地铁施工盾构机开仓气体安全检测分析

盾构法隧道施工在我国城市地铁建设中得到了广泛应用,也是高技术、高风险的施工工法。由于受通过地层地质条件及刀具耐磨情况的影响,盾构机需要不定期地进仓进行检查或换刀作业,仓内存在的易燃易爆气体有害气体直接影响作业人员安全,可能窒息和一氧化碳中毒等安全事故发生。结合华南某城市地铁隧道盾构施工的实际情况,应用数值统计分析方法,对仓内甲烷、一氧化碳、二氧化碳、二氧化硫、氧气、硫化氢、氨气、氮氧化物等八种气体检测结果进行统计,根据统计结果,初步分析了地铁工程隧道盾构法施工盾构机仓内的气体浓度情况及其危害存在的可能性,论述作业人员进仓施工可能遇到的安全风险,提出预防窒息和一氧化碳中毒事故发生的安全防范措施,为地铁隧道盾构法现场管理及施工安全提供参考。     

Airbag for the closing of pipelines on explosions and leakages

This paper is a result of international effort aimed at the construction of a device for quick closing of pipelines in the case of explosion propagation and/or chemical leakage. Such a problem exists in industries where flammable substances are transported by pipelines. The basic solution principle was the idea to use airbags similar to those utilized in cars.

Two pipeline applications were taken into consideration: a low-pressure module able to suppress explosion propagation and a high-pressure module to stop leakages from, e.g. natural gas pipeline capable to be used for duct diameters up to 0.6 m, pressures up to 5 MPa and reaction times of 50 ms. It was necessary to construct a new airbag, capable of withstanding up to 10 bar pressure. The choice of material was critical to ensure sufficient strength and chemical resistance while retaining impermeability.

CFD modeling of the bag deployment into a pipe flow and analysis of the bag shapes was also completed. Two gas generators were constructed and tested with novel propellant materials.

Different airbag models were tested to evaluate their effectiveness. Risk analysis approach was applied to evaluate the safety and economic benefits of the new technology in different fields of application.     

Dilution with air to minimise consequences of toxic/flammable gas releases

Dilution has long been considered a solution to many problems of toxic/flammable material releases. It implies diluting to a concentration that is below physiologically dangerous levels for a toxic substance (generally below TLV), or to a level below LFL for a flammable material release, ensuring that the process adopted for dilution does not itself enhance the risks.

In this paper, we discuss the dilution of a gaseous release by deliberate and cautious mixing with air to reduce its concentration to a harmless level. The idea bears its origin to the Bhopal Gas Tragedy where some families saved themselves by turning the ceiling fans on when MIC reached their bedrooms at the dead of very cold night on December 2–3, 1984. The air pushed in by the fans diluted the MIC to below the harm level.

Some of the advantages of using air dilution are: no cost of air, no air storage needed, no need to treat the air after use as in case of water curtains; required equipment, its maintenance and staff training in its use are very likely to cost less than in other ways of handling a release.

Air dilution may not be feasible in all cases, such as gaseous release within a congested equipment layout, release that forms a liquid pool, etc. The method needs to be evaluated for each case.     

Assessing the influence of real releases on explosions: Selected results from large-scale experiments

The research activities in the project Assessing the Influence of Real Releases on Explosions (AIRRE) included a unique series of large-scale explosion experiments with high-momentum jet releases directed into congested geometries with subsequent ignition. The primary objective for the AIRRE project was to gain improved understanding of the effect that realistic releases and turbulent flow conditions have on the consequences of accidental gas explosions in the petroleum industry. A secondary objective was to develop a methodology that can facilitate safe and optimal design of process facilities. This paper presents selected results from experiments involving ignition of a highly turbulent gas cloud, generated by a large-scale, pressurised release of natural gas. The paper gives an overview of the effect on maximum explosion overpressures of varying the ignition position relative to the release point of the jet and a congested region placed inside the flammable cloud, with either a high or a medium level of congestion. For two of the tests, involving a jet release and the medium congestion rig, the maximum overpressures significantly exceeded those obtained in a quiescent reference test. The paper presents detailed results for selected tests and discusses the effect of the initial flow field generated by realistic releases – including turbulence, net flow and concentration gradients – on relevant explosion phenomena.     

