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Explosion characteristics of LPG–air mixtures in closed vessels
Journal of Hazardous Materials 165 (2009) 1248–1252
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Explosion characteristics of LPG–air mixtures in closed vessels Domnina Razus a,∗ , Venera Brinzea a , Maria Mitu a , D. Oancea b a b
“Ilie Murgulescu” Institute of Physical Chemistry, 202 Spl. Independentei, 060021 Bucharest, Romania Department of Physical Chemistry, University of Bucharest, 4-12 Regina Elisabeta Blvd., 030018 Bucharest, Romania
a r t i c l e
i n f o
Article history: Received 8 January 2008 Received in revised form 19 September 2008 Accepted 22 October 2008 Available online 28 October 2008 Keywords: LPG (liquefied petroleum gas) Combustion Closed vessel Explosion pressure Rate of pressure rise Explosion time
a b s t r a c t The experimental study of explosive combustion of LPG (liquefied petroleum gas)–air mixtures at ambient initial temperature was performed in two closed vessels with central ignition, at various total initial pressures within 0.3–1.3 bar and various fuel/air ratios, within the flammability limits. The transient pressure-time records were used to determine several explosion characteristics of LPG–air: the peak explosion pressure, the explosion time (the time necessary to reach the peak pressure), the maximum rate of pressure rise and the severity factor. All explosion parameters are strongly dependent on initial pressure of fuel–air mixture and on fuel/air ratio. The explosion characteristics of LPG–air mixtures are discussed in comparison with data referring to the main components of LPG: propane and butane, obtained in identical conditions. © 2008 Elsevier B.V. All rights reserved.
1. Introduction LPG is extensively used nowadays, both as alternative fuel in automotive engines and as domestic fuel. In comparison with conventional engine fuels (gasoline and diesel), LPG is considered an attractive alternative fuel since its combustion in air is characterized by reduced emissions of NOx , CO and unburned hydrocarbons [1]. A thorough analysis and prediction of engines’ and/or combustors’ performance requires a systematic investigation of explosion characteristics of LPG in air in various conditions. Values of flammability limits for LPG in air were reported by Mishra and Rahman, for a LPG blend containing 70% propane and 30% butane [2]. Flame propagation in LPG–air mixtures with various equivalence ratios, in operating conditions close to a heavy-duty LPG engine (initial pressures between 1.5 and 4 bar, initial temperatures between 330 and 380 K) was studied by Lee and Ryu [3]. The authors examined the pressure and temperature influence on flame propagation speed for LPG–air, in comparison with propane–air mixtures, well characterized in this respect. Burning velocities of LPG–air without or with exhaust gas addition are also reported by Liao et al. [1,4] from optical records of flame front position, in experiments made at vari-
∗ Corresponding author. Tel.: +40 21 316 79 12; fax: +40 21 312 11 47. E-mail addresses: [email protected], [email protected] (D. Razus), [email protected] (V. Brinzea), maria [email protected] (M. Mitu), [email protected] (D. Oancea). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.10.082
ous initial pressures and temperatures in a 1.57 L closed vessel with rectangular profile and central ignition. The authors discuss their results versus similar data referring to propane– and i-butane–air mixtures [4]. Other experiments, made in a 2.46 L cylindrical vessel with central ignition used LPG– and propane–air mixtures with variable initial composition, pressure and temperature [5]. Values of several explosion characteristics of closed vessel combustion (maximum explosion pressure, maximum rate of pressure rise, time to maximum pressure, time to maximum rate of pressure rise, ignition delay period, severity factors) are given for both fuels and examined against initial pressure, temperature and composition. For all explosion parameters, the dependencies on initial pressure, temperature and equivalence ratio are given both as diagrams and fit equations, able to be used for comparison with other sets of data. Burning velocities of propane–, n-butane– and LPG–air mixtures, determined from pressure-variation during the early stage of propagation in a closed spherical vessel with central ignition by means of cubic law coefficients, were recently reported for mixtures of variable initial composition and pressure [6]. The present work completes the data on burning velocities [6] with values of explosion parameters of LPG–air in two highly symmetrical closed vessels, at room temperature, at various initial pressures within 0.3–1.3 bar and various [LPG]/[O2 ] ratios: the peak (maximum) explosion pressures pmax , the times to peak pressures max , the maximum rates of pressure rise (dp/dt)max and the severity factors KG . LPG is a blend used in Romania for domestic use containing 12 vol.% propane, 87 vol.% butane, 1 vol.% pentane. The
D. Razus et al. / Journal of Hazardous Materials 165 (2009) 1248–1252
Nomenclature h K p R t V
height severity factor pressure radius time volume
