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Tests to determine the ignition of dust by brush discharges
Journal of Electrostatics, 30 (1993) 115-122 Elsevier
115
Tests to determine the ignition of dust by brush discharges K. Schwenzfeuer and M.Glor Ciba-Geigy Ltd., CH-4002 Basle, Switzerland
Abstract The equivalent energy of brush discharges for the ignition of flammable gases has been determined in earlier work as 1 to 4 mJ. Although this lies in the region of the minimum ignition energy of several dusts, until today no dust ignitions due to brush discharges could be observed. The present work describes the successful ignition of sulphur by brush discharges with separation of the discharge zone and dust ignition. Estimation of the resultant energies is in good agreement with the determined equivalent energy.
1. Introduction Electrostatic discharges leading to explosions are a frequent cause of accidents in industry. Depending on the geometry and conductivity of the installation parts and products involved, six different types of discharge can be distinguished: Spark discharge, brush discharge, corona discharge, propagating brush discharge, lightning-like discharge and discharge from bulked powder. An important step today in the assessment of ignition hazards resulting from electrostatic charging involves an estimation of the probability of occurrence of one of these discharges and a comparison of its incendivity with the minimum ignition energy of any explosible atmosphere present. This requires a knowledge of the incendivity of the discharges, for example in the form of an equivalent energy [1]. In the case of brush discharges, earlier work by Gibson and Glor [1-3] has shown the equivalent energy for the ignition of propane/nitrogen/fir mixtures to lie between 1 and 4 mJ. The brush discharges thus lie in an energy range attained by the minimum ignition energy of several dusts, as Glor [4] has already demonstrated. This notwithstanding, until today it was generally thought (see, for instance, LObel [5]) that brush discharges were capable of igniting only flammable gases and not dusts. This contradiction is usually explained by the different behaviour of electrostatically charged insulators in the presence of dust. Investigations in the author's laboratory have shown time and again that with the same experimental arrangement the charge transfer in the individual brush discharges was lower in the presence of dust than in pure air or in gas/air mixtures. With a view to considering the worst-case scenario, a new type of experimental setup was constructed and used to perform ignition tests. 0304-3886/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
116
2. Measurement principle and experimental arrangement To prevent the type of interference observed by Edwards and Underwood [6] with the electric field of the charged plastic sheet due to the presence of dust deposits, in the present work the brush discharge and the dust ignition were spatially separated from each other. Fig. 1 shows a picture of the experimental setup.
Figure 1. A picture of the experimental setup.
Brush discharges were generated with a rotating polyethylene disk of diameter 50 cm and thickness 2 cm. This was electrostatically charged by rubbing with cat fur and discharged after half a turn using a spherical electrode. At ca. 20 revolutions per second, brush discharges of 20 Hz were produced at the electrode. The electrode was earthed with a resistance of 109 f~ to exclude the influence of the induced charge and connected to a spark gap. With a capacitance of the spherical electrode of 30 pF to earth and the associated leakage resistance, a time constant of 30 ms results. During this time, the induced charge leaks away without building up a field at the spark gap large enough for discharge. The brush discharge itself leads to breakdown of the spark gap. To measure the charge transfer of the spark discharge, an RC integrator is installed on the earthing side. With a capacitance of 25 nF and a resistance of 100 kf2, the integrator has a time constant of 2.5 ms. This suffices to record the discharge signal by means of an oscilloscope with time resolution of every single discharge. The error in the charge determination caused by use of an RC integrator with a small time constant has been reported by Mills and Haighton [7] as 5%. The effect of release of the image charges from the spherical electrode when the discharge occurs, see Chubb and Butterworth [8], has not been taken into account. Fig. 2 shows a schematic representation of the setup for the charge detection of a brush discharge and Fig. 3 a typical signal of the RC integrator.
