Gas explosion analysis of safety gap effect on the innovating FLNG vessel with a cylindrical platform

After investigating gas dispersion on a cylindrical Floating Liquefied Natural Gas (FLNG) platform (Li et al, 2016), this second article focuses on assessment of gas explosion by using Computational Fluid Dynamics (CFD). Gas explosion simulations are carried out to evaluate the explosion overpressure mitigating effect of safety gap. The Data-dump technique, which is an effective tool in resetting turbulence length scale in gas explosion overpressure calculation, is applied to ensure simulation accuracy for the congestion scenario with safety gap. Two sets of different safety gaps are designed to investigate the safety gap on the cylindrical FLNG platform, the overall results indicate that the safety gap is effective in reducing overpressure in two adjacent congestions. However, for the explosion scenario where the flame is propagating through several safety gaps to the far field congestion, the safety gap mitigates overpressure only in certain explosion protecting targets. Two series of artificial configurations are modeled to further investigate the explosion scenarios with more than two safety gaps in one direction. It is concluded that the optimal safety gap design in overpressure mitigation for the cylindrical FLNG platform is to balance the safety gap distance ratio in the congested regions.     

The acceleration of flames in tube explosions with two obstacles as a function of the obstacle separation distance

The separation distance (or pitch) between two successive obstacles or rows of obstacles is an important parameter in the acceleration of flame propagation and increase in explosion severity. Whilst this is generally recognised, it has received little specific attention by investigators. In this work a vented cylindrical vessel 162 mm in diameter 4.5 m long was used to study the effect of separation distance of two low blockage (30%) obstacles. The set up was demonstrated to produce overpressure through the fast flame speeds generated (i.e. in a similar mechanism to vapour cloud explosions). A worst case separation distance was found to be 1.75 m which produced close to 3 bar overpressure and a flame speed of about 500 m/s. These values were of the order of twice the overpressure and flame speed with a double obstacle separated 2.75 m (83 characteristic obstacle length scales) apart. The profile of effects with separation distance was shown to agree with the cold flow turbulence profile determined in cold flows by other researchers. However, the present results showed that the maximum effect in explosions is experienced further downstream than the position of maximum turbulence determined in the cold flow studies. It is suggested that this may be due to the convection of the turbulence profile by the propagating flame. The present results would suggest that in many previous studies of repeated obstacles the separation distance investigated might not have included the worst case set up, and therefore existing explosion protection guidelines may not be derived from worst case scenarios.     

独头巷道射流通风流场CFD模拟研究

独头巷道压入式通风是有限空间的受限贴附射流。独头巷道通风风流流场是通风理论和通风设计的基础。笔者根据流体动力学和射流理论 ,建立了独头巷道压入式受限贴附射流通风的紊流k -ε数学模型 ,分析了计算边界条件 ,并应用计算流体动力学 (CFD)的方法模拟了独头巷道射流通风流场 ,从理论上得出了独头巷道有限空间受限贴附射流通风的规律 ,为研究独头巷道合理并有效通风提供了新的理论依据。     

Accuracy improvement in evaluation of gas explosion overpressures in congestions with safety gaps

This paper reports a comparison of simulations and published data from experiments carried out by TNO Prins Maurits Laboratory on geometric configurations that involved safety gaps of various separation distances. The Computational Fluid Dynamics (CFD) based software – FLACS is utilized to conduct the numerical simulations. In the majority of cases, good agreement is found between the simulated results and those obtained by experiment in both the donor and acceptor modules. However, a large discrepancy in the overpressures in the acceptor module is seen when the size of the separation gap approaches one or two times of the module size. A Data-dump technique is used in this study to reset the turbulence length scale for these cases with different separation distances, five sets of explosion scenarios are then numerically simulated and the overpressures are compared with experimentally measured explosion overpressures. The overall results indicate that the software with the Data-dump technique is still an extremely effective tool when it comes to the evaluation of gas explosion overpressures in areas with large separation gaps.     