Greek letters time to peak pressure ˚ diameter Subscripts G referring to gas explosions max maximum value 0 initial value
data referring to LPG–air will be examined against literature data on LPG combustion in closed vessels [5,7] and compared to data referring to propane–air and n-butane–air, obtained in identical conditions [8,9]. The measured explosion parameters of LPG–air, in a wide range of initial conditions and in various vessels, are needed for promoting a database of explosion parameters of pure and mixed fuels. At the same time, the peak explosion pressures, the maximum rates of pressure rise, the explosion times and the severity factors are important values for vent area design [10,11] and for characterization of explosion transmission between interconnected vessels [12,13]. 2. Experimental Experiments were made in two explosion vessels with central ignition, tight at vacuum and at pressures up to 2.5 MPa: vessel S: a spherical vessel with the radius R = 5 cm and vessel C: a cylinder with height h = 15 cm and diameter ˚ = 10 cm. The initial pressure of explosive mixtures was measured using a strain gauge manometer Edwards EPSA-10HM. In each vessel, ignition was made with an inductive-capacitive sparks produced between stainless steel electrodes with rounded tips, using a spark gap of 3.5 mm. The sparks have energies between 2 and 5 mJ and approximately 1 ms duration. Explosions were monitored by means of a fast response pressure transducer (Kistler 601A, connected to a Charge Amplifier Kistler 5001N) and an ionization probe with the tip mounted 3 mm far from wall, in every explosion vessel. The signals from the charge amplifier and from the ionization probe amplifier were recorded with a Tektronix acquisition data system (TestLab 2505, acquisition card 25AA1) at 104 signals/s. A vacuum and gas-feed line, tight at pressures from 0.05 to 150 kPa, connects the vacuum pump, the gas cylinders with fuel and air, the metallic cylinder for mixture storage and the explosion vessels. The vacuum pump maintains a vacuum of 0.05 kPa in the explosion vessel, after each experiment. The experimental procedure consisted of evacuating the combustion vessel down to 0.05 kPa; the fuel–air mixture was then introduced, allowed to become quiescent and ignited. Minimum three experiments were performed for each initial condition of explosive mixture. More details were recently given [14,15]. The fuel–air mixtures were prepared in stainless steel cylinders at a total pressure of 400 kPa, by partial pressure method and used 24 h after mixing the components.
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LPG (Arpechim—Romania) contained 12 vol.% propane, 62.6 vol.% n-butane, 24.4 vol.% i-butane, 1 vol.% pentane. The examined mixtures had LPG concentration between 2.50 and 6.15 vol.%. 3. Data evaluation The derivatives (dp/dt) were obtained after smoothing the experimental p(t) data by Savitsky–Golay method, based on least squares quartic polynomial fitting across a moving window within the data. In all cases, we used a 10% smoothing level. 4. Results and discussion For each LPG–air mixture, the maximum explosion pressures are linearly correlated with initial pressures, p0 , within the range 30–130 kPa. An illustrative diagram is given in Fig. 1, where several results obtained in the spherical vessel are plotted. Similar data were obtained for the cylindrical vessel. From such correlations it is possible to calculate the peak pressure at any value of initial pressure even beyond the investigated range (as long as the process takes place as a deflagration). For both vessels, the slope and intercept of the linear correlations depend on the initial composition and the characteristics of explosion vessel (volume, height/diameter ratio, position of the ignition source), as already observed for mixtures of other hydrocarbons (methane, propene, n-butane, n-pentane) with air [14]. The slopes of these correlations are numerically close to the adiabatic explosion pressure pmax,ad ; their intercepts depend on both the heat lost by the burning mixture to the vessel and the vessel’s volume [14–16]. Information concerning the amount of heat lost during flame propagation in a closed vessel is also given in a recent contribution of Van den Schoor et al. [17] focused on combustion of very rich methane–air mixtures (in the vicinity of the upper flammability limit): the explosion pressure measured in centrally ignited explosions is higher as compared to explosions asymmetrically initiated, by an ignition source positioned 6 cm under central position. Measured explosion pressures of LPG–air in spherical vessel S and in cylindrical vessel C at ambient initial conditions, together with explosion pressures reported by other authors from experiments in cylindrical vessels, are plotted versus LPG concentration in Fig. 2. Larger heat losses in asymmetrical vessel C as compared to vessel S account for the lower peak pressures measured in vessel C as compared to vessel S. The observed differences between peak pressures we measured and literature data can be assigned mainly to differences in volume and shape (length to diameter
Fig. 1. Maximum explosion pressures measured during deflagration of LPG–air mixtures in spherical vessel S with central ignition.
