Concentration model for gas releases in buildings and the mitigation effect

In case of accidents involving releases of hazardous materials, calculating the gas dispersion is essential for assessing risks. In general, the leaked chemical is assumed to be instantly dispersed to the atmosphere if the leak occurs in the outdoor location. However, a different approach should be made for the incidents when sources are located inside a building. For the indoor release, the gas will be diluted prior to the release to the atmosphere and the gas release from a building to the atmosphere demands the application of another model before the dispersion calculation. The indoor release model calculates average indoor concentration and volumetric flowrate to the exterior. The model is fast and reasonably accurate compared to rigorous but time-consuming computational fluid dynamics (CFD) models. The model results were compared with experimental data, and CFD simulation results both with simple geometry to demonstrate validation and assess the performance of the indoor release model. Lastly, the behavior and effect of mitigation of indoor release were demonstrated by using the model results.     

Quantitative risk analysis of toxic gas release caused poisoning—A CFD and dose–response model combined approach

Some major toxic gas release accidents demonstrate the urgent need of a systematic risk analysis method for individuals exposed to toxic gases. A CFD numerical simulation and dose–response model combined approach has been proposed for quantitative analysis of acute toxic gas exposure threats. This method contains four steps: firstly, set up a CFD model and monitor points; secondly, solve CFD equations and predict the real-time concentration field of toxic gas releases and dispersions; thirdly, calculate the toxic dose according to gas concentration and exposure time; lastly, estimate expected fatalities using dose–response model. A case study of hydrogen sulfide releases from a gas gathering station has been carried out using a three dimension FLUENT model. Acute exposure fatalities have been evaluated firstly with a simplified ideal model which assumes workers stay at original exposure location without moving. Then a comparison has been made with a more realistic model which assumes workers start evacuating according to a prearranged course as soon as hydrogen sulfide detection system alarms. These two models represent the worst and best emergency response effects, respectively, and the analysis results demonstrate significant differences. Results indicate that the CFD and dose–response combined approach is a good way for estimating fatalities of individuals exposed to accidental toxic gas releases.     

Numerical analysis of toxic cloud generation and dispersion: A case study of the ethylene oxide spill

The present study examined the accidental spill of ethylene oxide, and a sensitivity analysis of the corresponding consequences was conducted using computational fluid dynamics (CFD). A validation of the gas dispersion CFD model against the experimental data sets included in the model evaluation protocol (MEP) was performed. The effect of the variability of the wind velocity on the extension of the hazardous areas and pool evaporation characteristics was evaluated. Additionally, the mitigation effects of the dike walls surrounding a spill were discussed. CFD simulation results have shown that the mitigation effect of dike walls is determined by their influence on both gas dispersion and pool evaporation and depends strongly on wind velocity in terms of toxic impact distances.     

A dynamic approach for evaluating the consequences of toxic gas dispersion in the chemical plants using CFD and evacuation modelling

Accidental releases of toxic gas in the chemical plants have caused significant harm to the exposed occupants. To evaluate the consequences of these accidents, a dynamic approach considering the gas dispersion and behavior evacuation modelling has been proposed in this paper. This approach is applied to a hypothetical scenario including an accidental chlorine release in a chemical plant. CFD technique is utilized to calculate the time-varying concentration filed and evacuation modelling is used to obtain the evacuation routes. The exposure concentrations in the evacuation routes are calculated by using the code of data query. The integrated concentration toxic load model and probit model are used to calculate the probability of mortality of each occupant by using the exposure concentrations. Based on this dynamic approach, a new concept of average probability of mortality (APM) has been proposed to quantify the consequences of different accidental scenarios. The results show that APM decreases when the required detection time decreases or emergency evacuation mode is implemented. The impact of the detection time on APM becomes small as the wind speed increases. The effect of emergency evacuation mode is more obvious when the release occurs in an outdoor space.     

