PR PR PR W W F M
Air arc gauge with Plasmarc and Mappgas; mixing tank Plastic products factory Blending tote Automobile brake pads and lining manufacturer Dust collector; air handling ducts Sports equipment manufacture Oven; dust collection system Grey and duct tile foundries Machine that grinds plastic pellets Plastic manufacturing Tire bin Tire recycling Milling equipment Mechanical rubber goods Oven e Dust collection system Wood products plant Conveyor Wood pelts manufacturer Workshop, pulverizer Rice process factory Workshop Aluminum products
2009 2010 2011 2012 2014
CHN CHN US CHN CHN
IN M M M M
Dust collection system Dust setting chamber Pipes, furnace Polishing workshop Polishing workshop
Chemical preparations Metal polishing Iron powder plant Metal polishing plant Metal polishing
Dead/ injured
Ignition sources
2d/2i 1d
Electrical sparks e
16d 9d/3i 2d 13d 38d/6i 37d/7i 3d 4d/5i e 11d/20i 2i 2d/6i 3d/2i 13d/16i 146d/114i
e Flame and direct heat Hot work e e e Impact sparks e Hot surface Self-heatingand smoldering e Impact sparks Impact sparks Electrical sparks self-ignition
PR: Plastic/rubber; W: Wood; F: Food; M: Metal; IN: Inorganic.
researched the influences of powder diameter on explosion features of micron-pyrite and pyrrhotine. Denkevits and Dorofeev (2005, 2006) investigated the explosibility of micron-graphite and micron-tungsten dusts along with mixtures for different particle size. Pilǎo et al. (2006) studied the explosion severity and ignition sensitivity of cork and air dust mixtures in an almost spherical apparatus. Azhagurajan et al. (2012) studied the MIEs of micron- and nano-size flash powders in fireworks industry with the change of electrode gap, electrode material, and dust concentration, dust composition, etc. Mittal (2013), Yuan et al. (2014), and Li et al. (2016b) conducted experiments to study the explosion characteristics of micron coal dust. Castellanos et al. (2014) investigated the effects of powder diameter polydispersity on the explosibility features of aluminum dust. Boskovic et al. (2015), Medina et al. (2015a, b), and Saeed et al. (2015) tested the explosibility characteristics of biomasses. All of the above research results have provided useful data to design proper and sufficient safety measures to mitigate or prevent dust explosion. To supply useful information for manufacturing and handling enterprises of conveyor rubber products, it is significant to investigate the explosion severity and ignition sensitivity parameters of conveyor rubber dust. The conveyor rubber dust samples used for the current works were supplied by Gates Unitta Power Transmission (Suzhou) Co. Ltd., a company manufacturing different kinds of conveyor belts composed mainly by synthetic rubbers. And during the cutting and polishing process, a large number of conveyor rubber powders with different particle sizes would be generated, and suspended by the momentum from processing machines and ventilation system. Therefore, the conveyor rubber dust mainly distributed in the workshops, ducts, and dust collectors. In the present paper, the objective was to test and elucidate the explosion characteristics of micron-size conveyor rubber dust. The influences of dust concentration and powder size of conveyor rubber dust on maximum explosion pressure (Pmax) and explosibility index (Kst) were studied, along with the influences of powder size on minimum explosion concentration (MEC) and the effects of dust concentration and particle diameter on MIE.
2. Methodology 2.1. Apparatuses and methods 2.1.1. Experimental apparatus for MEC and explosion severity parameters A normal 20 L stainless steel orbicular chamber (Fig. 1), produced by Northeastern University in Liaoning province, China and recommended by International Standard ISO6184/1 (International Organization for Standardization, 1985), was selected to test Pmax, maximum rate of pressure rise (dP/dt)max, and MEC of conveyor rubber dust. This device mainly consists of a spherical vessel,
Fig. 1. The 20 L spherical experimental apparatus. Notes: 1Water outlet, 2Pressure sensor, 3Manometer, 4Dust container, 5Air inlet, 6Ignition source, 7Rebound nozzle, 8Fast acting valve, 9Water inlet, 10Outlet (air, reaction products).
Y. Li et al. / Journal of Loss Prevention in the Process Industries 45 (2017) 173e181
control system, and data acquisition system. Before an explosion test, pre-weighed conveyor rubber dust was put into the dust container (volume: 0.6 L), and an igniter fixed at hub of the chamber. The safely sealed explosion vessel was vacuumed in part to 0.6 bar (gauge) and the pressure of pressurized air in the dust storage tank was 20 bar (gauge). Conveyor rubber powders and air were first diffused into the chamber when opened the solenoid valve between the sample container and vessel. The chemical igniter was then ignited after delaying 60 ms. Finally, the acquired data were analyzed by the computer. Once a measurement was accomplished, the vessel and dust storage tank were tidied by a vacuum sweeper in preparation for the next test. The test methods of Pmax, (dP/dt)max, and MEC of conveyor rubber dust were based on European Standard EN 140341: 2004 (Birtish Standards Institution, 2004), EN 140342: 2006 (Birtish Standards Institution, 2006a), and EN 140343: 2006 (Birtish Standards Institution, 2006b). A chemical ignitor with energy of 2 kJ was used in the experiments. Ambient temperature was 25 C. In the process of a test, the pressure change of conveyor rubber dust explosion was gauged and notes taken by a system that acquiring experimental data. The measurement range of the pressure sensor was 0e250 psi (17 bar). A normal curve of pressuretime evolution noted during conveyor rubber dust explosion is delineated in Fig. 2. Pressure evolution first began at 0.6 bar (gauge). The air blast applying for dispersing powders then started at 156 ms. Igniting started at 216 ms at an ambient condition of 1 bar (absolute). The value of Pex, for a single measurement at a certain concentration, is the highest explosion overpressure (gauge). (dP/dt)ex is maximum rate of pressure rise in one test (Mittal, 2014). A dust explosion occurred when the measured overpressure Pex 0.05 bar relative to the initial pressure Pi 0.02 bar. If the measured overpressure Pex 0.05 bar was relative to the initial pressure Pi 0.02 bar in three continuous tests, this sample was considered as an unexplosive dust at this concentration. According to (dP/dt)max and the volume of the spherical chamber (V), Kst an international common value can be calculated by Eq. (1):
Fig. 2. Pressuretime curve recorded during dust explosion experiments.
