An experimental study of flame acceleration and deflagration to detonation transition in representative process piping

The paper summarizes the results of experimental tests and accompanying analyses to investigate the factors that govern flame acceleration and potential transition to detonation in a relatively long unobstructed piping system. The overall aim of the work was to obtain sufficient experimental data so as to be able to develop and evaluate methodologies for classifying and predicting potential detonation flame acceleration and deflagration to detonation transition (DDT) hazard in industrial process pipes and mixtures. The present results show that the flame acceleration process in an unobstructed pipe exhibit three distinct phases: an initial establishment phase; a second rapid acceleration phase and a final transition to detonation phase. Test results with ethylene indicate that the acceleration process is not sensitive to initial pressure (all other parameters remaining constant) but can be sensitivity to initial pipe wall temperature or possibly mixture humidity. The presence of bends increases the local rate of turbulent combustion, an effect attributed to the additional turbulence generated downstream of the bend. For straight pipes, detonation was only observed to develop for hydrogen–air and ethylene–air mixtures. Detonation was not observed with methane, propane or acetone as fuel in the present piping apparatus.     

A violent,episodic vapour cloud explosion assessment: Deflagration-to-detonation transition

A large vapour cloud explosion (VCE) followed by a fire is one of the most dangerous and high consequence events that can occur in petrochemical facilities. The current process of safety practice in the industry in VCE assessment is to assume that all VCEs are deflagration. This assumption has been considered for nearly three decades. In recent years, major fire and VCE incidents in fuel storage depots gained considerable attention in extreme high explosion overpressure due to the transition from Deflagration to Detonation (DDT). Though the possibility of DDTs is lower than deflagrations, they have been identified in some of the most recent large-scale VCE incidents, including Buncefield (UK), 2005, San Juan explosion (US), 2009, and IOCL Jaipur (India), 2009 event. Such an incident established the need to understand not only VCE but also the importance of avoiding the escalation of minor incidents into much more devastating consequences.Despite decades of research, understanding of the fundamental physical mechanisms and governing factors of deflagration-to detonation transition (DDT) transition remains mostly elusive. An extreme multi-scale, multi-physics nature of this process uncertainly makes DDT one of the “Grand Challenge” problems of typical physics, and any significant developments toward its assured insistence would require revolutionary step forward in experiments, theory, and numerical modelling. Under certain circumstances, nevertheless, it is possible for DDT to occur, and this can be followed by a propagating detonation that quickly consumes the remaining detonable cloud. In a detonable cloud, a detonation creates the worst accident that can happen. Because detonation overpressures are much higher than those in a deflagration and continue through the entire detonable cloud, the damage from a DDT event is more severe. The consideration of detonation in hazard and risk assessment would identify new escalation potentials and recognize critical buildings impacted. This knowledge will allow more effective management of this hazard.The main conclusion from this paper is that detonations did occur in Jaipur accident at least part of the VCE accidents. The vapour cloud explosion could not have been caused by a deflagration alone, given the widespread occurrence of high overpressures and directional indicators in open uncongested areas containing the cloud. Additionally, the major incident has left many safety issues behind, which must be repeatedly addressed. It reveals that adequate safety measures were either underestimated or not accounted for seriously. This article highlights the aftermath of the IOCL Jaipur incident and addresses challenges put forward by it.     

氢气爆炸特性研究

本文研究、总结了氢气与空气(氢气与氧气)的混合物的爆炸特性.即氢气在空气中,在比较低燃烧界限的情况下,只有向上的传播和非常少的超压可以观测得到.正因为氢气的这种特性,将氢应用于科技将极大地推进社会进步,氢燃料将成为一种主要的能源.然而,氢技术应用的成功与否主要取决于氢使用的安全性.所以,必须掌握实际使用时氢气燃烧的性能.本文在日本过去十年实验数据的基础上,通过实验研究了氢气与空气混合物的燃点.研究了氢气、氧气混合物经氮气稀释后,按化学当量比例将不同浓度的氢气与空气进行混合,并得出了低温下的爆炸压力特性.随后,分别讨论了在初始压力下一致的情况下,试管直径相同的状况下,氢气与空气混合浓度相同的情况下,这三种爆轰传播限制之间的关系.得出了在空气中直接点燃的发生爆轰的最小试管直径,最小的装药量之间的关系,进行了爆轰危险性分级.最后,文章概括比较了氢与其他燃料的燃烧特性,评估了氢气燃烧过程中的危险与安全因素.     

