Experimental study on gas explosion and venting process in interconnected vessels

A pilot scale interconnected vessels experiment system was established, and the closed and vented gas explosion characteristics in the system were studied, using 10% methane–air mixture. Regularity of pressure variation in vessels and flame propagation in linked pipes was analyzed. Furthermore, the effects of transmission style, ignition position, pipe length, and initial pressure on explosion severity were discussed. For the closed explosion: explosion in interconnected vessels presents strongly destructive power to secondary vessel, especially transmission from the big vessel to the small one; the worst ignition position is shifting from ignition in the interconnected pipe to the walls of the two vessels; as far as ignition in big vessel is concerned, the peak pressure in secondary vessel increases with the pipe length much faster than that for ignition in small vessel; the peak pressures in two vessels are approximate linear functions of initial pressure. For the vented explosion: the transmission style and interconnected pipe length have significant impacts on the effect of venting on the protection; in order to obtain the better venting effect, the use of a divergent interconnected pipe from the big vessel to the small one in industry is advised and it is necessary to reduce the interconnected pipe length as far as possible or install flame arrester in the interconnected pipe.     

A study on the obstacle-induced variation of the gas explosion characteristics

A study on the variation of the gas explosion characteristics caused by the built-in obstacles was conducted in enclosed/vented gas explosion vessels. It has been well known that the obstacles in pipes and long ducts would accelerate the flame propagation, and cause the transition from deflagration to detonation. In this study, the explosion characteristics and the flame behavior of vented explosions and constant-volume explosions were investigated. Experiments were carried out in a 270-liter and 36-liter hexahedron vessels filled with LPG–air mixture. The explosion characteristics of the gas mixture were determined by using a strain-responding pressure transducer. The flame behavior was recorded by using a high-speed video camera. The shape and the size of the obstacle, and the gas concentration, were adjusted in the experiments.

It can be seen from the experimental results that, instead of being accelerated, the flame propagation inside the explosion vessel is decelerated by the plate obstacles fixed at the bottom of the vessel. Also, the characteristics of the enclosed explosion are not so affected by the built-in obstacles as those of the vented explosion are. It is believed that the eddy-induced turbulence behind the obstacle decelerates the flame propagation.     

连通容器气体密闭爆炸与泄爆过程的实验研究

利用球型容器与管道组合,开展连通容器气体爆炸与泄爆实验,分析连通条件下,火焰在管道中的传播过程及其对起爆容器和传爆容器的压力影响。实验结果表明:连通容器气体爆炸中,火焰从起爆容器到传爆容器传播经历了一段不断加速,但加速度不断减小的过程;泄爆过程中,火焰传播过程与密闭爆炸时基本一致。管道中火焰加速传播,使得传爆容器的爆炸压力和强度相较于作为起爆容器时均明显增加,危险更大,采用与起爆容器相同的泄爆面积,无法满足对连通容器中传爆容器的泄爆。同时,泄爆是一个快速的能量泄放过程应选择合理的泄爆方式,防止二次危害。     

FA型高炉煤气管道火焰捕器研制

研制满足高炉煤气管道阻火的火焰捕器.内径88 mm、199 mm和305 mm组合爆炸管道模拟实验结果表明,FA型火焰捕器的阻火性能良好,满足高炉煤气管道阻火的技术要求,各项技术性能指标达到了设计要求.     

Experimental investigation of gas explosion in single vessel and connected vessels

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.     

The influence of inert gas on limiting experimental safe gap of fuel-air mixtures at various initial pressures

The Maximum Experimental Safe Gap (MESG) is an important criterion to assess the propagation of flames through small gaps. This safety-related parameter is used to classify the flammable gases and vapors in explosion groups, which are fundamental to constructional explosion protection. It is used both, for the safe design of flameproof encapsulated devices as well as for selecting flame arresters appropriate to the individual application. The MESG of a fuel is determined experimentally according to the standard ISO/IEC 80079-20-1:2017 at normal conditions (20 °C, 1.0 bar) with air as oxidizing gas. The aim of this work is to investigate the effect of inert gas addition on the MESG in order to assess the effectiveness of inertization in constructional explosion protection. The term limiting experimental safe gap (S_G) is used for the result of these measurements. The fuel-air mixtures (fuels: hydrogen, ethylene, propene, methane) used as representatives for the explosion groups in flame arrester testing were chosen and diluted with inert gas (nitrogen, carbon dioxide) before testing. The dependence of the limiting experimental safe gap on the total initial pressure, amount and nature of inert additive is discussed. The initial pressure was varied up to 2.0 bar to include increased pressure conditions used in flame arrester testing. Apart from the well-known reciprocal dependence on the initial pressure, the added inert gas results in an exponential increase of S_G. This effect depends on the inertizing potential of the gas and is therefore different with nitrogen and carbon dioxide. The ranking of the fuels is the same as with MESG. As a result, various mixtures of the same limiting experimental safe gap can now be chosen and tested with an individual flame arrester to prove the concept of a constant and device-related limiting safe gap. The work was funded by BG-RCI in Heidelberg (PTB grant number 37056).     

