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Ignition investigation of methane–air mixtures by multiple capacitor discharges
Journal of Electrostatics 51}52 (2001) 395}401
Ignition investigation of methane}air mixtures by multiple capacitor discharges Leszek PtasinH ski, Tadeusz Z eglenH * Faculty of Electrical Engineering, Automatics, Computer Science and Electronics, University of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Krako& w, Poland
Abstract Multiple capacitor discharges with controlled parameters have been carried out. Methane} air mixtures of various compositions have been examined. Ignition tests were undertaken under conditions of continuous #ow of the mixture. The charge transferred in a single discharge pulse and the voltage before each pulse were recorded. Based on the values as above, the energy has been estimated. The measurements of discharge courses and incendivity testing were carried out under di!erent conditions, such as: time intervals between the successive pulses, methane concentration, capacity of the discharged capacitor and resistance of the decoupling resistor. 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrostatic discharges (ESD); Capacitor discharges; Incendivity of ESD; Flammable gases and vapours; Ignition
1. Introduction Ignition of #ammable atmospheres caused by multiple ESD, i.e., those that occur in the form of a pulse series may take place in di!erent processes of continuous charging [1,2] as well as when the grounded object approaches a charged insulating surface [3]. Attempts to introduce standard spark discharge tests for evaluating electrostatic hazards have been made for a long time. Recently, hopes related to this method have arisen in connection with this investigation of textile products containing core "bres, for which other tests are ine!ective [4]. Due to the di$culty in predicting the
* Corresponding author. Fax: #48-12-634-57-21. E-mail address: [email protected] (T. Z eglenH ). 0304-3886/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 0 8 8 - 2
396
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
incendivity of ESD by the laws of plasma physics a phenomenological approach is commonly used [5]. ESD incendivity depends upon the composition and temperature of the #ammable mixture, the energy of the discharge and the distribution of this energy in space and time. A capacitor spark discharge is a commonly accepted means for measuring ignition energies. For a given #ammable material the energy for ignition is a function of its concentration in air or oxygen [6,7]. The lowest electrical energy stored in a capacitor which upon discharge is su$cient to e!ect ignition of the most ignitable atmosphere under speci"ed test conditions is called minimum ignition energy (MIE) [8]. It is assumed that successive discharges and ignitions are independent phenomena. Multiple ESD may, however, cause the e!ect of energy cumulation in particular space points that being conductive to initiation of ignition. The same phenomenon may also occur in the case of minimal ignition energy determination, based upon capacitor spark discharge examination. Investigations described in the following are aimed at checking this e!ect for methane}air mixtures, taking advantage of the possibilities rendered by the up-to-date measurement apparatus [9], as well as at the examination of the e!ect of chosen factors on the traces of characteristics of energy of individual discharge impulses in function of time and their incendivity.
2. Experimental set-up and measuring procedure Investigations were carried out on the experimental set-up presented in Fig. 1. Tests of in#ammation of the examined explosive mixture were carried out in the ignition chamber ICh. Test mixtures of a precisely determined composition and #ow rate were produced in a special arrangement Bronk}Horst Mass Flow. The ignition chamber of 0.6 dm capacity is in the form of a cylinder. The electrodes of the chamber are made of stainless steel, spherical in shape and 10 mm in diameter. Measurement of the distance between the electrodes was carried out within the range of 0}10 mm with resolution to 0.01 mm. At the inlet and outlet of the chamber the #ame safety devices
Fig. 1. Schematic diagram of the experimental set-up; HVS: high voltage DC source, ICh: ignition chamber, VD: voltage divider, PChC: pulse charge converter, SU: separating unit; WBK512: DSP-based multichannel acquisition system; T: transoptor.
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
397
are installed. Ignition of the mixture causes pressure increase in the chamber and activates the pressure gauge which, by means of an electrovalve, cuts o! the mixture supply. High voltage DC source HVS, capacitor C and decoupling resistor R form the generator of $15 kV voltage. Regulation of the supplier voltage similar to the change of decoupling resistance permits control of repetition frequency of impulse discharges and change of the charged capacity or distance of a spark-gap electrodes causes change of the amplitude of impulses. A special programme permits generation of an a priori assumed number of these impulses. Discharges were recorded using a high-resolution microcomputer system described in [9], working on two-channel option with multiple triggering by successive impulses, with 500 s resolution, making recording of 1600 impulses possible. In one channel charges of particular discharge impulses and time intervals between them were recorded, whereas in the other one voltage proceeding each discharge was recorded. Based on the measurement of these two quantities, energy of an individual impulse was determined. The measurement process starts at the moment of activation of the digital signal of output of the modulus WBK512 which controls the high voltage generator. The time of occurrence of each trigger is remembered based on the reading of the real time clock. The recording process gets completed automatically after the previously declared number of discharge impulses has been reached. The obtained results are recorded on a hard disc in the form of text sets. Safety of the service system is guaranteed by systems of analogous galvanic separation SU as well as by the transoptor T which controls the digital signal.