Utilization of regression technique to develop a predictive model for hazard radius from release of typical methane-rich natural gas

The aim of hazardous area classification around equipment handling or storing of flammable fluids is to avoid the ignition of those releases that may occur from time to time in the operation of these equipment. There is a point source approach for the classification of hazardous areas which can estimate hazard radius by using hole size and release pressure. Methane-rich natural gas is widely used or produced in the process industries. Till date, there exist no reference that represents hazard radii for the wide range of possible hole sizes and release pressures of this fluid. The aim of the present study was to propose a predictive model for estimation of hazard radii due to releases of typical methane-rich natural gas based on hole size and release pressure. In this study, a complete database of hazard radii due to a broad range of hole sizes and release pressures was provided using available discharge and dispersion models. A regression-based model for estimation of hazard radii was developed based on the provided database. Performance investigation of the proposed model and a case study showed that the results are reliable with an acceptable standard error.     

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.     

Validation of FLACS against experimental data sets from the model evaluation database for LNG vapor dispersion

The siting of facilities handling liquefied natural gas (LNG), whether for liquefaction, storage or regasification purposes, requires the hazards from potential releases to be evaluated. One of the consequences of an LNG release is the creation of a flammable vapor cloud, that may be pushed beyond the facility boundaries by the wind and thus present a hazard to the public. Therefore, numerical models are required to determine the footprint that may be covered by a flammable vapor cloud as a result of an LNG release. Several new models have been used in recent years for this type of simulations. This prompted the development of the “Model evaluation protocol for LNG vapor dispersion models” (MEP): a procedure aimed at evaluating quantitatively the ability of a model to accurately predict the dispersion of an LNG vapor cloud.This paper summarizes the MEP requirements and presents the results obtained from the application of the MEP to a computational fluid dynamics (CFD) model – FLACS. The entire set of 33 experiments included in the model validation database were simulated using FLACS. The simulation results are reported and compared with the experimental data. A set of statistical performance measures are calculated based on the FLACS simulation results and compared with the acceptability criteria established in the MEP. The results of the evaluation demonstrate that FLACS can be considered a suitable model to accurately simulate the dispersion of vapor from an LNG release.     

Safety analysis of instantaneous release of compressed natural gas from a cylinder

This study presents a numerical model to analyze the sudden failure of compressed natural gas (CNG) cylinder onboard a CNG vehicle. The model is developed using COMSOL. It accounts for the real gas effects, physical energy, and combustion of the flammable gas. The model is tested using experimental data.The study highlight compression energy as one of the serious concern. An unintentional rupture of a compressed cylinder filled with natural gas would generate a rapid energy release in the form of the pressure energy (blast). The release of energy and gas would cause rapid mixing and generate overpressure and may also cause flash fire. A detailed failure frequency analysis is also done to analyze the effectiveness of barriers. This study identifies critical points for the safe operation of the CNG system onboard a vehicle.     

Dynamic pressure induced by a methane–air explosion in a coal mine

The hazardous effect of dynamic pressure and strong gas flows induced by a methane–air mixture explosion in underground coal mines is studied. The dynamic pressure effect of a methane–air explosion was analyzed by numerical simulation, in a duct and tunnel. Compared to the overpressure generated by an explosion that can act on a body, the dynamic pressure caused by the high-speed flow of the gaseous combustion products can cause serious damage as well. At the structural opening of a coal mine, the destruction caused by the dynamic pressure induced by a methane–air explosion is more serious than the overpressure. For a tube or tunnel partially filled by a methane–air mixture, the dynamic pressure is lower than the overpressure in the region occupied by the flammable mixture. Beyond the premixed region, the dynamic pressure is of the same order of magnitude as the overpressure.     