Investigations of flame front propagation between interconnected process vessels. Development of a new flame front propagation time prediction model

For the case where a dust or gas explosion can occur in a connected process vessel, it would be useful, for the purpose of designing protection measures and also for assessing the existing protection measures such as the correct placement, to have a tool to estimate the time for flame front propagation along the connecting pipe. Measurements of data from large-scale explosion tests in industrially relevant process vessels are reported. To determine the flame front propagation time, either a 1 m³ or a 4.25 m³ primary process vessel was connected via a pipe to a mechanically or pneumatically fed 9.4 m³ secondary silo. The explosion propagation started after ignition of a maize starch/air mixture in the primary vessel. No additional dust was present along the connecting pipe. Systematic investigations of the explosion data have shown a relationship between the flame front propagating time and the reduced explosion over-pressure of the primary explosion vessel for both vessel volumes. Furthermore, it was possible to validate this theory by using explosion data from previous investigations. Using the data, a flame front propagation time prediction model was developed which is applicable for:

• •gas and dust explosions up to a K value of 100 and 200 bar m s⁻¹, respectively, and a maximum reduced explosion over-pressure of up to 7 bar;
• •explosion vessel volumes of 0.5, 1, 4.25 and 9.4 m³, independent of whether they are closed or vented;
• •connecting pipes of pneumatic systems with diameters of 100–200 mm and an air velocity up to 30 m s⁻¹;
• •open ended pipes and pipes of interconnected vessels with a diameter equal to or greater than 100 mm;
• •lengths of connecting pipe of at least 2.5–7 m.

     

Effect of spark duration on explosion parameters of methane/air mixtures in closed vessels

The paper outlines an experimental study on influence of the spark duration and the vessel volume on explosion parameters of premixed methane–air mixtures in the closed explosion vessels. The main findings from these experiments are: For the weaker ignition the spark durations in the range from 6.5 μs to 40.6 μs had little impact on explosion parameters for premixed methane–air mixtures in the 5 L vessel or 20 L vessel; For the same ignitions and volume fractions of methane in air the explosion pressures and the flame temperatures in both vessels of 5 L and 20 L were approximately the same, but the rates of pressure rises in both vessels of 5 L and 20 L were different; The explosion indexes obtained from the measured pressure time histories for both vessels of 5 L and 20 L were approximately equal; For the weaker ignition with the fixed spark duration 45 μs the ignition energies in the range from 54 mJ to 430 mJ had little impact on the explosion parameters; For the same ignition and the volume fractions of methane in air, the vessel volumes had a significant impact on the flame temperatures near the vessel wall; The flame temperatures near the vessel wall decreased as the vessel volumes increased.     

初始压力对连通容器甲烷-空气混合物泄爆压力的影响

建立球形容器与管道、2个球形容器与管道组成的2种形式的连通容器试验装置,研究初始压力对连通容器甲烷-空气混合物泄爆压力的影响。结果表明:连通容器内泄爆超压随初始压力增加而增大,并与初始压力近似成线性关系;对于2个球形容器与管道组成的连通容器,起爆容器的泄爆超压始终小于传爆容器;泄爆方式和点火方式对连通容器泄爆超压有较大影响,大容器点火时,2个容器的泄爆压力差随初始压力增加而增大,但小容器点火时,2个容器的泄爆压力差随初始压力的增加变化较小;初始压力对不同结构和尺寸的连通容器的泄爆压力的影响不同,当令初始压力对大容器点火时,小容器内泄爆压力受影响最大,而当对单球形容器与管道组成的连通容器的小容器点火时,小容器内泄爆压力受影响最小。     

密闭空间内天然气混合及爆炸传播规律研究

为研究密闭容器内甲烷-空气不均匀分布对混合气体燃烧的影响,将数值模拟和实验相结合,发现在重力作用下混合气体浓度分布不均匀,长径比越大的容器,混合气体浓度分布梯度越大。混合气体浓度分布影响气体火焰传播规律。宏观浓度为5%的甲烷与空气混合后,容器上部甲烷浓度高于5%,在该处点火时非均匀混合甲烷-空气火焰传播较快,非均匀混合气体的爆炸压力比均匀混合气体压力上升快,且分层混合气体的超压峰值高于均匀混合气体的值。由于浓度分布不均匀,点火位置影响甲烷/空气火焰传播的规律。     

Effects of vacuum levels,carbon dioxide,and structural sizes of linked vessels on dump vessel vented