Kst ¼ ðdP=dtÞmax V 3
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(1)
2.1.2. Experimental apparatuses for MIE According to the European Standard EN 13821: 2002 (Birtish Standards Institution, 2002), two types of modified 1.2-L Hartmann tubes made of glass were employed as the apparatus for MIE. The test coverage of one device ranges from 1 to 2000 mJ. The other only provides ignition energy of 4 and 10 J. Fig. 3 shows a schematic diagram of the modified Hartmann apparatus. These apparatuses consist of Hartmann tube, control system, high voltage unit, dust diffusion unit, electronic pneumatic control unit, etc. The dust diffusion and ignition are implemented in the Hartmann tube. The pressure of the pressurized air stored in the gas container was 70 bar. When the electronic pneumatic valve was opened, the conveyor rubber dust at the bottom of the tube was dispersed into the tube. Then the conveyor rubber dust was ignited by electrical spark triggered by discharge of the high voltage unit. Inductance of the discharge circuit was 1 mH. Moreover, the electrode gap was 6 mm. Furthermore, the time from dispersion to ignition was 120 ms. After ignition, if the conveyor rubber dust exploded and flame propagation appeared, a dust cloud explosion was induced; if no flame propagation was seen in ten continual tests, this ignition energy could not induce a dust cloud explosion. To obtain MIEs, the tests were accomplished by starting at a relatively high spark energy level. Then the spark energy level was reduced in steps until no ignition occurred for ten ignition trials. 2.2. Samples and particle size characteristics The particle size distribution of original dust samples used for the current works from workshops was measured by laser diffraction analyzer. And the particle size ranges from 30 to 500 mm. Meanwhile, many researchers have found that Pmax and Kst decreased with increasing particle size. Therefore, finer conveyor rubber powders may have more significant hazards. And we prepared testing samples by sieving. Then particle diameter distributions of conveyor rubber dust were confirmed by a laser diffraction analyzer and distinguished by d50 recorded respectively by 48, 75, and 120 mm. The powder diameter distributions of conveyor rubber
Fig. 3. Schematic diagram of the modified Hartmann apparatus.
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Fig. 5. SEM photograph of conveyor rubber powder.
irregular. In addition, the surface of the conveyor rubber particles was flocculent. 3. Results and discussion 3.1. Dust concentration effects on Pmax and Kst
Fig. 4. Particle size distributions of samples.
powder samples are shown in Fig. 4. Besides, prior to explosion tests, to obtain relatively dry samples, the conveyor rubber dust was dried by a vacuum drying box. Fig. 5 shows a photograph of conveyor rubber powders by scanning electron microscope (SEM). It can be seen that the shapes of the conveyor rubber particles were
Explosion severity can be characterized by Pmax and Kst. It is vital for safety design and explosion prevention to understand the explosion severity parameters of conveyor rubber dust. The conveyor rubber dust concentrations for the tests were 100, 200, 300, 400, 500, 600, and 750 g/m3. The evolution of Pmax and Kst vs. dust concentration is illustrated in Figs. 6 and 7, respectively. In Fig. 6, it can be seen that Pmax increased first and then decreased as dust concentration increased. Note that the Pmax was 4.67 and 4.54 bar for the conveyor rubber dust with the particle size of 48 and 75 mm, respectively. When the particle size increased to 120 mm, the Pmax decreased to 3.91 bar. From Fig. 7, one notice that Kst increased first and then decreased with the increase of dust concentration, too. Interestingly, the Kst was 25 and 24.14 bar m/s for the conveyor rubber dust with the particle size of 48 and 75 mm, respectively. When the particle size increased to 120 mm, the Kst decreased to 21.95 bar m/s. Table 2 lists the explosion characteristics of several common dust samples gleaned from the literature. As shown, conveyor rubber powders had much lower Pmax and Kst than most of the dust samples, such as coal and magnesium. Only the explosion severity parameters of iron dust were lower than conveyor rubber dust. According to Cashdollar (1996, 2000), Cashdollar and Zlochower (2007), Li et al. (2011), and Mittal (2014), the Pmax and Kst reached its maximal values when dust concentration ranged from 800 to 1500 g/m3 for magnesium, aluminum, iron, and coal dust. Nonetheless, the concentration ranging from 300 to 500 g/m3 was lower for conveyor rubber dust when reaching the maximum Pmax and Kst. Consequently, the explosion severity of conveyor rubber powders was weaker than most of the common dusts, such as coal, magnesium, and aluminum dust. Nevertheless, conveyor rubber dust is a kind of combustible dust. If we do not pay attention to it and take proactive measures to prevent or mitigate its accumulation, there exists the risk of a conveyor rubber dust explosion. 3.2. Particle size effects on Pmax and Kst The evolution of Pmax and Kst vs. particle size is plotted in Figs. 8 and 9. The conveyor rubber dust concentrations were 300, 500, and 750 g/m3, respectively. Figs. 8 and 9 show powder diameters had prominent influences on Pmax and Kst. The Pmax decreased faster
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Fig. 6. Pmax of conveyor rubber dust with different dust concentrations. Fig. 8. Pmax of conveyor rubber dust with different particle sizes.
and the Kst decreased slowly with the particle size increasing when the dust concentration varied from 300 to 750 g/m3. The effects of particle size on Pmax and Kst were investigated by Cashdollar (1996), Soundararajan et al. (1996), Nifukua et al. (2007), Wu et al. (2010), Li et al. (2011), Amyotte et al. (2012), and Mittal (2014). They found that Pmax and Kst decreased with increasing particle size. Meanwhile, finer size conveyor rubber powders were more intrinsically hazardous. They also found that the reason why Pmax and Kst increased with the particle size decreasing was that the specific surface area of powders decreased as the particle size increased. Furthermore, powders usually agglomerate. This characteristic has influences on the powder diffusion, distribution, ignition, and deflagration. The results in the present work are in accordance with the above researches. According to the SEM photograph for conveyor rubber powders, we estimated that the specific surface area of conveyor rubber powders increased as the particle size decreased. Meanwhile, the rate of thermal Fig. 7. Kst of conveyor rubber dust with different dust concentrations.