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D. Razus et al. / Journal of Hazardous Materials 165 (2009) 1248–1252
Fig. 3. Maximum rates of pressure rise measured during deflagration of LPG–air mixtures in spherical vessel S with central ignition. Fig. 2. Maximum explosion pressures of LPG–air mixtures, in closed vessels, at p0 = 101 kPa and T0 = 298 K: (1) present data: spherical vessel S (V = 0.52 L); (2) present data: vessel C (V = 1.12 L); (3) Ref. [5]: cylindrical vessel (V = 2.46 L); (4) Ref. [7]: cylindrical vessel (V = 270 L).
ratio) of explosion vessels, but also to the different composition of the examined LPG blends. According to our measurements, the highest value of explosion pressure for LPG–air at p0 = 101 kPa and T0 = 298 K is 922 kPa, recorded in spherical vessel S; in vessel C, the highest explosion pressure is 853 kPa (central ignition in both vessels). Compared to this, Huzayyin et al. [5] measured the highest value of explosion pressure as 800 kPa, in a cylindrical vessel of volume V = 2.46 L (˚ = 144.5 mm; h = 150 mm) with central ignition, using a LPG blend formed from C3 H8 (26.41 vol.%), n-C4 H10 (47.22 vol.%), iC4 H10 (26.32 vol.%) and 0.05 vol.% higher hydrocarbons. Oh et al. [7] reported 680 kPa from measurements in a rectangular-profile vessel with V = 270 L (0.6 m × 0.45 m × 1 m) with side ignition, without mentioning the composition of used LPG. For the main components of LPG, the maximum peak pressures measured in vessel S were 910 kPa (propane) [9] and 925 kPa (n-butane) [8]. In vessel C, the maximum peak pressures were 827 kPa (propane) [9] and 860 kPa (n-butane) [8]. These values range well within reported data in literature: 890 kPa for propane–air and 900 kPa for nbutane–air measured in a 5-L spherical vessel with central ignition [18]; 940 kPa for both propane– and n-butane–air, measured in a 20-L spherical vessel with central ignition [19]; 840 kPa measured for propane–air in a 1-L cylindrical vessel (˚ = 90 mm; h = 45 mm) [20]. The maximum rates of pressure rise reached in the two explosion vessels S and C follow similar trends when examined against initial pressure and LPG concentration. The corresponding plots are given in Figs. 3 and 4. In both vessels, the highest rates of pressure rise are observed at LPG concentrations between 4.3 and 4.6 vol.%, the same concentration range where highest peak pressures were measured. This concentration range is quite the same for several LPG blends, as seen from Fig. 4 where results from several authors were plotted. The peak values of pressure rise rates in the diagrams of Fig. 4 vary between 25 and 128 MPa/s, due to the differences of experimental conditions: a spherical and a cylindrical vessel with central ignition—the present data; cylindrical vessel with central ignition—data from Ref. [5]; cylindrical vessel with side ignition—Ref. [7]. In cylindrical vessel C (a slightly elongated vessel, with ˚/h = 1.5) the recorded (dp/dt)max have lower values as compared to spherical vessel S due to larger heat losses before reaching the peak explosion pressure, at all examined composi-
tions. The highest measured pressure rise rates of LPG–air mixtures are 128 MPa/s in vessel S and 44 MPa/s in vessel C. Data referring to propane–air and n-butane–air mixtures, determined in identical experimental conditions, are given in Table 1. The explosion parameters characteristic to LPG values listed in Table 1 are slightly outside the region between propane and butane; some parameters characteristic to LPG are closer to those of propane and the others are closer to n-butane. The observed differences lie however within the range of standard errors, for each of the measured parameters. The lowest values of explosion time max (the time from ignition till the peak pressure) are recorded at LPG concentrations between 4.2 and 4.4 vol.%, for both explosion vessels and are also given in Table 1. In Fig. 5, the times to peak pressure from our measurements at ambient initial conditions are plotted together with data obtained for LPG–air explosions in a 2.46-L cylindrical vessel [7]. Examination of the present data showed that initial pressure variation, within 30 and 130 kPa, has little influence upon values of time to peak pressure, in both explosion vessels. From the maximum rates of pressure rise, the severity factors KG were calculated as: KG =
dp dt
max
√ 3 V
(1)
Fig. 4. Maximum rates of pressure rise of LPG–air explosions in closed vessels, at p0 = 101 kPa and T0 = 298 K: (1) present data: spherical vessel S (V = 0.52 L); (2) present data: vessel C (V = 1.12 L); (3) Ref. [5]: cylindrical vessel (V = 2.46 L); (4) Ref. [7]: cylindrical vessel (V = 270 L).
D. Razus et al. / Journal of Hazardous Materials 165 (2009) 1248–1252
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Table 1 Average values and standard errors of explosion characteristic parameters for the most reactive fuel–air mixtures in vessels S and C, at p0 = 101 kPa and T0 = 298 Ka .
pmax (kPa) max (ms) (dp/dt)max (MPa/s) KG (bar m/s) pmax (kPa) max (ms) (dp/dt)max (MPa/s) KG (bar m/s) Concentration range (vol.%) for extreme values of flammability parameters a
C3 H8 [9]
n-C4 H10 [8]
LPG
Spherical vessel S 911 ± 12 25 ± 0.4 125 ± 5.0 100.7 ± 4.0
925 ± 12 23 ± 0.3 127 ± 5.5 102.3 ± 4.4
922 ± 13 26 ± 0.4 126 ± 5.5 101.5 ± 4.4
Cylindrical vessel C 827 ± 10 44 ± 0.5 43.6 ± 2.1 46.0 ± 0.2 4.0–4.5
860 ± 11 42 ± 0.5 41.2 ± 2.0 43.5 ± 0.2 3.6–4.3
853 ± 11 43 ± 0.5 43.0 ± 2.0 45.4 ± 0.2 4.2–4.7
Average values and standard errors were calculated from five experiments made in identical conditions.