The method of explosion venting is widely used in industrial explosion-proof design due to its simple operation, economical and practical features. A dump vessel vented platform was built. By changing the vacuum level and the gas in the dump vessels and the structural size of linked vessels, the pressure in the explosion vessel and the dump vessel was compared, and the influencing factors of explosion venting investigated. The main conclusions are as follows: In the explosion venting process, the higher the vacuum in the dump vessel, the smaller the pressure peak of the explosion vessel and the dump vessel, and the faster the explosion pressure is lowered. When the dump vessel is under the same vacuum level and the gas in the dump vessel is CO₂, the maximum pressure of the explosion vessel and the dump vessel is less than the maximum pressure when the containment medium is air. Under the same vacuum condition, the larger the volume ratio of the dump vessel and the explosion vessel, the smaller the pressure peak of the explosion vessel, the faster the explosion pressure drops, and the volume of the dump vessel reaches or exceeds the explosion vessel. Increasing the volume ratio of the containment vessel to the explosion vessel facilitates protection of the explosion vessel and the containment vessel. Under the same vacuum condition, when the gas explosion in 113 L vessel vents into 22 L vessel, the longer the length of the pipe, the greater the maximum pressure in the spherical vessel. When the gas explosion in 22 L vessel vents into 113 L dump vessel, as the pipeline grows, the maximum pressure in the two vessels decreases, but the reduction is not significant. In practical application, it is recommended to use a vacuum of 0.08Mpa or more for the dump vessel vented, and the containment medium is CO_2.In terms of the structural size of the container, it is recommended that the ratio of the receiving container to the explosion container be as large as possible, and the pipe length be as long.     

A new phenomenological model of gas explosion based on characteristics of flame surface

This paper presents a new model of explosion propagation in a closed vessel. The foundation of the formulation is a sub-model of turbulent burning velocity based on the assumption that the burning velocity of a turbulent wrinkled flame can be determined from the flame surface. In addition, model development includes simple sub-models of heat transfer and free convection. In order to verify the physics, the model was utilized to simulate the explosion of a methane–air mixture in two different test vessels. The results obtained by use of this new model were compared with results obtained by use of the classical model. While the simulations showed that both are accurate, the new model presented in this paper (called “flame surface model” for simplicity) is more flexible and can easily accommodate sub-models of different phenomena that can play an important role in fuel–air explosions.     

Dust explosion protection in linked vessels: guidance for containment and venting

Much industrial dust-handling plant consists of vessels connected by pipelines. If a dust explosion propagates through such a system, the overall explosion event can be more violent than if a single vessel only is involved, due to a combination of increased turbulence, pressure piling and a jet flame ignition source in the second vessel. This paper gives guidance, based on an extensive experimental programme reported previously, on two aspects of the protection of interconnected vessels: containment and venting.     

Wire-mesh inhibition of jet fire induced by explosion venting

Explosion venting is widely applied in industrial explosion-proof designs due to the convenient, economical and practical features of this method. Natural gas is usually stored in storage tanks. If the gas in the vessel is mixed with air and encounters an ignition source, explosion venting might occur, producing jet fire, generating new secondary derivative accidents and causing casualties and property losses. In this paper, a set of test platforms including wire-mesh suppression devices is established to study the inhibition of jet fire induced by explosion venting by wire mesh. The experimental research shows that a wire mesh significantly inhibits the jet fire induced by explosion venting. The flame propagation velocity and pressure clearly decrease with increasing numbers of wire-mesh layers. The wire-mesh structure significantly affects the flame propagation, and the more layers of mesh there are, the better the suppression effect is. The flame temperature gradually decreases with the addition of the wire mesh. The mesh size significantly affects the pressure propagation of explosion venting. The explosion pressure gradually decreases with the addition of the wire mesh. With increasing distance between the wire mesh and the explosion vent, the maximum temperature first increases and then decreases, and the maximum explosion pressure first decreases and then increases. In the case of single gas cloud, the flame suppression effect is the most obvious when the wire mesh is 0.2 m away from the explosion vent. In the case of double gas clouds, the flame suppression effect is the most significant when the distance between the wire mesh and the first gas cloud is 0.4 m.     

Fast flame speeds and rates of pressure rise in the initial period of gas explosions in large L/D cylindrical enclosures

Flame speeds and rates of pressure rise for gaseous explosions in a 76 mm diameter closed cylindrical vessel of large length to diameter ratio (L/D = 21.6), were quantitatively investigated. Methane, propane, ethylene and hydrogen mixtures with air were studied across their respective flammability ranges. Ignition was affected at one end of the vessel. Very fast flame speeds corresponding to high rates of pressure rise were measured in the initial 5–10% of the total explosion time. During this period 20–35% of the maximum explosion pressure was produced, and over half of the flame propagation distance was completed. Previous work has concentrated on the later stages of this type of explosion; the development of tulip flames, pressure wave effects and transition to turbulence. The initial fast phase is very important and should dominate considerations in pressure relief vent design for vessels of large L/D.     

