- Email: [email protected]
Experimental investigation of gas explosion in single vessel and connected vessels
Journal of Loss Prevention in the Process Industries xxx (2013) 1e6
Contents lists available at SciVerse ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Experimental investigation of gas explosion in single vessel and connected vessels Z.R. Wang*, M.Y. Pan, J.C. Jiang Jiangsu Key Laboratory of Urban and Industrial Safety, Institute of Safety Engineering, Nanjing University of Technology, Nanjing 210009, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2012 Received in revised form 16 April 2013 Accepted 17 April 2013
Gas explosion in connected vessels usually leads to high pressure and high rate of pressure increase which the vessels and pipes can not tolerate. Severe human casualties and property losses may occur due to the variation characteristics of gas explosion pressure in connected vessels. To determine gas explosion strength, an experimental testing system for methane and air mixture explosion in a single vessel, in a single vessel connected a pipe and in connected vessels has been set up. The experiment apparatus consisted of two spherical vessels of 350 mm and 600 mm in diameter, three connecting pipes of 89 mm in diameter and 6 m in length. First, the results of gas explosion pressure in a single vessel and connected vessels were compared and analyzed. And then the development of gas explosion, its changing characteristics and relevant influencing factors were analyzed. When gas explosion occurs in a single vessel, the maximum explosion pressure and pressure growth rate with ignition at the center of a spherical vessel are higher than those with ignition on the inner-wall of the vessel. In conclusion, besides ignition source on the inner wall, the ignition source at the center of the vessels must be avoided to reduce the damage level. When the gas mixture is ignited in the large vessel, the maximum explosion pressure and explosion pressure rising rate in the small vessel raise. And the maximum explosion pressure and pressure rising rate in connected vessels are higher than those in the single containment vessel. So whenever possible, some isolation techniques, such as fast-acting valves, rotary valves, etc., might be applied to reduce explosion strength in the integrated system. However, when the gas mixture is ignited in the small vessel, the maximum explosion pressures in the large vessel and in the small vessel both decrease. Moreover, the explosion pressure is lower than that in the single vessel. When gas explosion happens in a single vessel connected to a pipe, the maximum explosion pressure occurs at the end of the pipe if the gas mixture is ignited in the spherical vessel. Therefore, installing a pipe into the system can reduce the maximum explosion pressure, but it also causes the explosion pressure growth rate to increase. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Gas explosion Single vessel Connected vessels Explosion pressure Influencing factors
1. Introduction Connected vessels are commonly used for industrial processes, especially in the chemical and petrochemical industries, where pressurized vessels and equipment are usually connected with pipelines. These systems include various types of reactor, chemical containers, apparatus or process units (Singh, 1994). However, when gas explosion initially occurs in one vessel, which is the primary vessel, the pressure wave can be transmitted through pipelines to the adjacent vessel, which is the secondary vessel. As a result, secondary gas explosion may then occur in the secondary vessel resulting in more serious damage. In this condition, the gas
* Corresponding author. Tel./fax: þ86 25 83587423. E-mail addresses: [email protected], [email protected] (Z.R. Wang).
explosion strength in linked vessels is quite different from that in the single vessel. At present, there are national regulations and standards for gas explosion proofing and venting in pressurized single vessels. However, definite specifications on gas explosion proofing and venting in connected vessels remain unavailable (Holbrow, , Andrews, & Lunn, 1996). The main reason is limited researches on gas explosion in connected vessels and scared experimental data. Since 1981 gas explosion in connected vessels was researched (Bartknecht, 1981), much work has been developed and implemented to study the phenomenon of gaseous deflagration in integrated system (Bjerketvedt, Bakke, & Wingerden, 1997; Chen, 1999; Eckhoff, 2003; Hu, Pu, & Wan, 2001; Yu, Zhou, & Liu, 2004). Therefore, gas explosion testing apparatus for a sing vessel, a single connected to a pipe and connected vessels has been established to reveal different variation characteristics of gas explosion. The
0950-4230/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jlp.2013.04.007
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
2
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6
results serve as significant theoretical reference for guiding the safety design of explosion proofing, explosion venting and explosion suppression systems for connected vessels.
0.7
2. The testing system for gas explosion in single and connected vessels
0.5
The experimental apparatus for gas explosion in connected vessels is illustrated in Fig. 1, where two spherical vessels have respective diameters of 350 mm and 600 mm. The small vessel can also be used as a single vessel for the purpose of the experiment. The connecting pipe can be divided into three parts with each part of 89 mm in external diameter, 8 mm in thickness, and 2 m in length. The spherical vessel and connecting pipe are linked by welded flanges. Nozzles in spherical vessel and connecting pipe are used for setting pressure transmitters, vacuum manometers, spark plugs and a gas inlet and gas outlet on the spherical vessels. 2.2. Experimental measurement system A testing system was established to measure the explosion pressure of methane and air mixture. The measurement system consisted of a gas distribution system, an ignition device, a pressure measuring unit and a data acquisition and processing unit. In order to get the mixture of methane and air at particular concentrations, a distribution instrument named SY-9506 was used. First, a vacuum pump was used to take the air out of the experimental apparatus to a certain vacuum. Then the mixture was filled into the experimental apparatus through SY-9506. In order to mix the mixture and keep the air remained in the vessel or pipe, air booster pump named MPV02 was used for recycle mixing for 30 min. And then the gas mixture was kept still to allow the turbulence to dissipate for 12 h. Concurrently, CYG1401MF pressure transmitters were used to measure explosion pressure. A type of NP4-12 storage battery was employed as an electrical source for the high-tension spark plugs. Spark plugs of different lengths were used to ignite the gas mixture in the vessel. A type of JV5231 multi-channel data acquisition unit was adopted to collect synchronous data, and a data analysis software SignalView was used to process these data. 3. Gas explosion in single spherical vessel
Explosion pressure, MPa
2.1. Experimental apparatus
8% 9% 9.5% 10%
0.6
0.4
0.3
0.2
0.1
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Time after ignition, s
Fig. 2. Pressure versus time in the vessel at different gas concentrations.
