Vacuum chamber suppression of gas-explosion propagation in a tunnel

leonlineatwww.sciencedireJOURNAL OF CHINA UNIVERSITY OFScience dir(MININGTECHNOLOGYELSEVIERJ China Univ Mining Technol 18(2008)0337-0341www.clsevier.com/locate/jcumtVacuum chamber suppression of gas-explosionpropagation in a tunnelJIANG Shu-guang, WU Zheng-yan, LI Qing-hua, HE Xin-jian, SHAO Hao,QIN Jun-hui, WANG Lan-yun, HU Li-ming, LIN Bai-quanSchool of Safery Engineering, China Universiry of Mining Technology, Xuzhou, Jiangsu 221008, chinaAbstract: To control and reduce the harm of a gas explosion, a new method is proposed for suppressing gas-explosion propagationa tunnel by using a vacuum chamber. We studied the suppression effect on gas explosions by placing a vacuum chamber at dif-rent positions along the tunnel. The results indicate that: I)the vacuum chamber can absorb the explosion wave and explosionnergy as much as possible at the beginning of the gas explosion, and; 2)when the vacuum chamber is used the closer it is to theon source the more significant the suppression effect. In addition, by using the vacuum chamber: 1)the flame propagationvelocity decreases from ultrasonic to subsonic: 2)the flame propagation distance is remarkably shortened; 3)the maximumvalue of overpressure (Pm)decreases from 0.34 to 0. 17 MPa or less, and; 4)the impulse of the blast wave(n) decreases from 20 to 8kPa.s or lessKey words: vacuum chamber; liquid venting: flame quenching: pressure venting: impulse; explosion suppression1 IntroductionFive-Year Plan".However, the application of thesetechnologies introduces a significant workload in theQ For a long time gas explosions in the coal mines of form of equipment maintenance. As we all know, ex-hina have been the most serious problem affecting plosion suppression means not only suppression ofsafe production. Anti-explosion sheds, water tanks, flame propagation but also the suppression of the in-rock powder sheds, water bags and other tech- teraction between the flame and blast waves. So,niques- are widely applied in coal mines at home supported by the National Natural Science Founda-and abroad to control or reduce the harm from gas tion of China, the author, together with other colexplosions. Although these methods have played an leagues, has for the first time proposed a new methodmportant role in explosion prevention, they still have of suppressing gas explosions by using a vacuumsome shortcomings: 1) most of them act passively in chamberlo-7l. We have developed a vacuum chambersuppressing explosions and can only reduce the tem- that can be easily connected to the tunnel. a series ofperature of the flame; 2)the water in the water tanks experimental gas explosions were performed in aor the water bags is easily evaporated. Particularlysquare tunnel with a section area of 80 mmx80 mmmining districts with a large air volumes and high and a length of 17 mtemperatures, the water is easily mixed with dust re-To prove the suppression effect of the same vasulting in a failure to suppress flame propagation be- cuum chamber when connected to any side of thecause of the lack of water, and; 3)rock powders in tunnel, but at different positions, we selected me-the sheds will absorb moisture and become agglome- thane-air as the working medium. The gas explosionrated and unable to fly upwards to form a rock powd. experimental system developed by China Universityer mist, which may reduce or even prevent proper of Mining Technology was used to study and ana-performance for explosion-proofing. To overcome lyze the flame propagation velocity and the explosionthese disadvantages, automatic suppression technolo- pressure along the tunnel with or without the vacuumgies have been proposed and studied since the EighthReceived 12 January 2008: accepted 15 May 200中国煤化工ojects 50534090 and 50674090 supported by the National Natural Science FoundatCNMHtific and Technology Key Program of China, 2005CB221503 by the National Basic ReFoundation of China University of Mining&Correspondingauthor.Tel:+86-516-83885156E-mailaddress:jsguang@cumt.edu.cnJournal of China University of Mining& TechnologyVol, 18 No. 32 Experimental planperiments, to properly suppress gas-explosion propagation, we want the glass not to be broken when theThe experimental system is shown in Fig. 1vacuum is below 0. 