Table 2 Explosion characteristics for several common dust samples summarized from the literature. Sample
Pmax (bar)
Kst (bar m/s)
Nominal MEC (g/m3)
Particle size (mm)
Vessel volume
References
Prince mine coal Phalen mine coal Lingan mine coal Pittsburgh coal Pocahontas coal Kellingley coal Colombian coal Cork Fibrous wood Fibrous polyethylene Spherical polyethylene Spherical polyethylene Spherical polyethylene Aluminum Magnesium Magnesium Magnesium Titanium Iron Conveyor rubber Conveyor rubber Conveyor rubber
6.5 6.0 7.0 6.7 6.5 8.2 8.5 7.2 8.2 7.2 5.8 6.7 6.9 5.9 7 8.8 10.8 5.7 3.1 3.91 4.54 4.67
44 30 44 41 31 80 129 179 149 75 15 104 137 24.4 98 202 362 42 3.5 21.95 24.14 25
70 120 90 65 80 120 75 40 20 30 500 10 20 50 160 90 60 70 ~500 90 50 30
<125 <125 <125 <75 <75 D90 ¼ 85 D90 ¼ 65 D90 ¼ 280 <75 <75 <212 <75 <38 75 125 74 38 ~25 ~45 120 75 48
20 L 20 L 20 L 20 L 20 L 1 m3 1 m3 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L
Amyotte et al. (1991) Amyotte et al. (1991) Amyotte et al. (1991) Cashdollar (1996) Cashdollar (1996) Medina et al. (2015a,b) Medina et al. (2015a,b) Pilǎo et al. (2006) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Li et al. (2011) Mittal (2014) Mittal (2014) Mittal (2014) Cashdollar and Zlochower (2007) Cashdollar and Zlochower (2007) Present study Present study Present study
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Fig. 9. Kst of conveyor rubber dust with different particle sizes.
decomposition and evaporation of conveyor rubber dust increased with decreasing particle size. Accordingly, the concentration of flammable gas pyrolyzed by conveyor rubber particles increased as the particle size decreased. Therefore, Pmax and Kst decreased with the increasing of the particle size. 3.3. Particle size effects on MEC Explosion possibility could be characterized by MEC. It is crucial for industrial production to determine the MEC. Once MEC is determined, numerous measures can be taken to alleviate the ambient dust composition, so as to avoid the occurrence of a dust explosion from ignition sources. A certain dust concentration was selected as an initial test concentration. If Pex 0.5 bar (gauge), the explosion measurement endured with decreasing the dust concentration level in 10 g/m3 until the three continuous measured overpressures were all smaller than 0.5 bar (gauge). The tested MEC of powder samples Cmin was between Ca (the maximum dust concentration of three continuous measurements when Pex 0.5 bar) and Cb (the minimum dust concentration of the three continuous measurements when
Fig. 10. MEC of conveyor rubber dust vs. particle size.
Pex 0.5 bar), so Ca < Cmin < Cb. In this study, 100 g/m3 was selected as the initial test concentration, because the conveyor rubber dust explosion occurred at 100 g/m3. Fig. 10 depicts the MEC of conveyor rubber dust vs. particle size. The MEC values for different particle sizes are summarized in Table 2. It can be seen that experimentally measured MEC of conveyor rubber dust was 30, 50, and 90 g/m3, respectively, for the particle size of 48, 75, and 120 mm. These results indicated MEC decreased as the powder diameter decreased. The reason is that the specific surface area increased as the powder size decreased, and conveyor rubber powders contact more fully with oxygen. Therefore, the chemical reaction was more intense between oxygen and conveyor rubber powders. Accordingly, less conveyor rubber powder was required when an explosion was triggered when the particle size was smaller. Consequently, MEC reduced with the decreasing with particle size. The fourth column in Table 2 reports the nominal MEC for several common dust samples. By comparison, the MEC for conveyor rubber dust was lower than several common metals and higher than cork and fibrous wood. Meanwhile, a conveyor rubber dust explosion can occur at a very low concentration. Consequently, in plants for manufacturing and processing conveyor rubber products, more stringent preventive measures must be taken than industrial plants for metals to prevent or mitigate the accumulation of conveyor rubber dust. 3.4. Ignition sensitivity Ignition sensitivity can be characterized scientifically by the MIE of a dust cloud. The dangerous situations of dust processing equipment and workplaces can be judged by MIE. Meanwhile, according to MIE, preventive and mitigative measures can be designed against a hazardous scenario. As planned, the conveyor rubber dust concentrations for tests were 150, 300, 450, 600, and 750 g/m3. Fig. 11 shows experimental MIEs vs. dust concentration for different particle sizes. Experimental MIEs for conveyor rubber dust are given in Table 3. It can be seen that only the conveyor rubber dust with the particle sizes of 48 and 75 mm was ignited by the modified 1.2-L Hartmann tube of which the test coverage ranges from 1 to 2000 mJ when the dust concentration reached 750 g/m3. However, the other conveyor rubber dust samples could not be ignited by the ignition energy of 2000 mJ. Due to the limitations of the apparatus, only the approximate range of ignition energy can be measured by the second modified 1.2-L Hartmann tube, which generated ignition energy of 4 and 10 J for the other conveyor rubber dust samples. If a conveyor rubber dust sample was ignited by the spark energy of 4 and 10 J, and was not ignited by 2 J, the ignition energy for this sample ranged from 2 to 4 J. If a conveyor rubber dust sample was ignited by the spark energy of 10 J, and was not ignited by 4 J, the ignition energy for this sample ranged from 4 to 10 J. It also can be seen that particle size and dust concentration had influences on MIE. For the dust concentration of 750 g/m3, MIE for the particle size of 120, 75, and 48 mm was 2.0e4.0, 1.5e1.6, 1.1e1.2 J, respectively. The larger the particle size, the greater the MIE. Furthermore, for a particle size of 75 mm, it seems that MIE decreased as the concentration of conveyor rubber dust increased. It may be because the particle ignited by spark per unit volume increased when the dust concentration increased. Table 4 summarizes the MIEs of several dust samples. It appears that MIE of conveyor rubber dust is much higher than some metals, fibrous wood, and polyethylene. If the MIE of a dust is smaller than or equal 10 mJ, it can be ignited by collision, friction, and attrition (Christoph and Richard, 1995). Therefore, conveyor rubber dust cannot be ignited by collision, friction, and attrition.
Y. Li et al. / Journal of Loss Prevention in the Process Industries 45 (2017) 173e181
Fig. 11. Experimental MIEs vs. dust concentration for different particle sizes.
