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Ignition of a dust layer by a constant heat flux
Journal of Loss Prevention in the Process Industries 16 (2003) 243–248 www.elsevier.com/locate/jlp
Ignition of a dust layer by a constant heat flux K. Lebecki ∗, Z. Dyduch, A. Fibich, J. S´liz˙ Experimental Mine “Barbara” of Central Mining Institute, PL-43-190 Mikoło´w, Katowice, Poland
Abstract In real conditions, the surface temperature of an equipment enclosure covered with a combustible dust layer can significantly rise due to insulating properties of the dust layer. To assess this effect, the measurements of minimum ignition temperature of dust layer at constant temperature of the heated plate tt min (standard method) and the same ignition temperature at constant rate of heat generation th min for two coal dusts were made. Dust layers of thickness between 5 and 50 mm were tested. For each dust, tt min was higher than th min for every tested thickness of the layer. The difference was biggest for thin layers and decreased with increase of the layer thickness. The results suggest a deficiency of the standard procedure of measuring minimum ignition temperature of a dust layer. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Dust layer; Ignition temperature
1. Introduction Ignition of a dust layer is one the most frequent causes of dust explosions in an industry. In many cases, the real conditions are such that it is impossible to avoid the accumulation of combustible dust on surfaces. The only way to avoid the risk of ignition is to prevent the equipment housing from heating up to dangerous temperatures. How easily the dust layer can be ignited depends on the amount of heat supplied to the layer, surface configuration and properties of the dust itself—minimum temperature at which ignition begins. Usually, the heat transport occurs when the dust is deposited on a hot horizontal surface, e.g. casing of electric equipment, mechanical machinery, etc. Susceptibility of a dust layer to ignition is described worldwide by the minimum ignition temperature of the dust layer. Recently, in the USA, the ASTM E2021-01 (2003) published its dust layer ignition procedure based on the experimental work of Miron and Lazzara (1988). According to the European Standard EN 50281-2-1 (1999), the value of minimum ignition temperature is determined for a 5 mm dust layer. The standard introduces an additional safety margin. Minimum ignition
∗
Corresponding author. Fax: +48-32-3246516. E-mail address: [email protected] (K. Lebecki).
temperature of dust layer reduced by 75 K is regarded as a safe temperature of a hot surface. The standard also states that the maximum allowable temperature of external surfaces of equipment’s housing must be further reduced if the thickness of the dust layer is greater than 5 mm. Evaluated this way, the maximum allowable temperature is then related to actual conditions where dust layers can be formed. Usually, this is simply the actual highest possible temperature of the equipment enclosure. However, as some authors point out (Eckhoff, 2000), direct comparison of maximum allowable temperature of external surfaces of equipment’s housing with its actual highest possible temperature is not a well justified procedure. This can be done only when the dust layers that may accumulate on the enclosure surfaces are so thin that they do not significantly change the surface temperature of the equipment. In case of a thick dust layer, the conditions are different. The surface temperature of the enclosure is not a constant inherent property of the enclosure, but is a result of heat transfer between the heat-generating element inside the enclosure and the surroundings. The parameter that can more likely be regarded constant is the rate of heat generation by the element. When the enclosure surface is covered by a dust layer, conditions of heat exchange through the enclosure change as a result of the layer’s thermal insulating properties. As a consequence, the surface temperature of the enclosure rises and may exceed the maximum value
0950-4230/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0950-4230(03)00041-X
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K. Lebecki et al. / Journal of Loss Prevention in the Process Industries 16 (2003) 243–248
Nomenclature d D t td th th min ts tt tt min
thickness of the dust layer diameter of the dust layer in horizontal direction time transient temperature in the dust layer temperature of the heated plate minimum ignition temperature evaluated at constant rate of heat generation maximum permissible temperature (EN 50281-1-2, 1998) temperature of the heated plate in a test at constant temperature of the heated plate minimum ignition temperature evaluated at constant temperature of the heated plate
defined by its nominal temperature. Therefore, the dust layer can ignite even if its minimum ignition temperature determined in the standard test is higher than nominal temperature of the equipment enclosure. The problem described above was addressed in the new release of the European Standard concerning electrical equipment designed for use in the presence of dust (EN 50281-1-2, 1998). The standard includes a method for determining maximum allowable temperature of the surface of equipment’s casing for dust layers up to 50 mm thick based on the value of the minimum ignition temperature evaluated for a 5 mm dust layer. The method can be used when the minimum ignition temperature of the 5 mm dust layer is equal to or higher than 250 °C. If the temperature is lower or the thickness of the layer is greater than 50 mm, then direct measurements are recommended. The aim of the present work was to assess the difference in minimum ignition temperature of dust layer measured at constant temperature of the heated plate (CTHP), standard method, and the same ignition temperature but measured at constant rate of heat generation (CRHG). The latter condition was achieved by attaching the heating element of standard test apparatus directly to constant voltage power supply and assuming that temperature rise of heated plate does not significantly influence parameters of the heating element.