117
'
'
plastic sheet
oscilloscope
Figure 2. Schematic experimental arrangement for charge detection of the brush discharges. The individual quantities are: C1 = 25 nF, R1 = 100 kf2, R2 = 1 G~. The spark gap is 5 m m
25 ~' 20 ¢O)
b515 10
0
2
4
6
8 10 Time [ms]
Figure 3. Typical signal on the oscilloscope in a charge transfer via the electrodes of the Hartmann tube for a brush discharge.
To find the optimum diameter of the spherical electrode, 5 different spheres were tested at a variable distance from the plastic sheet. Fig. 4 shows a plot of the charge determined at an optimum distance from the sheet for the different diameters together with the positive and
118 negative spread. For the ignition experiments, the sphere with diameter 40 mm separated from the sheet by 30 mm proved particularly suitable with this experimental setup
400 O .=. 300 tO
200 100
20
~
I
30
40
I
.....
I
I
60 70 50 80 Sphere Diameter [mm]
Figure 4. The measured charge transfer of brush discharges as a function of the electrode diameter. A mean value (large circle) is shown together with the positive and negative spread (small diamonds).
In the dust ignition experiments, the spark gap was located in an open Hartmann tube of content 1.4 L fitted with a mushroom-shaped nozzle for dust distribution. The spark gap for ignition of the dust comprised two pointed tungsten electrodes with a free separation of 5 ram. To initiate a dust ignition by the discharge, the dust was dispersed in the Hartmann tube for 2 seconds at 5-second intervals through the mushroom-shaped nozzle by compressed air at ca. 1.5 bar.
3. Determination of the equivalent energy Although the brush discharges with the selected experimental setup have the form of a continuous spark with a repetition rate of 20 Hz, the range of the measured charges shows a considerable scatter. Fig. 5 shows this for the optimum electrode arrangement. If the energy is estimated using the equation E
-
1 Q2 2C
with C = 30 pF
a mean energy of the generated discharges of 1.31 ± 0.37 mJ is found.
( 1)
119 25
o~
20
z 10 5 O0~ 100
.
. . 200
.
. . 300
.
. 400 500 Charge [nC] w w v w w w w
Figure 5. Frequency distribution of the measured charge transfers. The sum of all measurements is set equal to 100%.
To verify this roughly estimated value, the equivalent energy for the ignition of a propane/nitrogen/air mixture was determined using the method described by Glor [2]. Variation of the nitrogen concentration in the gas mixture was used to change the minimum ignition energy. Fig. 6 shows the relation between the nitrogen concentration and the minimum ignition energy and Fig. 7 the ignition probability as a function of the nitrogen concentration. From the ignition probability, an equivalent energy of between 0.8 and 1.2 mJ for the brush discharges generated using the present setup can be deduced.
4 E3.5 3 2.5 C
o
2
E
1
IE 0.5 t'-
o
i
14
16
i
i
i
i
J
I
i
i
i
i
i
i
i
i
i
18 20 22 24 26 28 30 32 N2 Concentration [%]
Figure 6. Minimum ignition energy of a propane/nitrogen/air mixture as a function of the nitrogen concentration (taken from Glor [2]).
120
lO0, ~80 ..(3
~ 60 £ ~40 ..Q
tO
"~ 20 C~ "v"
6
18
20
22
24
26
28
N2 Concentration [%] Figure 7. Plot showing the ignition probability as a function of the nitrogen concentration in the propane/nitrogen/air mixture.
4. Ignition experiments with polyethylene wax and sulphur Determination of the minimum ignition energy using the standard method [9] in the modified Hartmann tube gave a value for polyethylene wax between 1 and 3 mJ and for sulphur a value < 1 mJ. Fig. 8 shows the dependence of the ignition energy on the dust concentration for polyethylene wax determined by the standard method. In the case of sulphur, over the entire concentration range investigated an ignition occurred at the lowest possible ignition energy of 1 mJ.
12 •
El0 ~ 0
Ignition
!