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Table 3 Experimental MIEs for conveyor rubber dust. Particle size (mm)
Dust concentration (g/ Ignition energy Particle size m3) (J) (mm)
Dust concentration (g/ Ignition energy Particle size m3) (J) (mm)
Dust concentration (g/ Ignition energy m3) (J)
120 120 120 120 120
150 300 450 600 750
150 300 450 600 750
150 300 450 600 750
4.0e10.0 4.0e10.0 2.0e4.0 2.0e4.0 2.0e4.0
75 75 75 75 75
Table 4 MIEs for different dust samples. Material
Particle size (mm)
MIE (mJ)
References
Magnesium Magnesium Magnesium Aluminum Titanium Fibrous wood Fibrous polyethylene Spherical polyethylene Spherical polyethylene Spherical polyethylene
125 74 38 >37 45 <75 <75 <212 <75 <38
120 50 10 48 21.91 10e30 10e30 >1000 10e30 10e30
Mittal (2014) Mittal (2014) Mittal (2014) Nifukua et al. (2007) Wu et al. (2009) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012)
Table 5 Dust explosion class standard in accordance with ISO 6184/1 (International Organization for Standardization, 1985). Dust explosion class
Kst/(bar m/s)
St1 St2 St3
0
4.0e10.0 2.0e4.0 2.0e4.0 2.0e4.0 1.5e1.6
48 48 48 48 48
2.0e4.0 2.0e4.0 2.0e4.0 2.0e4.0 1.1e1.2
decreased with the increase of particle size when the dust concentration varied from 300 to 750 g/m3. (3) The MEC of conveyor rubber dust was 30, 50, and 90 g/m3 for the corresponding particle size of 48, 75, and 120 mm. In addition, the MEC for conveyor rubber dust was lower than several common metals, but higher than cork and fibrous wood. Consequently, in plants for manufacturing and processing conveyor rubber products, more stringent preventive measures than industrial plants for metals must be seriously taken to prevent or mitigate the accumulation of conveyor rubber dust. (4) The particle size and dust concentration had influences on MIE. However, the MIE of conveyor rubber dust was much higher than 10 mJ. Therefore, conveyor rubber dust cannot be ignited by collision, friction, and attrition. Acknowledgements The authors are grateful for the financial support given by the National Key Research and Development Plan (No. 2016YFC0800102) and key project of National Natural Science Foundation of China (No. 21436006), along with the open project of Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control.
3.5. Conveyor rubber dust explosion class References Table 5 presents the dust explosion class standard in accordance with ISO6184/1 (International Organization for Standardization, 1985). According to the ISO6184/1, the conveyor rubber dust for different particle sizes falls into class St1, with weak or medium explosive power. Due to the explosion risk of conveyor rubber dust, when designing the plants for producing and processing conveyor rubber products, preventive and mitigative measures for conveyor rubber dust explosion must be seriously considered. During production, avoiding the formation of flammable conveyor rubber dust cloud is a crucial task. 4. Conclusions A standard 20 L stainless steel spherical vessel and two types of modified 1.2-L Hartmann tubes were applied to investigate the explosion characteristics of conveyor rubber dust with various particle sizes and dust concentrations. Moreover, the conveyor rubber dust explosion class was also analyzed. The following conclusions can be drawn from the experimental results. (1) The explosion severity of conveyor rubber powders is weaker than some common dust, such as coal, magnesium, and aluminum dust. Nevertheless, conveyor rubber dust is a kind of combustible dust. According to the ISO6184/1 (International Organization for Standardization, 1985) dust explosion class standard, the conveyor rubber dust for different particle sizes falls into class St1. (2) The effects of particle size on Pmax and Kst in the present work were in accordance with former studies. The Pmax and Kst
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Y. Li et al. / Journal of Loss Prevention in the Process Industries 45 (2017) 173e181 of fine graphite and tungsten dusts and their mixtures. Fusion Eng. Des. 75e79, 1135e1139. Denkevits, A., Dorofeev, S., 2006. Explosibility of fine graphite and tungsten dusts and their mixtures. J. Loss Prev. Process Ind. 19, 174e180. Gao, W., Yu, J., Zhang, X., Li, J., Wang, B., 2015. Characteristics of vented nanopolymethyl methacrylate dust explosions. Powder Technol. 283, 406e414. International Organization for Standardization, 1985. Explosion Protection SystemsPart 1: Determination of Explosion Indices of Combustible Dusts in Air. ISO 6184/1. Li, Q., Lin, B., Li, W., Zhai, C., Zhu, C., 2011. Explosion characteristics of nanoaluminum powder-air mixtures in 20 L spherical vessels. Powder Technol. 212, 303e309. Li, G., Yang, H.-X., Yuan, C.-M., Eckhoff, R.K., 2016a. A catastrophic aluminium-alloy dust explosion in China. J. Loss Prev. Process Ind. 39, 121e130. Li, Q., Wang, K., Zheng, Y., Ruan, M., Mei, X., Lin, B., 2016b. Experimental research of particle size and size dispersity on the explosibility characteristics of coal dust. Powder Technol. 292, 290e297. Marmo, L., Cavallero, D., 2008. Minimum ignition energy of nylon fibres. J. Loss Prev. Process Ind. 21, 512e517. Mittal, M., 2013. Limiting oxygen concentration for coal dusts for explosion hazard analysis and safety. J. Loss Prev. Process Ind. 26, 1106e1112. Mittal, M., 2014. Explosion characteristics of micron- and nano-size magnesium powders. J. Loss Prev. Process Ind. 27, 55e64. Medina, C.H., MacCoitir, B., Sattar, H., Slatter, D.J.F., Phylaktou, H.N., Andrews, G.E., et al., 2015a. Comparison of the explosion characteristics and flame speeds of pulverised coals and biomass in the ISO standard 1m3 dust explosion equipment. Fuel 151, 91e101.