1 MPa in the chamber. Otherwisewe want the glass to quickly break into pieces themoment the gas explodes. Many experiments provedthat a 4 mm thick glass with the same area can meetthese requirementsA cylindrical steel vacuum chamber 300 mm indiameter and 500 mm long was used in the experiments In practice, the vacuum chamber is connectedto the side of the tunnel to allow normal productionFig 1 Gas explosion experimental systemand ventilation in the tunnels(Fig. 2).system; 5. Vacuum chamber; 6. Gas7. Pressure test system; 8. Dynamic data acquisition system;9. Flame velocity test systemThe system mainly consists of a gas explosion tube,a vacuum chamber, a dynamic data collection andanalysis system, a flame velocity test system, a pres-sure test system, an air distribution system and thecombustible-gas ignition device. The dotted linerepresents a weak barrier, which is made of glass andso is very easily broken by an explosion, between thevacuum chamber and the tunnelosion tunneland the tunnelThe tunnel used in this experiment was 17 m inI Explosion tunnel; 2. Frangible cover: 3. Vacuum chamberlength with a cross section of 80 mmx80 mm During 2.3 Measuring explosion parametersan experiment we sealed the ignition source end andopened the outlet end of the tunnel. when evacuatingTwo important parameters, the flame propagationand filling the mixed gas into the tunnel we sealed the velocity and the explosion pressure, were measuredoutlet endwith a flame transducer(a photosensitive transistor)2.2 Vacuum chamberand a pressure transducer type CYG1402 A dynamicdata collation system type tsT3000 was also usedThe vacuum chamber was composed of a steel The signal output from the flame and pressure trans-shell, the frangible cover, a flange and some other ducers was connected to the TST3000 system. Theparts. One side of the chamber was sealed with a TST3000 system automatically processes and dis-flange and the other side was sealed with a plays the data according to preset conditions, such asself-designed flange holding the frangible cover, sampling length, sampling rate and so on. Synchronwhich is put in place during evacuation of the cham- ous sampling of the flame and pressure signals at theber. The frangible cover is the interface between the moment of the gas explosion was achieved by con-acuumchamber and the tunnel. The frangible cover necting pin 5 of the COM port of the TST3000 sysmust bear a certain pressure. Once the flammable tem to the signal pin of one flame transducer locatedmixed gas explodes in the tunnel the frangible cover very close to the ignition source. Once the flamebreaks immediately due to the overpressure on the transducer is triggered the other ones will also betunnel side and the vacuum on the other side. Burned triggered instantaneouslyand unburned gas, together with the increasing energy,Six pressure measurement points and eleven flameis quickly sucked into the vacuum chamber once the measurement points are located along the experimen-cover breaks( Call the process an effusion effect). tal tunnel. Experiments showed that the explosionThen the explosion can not propagatepower was so great that we could feel a slight shakingTempered glass is used for the frangible cover. The of the ground. The explosion pressure is much higherforce to break the glass depends on not only the ex- than 1. 2 MPa at a distance along the tunnel 10plosion pressure but also on the area and the thick from the ignition source when the methane concen-ness of the glass. Reference [8] indicates that: 1)for a trati3%. The pressurecertain thickness of glass the failure pressure increastran中国煤某某化二nt can read to 1.5es with a decrease in the area of the glass, and; 2)for MPC NMHGwere damaged andthe same area of glass the failure pressure of thick did notpressure exlass is higher than that of thinner glass. In these ex- ceeded their rating. To overcome this problem theJIANG Shu-et alVacuum chamber suppression of gas-explosion propagation in a tunneltransducers were only located along the tunnel further propagates at subsonic velocity and no outburst ofthan 10m from the ignition sourceflame is seen at the outlet. In addition the closer thecitor was used for ignition. The energy stored in the the flame propagation distance. The flame praterA high voltage spark from charge stored in a capa- vacuum chamber is to the ignition source the shecapacitor was 20-100 J. When the experimental sys- tion distance is shown in Table 1tem is ready and the 9.3% methane charge fills theWe conclude that the vacuum chamber can sup-tunnel the flame test system starts to work and the press flame propagation, shortening the propagationdata acquisition system sends its control signals. The distance of the flame and easily quenching the flameignition source is then ignited and the resulting ex when it is located very close to the ignition source.plosion parameters are measured.This is because: 1)the vacuum chamber can quicklysuck the burning and unburned gas from the tunnel3 Results and discussionafter the glass is broken by the explosive blast wave;2)the flame in the lower tunnel, from the glass plateTo study the explosion suppression effect of the to the outlet of the tunnel, is not well developed, and;vacuum chamber located at different positions expe- 3)the flame will die out in the upper tunnel, from theriments with or without the vacuum chamber were glass plate to the ignition source, since cold air keepscarried out. The chamber was located 2. 4. 