Vent Sizing Criteria for Partial Volume Deflagration

The accidental spill of volatile solvents or the release of flammable gases within equipment and buildings is likely to form fuel concentration gradients unless efficient mixing is provided. As a consequence, even small amounts of fuel can form flammable clouds, and partial volume deflagrations may occur. Nevertheless, few indications are given in international guidelines for vent sizing and only over-conservative well-mixed stoichiometric assumptions are used. In this paper, we propose a predictive methodology for the evaluation of the dynamics of partial volume deflagration, aiming at defining useful correlations for the design of vent devices, starting from the fundamental equation for the rate of pressure rise and flame propagation in closed vessel. We define a ‘stratified gas deflagration index’ K_G(m), where m is the filling ratio, and use it with the most common design equations for vent sizing. The approach has been validated by means of a CFD code for the simulation of stratified laminar methane–air explosion by varying both filling ratio and volume.     

Turbulent deflagrations of mildly flammable refrigerant-air mixtures

With current concerns around global climate change, new hydrofluorocarbons with low Global Warming Potential (GWP) are being evaluated as alternative refrigerants. These alternative refrigerants, however, may be mildly flammable (as defined by the A2L safety group classification) and pose safety concerns for the heating, ventilation, air conditioning, and refrigeration (HVAC/R) industry. Consequently, careful assessments of different flammability characteristics and risks for these refrigerants are essential for their safe use in actual applications. In this study, deflagration propagation measurements for different mildly flammable refrigerants, including difluoromethane (R-32) and 2,3,3,3-tetrafluoropropene (R-1234yf), were undertaken in different geometries including a 9.1-m long conduit test rig and a closed cubical 12.5 m³ volume. Different tests were conducted for full volume deflagrations as well as with and without obstructions. Turbulent deflagration speeds for well-mixed, refrigerant-air mixtures have been shown to be orders of magnitude larger than their corresponding laminar flame speed values that are used in classifying flammable refrigerants in safety standards. Testing has also quantified the resulting severity as measured by the event overpressure which was shown to worsen with increased congestion or confinement as a consequence of increased induced turbulence. This work illustrates the importance for severity evaluations for actual large-scale or congested geometries of concern in practical applications. Even for mildly flammable refrigerants characterized by laminar flame speeds <2 cm/s, which is lower than the 10 cm/s limit for A2L refrigerants, relatively fast deflagrations can be generated for very congested geometries where downstream turbulence is generated as the flame front passes over obstacles in these situations.     

Simulation of dust explosions in complex geometries with experimental input from standardized tests

The present paper describes the development of a new CFD-code (DESC) for the assessment of accidental hazards arising from dust explosions in complex geometries. The approach followed entails the estimation of the laminar burning velocity of dust clouds from standardized laboratory-scale tests, and its subsequent use as input to the combustion model incorporated in DESC. The methodology used to obtain the laminar burning velocities is demonstrated by igniting turbulent propane-air mixtures to deflagration in a standard 20-litre USBM-vessel, and extracting the laminar burning velocity from the pressure–time curves; the results are compared with literature data. Laminar burning velocities for clouds of maize starch dust in air were estimated following the same procedure, and the resulting empirical model was used to simulate dust explosions in a 236-m³ silo.     

Differences and similarities of gas and dust explosions: A critical evaluation of the European ‘ATEX’ directives in relation to dusts

Explosive gas mixtures and explosive dust clouds, once existing, exhibit similar ignition and combustion features. However, there are two basic differences between dusts and gases which are of substantially greater significance in design of safety standards than these similarities. Firstly, the physics of generation and up-keeping of dust clouds and premixed gas/vapour clouds are substantially different. This means that in most situations where accidental explosive gas clouds may be produced quite readily, generation of explosive dust clouds would be highly unlikely. Secondly, contrary to premixed gas flame propagation, the propagation of flames in dust/air mixtures is not limited only to the flammable dust concentration range of dynamic clouds. The state of stagnant layers/deposits offers an additional discrete possibility of flame propagation.

