System dynamics research of non-adaptive evacuation psychology in toxic gas leakage emergencies of chemical park

Chemical industry park plays an important role in optimizing the allocation of resources, but an emergency may make a great deal of personnel casualty and property loss. Many casualties are not the result of accidents but are caused by extreme behavior because of the non-adaptive psychology of the evacuees. Panic is one of the non-adaptive psychological behaviors during an evacuation, which is influenced by a variety of factors. Based on the consideration of the disaster environment and the evolution of crowd emotion, a system dynamics model of panic spread is established by using Anylogic software, and simulation experiments are carried out for different disaster severity, visibility, and groups. The results show that the number of people with severe panic increase when visibility decreases or disaster diffuses. Besides, the appropriate proportion of groups can effectively reduce the cooling time of crowd and ease the fears. However, continue to increase the number of groups has no significant effect on the panic control. This work can provide some reasonable guidance for regional emergency evacuation in chemical park.     

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.     

CFD analysis of the dispersion of toxic materials in road tunnels

The present work is aimed at analyzing the evolution of accidental scenarios deriving from the release of toxic materials inside a tunnel. This scenario, compared to the more frequently investigated cases of fire, followed by smoke dispersion, may involve a large variety of common products characterized by widely differing physical properties; nonetheless it has been analysed in the literature less than expected. The present study compares the dispersion of two common toxic chemicals (chlorine and ammonia), in order to derive some preliminary information about the influence of the physical properties and the release rate. A reference road tunnel geometry is assumed, while the release occurs from ground level, at the centre of one lane and in the middle of the tunnel. Two study cases involving a road tanker, transporting the product as liquefied gas under pressure, were considered: a catastrophic release, from a 220 mm hole, emptying the tanker in a few tens seconds (case A), and a continuous release, from a much smaller hole (15 mm), lasting 5 min (case B). For the sake of simplicity, the release is assumed to be in gaseous phase; the dispersion of the toxic is simulated for the 5 min period following the start of the release using a CFD (Computational Fluid Dynamics) analysis, according to an RANS (Reynolds-Averaged Navier–Stokes) approach with the standard k–ε turbulence model, assuming no ventilation conditions. Structured curvilinear grids with hexaedric cells, refined according to the local concentration gradient, are used. For case A scenarios, especially for the whole release duration, dispersion is mainly governed by the “plug-flow” effect caused by the large volume of toxic entering the tunnel in a rather short time; then, the role of diffusivity and gravity becomes more important. Chlorine, heavier than air and with lower diffusivity than ammonia, progressively accumulates towards the floor; the dispersion of ammonia, which is lighter than air, appears more influenced by diffusivity than by gravity, since a limited stratification is observed. These trends are more evident for case B scenarios, where the toxic flow rates are much lower. It is expected the results will give some useful insight into the dispersion phenomenon within highly confined spaces and maybe also provide some suggestion about ventilation systems design and emergency procedures.     

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.     

Risk analysis of a cross-regional toxic chemical disaster by using the integrated mesoscale and microscale consequence analysis model

The rapidly growing capacity and scale of the world's petrochemical industries have forced many plants to have an even larger amount of hazardous substances. Once a serious leak occurs, the outcome of the effect zone could be very large or even uncontrollable just like the Bhopal disaster. In order to assess the risk of a cross-regional damage, this study aims to develop a model that can combine the benefits of both CFD model of the microscale simulation and the Gaussian dispersion model of the mesoscale simulation.The developed integrated model is employed on a toxic chemical tank leak accident of a process plant within an industrial park in order to explore the consequences and the risk of the toxic gas dispersion on three different scopes; one is the accident site, the second is the long-distance transmission route of the mesoscale area and the third is a target city. According to the simulation's results, it is obvious that the complexity of the structure surrounding the leaking tank will eventually affect the maximum ground concentration, the cloud shapes and cloud dilution rate, while the released gas is under dispersion. On the other hand, since the simple Gaussian dispersion model doesn't consider the above impacts, its calculation results will have many differences as compared to the realistic situation. This integrated model can be used as a tool for estimating the risk on a microscale or mesoscale areas and it can produce better results when an environmental impact analysis is required for a larger hazardous chemical process.     