of found correlations are influenced by the amount of heat losses from the burned gas to the vessel. The obtained correlations allow the calculation of peak pressure at any value of initial pressure even beyond the investigated range (as long as the process takes place as a deflagration), which is important in formulating safety recommendations for ambient conditions different from standard. Linear correlations were also found between the maximum rates of pressure rise and initial pressure, in both vessels. The peak pressures, the maximum rates of pressure rise and the deflagration index of LPG–air have maximum at concentrations higher than stoichiometric in both enclosures. The extreme values of all explosion parameters of LPG–air mixtures are close to the characteristic values of propane– and n-butane–air, measured or calculated in identical initial conditions. Their values are needed for promoting a database of explosion parameters of pure and mixed fuels, as it is known that the composition of LPG varies from country to country. Fig. 5. The time necessary to reach the peak pressure in three closed vessels, at p0 = 101 kPa and T0 = 298 K.
For LPG–air, the highest value of the severity factor (deflagration index) is KG = 102 bar m/s in spherical vessel S and KG = 45 bar m/s, in cylindrical vessel C. The values of the deflagration index of LPG–air in cylindrical vessel C are quite close to the values reported for another cylindrical vessel in Ref. [7]; compared to them, the deflagration index of explosions in the spherical vessel is approximately two times higher, for every concentration. In both cylindrical vessels, the heat losses start earlier as compared to spherical vessel and reach important values much before combustion is finished. Accordingly, the rate of heat release is lower in cylindrical vessels as compared to spherical ones, leading to a “milder” explosive combustion and to lower rates of pressure rise and lower severity factors. Reference values from literature, for propane–air, are: KG = 100 bar m/s determined in a spherical vessel of volume V = 5 L [18]; KG = 76 bar m/s determined in a cylindrical vessel with volume V = 22 L and ˚/h = 1.12 [21]; KG = 150 bar m/s determined in a cylindrical vessel with volume V = 120 L and ˚/h = 0.85 [22]. Linear correlations were found between the severity factor and initial pressure of each flammable LPG–air mixture. 5. Conclusions The propagation of LPG–air explosions in two closed vessels with central ignition was studied, using LPG–air mixtures with variable composition, at various initial pressures. In explosions of quiescent mixtures in both vessels, the peak pressures are linear functions on total initial pressure, at constant initial temperature and fuel/oxygen ratio. The slope and intercept
Acknowledgements The results reported in the present contribution were partly financed by the Romanian Ministry of Education and Research through Grant 42/2005-2007 awarded to the Institute of Physical Chemistry Bucharest. References [1] S.Y. Liao, D.M. Jiang, Q. Cheng, J. Gao, Z.H. Huang, Y. Hu, Correlations for laminar burning velocities of liquefied petroleum gas–air mixtures, Energy Conv. Manage. 46 (2005) 3175–3184. [2] D. Mishra, A. Rahman, An experimental study of flammability limits of LPG/air mixtures, Fuel 82 (2003) 863–866. [3] K. Lee, J. Ryu, An experimental study of the flame propagation and combustion characteristics of LPG fuel, Fuel 84 (2005) 1116–1127. [4] S. Liao, D. Jiang, J. Gao, Z. Huang, Q. Cheng, Measurements of Markstein numbers and laminar burning velocities for liquefied petroleum gas–air mixtures, Fuel 83 (2004) 1281–1288. [5] A. Huzayyin, H. Moneib, M. Shehatta, A. Attia, Laminar burning velocity and explosion index of LPG–air and propane–air mixtures, Fuel 87 (2007) 39–57. [6] D. Razus, D. Oancea, V. Brinzea, M. Mitu, V. Munteanu, Experimental and computational study of flame propagation in propane–, n-butane– and liquefied petroleum gas–air mixtures, in: Proceedings of the 7th European Comb. Meeting, Chania, Greece, April 11–13, 2007, Paper VI-9. [7] K.H. Oh, H. Kim, J.B. Kim, S.E. Lee, A study on the obstacle-induced variation of the gas explosion characteristics, J. Loss Prev. Process Ind. 14 (2001) 597–602. [8] D. Razus, M. Mitu, V. Brinzea, D. Oancea, Pressure evolution during confined deflagration of n-butane/air mixtures, Rev. Chim. (Bucuresti) 58 (2007) 1170–1175. [9] V. Brinzea, D. Razus, D. Oancea, Temperature and pressure influence on the maximum rate of pressure rise and explosion index of closed-vessel propane–air explosions, in: Proceedings of the 32nd International Symposium on Combustion, Montreal, Canada, August, 2008, poster W4P198. [10] NFPA 68, Guide for Venting Deflagrations, 1998 Edition, National Fire Protection Association.
