Design of a system for real-time modelling of the dispersion of hazardous gas releases in industrial plants 2. Accidental releases from storage installations

This paper is the second part of a research programme concerning the modelling capabilities of accidental releases of heavier-than-air toxic gases. The existing theory, which includes the strength of the source and the subsequent development of the released cloud under representative environmental conditions, is described. Comparison of the ZZB-2 system predictions with field data from the Desert Tortoise and Lyme Bay V, ammonia and chlorine releases, shows excellent agreement at distances between ≈ 200 m and a few kilometres from the source. The correlation between observed and predicted cloud concentrations, was in all cases significant at a confidence level better than 95%.     

The use of water sprays for mitigating chlorine gaseous releases escaping from a storage shed

Accidental releases of toxic gases in partially confined spaces, like a storage shed, can sometimes be controlled by water sprays. This paper presents the results of experimental field tests during which various water sprays were used to mitigate chlorine gaseous releases. The releases (source strength: 1–4 kg/min) simulated a loss of containment occurring at an industrial chlorine storage installation (5 m³). The mitigation performances of different water sprays were investigated for diverse configurations, and under various atmospheric conditions. The best chlorine concentration reduction was achieved close to the source by a mobile upward water spray, with a maximum concentration reduction of a factor 10 at a distance of 5 m downstream from the source, and for a release flow rate of about 2 kg/min. The good performances of a fixed downward flat fan water spray were also pointed out (mean concentration reduction of a factor 2–5 for the whole series of experiments carried out), with an optimum of effectiveness at a distance of 10 m downstream from the source. In low wind speed conditions (U₁₀<1 m/s), the downward flat fan water spray was more effective for weak release flow rates. The mitigation effectiveness by absorption remained slight (<3%).     

Reduction of chlorine concentrations by using a greenbelt

This paper presents the experimental results of using a greenbelt to mitigate accidental releases of chlorine from a small storage installation. During the field tests carried out with the shrub Thuya orientalis, Linnaeus (oriental thuya), the ability of such a barrier of vegetation to mitigate accidental releases by reducing the chlorine downstream concentrations was investigated. Visible injuries to the shrubs exposed to chlorine have been noticed, since the damaged areas exhibited typical symptoms with needles necrosis and a burnt orange-brown color. Early defoliation was observed in the vegetation areas directly exposed to the gas plume. It was shown that in the presence of the greenbelt, the chlorine concentrations were lessened by a factor of 4 at a distance of 5 m downstream from the source. This dilution ability vanished swiftly with the distance downstream from the greenbelt.     

A new methodology to estimate the gas ascending time from underwater gas releases

Accidental subsea gas releases can pose a threat to people, equipment, and facilities since gas can be toxic or flammable at the concentrations in which the leak occurs. The accurate prediction of the behavior of the gas plume formed in the leaks can be fundamental to the development of techniques of accident prevention or, in some cases, remediation measures, avoiding the emergence of more serious consequences. Among the different ways to analyze the behavior of gas plumes formed underwater, the Computational Fluid Dynamics (CFD) tool stands out for allowing the study of plume behavior to be done in a safer, simpler, and less expensive way, if compared to experimental studies. Inspired by the accidental release of the subsea gas scenario, this work validated a CFD setup of a 2D two-phase air–water flow using the VOF method in Ansys Fluent. The use of the VOF method differs this work from other works that use a hybrid Eulerian–Lagrangian methodology to model such types of flow. In this validation, simulations with a 9 m base tank, and 7 m water depth, and 0.050, 0.100, and 0.450 m${}^3$/s gas flow were performed. The simulated data were compared to experimental results available in literature. After the validation of the setup, a study was carried out varying the size of the leak to 0.24 and 0.17 m, and the gas flow from 0.006 to 0.150 m${}^3$/s aiming to verify how some plume characteristics are affected by the changes. Finally, following the directions from literature for analyzing the ascending gas behavior, and combining it with a dimensional analysis of the data, we proposed a mathematical model for calculating the gas ascending time using only properties of the gas leak. With future modifications of the proposed methodology, we hope that soon it will be possible to simulate gas releases under more realistic conditions. Even so, the findings of this work are already a significant step forward in the study of underwater gas releases.     