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%.     

Validation of geometry modelling approaches for offshore gas dispersion simulations

Computational Fluid Dynamics (CFD) codes are widely used for gas dispersion studies on offshore installations. The majority of these codes use single-block Cartesian grids with the porosity/distributed-resistance (PDR) approach to model small geometric details. Computational cost of this approach is low since small-scale obstacles are not resolved on the computational mesh. However, there are some uncertainties regarding this approach, especially in terms of grid dependency and turbulence generated from complex objects. An alternative approach, which can be implemented in general-purpose CFD codes, is to use body-fitted grids for medium to large-scale objects whilst combining multiple small-scale obstacles in close proximity and using porous media models to represent blockage effects. This approach is validated in this study, by comparing numerical predictions with large-scale gas dispersion experiments carried out in DNV GL's Spadeadam test site. Gas concentrations and gas cloud volumes obtained from simulations are compared with measurements. These simulations are performed using the commercially available ANSYS CFX, which is a general-purpose CFD code. For comparison, further simulations are performed using CFX where small-scale objects are explicitly resolved. The aim of this work is to evaluate the accuracy and efficiency of these different geometry modelling approaches.     

3-D dispersion model for simulation of accidental toxic gas releases in a metropolitan area

Computational fluid dynamic (CFD) simulations were performed to assess the potential chlorine leak scenario in the super-urban area of South Korea, where the human population density is very high and numerous buildings exist near operational water treatment facilities. Flame acceleration simulator (FLACS) was used to predict the consequence from accidental chlorine releases out of one of the water treatment facilities for the nearby area having a size of 5 km × 3 km approximately. The ability to precisely implement 3-D geometries is crucial for a successful 3-D simulation. Thus, a method was proposed to rapidly and accurately implement geometry by importing computer aided-design (CAD) files provided by a government agency, and processing them using Auto CAD and MicroStation software programs. An accidental release from an 18-ton tank was simulated with three different wind directions to determine the expected evacuation distances. Results from the study showed that the endpoint distances varied depending on the density and arrangement of the buildings. Moreover, we employed physical barriers with varying heights for mitigating the effects of toxic gas releases and simulated how effectively they decreased the concentration of released chlorine.     

A design basis for the operational modelling of atmospheric dispersion

Based on the latest practices at the Institut de Protection et de Sûreté Nucléaire (IPSN) of the Commissariat à l'Energie Atomique (CEA), this paper presents the basic elements used for the simple modelling of hypothetical atmospheric pollution resulting from transient or continuous discharge. The effects of given kinetics under various weather conditions, which are not necessarily stationary or uniform, but are likely to occur with little or no wind are also considered. Discharges are considered as sequences of instantaneous puffs. The parameters deduced experimentally or from observations are functions of the transfer time, and cover all time and space scales. The restrictions of use are indicated, especially concerning heavy gases. Finally, simple formulae are proposed for concentrations and depositions so as to be able to make a rapid estimation of the orders of magnitude with almost no computation.     

Numerical modelling for effect of water curtain in mitigating toxic gas release

Hydrogen fluoride and ammonia are widely used in chemical industries. Both substances are hazardous and frequently a source of leakage accidents. Since a hydrogen fluoride release accident occurred in Gumi, S. Korea (2012), the Korea Occupational Safety and Health Agency (KOSHA) has emphasized that special attention and management are needed with respect to toxic substances. For post-release mitigation, a water curtain is known as one of the most effective and economical systems. In this study, a computational fluid dynamics (CFD) program was used to identify the effect of using a water curtain as a mitigation system for toxic substances that leak out from industrial facilities. Simulations were conducted to analyze how effectively a water curtain could mitigate the dispersion of toxic substances. To verify the simulation's accuracy, the INERIS Ammonia dispersion experiment and Goldfish experiment were simulated and compared. Various water curtains were applied to the simulated field experiment to confirm the mitigation factors with toxic substances. The results show that the simulations and experiments are consistent and that the dispersion of toxic substances can be mitigated by water curtains in certain circumstances.     

