Partial inerting—an additional degree of freedom in dust explosion protection

Partial inerting—an additional degree of freedom in dust explosion protection

Journal of Loss Prevention in the Process Industries 17 (2004) 187–193 www.elsevier.com/locate/jlp Partial inerting—an additional degree of freedom i...

208KB Sizes 0 Downloads 55 Views

Journal of Loss Prevention in the Process Industries 17 (2004) 187–193 www.elsevier.com/locate/jlp

Partial inerting—an additional degree of freedom in dust explosion protection Rolf K. Eckhoff  Process Technology Programme, Physics Department, University of Bergen, Allegaten 55, 5007 Bergen, Norway

Abstract When applying partial inerting the gas (most often air) in which the explosible dust is dispersed is mixed with a fraction of inert gas (e.g. nitrogen) considerably smaller than that required for complete inerting. This reduces both the explosibility and the ignition sensitivity of the dust cloud. The effects on KSt (explosion violence) and MIE (minimum ignition energy) are particularly pronounced. This can facilitate the implementation of conventional protective methods that would otherwise have been difficult to use. The purpose of the present paper is to draw further attention to the additional degree of freedom that partial inerting offers in dust explosion protection. By using published data for coal dust and the new European CEN standard for vent sizing, it is shown that the minimum required areas for explosion venting are reduced considerably, due to reduced KSt and Pmax values, by even a moderate reduction in the content of oxygen in the atmosphere. It is also shown, using a qualitative probabilistic argument, how the marked increase of MIE obtained by partial inerting would justify a further reduction of minimum required vent areas. # 2003 Elsevier Ltd. All rights reserved. Keywords: Dust explosions; Dust clouds; Partial inerting; Explosion venting; Explosion violence; Ignition sensitivity; KSt; Pmax; MIE; LOC

1. Introduction Sometimes it is not feasible, or at least not practical, to accommodate any single basic dust explosion protection technique (venting, automatic suppression, full confinement), and/or automatically triggered isolation system, to process equipment in the process industries. One is then often compelled to adopt complete inerting, which can be very expensive. Increasing focus has been, therefore, on the possibilities of combining various methods of protection to obtain feasible solutions in otherwise difficult cases. For example, Siwek (1992) and Sliz, Lebecki and Dyduch (1993) described experiments in which combinations of explosion venting and explosion suppression were employed successfully in situations where it would not be straightforward to employ just one of the two methods. It appears that the potential of using partial inerting as an integral part of combined solutions to dust explosion protection has not been exploited in practice 

Tel.: +47-55-582858; fax: +47-55-589440. E-mail address: rolf.eckhoff@fi.uib.no (R.K. Eckhoff).

0950-4230/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2003.11.004

to the extent that one would perhaps anticipate. The purpose of the present paper is to draw further attention to the additional degree of freedom that this method offers. The essence of partial inerting is as follows: when the oxygen content of the atmosphere is reduced by mixing inert gas with the air, both the ignition sensitivity and the explosion violence of the dust cloud are reduced. In some cases, this can make it possible to implement one of the classical protective methods, i.e. full confinement, explosion venting, or automatic explosion suppression, and/or automatically triggered isolation systems, when this would otherwise have been unfeasible.

2. Influence of oxygen content in oxidizer gas on explosibility and ignitability of dust clouds 2.1. Explosion violence Wiemann (1984, 1987a, 1987b) investigated the influence of the oxygen content of air þ nitrogen on the maximum pressure and maximum rate of pressure rise in coal dust explosions in a 1 m3 closed vessel. Some