Numerical simulation of LNG dispersion under two-phase release conditions

The use of LNG (liquefied natural gas) as fuel brings up issues regarding safety and acceptable risk. The potential hazards associated with an accidental LNG spill should be evaluated, and a useful tool in LNG safety assessment is computational fluid dynamics (CFD) simulation. In this paper, the ADREA-HF code has been applied to simulate LNG dispersion in open-obstructed environment based on Falcon Series Experiments. During these experiments LNG was released and dispersed over water surface. The spill area is confined with a billboard upwind of the water pond. FA1 trial was chosen to be simulated, because its release and weather conditions (high total spill volume and release rate, low wind speed) allow the gravitational force to influence the cold, dense vapor cloud and can be considered as a benchmark for LNG dispersion in fenced area. The source was modeled with two different approaches: as vapor pool and as two phase jet and the predicted methane concentration at sensors' location was compared with the experimental one. It is verified that the source model affect to a great extent the LNG dispersion and the best case was the one modeling the source as two phase jet. However, the numerical results in the case of two phase jet source underestimate the methane concentration for most of the sensors. Finally, the paper discusses the effect of neglecting the ?9.3° experimental wind direction, which leads to the symmetry assumption with respect to wind and therefore less computational costs. It was found that this effect is small in case of a jet source but large in the case of a pool source.     

Ventilation theory and dispersion modelling applied to hazardous area classification

Critical formulae given in the current Explosive Atmospheres Hazardous Area Classification Standard IEC 60079-10-1 (2008) [BS EN 60079-10-1, 2009] to determine the expected gas cloud volume which is used to determine area classification do not have any scientific justification. The standard does allow the alternative use of Computational Fluid Dynamics (CFD) methods, which serve to compound the concern with these formulae: the predicted volume of the gas cloud from CFD models being several orders of magnitude smaller than that given by the formulae in question. To resolve such major discrepancies, replacement of the current formulae with a scientifically validated approach is proposed. Integral models of dispersion and ventilation have been used routinely for many years in the analysis of major hazards in the chemical industry. This paper presents an adaptation of these models to determine the expected volume of a gas cloud arising from a release of gas from a pressurised source. A very simple integral jet model is presented for outdoor dispersion, extended to the case of indoor dispersion, from which the volume of the gas cloud is derived. The single free parameter, an entrainment coefficient, is fixed by comparison with data on a free jet, and then predictions of the model are compared with CFD calculations (which themselves have been validated against experimental data) for dispersion within an enclosed volume. The results of this simple integral model are seen to agree very well with the CFD predictions. The methodology presented here is therefore proposed as a scientifically validated approach to Hazardous Area Classification.     

Dynamic simulation of hazard analysis of radiations from LNG pool fire

Because of its highly flammable nature, any accidental release of liquefied natural gas (LNG) could possibly pose significant fire hazard. In this study, a computational fluid dynamics (CFD) model was used to analyze this hazard around an existing LNG station. By assuming an LNG pool fire occurring in an impoundment area, dynamic simulations of flame development have been carried out. In order to provide more reliable simulation results, a study was first conducted to determine the mesh independence and suitable time step. The results of CFD simulations were also compared with those using the commonly-used phenomenological model. The simulation results showed that LNG tanks in the neighbor dike area could withstand the received radiant heat flux, and the areas involving human activities, such as security office and public area, were also secure enough for people to escape from the hazards. LNG vaporizers, which are often located close to tank area, could possibly receive relatively higher radiant heat flux. High temperature achieved on vaporizers could cause material failure. CFD calculations have also indicated that increasing the spacing distance or using flowing water curtain could reduce this temperature. It is concluded that CFD method is significantly more effective to account for LNG hazard analysis and provide realistic results for complicated scenarios, thus providing meaningful information for safety consideration.     

Modeling the two-phase cloud evolution from instantaneous flashing release using CFD

Computational Fluid Dynamics (CFD) approach has been successfully applied to simulate the small-scale instantaneous flashing release experiment by Pettitt. A model for dispersion of the release event is provided based on relevant theories and existing experimental data. An application of the CFD method to the dispersion simulation is illustrated. Furthermore, a new methodology based on discrete phase model for setting computational initial conditions is provided. An initial expansion and subsequent turbulence dispersion can be characteristically identified from both volume and temperature variation of the cloud obtained by the simulation. The possible mechanism for these phenomena has also been discussed and analyzed. The study deepens the understanding of the physical process of this event and provides one more reliable tool for relevant safety systems.     