3. Results of experiments About 300 tests of methane}air mixture ignition connected with discharge registration were carried out. The results presented subsequently, in the form of examples, were obtained at constant distance between the electrodes of the spark-gap, this being 2 mm [7] for 300 impulses of capacitor discharges. Investigations were carried out at various quantities of resistance R of the resistor R and capacity C of the generator which is the sum of capacities of the capacitor C , electrodes and connecting leads. Exemplary characteristics of the measured parameters of discharge impulses, charge Q, voltage U and energy E calculated on their basis, recorded in the case of occurrence of test mixture ignition, are presented in Fig. 2. Ignition was marked with a triangular sign 䉲. The average discharge frequency before igniting ESD pulse f was calculated. It follows from Fig. 2 that it seems justi"ed to make use of the quantity of the charge transferred in discharge, instead of energy, which is more di$cult to be determined. Fig. 3 shows, for comparison, the traces of energy of discharges in the experiment carried out under the same conditions as before but without the occurrence of ignition. Comparing Fig. 2(c) and Fig. 3, it may be noticed that ignition of the mixture is associated with the decrease in energy of consequent discharge impulses. This e!ect permits to observe the number of impulses before the ocurrence of ignition. Analogical
398
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
Fig. 2. Time characteristics of charge (Q), voltage (U) and energy (E) of igniting capacitor discharges, received in ignition test of mixture: 10.5% methane and 89.5% air; R"200 M, C"65 pF, f "20.4 1/s.
Fig. 3. Time-energy characteristics of nonigniting multiple capacitor discharges; test conditions as in Fig. 2; f "46.2 1/s.
experiments carried out at a lower value of resistance R, hence, shorter time intervals between consequent impulses, showed that the formerly noticed properties of the recorded characteristics occur here also, though, their traces are di!erent. Diminution of the discharged capacity C only to the natural capacity of the system, by excluding the capacitor C , results in obtaining characteristics of discharges more di$cult for interpretation. This was shown, in the form of an example, in Figs. 4(a) and (b). But still on, there are signi"cant di!erences between characteristics obtained in the case of ignition and lack of it. The traces of characteristics of capacitor igniting discharges depend also on the percentage composition of the mixture. This is illustrated in Fig. 5(a)}(c). The results can be interpreted as e!ect of energy cumulation of successive discharge impulses. This e!ect leads to the ignition only in consequence of generation of series of
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
399
Fig. 4. Characteristics of capacitor discharges in the mixture of 11% methane and 89% air; R"15 M, C"3.5 pF; (a) ignition occurred; (b) ignition did not occur.
Fig. 5. Time}energy characteristics of capacitor discharges that ignited di!erent methane}air mixtures; R"200 M, C"3.5 pF; (a) 8.5% methane and 90% air; (b) 9.5% methane and 90.5% air, f "32.8 1/s; (c) 10% methane and 90% air, f "48 1/s.
discharge impulses; at the same time, till the occurrence of ignition, their number increases with an increase in methane concentration, hence, minimal ignition energy. Initiation of explosive mixture ignition is a stochastic process. Therefore, for the examination of in#uence of time interval between impulses of discharges on incendivity of multiple capacitor discharges, histograms were elaborated; these are shown in Fig. 6. As it follows from Fig. 6, decrease in resistance R*synonymous with shortening of the time intervals between discharge impulses*causes a greater ignition possibility of mixtures of methane}air of higher concentration, hence higher minimal ignition energy. This e!ect also con"rms the hypothesis of cumulation of energy of consequent impulses of multiple discharges inside the ignition chamber.
400
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
Fig. 6. Probability of occurrence of ignition of methane}air mixtures in dependence on percentage composition of the mixture and resistance R; C"3.5 pF. Table 1 Fig
2 3 4(a) 4(b) 5(a) 5(b) 5(c)
Conditions of experiment
Energy
R (M)
C (pF)
E"0.5UQ (mJ) E"0.5UC (mJ)
200 200 15 15 200 200 200
65 65 3.5 3.5 3.5 3.5 3.5
5.1 5,55 1.35 1.4 1.55 1.6 1.78
4.2 4.85 0.3 0.3 0.27 0.28 0.29
For measurement results presented in Fig. 2}5, calculations of energy E were performed additionally based on the value of voltage U and discharged capacity C, according to [6,8] and were compared by the use of value E determined on the basis of the measured values of voltage U and maximal charge of impulse Q . The results are
listed in Table 1. It follows from the data in Table 1 that, particularly for the minimal value of capacity C (3.5 pF), values E and E di!er signi"cantly from each other. Most likely it is due to the decrease in the voltage value U during ESD time.