7. ll or 14 going upward making it difficult to ignite the un-m from the ignition sourceburned gas3.1 Suppression effect3.2 Suppression effect of the vacuum chamberFig. 3 shows the flame propagation velocity versuson explosive blast wavesdistance along the tunnel with or without the vacuumThe damage from an explosion generally dependschamber. The methane concentration was 9.3%. In on the loads on an object caused by the explosiveFig 3 the"NV"curve is the flame propagation veloc- impulse from the blast wave. The rules for damageity versus distance when there is no vacuum chamber from blast waves are currently thought to be: I)theconnected to the tunnel. The other curves are the re- overpressure rule; 2)the impulse rule, and; 3)thesults when the vacuum chamber was located at 2, 4. 7, overpressure-impulse rule(P-I rule).The overpres-Il or 14 m from the ignition source.sure rule ignores response time. Studies indicate thatconcerning object damage when t> 10T(r is the timedelay of the blast wave; T is the self-vibration period。.2mof the object). The impulse rule considers the effect07mof both overpressure and positive time delay. Alhouge impulse rule is more comprehensive thanthe overpressure rule, it ignores the fact that there is acritical pressure for object damage. If the pressure islower than the lowest critical pressure, then the objectwill not be damaged even with a long working time0414524539495645895105135144165and great impulse. Moreover, studies indicate that: 1)Distance from the ignition source location(m)different explosion sources result in different explo-Fig. 3 Flame propagation velocity versus distance with orithout the vacuum chambersion wave shapes; 2)damage for the same values of Imay be obviously different, and; 3) the specific im-TableI Flame and overpressure parameters with the vacuum pulse will play a crucial role in object damage onlyhamber located at different positions from the ignition sourcefor t s T/4. Obviously, the P-I rule is more reasonaDistance of famTime delay rangevacuum chamber(m) propagation(m)The area enclosed by the overpressure-time curve74-1is called 1. Explosive blast waves consist of both pos15.146.8itive and negative pressures. The impulse thus has226-502both a positive and a negative phase. The negativeI1104534.3-70.14pressure acts on destroyed objects causing even more40.8-903serious damages. If we can show that the vacuumchamber can decrease the damage from the positivechamber is not present the flame propagates at ultra- cse ofFrom Fig 3 we can see that: 1)when the vacuum phawe will conclude that the vacuum中国煤化工 ffect on gas explo-sonic velocity, the flame propagationvelocity reacheson the gas explo-2500 m/s, resulting in a detonation with the flame sionCNMH Gective of positivebursting out from the tunnel outlet, and; 2)when the overpressure and positive impulse. The pressure andvacuum chamber is connected to the tunnel the flame impulse mentioned below are the positive phase3.2.1 Suppression effect of the vacuum chamber on ratio of L/d>89)is more than 1.2 MPa. The wholehe explosion pressureexplosion lasts for 20 ms at this point. Obviously, theFigs. 4a to 4f show curves of overpressure versus explosion in the tunnel without the vacuum chambertime of a gas explosion in the tunnel with the vacuum will cause serious damagechamber at 2, 4, 7, ll or 14 m from the ignition From Figs. 4a to 4f we see that when the vacuumsource, or without any vacuum chamber. In Fig. 4 the chamber is connected to the tunnel the pm value at thepoints Pl to P6 are six measurement points along the same measurement point is very small, varying fromtunnel and Pm is the overpressure value(MPa). The 0.08 to 0. 16 MPa. Now the development of the gasthin lines are the overpressure versus time curves of explosion only shows the leading shock and thegas explosions without the vacuum chamber. The pressure-decay stages. However, the time delay of thethick lines are the data when the vacuum chamberoverpressure becomes longer when the vacuumpresent.chamber is far away from the ignition source. TheIn Fig. 4, the development of a gas explosion time delay of the overpressure with the vacuumwithout the vacuum chamber goes through three chamber at five different positions is shown in Tablestages:the pressure-growth stage, the high pressure 1. For similar pressure ranges the shorter the timeregion and the pressure-decay stage. The overpres- delay of the overpressure is, the better the observedsure lasts for about 40-95 ms. The Pm value at every suppression effect. Obviously, the suppression effectmeasurement point is greater than 0.3 MPa. The Pm will get better when the vacuum chamber is veryvalue at the measurement point P6(a length-radius close to the ignition source483[P30.084080000B00002690500750.100012501500365P00015002000.2500.30000s01001502005030Fig 4 Comparison of overpressure versus time for gas explosions with or without the vacuum chamberThe reason