环己烷泄漏事故模拟与风险控制措施研究

环己烷具有闪点低、爆炸极限宽等特点,一旦发生泄漏,着火爆炸的危险性随时存在。利用计算流体动力学模拟的方法对工程项目中环己烷的泄漏事故进行模拟及风险分析,建立环己酮生产装置的全尺寸三维模型并进行仿真计算,模拟了不同泄漏场景所形成的环己烷可燃气体云团瞬态发展过程及影响范围,并对建构筑物的布局对可燃气体云团的扩散行为的影响进行研究。研究结果表明,通过优化建构筑物布局,可有效降低该装置环己烷的燃爆风险,为企业相关装置的总图布置及环己烷泄漏的安全监控和应急响应提供有价值的参考数据。     

Modified porosity approach and laminar flamelet modelling for advanced simulation of accidental explosions

The production of oil and gas is an inherently hazardous task. Therefore it is crucial to provide reliable estimates of the risks involved. The major contributors to the risk level of an offshore installation, for example, arise from accidents involving explosion and fire. Computational Fluid Dynamics (CFD) can be a powerful tool to help with the calculation of accidental explosion scenarios. In this context, the present work suggests a novel implementation of a model based on a modified Porosity Distributed Resistance (MPDR) approach within an unstructured 3D Navier–Stokes solver. The model operates by representing parts of the filtered geometry from the original model through porosity values attributed to an unstructured tetrahedral mesh. Extra resistance terms are added in the momentum equation as well as extra sources of turbulence. Two extra sources of turbulence are modelled. The first of these is due to the shear layers of the non-resolved obstacles, whilst the second is due to the presence of wakes behind the non-resolved obstacles. Results for 2D and 3D test cases are compared against both experimental data and simulations with fully-resolved geometry and good agreement is observed.     

Analysis of gas dispersion and ventilation within a comprehensive CAD model of an offshore platform via computational fluid dynamics

Maintaining an adequate air flow with a desired air quality that is free from hazardous gases is among the most important actions taken toward the improvement of safety in any process plant. Due to the increased focus on the consequences of existing hazardous material on safety, health, and the environment, air quality and sufficient ventilation within a plant has been increasingly considered in the design stage. This paper investigates and analyzes methane and hydrogen sulfite dispersion and the effect of air ventilation within a CAD model of an offshore platform using computation fluids dynamics (CFD). In addition, this method and its principals could be utilized in any other hazardous environment. Simulations of possible hazardous events along with solutions for preventing or reducing their probability are presented to better assess the data. These investigations are performed by considering hypothetical hazardous scenarios which consist of gas leakages from pipes and process equipment under different conditions. After drafting a precise and highly detailed CAD model of the plant and performing CFD simulations on this model, the results of gas behaviors, dispersion, distribution, accumulation, and its possible hazards are investigated and analyzed. The larger amount of details of the actual plant model in CFD simulation are obtained by using a combination of different methods and software. These include PDMS for 3-D drawing of the plan, Rinoceros for geometrical integration of the process equipment and facilities, and Sharc Harpoon which meshes the model. Moreover, the probability of inducing ignitable or toxic concentration of gases within the atmosphere and air ventilation of the unit is considered by these investigations.     

LES – DFSD modelling of vented hydrogen explosions in a small-scale combustion chamber

Accidental explosions are a plausible danger to the chemical process industries. In the event of a gas explosion, any obstacles placed within the path of the flame generate turbulence, which accelerates the transient flame and raises explosion overpressure, posing a safety hazard. This paper presents numerical studies using an in-house computational fluid dynamics (CFD) model for lean premixed hydrogen/air flame propagations with an equivalence ratio of 0.7. A laboratory-scale combustion chamber is used with repeated solid obstacles. The transient compressible large eddy simulation (LES) modelling technique combined with a dynamic flame surface density (DFSD) combustion model is used to carry out the numerical simulations in three-dimensional space. The study presented uses eight different baffle configurations with two solid obstructions, which have area blockage ratios of 0.24 and 0.5. The flame speed, maximum rate of pressure-rise as well as peak overpressure magnitude and timing are presented and discussed. Numerical results are validated against available published experimental data. It is concluded that, increasing the solid obstacle area blockage ratio and the number of consecutive baffles results in a raised maximum rate of pressure rise, higher peak explosion overpressure and faster flame propagation. Future model development would require more experimental data, probably in a more congested configuration.     