-q~ No Ignition
8
C
w ¢,-
6
0
,'- 4 2
0
I
0
200
I
~
i
i
i
i
i
I
L
400 600 800 1000 1 !00 Dust Concentration [g/m3]
Figure 8. Minimum ignition energy as a function of the dust concentration for polyethylene wax measured in the modified Hartmann tube. The triangles represent a dust ignition, the circles no ignition.
121 The determination of the equivalent energy of the generated brush discharges shows that the available energy only just suffices to ignite the optimum concentration of polyethylene wax. This is why no ignition could be observed with this dust. For sulphur the minimum ignition energy is less than 1 mJ when it is in the form of a fine powder. The energy of the brush discharges is thus sufficient and reproducible dust ignitions of sulphur could be produced in the apparatus used. The frequency with which a dust ignition is caused by a certain charge is shown in Fig. 9. The decrease of the number of ignitions above 280 nC is due to the fact, that discharges with a charge transfer above 280 nC occurred less frequently (see Fig. 5).
30
1 O0
200
300
400 500 Charge [nC]
Figure 9. Frequency of a charge transfer measurement in successful ignition of sulphur dust. The sum of all dust ignitions is set equal to 100%.
5. Discussion of the results
A comparison of Figs 5 and 9 shows that discharges of 220 nC and less do not have sufficient energy to ignite sulphur. According to equation (1), this corresponds to an energy value of 0.8 mJ. For discharges above 280 nC, the charge distribution corresponds to a good approximation to the ignition frequency leading to the conclusion that every discharge greater than 280 nC will result in an ignition of sulphur. From equation (1) this corresponds to an energy value of 1.3 mJ. In the range between 220 nC and 280 nC, or 0.8 mJ to 1.3 mJ, the ignition probability is greater than 0% and reaches 100%. These results are in agreement with the determination of the minimum ignition energy for sulphur in the modified Hartmann tube and with the determination of the equivalent energy of the brush discharges by ignition of a propane/nitrogen/air mixture. To ignite polyethylene wax by brush discharges using the current experimental arrangement, it would be necessary to increase the charge transfer. This can be done by giving the polyethylene sheet a higher charge, possibly by lowering the room temperature. A better alternative involves increasing
122 the chargeable polyethylene area. Gibson and Harper [ 10] have already shown that an increase in the charged area leads to an increase in the incendivity of flammable gases by brush discharges.
6. Conclusions
The authors are well aware that the experimental procedure described here differs in several respects to industrial situations. Formation of the brush discharge is not inhibited by the presence of a dust cloud and the energy distribution in the spark gap with respect to space and time is not the same as in the brush discharge at the spherical electrode. Inferences for industrial practice are thus not yet possible. However, the sole aim of the present study was to initiate a dust ignition under idealised conditions. This aim has been realised. Further experiments are in hand to match the procedure to industrial practice (for example, generation of a dust cloud at the site of the brush discharge). It is hoped that such a procedure will show that, starting from a positive result (dust ignition) due to an actual industrial influence, conditions can be found under which ignition no longer occurs. This approach is eminently more trustworthy from the safety engineering viewpoint than numerous unsuccessful ignition experiments under conditions resembling those found in practice.
7. References
10
N. Gibson and F.C. Lloyd, British Journal of Appl. Physics., 16 (1965) 1619. M. Glor, Journal of Electrostatics, 10 (1981) 327-332. M. Glor, Electrostatic Hazards in Powder Handling, Research Studies Press Ltd., 1988. M. Glor, Journal of Electrostatics, 16 (1984) 175-191. W. Lrbel, Staub - Reinhaltung der Lufi, 51 (1991) 413-415. H.R. Edwards and M.C. Underwood, Journal of Electrostatics, 15 (1984) 123-125. J.S. Mills and EJ. Haighton, Journal of Electrostatics, 13 (1982) 91-97. J.N. Chubb and G.J. Butterworth, Journal of Electrostatics, 13 (1982) 209-214. W. Berthold (Editor), Fortschrittberichte VDI, No. 134, VDI-Verlag D(isseldorf, 1987. N. Gibson and D.J. Harper, Journal of Electrostatics, 21 (1988) 27-36.