maximum pressure and pressure increase rate occurred when the concentration of methane was 9.5%, which was slightly higher than the stoichiometric level of 9%. At this concentration, complete combustion of methane in the air happened which can release a large amount of heat. This also results in the highest explosion strength. Both lower maximum explosion pressures and lower maximum pressure rising rate occurred if the concentration of methane is other than 9%. 3.2. Gas explosion with different ignition locations Fig. 3 shows that both the maximum of pressure and maximum pressure increase rate in the center-ignited vessel was much higher than that in the inner-wall ignited vessel. Because of the cooling effect of the wall, the combustion rate of methane with inner-wall ignition is lower than that with center-ignition. 3.3. Gas explosion under different initial pressure Pressures versus time under different initial pressures are shown in Fig. 4. The initial pressures were 0.05 MPa, 0.02 MPa, 0 MPa and 0.1 MPa respectively. The higher the initial pressure is, the higher the maximum gas explosion pressure and the higher the maximum pressure increase rate. These results were consistent
The gas explosion experiment was carried out in a spherical vessel with a volume of 22 L, as depicted in Fig. 1. 0.6
3.1. Gas explosion with different gas concentrations 0.5
Explosion pressure, MPa
The trends in explosion pressure versus time given varying concentrations of methane (at 8%, 9%, 9.5%, 10% and 11%) are shown in Fig. 2. Explosion pressure and the pressure increase rate varied considerably with the changes in methane concentration. The
0.4
0.3
0.2
wall ignition center ignition
0.1
Fig. 1. The experimental testing system for gas explosion in connected vessels. A e spherical vessel (113 L); B e connecting pipe; C e spherical vessel (22 L); 1, 8 e gas inlet and outlet; 2, 7, 9, 10 e pressure transmitter; 3, 6 e spark plug; 4, 5 e vacuum manometer.
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Time after ignition, s
Fig. 3. Pressure versus time in the vessel with different ignition locations.
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6 1.0
0.8
-0.05MPa -0.02MPa 0MPa 0.1MPa
113L big vessel 22L small vessel
0.7 0.6
Explosion pressure, MPa
0.8
Explosion pressure, MPa
3
0.6
0.4
0.5 0.4 0.3 0.2
0.2
0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.0
Time after ignition, s
0.2
0.4
0.6
0.8
1.0
1.2
Time after ignition, s
Fig. 4. Pressure versus time in the vessel under different initial pressures.
with the theoretical analysis in the literature; “The final explosion pressure and pressure increase rate are proportional to the initial pressure” (Bartknecht, 1981). This might be caused by higher initial pressure which can lead to higher molecule density and shorter intermolecular distance. On this basis, the probability of a reaction among activated molecules will be greater and the reaction speed will be faster.
Fig. 6. Pressure versus time in the connected vessels with inner-wall ignition in the large vessel.
Gas explosion experiments in connected vessels were carried out in a unit consisting of two spherical vessels connected by a pipe that was 2 m in length, as shown in Fig. 1.
increase rates in the small vessel. The cube root law (Bartknecht, 1981) can be applied to single vessels, but not to connected vessels. To prevent such gas explosion, the most effective method is explosion isolation besides explosion-resistant structural design for vessels and pipelines. Flameproof equipment can be installed at the connecting joint of the vessel and the pipeline to prevent the flames in linked vessels from transmitting through the pipeline. The results also show that gas explosion develops laminar flow conditions at the preliminary stages which leads to lower pressure increasing rate. Therefore, the slow rates of combustion allow explosion proofing to prevent explosion transmission.
4.1. Comparison with gas explosion in a single vessel
4.2. The influence of ignition location
The comparison between gas explosion in connected vessels and a single vessel with inner-wall ignition under the same conditions is shown in Fig. 5. Both the maximum explosion pressure and pressure increase rate in connected vessels were quite different from those in the single vessel. The primary reason is the accelerated spread of the flame in the pipe. Turbulence in linked vessels can result in a violent combustion. Therefore, the gas explosion in connected vessels can cause higher pressure levels and pressure
The trends in explosion pressure versus time for different ignition locations are shown in Fig. 6 and Fig. 7. The figure demonstrated that explosion pressure was influenced by ignition locations. When the flammable gas mixture is ignited in the small vessel, the maximum pressure both in the large vessel and in the small vessel is lower than that in the single vessel. However when the gas mixture is ignited in the large vessel, the maximum explosion pressure and explosion rising rate in the small vessel is
4. Gas explosion in connected vessels
0.35 0.8
113L big vessel 22L small vessel
0.7
Explosion pressure, MPa
22L single vessel 0.6
Explosion pressure, MPa
113L big vessel 22L small vessel
0.30
0.5 0.4 0.3
0.25
0.20
0.15
0.10
0.2 0.05 0.1 0.0 0.0
0.00 0.0 0.2
0.4
0.6
0.8
1.0
1.2
0.2
0.4
0.6
0.8
1.0
Time after ignition, s
Time after ignition, s
Fig. 5. Pressure versus time in the single and connected vessels.