Toxic release of chlorine and off-site emergency scenario – A case study

Release of chlorine gas causes deaths and injuries to workers and the public, resulting in the evacuation of communities and adversely affecting the environment as a whole. The off-site emergency plan is an integral part of any major hazard control system. This paper highlights some salient features of the emergency scenario from a chemical plant, which ultimately lead to fatal consequences all around upon releases of toxic chlorine gas. A typical scenario illustrating the dispersion model of chlorine (for three isopleths concentration) has been estimated by Complex Hazards Air Release Model (CHARM) software package. The enlarged form of this model diagram has been outlined on the area map of the study area for contingency planning. As a broad guide line to the district authorities for contingency planning, evacuation time has also been calculated with reference to a concentration level of 3 ppm chlorine.     

Influence of wind speed profile and roughness parameters on the downwind extension of vulnerable zones during dispersion of toxic dense gases

The accidental release of toxic chemicals, which are heavier than air and stored under pressure, may create an emergency scenario in an industrial plant. The extension of vulnerable distance in the downwind direction is an important criterion in framing an emergency management plan of that industrial area. There are several studies showing the role of surface level meteorological and topographical features on its propagation and dispersion just after its release from a container. In the present study, vertical variation of wind speed in the atmospheric boundary layer and surface roughness parameter have been considered to study their roles on the impact of downwind extension of vulnerable distances. A catastrophic release from a tonner having 900 kg of liquid chlorine has been considered, and SAFETI Micro developed by DNV Technica, UK has been utilized in the consequence analysis of this release. The analysis results have been explained for various atmospheric stability classes and surface wind speeds.     

Key airborne concentrations of chemicals for emergency response planning in HAZMAT road transportation- margin of safety or survival

The release of chemicals due to road transportation accidents could have adverse consequences such as fatality, physical and financial loss and environmental damage. The purpose of this study was to determine the suitable margin of safety/survival of individuals in HAZMAT road transportation accidents for use in Emergency Response Planning (ERP). In this study, at first, the safety margin and survival margin were defined and proposed. Then, as a case study, the chemical tanker trucks in Iran's road transport fleet were investigated and the full bore rupture of the tanker trucks was considered as the selective scenario. Eventually, safety margin and survival margin were determined using ALOHA and PHAST software and the Chemical Exposure Index (CEI). The results showed that using the CEI, among the selected chemicals, ammonia, chlorine and 1,3-butadiene had the highest chemical release potential with the exposure indices of 597, 548 and 284, respectively, and need further investigation. The possible safety margin obtained in this study was distances over 5100 m (using ALOHA software for ammonia) and 10,983 m (using PHAST software for chlorine). On the other hand, the survival margin was distances over 980 m away from the accident area (using ALOHA software for ammonia) and 620 m away from the accident area (using the PHAST software for chlorine). The results of this study indicate that determining the safety and survival margins surrounding the tanker trucks and containers of chemicals is a critical issue for the emergency response planning and determining the standards of road's safety and survival margins. On the other hand, due to the obtainment of different results by various methods and software, in road accidents, the highest hazard distance is suggested to be considered to determine the safety margin (distances longer than 10,983 m) and survival margin (distances longer than 980 m) for ERP, regardless of the type of used material and software.     

Modelling and control of the dispersion of hazardous heavy gases

Dispersion of several common `heavy' gases (ethylene, propylene, ammonia, and chlorine) has been modelled on the basis of modifications in plume path theory. The model takes into account, among other things, the variations in temperature, density, and specific heat during the movement of the heavy gas plume. The effects of wind speed, density of the gas, and venting speed on the plume dispersion have been simulated. Based on the simulations a set of empirical equations has been developed. The equations have been validated by theoretical as well as experimental studies.Studies have also been carried out to simulate the effect of venting speed (manipulated by injecting hot air with the released gas) on the plume dispersion. The study reveals that the effect of venting speed on dispersion is very pronounced and can be used to reduce the risk posed by the accidental luxurious release of toxic/flammable gases. For example an increase of 20% in venting speed of chlorine (54.1 m/s) can reduce the distance up to which toxic concentration would occur by about 1100 meters.     