Numerical study on the initial expansion of two-phase cloud from an instantaneous release

In industries some dangerous liquefied gases may accidentally release and it may form a flammable or toxic mixture after mixing with air. One tool that is being developed in industry for two-phase cloud dispersion modeling is computational fluid dynamics (CFD). In this paper, the dispersion processes of different dangerous materials including liquefied chlorine, liquefied ammonia and liquefied petroleum gas were simulated in the same condition to analyze the characteristics of the initial expansion processes by CFD tool. The heat and mass transfer between droplets and the vapor after an instantaneous release event was calculated by using the Eulerian–Lagrangian method. The results from a number of 3-D CFD based studies were compared with the available small-scale experimental results. The results show that the present model and numerical simulation are reliable.     

CFD modelling of hydrogen release, dispersion and combustion for automotive scenarios

The paper describes the analysis of the potential effects of releases from compressed gaseous hydrogen systems on commercial vehicles in urban and tunnel environments using computational fluid dynamics (CFD). Comparative releases from compressed natural gas systems are also included in the analysis.

This study is restricted to typical non-articulated single deck city buses. Hydrogen releases are considered from storage systems with nominal working pressures of 20, 35 and 70 MPa, and a comparative natural gas release (20 MPa). The cases investigated are based on the assumptions that either fire causes a release via a thermally activated pressure relief device(s) (PRD) and that the released gas vents without immediately igniting, or that a PRD fails. Various release strategies were taken into account. For each configuration some worst-case scenarios are considered.

By far the most critical case investigated in the urban environment, is a rapid release of the entire hydrogen or natural gas storage system such as the simultaneous opening of all PRDs. If ignition occurs, the effects could be expected to be similar to the 1983 Stockholm hydrogen accident [Venetsanos, A. G., Huld, T., Adams, P., & Bartzis, J. G. (2003). Source, dispersion and combustion modelling of an accidental release of hydrogen in an urban environment. Journal of Hazardous Materials, A105, 1–25]. In the cases where the hydrogen release is restricted, for example, by venting through a single PRD, the effects are relatively minor and localised close to the area of the flammable cloud. With increasing hydrogen storage pressure, the maximum energy available in a flammable cloud after a release increases, as do the predicted overpressures resulting from combustion. Even in the relatively confined environment considered, the effects on the combustion regime are closer to what would be expected in a more open environment, i.e. a slow deflagration should be expected.

Among the cases studied the most severe one was a rapid release of the entire hydrogen (40 kg) or natural gas (168 kg) storage system within the confines of a tunnel. In this case there was minimal difference between a release from a 20 MPa natural gas system or a 20 MPa hydrogen system, however, a similar release from a 35 MPa hydrogen system was significantly more severe and particularly in terms of predicted overpressures. The present study has also highlighted that the ignition point significantly affects the combustion regime in confined environments. The results have indicated that critical cases in tunnels may tend towards a fast deflagration, or where there are turbulence generating features, e.g. multiple obstacles, there is the possibility that the combustion regime could progress to a detonation.

When comparing the urban and tunnel environments, a similar release of hydrogen is significantly more severe in a tunnel, and the energy available in the flammable cloud is greater and remains for a longer period in tunnels. When comparing hydrogen and natural gas releases, for the cases and environments investigated and within the limits of the assumptions, it appears that hydrogen requires different mitigation measures in order that the potential effects are similar to those of natural gas in case of an accident. With respect to a PRD opening strategy, hydrogen storage systems should be designed to avoid simultaneous opening of all PRD, and that for the consequences of the released energy to be mitigated, either the number of PRDs opening should be limited or their vents to atmosphere should be restricted (the latter point would require validation by a comprehensive risk assessment).     