2. Dust properties For the tests, two coal dusts were chosen: coal dust Barbara d40 (brb), regarded as a reference dust at Experimental Mine “Barbara”, and a coal dust from Zofio´ wka (zfk) colliery. Table 1 presents the properties of the chosen dusts. The two dusts mainly differ in volatile and ash content. Also, the mass fractions of finer grains are greater amounts in zfk dust. The remaining presented parameters are similar for both dusts.
3. The equipment and testing method For the tests, a standard apparatus described in EN 50281-2-1 (1999) was used. The apparatus consisted of a steel plate 200 mm in diameter. The plate was heated electrically and its temperature was controlled by a thermocouple mounted in the plate near its centre. Another thermocouple mounted in a similar way was used to record the temperature of the surface during a test. Dust layers were prepared by filling the cavity formed by placing a steel ring of chosen height on the heated plate and levelling the layer to the top of the ring. To measure temperature inside the layer, a thermocouple stretched across the heated plate, parallel to its surface, was used. The thermocouple junction was positioned over the centre of the plate. For the tests at CTHP, the standard procedure described in EN 50281-2-1 (1999) was used. In all the tests with layers thicker than 5 mm, a junction of the thermocouple measuring temperature inside the layer was positioned at a distance 2/3 of the layer thickness from the hot plate surface. Minor adjustments were however necessary to perform tests at CRHG. The heating element of the apparatus was directly connected to a power supply with adjustable voltage output. Constant rate of heat production was achieved by keeping constant the voltage of the power supply. The assumption was made that the temperature rise of heated plate, within the range resulting from the dust layer insulation effect, does not significantly influence the parameters of the heating element. The adjustment of the apparatus also influenced the testing procedure. At the beginning of each test, the desired voltage of power supply was chosen. After temperature of the plate had reached its steady value, th temperature recording was activated and dust layer of desired thickness was formed on the plate. No further adjustments of the voltage were applied during the test. In all tests, as an ignition criterion, the criterion intro-
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Table 1 Properties of dusts tested brb Percentage of mass fraction passing the sieve of: 0.200 mm 100.0 0.100 mm 81.5 0.075 mm 76.0 0.063 mm 72.5 Average ash content A, % Average volatile content V, % Average moisture Wc, % Bulk density s, kg/m3 Specific heat c, J/kg K Heat of combustion Qs, kJ/kg
13.9 39.2 5.7 542.0 1323.0 24396.0
zfk
100.0 71.8 61.3 58.6 3.8 27.5 1.6 543.0 1398.0 34200.0
duced in EN 50281-2-1 (1999) was used. According to this criterion, the ignition shall be considered to have occurred if visible glowing or flaming is observed or a temperature in the layer of 450 °C is measured or a temperature rise of 250 K above the temperature of the heated plate is measured. However, in tests at CRHG, the plate temperature changes during the experiment. Therefore, to calculate temperature difference between the heated plate and the dust layer in these tests, transient values of the plate temperature were used. Nonetheless, to determine minimum ignition temperature of the layer, the value of initial temperature of the plate th was used. 4. Results of the tests at CRHG 4.1. Temperature changes in dust layer In Fig. 1, typical results for thin (5 mm) and thick (50 mm) dust layers are presented. In both cases, the layers ignited (visible glowing was observed); however, time scales are different. The thin layer ignited after about 14 min, while the thick one took over 3 h. In both