The two European Directives 94/9/EC (1994) and 1999/92/EC (1999) primarily address gases/vapours, whereas the particular properties of dusts are not addressed adequately. Some recent IEC and European dust standards resulting from this deficiency are discussed, and the need for revising the two directives accordingly is emphasized.     

Process of accidental explosions at a refuse derived fuel storage

Accidental gas explosions occurred at a refuse-derived-fuel (RDF) storage in Japan, and two fire fighters on duty were dead. The flammable gases, which caused the gas explosions generated during a RDF fire. It means that gas explosions could occur in the use of solid fuels under certain conditions. This study has been conducted for exploring the process to gas explosions in the RDF storage. The temperature at a part of the RDF pile in the storage was inferred to spontaneously increase, and the prediction of the temperature increase was attempted on the basis of the Frank-Kamenetskii theory. It was shown that the critical temperature of RDF for spontaneous temperature rise depends on the size of the pile. Larger the pile, lower the critical temperature. The possibility of accumulation of flammable gas in the space of the RDF storage is discussed. It is indicated that the spread rate of thermal wave is slow and a high temperature region likely established. After the RDF pile ignites, the oxygen concentration near the burning site becomes low and the flammable species components in the generated gas increases. Those species pass through surrounding low temperature region and come out into the space over the RDF pile without combustion. An explosion would occur when a fresh air comes into the storage, mixes with the flammable gas coming out from the pile to form a flammable mixture, and then the flammable mixture ignites. The most effective means to prevent accidental explosions is to avoid spontaneous ignition by cooling the heated RDF. If spontaneous ignition occurs, elimination of flammable gases from the storage should be strongly recommended.     

重气扩散的数值模拟

易燃易爆有毒物质泄漏事故常形成比空气重的气云 ,国外已开发了大量不同复杂程度的重气扩散模型 ,其中数值模拟已成为快速发展的领域。本文综述了重气扩散的湍流模型 ;还描述了数值模拟的计算方法及其准确性 ;最后分析了目前存在的问题 ,提出了今后的发展方向。     

Flamelet surface density modelling of turbulent deflagrating flames in vented explosions

The performance of two reaction rate models based on the laminar flamelet concept have been examined by calculating the behaviour of turbulent flame deflagration inside a semi-confined explosion tube. The models formulate the mean rate of reaction as a function of a transport equation for the flamelet surface density. The difference in the models is in modelling the source/sink terms of the flamelet surface density transport equation. The models are validated using laser diagnostics of flame deflagration in methane–air flammable mixture. The predictions are compared with experimental results for propagation, pressure history and flame speed. Sensitivity to cross-flow effects are investigated through comparison between two- and three-dimensional calculations. The numerically simulated results show that experimental trends are well reproduced by both models.     

Lithium-ion energy storage battery explosion incidents

Utility-scale lithium-ion energy storage batteries are being installed at an accelerating rate in many parts of the world. Some of these batteries have experienced troubling fires and explosions. There have been two types of explosions; flammable gas explosions due to gases generated in battery thermal runaways, and electrical arc explosions leading to structural failure of battery electrical enclosures. The thermal runaway gas explosion scenarios, which can be initiated by various electrical faults, can be either prompt ignitions soon after a large flammable gas mixture is formed, or delayed ignitions associated with late entry of air and/or loss of gaseous fire suppression agent. The electrical explosions have entailed inadequate electrical protection to prevent high energy arcs within electrical boxes vulnerable to arc induced high pressures and thermal loads. Estimates of both deflagration pressures and arc explosion pressures are described along with their incident implications.     