Fig. 7. Pressure versus time in the connected vessels with inner-wall ignition in the small vessel.
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6 0.7
0.8
0.6
0.7
0.08MPa 0.04MPa 0.02MPa 0MPa
0.5
Explosion pressure,MPa
Explosion pressure, MPa
4
0.4
0.3
0.2
0.6 0.5 0.4 0.3 0.2
0.1
0.0 0.0
9% 10% 11%
0.1
0.2
0.4
0.6
0.8
0.0 0.0
1.0
0.2
0.4
Time after ignition, s
0.6
0.8
1.0
1.2
Time after ignition, s
Fig. 8. Pressure versus time in the small vessel at different initial pressures.
Fig. 10. Pressure versus time in the small vessel at different gas concentrations.
higher than that in the single vessel. Therefore, ignition in the large vessel can result in much more intense explosion strength in connected vessels. Because when the ignition occurs at the large vessel, some degree of gas venting effect from the big vessel into the small vessel through the pipe takes place. Moreover, the acceleration of flame in the pipe and the turbulent jet combustion in the small vessel strengthens the explosion intensity. While, if the ignition is at the small vessel, the gas venting effect is not so obvious.
From Figs. 10 and 11, the second leap in explosion pressure was observed in connected vessels, which might be caused by explosion wave oscillation. Because of a bounce in the shock wave and its reflecting effects, the explosion pressure increases and then a second bounce in the shock wave develops.
4.3. The influence of initial pressure Fig. 8 and Fig. 9 demonstrated that the maximum explosion pressure increased both in the large vessel and in the small vessel if the initial pressure increased. Moreover, the maximum pressure increase rate also increased under such conditions. The reason for this phenomenon coincides with the explanation discussed in Section 3.3. 4.4. The influence of initial concentration
0.30
9% 10% 11%
0.25
Explosion pressure, MPa
Explosion pressure, MPa
Explosion pressures versus time in the spherical vessel and in the pipe with different initial pressures are shown in Fig. 12 and Fig. 13. Fig. 12 demonstrated that the maximum explosion pressures
0.08MPa 0.04MPa 0.02MPa 0MPa
0.5
0.4
0.3
0.2
0.20
0.15
0.10
0.05
0.1
0.0 0 .0
The style of the containment vessel was changed when the vessel was linked by a pipeline. Therefore, a study of the effects of the connecting pipeline was conducted. As shown in Fig. 1, a spherical vessel with a diameter of 350 mm was used in the experiment, and three pipes with 2 m in length and 89 mm in diameter were connected to the spherical vessel. Two data acquisition points were set in the installation, with one point in the spherical vessel and the other point at the end of the pipe. The concentration of methane in air was 9% and the initial pressures were 0.02 MPa, 0 MPa, 0.02 MPa, 0.05 MPa and 0.1 MPa respectively. 5.1. The influence of initial pressure
Fig. 10 and Fig. 11 show that maximum explosion pressure was obtained in both the large vessel and small vessel, if the concentration was the chemical equivalent concentration of 9%. This was different from the case shown in Section 3.1. 0.6
5. Gas explosion in the spherical vessel-pipeline
0 .2
0 .4
0 .6
0 .8
Time after ignition, s
Fig. 9. Pressure versus time in the large vessel at different initial pressures.
1 .0
0.00 0.0
0.2
0.4
0.6
0.8
1.0
Time after ignition, s
Fig. 11. Pressure versus time in the big vessel at different gas concentrations.
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6
Explosion pressure,MPa
0.8 Explosion pressure, MPa
0.4
-0.02MPa 0MPa 0.02MPa 0.05MPa 0.1MPa
1.0
0.6 0.4 0.2 0.0 -0.2 0.00
5
spherical vessel spherical vessel with a section of pipe spherical vessel with three sections of pipes
0.2
0.0
0.05
0.10
0.15
0.20
0.2
0.00
Time after ignition, s
0.05
0.10
0.15
0.20
Time after ignition,s Fig. 12. Pressure versus time in the spherical vessel with different initial pressures. Fig. 14. The influence of pipelines on explosion pressure in the spherical vessel.