4. Closing remarks and conclusions The measurements carried out con"rmed the hypothesis that in the investigations of minimal ignition energy consideration of speci"city of multiple electric discharges and the application of recording of individual impulses of discharges are useful. The obtained characteristics Q(t), ;(t) and E(t) are practically identical, which con"rms the assumption of allowance to receive the transferred charge as a criterial quantity in the investigations of electrical spark discharges incendivity. The traces of characteristics of capacitor multiple discharges re#ect the e!ect of ignition initiation as well as*most probably*the moment of its origin.
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
401
Probability of occurrence of methane}air ignition increases with the decrease in mean time interval between the impulses; this may be explained by the e!ect of test mixture energy cumulating.
Acknowledgements Authors are grateful to Test Mine Barbara management for rendering Bronk} Horst Mass Flow installation and the sta! for their help in the laboratory stand preparation. The investigations were sponsored by the State Committee for Scienti"c Research, Warsaw (contract No.: 8T10C01012).
References [1] [2] [3] [4] [5] [6]
K. Schwenzfeuer, M. Glor, Inst. Phys. Conf. Ser., No. 143, 1995, pp. 125}128. L. PtasinH ski, T. Z eglenH , A. Gajewski, Inst. Phys. Conf. Ser., No. 163, 1999, pp. 367}370. Y. Takahashi, H. Sumida, H. Fukai, X. Ji, S. Kobayashi, J. Electrost. 24 (1990) 185}196. EN 1149-1, 1995. M. Glor, Inst. Phys. Conf. Ser., No. 163, 1999, pp. 199}206. B. Levis, G. von Elbe, Combustion, Flames and Explosions of Gases, 3rd Edition, Academic Press, New York, 1987. [7] BS 5958: Part 1, 1991. [8] EN 1127-1, 1997. [9] L. PtasinH ski, T. Z eglenH , J. Gajda, Inst. Phys. Conf. Ser., No. 163, 1999, pp. 225}228.
Ignition investigation of methane}air mixtures by multiple capacitor discharges Leszek PtasinH ski, Tadeusz Z eglenH * Faculty of Electrical Engineering, Automatics, Computer Science and Electronics, University of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Krako& w, Poland
Abstract Multiple capacitor discharges with controlled parameters have been carried out. Methane} air mixtures of various compositions have been examined. Ignition tests were undertaken under conditions of continuous #ow of the mixture. The charge transferred in a single discharge pulse and the voltage before each pulse were recorded. Based on the values as above, the energy has been estimated. The measurements of discharge courses and incendivity testing were carried out under di!erent conditions, such as: time intervals between the successive pulses, methane concentration, capacity of the discharged capacitor and resistance of the decoupling resistor. 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrostatic discharges (ESD); Capacitor discharges; Incendivity of ESD; Flammable gases and vapours; Ignition
1. Introduction Ignition of #ammable atmospheres caused by multiple ESD, i.e., those that occur in the form of a pulse series may take place in di!erent processes of continuous charging [1,2] as well as when the grounded object approaches a charged insulating surface [3]. Attempts to introduce standard spark discharge tests for evaluating electrostatic hazards have been made for a long time. Recently, hopes related to this method have arisen in connection with this investigation of textile products containing core "bres, for which other tests are ine!ective [4]. Due to the di$culty in predicting the
* Corresponding author. Fax: #48-12-634-57-21. E-mail address: [email protected] (T. Z eglenH ). 0304-3886/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 0 8 8 - 2
396
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
incendivity of ESD by the laws of plasma physics a phenomenological approach is commonly used [5]. ESD incendivity depends upon the composition and temperature of the #ammable mixture, the energy of the discharge and the distribution of this energy in space and time. A capacitor spark discharge is a commonly accepted means for measuring ignition energies. For a given #ammable material the energy for ignition is a function of its concentration in air or oxygen [6,7]. The lowest electrical energy stored in a capacitor which upon discharge is su$cient to e!ect ignition of the most ignitable atmosphere under speci"ed test conditions is called minimum ignition energy (MIE) [8]. It is assumed that successive discharges and ignitions are independent phenomena. Multiple ESD may, however, cause the e!ect of energy cumulation in particular space points that being conductive to initiation of ignition. The same phenomenon may also occur in the case of minimal ignition energy determination, based upon capacitor spark discharge examination. Investigations described in the following are aimed at checking this e!ect for methane}air mixtures, taking advantage of the possibilities rendered by the up-to-date measurement apparatus [9], as well as at the examination of the e!ect of chosen factors on the traces of characteristics of energy of individual discharge impulses in function of time and their incendivity.