When solids meet solids: A glimpse into dust mixture explosions

Mixing an inert solid or a less flammable compound with a combustible dust can be regarded as a direct application of the inherent safety principle of moderation. An experimental investigation was carried out to determine the evolution of the ignition sensitivity and the explosion severity of such various mixtures as a function of their compositions. It demonstrates that the introduction of small amounts of highly combustible powders (such as sulphur or nicotinic acid) to a less flammable dust (such as microcrystalline cellulose or carbon black) can strongly influence the ignition sensitivity as well as the explosion severity.It has notably been shown that the ignition sensitivity of solid/solid mixtures significantly rises up when only 10–5%wt. of highly flammable dust is introduced. Simple models can often be applied to estimate the minimum ignition energy, minimum ignition temperature and minimum explosive concentration of such mixtures. Concerning the dust explosivity, three cases have been studied: mixtures of combustibles dusts without reaction, dusts with reactions between the powders, combustible dusts with inert solid. If the evolution of the maximum explosion pressure can be estimated by using thermodynamic calculations, the maximum rate of pressure rise is more difficult to predict with simple models, and both combustion kinetics and hydrodynamics of the dust clouds should be taken into account. These results were also extended to flammable dust/solid inertant mixture. They clearly show that the concentration of solid inertant at which the ignition is not observed anymore could reach 95%wt. As a consequence, the common recommendation of solid inertant introduction up to 50–80%wt. to prevent dust explosion/ignition should be reconsidered.     

Flammability of methane,propane, and hydrogen gases

This paper reports the results of flammability studies for methane, propane, hydrogen, and deuterium gases in air conducted by the Pittsburgh Research Laboratory. Knowledge of the explosion hazards of these gases is important to the coal mining industry and to other industries that produce or use flammable gases. The experimental research was conducted in 20 L and 120 L closed explosion chambers under both quiescent and turbulent conditions, using both electric spark and pyrotechnic ignition sources. The data reported here generally confirm the data of previous investigators, but they are more comprehensive than those reported previously. The results illustrate the complications associated with buoyancy, turbulence, selective diffusion, and ignitor strength versus chamber size. Although the lower flammable limits (LFLs) are well defined for methane (CH₄) and propane (C₃H₈), the LFLs for hydrogen (H₂) and its heavier isotope deuterium (D₂) are much more dependent on the limit criterion chosen. A similar behavior is observed for the upper flammable limit of propane. The data presented include lower and upper flammable limits, maximum pressures, and maximum rates of pressure rise. The rates of pressure rise, even when normalized by the cube root of the chamber volume (V^1/3), are shown to be sensitive to chamber size.     

可燃制冷剂泄漏及爆炸危害评价的研究

建立了计算可燃工质泄漏后在房间内可燃工质浓度变化的模型。计算结果表明,小型空调器可燃工质泄漏引发的爆炸隐患仅局限于空调系统附近的局部区域内。用压力－冲量准则,对可燃工质泄漏产生的最大爆炸当量、危害范围与危害等级进行分析研究。结果表明,使用充装量很小的可燃制冷剂,泄漏产生爆炸的危险性很小,火灾隐患是主要的。     

Study of dust cloud behaviour in the modified Hartmann tube using the image subtraction method (ISM)