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Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Explosion characteristics of micron-size conveyor rubber dust Yunhao Li a, Feifei Liu a, Qingwu Zhang a, *, Yuan Yu a, **, Chi-Min Shu b, Juncheng Jiang a a Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control, College of Safety Science and Engineering, Nanjing Tech University, Mail Box 13, No. 200 North Zhongshan Rd., Nanjing, 210009, China b Doctoral Program, Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2016 Received in revised form 19 December 2016 Accepted 19 December 2016 Available online 21 December 2016
Explosion characteristics of micron-size conveyor rubber dust were determined employing a normal 20 L stainless steel orbicular chamber and two types of modified 1.2-L Hartmann tubes. Maximum explosion pressure (Pmax), maximum rate of pressure rise (dP/dt)max, and minimum explosion concentration (MEC) were tested in the spherical chamber. The results indicated that the explosion severity of conveyor rubber powders is weaker than some common dust, such as coal, magnesium, and aluminum dust. However, conveyor rubber dust is a kind of combustible dust. Furthermore, the explosion severity increased as the particle size decreased from 120 to 48 mm. The MEC decreased from 90 to 30 g/m3 on decreasing particle size from 120 to 48 mm. Moreover, the MEC of conveyor rubber dust was 30, 50 and 90 g/m3 for the particle size of 48, 75, and 120 mm. The minimum ignition energy (MIE) was measured by two types of modified 1.2liter Hartmann tubes. The results indicated that particle size and dust concentration had influences on MIE. However, the MIE of conveyor rubber dust was much higher than 10 mJ. Accordingly, conveyor rubber dust cannot be ignited by collision, friction, and attrition. The experimental data presented could be useful for process plants that manufacturing conveyor rubber to evaluate explosibility of their conveyor rubber powders and propose/design adequate safety measures. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Micron-size conveyor rubber dust Hartmann tube Maximum rate of pressure rise Explosion severity Particle size Minimum ignition energy
1. Introduction Synthetic rubber is widely used in the production of tires, rubber hoses, and conveyor belts, etc. The European Union, United States, and China are dominant consumers for synthetic rubber, and the demand for synthetic rubber has been growing swiftly. According to statistics, the total production of tyres, which are manufactured in about 90 plants in the EU, was ca. 355 million in 2014, corresponding to 24% of the total world production. Meanwhile, synthetic rubber is a main production material of tyres (Torretta et al., 2015). In addition, there are numerous manufacturing and processing enterprises for other rubber products. Much rubber dust is generated in the process of manufacturing and processing of tyres as well as other rubber products. Combustible dusts can be found in various industries, and dust explosions present substantial threats to people, assets, and the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Y. Yu). http://dx.doi.org/10.1016/j.jlp.2016.12.005 0950-4230/© 2016 Elsevier Ltd. All rights reserved.
environment (Yuan et al., 2015), such as the catastrophic aluminum-alloy dust explosion which killed 146 persons in China (Li et al., 2016a). Table 1 lists several serious disasters caused by dust explosion (Yuan et al., 2015). Accordingly, it is necessary to explore the explosion severity and ignition sensitivity parameters for the conveyor rubber dust. Conveyor rubber is essentially a kind of polymer. There exists some information on explosion characteristics of polymer powders (Marmo and Cavallero, 2008; Amyotte et al., 2012; Gao et al., 2015; Addai et al., 2015, 2016). Amyotte et al. (2012) conducted experiments to determine the explosion severity and ignition sensitivity parameters for fibrous wood and polyethylene. Gao et al. (2015) studied the pressure characteristics of vented 100 and 800 nm polymethyl methacrylate (PMMA) dust explosions with different venting diameters. Addai et al. (2015) tested the explosion behavior of corn starch, and investigated the effects of methane and acetone on the explosion behavior of corn starch. Addai et al. (2016) determined the minimum ignition energy (MIE) of starch, wheat flour, protein, polypropylene, and dextrin. Marmo and Cavallero (2008) tested the MIE of fibrous nylon dust cloud. For micron-size dust, Soundararajan et al. (1996) experimentally
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Table 1 Accidents caused by dust explosion (Yuan et al., 2015). Date Country Material Equipment involved
Types of industries
1993 US 1996 US
PR PR
1998 1999 1999 2002 2003 2003 2004 2007 2007 2009
US US US US US US US CHN CHN CHN
PR PR PR PR PR PR W W F M
Air arc gauge with Plasmarc and Mappgas; mixing tank Plastic products factory Blending tote Automobile brake pads and lining manufacturer Dust collector; air handling ducts Sports equipment manufacture Oven; dust collection system Grey and duct tile foundries Machine that grinds plastic pellets Plastic manufacturing Tire bin Tire recycling Milling equipment Mechanical rubber goods Oven e Dust collection system Wood products plant Conveyor Wood pelts manufacturer Workshop, pulverizer Rice process factory Workshop Aluminum products
2009 2010 2011 2012 2014
CHN CHN US CHN CHN
IN M M M M
Dust collection system Dust setting chamber Pipes, furnace Polishing workshop Polishing workshop
Chemical preparations Metal polishing Iron powder plant Metal polishing plant Metal polishing
Dead/ injured
Ignition sources
2d/2i 1d
Electrical sparks e
16d 9d/3i 2d 13d 38d/6i 37d/7i 3d 4d/5i e 11d/20i 2i 2d/6i 3d/2i 13d/16i 146d/114i
e Flame and direct heat Hot work e e e Impact sparks e Hot surface Self-heatingand smoldering e Impact sparks Impact sparks Electrical sparks self-ignition
PR: Plastic/rubber; W: Wood; F: Food; M: Metal; IN: Inorganic.
researched the influences of powder diameter on explosion features of micron-pyrite and pyrrhotine. Denkevits and Dorofeev (2005, 2006) investigated the explosibility of micron-graphite and micron-tungsten dusts along with mixtures for different particle size. Pilǎo et al. (2006) studied the explosion severity and ignition sensitivity of cork and air dust mixtures in an almost spherical apparatus. Azhagurajan et al. (2012) studied the MIEs of micron- and nano-size flash powders in fireworks industry with the change of electrode gap, electrode material, and dust concentration, dust composition, etc. Mittal (2013), Yuan et al. (2014), and Li et al. (2016b) conducted experiments to study the explosion characteristics of micron coal dust. Castellanos et al. (2014) investigated the effects of powder diameter polydispersity on the explosibility features of aluminum dust. Boskovic et al. (2015), Medina et al. (2015a, b), and Saeed et al. (2015) tested the explosibility characteristics of biomasses. All of the above research results have provided useful data to design proper and sufficient safety measures to mitigate or prevent dust explosion. To supply useful information for manufacturing and handling enterprises of conveyor rubber products, it is significant to investigate the explosion severity and ignition sensitivity parameters of conveyor rubber dust. The conveyor rubber dust samples used for the current works were supplied by Gates Unitta Power Transmission (Suzhou) Co. Ltd., a company manufacturing different kinds of conveyor belts composed mainly by synthetic rubbers. And during the cutting and polishing process, a large number of conveyor rubber powders with different particle sizes would be generated, and suspended by the momentum from processing machines and ventilation system. Therefore, the conveyor rubber dust mainly distributed in the workshops, ducts, and dust collectors. In the present paper, the objective was to test and elucidate the explosion characteristics of micron-size conveyor rubber dust. The influences of dust concentration and powder size of conveyor rubber dust on maximum explosion pressure (Pmax) and explosibility index (Kst) were studied, along with the influences of powder size on minimum explosion concentration (MEC) and the effects of dust concentration and particle diameter on MIE.