in the spherical vessel were 0.17 MPa, 0.19 MPa, 0.25 MPa, 0.29 MPa and 1.07 MPa respectively. The maximum explosion pressure increases with the increment in initial pressure. When the initial pressure is 0.1 MPa, a much higher explosion pressure and explosion rising rate occurs in the spherical vessel. The maximum explosion pressure is lower than that in the single vessel when the initial pressure is lower than 0.05 MPa. This might be a result of the venting effect of the connecting pipe. From Fig. 13, explosion pressure oscillations occur at the end of the pipeline, with a large shock wave developing in the first two peaks where the maximum pressure values go up to 0.5 MPa, 0.65 MPa, 0.73 MPa, 0.81 MPa and 0.91 MPa respectively. When the initial pressure is 0.05 MPa or 0.1 MPa, the maximum pressure of the second explosion is greater than that of the initial explosion. This indicates that the value of the second pressure peak is higher than that of the first pressure peak when the initial pressure is higher than 0.05 MPa. In conclusion, the maximum explosion pressure in the pipe increases with the increment of the initial pressure, and the maximum explosion pressure occurs at the end of the pipeline. The reason for the phenomena is that the remaining unburnt gas is compressed by the shock wave to enter the pipeline, which results in a violent gas explosion. In addition, the combustion of the compressed unburnt gas mixture can accelerate the turbulent flow in the pipeline. 5.2. The influence of the connecting pipe Fig. 14 represents pressure changes versus time in a single spherical vessel without pipelines, a vessel connected by one 1.0
-0.02MPa 0MPa 0.02MPa 0.05MPa 0.1MPa
Explosion pressure,MPa
0.8
0.6
0.4
0.2
0.0 0.00
0.05
0.10
0.15
0.20
0.25
Time after igniton,s Fig. 13. Pressure versus time at the end of the pipe with different initial pressures.
pipeline and a vessel connected by three pipelines. The maximum pressures were 0.39 MPa, 0.16 MPa and 0.19 MPa respectively. For spherical vessels with additional pipelines, the maximum explosion pressure drops to 0.23 MPa and pressure wave oscillation also initiates. When the length of the pipeline is 6 m, a negative pressure occurs. Therefore, transmission and oscillation of pressure waves have a significant effect on explosion intensity. The total time to get to the maximum explosion pressure in the vessel without any additional pipeline is two to three times of that in the vessel connected with one or three additional pipelines. This indicates that the additional pipeline causes an acceleration of flame transition in the vessel. 6. Conclusions In the case of gas explosion in a single vessel, when gas concentration is slightly higher than the concentration of the stoichiometric ratio, the maximum explosion pressure and pressure increase rate occurs. The maximum explosion pressure and pressure growth rate with ignition at the center of a spherical vessel is higher than those with ignition on the inner-wall of the vessel. The higher the initial pressure is, the higher the maximum explosion pressure and pressure growth rate. In conclusion, besides ignition source on the inner wall, the ignition source at the center of the vessels must be avoided to reduce the damage level. In the case of gas explosion in connected vessels, when the gas mixture is ignited in the large vessel, the small vessel produces a higher explosion pressure and pressure increase rate. However, when the gas mixture is ignited in the small vessel, the maximum explosion pressure both in the large vessel and in the small vessel reduces. And the maximum explosion pressure in both vessels is lower than that in the single vessel. When the gas mixture is ignited in the large vessel, the maximum explosion pressure in both vessels raises with an increment in initial pressure. Whether the gas mixture is ignited in the large vessel, the maximum explosion pressure and pressure growth rate occurs when the gas concentration is consistent with the chemical equivalent concentration. Regardless of the position of ignition, the explosion pressure growth rate in the system is always higher than that in the single vessel. So whenever possible, some isolation techniques, such as fast-acting valves, rotary valves, etc., might be applied to reduce explosion strength in the integrated system. In the case of gas explosion in a single vessel connected to a pipe, due to the oscillation effects of the explosion shock wave, negative pressure occurs in the spherical vessel. When the initial
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
6
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6
pressure is lower than a certain value, the maximum explosion pressure in the system is low, which is even lower than that in the single vessel. The maximum explosion pressure occurs at the end of the pipeline. In conclusion, installing pipelines to a vessel can reduce the maximum explosion pressure in the vessel, but it also raises the explosion pressure growth rate.
Acknowledgments The authors are grateful for the support given by National Natural Science Foundation of China under Grant No. 50904037, and the Natural Science Foundation for Universities in Jiangsu Province under Grant Nos. 11KJA620001 and 10KJB620001.
References Bartknecht, W. (1981). Explosion course prevention protection. Berlin: Springer-Verlag. Bjerketvedt, D., Bakke, J. R., & Wingerden, K. V. (1997). Gas explosion handbook. Journal of Hazardous Materials, 52, 1e150. Chen, A. P. (1999). The experimental research of the flowing gas explosion in pipelines. Explode and Attack, 4, 347e352. Eckhoff, R. K. (2003). Dust explosions in process industries (1st ed.). (pp. 90e154) Burlington: Gulf Professional Publishing. Holbrow, P., Andrews, S., & Lunn, G. A. (1996). Dust explosion in interconnected vented vessels. Journal of Loss Prevention in the Process Industries, 1, 91e103. Hu, J., Pu, Y. K., & Wan, S. X. (2001). The experimental study of the process of the development of cylindrical container vent opening pressure. Explode and Attack, 1, 47e52. Singh, J. (1994). Gas explosions in inter-connected-vessels: pressure piling. In ChemE hazards XII symposium (pp. 19e21). Yu, J. L., Zhou, C., & Liu, R. J. (2004). The experimental research of premixed gas deflagrating process in pipelines. Petroleum and Natural Gas Chemical Engineering, 6, 453e457.