188

R.K. Eckhoff / Journal of Loss Prevention in the Process Industries 17 (2004) 187–193

results are given in Fig. 1 and show that both the explosion pressure and the rate of pressure rise decreased systematically with decreasing oxygen content. Furthermore, the explosive dust concentration range was narrowed, in particular on the fuel-rich side. The reduction of the maximum explosion pressure was approximately proportional to the reduction of the oxygen content, as would be expected from thermodynamic considerations. In the case of the maximum rate of pressure rise, i.e. the KSt value, the effect was considerably stronger. The data in the lower part of Fig. 1 show that the maximum rate of pressure rise decreased approximately linearly with decreasing oxygen content, down to zero at 11 vol.% oxygen, the limiting oxygen concentration (LOC) for sustained flame propagation through the dust cloud. The data in Fig. 1 were obtained with dust clouds having an initial temperature v of 150 C. Wiemann (1987b) also provided similar data for the same coal dust at lower temperatures. Some v data for 50 C is given in Table 1.

Table 1 Peak values of maximum explosion pressure and maximum rate of pressure rise generated by brown coal dust explosions in atmospheres of reduced oxygen content, in a 1 m3 standard closed explosion vessel v (ISO). Initial conditions of dust cloud: 50 C and atmospheric pressure. From Wiemann (1987b) Vol.% oxygen

Max. explosion pressure (bar(g))

KSt (bar m/s)

21 18 16

8.1 7.4 6.8

124 83 55

Fig. 2 shows some earlier results from the work of Hartmann (1948), which, in spite of a weak ignition source, confirm the trends in the data of Wiemann in Fig. 1 and Table 1. The results of Devlikanov, Kuzmenko and Poletaev (1995) for nutrient yeast dust are in good agreement with those of Wiemann (1984, 1987a, 1987b) and Hartmann (1948). The data in Fig. 3, from experiments by Walther and Schacke (1986), show that the LOC of clouds of a polymer powder was independent of the initial cloud pressure over the range 1–4 bar (abs.). 2.2. Explosive range of dust concentrations The influence of the oxygen content of the oxidizing gas on the minimum explosive dust concentration (MEC) was studied by Hertzberg and Cashdollar (1987). Because the ignition source used may have been

Fig. 1. Influence of oxygen content of gas on maximum explosion pressure and maximum rate of pressure rise of brown coal dust for various dust concentrations. Nitrogen was used as inert gas. 1 m3 stanv dard explosion vessel (ISO). Initial conditions of dust clouds: 150 C and atmospheric pressure (from Wiemann, 1984, 1987a, 1987b).

Fig. 2. Influence of oxygen content of gas on maximum pressure and maximum rate of pressure rise in explosions of 100 g/m3 of <74 lm ethyl cellulose moulding powder in the 1.2 l Hartmann bomb (from Hartmann, 1948).

R.K. Eckhoff / Journal of Loss Prevention in the Process Industries 17 (2004) 187–193

Fig. 3. Influence of oxygen content of gas on maximum explosion pressure for a polymer powder for various initial pressures. 1 m3 closed ISO vessel (from Walther & Schacke, 1986).

189

Fig. 4. Influence of oxygen content of gas on minimum explosive concentration of a high volatile content coal dust versus particle size (from Hertzberg & Cashdollar, 1987).

rather weak, the data obtained may not be useable for the design of practical dust explosion protection systems. Some results are given in Fig. 4. For particles smaller than about 10 lm, a reduction of the oxygen content from that of air to 15.5% caused only a moderate increase, from 130 to 160 g/m3, of the minimum explosive concentration. However, as the mean particle size increased, the influence of reducing the oxygen content became more pronounced. Hence, at a mean particle size of 40 lm, lowering the oxygen content from that of air to 15.5 vol.% raised MEC from 135 to 300 g/m3. Fig. 1 also suggests that the maximum explosive concentration of dusts, as measured in the 1 m3 vessel, drops substantially when the oxygen content of the atmosphere is reduced. 2.3. Ignitability Fig. 5 illustrates the influence of the oxygen content of the gas phase on the minimum ignition temperature of a dust cloud. For the <74 lm Pittsburgh coal dust v tested, there was only a moderate increase, from 610 C v in air to 730 C in 10 vol.% oxygen. This conclusion was supported by Zeeuwen (1996), who found that minimum ignition temperatures of dust clouds and layers did not increase dramatically when reducing the oxygen content of the atmosphere.