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[11] D. Razus, U. Krause, Empirical and semi-empirical calculation methods for venting of gas explosions, Fire Safety J. 36 (2001) 1–23. [12] D. Razus, D. Oancea, F. Chirila, N.I. Ionescu, Transmission of an explosion between linked vessels, Fire Safety J. 38 (2003) 147–163. [13] A. Benedetto, E. Salzano, G. Russo, Predicting pressure piling by semi-empirical correlations, Fire Safety J. 40 (2005) 282–298. [14] D. Razus, C. Movileanu, V. Brinzea, D. Oancea, Explosion pressures of hydrocarbon–air mixtures in closed vessels, J. Hazard. Mater. 135 (2006) 58–65. [15] D. Razus, C. Movileanu, D. Oancea, The rate of pressure rise of gaseous propylene–air explosions in spherical and cylindrical enclosures, J. Hazard. Mater. 139 (2007) 1–9. [16] D. Oancea, V. Gosa, N.I. Ionescu, D. Popescu, An experimental method for the measurement of adiabatic maximum pressure during an explosive gaseous combustion, Rev. Roumaine Chim. 30 (1985) 767–776.
[17] F. Van den Schoor, F. Norman, F. Verplaetsen, Influence of the ignition source location on the determination of the explosion pressure at elevated initial pressures, J. Loss Prev. Process Ind. 19 (2006) 459–462. [18] W. Bartknecht, G. Zwahlen, Explosionsschutz, Grundlagen und Anwendung, Springer-Verlag, Berlin, 1993 (Chapter 2). [19] E. Brandes, W. Möller, Sicherheitstechnische Kenngröen, Band 1: Brennbare Flüssigkeiten und Gase, Wirtschaftsverlag NW, Bremerhaven (2003). [20] Y. Tanaka, Numerical simulations for combustion of quiescent and turbulent mixtures in confined vessels, Comb. Flame 75 (1989) 123–138. [21] J. Senecal, P. Beaulieu, KG: new data and analysis, Proc. Saf. Prog. 17 (1998) 9–15. [22] K. Cashdollar, I. Zlochower, G. Green, R. Thomas, M. Hertzberg, Flammability of methane, propane, and hydrogen gases, J. Loss Prev. Process Ind. 13 (2000) 327–340.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Explosion characteristics of LPG–air mixtures in closed vessels Domnina Razus a,∗ , Venera Brinzea a , Maria Mitu a , D. Oancea b a b
“Ilie Murgulescu” Institute of Physical Chemistry, 202 Spl. Independentei, 060021 Bucharest, Romania Department of Physical Chemistry, University of Bucharest, 4-12 Regina Elisabeta Blvd., 030018 Bucharest, Romania
a r t i c l e
i n f o
Article history: Received 8 January 2008 Received in revised form 19 September 2008 Accepted 22 October 2008 Available online 28 October 2008 Keywords: LPG (liquefied petroleum gas) Combustion Closed vessel Explosion pressure Rate of pressure rise Explosion time
a b s t r a c t The experimental study of explosive combustion of LPG (liquefied petroleum gas)–air mixtures at ambient initial temperature was performed in two closed vessels with central ignition, at various total initial pressures within 0.3–1.3 bar and various fuel/air ratios, within the flammability limits. The transient pressure-time records were used to determine several explosion characteristics of LPG–air: the peak explosion pressure, the explosion time (the time necessary to reach the peak pressure), the maximum rate of pressure rise and the severity factor. All explosion parameters are strongly dependent on initial pressure of fuel–air mixture and on fuel/air ratio. The explosion characteristics of LPG–air mixtures are discussed in comparison with data referring to the main components of LPG: propane and butane, obtained in identical conditions. © 2008 Elsevier B.V. All rights reserved.