cases, a rise of heated plate temperature is apparent and does not exceed 50 K. A part of this rise can be attributed to heat generated after ignition of the dust layer. In Fig. 1b after about 50 min, the th curve is nearly flat showing almost no further influence of heat generated in the layer. In the case of the thin layer, the ignition took place too early to obtain the plate temperature stabilisation. For both the tested dusts and dust layers of 5, 10, 20, 25 and 50 mm, minimum ignition temperatures were determined at constant rate of heat flux across the heated plate. Results of those tests are summarised in Figs. 2 and 3. Relations between minimum ignition temperature th min and the thickness of the dust layer d are qualitatively similar to those usually obtained in tests at CTHP (e.g. Hensel & John, 1993). For increasing layer thick-
Fig. 1. Temperature changes in typical tests with thin and thick layers at CRHG (a) d = 5 mm, (b) d = 50 mm.
ness, th min initially decreases very fast. Then, for greater values of d, the curve flattens. For zfk dust, the difference in th min for 25 and 50 mm dust layer is only 2 °C. This is well within measurement accuracy of the tests. The same behaviour for brb dust should be expected at layer thickness greater than 50 mm.
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Fig. 2. Minimum ignition temperature of brb dust vs. dust layer thickness at CRHG.
Fig. 3. Minimum ignition temperature of zfk dust vs. dust layer thickness at CRHG.
5. Comparison with the results at CTHP To compare the results presented above with the ones usually obtained by use of the standard procedure, tests similar to those described in Section 4.1 were performed at CTHP. In the tests, minimum ignition temperatures of dust layers tt min for different thicknesses were evaluated for brb and zfk dusts. The results of this set of tests are presented in Figs. 4 and 5, together with those at CRHG (th min). The difference between the results obtained with the two different methods, at CRHG and at CTHP, is apparent. For both dusts, the values of th min are much lower than the corresponding values of tt min. Surprisingly, the difference is bigger for small depths of the layer d and decreases when the layer gets thicker. It is even easier to see in Fig. 6, where this difference is plotted as a function of the layer thickness d. For the thin layers, the difference reaches 140 °C for brb dust and over 120 °C
Fig. 4. Minimum ignition temperature of brb dust vs. dust layer thickness at CTHP and CRHG. The line represents recommendations for the corresponding maximum permissible temperature based on CTHP tests.
Fig. 5. Minimum ignition temperature of zfk dust vs. dust layer thickness at CTHP and CRHG. The line represents recommendations for the corresponding maximum permissible temperature based on CTHP tests.
for zfk dust. For the thick layers, there are some indications that the difference stabilises on the level of 30– 50 °C. This difference can be directly attributed to the rise of heating plate temperature in experiments at CRHG (see Fig. 1b). The ignition in the dust layer starts at a certain distance from the heated plate. At that point, chemical reaction causes local increase of temperature and heat conduction towards both surfaces of the layer. In the tests at CRHG, the value of heat flux entering the dust layer is constant during the whole experiment. This prevents heat losses through the boundary being in contact with the heated plate. In the tests at CTHP, the situation is different. At a certain moment the heated plate starts to act as a heat sink and reduces temperature at the ignition point. The influence of this phenomenon on the ignition
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6. Discussion
Fig. 6. The difference in minimum ignition temperatures of dust layers evaluated at CTHP and CRHG.