常压及低压下锂电池热失控随数量变化特性

为明确在地面常压环境和商用飞机巡航高度低气压环境下锂电池热失控火灾危险特性随电池数量的变化关系,分别于95 kPa地面常压环境和20 kPa低压环境下,开展不同电池数量梯度的热失控试验,测量热释放速率,总热释放量,烟气温度,CO、CO2和碳氢等气体的实时体积分数.结果表明:最高热释放速率和总热释放量与电池数量均呈幂函数...     

穿越城市生活区的天然气管道泄漏连锁爆燃后果评估研究*

为评估城市天然气管道泄漏连锁爆燃事故后果,基于计算流体力学(CFD)方法构建穿越城市区域的天然气管道泄漏连锁爆燃后果预测与评估模型,以某城市生活区域为例,在城市生活区域建筑物内风场流动计算的基础上,模拟风场作用下可燃气体在城市建筑物空间内的运移规律,预测可燃气云的积聚区域;考虑意外点火的情况,计算城市生活区域内可燃气云爆燃灾害特征,预测爆燃超压、热辐射和高温的影响。研究结果表明:由于建筑物之间的阻挡与反射作用,建筑物下风向有明显的低风速区域,并在一定时间段后扩散过程趋于稳定;在爆燃火焰作用下,高温和热辐射会造成建筑物部分钢结构发生失效变形。     

可燃气体储罐区泄漏危险性定量分析

对位于某城市中心附近的可燃气体储罐区的气体泄漏危险性进行了分析,求出了下风向最大可燃范围和中毒范围.进行灵敏度分析以便识别风速、泄漏面积对泄漏危险性的影响.分析结果显示,风速、泄漏面积对泄漏危险性有显著影响.随着泄漏面积增大,下风向最大可燃范围增大;随着风速的增大,下风向最大可燃范围则减小.最后提出了若干安全措施的建议.     

Modelling the mitigation of a hydrogen deflagration in a nuclear waste silo ullage with water fog

During the decommissioning of certain legacy nuclear waste storage plants it is possible that significant releases of hydrogen gas could occur. Such an event could result in the formation of a flammable mixture within the silo ullage and, hence, the potential risk of ignition and deflagration occurring, threatening the structural integrity of the silo. Very fine water mist fogs have been suggested as a possible method of mitigating the overpressure rise, should a hydrogen–air deflagration occur. In the work presented here, the FLACS CFD code has been used to predict the potential explosion overpressure reduction that might be achieved using water fog mitigation for a range of scenarios where a hydrogen–air mixture, of a pre-specified concentration (containing 800 L of hydrogen), uniformly fills a volume located in a model silo ullage space, and is ignited giving rise to a vented deflagration. The simulation results suggest that water fog could significantly reduce the peak explosion overpressure, in a silo ullage, for lower concentration hydrogen–air mixtures up to 20%, but would require very high fog densities to be achieved to mitigate 30% hydrogen–air mixtures.     

Source term estimation of natural gas leakage in utility tunnel by combining CFD and Bayesian inference method

As an effective way to construct and maintain various life pipelines in urban areas and industrial parks, the underground utility tunnel has been developed rapidly in China in recent years. However, the natural gas pipeline leakage in a utility tunnel may cause fire, explosion or other coupling disastrous accidents that could result in fatal consequences. The effective source term estimation (STE) of natural gas leakage can provide technical supports for emergency response during natural gas leakage accidents in utility tunnels. In this paper, a STE model with the combination of gas transport model, Bayesian inference and slice sampling method is proposed to estimate the source parameters of natural gas leakage in underground utility tunnels. The observed data can be integrated into the gas transport model and realize the inversion of natural gas leakage location and release rates. The parameter sensitivity analysis is presented to evaluate the robustness of the proposed model with good practicability, and the gas sensor layouts in the utility tunnel are analyzed and optimized. The spatio-temporal distribution of the leaked gas could be well predicted based on the estimation source parameters by the proposed STE model. The results show that the proposed model is an alternative and effective tool to provide technical supports for loss prevention and mitigation for natural gas leakage accidents in urban utility tunnels.     