Fig. 5. Influence of oxygen content of gas on minimum ignition temperature of <74 lm Pittsburgh coal dust in the Godbert–Greenwald furnace (from Hartmann, 1948).

190

R.K. Eckhoff / Journal of Loss Prevention in the Process Industries 17 (2004) 187–193

permissible explosion overpressure in the vented enclosure, Pred, the opening pressure of the vent cover, Pstat, the volume V of the enclosure, and the length-todiameter ratio of the enclosure, L/D. By applying the appropriate equation to the data in Table 1, the effect on minimum vent areas of reducing the oxygen content of the atmosphere can be assessed. The results are given in Table 2. The data in Table 2 show that, for the limited range of parameters investigated, a reduction of the oxygen content from that of air (21 vol.%) to 18 vol.% reduces the required vent areas by a factor of about 0.61 on average, i.e. by about 40%. The corresponding factor for a reduction from 21 to 16 vol.% is 0.37, i.e. more than a 60% reduction of the minimum necessary vent area. Table 2 Permissible reduction of minimum required dust explosion vent areas for various enclosures and dusts due to reduced oxygen content of the atmosphere, calculated by applying the appropriate vent area calculation equation in CEN (2002a) to the coal dust data in Table 1. Pred ¼ 0:3 bar(g) and Pstat ¼ 0:1 bar(g) in all calculated cases Fig. 6. Influence of oxygen content of gas on minimum ignition energy of dust clouds (from Glarner, 1984).

The much more pronounced influence of the oxygen content of the gas on the minimum ignition energy of dust clouds (MIE) is illustrated by Glarner’s (1984) data for some organic dusts in Fig. 6. These data show that MIE rises by 2–6 orders of magnitude when the oxygen content of the atmosphere is reduced from the 21 vol.% of air to 10 vol.%. Additional experimental data, supporting the results of Glarner, were produced in the joint European research programme CREDIT (1995). Hence, Zeeuwen (1996) confirmed that even modest reductions of the oxygen concentration in the atmosphere by partial inerting can increase the MIE of dust clouds substantially. Glor and Schwenzfeuer (1996, 1999) provided further information on the influence of the oxygen content of the atmosphere on MIE.

O2 content Pmax (vol.%) (bar(g))

KSt V (m3) (bar m/s)

L/D

Vent area (m2)

21 18 16

8.1 7.4 6.8

124 83 55

1 1 1

1 1 1

0.065 0.040 0.024

21 18 16

8.1 7.4 6.8

124 83 55

10 10 10

1 1 1

0.37 0.23 0.14

21 18 16

8.1 7.4 6.8

124 83 55

100 100 100

1 1 1

2.1 1.28 0.78

21 18 16

8.1 7.4 6.8

124 83 55

1 1 1

2 2 2

0.12 0.076 0.046

21 18 16

8.1 7.4 6.8

124 83 55

10 10 10

2 2 2

0.70 0.43 0.26

21 18 16

8.1 7.4 6.8

124 83 55

100 100 100

2 2 2

4.0 2.4 1.5

21 18 16

8.1 7.4 6.8

124 83 55

1 1 1

6 6 6

0.22 0.13 0.08

21 18 16

8.1 7.4 6.8

124 83 55

10 10 10

6 6 6

1.23 0.75 0.46

21 18 16

8.1 7.4 6.8

124 83 55

100 100 100

6 6 6

7.0 4.3 2.6

3. Effect of partial inerting on required vent areas 3.1. An example using published experimental KSt and Pmax data (Table 1) The new draft European guideline CEN (2002a) for sizing explosion vents provides empirical equations by which minimum acceptable vent areas for dust explosions can be calculated. The parameters included in the equations are KSt and Pmax for the actual dust in air, as measured in standard closed-bomb tests, the maximum

