Experimental investigation of external explosion in the venting process

Experimental investigation of external explosion in the venting process 1 first-author †E-mail:duzhimin@sjtu.edu.cn †E-mail:duzhimin@sjtu.edu.cn duzhimin@sjtu.edu.cn 1 2 2 Department of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Department of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200030, China School of Powering Engineering, Nanjing University of Science and Technology, Nanjing 210094, China School of Powering Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Experimental investigations were conducted on the process of combustion and explosion vent in a 200 mm (diameter)×400 mm (length) vertical cylindrical vessel. When CH 4 -air mixture gases were used and the vent diameter was 55 mm, conditions of Φ (equivalent ratio)=0.8, Φ =1.0 and Φ =1.3 and two ignition positions (at the cylinder center and bottom) were selected. The venting processes and the correlated factors are discussed in this paper. Venting process CH 4 -air External explosion Correlated factors INTRODUCTION 1 Venting technology provides possible protective measures against the effect of explosions. The essential point is to release the high pressure through proper venting to ensure pressure vessel safety. In a combustion system, when abnormal pressure occurs and the venting orifice is opened, a lot of combustible mixtures venting from the vessel lead to a series of changes inside and outside the vessel, and may result in serious explosion disasters. During the venting process, the explosion which occurs outside the vessel is called external explosion. The combustible mixtures venting outside of the vessel are ignited by the jet flame and then led to external explosion. External explosion is the abnormal situation during the venting process. According to the study of Harrison and Eyre ( 1987 1987 ), external explosions can not only produce explosion disasters outside of the vessel, but also make the inside pressure of the vessel rise. In essence, external explosion is the transit from deflagration to detonation (DDT). So the DDT mechanism has received much attention in recent years. In order to study the mechanism of external explosions, many investigations (Catlin, 1991 1991 ; Carnasciali et al., 1991 1991 ; Wu and Swithenbank, 1992 1992 ) were concentrated on intensified experiment conditions. In those experiments, the venting vessel was connected with another large vessel full of combustible gas mixture, and the combustible mixture vented to the large vessel. The investigations were aimed at studying when and how the external explosions occur by measuring the pressure and displaying the flow field. Catlin ( 1991 1991 )’s study was typical. He found two impulse peaks of super pressure. The second impulse pressure peak was generated by DDT of combustible mixture outside the vessel. Holbrow et al.( 1996 1996 )’s study on the behavior of dust explosions in linked vented vessel systems revealed that the passage of flame from the primary ignition vessel could result in a secondary explosion producing a much higher pressure than expected from a vented single vessel. They suggested that the effects of secondary explosion were dependent on some factors such as the vent area, the pipe diameter, and so on. Thomas and Jones ( 2000 2000 ) employed a conventional wave tube to introduce a pre-ignited flame bubble from a circular pipe into a large vessel full of CH 4 -O 2 mixture. They used Schlieren photography to shoot the venting process, and the occurring process of explosion could be clearly observed from the pictures. Razus and Krause ( 2001 2001 ) suggested that the key problem in venting is the appropriate design of the vent area necessary for effective release of the material. They presented a review of different calculation methods, their ranges of validity, their physical background and applicability. Hu and Pu ( 2001 2001 ) conducted experimental investigations on explosion occurring at the vent in a 0.025 m 3 vertical cylindrical vessel. They used 4.1% propane and 9.5% methane air mixtures with central bottom spark ignition. When vent explosions occurred at different failure pressure through different vent areas, pressure histories were obtained and possible mechanisms for vent explosions were discussed. Since external explosion is a typical explosion disaster, it is necessary to study its mechanism. In this work, during the venting process of the experiment, external explosion was observed and its mechanism was studied. For a reactive system, however, the venting process is very complicated and is affected by many correlated factors such as Φ , ignition position, venting orifice diameter and failure pressure. These correlated factors are discussed below. EXPERIMENT SYSTEM 1 The experiment system (Fig. 1 1 ) includes venting vessel, gases mixing system, ignition device, pressure measurement and flow field display systems. The length of the vessel was 400 mm and its diameter was 200 mm. There was a series of orifices on the vessel for fitting sensors. One of the ends of the vessel was closed, while the other end was open with various area circular flanges where various thickness