Safety devices for gas welding, cutting and allied processes

This paper describes the results of extensive research to determine the limit of safety against flame transmission for sintered metal flame arrester elements when stressed by a flashback in a fuel-gas/oxygen mixture and when stressed through a stabilized burning of a flowing mixture of fuel-gas and air and of fuel-gas and oxygen at the sintered metal element. On the basis of the results of these investigations, the limit of safety against flame transmission for sintered metal flame arrester elements can be estimated and the conditions for testing can be specified. An analysis of the protection of gas outlets on gas distribution lines for welding, cutting and allied processes has been carried out, resulting in recommendations for necessary changes to the regulations for testing of safety devices with sintered metal flame arrester elements, which should be made in the next revision of the corresponding technical regulations for acetylene installations and calcium carbide stores (TRAC), and the standards DIN 8521, EN 730 and ISO 5175.     

Determination of the performance limits of flame arresters at increased oxygen concentrations

This investigation shows how an increased oxygen concentration influences the performance limits of crimped ribbon deflagration flame arresters at elevated pressures. An evaluation of the maximum experimental safe gap (MESG) as reliable criterion for describing the performance limits under non-atmospheric conditions is given. Measurements of MESGs and flame arrester performance tests were performed. Various fuel/oxygen/air mixtures containing ethylene and propane were used as testing gases. Former studies on the pressure dependence and the influence of oxygen on the MESG were initially confirmed. Furthermore, performance tests using a commercial deflagration flame arrester revealed that such a flame arrester may prevent flame transmission also at non-atmospheric conditions within a limited range. For various oxygen concentrations the performance limits were reached at the same MESG. Hence, it can be assumed that a flame arrester possesses a device- and fuel-specific maximum experimental safe gap for a specific gas mixture in different concentrations and at different pressures. This performance-related maximum safe gap can be used as a parameter for estimating and describing the performance limits of a flame arrester. It offers an attempt to simplify the testing and qualification of deflagration flame arresters for non-atmospheric conditions.     

水雾作用下气体爆炸火焰传播的实验研究

利用自行设计的全程透明的火焰加速管和水喷雾系统，对不同水雾条件下的气体火焰传播现象进行了实验研究。运用光电传感器与数字摄像技术分析了不同浓度的甲烷在不同水雾条件下的火焰传播速度、火焰阵面轨迹以及火焰结构特性，并通过对传播火焰反应区温度的测量，探讨了水雾抑制气体爆炸火焰传播的内在机理及所需的条件。实验结果表明：由于水雾作用于火焰反应区，降低了火焰反应区内的温度和气体燃烧速度，延长火焰阵面的预热区，减缓火焰阵面传热与传质的进行，从而使传播火焰得以抑制。水雾对气体爆炸火焰传播的抑制效果与水雾流量速度、雾区浓度以及火焰到达水雾区的火焰传播速度有关。     

大尺度障碍物与泄爆面对天然气内爆炸的协同作用规律研究

为阐释民用建筑内部大尺度物品与门窗等泄爆面对天然气爆炸灾害的协同作用机制，基于典型厨房空间布局及内部物品特征，借助计算流体动力学技术研究了不同泄爆面开启压力和不同大尺度障碍物体积阻塞率条件下天然气内爆炸火焰速度、爆炸超压的分布规律。研究结果表明:大尺度障碍物与泄爆面对室内天然气爆炸过程具有显著的协同作用，共同促进火焰速度与爆炸超压的显著增长，并缩短峰值超压到达时间；大尺度障碍物的存在虽然显著降低了室内天然气的体积，但从增加房间内湍流源和相对长径比的角度进一步促进了泄爆效应；大尺度障碍物与泄爆面协同作用下，室内火焰速度呈现明显的阶段性特征，并在泄爆面附近发生波动。研究结论可为民用建筑物内气体爆炸事故调查分析和灾害评估提供科学依据。     

The role of turbulence in the validity of the cubic relationship

The effect of turbulence on unsteady premixed flame propagation and associated pressure rise during explosion of stoichiometric CH₄/air in closed spherical vessels of different size was investigated by means of CFD simulation. Computations were run by varying the vessel volume from 20 l to 200 l and to 1 m³.Numerical results have shown that, at fixed initial conditions, the turbulence kinetic energy induced by the propagating flame increases with increasing vessel volume. It has been demonstrated that the cubic relationship does not apply. Under the conditions investigated, a correction to the cubic relationship has been proposed to take into account the effect of the vessel volume on turbulence.