1. Introduction LPG is extensively used nowadays, both as alternative fuel in automotive engines and as domestic fuel. In comparison with conventional engine fuels (gasoline and diesel), LPG is considered an attractive alternative fuel since its combustion in air is characterized by reduced emissions of NOx , CO and unburned hydrocarbons [1]. A thorough analysis and prediction of engines’ and/or combustors’ performance requires a systematic investigation of explosion characteristics of LPG in air in various conditions. Values of flammability limits for LPG in air were reported by Mishra and Rahman, for a LPG blend containing 70% propane and 30% butane [2]. Flame propagation in LPG–air mixtures with various equivalence ratios, in operating conditions close to a heavy-duty LPG engine (initial pressures between 1.5 and 4 bar, initial temperatures between 330 and 380 K) was studied by Lee and Ryu [3]. The authors examined the pressure and temperature influence on flame propagation speed for LPG–air, in comparison with propane–air mixtures, well characterized in this respect. Burning velocities of LPG–air without or with exhaust gas addition are also reported by Liao et al. [1,4] from optical records of flame front position, in experiments made at vari-
∗ Corresponding author. Tel.: +40 21 316 79 12; fax: +40 21 312 11 47. E-mail addresses: [email protected], [email protected] (D. Razus), [email protected] (V. Brinzea), maria [email protected] (M. Mitu), [email protected] (D. Oancea). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.10.082
ous initial pressures and temperatures in a 1.57 L closed vessel with rectangular profile and central ignition. The authors discuss their results versus similar data referring to propane– and i-butane–air mixtures [4]. Other experiments, made in a 2.46 L cylindrical vessel with central ignition used LPG– and propane–air mixtures with variable initial composition, pressure and temperature [5]. Values of several explosion characteristics of closed vessel combustion (maximum explosion pressure, maximum rate of pressure rise, time to maximum pressure, time to maximum rate of pressure rise, ignition delay period, severity factors) are given for both fuels and examined against initial pressure, temperature and composition. For all explosion parameters, the dependencies on initial pressure, temperature and equivalence ratio are given both as diagrams and fit equations, able to be used for comparison with other sets of data. Burning velocities of propane–, n-butane– and LPG–air mixtures, determined from pressure-variation during the early stage of propagation in a closed spherical vessel with central ignition by means of cubic law coefficients, were recently reported for mixtures of variable initial composition and pressure [6]. The present work completes the data on burning velocities [6] with values of explosion parameters of LPG–air in two highly symmetrical closed vessels, at room temperature, at various initial pressures within 0.3–1.3 bar and various [LPG]/[O2 ] ratios: the peak (maximum) explosion pressures pmax , the times to peak pressures max , the maximum rates of pressure rise (dp/dt)max and the severity factors KG . LPG is a blend used in Romania for domestic use containing 12 vol.% propane, 87 vol.% butane, 1 vol.% pentane. The
D. Razus et al. / Journal of Hazardous Materials 165 (2009) 1248–1252
Nomenclature h K p R t V
height severity factor pressure radius time volume
Greek letters time to peak pressure ˚ diameter Subscripts G referring to gas explosions max maximum value 0 initial value
data referring to LPG–air will be examined against literature data on LPG combustion in closed vessels [5,7] and compared to data referring to propane–air and n-butane–air, obtained in identical conditions [8,9]. The measured explosion parameters of LPG–air, in a wide range of initial conditions and in various vessels, are needed for promoting a database of explosion parameters of pure and mixed fuels. At the same time, the peak explosion pressures, the maximum rates of pressure rise, the explosion times and the severity factors are important values for vent area design [10,11] and for characterization of explosion transmission between interconnected vessels [12,13]. 2. Experimental Experiments were made in two explosion vessels with central ignition, tight at vacuum and at pressures up to 2.5 MPa: vessel S: a spherical vessel with the radius R = 5 cm and vessel C: a cylinder with height h = 15 cm and diameter ˚ = 10 cm. The initial pressure of explosive mixtures was measured using a strain gauge manometer Edwards EPSA-10HM. In each vessel, ignition was made with an inductive-capacitive sparks produced between stainless steel electrodes with rounded tips, using a spark gap of 3.5 mm. The sparks have energies between 2 and 5 mJ and approximately 1 ms duration. Explosions were monitored by means of a fast response pressure transducer (Kistler 601A, connected to a Charge Amplifier Kistler 5001N) and an ionization probe with the tip mounted 3 mm far from wall, in every explosion vessel. The signals from the charge amplifier and from the ionization probe amplifier were recorded with a Tektronix acquisition data system (TestLab 2505, acquisition card 25AA1) at 104 signals/s. A vacuum and gas-feed line, tight at pressures from 0.05 to 150 kPa, connects the vacuum pump, the gas cylinders with fuel and air, the metallic cylinder for mixture storage and the explosion vessels. The vacuum pump maintains a vacuum of 0.05 kPa in the explosion vessel, after each experiment. The experimental procedure consisted of evacuating the combustion vessel down to 0.05 kPa; the fuel–air mixture was then introduced, allowed to become quiescent and ignited. Minimum three experiments were performed for each initial condition of explosive mixture. More details were recently given [14,15]. The fuel–air mixtures were prepared in stainless steel cylinders at a total pressure of 400 kPa, by partial pressure method and used 24 h after mixing the components.