R.K. Eckhoff / Journal of Loss Prevention in the Process Industries 17 (2004) 187–193

3.2. Example using first-order correlations for estimating reduced Pmax and KSt Pmax and KSt values in air, as well as LOC values, for a variety of dusts have been published, e.g. by Beck, Glienke and Mo¨hlmann (1997). Some of these data are also included in Eckhoff (2003). LOC is the lower limiting oxygen concentration in the atmosphere at which a flame can propagate through a cloud of the dust in question at normal temperature and atmospheric pressure. The data in Fig. 1 and Table 2 suggest that the following first-order correlations apply, and may be used for estimating reduced Pmax and KSt values for the dust in question: Pmax;C ¼ Pmax;21 ðC=21Þ

ð1Þ

KSt;C ¼ KSt;21 ðC  LOCÞ=ð21  LOCÞ

ð2Þ

Here, C is the actual oxygen concentration in the atmosphere, in vol.%. The equations only apply for C> LOC. In the case of an aluminium powder tested by Beck et al. (1997), with a Pmax,21 of 12.5 bar(g), KSt,21 of 400 bar m/s, and LOC equal to 6 vol.%, Eqs. (1) and (2) indicate that partial inerting to, for example, 17 vol.% reduces KSt to below 300 bar m/s and Pmax to about 10 bar(g). Eq. (2) shows that the higher the LOC value is, the larger the relative reduction in KSt will be for a given value of C.

191

distributed at random along the time axis. The resulting distribution of the frequencies of the various Pmax values is illustrated by the bell-shaped distributions in Fig. 7. If the entire distribution of expected maximum explosion pressures Pmax falls below the maximum permissible pressure Pred, the vent area is unnecessarily large. On the other hand, if an unacceptable fraction of all explosions generate destructive pressures (Pmax >Pred ), the vent is too small. Assume (disregarding that consequences of destructive explosions may vary from explosion to explosion) that the overall explosion risk is represented by the total number of destructive explosions in the one-million-year period. Let N be the maximum acceptable total number of destructive explosions. The upper part of Fig. 7 then illustrates that, with inadequate ignition source prevention and bad housekeeping, the vent area A1 is just sufficient to keep N at the maximum acceptable level. (It may be argued that improving housekeeping standards will not contribute much to reducing the total number of primary explosions inside process

3.3. Potential for further reduction of dust explosion vent areas due to increased MIE The probabilistic aspect of dust explosion venting is discussed more extensively in Chapter 6 of Eckhoff (2003). Consider a process unit in an industrial plant in which a combustible powder is produced or handled. The process unit can be a mill, a fluidized bed, a bucket elevator, a cyclone, a storage silo or any other enclosure in which explosive dust clouds may occur. The process unit is equipped with a vent opening. Assume that the plant is operated for a period of one million years without any systematic changes in technology, operating and maintenance procedures, knowledge and attitudes of personnel, or in any other factor that might influence the ways in which dust clouds are generated and potential ignition sources arise. Due to the remaining, constant random variation of the totality of momentary conditions, a certain finite number of accidental dust explosions will occur during the onemillion-year period. Depending on the actual momentary circumstances, some of these will only be weak ‘puffs’, whereas others will be more violent. Some will be destructive. Because ‘status-quo’ conditions are reestablished after each incident, whether destructive or non-destructive, the maximum pressures Pmax in all the explosions during the one million years will be

Fig. 7. Illustration of the reduction of the required vent area resulting from a reduction of the overall probability of dust cloud ignition, and/or improvement of housekeeping standards. N is the maximum acceptable number of destructive explosions per one million years.

