Use of water spray curtain to disperse LNG vapor clouds

Effective safety measures to prevent and mitigate the consequences of an accidental release of flammable LNG are critical. Water spray curtain is currently recognized as an effective technique to control and mitigate various hazards in the industries. It has been used to absorb, dilute and disperse both toxic and flammable vapor cloud. It is also used as protection against heat radiation, in case of fighting vapor cloud fire. Water curtain has also been considered as one of the most economic and promising LNG vapor cloud control techniques. Water curtains are expected to enhance LNG vapor cloud dispersion mainly through mechanical effects, dilution, and thermal effects. The actual phenomena involved in LNG vapor and water curtain interaction were not clearly established from previous research. LNG spill experiments have been performed at the Brayton Fire Training Field at Texas A&M University (TAMU) to understand the effect of water curtain in controlling and dispersing LNG vapor cloud. This paper summarizes experimental methodology and presents data from two water curtain tests. The analysis of the test results are also presented to identify the effectiveness of these two types of water spray curtains in enhancing the LNG vapor cloud dispersion.     

LNG accident dynamic simulation: Application for hazardous consequence reduction

The evaluation of exclusion (hazard) zones around the LNG stations is essential for risk assessment in LNG industry. In this study, computational fluid dynamics (CFD) simulations have been conducted for the two potential hazards, LNG flammable vapor dispersion and LNG pool fire radiation, respectively, to evaluate the exclusion zones. The spatial and temporal distribution of hazard in complex spill scenario has been taken into account in the CFD model. Experimental data from Falcon and Montoir field tests have been used to validate the simulation results. With the valid CFD model, the mitigation of the vapor dispersion with spray water curtains and the pool fire with high expansion foam were investigated. The spray water curtains were studied as a shield to prevent LNG vapor dispersing, and two types of water spray curtain, flat and cone, were analyzed to show their performance for reduction and minimization of the hazard influencing distance and area. The high expansion foam firefighting process was studied with dynamic simulation of the foam action, and the characteristics of the foam action on the reduction of LNG vaporization rate, vapor cloud and flame size as well as the thermal radiation hazard were analyzed and discussed.     

Key parametric analysis on designing an effective forced mitigation system for LNG spill emergency

Concerns over public safety and security of a potential liquefied natural gas (LNG) spill have promoted the need for continued improvement of safety measures for LNG facilities. The mitigation techniques have been recognized as one of the areas that require further investigation to determine the public safety impact of an LNG spill. Forced mitigation of LNG vapors using a water curtain system has been proven to be effective in reducing the vapor concentration by enhancing the dispersion. Currently, no engineering criteria for designing an effective water curtain system are available, mainly due to a lack of understanding of the complex droplet–vapor interaction. This work applies computational fluid dynamics (CFD) modeling to evaluate various key design parameters involved in the LNG forced mitigation using an upwards-oriented full-cone water spray. An LNG forced dispersion model based on a Eulerian–Lagrangian approach was applied to solve the physical interactions of the droplet–vapor system by taking into account the various effects of the droplets (discrete phase) on the air–vapor mixture (continuous phase). The effects of different droplet sizes, droplet temperatures, air entrainment rates, and installation configurations of water spray applications on LNG vapor behavior are investigated. Finally, the potential of applying CFD modeling in providing guidance for setting up the design criteria for an effective forced mitigation system as an integrated safety element for LNG facilities is discussed.     

Influence of active mitigation barriers on LNG dispersion

In recent years, particular interest has been direct to the issues of risk associated with the storage, transport and use of Liquefied Natural Gas (LNG) due to the increasing consideration that it is receiving for energy applications. Consequently, a series of experimental and modeling studies to analyze the behavior of LNG have been carried out to collect an archive of evaporation, dispersion and combustion information, and several mathematical models have been developed to represent LNG dispersion in realistic environments and to design mitigation barriers.This work uses Computational Fluid Dynamics codes to model the dispersion of a dense gas in the atmosphere after accidental release. In particular, it will study the dispersion of LNG due to accidental breakages of a pipeline and it will analyze how it is possible to mitigate the dispersing cloud through walls and curtains of water vapor and air, also providing a criterion for the design of such curtains.     

Numerical simulation of water curtain application for ammonia release dispersion

Using water curtain system to forced mitigate ammonia vapor cloud has been proven to be an effective measure. Currently, no engineering guidelines for designing an effective water curtain system are available, due to lack of understanding of complex interactions between ammonia vapor cloud and water droplets, especially the understanding of ammonia absorption into water droplets. This paper presents numerical calculations to reproduce the continuous ammonia release dispersion with and without the mitigating influence of a downwind water curtain using computational fluid dynamic (CFD) software ANSYS Fluent 14.0. The turbulence models k–ɛ and RNG were used to simulate the ammonia cloud dispersion without downwind water curtain. The simulated results were compared with literature using the statistical performance indicators. The RNG model represents better agreement with the experimental data and the k–ɛ model generates a slightly lesser result. The RNG model coupled with Lagrangian discrete phase model (DPM) was used to simulate the dilution effectiveness of the water curtain system. The ammonia absorption was taken into account by means of user-defined functions (UDF). The simulated effectiveness of water curtains has good agreements with the experimental results. The effectiveness of water mitigation system with and without the ammonia absorption was compared. The results display that the effectiveness mainly depends on the strong air entrainment enhanced by water droplets movement and the ammonia absorption also enhances the effectiveness of water curtain mitigation system. The study indicates that the CFD code can be satisfactorily applied in design criteria for an effective mitigation system.     