温压炸药弹内爆轰特性试验研究

研究温压炸药爆炸初期弹内爆炸波作用过程,可以进一步分析温压炸药分散爆轰的作用机理,为温压炸药武器设计和系统优化提供可靠依据.设计了用测时法研究温压炸药爆轰波传播速度的试验装置,并用探针对相同组分和密度条件下、温压战斗部弹内爆炸初期的爆炸作用过程进行了对比试验,记录了触发时间信号,计算并分析了温压炸药弹内爆炸波的传播特性,得到了波阵面与弹径方向夹角α的变化规律.试验结果表明,温压炸药的爆轰波传播速度为4.632km/s,其弹内爆轰波的传播特性与中心高能分散药的爆轰波有密切联系,通过合理设计弹体结构和比药量,可以实现温压炸药的分散爆轰,提高温压炸药的爆炸威力.     

Deflagration to detonation transition in a vapour cloud explosion in open but congested space: Large scale test

The paper reviews large scale experiments with various fuels in air where successful deflagration to detonation transition (DDT) took place. This includes a recent experiment disclosed in the Buncefield R&D program, where DDT developed in the propane/air mixture. The DDT occurred in branches of deciduous trees in a premixed stagnant mixture. An internal R&D investigation programme was initiated to better understand the phenomena. A large scale experiment in an open space with ethane air mixture is presented in the paper. The premixed mixture was ignited at the edge of the congested three-dimensional rigs which consisted of vertical and horizontal pipes. After ignition, the flame accelerated in the congestion and transitioned to detonation at the end of congestion. Stable detonation propagated through the remaining open and uncongested space.The flame acceleration process leading to DDT is scale dependent. It also depends on many parameters leading to a large investigation array and, significant cost. However, such R&D efforts aimed toward a safer plant design, i.e. the prevention of occurrence of a major accident, are a small fraction of a real accident cost.     

Propagation and intensity of blast wave from hydrogen pipe rupture due to internal detonation

The coupled fluid-structure-rupture model was developed to study the propagation and intensity of blast wave from hydrogen pipe rupture due to internal detonation. The dynamic rupture of pipe and propagation of blast wave were well coupled together in every timestep during the simulation. The numerical model was validated with experiments in terms of both typical rupture profiles and blast overpressures. Results reveal that crack branching of pipe can dramatically increase the rupture opening rate which controls the intensity and shape of the resultant blast wave. Due to the process of crack initiation and extension, the blast wave out of the pipe first forms and then is strengthened by the subsequent compression waves. This makes the maximum peak overpressure appears at a certain standoff distance above the rupture. Despite consuming some percentages of energy, the dynamic rupture of pipe generally presents positive effects (up to 2–3 times) on the blast wave intensity along the jetting direction due to the convergence effect of rupture opening on the release of internal high-pressure gas. Finally, through defining normalized overpressure and impulse based on the same hydrogen detonation in open spaces, the quantitative influences of pipe rupture on the blast wave intensity in cases of different detonation pressures and standoff distances are clarified.     

高硫油炼制企业H_2S中毒风险分析

随着企业炼油规模不断扩大及原油中含硫量不断提高,炼油企业中H2S中毒事故风险迅速增大。在对高硫油炼制企业H2S的分布及危险源状况分析的基础上,指出炼油企业存在两类H2S危险源,即含有H2S的气体和含有H2S的酸性水,炼油装置产出含H2S的气体中H2S含量一般在1%～92%,酸性水中H2S含量一般在0.01%～4.0%;介绍了不同H2S浓度等级的空气可对人产生的毒性危害后果,H2S在空气中数分钟内致人死亡的浓度为1500mg/m3;对炼油企业重大酸性气泄漏事故、酸性水泄漏事故进行定量毒性危害计算和分析,给出H2S毒性危害低危区、高危区及迅速致死区达到的下风距离及覆盖的区域面积,重大酸性气、酸性水泄漏事故迅速致死浓度可达到下风距离分别为300m和89m;指出H2S中毒已成为高硫油炼制企业的重大风险,应该加强安全措施,重点防控。     

柴油环卫车电动化替代环境效益分析及可行性评估

以环卫车为研究对象,通过使用阶段柴油环卫车和电动环卫车污染排放强度对比,分析电动化替代所产生的环境效益。应用模糊数学模型方法从技术、经济、配套设施、污染减排效益方面,综合评估环卫车电动化替代可行性。结果表明:电动环卫车较柴油环卫车具有显著的减排效果,主要大气污染物排放可减少95.5%。影响柴油环卫车电动化替代的关键因素在电池动力性能、续航能力、经济成本和配套设施充电时长等方面。模糊数学模型的评估结果表明,50%柴油环卫车进行电动化替代是最佳方案,能较好地平衡经济性和大气环境保护的公共利益。未来提高电动化替代比例,还需依靠科技创新改进电动环卫车电池技术和动力等性能,降低购置成本,加强充电桩等配套基础设施的完善。     

Experimental study on DDT for hydrogen–methane–air mixtures in tube with obstacles