films were used to obturate the vessel. The methane and air in the mixing system were mixed in proportion in advance. The main measuring methods of the venting process experiment were pressure measurement and flow field measurement. A series of Kistler pressure sensors were used in the experiment (Table 1 1 ). And the deviation angle between all these sensors and the axis of the cylinder was 30° (Du, 2003 2003 ). The first sensor above the venting orifice was used as datum mark. The sensor signals were amplified and input to the JV5200 transient register. YA-16 high speed spark photograph system was used to get the images of the flow field. The position refers to the distance between the sensor and the vessel orifice Number Position * * Model Scale 1 Orifice 211M 500 PSI 2 100 mm 211B 50 PSI 3 200 mm 211B 50 PSI 4 300 mm 211B 50 PSI 5 400 mm 211B 50 PSI 6 Bottom 211M 1000 PSI 7 Foreside 211M 1000 PSI After the venting vessel was vacuumed, the methane and air mixture were introduced into the vessel. The initial pressure was set to the pressure of the environment. Then the combustible gases mixture was ignited. During the venting process, the pressure measurement system records the various pressure processes inside and outside the vessel, while the YA-16 photographic system shoots the images of the flow field. RESULTS AND DISCUSS 1 Normal venting process 2 The normal venting process without abnormal pressure should prevail during the venting process. The method to judge whether the venting process was normal was to examine whether there was only one pressure peak in the P - t curves and one wave in the pictures. For example, when Φ =0.8, initial pressure was 0.101325 MPa, venting diameter was 55 mm, ignition position was at the center of the vessel, and the failure pressure of the film was 0.3 MPa, external explosion did not occur as proven by comparing the P - t curves with the photos. The normal venting process is shown in Fig. 2 2 . External explosion 2 The typical external explosion process is different from the normal venting one. Fig. 3 3 shows that there are two pressure peaks in the P - t curves and two waves in the pictures. The first wave was the initial wave P 1 caused by the breaking of the orifice film. The second was the external explosion wave P 2 generated after venting. The experiment conditions of Fig. 3 3 were selected as follows: Φ =1.0, failure pressure=0.101325 MPa, the diameter of the venting orifice was set at 55 mm, and the ignition position was set at the vessel bottom. Next, the growing process of the explosion waves P 1 and P 2 was analyzed. The initial wave P 1 moved from the venting orifice to the outside space and reached the second, third, fourth and fifth sensor in 320 μs, 520 μs, 740 μs and 940 μs respectively. The external explosion wave P 2 that followed the initial explosion wave P 1 reached the second, third, fourth and fifth sensor in 400 μs, 580 μs, 820 μs and 1020 μs respectively. At the same time, the pressure of the external explosion wave P 2 was about 19.7 kPa, 21.5 kPa, 16.8 kPa and 10.6 kPa respectively. Different from the rapid attenuation of P 1 , the pressure of P 2 increased at first, and then began to attenuate after a few microseconds. This means that during the venting process, the reigniting outside the vessel may not only compensate for the energy attenuation, but also increase the pressure inside and outside the vessel. The curves and the pictures show that the time interval for the two explosion waves passing the same sensor was shortened, and was about 300 μs, 250 μs, 200 μs and 100 μs respectively when the explosion waves P 1 and P 2 passed by the second, third, fourth and fifth sensor. So it can be concluded that P 2 was chasing after P 1 . According to this trend, it is expected that the wave P 2 may possibly catch up on the initial wave P 1 to some extent. In addition, it can be concluded from the images and the curves that the third explosion wave can be found. Comparison of the second sensor P - t curve with the third, fourth and fifth sensor P - t curves showed the occurring process of the third external explosion P 3 . The second P - t curve showed that P 3 had not emerged, while the third sensor P - t curve showed that P 3 had formed clearly. So it can be concluded that P 3 was generated at the location between the second sensor and the third one. What was more, the pressure value of P 3 was only about 8 kPa when it reached the third sensor (shortly after its occurring), while it was about 30 kPa when it reached the fourth sensor. The pressure value of P 3 was increased by about four times. And then, P 3 began to attenuate. When it reached the fifth sensor, the pressure value of the explosion wave P 3 was 17 kPa. Explosion occurs at the orifice 2 During the venting experiments, besides the external explosion, another abnormal phenomenon that was observed was the explosion at the vessel orifice. The P - t curve of the first sensor (Fig. 4 4 ) showed that the value of the pressure at the venting orifice was about 5000 kPa. This condition was different from the normal venting condition and the external explosion condition. Correlated factors 2 1. Influence of Φ 3 There was