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
Contents lists available at SciVerse ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Experimental investigation of gas explosion in single vessel and connected vessels Z.R. Wang*, M.Y. Pan, J.C. Jiang Jiangsu Key Laboratory of Urban and Industrial Safety, Institute of Safety Engineering, Nanjing University of Technology, Nanjing 210009, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2012 Received in revised form 16 April 2013 Accepted 17 April 2013
Gas explosion in connected vessels usually leads to high pressure and high rate of pressure increase which the vessels and pipes can not tolerate. Severe human casualties and property losses may occur due to the variation characteristics of gas explosion pressure in connected vessels. To determine gas explosion strength, an experimental testing system for methane and air mixture explosion in a single vessel, in a single vessel connected a pipe and in connected vessels has been set up. The experiment apparatus consisted of two spherical vessels of 350 mm and 600 mm in diameter, three connecting pipes of 89 mm in diameter and 6 m in length. First, the results of gas explosion pressure in a single vessel and connected vessels were compared and analyzed. And then the development of gas explosion, its changing characteristics and relevant influencing factors were analyzed. When gas explosion occurs in a single vessel, the maximum explosion pressure and pressure growth rate with ignition at the center of a spherical vessel are higher than those with ignition on the inner-wall of the vessel. In conclusion, besides ignition source on the inner wall, the ignition source at the center of the vessels must be avoided to reduce the damage level. When the gas mixture is ignited in the large vessel, the maximum explosion pressure and explosion pressure rising rate in the small vessel raise. And the maximum explosion pressure and pressure rising rate in connected vessels are higher than those in the single containment vessel. So whenever possible, some isolation techniques, such as fast-acting valves, rotary valves, etc., might be applied to reduce explosion strength in the integrated system. However, when the gas mixture is ignited in the small vessel, the maximum explosion pressures in the large vessel and in the small vessel both decrease. Moreover, the explosion pressure is lower than that in the single vessel. When gas explosion happens in a single vessel connected to a pipe, the maximum explosion pressure occurs at the end of the pipe if the gas mixture is ignited in the spherical vessel. Therefore, installing a pipe into the system can reduce the maximum explosion pressure, but it also causes the explosion pressure growth rate to increase. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Gas explosion Single vessel Connected vessels Explosion pressure Influencing factors
1. Introduction Connected vessels are commonly used for industrial processes, especially in the chemical and petrochemical industries, where pressurized vessels and equipment are usually connected with pipelines. These systems include various types of reactor, chemical containers, apparatus or process units (Singh, 1994). However, when gas explosion initially occurs in one vessel, which is the primary vessel, the pressure wave can be transmitted through pipelines to the adjacent vessel, which is the secondary vessel. As a result, secondary gas explosion may then occur in the secondary vessel resulting in more serious damage. In this condition, the gas
* Corresponding author. Tel./fax: þ86 25 83587423. E-mail addresses: [email protected], [email protected] (Z.R. Wang).
explosion strength in linked vessels is quite different from that in the single vessel. At present, there are national regulations and standards for gas explosion proofing and venting in pressurized single vessels. However, definite specifications on gas explosion proofing and venting in connected vessels remain unavailable (Holbrow, , Andrews, & Lunn, 1996). The main reason is limited researches on gas explosion in connected vessels and scared experimental data. Since 1981 gas explosion in connected vessels was researched (Bartknecht, 1981), much work has been developed and implemented to study the phenomenon of gaseous deflagration in integrated system (Bjerketvedt, Bakke, & Wingerden, 1997; Chen, 1999; Eckhoff, 2003; Hu, Pu, & Wan, 2001; Yu, Zhou, & Liu, 2004). Therefore, gas explosion testing apparatus for a sing vessel, a single connected to a pipe and connected vessels has been established to reveal different variation characteristics of gas explosion. The
0950-4230/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jlp.2013.04.007
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
2
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6
results serve as significant theoretical reference for guiding the safety design of explosion proofing, explosion venting and explosion suppression systems for connected vessels.
0.7
2. The testing system for gas explosion in single and connected vessels
0.5
The experimental apparatus for gas explosion in connected vessels is illustrated in Fig. 1, where two spherical vessels have respective diameters of 350 mm and 600 mm. The small vessel can also be used as a single vessel for the purpose of the experiment. The connecting pipe can be divided into three parts with each part of 89 mm in external diameter, 8 mm in thickness, and 2 m in length. The spherical vessel and connecting pipe are linked by welded flanges. Nozzles in spherical vessel and connecting pipe are used for setting pressure transmitters, vacuum manometers, spark plugs and a gas inlet and gas outlet on the spherical vessels. 2.2. Experimental measurement system A testing system was established to measure the explosion pressure of methane and air mixture. The measurement system consisted of a gas distribution system, an ignition device, a pressure measuring unit and a data acquisition and processing unit. In order to get the mixture of methane and air at particular concentrations, a distribution instrument named SY-9506 was used. First, a vacuum pump was used to take the air out of the experimental apparatus to a certain vacuum. Then the mixture was filled into the experimental apparatus through SY-9506. In order to mix the mixture and keep the air remained in the vessel or pipe, air booster pump named MPV02 was used for recycle mixing for 30 min. And then the gas mixture was kept still to allow the turbulence to dissipate for 12 h. Concurrently, CYG1401MF pressure transmitters were used to measure explosion pressure. A type of NP4-12 storage battery was employed as an electrical source for the high-tension spark plugs. Spark plugs of different lengths were used to ignite the gas mixture in the vessel. A type of JV5231 multi-channel data acquisition unit was adopted to collect synchronous data, and a data analysis software SignalView was used to process these data. 3. Gas explosion in single spherical vessel
Explosion pressure, MPa
2.1. Experimental apparatus
8% 9% 9.5% 10%
0.6
0.4
0.3
0.2
0.1
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Time after ignition, s
Fig. 2. Pressure versus time in the vessel at different gas concentrations.