depends on the distance between the ignition point and the heated plate. The bigger the distance, i.e. the thicker the layer, the weaker the influence of the heated plate acting as a heat sink. As mentioned in Section 1, the European Standard concerning electrical equipment designed for use in presence of dust EN 50281-1-2 (1998) includes a method for determining maximum permissible temperature ts of the surface of equipment’s casing for dust layers up to 50 mm thick. The value of ts depends on the value of the minimum ignition temperature evaluated for a 5 mm dust layer and the thickness of the actual layer. In Figs. 4 and 5, the recommended values of ts for dusts having minimum ignition temperature of 5 mm layer in the range between 250 and 320 °C are plotted with a line. For the thin layers, the line runs very close to experimental values of th min and for brb dust, it coincides with the experimental points. Moving towards the thicker layer, the standard recommends significantly lower values of ts than all results obtained in this experiment. It should be emphasised that in that range of d, the heat flow is qualitatively different than that in the case of thin layers. For the thin layers, the heat flow is basically one-dimensional—the layer can be treated as infinitive. However, when the ratio D/d layer becomes small, the heat flow becomes two-dimensional and the influence of the ring forming a cavity where the dust is placed cannot be neglected. Anthony and Field (1975) suggested that the diameter of the layer should be at least five times its thickness. There are some indications that the minimum ignition temperature of a thick “infinite” dust layer can be significantly lower than that obtained by use of the standard apparatus. It is important to point out that the values of tt min for both dusts tested are close to the upper limit of the range. For dusts with tt min slightly above 250 °C, one can expect the experimental points below the curve recommended in EN 50281-1-2 (1998).
At present, the minimum ignition temperature of a dust layer evaluated at CTHP is commonly used to assess the ignition hazard connected with hot horizontal surfaces of equipment’s casing. However, as discussed in Section 1, constant temperature of the casing when covered with a dust layer is barely possible in practical situations. More realistic conditions can be achieved in tests at CRHG. The comparison of the results obtained by use of both the methods suggests that the values of minimum ignition temperature evaluated at CRHG are much lower than those evaluated by use of the standard procedure (CTHP). In some cases, even an addition of the safety margin recommended by the European Standard EN 50281-1-2 (1998) may be insufficient to compensate the difference. If this is the case, then probably the standard test at CTHP should be replaced by the tests at CRHG in assessment of the above mentioned ignition hazard. This would raise a question on how to correlate the equipment’s working condition with the experimental set-up when the surface temperature of the casing is no longer an adequate parameter. It seems that the correlation can be established by measuring heat flux at all surfaces of the casing. This should be relatively easy with the use of heat flux detectors of proper type. Conducting tests at CRHG (constant, known heat flux across the surface of the heating plate) would result in obtaining more reliable estimation of ignition hazard of the dust layer on the tested equipment. There certainly exists another, more direct option. For particular equipment, the temperature of its casing covered with an appropriate dust layer of thickness expected in practice can be measured during the equipment’s regular working conditions. However, this approach seems impractical and in some cases impossible to achieve.
7. Conclusions 1. Minimum ignition temperatures of dust layers measured at CTHP tt min and at CRHG th min for two coal dusts appeared to be significantly different. For each thickness of the layer, tt min is higher than th min. 2. The difference between tt min and th min is greatest for thin layers and decreases when the layer gets thicker. 3. These results suggest that the standard procedure currently used to assess the ignition hazard connected with hot horizontal surfaces of equipment’s casing in practical cases cannot be fully justified. Further experimental work is required before decisive conclusions can be drawn.