192

R.K. Eckhoff / Journal of Loss Prevention in the Process Industries 17 (2004) 187–193

equipment, because improving housekeeping mainly reduces the likelihood of secondary explosions outside process equipment. On the other hand, because this reduces the average consequences of the explosions, the ultimate real explosion risk is also reduced.) However, the core of the present argument is that the total number of destructive explosions can be reduced significantly if the probability of dust cloud ignition is reduced. This is illustrated in the lower part of Fig. 7, where the area under the bell-shaped curve representing the vent area A1 is substantially smaller than the area under the curve in the upper part of Fig. 7. On the assumption that the reduction of the total number of explosions does not influence the shape of the frequency distribution of Pmax, the reduction of the total number of explosions results in a proportional reduction of the total number of destructive explosions to a value considerable smaller than the maximum acceptable value N. This means that the vent area A2 required to maintain the total number of destructive explosions at N, under the new circumstances of reduced ignition probability, is smaller than A1. This is illustrated in the lower part of Fig. 7. A smaller vent means that, on the whole, the maximum explosion pressures in the vented enclosure will increase, i.e. there will be a shift of the distribution of Pmax towards higher pressures, whilst the total number of explosions, i.e. the area under the curve, remains unchanged. However, as a result of this shift, the total number of destructive explosions increases. With a reduction of the vent area to A2, this number has again retained the maximum permissible value N. Because, as illustrated in Fig. 6, partial inerting raises MIE values substantially, it seems reasonable to expect that the overall ignition probability, i.e. the total number of expected explosions in the hypothetical one-million-year period, will decrease. In the case of dusts with very low MIEs in air, the reduction is likely to be particularly pronounced, because of the substantial reduction of the electric/electrostatic discharge ignition hazard. In situations where electric/electrostatic discharge ignition constitutes a significant part of the total ignition hazard, this in principle opens up for a further reduction of required vent areas, beyond the reduction resulting from the reduced standard dust test parameters Pmax and KSt.

4. Conclusions 1. A significant reduction of the minimum vent areas required for dust explosion venting, resulting from partial inerting, has been demonstrated by applying Wiemann’s (1987b) explosibility data for a brown coal dust to the new European venting standard CEN (2002a). In the examples considered, a reduction

of the oxygen content from that of air (21 vol.%) to 18 vol.% reduced the required vent areas by a factor of about 0.6. The corresponding factor for a reduction of the oxygen content from 21 to 16 vol.% was 0.37. 2. Data for KSt and Pmax at reduced oxygen concentrations are scarce, but first-order estimates for oxygen concentrations in the range from 21 vol.% to LOC may be obtained from simple linear relationships supported by the limited experimental data available (Eqs. (1) and (2)). 3. By using a probabilistic argument, it was shown that the pronounced reduction of the ignition sensitivity of the dust cloud (substantial increase of MIE) due to reduced oxygen concentration may justify a further reduction of the minimum required vent area, beyond the reduction due to reduced KSt and Pmax values. 4. It is expected that favourable effects of partial inerting will also be encountered in the design of fully confined systems, automatic explosion suppression systems, and systems for automatically triggered explosion isolation. The straightforward effect on design pressures for fully confined systems is indicated by Eq. (1). CEN (2002b) gives general guidance for designing automatic suppression systems, but additional technical details on the design and performance of the specific suppression system used are required for quantifying the effect of partial inerting.