A method for simulation of vapour cloud explosions based on computational fluid dynamics (CFD)

The effectiveness of the application of CFD to vapour cloud explosion (VCE) modelling depends on the accuracy with which geometrical details of the obstacles likely to be encountered by the vapour cloud are represented and the correctness with which turbulence is predicted. This is because the severity of a VCE strongly depends on the types of obstacles encountered by the cloud undergoing combustion; the turbulence generated by the obstacles influences flame speed and feeds the process of explosion through enhanced mixing of fuel and oxidant. In this paper a CFD-based method is proposed on the basis of the author’s finding that among the various models available for assessing turbulence, the realizable k-? model yields results closer to experimental findings than the other, more frequently used, turbulence models if used in conjunction with the eddy-dissipation model. The applicability of the method has been demonstrated in simulating the dispersion and ignition of a typical vapour cloud formed as a result of a spill from a liquid petroleum gas (LPG) tank situated in a refinery. The simulation made it possible to assess the overpressures resulting from the combustion of the flammable vapour cloud. The phenomenon of flame acceleration, which is a characteristic of combustion enhanced in the presence of obstacles, was clearly observed. Comparison of the results with an oft-used commercial software reveals that the present CFD-based method achieves a more realistic simulation of the VCE phenomena.     

Review of the DESC project

Dust Explosion Simulation Code (DESC) was a project supported by the European Commission under the Fifth Framework Programme. The main purpose of the project was to develop a simulation tool based on computational fluid dynamics (CFD) that could predict the potential consequences of industrial dust explosions in complex geometries. Partners in the DESC consortium performed experimental work on a wide range of topics related to dust explosions, including dust lifting by flow or shock waves, flame propagation in vertical pipes, dispersion-induced turbulence and flame propagation in closed vessels, dust explosions in closed and vented interconnected vessel systems, and measurements in real process plants. The new CFD code DESC is based on the existing CFD code FLame ACceleration Simulator (FLACS) for gas explosions. The modelling approach adopted in the first version entails the extraction of combustion parameters from pressure–time histories measured in standardized 20-l explosion vessels. The present paper summarizes the main experimental results obtained during the DESC project, with a view to their relevance regarding dust explosion modelling, and describes the modelling of flow and combustion in the first version of the DESC code. Capabilities and limitations of the code are discussed, both in light of its ability to reproduce experimental results, and as a practical tool in the field of dust explosion safety.     

综合管廊燃气舱结构形式对燃气爆炸超压的影响*

为研究地下综合管廊燃气舱结构形式对燃气爆炸超压的影响,采用数值模拟的方法,改变燃气舱高度,通风分区长度和局部开口大小,分析不同情况下的燃气爆炸超压变化规律。结果表明:冲击波传播速度随燃气舱高度的增加而减小,随着高度的增加,超压峰值曲线由“驼峰状”逐渐变为两端高中间低的“盆形”,爆炸过程产生的最大超压与高度成反比关系。超压峰值在340 m处接近0 kPa,延长通风分区并不会增加超压峰值,可以在考虑防火的要求下根据实际情况适当延长通风分区的长度。局部开口的存在使得爆炸气流能够自由泄压,超压峰值与开口的大小成反比关系。     

Prediction of distance to maximum intensity of turbulence generated by grid plate obstacles in explosion-induced flows