no doubt that Φ affected the venting process. And various value of Φ may lead to different venting results. When the venting diameter was 55 mm, conditions ( Φ =1.0, Φ =0.8 and Φ =1.3) were investigated and the results are shown in Table 2 2 . For Φ =1.0, external explosions occurred 17 times (among 46 times) comprising thirty-seven percent. While for Φ =0.8, only one (among 7 times) external explosion occurred. Moreover, none (among 5 times) occurred when Φ =1.3. The statistic results showed that when Φ =1.0, the external explosion occurred easily, while for Φ =0.8/1.3, it was more difficult. Φ Experiment (times) Normal venting (times) External explosion (times) Orifice explosion (times) Others (times) 1.0 46 16 17 6 7 0.8 7 4 1 2 0 1.3 5 4 0 1 0 Fig. 5 5 and Fig. 6 6 are two examples of external explosion phenomenon when Φ =1.0. The curves and pictures in Fig. 5 5 showing two pressure peaks and two waves indicate occurrence of external explosion. Especially from Fig. 6 6 , the developing process of the external explosion wave can be observed clearly. 2. Influence of ignition position 3 Our experiments showed that the ignition position affected the venting process; and that various ignition positions led to different venting results. When Φ =1.0 and venting diameter was 55 mm, two kinds of conditions (vessel center ignition and bottom ignition) were investigated and the results are shown in Table 3 3 . For center ignition, external explosions occurred 6 times (among 30 times) comprising twenty percent. For bottom ignition, external explosions occurred 12 times (among 28 times) comprising forty-three percent. The statistical results showed that igniting at the vessel bottom, rather vessel center, led to external explosion easily. Ignition position Experiment (times) Normal venting (times) External explosion (times) Orifice explosion (times) Others (times) Vessel center 30 15 6 4 5 Vessel bottom 28 9 12 5 2 Fig. 7 7 is an example when the ignition position was selected at the vessel bottom. The curves and pictures showing two pressure peaks and waves indicate external explosion occurred. 3. Influence of failure pressure 3 Failure pressure would also affect the venting process. When Φ =1.0 and venting diameter was 55 mm, two kinds of conditions (low failure pressure and high failure pressure) were investigated and the results are shown in Table 4 4 . In the former condition, there were 6 times (among 17 times) when external explosion occurred, comprising thirty-five percent. In the latter condition, however, there was only one time (among 14 times) when external explosion occurred, comprising seven percent. The statistical results showed that low failure pressure, rather than high failure pressure, easily led to external explosion. Failure pressure Experiment (times) Normal venting (times) External explosion (times) Others (times) Low 17 8 6 3 High 14 7 1 6 Fig. 8 8 is an example when failure pressure was low. From the curves and pictures with two pressure peaks and waves, it can be concluded that external explosion occurred. CONCLUSION 1 External explosion is the abnormal condition during the venting process. The method to judge whether external explosion occurred is to examine whether there are two pressure peaks in the P - t curves and two explosion waves in the figures. The external explosion process observed in this experiment is discussed in this paper. Another abnormal phenomenon found was that the explosion occurred at the venting orifice. Some correlated factors (such as Φ , ignition position and failure pressure) affecting the venting process are discussed in this paper. Three conclusions were drawn: First, the venting process was remarkably influenced by Φ . When Φ equals 1.0, the external explosion would easily occur, but when Φ equals 0.8 and 1.3, the external explosion does not easily occur. Second, the ignition position influences the venting process. When the ignition point is at the bottom of the vessel, external explosion easily occurs. However, when the explosion is ignited at the center of the vessel, external explosion becomes relatively difficult. Finally, failure pressure affects the venting process. Experiments confirmed that low failure pressure, as compared with what was found for high failure pressure, would more easily lead to external explosion. In essence, when the failure pressure is low, as compared with the condition when failure pressure is high, more combustible mixture vents from the vessel to the outside space because the venting orifice is opened earlier. And the unburned combustible mixture can be reignited more easily and then leads to external explosion. References F Carnasciali , JHS Lee , R Knystautas , F Fineschi . Turbulent jet initiation of detonation . Combustion and Flame , 1991 . 84 170 - 180 . . Turbulent jet initiation of detonation CA Catlin . Scale effects on the external combustion caused by venting of a confined explosion . Combustion and Flame , 1991 . 83 399 - 411 . . Scale effects on the external combustion caused by venting of a confined explosion ZM Du . Study of External Explosion Phenomenon During the Venting Process , 2003 . 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