115
Tests to determine the ignition of dust by brush discharges K. Schwenzfeuer and M.Glor Ciba-Geigy Ltd., CH-4002 Basle, Switzerland
Abstract The equivalent energy of brush discharges for the ignition of flammable gases has been determined in earlier work as 1 to 4 mJ. Although this lies in the region of the minimum ignition energy of several dusts, until today no dust ignitions due to brush discharges could be observed. The present work describes the successful ignition of sulphur by brush discharges with separation of the discharge zone and dust ignition. Estimation of the resultant energies is in good agreement with the determined equivalent energy.
1. Introduction Electrostatic discharges leading to explosions are a frequent cause of accidents in industry. Depending on the geometry and conductivity of the installation parts and products involved, six different types of discharge can be distinguished: Spark discharge, brush discharge, corona discharge, propagating brush discharge, lightning-like discharge and discharge from bulked powder. An important step today in the assessment of ignition hazards resulting from electrostatic charging involves an estimation of the probability of occurrence of one of these discharges and a comparison of its incendivity with the minimum ignition energy of any explosible atmosphere present. This requires a knowledge of the incendivity of the discharges, for example in the form of an equivalent energy [1]. In the case of brush discharges, earlier work by Gibson and Glor [1-3] has shown the equivalent energy for the ignition of propane/nitrogen/fir mixtures to lie between 1 and 4 mJ. The brush discharges thus lie in an energy range attained by the minimum ignition energy of several dusts, as Glor [4] has already demonstrated. This notwithstanding, until today it was generally thought (see, for instance, LObel [5]) that brush discharges were capable of igniting only flammable gases and not dusts. This contradiction is usually explained by the different behaviour of electrostatically charged insulators in the presence of dust. Investigations in the author's laboratory have shown time and again that with the same experimental arrangement the charge transfer in the individual brush discharges was lower in the presence of dust than in pure air or in gas/air mixtures. With a view to considering the worst-case scenario, a new type of experimental setup was constructed and used to perform ignition tests. 0304-3886/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
116
2. Measurement principle and experimental arrangement To prevent the type of interference observed by Edwards and Underwood [6] with the electric field of the charged plastic sheet due to the presence of dust deposits, in the present work the brush discharge and the dust ignition were spatially separated from each other. Fig. 1 shows a picture of the experimental setup.
Figure 1. A picture of the experimental setup.
Brush discharges were generated with a rotating polyethylene disk of diameter 50 cm and thickness 2 cm. This was electrostatically charged by rubbing with cat fur and discharged after half a turn using a spherical electrode. At ca. 20 revolutions per second, brush discharges of 20 Hz were produced at the electrode. The electrode was earthed with a resistance of 109 f~ to exclude the influence of the induced charge and connected to a spark gap. With a capacitance of the spherical electrode of 30 pF to earth and the associated leakage resistance, a time constant of 30 ms results. During this time, the induced charge leaks away without building up a field at the spark gap large enough for discharge. The brush discharge itself leads to breakdown of the spark gap. To measure the charge transfer of the spark discharge, an RC integrator is installed on the earthing side. With a capacitance of 25 nF and a resistance of 100 kf2, the integrator has a time constant of 2.5 ms. This suffices to record the discharge signal by means of an oscilloscope with time resolution of every single discharge. The error in the charge determination caused by use of an RC integrator with a small time constant has been reported by Mills and Haighton [7] as 5%. The effect of release of the image charges from the spherical electrode when the discharge occurs, see Chubb and Butterworth [8], has not been taken into account. Fig. 2 shows a schematic representation of the setup for the charge detection of a brush discharge and Fig. 3 a typical signal of the RC integrator.