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References Anthony, E. J., & Field, P. (1975). An apparatus and method for determining the self-ignition temperature of dust layers. Fire Research Station, Borehamwood, UK: Department of the Environment and Fire Offices’ Committee Joint Fire Research Organisation FRO/47/015. ASTM E2021-01 (2003). Standard test method for hot-surface ignition temperature of dust layers. Miron, Y., & Lazzara, Ch. (1988). Hot-surface ignition temperatures of dust layers. Fire and Materials, 12, 115–126. EN 50281-1-2 (1998). Electrical apparatus for use in the presence of
combustible dust—part 1-2: electrical apparatus protected by enclosures. Selection, installation and maintenance. EN 50281-2-1 (1999). Electrical apparatus for use in the presence of combustible dust—part 2-1: test methods—methods of determining minimum ignition temperatures. Eckhoff, R. K. (2000). Design of electrical equipment for areas containing combustible dust. Why dust standards cannot be extensively harmonised with gas standards. Journal of Loss Prevention in the Process Industries, 13, 3–5. Hensel, W., & John, W. (1993). The dependence of minimum ignition temperature of dust layer upon layer thickness. In P. Wolan´ ski (Ed.), Proceedings of the fifth international colloquium on dust explosions, Pułtusk. (p. 35). Warsaw: Oficyna Wydawnicza Politechniki Warszawskiej.
Ignition of a dust layer by a constant heat flux K. Lebecki ∗, Z. Dyduch, A. Fibich, J. S´liz˙ Experimental Mine “Barbara” of Central Mining Institute, PL-43-190 Mikoło´w, Katowice, Poland
Abstract In real conditions, the surface temperature of an equipment enclosure covered with a combustible dust layer can significantly rise due to insulating properties of the dust layer. To assess this effect, the measurements of minimum ignition temperature of dust layer at constant temperature of the heated plate tt min (standard method) and the same ignition temperature at constant rate of heat generation th min for two coal dusts were made. Dust layers of thickness between 5 and 50 mm were tested. For each dust, tt min was higher than th min for every tested thickness of the layer. The difference was biggest for thin layers and decreased with increase of the layer thickness. The results suggest a deficiency of the standard procedure of measuring minimum ignition temperature of a dust layer. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Dust layer; Ignition temperature
1. Introduction Ignition of a dust layer is one the most frequent causes of dust explosions in an industry. In many cases, the real conditions are such that it is impossible to avoid the accumulation of combustible dust on surfaces. The only way to avoid the risk of ignition is to prevent the equipment housing from heating up to dangerous temperatures. How easily the dust layer can be ignited depends on the amount of heat supplied to the layer, surface configuration and properties of the dust itself—minimum temperature at which ignition begins. Usually, the heat transport occurs when the dust is deposited on a hot horizontal surface, e.g. casing of electric equipment, mechanical machinery, etc. Susceptibility of a dust layer to ignition is described worldwide by the minimum ignition temperature of the dust layer. Recently, in the USA, the ASTM E2021-01 (2003) published its dust layer ignition procedure based on the experimental work of Miron and Lazzara (1988). According to the European Standard EN 50281-2-1 (1999), the value of minimum ignition temperature is determined for a 5 mm dust layer. The standard introduces an additional safety margin. Minimum ignition
∗
Corresponding author. Fax: +48-32-3246516. E-mail address: [email protected] (K. Lebecki).
temperature of dust layer reduced by 75 K is regarded as a safe temperature of a hot surface. The standard also states that the maximum allowable temperature of external surfaces of equipment’s housing must be further reduced if the thickness of the dust layer is greater than 5 mm. Evaluated this way, the maximum allowable temperature is then related to actual conditions where dust layers can be formed. Usually, this is simply the actual highest possible temperature of the equipment enclosure. However, as some authors point out (Eckhoff, 2000), direct comparison of maximum allowable temperature of external surfaces of equipment’s housing with its actual highest possible temperature is not a well justified procedure. This can be done only when the dust layers that may accumulate on the enclosure surfaces are so thin that they do not significantly change the surface temperature of the equipment. In case of a thick dust layer, the conditions are different. The surface temperature of the enclosure is not a constant inherent property of the enclosure, but is a result of heat transfer between the heat-generating element inside the enclosure and the surroundings. The parameter that can more likely be regarded constant is the rate of heat generation by the element. When the enclosure surface is covered by a dust layer, conditions of heat exchange through the enclosure change as a result of the layer’s thermal insulating properties. As a consequence, the surface temperature of the enclosure rises and may exceed the maximum value