maximum pressure and pressure increase rate occurred when the concentration of methane was 9.5%, which was slightly higher than the stoichiometric level of 9%. At this concentration, complete combustion of methane in the air happened which can release a large amount of heat. This also results in the highest explosion strength. Both lower maximum explosion pressures and lower maximum pressure rising rate occurred if the concentration of methane is other than 9%. 3.2. Gas explosion with different ignition locations Fig. 3 shows that both the maximum of pressure and maximum pressure increase rate in the center-ignited vessel was much higher than that in the inner-wall ignited vessel. Because of the cooling effect of the wall, the combustion rate of methane with inner-wall ignition is lower than that with center-ignition. 3.3. Gas explosion under different initial pressure Pressures versus time under different initial pressures are shown in Fig. 4. The initial pressures were 0.05 MPa, 0.02 MPa, 0 MPa and 0.1 MPa respectively. The higher the initial pressure is, the higher the maximum gas explosion pressure and the higher the maximum pressure increase rate. These results were consistent
The gas explosion experiment was carried out in a spherical vessel with a volume of 22 L, as depicted in Fig. 1. 0.6
3.1. Gas explosion with different gas concentrations 0.5
Explosion pressure, MPa
The trends in explosion pressure versus time given varying concentrations of methane (at 8%, 9%, 9.5%, 10% and 11%) are shown in Fig. 2. Explosion pressure and the pressure increase rate varied considerably with the changes in methane concentration. The
0.4
0.3
0.2
wall ignition center ignition
0.1
Fig. 1. The experimental testing system for gas explosion in connected vessels. A e spherical vessel (113 L); B e connecting pipe; C e spherical vessel (22 L); 1, 8 e gas inlet and outlet; 2, 7, 9, 10 e pressure transmitter; 3, 6 e spark plug; 4, 5 e vacuum manometer.
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Time after ignition, s
Fig. 3. Pressure versus time in the vessel with different ignition locations.
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6 1.0
0.8
-0.05MPa -0.02MPa 0MPa 0.1MPa
113L big vessel 22L small vessel
0.7 0.6
Explosion pressure, MPa
0.8
Explosion pressure, MPa
3
0.6
0.4
0.5 0.4 0.3 0.2
0.2
0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.0
Time after ignition, s
0.2
0.4
0.6
0.8
1.0
1.2
Time after ignition, s
Fig. 4. Pressure versus time in the vessel under different initial pressures.
with the theoretical analysis in the literature; “The final explosion pressure and pressure increase rate are proportional to the initial pressure” (Bartknecht, 1981). This might be caused by higher initial pressure which can lead to higher molecule density and shorter intermolecular distance. On this basis, the probability of a reaction among activated molecules will be greater and the reaction speed will be faster.
Fig. 6. Pressure versus time in the connected vessels with inner-wall ignition in the large vessel.
Gas explosion experiments in connected vessels were carried out in a unit consisting of two spherical vessels connected by a pipe that was 2 m in length, as shown in Fig. 1.
increase rates in the small vessel. The cube root law (Bartknecht, 1981) can be applied to single vessels, but not to connected vessels. To prevent such gas explosion, the most effective method is explosion isolation besides explosion-resistant structural design for vessels and pipelines. Flameproof equipment can be installed at the connecting joint of the vessel and the pipeline to prevent the flames in linked vessels from transmitting through the pipeline. The results also show that gas explosion develops laminar flow conditions at the preliminary stages which leads to lower pressure increasing rate. Therefore, the slow rates of combustion allow explosion proofing to prevent explosion transmission.
4.1. Comparison with gas explosion in a single vessel
4.2. The influence of ignition location
The comparison between gas explosion in connected vessels and a single vessel with inner-wall ignition under the same conditions is shown in Fig. 5. Both the maximum explosion pressure and pressure increase rate in connected vessels were quite different from those in the single vessel. The primary reason is the accelerated spread of the flame in the pipe. Turbulence in linked vessels can result in a violent combustion. Therefore, the gas explosion in connected vessels can cause higher pressure levels and pressure
The trends in explosion pressure versus time for different ignition locations are shown in Fig. 6 and Fig. 7. The figure demonstrated that explosion pressure was influenced by ignition locations. When the flammable gas mixture is ignited in the small vessel, the maximum pressure both in the large vessel and in the small vessel is lower than that in the single vessel. However when the gas mixture is ignited in the large vessel, the maximum explosion pressure and explosion rising rate in the small vessel is
4. Gas explosion in connected vessels
0.35 0.8
113L big vessel 22L small vessel
0.7
Explosion pressure, MPa
22L single vessel 0.6
Explosion pressure, MPa
113L big vessel 22L small vessel
0.30
0.5 0.4 0.3
0.25
0.20
0.15
0.10
0.2 0.05 0.1 0.0 0.0
0.00 0.0 0.2
0.4
0.6
0.8
1.0
1.2
0.2
0.4
0.6
0.8
1.0
Time after ignition, s
Time after ignition, s
Fig. 5. Pressure versus time in the single and connected vessels.
