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Incendiary characteristics of electrostatic discharge for dust and gas explosion
Journal of Loss Prevention in the Process Industries 14 (2001) 547–551 www.elsevier.com/locate/jlp
Incendiary characteristics of electrostatic discharge for dust and gas explosion M. Nifuku *, H. Katoh National Institute of Advanced Industrial Science and Technology, Onogawa 16-1, Tsukuba, Ibaraki 305-8569, Japan
Abstract Paying attention to the ignition potentiality of static electricity, the relation between the discharge characteristics and the ignition of a dust cloud and the gas produced was studied, applying an electrical power supply of which the electrical circuit is adjustable. The effect of ignition characteristics on dust and gas explosions was investigated. The results of the study indicate that the probability of an explosion is influenced by the minimum ignition energy, spark duration time, feeding rate of ignition energy, circuit capacitance, ignition voltage, etc. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Electrostatic charge; Dust cloud; Ignition source; Dust explosion
1. Introduction In industry, various kinds of powders and liquids are handled in many forms. Electrostatic charge is always present in such processes. There is always potential danger that the charge might ignite the dust cloud and gas produced. However, there remains much to be investigated to elucidate ignition due to electrostatic charge. Ignition phenomena of dust and gas are influenced by the generating characteristics of the ignition sources. Empirically, all electrostatic discharges do not necessarily ignite a dust cloud and/or gas. It is necessary to investigate the details of the ignition in order to maintain safe operation of these processes. The incendiary character of a dust cloud/gas is influenced by the electrostatic discharge characteristics (International Electrotechnical Commission, 1984; Tanaka & Sugawara, 1971; Tanaka & Ichikawa 1976, 1977). The discharge characteristics depend on many factors such as total charge, capacitance of the charged body, shape of discharge points (geometrical dimensions), accumulated voltage, etc. Gas, in particular, is very sensitive to electrical discharge characteristics, and it is important to ascertain details of its ignitability.
* Corresponding author. Fax: +81-298-61-8791. E-mail address: [email protected] (M. Nifuku).
In this research, relations between the discharge characteristics and the ignition of a dust cloud and gas were studied, applying an electrical power supply of which the electrical circuit is adjustable. The effect of ignition characteristics on dust and gas explosions was investigated.
2. Experimental For the incendiary investigation of a dust explosion, the equipment used was the Hartman explosion apparatus and an ignition energy power supply, as shown in Figs. 1 and 2. Fig. 1 shows the Hartman explosion apparatus. The diameter of the explosion tube is about 6.5 cm, the height about 33 cm and the volume about 1.1 l. Fig. 2 details the ignition energy power supply. The power supply produces various types of ignition source. The ignition source is produced by the discharge of highvoltage electricity with arranged discharge duration time. The ignition duration is controllable from 100 µs to 10 ms. The maximum discharge energy is 1.4 J. The ignition energy is controlled by changing the discharge electric current and the discharge pulse width. The capacitance of the capacitor in the apparatus is 100 µF. Besides the apparatus described in Fig. 2, a simplified DC power supply–capacitor device with thyratron switch was also applied. This represents the common electro-
0950-4230/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 4 2 3 0 ( 0 1 ) 0 0 0 4 6 - 8
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M. Nifuku, H. Katoh / Journal of Loss Prevention in the Process Industries 14 (2001) 547–551
Fig. 3. Electric current and voltage waves without discharge regulation.
Fig. 1.
Experimental apparatus for dust explosion.
static discharge produced in industry. The thyratron was used to minimize the energy loss by the high-voltage switching. In Fig. 3, which shows a discharging waveform, it can be seen that the electric current is steadily decreased. Fig. 4 shows an example of the discharge waveform. It can be seen that the discharge voltage and current maintain the same levels of discharge. This is advantageous in analyzing the incendiary characteristics because the ignition energy supply is maintained at the same level.
Fig. 2.