0950-4230/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0950-4230(03)00041-X
244
K. Lebecki et al. / Journal of Loss Prevention in the Process Industries 16 (2003) 243–248
Nomenclature d D t td th th min ts tt tt min
thickness of the dust layer diameter of the dust layer in horizontal direction time transient temperature in the dust layer temperature of the heated plate minimum ignition temperature evaluated at constant rate of heat generation maximum permissible temperature (EN 50281-1-2, 1998) temperature of the heated plate in a test at constant temperature of the heated plate minimum ignition temperature evaluated at constant temperature of the heated plate
defined by its nominal temperature. Therefore, the dust layer can ignite even if its minimum ignition temperature determined in the standard test is higher than nominal temperature of the equipment enclosure. The problem described above was addressed in the new release of the European Standard concerning electrical equipment designed for use in the presence of dust (EN 50281-1-2, 1998). The standard includes a method for determining maximum allowable temperature of the surface of equipment’s casing for dust layers up to 50 mm thick based on the value of the minimum ignition temperature evaluated for a 5 mm dust layer. The method can be used when the minimum ignition temperature of the 5 mm dust layer is equal to or higher than 250 °C. If the temperature is lower or the thickness of the layer is greater than 50 mm, then direct measurements are recommended. The aim of the present work was to assess the difference in minimum ignition temperature of dust layer measured at constant temperature of the heated plate (CTHP), standard method, and the same ignition temperature but measured at constant rate of heat generation (CRHG). The latter condition was achieved by attaching the heating element of standard test apparatus directly to constant voltage power supply and assuming that temperature rise of heated plate does not significantly influence parameters of the heating element.
2. Dust properties For the tests, two coal dusts were chosen: coal dust Barbara d40 (brb), regarded as a reference dust at Experimental Mine “Barbara”, and a coal dust from Zofio´ wka (zfk) colliery. Table 1 presents the properties of the chosen dusts. The two dusts mainly differ in volatile and ash content. Also, the mass fractions of finer grains are greater amounts in zfk dust. The remaining presented parameters are similar for both dusts.
3. The equipment and testing method For the tests, a standard apparatus described in EN 50281-2-1 (1999) was used. The apparatus consisted of a steel plate 200 mm in diameter. The plate was heated electrically and its temperature was controlled by a thermocouple mounted in the plate near its centre. Another thermocouple mounted in a similar way was used to record the temperature of the surface during a test. Dust layers were prepared by filling the cavity formed by placing a steel ring of chosen height on the heated plate and levelling the layer to the top of the ring. To measure temperature inside the layer, a thermocouple stretched across the heated plate, parallel to its surface, was used. The thermocouple junction was positioned over the centre of the plate. For the tests at CTHP, the standard procedure described in EN 50281-2-1 (1999) was used. In all the tests with layers thicker than 5 mm, a junction of the thermocouple measuring temperature inside the layer was positioned at a distance 2/3 of the layer thickness from the hot plate surface. Minor adjustments were however necessary to perform tests at CRHG. The heating element of the apparatus was directly connected to a power supply with adjustable voltage output. Constant rate of heat production was achieved by keeping constant the voltage of the power supply. The assumption was made that the temperature rise of heated plate, within the range resulting from the dust layer insulation effect, does not significantly influence the parameters of the heating element. The adjustment of the apparatus also influenced the testing procedure. At the beginning of each test, the desired voltage of power supply was chosen. After temperature of the plate had reached its steady value, th temperature recording was activated and dust layer of desired thickness was formed on the plate. No further adjustments of the voltage were applied during the test. In all tests, as an ignition criterion, the criterion intro-