Fig. 7. Pressure versus time in the connected vessels with inner-wall ignition in the small vessel.
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6 0.7
0.8
0.6
0.7
0.08MPa 0.04MPa 0.02MPa 0MPa
0.5
Explosion pressure,MPa
Explosion pressure, MPa
4
0.4
0.3
0.2
0.6 0.5 0.4 0.3 0.2
0.1
0.0 0.0
9% 10% 11%
0.1
0.2
0.4
0.6
0.8
0.0 0.0
1.0
0.2
0.4
Time after ignition, s
0.6
0.8
1.0
1.2
Time after ignition, s
Fig. 8. Pressure versus time in the small vessel at different initial pressures.
Fig. 10. Pressure versus time in the small vessel at different gas concentrations.
higher than that in the single vessel. Therefore, ignition in the large vessel can result in much more intense explosion strength in connected vessels. Because when the ignition occurs at the large vessel, some degree of gas venting effect from the big vessel into the small vessel through the pipe takes place. Moreover, the acceleration of flame in the pipe and the turbulent jet combustion in the small vessel strengthens the explosion intensity. While, if the ignition is at the small vessel, the gas venting effect is not so obvious.
From Figs. 10 and 11, the second leap in explosion pressure was observed in connected vessels, which might be caused by explosion wave oscillation. Because of a bounce in the shock wave and its reflecting effects, the explosion pressure increases and then a second bounce in the shock wave develops.
4.3. The influence of initial pressure Fig. 8 and Fig. 9 demonstrated that the maximum explosion pressure increased both in the large vessel and in the small vessel if the initial pressure increased. Moreover, the maximum pressure increase rate also increased under such conditions. The reason for this phenomenon coincides with the explanation discussed in Section 3.3. 4.4. The influence of initial concentration
0.30
9% 10% 11%
0.25
Explosion pressure, MPa
Explosion pressure, MPa
Explosion pressures versus time in the spherical vessel and in the pipe with different initial pressures are shown in Fig. 12 and Fig. 13. Fig. 12 demonstrated that the maximum explosion pressures
0.08MPa 0.04MPa 0.02MPa 0MPa
0.5
0.4
0.3
0.2
0.20
0.15
0.10
0.05
0.1
0.0 0 .0
The style of the containment vessel was changed when the vessel was linked by a pipeline. Therefore, a study of the effects of the connecting pipeline was conducted. As shown in Fig. 1, a spherical vessel with a diameter of 350 mm was used in the experiment, and three pipes with 2 m in length and 89 mm in diameter were connected to the spherical vessel. Two data acquisition points were set in the installation, with one point in the spherical vessel and the other point at the end of the pipe. The concentration of methane in air was 9% and the initial pressures were 0.02 MPa, 0 MPa, 0.02 MPa, 0.05 MPa and 0.1 MPa respectively. 5.1. The influence of initial pressure
Fig. 10 and Fig. 11 show that maximum explosion pressure was obtained in both the large vessel and small vessel, if the concentration was the chemical equivalent concentration of 9%. This was different from the case shown in Section 3.1. 0.6
5. Gas explosion in the spherical vessel-pipeline
0 .2
0 .4
0 .6
0 .8
Time after ignition, s
Fig. 9. Pressure versus time in the large vessel at different initial pressures.
1 .0
0.00 0.0
0.2
0.4
0.6
0.8
1.0
Time after ignition, s
Fig. 11. Pressure versus time in the big vessel at different gas concentrations.
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6
Explosion pressure,MPa
0.8 Explosion pressure, MPa
0.4
-0.02MPa 0MPa 0.02MPa 0.05MPa 0.1MPa
1.0
0.6 0.4 0.2 0.0 -0.2 0.00
5
spherical vessel spherical vessel with a section of pipe spherical vessel with three sections of pipes
0.2
0.0
0.05
0.10
0.15
0.20
0.2
0.00
Time after ignition, s
0.05
0.10
0.15
0.20
Time after ignition,s Fig. 12. Pressure versus time in the spherical vessel with different initial pressures. Fig. 14. The influence of pipelines on explosion pressure in the spherical vessel.