The test powders were coal dust (mean particle size 70 µm, volatile content 38%, water content 4% and calorific value 6500 cal/g), zirconium and zircaloy-4. The minimum explosive concentration of coal dust is about 50 g/m3. Fig. 5 shows the apparatus used to ascertain the incendiary characteristics of the gas explosion. The diameter of the explosion chamber is 8.2 cm and the height 20 cm. The ignition source is provided by discharging electricity after charging the capacitor and operating the vacuum switch. The ignition electrode is needle-like (tungsten rod, top angle is about 8°, diameter 2 mm). The ignition energy is supplied into the ignition electrode after the explosion chamber is filled with sample gas. The sample gases applied are methane gas–air mixture (methane 8.3 vol.%) and propane gas–air mixture (propane 5.25 vol.%). The capacitance is changed from 33 to 10,000
Block diagram for the measurement of ignition energy.
M. Nifuku, H. Katoh / Journal of Loss Prevention in the Process Industries 14 (2001) 547–551
Fig. 4.
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Discharging waveform.
Fig. 6.
Relation between particle size and ignition energy.
Fig. 5. Experimental apparatus for the measurement of ignition energy by capacitive spark.
pF. The maximum charging voltage for the capacitor is 20 kV.
3. Results and discussion Since the powders used in industry have a wide ranging size distribution, which is one of the key factors for the explosibility of dust, the authors investigated the influence of the particle size on the ignition energy. The result is shown in Fig. 6. The influence of particle size is shown clearly. The smaller the particle size (in other words, the larger the specific surface area), the smaller the ignition energy. As is already known, ignition energy has a big influence on dust explosion. If a larger amount of energy is supplied to particles, it is easier for the oxidation of particles to take place. Fig. 7 shows the influence of ignition energy on the explosibility of powders. As is clearly shown, the minimum explosive concentration decreases
Fig. 7. Minimum explosive concentration as a function of igniting energy.
with increasing ignition energy. This relation becomes saturated at a certain amount of ignition energy. Thus, the minimum ignition energy is obtained. In the process of coal dust explosion, flammable substances come out of the coal particles and the explosive mixture is produced. The mixture then receives heat energy and ignites. This reaction continues in the case
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M. Nifuku, H. Katoh / Journal of Loss Prevention in the Process Industries 14 (2001) 547–551
Fig. 8.
Effect of spark duration time on the ignition energy.
of coal dust explosion. Therefore, the contact time between the coal dust cloud and the ignition source influences the formation of the explosive mixture, ignition, development of the explosion, etc. Fig. 8 shows this relation (coal dust cloud concentration is 150 g/m3). It can be seen that the ignition energy decreases with increasing spark duration time and that the contact time has a big influence on the ignitability of the coal dust cloud. Fig. 9 shows the relation between the ignition energy and the ignition probability of the coal dust cloud. The
Fig. 9. Relation between ignition energy and probability for explosion when ignition duration is varied under constant coal dust concentration.
probability is increased with increasing ignition energy. The probability, however, is not significantly influenced by the ignition spark duration time when it is 2–10 ms and the coal dust cloud concentration is about 150 g/m3. However, the formation of the flammable mixture around the coal particle, the ignition and the propagation of these processes is influenced by the method of thermal energy supply, as depicted in Fig. 10. It can be seen that the longer the ignition duration, the bigger the ignition probability, even with a similar feeding rate of ignition energy. Furthermore, the sample with a smaller ignition duration has a smaller ignition probability, even when the feeding rate of the ignition energy is increased. These findings demonstrate that the ignitability of a coal dust cloud is greatly influenced by the method of ignition energy feeding. Specifications for the ignitability of gases are given in IEC Publication 79-11, in the capacitance ranges of 0.01–10,000 µF and igniting voltages of 5–1000 V (International Electrotechnical Commission, 1984). However, there are cases in actual industrial situations with less capacitance and larger voltage where static electrification occurs. The ignitability of the sample gases are shown in Fig. 11 in view of this factor. It is shown that the minimum ignition voltage decreases and the minimum ignition energy increases with increasing circuit capacitance up to about 500 pF. The minimum ignition voltage becomes constant (about 4 kV) for a capacitance of more than about 1000 pF. For a charging voltage of less than about 4 kV to the capacitor, a spark is not produced. Therefore, the energy produced by the spark discharge depends on the capacitance of the discharge circuit. This implies that certain values of the voltage in the ignition source and the energy, corresponding to the capacitance in the charging system, are
Fig. 10. Relation between feeding rate of ignition energy and probability for explosion when ignition duration is varied under constant coal dust concentration.