CFD-based simulation of dense gas dispersion in presence of obstacles

Quantification of spatial and temporal concentration profiles of vapor clouds resulting from accidental loss of containment of toxic and/or flammable substances is of great importance as correct prediction of spatial and temporal profiles can not only help in designing mitigation/prevention equipment such as gas detection alarms and shutdown procedures but also help decide on modifications that may help prevent any escalation of the event.The most commonly used models - SLAB (Ermak, 1990), HEGADAS (Colenbrander, 1980), DEGADIS (Spicer & Havens, 1989), HGSYSTEM (Witlox & McFarlane, 1994), PHAST (DNV, 2007), ALOHA (EPA & NOAA, 2007), SCIPUFF (Sykes, Parker, Henn, & Chowdhury, 2007), TRACE (SAFER Systems, 2009), etc. - for simulation of dense gas dispersion consider the dispersion over a flat featureless plain and are unable to consider the effect of presence of obstacles in the path of dispersing medium. In this context, computational fluid dynamics (CFD) has been recognized as a potent tool for realistic estimation of consequence of accidental loss of containment because of its ability to take into account the effect of complex terrain and obstacles present in the path of dispersing fluid.The key to a successful application of CFD in dispersion simulation lies in the accuracy with which the effect of turbulence generated due to the presence of obstacles is assessed. Hence a correct choice of the most appropriate turbulence model is crucial to a successful implementation of CFD in the modeling and simulation of dispersion of toxic and/or flammable substances.In this paper an attempt has been made to employ CFD in the assessment of heavy gas dispersion in presence of obstacles. For this purpose several turbulence models were studied for simulating the experiments conducted earlier by Health and Safety Executive, (HSE) U.K. at Thorney Island, USA (Lees, 2005). From the various experiments done at that time, the findings of Trial 26 have been used by us to see which turbulence model enables the best fit of the CFD simulation with the actual findings. It is found that the realizable k-? model was the most apt and enabled the closest prediction of the actual findings in terms of spatial and temporal concentration profiles. It was also able to capture the phenomenon of gravity slumping associated with dense gas dispersion.     

Fused CFD-interpolation model for real-time prediction of hazardous gas dispersion in emergency rescue

Accidental releases of hazardous gases in chemical industries can pose great threats to public security. The computational fluid dynamics (CFD) model is commonly applied to predict gas dispersion in complex structured areas. It can provide good accuracy but it is too time-consuming to be used in emergency response. To reduce computation time while keep acceptable accuracy, this paper proposes several fused CFD-interpolation models which combine CFD model with different interpolation methods. Spline, linear and nearest interpolation methods are used. A CFD simulations database is created ahead of time which can be quickly recalled for emergency usage and unknown situations can be predicted instantly by interpolation methods instead of time-consuming CFD model. Fused models were applied to a case study involving a hypothetical propane release with varying conditions and validated against CFD model. The validation shows that prediction accuracy of these fusion models is acceptable. Among these models, CFD-Spline interpolation model performs best. It is faster than CFD model by a factor of 75 and is potentially a good method to be applied to real-time prediction.     

Y型通风下采空区瓦斯运移规律及治理研究

为了更好的研究Y型通风系统下的采空区的瓦斯流动和涌出规律,针对综放面Y型通风系统特点,建立了Y型通风采空区流场模拟的计算流体力学模型。通过数值模拟,系统研究了Y型通风采空区流场和瓦斯运移规律,对比分析了Y型通风和U型通风条件下的采空区流场及瓦斯运移特征,并将其应用于15120高瓦斯综采工作面的Y型通风系统中,根据现场的实际情况建立对应的CFD模型,得出Y型通风系统下采空区瓦斯流动及分布规律,数值模拟结果与现场大量观测数据相吻合,为瓦斯治理和通风系统优化提供理论依据。研究表明,采用Y型通风系统可消除采空区向上隅角的集中漏风,从而有效解决了U型通风上隅角瓦斯积聚和回风巷中的瓦斯。     

Evaluation of gas release rate through holes in pipelines

A mathematical model of an accidental gas release in a long transmission pipeline is presented in terms of computational fluid mechanics. It was found that the hole model is suitable for the release of gas through a small hole, while the pipe model is suitable for the gas release through a hole corresponding to the complete breaking of the pipe. In this paper, a new model was proposed for a hole that lies between both these situations. The results of the example show that when the initial inside pressure is higher than 1.5 MPA, the mass of gas released during the sonic flow is more than 90% of the total mass of gas released. The average release rate of the total release process could be substituted by the average release rate of the sonic flow, or by 30% of the initial release rate. This approximation would become more accurate with the increase in the initial inside pressure.     