An experimental study of flame propagation, acceleration and transition to detonation in stoichiometric hydrogen–methane–air mixtures in 6 m long tube filled with obstacles located at different configurations was performed. The initial conditions of the hydrogen–methane–air mixtures were 1 atm and 293 K. Four different cases of obstacle blockage ratio (BR) 0.7, 0.6, 0.5 and 0.4 and three cases of obstacle spacing were used. The wave propagation was monitored by piezoelectric pressure transducers PCB. Pressure transducers were located at different positions along the channel to collect data concerning DDT and detonation development. Tested mixtures were ignited by a weak electric spark at one end of the tube. Detonation cell sizes were measured using smoked foil technique and analyzed with Matlab image processing toolbox. As a result of the experiments the deflagration and detonation regimes and velocities of flame propagation in the obstructed tube were determined.     

DDT and detonation propagation limits in an obstacle filled tube

Experiments with hydrogen–air and ethylene–air mixtures at atmospheric pressure were carried out in a 6.1 m long, 0.1 m diameter tube with different obstacle configurations and ignition types. Classical DDT experiments were performed with the first part of the tube filled with equally spaced 75 mm (44% area blockage ratio) orifice-plates. The DDT limits, defining the so-called quasi-detonation regime, where the wave propagates at a velocity above the speed of sound in the products, were found to be well correlated with d/λ = 1, where d is orifice-plate diameter and λ is the detonation cell size. The only exception was the rich ethylene limit where d/λ = 1.9 was found. In a second experiment detonation propagation limits were measured by transmitting a CJ detonation wave into an obstacle filled (same equally spaced 44% orifice plates) section of the tube. An oxy-acetylene driver promptly initiated a detonation wave at one end. In this experiment the quasi-detonation propagation limits were found to agree very well with the d/λ = 1 correlation. This indicates that the d/λ = 1 represents a propagation limit. In general, one can conclude that the classical DDT limits measured in an orifice-plate filled tube are governed by the wave propagation mechanism, independent of detonation initiation (DDT process) that can occur locally in the obstacles outside these limits. For rich mixtures, transmission of the quasi-detonation into the smooth tube resulted in CJ detonation wave. However, in a narrow range of mixtures on the lean side, the detonation failed to transmit in the smooth tube. This highlights the critical role that shock reflection plays in the propagation of quasi-detonation waves.     

丁烷与空气混合物的爆炸性能测定

在不同条件下,对丁烷与空气混合物进行爆炸实验,由微机数采测试系统测定其爆炸参数（爆轰波压力、爆轰波传播速度等）、爆轰极限以及当其形成爆轰时所需的临界起爆能。通过实验测定,为评价丁烷与空气混合物的安全性能提供重要依据     

Experimental investigation of fast flame propagation in stratified hydrogen–air mixtures in semi-confined flat layers

This paper presents results of an experimental investigation on fast flame propagation and the deflagration-to-detonation transition (DDT) and following detonation propagation in a semi-confined flat layer filled with stratified hydrogen–air mixtures. The experiments were performed in a transparent, rectangular channel open from below. The combustion channel has a width of 0.3 m and a length of 2.5 m. The effective layer thickness in the channel was varied by using different linear hydrogen concentration gradients. The method to create quasi-linear hydrogen concentration gradients that differ in the range and slope is also presented. The ignited mixtures were accelerated quickly to sonic flame speed in the first obstructed part of the channel. The interaction of the fast flame propagation with different obstacle set-ups was studied in the second part of the channel. The experimental results show an initiation of DDT by one additional metal grid in the obstructed semi-confined flat layer. Detonation propagation and failed detonation propagation were observed in obstructed and unobstructed parts of the channel.     

Experimental study on detonation flame penetrating through flame arrester

In-line detonation flame arresters are important safety apparatus to prevent group tank fires caused by the spreading of fire through vapor connection lines. In this study, a DN50 experimental apparatus aimed at the detonation flame penetration characteristics and failure mechanisms in a flame arrester was set up, and a series of experiments were carried out with 6.6% C2H4 and air mixture. Pressure, and velocity of flame penetrating through flame arrester housing and filters were analyzed. Experimental results showed that the attenuation of pressure and velocity was proportional to the thickness of the filters. Two failure modes of the fire-extinguishing process in the flame arrester were captured directly with a high-speed camera. In Mode I, the detonation flame could go straight through the flame arrester filters when the filters were too thin. In Mode II, when the filters were not sufficiently thick, the remained shock wave pressure of detonation flame was still several times of the initial pressure and could rise sharply at the downstream contraction section, resulting in that the flammable gas at the downstream transition section could be compressed and reignited even the flame had been extinguished by filters. These conclusions are helpful to reveal the nature of failure modes of fire-extinguishing process and design flame arresters with high fire-resisting performance by structure improved.     