The interaction of unburnt gas flow induced in an explosion with an obstacle results in the production of turbulence downstream of the obstacle and the acceleration of the flame when it reaches this turbulence. Currently, there are inadequate experimental measurements of these turbulent flows in gas explosions due to transient nature of explosion flows and the connected harsh conditions. Hence, majority of measurements of turbulent properties downstream of obstacles are done using steady-state flows rather than transient flows. Consequently, an empirical based correlation to predict distance to maximum intensity of turbulence downstream of an obstacle in an explosion-induced flow using the available steady state experiments was developed in this study. The correlation would serve as a prerequisite for determining an optimum spacing between obstacles thereby determining worst case gas explosions overpressure and flame speeds. Using a limited experimental work on systematic study of obstacle spacing, the correlation was validated against 13 different test conditions. A ratio of the optimum spacing from the experiment, x_exp to the predicted optimum spacing, x_pred for all the tests was between 2-4. This shows that a factor of three higher than the x_pred would be required to produce optimum obstacle spacing that will lead to maximum explosion severity. In planning the layout of new installations, it is appropriate to identify the relevant worst case obstacle separation in order to avoid it. In assessing the risk to existing installations and taking appropriate mitigation measures it is important to evaluate such risk on the basis of a clear understanding of the effects of separation distance and congestion. It is therefore suggested that the various new correlations obtained from this work be subjected to further rigorous validation from relevant experimental data prior to been applied as design tools.     

Initial phase modelling in numerical explosion applied to process safety

The utilisation of computational fluid dynamics (CFD) in process safety has increased significantly in recent years. The modelling of accidental explosion via CFD has in many cases replaced the classical Multi Energy and Brake Strehlow methods. The benefits obtained with CFD modelling can be diminished if proper modelling of the initial phase of explosion is neglected. In the early stages of an explosion, the flame propagates in a quasi-laminar regime. Proper modelling of the initial laminar phase is a key aspect in order to predict the peak pressure and the time to peak pressure. The present work suggests a modelling approach for the initial laminar phase in explosion scenarios. Findings are compared with experimental data for two classical explosion test cases which resemble the common features in chemical process areas (confinement and congestion). A detailed analysis of the threshold for the transition from laminar to turbulent regime is also carried out. The modelling is implemented in a fully 3D Navier–Stokes compressible formulation. Combustion is treated using a laminar flamelet approach based on the Bray, Moss and Libby (BML) formulation. A novel modified porosity approach developed for the unstructured solver is also considered. Results agree satisfactorily with experiments and the modelling is found to be robust.     

Fast and tiny: A model for the flame propagation of nanopowders

To avoid the influence of external parameters, such as the vessel volume or the initial turbulence, the explosion severity should be determined from intrinsic properties of the fuel-air mixture. Therefore, the flame propagation of gaseous mixtures is often studied in order to estimate their laminar burning velocity, which is both independent of external factors and a useful input for CFD simulation. Experimentally, this parameter is difficult to evaluate when it comes to dust explosion, due to the inherent turbulence during the dispersion of the cloud. However, the low inertia of nanoparticles allows performing tests at very low turbulence without sedimentation. Knowledge on flame propagation concerning nanoparticles may then be modelled and, under certain conditions, extrapolated to microparticles, for which an experimental measurement is a delicate task. This work focuses on a nanocellulose with primary fiber dimensions of 3 nm width and 70 nm length. A one-dimensional model was developed to estimate the flame velocity of a nanocellulose explosion, based on an existing model already validated for hybrid mixtures of gas and carbonaceous nanopowders similar to soot. Assuming the fast devolatilization of organic nanopowders, the chemical reactions considered are limited to the combustion of the pyrolysis gases. The finite volume method was used to solve the mass and energy balances equations and mass reactions rates constituting the numerical system. Finally, the radiative heat transfer was also considered, highlighting the influence of the total surface area of the particles on the thermal radiation. Flame velocities of nanocellulose from 17.5 to 20.8 cm/s were obtained numerically depending on the radiative heat transfer, which proves a good agreement with the values around 21 cm/s measured experimentally by flame visualization and allows the validation of the model for nanoparticles.     