On the non-monotonic wind influence on flammable gas cloud from CFD simulations for hazardous area classification

Hazardous areas are defined as a result of a variety of variables as storage temperature, pressure, leak orifice size, physical properties of flammable substance, and wind characteristics. The potential formation of an explosive atmosphere must be accurately assessed to ensure process safety. Therefore, computational fluid dynamics (CFD) arises as an important tool for accurate predictions as recommended by the international standard IEC 60079-10-1 (2015). This study aims to analyze the influence of wind velocity magnitude and direction on the hazardous area classification. The authors evaluated the extent and volume for methane, propane, and hydrogen leakages from a CFD model. For each flammable gas, the wind velocity magnitude and direction were regularly varied. The outcomes show that the behavior of the plume size as the wind varies mainly depends on the gas concentration. Counter-flow wind directions lead to zero relative velocity closer to the release point, which concentrates the gas, and wind in the release direction promotes a higher dilution of the gas cloud increasing the hazardous extent while decreases the volume. As a consequence, the wind also influences the zone type, which was accurately predicted from CFD simulations and significant differences were found when compared to the standard analyses. These differences are, to some extent, related to the consideration of wind velocity effects on the gas jet release.     

A preliminary explore to the forced ventilation on the toxic gas release/dispersion and the hazard mitigation within a petrochemical plant

More than thirty-five years ago, the Bhopal disaster shook the whole world and investigators found out that many people survived just because they turned on the fans in their bedrooms. It was postulated that the forced ventilation played an important role in diluting the toxic gas and saved these people. In order to provide evidence to solve this old mystery, this research employed FLACS software to assess the hazardous degree of a toxic gas (hydrogen sulfide) leakage within a petrochemical process. Series of gas dispersion simulations were performed to actualize the hazardous characteristics and the corresponding risks of the release accident. The study shows that the hazardous level and the hazard range can be greatly influenced when parameters, such as the gas leakage circumstances (atmospheric conditions and wind speed) and the mitigation measures (direction of fans and their speed) are altered.By using explosion-proof fans in different positions and ventilation directions, combined with the natural wind in a certain direction, this research attempts to detect the best combination from various mitigation designs and to compare the influence of fan directions on hazard mitigation. It is also the first time of its kind to simulate the effect of forced ventilation on hazard mitigation within a process plant. The results show that the hazardous level of a toxic release can be effectively alleviated, when the direction of the mechanical ventilation is against the natural wind direction. With the help of the CFD simulation and the quantitative risk analysis technique, different loss prevention strategies can be tested via this method in order to establish a safer working environment.     

CFD simulations of dust dispersion in the 1 m3 explosion vessel

According to standard procedures, flammability and explosion parameters for dusts and dust mixtures are evaluated in 20 L and/or 1 m³ vessels, with equivalent results provided a correct ignition delay time (60 ms in the 20 L vessel; 600 ms in the 1 m³ vessel). In this work, CFD simulations of flow field and dust concentration distribution in the 1 m³ spherical vessel are performed, and the results compared to the data previously obtained for the 20 L. It has been found that in the 1 m³ vessel, the spatial distribution of the turbulent kinetic energy is lower and much more uniform. Concerning the dust distribution, as in the case of the 20 L, dust is mainly concentrated at the outer zones of the vortices generated inside the vessel. Furthermore, an incomplete feeding is attained, with most of the dust trapped in the perforated annular nozzle. Starting from the maps of dust concentration and turbulent kinetic energy, the deflagration index K_St is calculated in both vessels. In the conditions of the present work, the K_St is found to be 2.4 times higher in the 20 L than in the 1 m³ vessel.     

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.     