117
'
'
plastic sheet
oscilloscope
Figure 2. Schematic experimental arrangement for charge detection of the brush discharges. The individual quantities are: C1 = 25 nF, R1 = 100 kf2, R2 = 1 G~. The spark gap is 5 m m
25 ~' 20 ¢O)
b515 10
0
2
4
6
8 10 Time [ms]
Figure 3. Typical signal on the oscilloscope in a charge transfer via the electrodes of the Hartmann tube for a brush discharge.
To find the optimum diameter of the spherical electrode, 5 different spheres were tested at a variable distance from the plastic sheet. Fig. 4 shows a plot of the charge determined at an optimum distance from the sheet for the different diameters together with the positive and
118 negative spread. For the ignition experiments, the sphere with diameter 40 mm separated from the sheet by 30 mm proved particularly suitable with this experimental setup
400 O .=. 300 tO
200 100
20
~
I
30
40
I
.....
I
I
60 70 50 80 Sphere Diameter [mm]
Figure 4. The measured charge transfer of brush discharges as a function of the electrode diameter. A mean value (large circle) is shown together with the positive and negative spread (small diamonds).
In the dust ignition experiments, the spark gap was located in an open Hartmann tube of content 1.4 L fitted with a mushroom-shaped nozzle for dust distribution. The spark gap for ignition of the dust comprised two pointed tungsten electrodes with a free separation of 5 ram. To initiate a dust ignition by the discharge, the dust was dispersed in the Hartmann tube for 2 seconds at 5-second intervals through the mushroom-shaped nozzle by compressed air at ca. 1.5 bar.
3. Determination of the equivalent energy Although the brush discharges with the selected experimental setup have the form of a continuous spark with a repetition rate of 20 Hz, the range of the measured charges shows a considerable scatter. Fig. 5 shows this for the optimum electrode arrangement. If the energy is estimated using the equation E
-
1 Q2 2C
with C = 30 pF
a mean energy of the generated discharges of 1.31 ± 0.37 mJ is found.
( 1)
119 25
o~
20
z 10 5 O0~ 100
.
. . 200
.
. . 300
.
. 400 500 Charge [nC] w w v w w w w
Figure 5. Frequency distribution of the measured charge transfers. The sum of all measurements is set equal to 100%.
To verify this roughly estimated value, the equivalent energy for the ignition of a propane/nitrogen/air mixture was determined using the method described by Glor [2]. Variation of the nitrogen concentration in the gas mixture was used to change the minimum ignition energy. Fig. 6 shows the relation between the nitrogen concentration and the minimum ignition energy and Fig. 7 the ignition probability as a function of the nitrogen concentration. From the ignition probability, an equivalent energy of between 0.8 and 1.2 mJ for the brush discharges generated using the present setup can be deduced.
4 E3.5 3 2.5 C
o
2
E
1
IE 0.5 t'-
o
i
14
16
i
i
i
i
J
I
i
i
i
i
i
i
i
i
i
18 20 22 24 26 28 30 32 N2 Concentration [%]
Figure 6. Minimum ignition energy of a propane/nitrogen/air mixture as a function of the nitrogen concentration (taken from Glor [2]).
120
lO0, ~80 ..(3
~ 60 £ ~40 ..Q
tO
"~ 20 C~ "v"
6
18
20
22
24
26
28
N2 Concentration [%] Figure 7. Plot showing the ignition probability as a function of the nitrogen concentration in the propane/nitrogen/air mixture.
4. Ignition experiments with polyethylene wax and sulphur Determination of the minimum ignition energy using the standard method [9] in the modified Hartmann tube gave a value for polyethylene wax between 1 and 3 mJ and for sulphur a value < 1 mJ. Fig. 8 shows the dependence of the ignition energy on the dust concentration for polyethylene wax determined by the standard method. In the case of sulphur, over the entire concentration range investigated an ignition occurred at the lowest possible ignition energy of 1 mJ.
12 •
El0 ~ 0
Ignition
!
-q~ No Ignition
8
C
w ¢,-
6
0
,'- 4 2
0
I
0
200
I
~
i
i
i
i
i
I
L
400 600 800 1000 1 !00 Dust Concentration [g/m3]
Figure 8. Minimum ignition energy as a function of the dust concentration for polyethylene wax measured in the modified Hartmann tube. The triangles represent a dust ignition, the circles no ignition.