K. Lebecki et al. / Journal of Loss Prevention in the Process Industries 16 (2003) 243–248
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Table 1 Properties of dusts tested brb Percentage of mass fraction passing the sieve of: 0.200 mm 100.0 0.100 mm 81.5 0.075 mm 76.0 0.063 mm 72.5 Average ash content A, % Average volatile content V, % Average moisture Wc, % Bulk density s, kg/m3 Specific heat c, J/kg K Heat of combustion Qs, kJ/kg
13.9 39.2 5.7 542.0 1323.0 24396.0
zfk
100.0 71.8 61.3 58.6 3.8 27.5 1.6 543.0 1398.0 34200.0
duced in EN 50281-2-1 (1999) was used. According to this criterion, the ignition shall be considered to have occurred if visible glowing or flaming is observed or a temperature in the layer of 450 °C is measured or a temperature rise of 250 K above the temperature of the heated plate is measured. However, in tests at CRHG, the plate temperature changes during the experiment. Therefore, to calculate temperature difference between the heated plate and the dust layer in these tests, transient values of the plate temperature were used. Nonetheless, to determine minimum ignition temperature of the layer, the value of initial temperature of the plate th was used. 4. Results of the tests at CRHG 4.1. Temperature changes in dust layer In Fig. 1, typical results for thin (5 mm) and thick (50 mm) dust layers are presented. In both cases, the layers ignited (visible glowing was observed); however, time scales are different. The thin layer ignited after about 14 min, while the thick one took over 3 h. In both cases, a rise of heated plate temperature is apparent and does not exceed 50 K. A part of this rise can be attributed to heat generated after ignition of the dust layer. In Fig. 1b after about 50 min, the th curve is nearly flat showing almost no further influence of heat generated in the layer. In the case of the thin layer, the ignition took place too early to obtain the plate temperature stabilisation. For both the tested dusts and dust layers of 5, 10, 20, 25 and 50 mm, minimum ignition temperatures were determined at constant rate of heat flux across the heated plate. Results of those tests are summarised in Figs. 2 and 3. Relations between minimum ignition temperature th min and the thickness of the dust layer d are qualitatively similar to those usually obtained in tests at CTHP (e.g. Hensel & John, 1993). For increasing layer thick-
Fig. 1. Temperature changes in typical tests with thin and thick layers at CRHG (a) d = 5 mm, (b) d = 50 mm.
ness, th min initially decreases very fast. Then, for greater values of d, the curve flattens. For zfk dust, the difference in th min for 25 and 50 mm dust layer is only 2 °C. This is well within measurement accuracy of the tests. The same behaviour for brb dust should be expected at layer thickness greater than 50 mm.
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Fig. 2. Minimum ignition temperature of brb dust vs. dust layer thickness at CRHG.
Fig. 3. Minimum ignition temperature of zfk dust vs. dust layer thickness at CRHG.
5. Comparison with the results at CTHP To compare the results presented above with the ones usually obtained by use of the standard procedure, tests similar to those described in Section 4.1 were performed at CTHP. In the tests, minimum ignition temperatures of dust layers tt min for different thicknesses were evaluated for brb and zfk dusts. The results of this set of tests are presented in Figs. 4 and 5, together with those at CRHG (th min). The difference between the results obtained with the two different methods, at CRHG and at CTHP, is apparent. For both dusts, the values of th min are much lower than the corresponding values of tt min. Surprisingly, the difference is bigger for small depths of the layer d and decreases when the layer gets thicker. It is even easier to see in Fig. 6, where this difference is plotted as a function of the layer thickness d. For the thin layers, the difference reaches 140 °C for brb dust and over 120 °C
Fig. 4. Minimum ignition temperature of brb dust vs. dust layer thickness at CTHP and CRHG. The line represents recommendations for the corresponding maximum permissible temperature based on CTHP tests.