A dispersion of fine particles in the air is needed for a dust explosion to occur since an explosion is the fast combustion of particles in the air. When particles are poorly dispersed, agglomerated, or their concentration is low, the combustion velocity decreases, and deflagration would not occur. The combustion rate is strictly related to dust concentration. Therefore, the maximum explosion pressure rise occurs at dust concentration close to stoichiometric. Conversely, Minimum Explosion Concentration (MEC) is the lower limit at which self-sustained combustion and a pressure rise are possible. Dust explosion tests are designed to reproduce the dispersion and generation of dust clouds in industrial ambiences by using dispersion devices activated by pressurised air pulses. The resulting dust cloud, which has a marked transient character, is considered representative of real clouds by current standards. Over time, several studies have been carried out to optimise these devices (e.g. to reduce the inhomogeneity of the cloud in the 20 L sphere). The Minimum Ignition Energy (MIE) of dust is measured using the Mike3 modified Hartmann tube, where the ignition attempt is made 60–180 ms after dust dispersion regardless of dust characteristics.This work investigates the dust clouds’ actual behaviour inside the modified Hartmann tube before ignition using high-velocity video movies and a new image post-treatment method called Image Subtraction Method (ISM). Movies are recorded with high-speed cameras at a framerate of 2000 fps and elaborated with an on-purpose developed LabVIEW® code. Concentration (mass per volume) and dispersion pressure are varied to evaluate their effect on dust clouds. Maise starch, iron powder and silica powder are chosen to investigate the effect of particle density and size on the cloud structure and turbulence. This approach will help to investigate the structure of the dust cloud, the shape and size of the particle lumps and the change in dust concentration over time. In addition, information on the actual concentration and cloud turbulence at the ignition location and delay time were obtained, which may help identify the local turbulence scale and widen the characterisation of the cloud generated in the Hartmann tube.     

Laminar burning velocities of hydrogen–air mixtures from closed vessel gas explosions

The laminar burning velocity of hydrogen–air mixtures was determined from pressure variations in a windowless explosion vessel. Initially, quiescent hydrogen–air mixtures of an equivalence ratio of 0.5–3.0 were ignited to deflagration in a 169 ml cylindrical vessel at initial conditions of 1 bar and 293 K. The behavior of the pressure was measured as a function of time and this information was subsequently exploited by fitting an integral balance model to it. The resulting laminar burning velocities are seen to fall within the band of experimental data reported by previous researchers and to be close to values computed with a detailed kinetics model. With mixtures of an equivalence ratio larger than 0.75, it was observed that more advanced methods that take flame stretch effects into account have no significant advantage over the methodology followed in the present work. At an equivalence ratio of less than 0.75, the laminar burning velocity obtained by the latter was found to be higher than that produced by the former, but at the same time close enough to the unstretched laminar burning velocity to be considered as an acceptable conservative estimate for purposes related to fire and explosion safety. It was furthermore observed that the experimental pressure–time curves of deflagrating hydrogen–air mixtures contained pressure oscillations of a magnitude in the order of 0.25 bar. This phenomenon is explained by considering the velocity of the burnt mixture induced by the expansion of combusting fluid layers adjacent to the wall.     

可燃制冷剂空调用开关元件点燃危险及防爆试验分析

随着全球范围对环境问题的日益重视,R290,R32等新型制冷剂因优良的环保性能和节能效果被广泛应用于空调行业,但其属于IIA级爆炸性气体,为了进一步分析其运行过程中的防爆安全性能。通过文献数据和测试分析,可燃制冷剂空调电气元件的点燃源主要来自其工作过程中产生的电气火花。其中空调开关元件最容易产生电火花、电弧,且通过试验验证这些火花或电弧能点爆（6.5±0.5）%乙烯/空气混合物。综合IEC 60335-2-40,GB 4706.32标准要求和空调开关元件的结构特征,分析确定其应符合“nC”型防爆技术要求,并能通过“nC”型爆炸试验。针对开关元件结构尺寸小、存在爆炸试验中爆炸性混合气体多次置换和负载通、断电操作的难点问题,提出一套合理可行的爆炸试验实施方案,并通过测试应用得到验证。     