the vacuum chamber suppresses the the flalong the tunnel andexplosion is mainly the pressure decrease caused by the中国煤化工isow. Then thethe effusion effect. The reason explosion suppression pressdiffers for different locations of the vacuum chamber shortCNMHG reques a veryre intimately related to flame propagation characte- ber is far away from the ignition source the distanceristics. Near the ignition source the chamber quenches of flame quenching along the tunnel is shorter and theJIANG ShuVacuum chambeof gas-explosion propagation in a tunnelflame causes a rapid pressure increase. Accordingly, quenching along the tunnel is as great as 3/4 of thethe pressure decrease stage takes a longer time to oc- tunnel length. And the distance becomes greater whenthe vacuum chamber is very close to the ignition3. 2. 2 Effect of the vacuum chamber on explosive sourceblast waves2)The vacuum chamber suppresses the explosiveTable 2 shows I values along the tunnel without the blast wave. when there is no vacuum chamber Pm ofvacuum chamber or with it located at five different the explosive blast wave is as high as 1. 4 MPa and Ipositions along the tunnel: 2, 4, 7, Il or 14 m In Ta- values are generally about 20 kPa.s. After the vacuumble 2 it is clear that when there is no vacuum chamber chamber is connected to the tunnel, Pm of the blast isthe pressure at the end of the tunnel (UD=120) less than 0.16 MPa and the I values are less than 8reaches a very high value in a short time because of kPa s. The attenuation of explosion power will bedetonation. The value I at this measurement point is enhanced when the vacuum chamber is close to thejust about 6 kPa.S, and I values at the other measuregnition source.ment points are typically about 20 kPas. When thevacuum chamber is connected to the tunnel at the 2 m Acknowledgementsposition the I values are the smallest observed. Theyrange from 0.5 to 0.8 kPas. The value I at every The work described in this paper is a part of themeasurement point increases as the vacuum chamber project, The basic research of gas disaster evolutionis moved to other positions along the tunnel. But I is and prevention in the coal mine, and,'Research onalways smaller than without the presence of the va- the suppression mechanism of the metal mesh con-struction to the coupling effect between the gas exTable 2 Comparisons of the values in the tunnel with or plosion flame and the pressure wave, which is sup.ported by the National Science Foundation of ChinaI m14 mReferences2022.140.522.172.106435833022l60[1 Zhou D B. Passive water bag shed and its application.L94222Coal Engineer, 1993(2): 1-8.(In Chinese)1357220106[2] Fei G Y, Wang C. Waterproof rock powe1.745.21pressing coal dust explosion and propagation. Coal Er720993.33gineer, 1995(6): 10-11, 24(In Chinese)082[3] Lu SX, He J, Yu C H, et al. Mechanism of gas explosionsuppression by water. Journal of China Coal Society,Based on the P-I rule, we can conclude that: 1)the1998,23(4):417-421.( In Chinessuppression effect on the gas explosion is very ob[4] Pan F. Application on isolation(suppression)apparatusvious; 2)the attenuation of the explosive power isfor gas explosion in Pingdingshan coalfield. Safery inCoal Mines, 2005, 36(8): 15-18.( In Chinese)greatest when the vacuum chamber is connected to[5] Cai Z Q, Xia ZZ, Liao J Q. Research on model ZYB-Sthe tunnel 2 m from the ignition source; 3)the attenautomatic explosion suppressing unit of real time airuation of the explosive power decreases as the dis-generation type. Mining Safery Environmental Protectance between the vacuum chamber and the ignition (6) Jiang s ou x M, Wu Z Y. Method and Apparaus totunnel with the vacuum chamber present is much lessCombustible Materials Explosion Suppression, Chinesepatent:200710020127X,2007.85.(lnCthan without it[7] Wu Z Y. Study on the Vacuum Cavity Suppressing GasThe reasons are: 1)the flame in the lower tunnelExplosion IPh. D dissertation]. Xuzhou: China Universityfrom the glass plate to the end is not well developed [8] Wang B X, Zhang Y H MaK x Influences of the ventof Mining and Technology, 2007. (In Chinese)and the flame goes out, and; 2)the flame in the uppertunnel from the glass plate to the ignition source diesout because cold gas flows upwardChemical Engineering Design, 1992(2): 36-39.(In Chi-ene4 Conclusionswaves. Science and Technology of Labour Protection,1999,193):38-41.( In Chinese)1)The vacuum chamber has a quenching effect on [101 Yu d M. The Appraisal of Major Risk Sources and Re-flame propagation. When there is no vacuum cham-searchesSeverity of Fire and Explosion Accidentber a detonation will take place and the flame spreads[Phnl. Beiiing: Beiiing Institute of Tech中国煤化工along the whole tunnel bursting from the outlet. [lllWhen the vacuum chamber has been connected to theCNMHGIPhD dism2仙side of the tunnel the flame will propagate at subsonicvelocity and then quench. The distance of flame