Accident modeling of toxic gas-containing flammable gas release and explosion on an offshore platform

Toxic gas-containing flammable gas leak can lead to poisoning accidents as well as explosion accidents once the ignition source appears. Many attempts have been made to evaluate and mitigate the adverse effects of these accidents. All these efforts are instructive and valuable for risk assessment and risk management towards the poisoning effect and explosion effect. However, these analyses assessed the poisoning effect and explosion effect separately, ignoring that these two kinds of hazard effects may happen simultaneously. Accordingly, an integrated methodology is proposed to evaluate the consequences of toxic gas-containing flammable gas leakage and explosion accident, in which a risk-based concept and the grid-based concept are adopted to combine the effects. The approach is applied to a hypothetical accident scenario concerning an H₂S-containing natural gas leakage and explosion accident on an offshore platform. The dispersion behavior and accumulation characteristics of released gas as well as the subsequent vapor cloud explosion (VCE) are modeled by Computational Fluid Dynamics (CFD) code Flame Acceleration Simulator (FLACS). This approach is concise and efficient for practical engineering applications. And it helps to develop safety measures and improve the emergency response plan.     

Comparing tools of varying complexity for calculating the gas dispersion

When handling flammable and/or toxic liquids or gases, the gas dispersion following a release of substance is a scenario to be considered in the risk assessment to determine the lower flammability distance (LFD) and toxicity thresholds. In this work a comparison of different gas dispersion tools of varying complexity ranging from a simple Gaussian model over a boundary layer model (BLM) and a Lagrangian model to CFD (in this case ANSYS CFX v14) is presented. The BLM covers the special case of liquid releases with formation of a pool. It does not only solve the gas dispersion but also calculates the evaporating mass flow out of the pool. The simulation values are compared to each other and to experimental data resulting mainly from our own open air experiments covering the near field and carried out on the Test Site Technical Safety of BAM (BAM-TTS) for different release types (pool evaporation, gas release) and topologies. Other validation data were taken from literature and cover large scale experiments in the range of several 100 m.     

Analysis of meteorological parameters for dense gas dispersion using mesoscale models

The current study focuses on characterizing the atmospheric details required for dense gas dispersion analysis resulting from release of cryogenic liquids like LNG. The study investigates the effectiveness of coupling the prognostic MM5 mesoscale model with the CALMET diagnostic model for producing meteorological conditions that is characteristic of dry and arid regions like Qatar, with non-neutral boundary conditions. MM5-CALMET wind fields and temperature data are compared with the meteorological field observations from the Doha International Airport (DIA) on a monthly basis, daily basis and hourly basis to study the effect of different averaging periods. The monthly averages replicate the annual patterns of meteorological parameters very well. However difference in observation and model are observed for wind speed and wind direction variable. The daily averages obtained from the model are in good agreement with the observation for wind speed and temperature. For hourly averaging, the model is found capable of mimicking the temperature of a given location, but not wind speed and direction. The prediction of wind direction parameter using MM5-CALMET is moderate for any averaging period. The sub-optimal performance of wind direction variable is attributed to grid resolution of vertical and horizontal layers of MM5-CALMET model. Additionally a case study is performed to illustrate the effect of variation of meteorological parameters on the lower flammability limits (LFL) resulting from flammable dense gas release of LNG. The case study demonstrates the issues that arise in a risk analysis study when “wrong” meteorological data could be used. The overall study indicates that utilizing the coarsest prognostic meteorological model output in a diagnostic model provides an effective option for generating meteorological inputs for dispersion studies.     

Improvement in the prediction of gasoline pool fire behaviour: CFD modelling and validation

The development and implementation of mathematical models through Computational Fluid Dynamics (CFD) techniques has been acknowledged as a promising tool for the prediction of hydrocarbon pool fires behaviours. In this sense, different approaches, with different assumptions and simplifications, and accounting for different phenomena, have been developed in the literature. However, the deviations in the predictions of the experimentally determined parameters, such as temperatures profiles, flame heights and radiative heat flux, by the implemented models are still high. Therefore, the implementation of these models to predict combustion phenomena and flame behaviours for various scenarios is limited. In this work, the software C3D is used to model gasoline pool fires of different diameters, and under different wind conditions, in order to improve the quality of the predictions of the flame behaviour. The modelled cases correspond to the experimental studies reported in literature. The results from the implemented model show an improved predictive quality when compared with other modelling works reported on literature for the same experimental cases. The deviations in the time averaged temperature, flame height, surface emissive power and radiative heat flux, has been calculated to be 5.0%, 0.05%, 6.32% and 3.82%, respectively.