Thermal interaction analysis in pipeline systems: A case study

The assessment of the consequences of high pressure releases of flammable gases is a fundamental requirement for the safe design and operation of industrial installations, plants and pipework. A scenario of interest concerns a high pressure jet-fire following the ignition of a gas jet release which results in a thermal loading to the surroundings and possibly leads to accident escalation.

In the present paper, a case study is presented: two parallel-laid natural gas pipelines have been considered, the accidental scenarios which may possibly occur as a consequence of a pipeline failure have been discussed and the thermal effects caused by the jet-fire developing from different rupture sizes have been assessed.

Three scenarios have been analyzed, considering the pipelines being within a highly congested area: (i) large failure and vertical jet with detached flame; (ii) small failure with jet fire directly impinging on the parallel pipeline; (iii) small failure with pipeline engulfed within fire.

Once the temperature gradient through the pipeline wall has been found, the stresses deriving from pressure load and steel differential expansion have been analytically calculated and compared with the yielding stresses at the temperature achieved by the pipeline wall.

In the first scenario the pipeline is able to resist without major problems; in the second case the pipeline rupture is likely to occur; in the third scenario the pipeline resists to the applied loads but with a low margin to yielding.

It is understood that the analysis results are very much dependent on the utilized hypotheses, therefore a sensitivity analysis was performed in order to assess the variation of the results as a function of the variation of problem data; this analysis identifies the large influence of the parameters on the final result.     

A CFD based explosion risk analysis methodology using time varying release rates in dispersion simulations

A full probabilistic Explosion Risk Analysis (ERA) is commonly used to establish overpressure exceedance curves for offshore facilities. This involves modelling a large number of gas dispersion and explosion scenarios. Capturing the time dependant build up and decay of a flammable gas cloud size along with its shape and location are important parameters that can govern the results of an ERA. Dispersion simulations using Computational Fluid Dynamics (CFD) are generally carried out in detailed ERA studies to obtain these pieces of information. However, these dispersion simulations are typically modelled with constant release rates leading to steady state results. The basic assumption used here is that the flammable gas cloud build up rate from these constant release rate dispersion simulations would mimic the actual transient cloud build up rate from a time varying release rate. This assumption does not correctly capture the physical phenomena of transient gas releases and their subsequent dispersion and may lead to very conservative results. This in turn results in potential over design of facilities with implications on time, materials and cost of a project.In the current work, an ERA methodology is proposed that uses time varying release rates as an input in the CFD dispersion simulations to obtain the fully transient flammable gas cloud build-up and decay, while ensuring the total time required to perform the ERA study is also reduced. It was found that the proposed ERA methodology leads to improved accuracy in dispersion results, steeper overpressure exceedance curves and a significant reduction in the Design Accidental Load (DAL) values whilst still maintaining some conservatism and also reducing the total time required to perform an ERA study.     

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.     

水泥窑用低氮燃烧器燃烧特性的试验研究

基于水泥生产线NO_x排放机理及控制方法,设计、研发了一种新型水泥窑用低氮燃烧器,并分析了天然气、煤制气及混合气的燃烧特性及其污染物的排放特性。结果表明:在过量空气系数α为1.2、二次风占比β为0.6时,新型低氮燃烧器燃烧天然气的效果较好,NO_x排放量为53 mg/m³,CO排放量为22 mg/m³;在相同燃烧条件下,煤制气燃烧温度及NO_x排放量比天然气高,CO排放量低于天然气;在过量空气系数α为1.2、二次风占比β为0.8时,NO_x排放量为57 mg/m³,CO排放量为6 mg/m³;天然气和煤制气掺烧时,其燃烧特性介于两者之间,且掺烧煤制气可提高天然气的燃烧速率。