Quantification of source-level turbulence during LNG spills onto a water pond

The use of computational fluid dynamics (CFD) models to simulate LNG vapor dispersion scenarios has been growing steadily over the last few years, with applications to LNG spills on land as well as on water. Before a CFD model may be used to predict the vapor dispersion hazard distances for a hypothetical LNG spill scenario, it is necessary for the model to be validated with respect to relevant experimental data. As part of a joint-industry project aimed at validating the CFD methodology, the LNG vapor source term, including the turbulence level associated with the evaporation process vapors was quantified for one of the Falcon tests.This paper presents the method that was used to quantify the turbulent intensity of evaporating LNG, by analyzing the video images of one of the Falcon tests, which involved LNG spills onto a water pond. The measured rate of LNG pool growth and spreading and the quantified turbulence intensity that were obtained from the image analysis were used as the LNG vapor source term in the CFD model to simulate the Falcon-1 LNG spill test. Several CFD simulations were performed, using a vaporization flux of 0.127 kg/m² s, radial and outward spreading velocities of 1.53 and 0.55 m/s respectively, and a range of turbulence kinetic energy values between 2.9 and 28.8 m²/s². The resulting growth and spread of the vapor cloud within the impounded area and outside of it were found to match the observed behavior and the experimental measured data.The results of the analysis presented in this paper demonstrate that a detailed and accurate definition of the LNG vapor source term is critical in order for any vapor cloud dispersion simulation to provide useful and reliable results.     

On the transient flow in the 20-liter explosion sphere

The turbulence level in the 20-l explosion sphere, equipped with the Perforated Dispersion Ring, was measured by means of laser Doppler anemometry. The spatial homogeneity of the turbulence was investigated by performing velocity measurements at various locations in the transient flow field. Directional isotropy was investigated by measuring two independent components of the instantaneous velocity. The transient turbulence level could be correlated by a decay law of the form

in which the exponent, n, assumes a constant value of 1.49±0.02 in the period between 60 and 200 ms after the start of the injection process. In this time interval the turbulence was also observed to be homogeneous and practically isotropic. The results of this investigation imply that the turbulence level in the 20-l explosion sphere at the prescribed ignition delay time of ms is not equal to the turbulence level in the 1 m³-vessel. Hence, these results call into question the widely held belief that the cube-root-law may be used to predict the severity of industrial dust explosions on the basis of dust explosion severities measured in laboratory test vessels.     

Analysis of dust distribution in silo during axial filling using computational fluid dynamics: Assessment on dust explosion likelihood

In this study, the dust distribution in a silo during axial filling was modelled using a commercial computational fluid dynamics (CFD) code. The work focused on the dust concentration distribution in the silo, for evaluating the likelihood of a dust explosion in the silo. The simulation was conducted using a combination of renormalized (RNG) k-epsilon and discrete phase models, with standard pressure interpolation and a second order upwind scheme. The predicted dust concentration distribution showed a good agreement with experimental data adopted from the literature. It was found that the dust concentration distribution was influenced by mean velocity and turbulence flow. The simulation results suggest that the cornstarch concentration inside the silo was always above the lower explosion limit (LEL), hence requiring a mitigating action or a control system to reduce the explosion risk.     

局部扰动对主坑道爆炸波发展的数值模拟与实验研究

在地下建筑物,如隧道、地下储油库、人防工程、地下物资仓库等里面,由主通道旁结分支通道是最常见的一种布置形式,是一种典型的复杂受限空间结构.一旦有可燃气体发生爆炸燃烧,爆炸压力波和火焰的传播将受众多因素的影响,其中局部扰动的影响是主要因素之一.本文通过实验和数值模拟的方法研究了油气混合物在该复杂受限空间中由弱点火引起爆炸燃烧的发展过程,湍流强度经旁接分支坑道后在主通道中的变化,以及爆炸压力波和火焰经局部扰动后的变化过程;并将数值模拟结果与实验结果进行了对比和综合分析,得到了与地下受限空间安全相关的重要结论.湍流强度是复杂受限空间中可燃气体爆炸燃烧发展过程的主要影响因素之一,局部扰动将增强爆炸流场的湍流强度,加速燃烧化学反应,能量的释放量和速率大大提高.这些能量的快速加入促进了高峰值压力波的形成,火焰也被加速,爆炸从此由弱转强,出现跃升.研究结果对地下受限空间爆炸过程的进一步研究以及爆炸灾害的预防都有参考价值.