Chlorine leakage from bonnet of a valve in a bullet—a case study

This paper deals with the accidental release of chlorine from bonnet of a valve in a bullet installed in a chloro-alkali industry, and the probable causes of the accident and the ensuing sequence of events. Emergency procedures are also discussed. Finally, in the conclusions reached at some useful recommendations, which has been drawn for industrial facilities handling chlorine.     

Are dispersion models suitable for simulating small gaseous chlorine releases?

Dispersion models are mostly validated on the basis of historical dispersion experiments. The latter imply large quantities of hazardous products (flammable or toxic gases), and are dedicated to study the dispersion of the resulting clouds on great distances from the source to reach a better knowledge of the different phases of gas dispersion (slumping, creeping, passive dispersion…).However, dispersion models have hardly been validated on small releases and therefore require more validation on small plumes of dangerous gases. Indeed, what is their reliability in case of accidents involving small amounts (e.g., chlorine leakages at swimming pools’ installations), and for small distances downwind the gas source? This information is of prime interest in so far as small releases are more likely to occur than larger ones.This paper reports on chlorine small-scale dispersion experiments and deals with the comparison between experimental data of ground level concentrations in the plume and predicted concentrations obtained from several dispersion models.     

Individual risk analysis of high-pressure natural gas pipelines

Transmission pipelines carrying natural gas are not typically within secure industrial sites, but are routed across land out of the ownership of the pipeline company. If the natural gas is accidentally released and ignited, the hazard distance associated with these pipelines to people and property is known to range from under 20 m for a smaller pipeline at lower pressure to up to over 300 m for a larger pipeline at higher pressure. Therefore, pipeline operators and regulators must address the associated public safety issues.This paper focuses on a method to explicitly calculate the individual risk of a transmission pipeline carrying natural gas. The method is based on reasonable accident scenarios for route planning related to the pipeline's proximity to the surrounding buildings. The minimum proximity distances between the pipeline and buildings are based on the rupture of the pipeline, with the distances chosen to correspond to a radiation level of approximately 32 kW/m². In the design criteria for steel pipelines for high-pressure gas transmission (IGE/TD/1), the minimum building proximity distances for rural areas are located between individual risk values of 10⁻⁵ and 10⁻⁶. Therefore, the risk from a natural gas transmission pipeline is low compared with risk at the building separated minimum distance from chemical industries.     

The impact of the 12 May 2008 Wenchuan earthquake on industrial facilities

This study describes the results of a field trip to the area affected by the 12 May, 2008, Wenchuan earthquake to analyse its impact on industrial facilities. The damage severity correlates well with the age of the plant, with older facilities having suffered more extensive and severe damage than those built more recently according to the latest design codes. The main cause of worker death and injury was the collapse of warehouses, office and manufacturing buildings. This concerned mostly concrete structures with insufficient confinement or poor reinforcement. The falling debris resulted in equipment damage and loss, as well as pipe severing and crushing. Pipes were also severed or bent when connected tanks were displaced or buildings collapsed. Numerous hazardous-materials releases occurred with spills being the dominant accident scenario. In some sites soil–liquefaction induced damage was evident, highlighting the need to consider potential site effects when selecting the location for a facility. The impact of the Wenchuan earthquake on chemical facilities confirms the findings from other earthquakes in terms of typical Natech damage and failure modes, as well as of hazardous-materials release potential and mechanisms.     

CGA G-13 large-scale silane release test – Part II. Unconfined silane–air explosions

A series of large-scale field trials to better understand the explosion characteristics of silane–air was conducted by the G-13 Silane Task Group under the direction of the Compressed Gas Association (CGA) and its guidelines. Silane was released from a high-pressure source into the open atmosphere, and overpressure measurements of unconfined silane–air explosions were taken at different locations away from the explosion centre. It was found that significant blast effects can result from relatively small releases of silane (around 0.1 kg). It is possible to achieve these small releases during an accidental discharge from a “pigtail” connection (a small-diameter coiled tube that connects a silane tube trailer to a process). Therefore, accidental silane explosions should be recognized as significant and possible events when handling silane. These results were also used in the proposed revision of ANSI/CGA G-13 Storage and Handling of Silane and Silane Mixtures.     