References Beck, H., Glienke, N., & Mo¨hlmann, C. (1997). Combustion and explosion characteristics of dusts. BIA-Report 13/97, Berufsgenossenschaftliches Institut fu¨r Arbeitsschutz, Sankt Augustin, Germany. CEN (2002a). Dust explosion venting protective systems. European Draft Standard prEN 14491, June 2002, CEN Management Centre, rue de Stassart, 36, B-1050 Brussels. CEN (2002b). Explosion suppression systems. European Draft Standard prEN 14373, March 2002, CEN Management Centre, rue de Stassart, 36, B-1050 Brussels. CREDIT (1995). Dust explosions: protecting people, equipment, buildings and environment. Proceedings of Conference in London, October 11–12, 1995London, UK: IBC Technical Services. Devlikanov, O., Kuzmenko, D. K., & Poletaev, N. L. (1995). Nitrogen dilution for explosion of nutrient yeast dust/air mixture. Fire Safety Journal, 25, 373. Eckhoff, R. K. (2003). Dust explosions in the process industries. (3rd ed.). Boston, USA: Gulf Professional Publishing/Elsevier. Glarner, Th. (1984). Mindestzu¨ndenergie—Einfluss der Temperatur. VDI-Berichte, Vol. 494. (pp. 109–118). Du¨sseldorf: VDI-Verlag GmbH. Glor, M., & Schwenzfeuer, K. (1996). Einfluss der Sauerstoffkonzentration auf die Mindestzu¨ndenergie von Stau¨ben. VDI-Berichte, Vol. 1272. (pp. 119–134). Du¨sseldorf, Germany: VDI-Verlag GmbH. Glor, M., & Schwenzfeuer, K. (1999). Einfluss der Sauerstoffkonzentration auf die Mindestzu¨ndenergie von Stau¨ben. Paper presented at the Dechema Jahrestagung 1999, in Wiesbaden, Germany.

R.K. Eckhoff / Journal of Loss Prevention in the Process Industries 17 (2004) 187–193 Hartmann, I. (1948). Recent research on the explosibility of dust dispersions. Industrial and Engineering Chemistry, 40, 752–758. Hertzberg, M., & Cashdollar, K. L. (1987). Introduction to dust explosions. In K. L. Cashdollar, & M. Hertzberg (Eds.), Industrial dust explosions (pp. 5–32). ASTM Special Technical Publication, Vol. 958. Philadelphia, USA: ASTM. Siwek, R. (1992). The combination of explosion venting and explosion suppression: explosion suppression in very small volumes. Proceedings of the 1st World Seminar on the Explosion Phenomenon and the Application of Explosion Protection Techniques in Practice, February 17–21, 1992, BrusselsKontich, Belgium: EuropEx. Sliz, J., Lebecki, K., & Dyduch, Z. (1993). Venting and suppression of grain dust explosions. Experiments in a 8 m3 chamber. Proceedings of the 5th International Colloquium in Dust Explosions, April 19–22, 1993, Pultusk near Wasaw, Poland. Walther, C.-D., & Schacke, H. (1986). Evaluation of dust explosion characteristics at reduced and elevated initial pressures (poster summary). Bayer AG, Leverkusen, F. R. Germany.

193

Wiemann, W. (1984). Einfluss der Temperatur auf Explosionskenngro¨ssen und Sauerstoffgrenzkonzentrationen. VDI-Berichte, Vol. 494. (pp. 89–97). Du¨sseldorf: VDI-Verlag GmbH. Wiemann, W. (1987a). Influence of temperature and pressure on the explosion characteristics of dust/air and dust/air/inert gas mixtures. In K. L. Cashdollar, & M. Hertzberg (Eds.), Industrial dust explosions (pp. 33–44). ASTM Special Technical Publication, Vol. 958. Philadelphia, USA: ASTM. Wiemann, W. (1987b). Untersuchung des Explosionsverhaltens von Methan und Kohlenstaub bei hohen Temperaturen unter vermindertem Sauerstoffgehalt. Final report project No. 321000, Westfa¨hlische Berggewerkschaftskasse, Bergbau-Versuchsstrecke, Institut fu¨r Explosionsschutz und Sprengtechnik, Beylingstrasse 65, 4600 Dortmund-Derne, Germany Zeeuwen, J. P. (1996). Dust explosion protection of grinding installations. Proceedings of 2nd World Seminar on the Explosion Phenomenon and on the Application of Explosion Protection Techniques in Practice, 4–8 March 1996, Gent, BelgiumKontich, Belgium: EuropEx.