M. Nifuku, H. Katoh / Journal of Loss Prevention in the Process Industries 14 (2001) 547–551
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mixture. 4. Conclusion The results of the study are concluded as follows. 1. The ignition of a dust cloud is influenced considerably by the particle size distribution, the feeding time of the ignition energy and the feeding rate of the ignition energy. 2. The ignitability of a combustible gas–air mixture depends on the capacitance of the electrical discharge system, as well as the minimum ignition voltage decreases and the ignition energy increases up to a particular level of the electrostatic capacitance. Certain values of the voltage and energy are necessary to ignite combustible gases by electrostatic discharge.
References
Fig. 11. Effect of circuit capacitance on the minimum ignition voltage and energy.
necessary for the ignition of a combustible gas–air
International Electrotechnical Commission. IEC Standard, Publication 79-11; 1984. p. 1–61. Tanaka R, Sugawara N. Safety for electrical equipment under artificial environments — low voltage d.c. spark ignition of atmospheric CH4–O2 and CH4–N2O mixtures and of solid combustibles in oxygen atmospheres. Research report of the Research Institute of Industrial Safety, RR-19-6; 1971. p. 1–11. Tanaka R, Ichikawa K. Safety for electrical equipment under artificial environments — ignition of flammable solid materials by low voltage inductive sparks. Research report of the Research Institute of Industrial Safety, RR-24-7; 1976. p. 1–8. Tanaka R, Ichikawa K. Safety for electrical equipment under artificial environments — ignition of flammable solid materials by low voltage capacitive or resistive sparks. Research report of the Research Institute of Industrial Safety, RR-26-2; 1977. p. 1–11.
Incendiary characteristics of electrostatic discharge for dust and gas explosion M. Nifuku *, H. Katoh National Institute of Advanced Industrial Science and Technology, Onogawa 16-1, Tsukuba, Ibaraki 305-8569, Japan
Abstract Paying attention to the ignition potentiality of static electricity, the relation between the discharge characteristics and the ignition of a dust cloud and the gas produced was studied, applying an electrical power supply of which the electrical circuit is adjustable. The effect of ignition characteristics on dust and gas explosions was investigated. The results of the study indicate that the probability of an explosion is influenced by the minimum ignition energy, spark duration time, feeding rate of ignition energy, circuit capacitance, ignition voltage, etc. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Electrostatic charge; Dust cloud; Ignition source; Dust explosion
1. Introduction In industry, various kinds of powders and liquids are handled in many forms. Electrostatic charge is always present in such processes. There is always potential danger that the charge might ignite the dust cloud and gas produced. However, there remains much to be investigated to elucidate ignition due to electrostatic charge. Ignition phenomena of dust and gas are influenced by the generating characteristics of the ignition sources. Empirically, all electrostatic discharges do not necessarily ignite a dust cloud and/or gas. It is necessary to investigate the details of the ignition in order to maintain safe operation of these processes. The incendiary character of a dust cloud/gas is influenced by the electrostatic discharge characteristics (International Electrotechnical Commission, 1984; Tanaka & Sugawara, 1971; Tanaka & Ichikawa 1976, 1977). The discharge characteristics depend on many factors such as total charge, capacitance of the charged body, shape of discharge points (geometrical dimensions), accumulated voltage, etc. Gas, in particular, is very sensitive to electrical discharge characteristics, and it is important to ascertain details of its ignitability.
* Corresponding author. Fax: +81-298-61-8791. E-mail address: [email protected] (M. Nifuku).