2. Methodology 2.1. Apparatuses and methods 2.1.1. Experimental apparatus for MEC and explosion severity parameters A normal 20 L stainless steel orbicular chamber (Fig. 1), produced by Northeastern University in Liaoning province, China and recommended by International Standard ISO6184/1 (International Organization for Standardization, 1985), was selected to test Pmax, maximum rate of pressure rise (dP/dt)max, and MEC of conveyor rubber dust. This device mainly consists of a spherical vessel,
Fig. 1. The 20 L spherical experimental apparatus. Notes: 1Water outlet, 2Pressure sensor, 3Manometer, 4Dust container, 5Air inlet, 6Ignition source, 7Rebound nozzle, 8Fast acting valve, 9Water inlet, 10Outlet (air, reaction products).
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control system, and data acquisition system. Before an explosion test, pre-weighed conveyor rubber dust was put into the dust container (volume: 0.6 L), and an igniter fixed at hub of the chamber. The safely sealed explosion vessel was vacuumed in part to 0.6 bar (gauge) and the pressure of pressurized air in the dust storage tank was 20 bar (gauge). Conveyor rubber powders and air were first diffused into the chamber when opened the solenoid valve between the sample container and vessel. The chemical igniter was then ignited after delaying 60 ms. Finally, the acquired data were analyzed by the computer. Once a measurement was accomplished, the vessel and dust storage tank were tidied by a vacuum sweeper in preparation for the next test. The test methods of Pmax, (dP/dt)max, and MEC of conveyor rubber dust were based on European Standard EN 140341: 2004 (Birtish Standards Institution, 2004), EN 140342: 2006 (Birtish Standards Institution, 2006a), and EN 140343: 2006 (Birtish Standards Institution, 2006b). A chemical ignitor with energy of 2 kJ was used in the experiments. Ambient temperature was 25 C. In the process of a test, the pressure change of conveyor rubber dust explosion was gauged and notes taken by a system that acquiring experimental data. The measurement range of the pressure sensor was 0e250 psi (17 bar). A normal curve of pressuretime evolution noted during conveyor rubber dust explosion is delineated in Fig. 2. Pressure evolution first began at 0.6 bar (gauge). The air blast applying for dispersing powders then started at 156 ms. Igniting started at 216 ms at an ambient condition of 1 bar (absolute). The value of Pex, for a single measurement at a certain concentration, is the highest explosion overpressure (gauge). (dP/dt)ex is maximum rate of pressure rise in one test (Mittal, 2014). A dust explosion occurred when the measured overpressure Pex 0.05 bar relative to the initial pressure Pi 0.02 bar. If the measured overpressure Pex 0.05 bar was relative to the initial pressure Pi 0.02 bar in three continuous tests, this sample was considered as an unexplosive dust at this concentration. According to (dP/dt)max and the volume of the spherical chamber (V), Kst an international common value can be calculated by Eq. (1):
Fig. 2. Pressuretime curve recorded during dust explosion experiments.
Kst ¼ ðdP=dtÞmax V 3
175
(1)
2.1.2. Experimental apparatuses for MIE According to the European Standard EN 13821: 2002 (Birtish Standards Institution, 2002), two types of modified 1.2-L Hartmann tubes made of glass were employed as the apparatus for MIE. The test coverage of one device ranges from 1 to 2000 mJ. The other only provides ignition energy of 4 and 10 J. Fig. 3 shows a schematic diagram of the modified Hartmann apparatus. These apparatuses consist of Hartmann tube, control system, high voltage unit, dust diffusion unit, electronic pneumatic control unit, etc. The dust diffusion and ignition are implemented in the Hartmann tube. The pressure of the pressurized air stored in the gas container was 70 bar. When the electronic pneumatic valve was opened, the conveyor rubber dust at the bottom of the tube was dispersed into the tube. Then the conveyor rubber dust was ignited by electrical spark triggered by discharge of the high voltage unit. Inductance of the discharge circuit was 1 mH. Moreover, the electrode gap was 6 mm. Furthermore, the time from dispersion to ignition was 120 ms. After ignition, if the conveyor rubber dust exploded and flame propagation appeared, a dust cloud explosion was induced; if no flame propagation was seen in ten continual tests, this ignition energy could not induce a dust cloud explosion. To obtain MIEs, the tests were accomplished by starting at a relatively high spark energy level. Then the spark energy level was reduced in steps until no ignition occurred for ten ignition trials. 2.2. Samples and particle size characteristics The particle size distribution of original dust samples used for the current works from workshops was measured by laser diffraction analyzer. And the particle size ranges from 30 to 500 mm. Meanwhile, many researchers have found that Pmax and Kst decreased with increasing particle size. Therefore, finer conveyor rubber powders may have more significant hazards. And we prepared testing samples by sieving. Then particle diameter distributions of conveyor rubber dust were confirmed by a laser diffraction analyzer and distinguished by d50 recorded respectively by 48, 75, and 120 mm. The powder diameter distributions of conveyor rubber
Fig. 3. Schematic diagram of the modified Hartmann apparatus.
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Fig. 5. SEM photograph of conveyor rubber powder.
irregular. In addition, the surface of the conveyor rubber particles was flocculent. 3. Results and discussion 3.1. Dust concentration effects on Pmax and Kst
Fig. 4. Particle size distributions of samples.