Real-time risk assessment of explosion on offshore platform using Bayesian network and CFD

Combustion or explosion accident resulting from accidental hydrocarbon release poses a severe threat to the offshore platform's operational safety. Much attention has been paid to the risk of an accident occurring over a long period, while the real-time risk that escalates from a primary accident to a serious one was ignored. In this study, a real-time risk assessment model is presented for risk analysis of release accidents, which may escalate into a combustion or explosion. The proposed model takes advantage of Fault Tree-Event Tree (FT-ET) to describe the accident scenario, and Bayesian network (BN) to obtain the initial probability of each consequence and describe the dependencies among safety barriers. Besides, Computational Fluid Dynamics (CFD) is applied to handle the relationship between gas dispersion and time-dependent risk. Ignition probability model that considering potential ignition sources, gas cloud, and time series are also integrated into this framework to explain the likelihood of accident evolution. A case of release accidents on a production platform is used to test the availability and effectiveness of the proposed methodology, which can be adopted for facilities layout optimization and ignition sources control.     

An alternative CFD tool for gas dispersion modelling of heavy gas

The numerical simulation of gas dispersion is of great importance in various areas of engineering such as optimisation, synthesis of chemical process, petroleum industry and process safety. The OpenFOAM (Open Field Operation and Manipulation) code is a free and open source computational fluid dynamics (CFD) program. The current research is focused on the development and customisation of a computational tool for handling gas dispersion of heavy gases, such a LNG and CO₂. The novel CFD tool relies on OpenFOAM framework. The core of the work is based on the OpenFOAM solver rhoReactingBuoyantFoam to handle gas dispersion. A series of CFD simulations has been performed for methane and CO₂. The source term of the former is modelled by HSM (Hybrid Switch Model). The model comprises contribution from HEM (Homogeneous Equilibrium Model) approach, frozen model and non-equilibrium model for CO₂ leak. The novel approach switches between equilibrium and non-equilibrium conditions based on the meta-stable parameter on the grounds of thermodynamics and experimental observations. Good agreement with experimental data is observed. Numerical findings for methane leakage from the proposed CFD tool are compared with experimental data and FLACS. Good agreement is observed.     

Numerical and experimental study on flame spread over conveyor belts in a large-scale tunnel

Conveyor belt fires in an underground mine pose a serious life threat to the miners. This paper presents numerical and experimental results characterizing a conveyor belt fire in a large-scale tunnel. A computational fluid dynamics (CFD) model was developed to simulate the flame spread over the conveyor belt in a mine entry. Thermogravimetric analysis (TGA) tests were conducted for the conveyor belt and results were used to estimate the kinetic properties for modeling the pyrolysis process of the conveyor belt burning. The CFD model was calibrated using results from the large-scale conveyor belt fire experiments. The comparison between simulation and test results shows that the CFD model is able to capture the major features of the flame spread over the conveyor belt. The predicted maximum heat release rate, and maximum smoke temperature are in good agreement with the large-scale tunnel fire test results. The calibrated CFD model can be used to predict the flame spread over a conveyor belt in a mine entry under different physical conditions and ventilation parameters to aid in the design of improved fire detection and suppression systems, mine rescue, and mine emergency planning.     

化学品突发性事故预测的不确定性分析

介绍化学品突发性事故预测的不确定性概率统计和模糊数学分析方法。对参数和模式的不确定性分别进行了讨论,计算机系统包括化学品常用数据库、事故识别模块,事故后果分析模块和不确定性分析模块,对液氯泄漏排放事故、液化气（丙烷）两相闪蒸爆炸和管道煤气（CO）泄漏伤害模式实例进行了分析。对连续源和瞬时源的讨论说明泄漏时间的确定对事故分析极为重要。