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LPG (Arpechim—Romania) contained 12 vol.% propane, 62.6 vol.% n-butane, 24.4 vol.% i-butane, 1 vol.% pentane. The examined mixtures had LPG concentration between 2.50 and 6.15 vol.%. 3. Data evaluation The derivatives (dp/dt) were obtained after smoothing the experimental p(t) data by Savitsky–Golay method, based on least squares quartic polynomial fitting across a moving window within the data. In all cases, we used a 10% smoothing level. 4. Results and discussion For each LPG–air mixture, the maximum explosion pressures are linearly correlated with initial pressures, p0 , within the range 30–130 kPa. An illustrative diagram is given in Fig. 1, where several results obtained in the spherical vessel are plotted. Similar data were obtained for the cylindrical vessel. From such correlations it is possible to calculate the peak pressure at any value of initial pressure even beyond the investigated range (as long as the process takes place as a deflagration). For both vessels, the slope and intercept of the linear correlations depend on the initial composition and the characteristics of explosion vessel (volume, height/diameter ratio, position of the ignition source), as already observed for mixtures of other hydrocarbons (methane, propene, n-butane, n-pentane) with air [14]. The slopes of these correlations are numerically close to the adiabatic explosion pressure pmax,ad ; their intercepts depend on both the heat lost by the burning mixture to the vessel and the vessel’s volume [14–16]. Information concerning the amount of heat lost during flame propagation in a closed vessel is also given in a recent contribution of Van den Schoor et al. [17] focused on combustion of very rich methane–air mixtures (in the vicinity of the upper flammability limit): the explosion pressure measured in centrally ignited explosions is higher as compared to explosions asymmetrically initiated, by an ignition source positioned 6 cm under central position. Measured explosion pressures of LPG–air in spherical vessel S and in cylindrical vessel C at ambient initial conditions, together with explosion pressures reported by other authors from experiments in cylindrical vessels, are plotted versus LPG concentration in Fig. 2. Larger heat losses in asymmetrical vessel C as compared to vessel S account for the lower peak pressures measured in vessel C as compared to vessel S. The observed differences between peak pressures we measured and literature data can be assigned mainly to differences in volume and shape (length to diameter
Fig. 1. Maximum explosion pressures measured during deflagration of LPG–air mixtures in spherical vessel S with central ignition.
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Fig. 3. Maximum rates of pressure rise measured during deflagration of LPG–air mixtures in spherical vessel S with central ignition. Fig. 2. Maximum explosion pressures of LPG–air mixtures, in closed vessels, at p0 = 101 kPa and T0 = 298 K: (1) present data: spherical vessel S (V = 0.52 L); (2) present data: vessel C (V = 1.12 L); (3) Ref. [5]: cylindrical vessel (V = 2.46 L); (4) Ref. [7]: cylindrical vessel (V = 270 L).
ratio) of explosion vessels, but also to the different composition of the examined LPG blends. According to our measurements, the highest value of explosion pressure for LPG–air at p0 = 101 kPa and T0 = 298 K is 922 kPa, recorded in spherical vessel S; in vessel C, the highest explosion pressure is 853 kPa (central ignition in both vessels). Compared to this, Huzayyin et al. [5] measured the highest value of explosion pressure as 800 kPa, in a cylindrical vessel of volume V = 2.46 L (˚ = 144.5 mm; h = 150 mm) with central ignition, using a LPG blend formed from C3 H8 (26.41 vol.%), n-C4 H10 (47.22 vol.%), iC4 H10 (26.32 vol.%) and 0.05 vol.% higher hydrocarbons. Oh et al. [7] reported 680 kPa from measurements in a rectangular-profile vessel with V = 270 L (0.6 m × 0.45 m × 1 m) with side ignition, without mentioning the composition of used LPG. For the main components of LPG, the maximum peak pressures measured in vessel S were 910 kPa (propane) [9] and 925 kPa (n-butane) [8]. In vessel C, the maximum peak pressures were 827 kPa (propane) [9] and 860 kPa (n-butane) [8]. These values range well within reported data in literature: 890 kPa for propane–air and 900 kPa for nbutane–air measured in a 5-L spherical vessel with central ignition [18]; 940 kPa for both propane– and n-butane–air, measured in a 20-L spherical vessel with central ignition [19]; 840 kPa measured for propane–air in a 1-L cylindrical vessel (˚ = 90 mm; h = 45 mm) [20]. The maximum rates of pressure rise reached in the two explosion vessels S and C follow similar trends when examined against initial pressure and LPG concentration. The corresponding plots are given in Figs. 3 and 4. In both vessels, the highest rates of pressure rise are observed at LPG concentrations between 4.3 and 4.6 vol.%, the same concentration range where highest peak pressures were measured. This concentration range is quite the same for several LPG blends, as seen from Fig. 4 where results from several authors were plotted. The peak values of pressure rise rates in the diagrams of Fig. 4 vary between 25 and 128 MPa/s, due to the differences of experimental conditions: a spherical and a cylindrical vessel with central ignition—the present data; cylindrical vessel with central ignition—data from Ref. [5]; cylindrical vessel with side ignition—Ref. [7]. In cylindrical vessel C (a slightly elongated vessel, with ˚/h = 1.5) the recorded (dp/dt)max have lower values as compared to spherical vessel S due to larger heat losses before reaching the peak explosion pressure, at all examined composi-
tions. The highest measured pressure rise rates of LPG–air mixtures are 128 MPa/s in vessel S and 44 MPa/s in vessel C. Data referring to propane–air and n-butane–air mixtures, determined in identical experimental conditions, are given in Table 1. The explosion parameters characteristic to LPG values listed in Table 1 are slightly outside the region between propane and butane; some parameters characteristic to LPG are closer to those of propane and the others are closer to n-butane. The observed differences lie however within the range of standard errors, for each of the measured parameters. The lowest values of explosion time max (the time from ignition till the peak pressure) are recorded at LPG concentrations between 4.2 and 4.4 vol.%, for both explosion vessels and are also given in Table 1. In Fig. 5, the times to peak pressure from our measurements at ambient initial conditions are plotted together with data obtained for LPG–air explosions in a 2.46-L cylindrical vessel [7]. Examination of the present data showed that initial pressure variation, within 30 and 130 kPa, has little influence upon values of time to peak pressure, in both explosion vessels. From the maximum rates of pressure rise, the severity factors KG were calculated as: KG =
dp dt
max
√ 3 V
(1)
Fig. 4. Maximum rates of pressure rise of LPG–air explosions in closed vessels, at p0 = 101 kPa and T0 = 298 K: (1) present data: spherical vessel S (V = 0.52 L); (2) present data: vessel C (V = 1.12 L); (3) Ref. [5]: cylindrical vessel (V = 2.46 L); (4) Ref. [7]: cylindrical vessel (V = 270 L).