Real-time response system for the prediction of the atmospheric transport of hazardous materials

Human urge of exploiting earth resources has resulted into unprecedented industrial development in the last century resulting into production of large quantities of hazardous chemicals. Chemical, petrochemical, nuclear, biomedical and pharmaceutical industrial accidents release large quantities of hazardous chemicals into the atmosphere. The accidental discharge during production or storage or transportation have subjected the population to be exposed to exceptionally high concentration levels of hazardous chemicals, taking them by surprise, unprepared with fatal consequences. An emergency planning organization has to be trained to combat this situation in the shortest possible time to minimize the number of causalities. The present paper focuses on computation of dispersion model, using emission source, accident location and online metrological data near to the sources, to provide necessary and accurate results swiftly. The predicted ground level concentrations with the hazardous nature of the chemical, speed and direction of plume, the emergency team will be supplied with all the information in graphical easy to grasp form, superimposed over a GIS map or the latest satellite image of the area.

The emergency team has to be trained for all past scenarios and their preparedness, response and actions must be practiced regularly to be able to abate chemical releases accidentally or intentionally.

Accidental releases of chlorine and ammonia gases in residential and industrial areas are simulated. The predicted ground level concentrations in the effected areas are shown after different time intervals. For low vapor pressure chemical, the dispersion time is large and concentration levels are low but persist for prolonged time while for volatile chemical, the concentrations are high in short time and recovering to safe environment is quick.     

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。     

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。     

Study on convection and dispersion characteristics of dense gases in urban environment considering trees

Although several studies on the dispersion of heavy toxic gas released from ruptured tanks on vehicles during transportation have considered complex terrain such as urban buildings, the influence of trees on the flow field in urban areas during gas dispersion tends to be ignored. In this study, a Computational Fluid Dynamics (CFD) model was proposed to investigate the characteristics of gas release and dispersion from loaded vehicle in the urban environment. In this model, the tree crown was treated as a porous medium, and the influence of drag due to the crown was incorporated into the model by a momentum source term through a user-defined function. In this study, the dynamic characteristics of chlorine (Cl₂) dispersion under the conditions of building distribution, tree species and porosities were comprehensively analysed, to cover the influence of urban complexity, leaf density, and tree planting configuration. The results show that compared with flat terrain, the presence of urban buildings will prolong the dense gas retention time and increase the dangerous distance. It is found that the horizontal dispersion distance can increase by 63% and the isosurface of 25 ppm hazardous gas can increase by 130% with the introduction of buildings. Compared with the terrain with only buildings, the introduction of arbors or shrubs can result in a 147% or 359% increase in the maximum concentration. Also, trees will prolong the dispersion duration. It is also found that the higher the porosity, the less the wind blocking effect, and the weaker the ability of capturing gas. The wind field affected by arbores and shrubs are different in height, and arbores capture more Cl₂. Planting short shrubs around buildings can effectively reduce the spread of harmful gases.     

Small-scale physical explosions in shock tubes in comparison with condensed high explosive detonations

A series of small-scale experiments involving physical explosions in a 1.6 l pressure vessel was carried out. Explosions were initiated by spontaneous rupture of an aluminium membrane on one side of the vessel at a pressure in the range 1–1.2 MPa. The pressure waves released were measured at different distances along two separate shock tubes, one 10 m long and 200 mm in diameter (closed at one end by the high pressure vessel) and the other 15 m long and 100 mm in diameter.TNT equivalency was used for predicting the blast wave characteristics after vessel rupture. TNT equivalency was used because equations for prediction of peak pressure and impulse of the blast wave in 1-D geometry after detonations of condensed explosives are known. Some experiments with an equivalent amount of real explosive were carried out for comparison with the theoretical and experimental data obtained. The applicability of the TNT equivalency method presented for calculations of maximum pressure and shock wave impulse generated after rupture of the pressure vessel in 1-D geometry is discussed.     

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.