In this research, relations between the discharge characteristics and the ignition of a dust cloud and gas were studied, applying an electrical power supply of which the electrical circuit is adjustable. The effect of ignition characteristics on dust and gas explosions was investigated.
2. Experimental For the incendiary investigation of a dust explosion, the equipment used was the Hartman explosion apparatus and an ignition energy power supply, as shown in Figs. 1 and 2. Fig. 1 shows the Hartman explosion apparatus. The diameter of the explosion tube is about 6.5 cm, the height about 33 cm and the volume about 1.1 l. Fig. 2 details the ignition energy power supply. The power supply produces various types of ignition source. The ignition source is produced by the discharge of highvoltage electricity with arranged discharge duration time. The ignition duration is controllable from 100 µs to 10 ms. The maximum discharge energy is 1.4 J. The ignition energy is controlled by changing the discharge electric current and the discharge pulse width. The capacitance of the capacitor in the apparatus is 100 µF. Besides the apparatus described in Fig. 2, a simplified DC power supply–capacitor device with thyratron switch was also applied. This represents the common electro-
0950-4230/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 4 2 3 0 ( 0 1 ) 0 0 0 4 6 - 8
548
M. Nifuku, H. Katoh / Journal of Loss Prevention in the Process Industries 14 (2001) 547–551
Fig. 3. Electric current and voltage waves without discharge regulation.
Fig. 1.
Experimental apparatus for dust explosion.
static discharge produced in industry. The thyratron was used to minimize the energy loss by the high-voltage switching. In Fig. 3, which shows a discharging waveform, it can be seen that the electric current is steadily decreased. Fig. 4 shows an example of the discharge waveform. It can be seen that the discharge voltage and current maintain the same levels of discharge. This is advantageous in analyzing the incendiary characteristics because the ignition energy supply is maintained at the same level.
Fig. 2.
The test powders were coal dust (mean particle size 70 µm, volatile content 38%, water content 4% and calorific value 6500 cal/g), zirconium and zircaloy-4. The minimum explosive concentration of coal dust is about 50 g/m3. Fig. 5 shows the apparatus used to ascertain the incendiary characteristics of the gas explosion. The diameter of the explosion chamber is 8.2 cm and the height 20 cm. The ignition source is provided by discharging electricity after charging the capacitor and operating the vacuum switch. The ignition electrode is needle-like (tungsten rod, top angle is about 8°, diameter 2 mm). The ignition energy is supplied into the ignition electrode after the explosion chamber is filled with sample gas. The sample gases applied are methane gas–air mixture (methane 8.3 vol.%) and propane gas–air mixture (propane 5.25 vol.%). The capacitance is changed from 33 to 10,000
Block diagram for the measurement of ignition energy.
M. Nifuku, H. Katoh / Journal of Loss Prevention in the Process Industries 14 (2001) 547–551
Fig. 4.
549
Discharging waveform.
Fig. 6.
Relation between particle size and ignition energy.
Fig. 5. Experimental apparatus for the measurement of ignition energy by capacitive spark.
pF. The maximum charging voltage for the capacitor is 20 kV.
3. Results and discussion Since the powders used in industry have a wide ranging size distribution, which is one of the key factors for the explosibility of dust, the authors investigated the influence of the particle size on the ignition energy. The result is shown in Fig. 6. The influence of particle size is shown clearly. The smaller the particle size (in other words, the larger the specific surface area), the smaller the ignition energy. As is already known, ignition energy has a big influence on dust explosion. If a larger amount of energy is supplied to particles, it is easier for the oxidation of particles to take place. Fig. 7 shows the influence of ignition energy on the explosibility of powders. As is clearly shown, the minimum explosive concentration decreases
Fig. 7. Minimum explosive concentration as a function of igniting energy.
with increasing ignition energy. This relation becomes saturated at a certain amount of ignition energy. Thus, the minimum ignition energy is obtained. In the process of coal dust explosion, flammable substances come out of the coal particles and the explosive mixture is produced. The mixture then receives heat energy and ignites. This reaction continues in the case
550
M. Nifuku, H. Katoh / Journal of Loss Prevention in the Process Industries 14 (2001) 547–551
Fig. 8.