powder samples are shown in Fig. 4. Besides, prior to explosion tests, to obtain relatively dry samples, the conveyor rubber dust was dried by a vacuum drying box. Fig. 5 shows a photograph of conveyor rubber powders by scanning electron microscope (SEM). It can be seen that the shapes of the conveyor rubber particles were
Explosion severity can be characterized by Pmax and Kst. It is vital for safety design and explosion prevention to understand the explosion severity parameters of conveyor rubber dust. The conveyor rubber dust concentrations for the tests were 100, 200, 300, 400, 500, 600, and 750 g/m3. The evolution of Pmax and Kst vs. dust concentration is illustrated in Figs. 6 and 7, respectively. In Fig. 6, it can be seen that Pmax increased first and then decreased as dust concentration increased. Note that the Pmax was 4.67 and 4.54 bar for the conveyor rubber dust with the particle size of 48 and 75 mm, respectively. When the particle size increased to 120 mm, the Pmax decreased to 3.91 bar. From Fig. 7, one notice that Kst increased first and then decreased with the increase of dust concentration, too. Interestingly, the Kst was 25 and 24.14 bar m/s for the conveyor rubber dust with the particle size of 48 and 75 mm, respectively. When the particle size increased to 120 mm, the Kst decreased to 21.95 bar m/s. Table 2 lists the explosion characteristics of several common dust samples gleaned from the literature. As shown, conveyor rubber powders had much lower Pmax and Kst than most of the dust samples, such as coal and magnesium. Only the explosion severity parameters of iron dust were lower than conveyor rubber dust. According to Cashdollar (1996, 2000), Cashdollar and Zlochower (2007), Li et al. (2011), and Mittal (2014), the Pmax and Kst reached its maximal values when dust concentration ranged from 800 to 1500 g/m3 for magnesium, aluminum, iron, and coal dust. Nonetheless, the concentration ranging from 300 to 500 g/m3 was lower for conveyor rubber dust when reaching the maximum Pmax and Kst. Consequently, the explosion severity of conveyor rubber powders was weaker than most of the common dusts, such as coal, magnesium, and aluminum dust. Nevertheless, conveyor rubber dust is a kind of combustible dust. If we do not pay attention to it and take proactive measures to prevent or mitigate its accumulation, there exists the risk of a conveyor rubber dust explosion. 3.2. Particle size effects on Pmax and Kst The evolution of Pmax and Kst vs. particle size is plotted in Figs. 8 and 9. The conveyor rubber dust concentrations were 300, 500, and 750 g/m3, respectively. Figs. 8 and 9 show powder diameters had prominent influences on Pmax and Kst. The Pmax decreased faster
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Fig. 6. Pmax of conveyor rubber dust with different dust concentrations. Fig. 8. Pmax of conveyor rubber dust with different particle sizes.
and the Kst decreased slowly with the particle size increasing when the dust concentration varied from 300 to 750 g/m3. The effects of particle size on Pmax and Kst were investigated by Cashdollar (1996), Soundararajan et al. (1996), Nifukua et al. (2007), Wu et al. (2010), Li et al. (2011), Amyotte et al. (2012), and Mittal (2014). They found that Pmax and Kst decreased with increasing particle size. Meanwhile, finer size conveyor rubber powders were more intrinsically hazardous. They also found that the reason why Pmax and Kst increased with the particle size decreasing was that the specific surface area of powders decreased as the particle size increased. Furthermore, powders usually agglomerate. This characteristic has influences on the powder diffusion, distribution, ignition, and deflagration. The results in the present work are in accordance with the above researches. According to the SEM photograph for conveyor rubber powders, we estimated that the specific surface area of conveyor rubber powders increased as the particle size decreased. Meanwhile, the rate of thermal Fig. 7. Kst of conveyor rubber dust with different dust concentrations.
Table 2 Explosion characteristics for several common dust samples summarized from the literature. Sample
Pmax (bar)
Kst (bar m/s)
Nominal MEC (g/m3)
Particle size (mm)
Vessel volume
References
Prince mine coal Phalen mine coal Lingan mine coal Pittsburgh coal Pocahontas coal Kellingley coal Colombian coal Cork Fibrous wood Fibrous polyethylene Spherical polyethylene Spherical polyethylene Spherical polyethylene Aluminum Magnesium Magnesium Magnesium Titanium Iron Conveyor rubber Conveyor rubber Conveyor rubber
6.5 6.0 7.0 6.7 6.5 8.2 8.5 7.2 8.2 7.2 5.8 6.7 6.9 5.9 7 8.8 10.8 5.7 3.1 3.91 4.54 4.67
44 30 44 41 31 80 129 179 149 75 15 104 137 24.4 98 202 362 42 3.5 21.95 24.14 25
70 120 90 65 80 120 75 40 20 30 500 10 20 50 160 90 60 70 ~500 90 50 30
<125 <125 <125 <75 <75 D90 ¼ 85 D90 ¼ 65 D90 ¼ 280 <75 <75 <212 <75 <38 75 125 74 38 ~25 ~45 120 75 48
20 L 20 L 20 L 20 L 20 L 1 m3 1 m3 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L 20 L
Amyotte et al. (1991) Amyotte et al. (1991) Amyotte et al. (1991) Cashdollar (1996) Cashdollar (1996) Medina et al. (2015a,b) Medina et al. (2015a,b) Pilǎo et al. (2006) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Li et al. (2011) Mittal (2014) Mittal (2014) Mittal (2014) Cashdollar and Zlochower (2007) Cashdollar and Zlochower (2007) Present study Present study Present study
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Fig. 9. Kst of conveyor rubber dust with different particle sizes.
decomposition and evaporation of conveyor rubber dust increased with decreasing particle size. Accordingly, the concentration of flammable gas pyrolyzed by conveyor rubber particles increased as the particle size decreased. Therefore, Pmax and Kst decreased with the increasing of the particle size. 3.3. Particle size effects on MEC Explosion possibility could be characterized by MEC. It is crucial for industrial production to determine the MEC. Once MEC is determined, numerous measures can be taken to alleviate the ambient dust composition, so as to avoid the occurrence of a dust explosion from ignition sources. A certain dust concentration was selected as an initial test concentration. If Pex 0.5 bar (gauge), the explosion measurement endured with decreasing the dust concentration level in 10 g/m3 until the three continuous measured overpressures were all smaller than 0.5 bar (gauge). The tested MEC of powder samples Cmin was between Ca (the maximum dust concentration of three continuous measurements when Pex 0.5 bar) and Cb (the minimum dust concentration of the three continuous measurements when
Fig. 10. MEC of conveyor rubber dust vs. particle size.