in the spherical vessel were 0.17 MPa, 0.19 MPa, 0.25 MPa, 0.29 MPa and 1.07 MPa respectively. The maximum explosion pressure increases with the increment in initial pressure. When the initial pressure is 0.1 MPa, a much higher explosion pressure and explosion rising rate occurs in the spherical vessel. The maximum explosion pressure is lower than that in the single vessel when the initial pressure is lower than 0.05 MPa. This might be a result of the venting effect of the connecting pipe. From Fig. 13, explosion pressure oscillations occur at the end of the pipeline, with a large shock wave developing in the first two peaks where the maximum pressure values go up to 0.5 MPa, 0.65 MPa, 0.73 MPa, 0.81 MPa and 0.91 MPa respectively. When the initial pressure is 0.05 MPa or 0.1 MPa, the maximum pressure of the second explosion is greater than that of the initial explosion. This indicates that the value of the second pressure peak is higher than that of the first pressure peak when the initial pressure is higher than 0.05 MPa. In conclusion, the maximum explosion pressure in the pipe increases with the increment of the initial pressure, and the maximum explosion pressure occurs at the end of the pipeline. The reason for the phenomena is that the remaining unburnt gas is compressed by the shock wave to enter the pipeline, which results in a violent gas explosion. In addition, the combustion of the compressed unburnt gas mixture can accelerate the turbulent flow in the pipeline. 5.2. The influence of the connecting pipe Fig. 14 represents pressure changes versus time in a single spherical vessel without pipelines, a vessel connected by one 1.0
-0.02MPa 0MPa 0.02MPa 0.05MPa 0.1MPa
Explosion pressure,MPa
0.8
0.6
0.4
0.2
0.0 0.00
0.05
0.10
0.15
0.20
0.25
Time after igniton,s Fig. 13. Pressure versus time at the end of the pipe with different initial pressures.
pipeline and a vessel connected by three pipelines. The maximum pressures were 0.39 MPa, 0.16 MPa and 0.19 MPa respectively. For spherical vessels with additional pipelines, the maximum explosion pressure drops to 0.23 MPa and pressure wave oscillation also initiates. When the length of the pipeline is 6 m, a negative pressure occurs. Therefore, transmission and oscillation of pressure waves have a significant effect on explosion intensity. The total time to get to the maximum explosion pressure in the vessel without any additional pipeline is two to three times of that in the vessel connected with one or three additional pipelines. This indicates that the additional pipeline causes an acceleration of flame transition in the vessel. 6. Conclusions In the case of gas explosion in a single vessel, when gas concentration is slightly higher than the concentration of the stoichiometric ratio, the maximum explosion pressure and pressure increase rate occurs. The maximum explosion pressure and pressure growth rate with ignition at the center of a spherical vessel is higher than those with ignition on the inner-wall of the vessel. The higher the initial pressure is, the higher the maximum explosion pressure and pressure growth rate. In conclusion, besides ignition source on the inner wall, the ignition source at the center of the vessels must be avoided to reduce the damage level. In the case of gas explosion in connected vessels, when the gas mixture is ignited in the large vessel, the small vessel produces a higher explosion pressure and pressure increase rate. However, when the gas mixture is ignited in the small vessel, the maximum explosion pressure both in the large vessel and in the small vessel reduces. And the maximum explosion pressure in both vessels is lower than that in the single vessel. When the gas mixture is ignited in the large vessel, the maximum explosion pressure in both vessels raises with an increment in initial pressure. Whether the gas mixture is ignited in the large vessel, the maximum explosion pressure and pressure growth rate occurs when the gas concentration is consistent with the chemical equivalent concentration. Regardless of the position of ignition, the explosion pressure growth rate in the system is always higher than that in the single vessel. So whenever possible, some isolation techniques, such as fast-acting valves, rotary valves, etc., might be applied to reduce explosion strength in the integrated system. In the case of gas explosion in a single vessel connected to a pipe, due to the oscillation effects of the explosion shock wave, negative pressure occurs in the spherical vessel. When the initial
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007
6
Z.R. Wang et al. / Journal of Loss Prevention in the Process Industries xxx (2013) 1e6
pressure is lower than a certain value, the maximum explosion pressure in the system is low, which is even lower than that in the single vessel. The maximum explosion pressure occurs at the end of the pipeline. In conclusion, installing pipelines to a vessel can reduce the maximum explosion pressure in the vessel, but it also raises the explosion pressure growth rate.
Acknowledgments The authors are grateful for the support given by National Natural Science Foundation of China under Grant No. 50904037, and the Natural Science Foundation for Universities in Jiangsu Province under Grant Nos. 11KJA620001 and 10KJB620001.
References Bartknecht, W. (1981). Explosion course prevention protection. Berlin: Springer-Verlag. Bjerketvedt, D., Bakke, J. R., & Wingerden, K. V. (1997). Gas explosion handbook. Journal of Hazardous Materials, 52, 1e150. Chen, A. P. (1999). The experimental research of the flowing gas explosion in pipelines. Explode and Attack, 4, 347e352. Eckhoff, R. K. (2003). Dust explosions in process industries (1st ed.). (pp. 90e154) Burlington: Gulf Professional Publishing. Holbrow, P., Andrews, S., & Lunn, G. A. (1996). Dust explosion in interconnected vented vessels. Journal of Loss Prevention in the Process Industries, 1, 91e103. Hu, J., Pu, Y. K., & Wan, S. X. (2001). The experimental study of the process of the development of cylindrical container vent opening pressure. Explode and Attack, 1, 47e52. Singh, J. (1994). Gas explosions in inter-connected-vessels: pressure piling. In ChemE hazards XII symposium (pp. 19e21). Yu, J. L., Zhou, C., & Liu, R. J. (2004). The experimental research of premixed gas deflagrating process in pipelines. Petroleum and Natural Gas Chemical Engineering, 6, 453e457.
Please cite this article in press as: Wang, Z. R., et al., Experimental investigation of gas explosion in single vessel and connected vessels, Journal of Loss Prevention in the Process Industries (2013), http://dx.doi.org/10.1016/j.jlp.2013.04.007