Effect of spark duration time on the ignition energy.
of coal dust explosion. Therefore, the contact time between the coal dust cloud and the ignition source influences the formation of the explosive mixture, ignition, development of the explosion, etc. Fig. 8 shows this relation (coal dust cloud concentration is 150 g/m3). It can be seen that the ignition energy decreases with increasing spark duration time and that the contact time has a big influence on the ignitability of the coal dust cloud. Fig. 9 shows the relation between the ignition energy and the ignition probability of the coal dust cloud. The
Fig. 9. Relation between ignition energy and probability for explosion when ignition duration is varied under constant coal dust concentration.
probability is increased with increasing ignition energy. The probability, however, is not significantly influenced by the ignition spark duration time when it is 2–10 ms and the coal dust cloud concentration is about 150 g/m3. However, the formation of the flammable mixture around the coal particle, the ignition and the propagation of these processes is influenced by the method of thermal energy supply, as depicted in Fig. 10. It can be seen that the longer the ignition duration, the bigger the ignition probability, even with a similar feeding rate of ignition energy. Furthermore, the sample with a smaller ignition duration has a smaller ignition probability, even when the feeding rate of the ignition energy is increased. These findings demonstrate that the ignitability of a coal dust cloud is greatly influenced by the method of ignition energy feeding. Specifications for the ignitability of gases are given in IEC Publication 79-11, in the capacitance ranges of 0.01–10,000 µF and igniting voltages of 5–1000 V (International Electrotechnical Commission, 1984). However, there are cases in actual industrial situations with less capacitance and larger voltage where static electrification occurs. The ignitability of the sample gases are shown in Fig. 11 in view of this factor. It is shown that the minimum ignition voltage decreases and the minimum ignition energy increases with increasing circuit capacitance up to about 500 pF. The minimum ignition voltage becomes constant (about 4 kV) for a capacitance of more than about 1000 pF. For a charging voltage of less than about 4 kV to the capacitor, a spark is not produced. Therefore, the energy produced by the spark discharge depends on the capacitance of the discharge circuit. This implies that certain values of the voltage in the ignition source and the energy, corresponding to the capacitance in the charging system, are
Fig. 10. Relation between feeding rate of ignition energy and probability for explosion when ignition duration is varied under constant coal dust concentration.
M. Nifuku, H. Katoh / Journal of Loss Prevention in the Process Industries 14 (2001) 547–551
551
mixture. 4. Conclusion The results of the study are concluded as follows. 1. The ignition of a dust cloud is influenced considerably by the particle size distribution, the feeding time of the ignition energy and the feeding rate of the ignition energy. 2. The ignitability of a combustible gas–air mixture depends on the capacitance of the electrical discharge system, as well as the minimum ignition voltage decreases and the ignition energy increases up to a particular level of the electrostatic capacitance. Certain values of the voltage and energy are necessary to ignite combustible gases by electrostatic discharge.
References
Fig. 11. Effect of circuit capacitance on the minimum ignition voltage and energy.
necessary for the ignition of a combustible gas–air
International Electrotechnical Commission. IEC Standard, Publication 79-11; 1984. p. 1–61. Tanaka R, Sugawara N. Safety for electrical equipment under artificial environments — low voltage d.c. spark ignition of atmospheric CH4–O2 and CH4–N2O mixtures and of solid combustibles in oxygen atmospheres. Research report of the Research Institute of Industrial Safety, RR-19-6; 1971. p. 1–11. Tanaka R, Ichikawa K. Safety for electrical equipment under artificial environments — ignition of flammable solid materials by low voltage inductive sparks. Research report of the Research Institute of Industrial Safety, RR-24-7; 1976. p. 1–8. Tanaka R, Ichikawa K. Safety for electrical equipment under artificial environments — ignition of flammable solid materials by low voltage capacitive or resistive sparks. Research report of the Research Institute of Industrial Safety, RR-26-2; 1977. p. 1–11.