2. Experimental set-up and measuring procedure Investigations were carried out on the experimental set-up presented in Fig. 1. Tests of in#ammation of the examined explosive mixture were carried out in the ignition chamber ICh. Test mixtures of a precisely determined composition and #ow rate were produced in a special arrangement Bronk}Horst Mass Flow. The ignition chamber of 0.6 dm capacity is in the form of a cylinder. The electrodes of the chamber are made of stainless steel, spherical in shape and 10 mm in diameter. Measurement of the distance between the electrodes was carried out within the range of 0}10 mm with resolution to 0.01 mm. At the inlet and outlet of the chamber the #ame safety devices
Fig. 1. Schematic diagram of the experimental set-up; HVS: high voltage DC source, ICh: ignition chamber, VD: voltage divider, PChC: pulse charge converter, SU: separating unit; WBK512: DSP-based multichannel acquisition system; T: transoptor.
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
397
are installed. Ignition of the mixture causes pressure increase in the chamber and activates the pressure gauge which, by means of an electrovalve, cuts o! the mixture supply. High voltage DC source HVS, capacitor C and decoupling resistor R form the generator of $15 kV voltage. Regulation of the supplier voltage similar to the change of decoupling resistance permits control of repetition frequency of impulse discharges and change of the charged capacity or distance of a spark-gap electrodes causes change of the amplitude of impulses. A special programme permits generation of an a priori assumed number of these impulses. Discharges were recorded using a high-resolution microcomputer system described in [9], working on two-channel option with multiple triggering by successive impulses, with 500 s resolution, making recording of 1600 impulses possible. In one channel charges of particular discharge impulses and time intervals between them were recorded, whereas in the other one voltage proceeding each discharge was recorded. Based on the measurement of these two quantities, energy of an individual impulse was determined. The measurement process starts at the moment of activation of the digital signal of output of the modulus WBK512 which controls the high voltage generator. The time of occurrence of each trigger is remembered based on the reading of the real time clock. The recording process gets completed automatically after the previously declared number of discharge impulses has been reached. The obtained results are recorded on a hard disc in the form of text sets. Safety of the service system is guaranteed by systems of analogous galvanic separation SU as well as by the transoptor T which controls the digital signal.
3. Results of experiments About 300 tests of methane}air mixture ignition connected with discharge registration were carried out. The results presented subsequently, in the form of examples, were obtained at constant distance between the electrodes of the spark-gap, this being 2 mm [7] for 300 impulses of capacitor discharges. Investigations were carried out at various quantities of resistance R of the resistor R and capacity C of the generator which is the sum of capacities of the capacitor C , electrodes and connecting leads. Exemplary characteristics of the measured parameters of discharge impulses, charge Q, voltage U and energy E calculated on their basis, recorded in the case of occurrence of test mixture ignition, are presented in Fig. 2. Ignition was marked with a triangular sign 䉲. The average discharge frequency before igniting ESD pulse f was calculated. It follows from Fig. 2 that it seems justi"ed to make use of the quantity of the charge transferred in discharge, instead of energy, which is more di$cult to be determined. Fig. 3 shows, for comparison, the traces of energy of discharges in the experiment carried out under the same conditions as before but without the occurrence of ignition. Comparing Fig. 2(c) and Fig. 3, it may be noticed that ignition of the mixture is associated with the decrease in energy of consequent discharge impulses. This e!ect permits to observe the number of impulses before the ocurrence of ignition. Analogical
398
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
Fig. 2. Time characteristics of charge (Q), voltage (U) and energy (E) of igniting capacitor discharges, received in ignition test of mixture: 10.5% methane and 89.5% air; R"200 M, C"65 pF, f "20.4 1/s.
Fig. 3. Time-energy characteristics of nonigniting multiple capacitor discharges; test conditions as in Fig. 2; f "46.2 1/s.
experiments carried out at a lower value of resistance R, hence, shorter time intervals between consequent impulses, showed that the formerly noticed properties of the recorded characteristics occur here also, though, their traces are di!erent. Diminution of the discharged capacity C only to the natural capacity of the system, by excluding the capacitor C , results in obtaining characteristics of discharges more di$cult for interpretation. This was shown, in the form of an example, in Figs. 4(a) and (b). But still on, there are signi"cant di!erences between characteristics obtained in the case of ignition and lack of it. The traces of characteristics of capacitor igniting discharges depend also on the percentage composition of the mixture. This is illustrated in Fig. 5(a)}(c). The results can be interpreted as e!ect of energy cumulation of successive discharge impulses. This e!ect leads to the ignition only in consequence of generation of series of
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
399
Fig. 4. Characteristics of capacitor discharges in the mixture of 11% methane and 89% air; R"15 M, C"3.5 pF; (a) ignition occurred; (b) ignition did not occur.