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Table 1 Average values and standard errors of explosion characteristic parameters for the most reactive fuel–air mixtures in vessels S and C, at p0 = 101 kPa and T0 = 298 Ka .
pmax (kPa) max (ms) (dp/dt)max (MPa/s) KG (bar m/s) pmax (kPa) max (ms) (dp/dt)max (MPa/s) KG (bar m/s) Concentration range (vol.%) for extreme values of flammability parameters a
C3 H8 [9]
n-C4 H10 [8]
LPG
Spherical vessel S 911 ± 12 25 ± 0.4 125 ± 5.0 100.7 ± 4.0
925 ± 12 23 ± 0.3 127 ± 5.5 102.3 ± 4.4
922 ± 13 26 ± 0.4 126 ± 5.5 101.5 ± 4.4
Cylindrical vessel C 827 ± 10 44 ± 0.5 43.6 ± 2.1 46.0 ± 0.2 4.0–4.5
860 ± 11 42 ± 0.5 41.2 ± 2.0 43.5 ± 0.2 3.6–4.3
853 ± 11 43 ± 0.5 43.0 ± 2.0 45.4 ± 0.2 4.2–4.7
Average values and standard errors were calculated from five experiments made in identical conditions.
of found correlations are influenced by the amount of heat losses from the burned gas to the vessel. The obtained correlations allow the calculation of peak pressure at any value of initial pressure even beyond the investigated range (as long as the process takes place as a deflagration), which is important in formulating safety recommendations for ambient conditions different from standard. Linear correlations were also found between the maximum rates of pressure rise and initial pressure, in both vessels. The peak pressures, the maximum rates of pressure rise and the deflagration index of LPG–air have maximum at concentrations higher than stoichiometric in both enclosures. The extreme values of all explosion parameters of LPG–air mixtures are close to the characteristic values of propane– and n-butane–air, measured or calculated in identical initial conditions. Their values are needed for promoting a database of explosion parameters of pure and mixed fuels, as it is known that the composition of LPG varies from country to country. Fig. 5. The time necessary to reach the peak pressure in three closed vessels, at p0 = 101 kPa and T0 = 298 K.
For LPG–air, the highest value of the severity factor (deflagration index) is KG = 102 bar m/s in spherical vessel S and KG = 45 bar m/s, in cylindrical vessel C. The values of the deflagration index of LPG–air in cylindrical vessel C are quite close to the values reported for another cylindrical vessel in Ref. [7]; compared to them, the deflagration index of explosions in the spherical vessel is approximately two times higher, for every concentration. In both cylindrical vessels, the heat losses start earlier as compared to spherical vessel and reach important values much before combustion is finished. Accordingly, the rate of heat release is lower in cylindrical vessels as compared to spherical ones, leading to a “milder” explosive combustion and to lower rates of pressure rise and lower severity factors. Reference values from literature, for propane–air, are: KG = 100 bar m/s determined in a spherical vessel of volume V = 5 L [18]; KG = 76 bar m/s determined in a cylindrical vessel with volume V = 22 L and ˚/h = 1.12 [21]; KG = 150 bar m/s determined in a cylindrical vessel with volume V = 120 L and ˚/h = 0.85 [22]. Linear correlations were found between the severity factor and initial pressure of each flammable LPG–air mixture. 5. Conclusions The propagation of LPG–air explosions in two closed vessels with central ignition was studied, using LPG–air mixtures with variable composition, at various initial pressures. In explosions of quiescent mixtures in both vessels, the peak pressures are linear functions on total initial pressure, at constant initial temperature and fuel/oxygen ratio. The slope and intercept
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