Pex 0.5 bar), so Ca < Cmin < Cb. In this study, 100 g/m3 was selected as the initial test concentration, because the conveyor rubber dust explosion occurred at 100 g/m3. Fig. 10 depicts the MEC of conveyor rubber dust vs. particle size. The MEC values for different particle sizes are summarized in Table 2. It can be seen that experimentally measured MEC of conveyor rubber dust was 30, 50, and 90 g/m3, respectively, for the particle size of 48, 75, and 120 mm. These results indicated MEC decreased as the powder diameter decreased. The reason is that the specific surface area increased as the powder size decreased, and conveyor rubber powders contact more fully with oxygen. Therefore, the chemical reaction was more intense between oxygen and conveyor rubber powders. Accordingly, less conveyor rubber powder was required when an explosion was triggered when the particle size was smaller. Consequently, MEC reduced with the decreasing with particle size. The fourth column in Table 2 reports the nominal MEC for several common dust samples. By comparison, the MEC for conveyor rubber dust was lower than several common metals and higher than cork and fibrous wood. Meanwhile, a conveyor rubber dust explosion can occur at a very low concentration. Consequently, in plants for manufacturing and processing conveyor rubber products, more stringent preventive measures must be taken than industrial plants for metals to prevent or mitigate the accumulation of conveyor rubber dust. 3.4. Ignition sensitivity Ignition sensitivity can be characterized scientifically by the MIE of a dust cloud. The dangerous situations of dust processing equipment and workplaces can be judged by MIE. Meanwhile, according to MIE, preventive and mitigative measures can be designed against a hazardous scenario. As planned, the conveyor rubber dust concentrations for tests were 150, 300, 450, 600, and 750 g/m3. Fig. 11 shows experimental MIEs vs. dust concentration for different particle sizes. Experimental MIEs for conveyor rubber dust are given in Table 3. It can be seen that only the conveyor rubber dust with the particle sizes of 48 and 75 mm was ignited by the modified 1.2-L Hartmann tube of which the test coverage ranges from 1 to 2000 mJ when the dust concentration reached 750 g/m3. However, the other conveyor rubber dust samples could not be ignited by the ignition energy of 2000 mJ. Due to the limitations of the apparatus, only the approximate range of ignition energy can be measured by the second modified 1.2-L Hartmann tube, which generated ignition energy of 4 and 10 J for the other conveyor rubber dust samples. If a conveyor rubber dust sample was ignited by the spark energy of 4 and 10 J, and was not ignited by 2 J, the ignition energy for this sample ranged from 2 to 4 J. If a conveyor rubber dust sample was ignited by the spark energy of 10 J, and was not ignited by 4 J, the ignition energy for this sample ranged from 4 to 10 J. It also can be seen that particle size and dust concentration had influences on MIE. For the dust concentration of 750 g/m3, MIE for the particle size of 120, 75, and 48 mm was 2.0e4.0, 1.5e1.6, 1.1e1.2 J, respectively. The larger the particle size, the greater the MIE. Furthermore, for a particle size of 75 mm, it seems that MIE decreased as the concentration of conveyor rubber dust increased. It may be because the particle ignited by spark per unit volume increased when the dust concentration increased. Table 4 summarizes the MIEs of several dust samples. It appears that MIE of conveyor rubber dust is much higher than some metals, fibrous wood, and polyethylene. If the MIE of a dust is smaller than or equal 10 mJ, it can be ignited by collision, friction, and attrition (Christoph and Richard, 1995). Therefore, conveyor rubber dust cannot be ignited by collision, friction, and attrition.
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Fig. 11. Experimental MIEs vs. dust concentration for different particle sizes.
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Table 3 Experimental MIEs for conveyor rubber dust. Particle size (mm)
Dust concentration (g/ Ignition energy Particle size m3) (J) (mm)
Dust concentration (g/ Ignition energy Particle size m3) (J) (mm)
Dust concentration (g/ Ignition energy m3) (J)
120 120 120 120 120
150 300 450 600 750
150 300 450 600 750
150 300 450 600 750
4.0e10.0 4.0e10.0 2.0e4.0 2.0e4.0 2.0e4.0
75 75 75 75 75
Table 4 MIEs for different dust samples. Material
Particle size (mm)
MIE (mJ)
References
Magnesium Magnesium Magnesium Aluminum Titanium Fibrous wood Fibrous polyethylene Spherical polyethylene Spherical polyethylene Spherical polyethylene
125 74 38 >37 45 <75 <75 <212 <75 <38
120 50 10 48 21.91 10e30 10e30 >1000 10e30 10e30
Mittal (2014) Mittal (2014) Mittal (2014) Nifukua et al. (2007) Wu et al. (2009) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012) Amyotte et al. (2012)
Table 5 Dust explosion class standard in accordance with ISO 6184/1 (International Organization for Standardization, 1985). Dust explosion class
Kst/(bar m/s)
St1 St2 St3
0
4.0e10.0 2.0e4.0 2.0e4.0 2.0e4.0 1.5e1.6
48 48 48 48 48
2.0e4.0 2.0e4.0 2.0e4.0 2.0e4.0 1.1e1.2
decreased with the increase of particle size when the dust concentration varied from 300 to 750 g/m3. (3) The MEC of conveyor rubber dust was 30, 50, and 90 g/m3 for the corresponding particle size of 48, 75, and 120 mm. In addition, the MEC for conveyor rubber dust was lower than several common metals, but higher than cork and fibrous wood. Consequently, in plants for manufacturing and processing conveyor rubber products, more stringent preventive measures than industrial plants for metals must be seriously taken to prevent or mitigate the accumulation of conveyor rubber dust. (4) The particle size and dust concentration had influences on MIE. However, the MIE of conveyor rubber dust was much higher than 10 mJ. Therefore, conveyor rubber dust cannot be ignited by collision, friction, and attrition. Acknowledgements The authors are grateful for the financial support given by the National Key Research and Development Plan (No. 2016YFC0800102) and key project of National Natural Science Foundation of China (No. 21436006), along with the open project of Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control.
3.5. Conveyor rubber dust explosion class References Table 5 presents the dust explosion class standard in accordance with ISO6184/1 (International Organization for Standardization, 1985). According to the ISO6184/1, the conveyor rubber dust for different particle sizes falls into class St1, with weak or medium explosive power. Due to the explosion risk of conveyor rubber dust, when designing the plants for producing and processing conveyor rubber products, preventive and mitigative measures for conveyor rubber dust explosion must be seriously considered. During production, avoiding the formation of flammable conveyor rubber dust cloud is a crucial task. 4. Conclusions A standard 20 L stainless steel spherical vessel and two types of modified 1.2-L Hartmann tubes were applied to investigate the explosion characteristics of conveyor rubber dust with various particle sizes and dust concentrations. Moreover, the conveyor rubber dust explosion class was also analyzed. The following conclusions can be drawn from the experimental results. (1) The explosion severity of conveyor rubber powders is weaker than some common dust, such as coal, magnesium, and aluminum dust. Nevertheless, conveyor rubber dust is a kind of combustible dust. According to the ISO6184/1 (International Organization for Standardization, 1985) dust explosion class standard, the conveyor rubber dust for different particle sizes falls into class St1. (2) The effects of particle size on Pmax and Kst in the present work were in accordance with former studies. The Pmax and Kst
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