A study on the effect of trees on gas explosions

The downstream as well as the upstream oil and gas industry has for a number of years been aware of the potential for flame acceleration and overpressure generation due to obstacles in gas clouds caused by leaks of flammable substances. To a large extent the obstacles were mainly considered to be equipment, piping, structure etc. typically found in many installations. For landbased installations there may however also be a potential for flame acceleration in regions of vegetation, like trees and bushes. This is likely to have been the case for the Buncefield explosion that occurred in 2005 (Buncefield Major Incident Investigation Board, 2008), which led to the work described in the present paper. The study contains both a numerical and an experimental part and was performed in the period 2006–2008 (Bakke and Brewerton, 2008, Van Wingerden and Wilkins, 2008).The numerical analysis consisted of modelling the Buncefield tank farm and the surrounding area with FLACS. The site itself was not significantly congested and it was not expected to give rise to high overpressures in case of a hydrocarbon leak. However, alongside the roads surrounding the site (Buncefield Lane and Cherry Tree Lane), dense vegetation in the form of trees and bushes was included in the model. This was based on a site survey (which was documented by video) performed in the summer of 2006.A large, shallow, heavier-than-air gas cloud was defined to cover part of the site and surroundings. Upon ignition a flame was established in the gas cloud. This flame accelerated through the trees along the surrounding roads, and resulted in high overpressures of several barg being generated by FLACS. This is to the authors’ knowledge the first time a possible effect of vegetation on explosions has been demonstrated by 3D analyses.As a consequence of these results, and since the software had been validated against typical industrial congestion rather than dense vegetation, a set of experiments to try to demonstrate if these effects were physical was carried out as well. The test volume consisted of a plastic tunnel, 20 m long with a semi-circular cross-section 3.2 m in diameter allowing for representing lanes of vegetation. The total volume of the tent was approximately 80.4 m³. The experimental programme involved different degrees of vegetation size, vegetation density (blocking ratio) and number of vegetation lanes (over the full length of the tunnel). The experiments were performed with stoichiometric propane–air mixtures resulting in continuously accelerating flames over the full length of the tunnel for some of the scenarios investigated.The main conclusions of the study are that trees can have an influence on flame acceleration in gas–air clouds, and that advanced models such as FLACS can be used to study such influence. More research is needed, however, because even if FLACS predicts flame acceleration in dense vegetation, no evidence exists that applying the code to trees rather than rigid obstacles provides results of acceptable accuracy.     

Un-ignited and ignited high pressure hydrogen releases: Concentration – turbulence mapping and overpressure effects

Safety studies for production and use of hydrogen reveal the importance of accurate prediction of the overpressure effects generated by delayed explosions of accidental high pressure hydrogen releases. Analysis of previous experimental work demonstrates the lack of measurements of turbulent intensities and lengthscales in the flammable envelope as well as the scarceness of accurate experimental data for explosion overpressures and flame speeds. AIR LIQUIDE, AREVA STOCKAGE ENERGIE and INERIS join in a collaborative project to study un-ignited and ignited high pressure releases of hydrogen.The purpose of this work is to map hydrogen flammable envelopes in terms of concentration, velocity and turbulence, and to characterize the flame behaviour and the associated overpressure. These experimental results (dispersion and explosion) are also compared with blind FLACS modelling.     

Behavior of buoyancy and momentum controlled hydrogen jets and flames emitted into the quiescent atmosphere

As an effort to improve the prediction of hydrogen dispersion in the atmosphere, effects of buoyancy acting on the hydrogen jets formed by releasing from high pressure vessels are investigated analytically and experimentally. For the analytical study, an integral analysis for buoyant jets and flames is carried out to yield the closed formula describing the jet and flame shapes, including their trajectory and horizontal and vertical lengths corresponding to the critical concentrations. Because the density of hydrogen after releasing from high pressure storage conditions is much close to that of helium than that of hydrogen at room temperature, helium is used as the hydrogen surrogate for the buoyant jet experiments, which were performed by visualizing the jet dispersing in the atmosphere up to the jet Reynolds number Re ～ 2400. The trajectories obtained by the integral analysis and experiments agree relatively well until the transition to turbulence occurs. A further estimate for jets and flames is made by using the integral analysis as a preliminary design to the experiments involving a much greater hydrogen release. Once the comparative investigation of the larger scale experiments with the integral analysis, we anticipate that a more universal hydrogen jet and flame data can be obtained, which perhaps leads to a better safety distance of hydrogen stations.