121 The determination of the equivalent energy of the generated brush discharges shows that the available energy only just suffices to ignite the optimum concentration of polyethylene wax. This is why no ignition could be observed with this dust. For sulphur the minimum ignition energy is less than 1 mJ when it is in the form of a fine powder. The energy of the brush discharges is thus sufficient and reproducible dust ignitions of sulphur could be produced in the apparatus used. The frequency with which a dust ignition is caused by a certain charge is shown in Fig. 9. The decrease of the number of ignitions above 280 nC is due to the fact, that discharges with a charge transfer above 280 nC occurred less frequently (see Fig. 5).
30
1 O0
200
300
400 500 Charge [nC]
Figure 9. Frequency of a charge transfer measurement in successful ignition of sulphur dust. The sum of all dust ignitions is set equal to 100%.
5. Discussion of the results
A comparison of Figs 5 and 9 shows that discharges of 220 nC and less do not have sufficient energy to ignite sulphur. According to equation (1), this corresponds to an energy value of 0.8 mJ. For discharges above 280 nC, the charge distribution corresponds to a good approximation to the ignition frequency leading to the conclusion that every discharge greater than 280 nC will result in an ignition of sulphur. From equation (1) this corresponds to an energy value of 1.3 mJ. In the range between 220 nC and 280 nC, or 0.8 mJ to 1.3 mJ, the ignition probability is greater than 0% and reaches 100%. These results are in agreement with the determination of the minimum ignition energy for sulphur in the modified Hartmann tube and with the determination of the equivalent energy of the brush discharges by ignition of a propane/nitrogen/air mixture. To ignite polyethylene wax by brush discharges using the current experimental arrangement, it would be necessary to increase the charge transfer. This can be done by giving the polyethylene sheet a higher charge, possibly by lowering the room temperature. A better alternative involves increasing
122 the chargeable polyethylene area. Gibson and Harper [ 10] have already shown that an increase in the charged area leads to an increase in the incendivity of flammable gases by brush discharges.
6. Conclusions
The authors are well aware that the experimental procedure described here differs in several respects to industrial situations. Formation of the brush discharge is not inhibited by the presence of a dust cloud and the energy distribution in the spark gap with respect to space and time is not the same as in the brush discharge at the spherical electrode. Inferences for industrial practice are thus not yet possible. However, the sole aim of the present study was to initiate a dust ignition under idealised conditions. This aim has been realised. Further experiments are in hand to match the procedure to industrial practice (for example, generation of a dust cloud at the site of the brush discharge). It is hoped that such a procedure will show that, starting from a positive result (dust ignition) due to an actual industrial influence, conditions can be found under which ignition no longer occurs. This approach is eminently more trustworthy from the safety engineering viewpoint than numerous unsuccessful ignition experiments under conditions resembling those found in practice.
7. References
10
N. Gibson and F.C. Lloyd, British Journal of Appl. Physics., 16 (1965) 1619. M. Glor, Journal of Electrostatics, 10 (1981) 327-332. M. Glor, Electrostatic Hazards in Powder Handling, Research Studies Press Ltd., 1988. M. Glor, Journal of Electrostatics, 16 (1984) 175-191. W. Lrbel, Staub - Reinhaltung der Lufi, 51 (1991) 413-415. H.R. Edwards and M.C. Underwood, Journal of Electrostatics, 15 (1984) 123-125. J.S. Mills and EJ. Haighton, Journal of Electrostatics, 13 (1982) 91-97. J.N. Chubb and G.J. Butterworth, Journal of Electrostatics, 13 (1982) 209-214. W. Berthold (Editor), Fortschrittberichte VDI, No. 134, VDI-Verlag D(isseldorf, 1987. N. Gibson and D.J. Harper, Journal of Electrostatics, 21 (1988) 27-36.