Fig. 5. Time}energy characteristics of capacitor discharges that ignited di!erent methane}air mixtures; R"200 M, C"3.5 pF; (a) 8.5% methane and 90% air; (b) 9.5% methane and 90.5% air, f "32.8 1/s; (c) 10% methane and 90% air, f "48 1/s.
discharge impulses; at the same time, till the occurrence of ignition, their number increases with an increase in methane concentration, hence, minimal ignition energy. Initiation of explosive mixture ignition is a stochastic process. Therefore, for the examination of in#uence of time interval between impulses of discharges on incendivity of multiple capacitor discharges, histograms were elaborated; these are shown in Fig. 6. As it follows from Fig. 6, decrease in resistance R*synonymous with shortening of the time intervals between discharge impulses*causes a greater ignition possibility of mixtures of methane}air of higher concentration, hence higher minimal ignition energy. This e!ect also con"rms the hypothesis of cumulation of energy of consequent impulses of multiple discharges inside the ignition chamber.
400
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
Fig. 6. Probability of occurrence of ignition of methane}air mixtures in dependence on percentage composition of the mixture and resistance R; C"3.5 pF. Table 1 Fig
2 3 4(a) 4(b) 5(a) 5(b) 5(c)
Conditions of experiment
Energy
R (M)
C (pF)
E"0.5UQ (mJ) E"0.5UC (mJ)
200 200 15 15 200 200 200
65 65 3.5 3.5 3.5 3.5 3.5
5.1 5,55 1.35 1.4 1.55 1.6 1.78
4.2 4.85 0.3 0.3 0.27 0.28 0.29
For measurement results presented in Fig. 2}5, calculations of energy E were performed additionally based on the value of voltage U and discharged capacity C, according to [6,8] and were compared by the use of value E determined on the basis of the measured values of voltage U and maximal charge of impulse Q . The results are
listed in Table 1. It follows from the data in Table 1 that, particularly for the minimal value of capacity C (3.5 pF), values E and E di!er signi"cantly from each other. Most likely it is due to the decrease in the voltage value U during ESD time.
4. Closing remarks and conclusions The measurements carried out con"rmed the hypothesis that in the investigations of minimal ignition energy consideration of speci"city of multiple electric discharges and the application of recording of individual impulses of discharges are useful. The obtained characteristics Q(t), ;(t) and E(t) are practically identical, which con"rms the assumption of allowance to receive the transferred charge as a criterial quantity in the investigations of electrical spark discharges incendivity. The traces of characteristics of capacitor multiple discharges re#ect the e!ect of ignition initiation as well as*most probably*the moment of its origin.
L. Ptasin& ski, T. ZQ eglen& / Journal of Electrostatics 51}52 (2001) 395}401
401
Probability of occurrence of methane}air ignition increases with the decrease in mean time interval between the impulses; this may be explained by the e!ect of test mixture energy cumulating.
Acknowledgements Authors are grateful to Test Mine Barbara management for rendering Bronk} Horst Mass Flow installation and the sta! for their help in the laboratory stand preparation. The investigations were sponsored by the State Committee for Scienti"c Research, Warsaw (contract No.: 8T10C01012).
References [1] [2] [3] [4] [5] [6]
K. Schwenzfeuer, M. Glor, Inst. Phys. Conf. Ser., No. 143, 1995, pp. 125}128. L. PtasinH ski, T. Z eglenH , A. Gajewski, Inst. Phys. Conf. Ser., No. 163, 1999, pp. 367}370. Y. Takahashi, H. Sumida, H. Fukai, X. Ji, S. Kobayashi, J. Electrost. 24 (1990) 185}196. EN 1149-1, 1995. M. Glor, Inst. Phys. Conf. Ser., No. 163, 1999, pp. 199}206. B. Levis, G. von Elbe, Combustion, Flames and Explosions of Gases, 3rd Edition, Academic Press, New York, 1987. [7] BS 5958: Part 1, 1991. [8] EN 1127-1, 1997. [9] L. PtasinH ski, T. Z eglenH , J. Gajda, Inst. Phys. Conf. Ser., No. 163, 1999, pp. 225}228.