工业粉尘“二次爆炸”过程研究

开展了工业粉尘“二次爆炸”过程实验室研究工作。对玉米淀粉、小麦粉等粮食粉尘进行了研究，得到了“二次爆炸”发展过程以及最后形成的爆轰波特性，还进一步研究了粉尘层冲击波卷扬过程和分析讨论了粉尘“二次爆炸”过程的影响因素。     

初始温度和初始压力对气体爆轰参数影响的研究

利用已有的气体爆炸模型和包含初始压力、初始温度的气体爆轰参数的计算公式,从理论上研究初始压力和初始温度对气体爆轰参数的影响情况。使用VisualBasic语言编写计算程序,将计算值与文献值进行对比,具有较好的一致性。以甲烷-空气混合物为例,计算在98000Pa,280～400K及298K,0.1～0.5MPa的气体爆轰参数。计算结果表明,初始压力一定,混合物的爆轰压随初始温度的升高而减小,爆轰波速增大;初始温度一定,混合物的爆轰压随初始压力的增大而增大,爆轰波速基本不变;在初始温度和初始压力两个影响因素中,初始压力对混合物爆轰参数的影响明显大于初始温度。     

爆炸事故及预防

为了促进我国的爆炸安全研究工作更深入发展,本文列出了利用爆炸激波管技术测定氢气、汽油、铝粉等可爆性物质的爆炸特性。研究表明:这些可爆性物质在一定条件都能形成破坏力极大的爆轰现象。实验确定了氢、汽油和氧混合物的可爆(轰)极限、可燃性极限、混合物临界初始压力等爆炸临界条件。控制可爆性物质的初始条件不超过其爆炸临界条件,能够防止爆轰或爆燃现象发生;添加不参加反应的物质(如氩气、氮气、水蒸汽等)能够使已达到爆炸条件的混合物阻爆。本文的数据可供有关部门参考。     

A model for estimating the impact of the domino effect on accident frequencies in quantitative risk assessments of storage facilities

It is well known that the domino effect can have a major impact on accidents in storage facilities, as it can increase the consequences of an initial event considerably. However, quantitative risk assessments (QRAs) do not usually take the domino effect into account in a detailed, systematic way, mostly because of its complexity and the difficulties involved in its incorporation. We have developed a simple method to include the domino effect in QRAs of storage facilities, by estimating the frequency with which new accidents will occur due to this phenomenon. The method has been programmed and implemented in two case studies. The results show that it can indeed be used to include the possibility of domino effect occurrence in a QRA. Furthermore, depending on the design of a facility, the domino effect can have a significant effect on the associated risk.     

Influence of non-reactive particle cloud on heterogeneous detonation propagation

An interaction of a detonation wave propagating in the cellular detonation mode with a cloud of inert particles is investigated numerically. The analysis of results allows the regimes of propagation of the heterogeneous plane Chapman–Jouguet and cellular detonations and their suppression to be identified. The influence of various parameters of the inert cloud is demonstrated. The critical length of the cloud sufficient for detonation suppression is determined. It is shown that the disperse composition and the nonuniform distribution of particles in the cloud are important parameters affecting the detonation propagation mode.     

Medium-scale experiments on vented hydrogen deflagration

A series of medium-scale experiments on vented hydrogen deflagration was carried out at the KIT test side in a chamber of 1 × 1 × 1 m³ size with different vent areas. The experimental program was divided in three series: (1) uniform hydrogen–air mixtures; (2) stratified hydrogen–air mixtures within the enclosure; (3) a layer deflagration of uniform mixture. Different uniform hydrogen–air mixtures from 7 to 18% hydrogen were tested with variable vent areas 0.01–1.0 m². One test was done for rich mixture with 50% H₂. To vary a gradient of concentration, all the experiments with a stratified hydrogen–air mixtures had about 4%H₂ at the bottom and 10 to 25% H₂ at the top of the enclosure. Measurement system consisted of a set of pressure sensors and thermocouples inside and outside the enclosure. Four cameras combined with a schlieren system (BOS) for visual observation of combustion process through transparent sidewalls were used. Four experiments were selected as benchmark experiments to compare them with four times larger scale FM Global tests (Bauwens et al., 2011) and to provide experimental data for further CFD modelling. The nature of external explosion leading to the multiple pressure peak structure was investigated in details. Current work addresses knowledge gaps regarding indoor hydrogen accumulations and vented deflagrations. The experiments carried out within this work attend to contribute the data for improved criteria for hydrogen–air mixture and enclosure parameters to avoid unacceptable explosion overpressure. Based on theoretical analysis and current experimental data a further vent sizing technology for hydrogen deflagrations in confined spaces should be developed, taking into account the peculiarities of hydrogen–air mixture deflagrations in presence of obstacles, concentration gradients of hydrogen–air mixtures, dimensions of a layer of flammable cloud, vent inertia, etc.