Fig. 5. Minimum ignition temperature of zfk dust vs. dust layer thickness at CTHP and CRHG. The line represents recommendations for the corresponding maximum permissible temperature based on CTHP tests.
for zfk dust. For the thick layers, there are some indications that the difference stabilises on the level of 30– 50 °C. This difference can be directly attributed to the rise of heating plate temperature in experiments at CRHG (see Fig. 1b). The ignition in the dust layer starts at a certain distance from the heated plate. At that point, chemical reaction causes local increase of temperature and heat conduction towards both surfaces of the layer. In the tests at CRHG, the value of heat flux entering the dust layer is constant during the whole experiment. This prevents heat losses through the boundary being in contact with the heated plate. In the tests at CTHP, the situation is different. At a certain moment the heated plate starts to act as a heat sink and reduces temperature at the ignition point. The influence of this phenomenon on the ignition
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6. Discussion
Fig. 6. The difference in minimum ignition temperatures of dust layers evaluated at CTHP and CRHG.
depends on the distance between the ignition point and the heated plate. The bigger the distance, i.e. the thicker the layer, the weaker the influence of the heated plate acting as a heat sink. As mentioned in Section 1, the European Standard concerning electrical equipment designed for use in presence of dust EN 50281-1-2 (1998) includes a method for determining maximum permissible temperature ts of the surface of equipment’s casing for dust layers up to 50 mm thick. The value of ts depends on the value of the minimum ignition temperature evaluated for a 5 mm dust layer and the thickness of the actual layer. In Figs. 4 and 5, the recommended values of ts for dusts having minimum ignition temperature of 5 mm layer in the range between 250 and 320 °C are plotted with a line. For the thin layers, the line runs very close to experimental values of th min and for brb dust, it coincides with the experimental points. Moving towards the thicker layer, the standard recommends significantly lower values of ts than all results obtained in this experiment. It should be emphasised that in that range of d, the heat flow is qualitatively different than that in the case of thin layers. For the thin layers, the heat flow is basically one-dimensional—the layer can be treated as infinitive. However, when the ratio D/d layer becomes small, the heat flow becomes two-dimensional and the influence of the ring forming a cavity where the dust is placed cannot be neglected. Anthony and Field (1975) suggested that the diameter of the layer should be at least five times its thickness. There are some indications that the minimum ignition temperature of a thick “infinite” dust layer can be significantly lower than that obtained by use of the standard apparatus. It is important to point out that the values of tt min for both dusts tested are close to the upper limit of the range. For dusts with tt min slightly above 250 °C, one can expect the experimental points below the curve recommended in EN 50281-1-2 (1998).
At present, the minimum ignition temperature of a dust layer evaluated at CTHP is commonly used to assess the ignition hazard connected with hot horizontal surfaces of equipment’s casing. However, as discussed in Section 1, constant temperature of the casing when covered with a dust layer is barely possible in practical situations. More realistic conditions can be achieved in tests at CRHG. The comparison of the results obtained by use of both the methods suggests that the values of minimum ignition temperature evaluated at CRHG are much lower than those evaluated by use of the standard procedure (CTHP). In some cases, even an addition of the safety margin recommended by the European Standard EN 50281-1-2 (1998) may be insufficient to compensate the difference. If this is the case, then probably the standard test at CTHP should be replaced by the tests at CRHG in assessment of the above mentioned ignition hazard. This would raise a question on how to correlate the equipment’s working condition with the experimental set-up when the surface temperature of the casing is no longer an adequate parameter. It seems that the correlation can be established by measuring heat flux at all surfaces of the casing. This should be relatively easy with the use of heat flux detectors of proper type. Conducting tests at CRHG (constant, known heat flux across the surface of the heating plate) would result in obtaining more reliable estimation of ignition hazard of the dust layer on the tested equipment. There certainly exists another, more direct option. For particular equipment, the temperature of its casing covered with an appropriate dust layer of thickness expected in practice can be measured during the equipment’s regular working conditions. However, this approach seems impractical and in some cases impossible to achieve.
7. Conclusions 1. Minimum ignition temperatures of dust layers measured at CTHP tt min and at CRHG th min for two coal dusts appeared to be significantly different. For each thickness of the layer, tt min is higher than th min. 2. The difference between tt min and th min is greatest for thin layers and decreases when the layer gets thicker. 3. These results suggest that the standard procedure currently used to assess the ignition hazard connected with hot horizontal surfaces of equipment’s casing in practical cases cannot be fully justified. Further experimental work is required before decisive conclusions can be drawn.
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