Developing a CFD heat transfer model for applying high expansion foam in an LNG spill

Liquefied natural gas (LNG) is widely used to cost-effectively store and transport natural gas. However, a spill of LNG can create a vapor cloud, which can potentially cause fire and explosion. High expansion (HEX) foam is recommended by the NFPA 11 to mitigate the vapor hazard and control LNG pool fire. In this study, the parameters that affect HEX foam performance were examined using lab-scale testing of foam temperature profile and computational fluid dynamics (CFD) modeling of heat transfer in vapor channels. A heat transfer model using ANSYS Fluent® was developed to estimate the minimum HEX foam height that allows the vapors from LNG spillage to disperse rapidly. We also performed a sensitivity analysis on the effect of the vaporization rate, the diameter of the vapor channel, and the heat transfer coefficient on the required minimum height of the HEX foam. It can be observed that at least 1.2 m of HEX foam in height are needed to achieve risk mitigation in a typical situation. The simulation results can be used not only for understanding the heat transfer mechanisms when applying HEX foam but also for suggesting to the LNG facility operator how much HEX foam they need for effective risk mitigation under different conditions.     

Medium scale LNG-related experiments and CFD simulation of water curtain

This work is a continuation of the experimental research on LNG releases and their consequence mitigation methods, which has been carried on by the Mary Kay O’Connor Process Safety Center (MKOPSC) at the Texas A&M University since 2004.A series of medium scale experiments to test the ability of the water curtains to hold up and disperse a vapor has been performed. Colored smoke has been used as an analog of the LNG vapor for easier tracking of the vapor path through the water curtain. The results and some analysis of the experimental data are presented. The CFD software FLACS (GexCon AS) was used to simulate the effects of the water curtains on vapor dispersion. The results of the simulations were compared with experimental data.     

Development of heat transfer and evaporation model of LNG covered by Hi-Ex foam

Natural gas is a kind of clean, efficient green energy source, which is used widely. Liquefied natural gas (LNG) is produced by cooling natural gas to −161 °C, at which it becomes the liquid. Once LNG was released, fire or explosion would happen when ignition source existed nearby. The high expansion foam (Hi-Ex foam) is believed to quickly blanket on the top of LNG spillage pool and warm the LNG vapor to lower the vapor cloud density at the ground level and raising vapor buoyancy. To identify the physical structure after it contacted with LN₂ and to develop heat transfer model, the small-scale field test with liquid nitrogen (LN₂) was designed. In experiment, three layers including frozen ice layer, frozen Hi-Ex layer and soft layer of Hi-Ex foam were observed at the steady state. By characterizing physical structure of the foam, formulas for calculating the surface of single foam bubble and counting foam film thickness were deduced. The micro heat transfer and evaporation model between cryogenic liquid and Hi-Ex foam was established. Indicating the physical structure of the frozen ice layer, there were a certain number of icicles below it. The heat transfer and evaporation mathematical model between the frozen ice layer and LNG was derived. Combining models above with the heat transfer between LNG, ground and cofferdam, the heat transfer and evaporation mathematical model of LNG covered by Hi-Ex foam was developed eventually. Finally, LN₂ evaporation rate calculated by this model was compared with the measured evaporation rate. The calculated results are 1.2–2.1 times of experimental results, which were acceptable in engineering and proved the model was reliable.     

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.     

n-Compartment mathematical model for transient evaluation of fluid curtains in mitigating chlorine releases

A simple n-compartment mathematical model is developed to study the effectiveness of fluid curtains and transient behaviour in mitigating the effects of an accidental chlorine release. The model is obtained considering chemical and physical absorption effectiveness of the toxic cloud by a reacting liquid curtain. The characteristics of the curtain and the evolution were deeply studied by means of replicated wind tunnel experimental runs, according to different operating parameters. An analytical solution of the model is presented. A fairly good agreement was verified between the model predictions and the original experimental results here presented.     

基于数值模拟的扇形水幕阻挡稀释氯气扩散的影响因素研究

在重气储罐区内设置喷射水幕是安全隔离、控制重气泄漏后扩散和减缓事故后果严重程度的重要措施之一。为此,利用计算流体力学(CFD)模型建立了氯气泄漏扩散模型,对扇形水幕阻挡稀释氯气扩散过程进行了动态模拟及影响因素分析,分别模拟了外界风速、水幕的喷射角度、水幕距泄漏源距离、水幕流量和水幕液滴直径等参数对氯气泄漏后扩散的影响情况。结果表明,合理地设置水幕能够有效阻挡氯气的扩散、缩短危险距离和减少危害面积。在大气稳定的情况下,外界风速、水幕的喷射角度、水幕距泄漏源距离、水幕流量等参数、水幕液滴直径是影响扇形水幕阻挡氯气扩散的重要因素。其中水幕距泄漏源距离和水幕流量2个因素对阻挡稀释效果的影响比较明显,水幕距泄漏源的距离越小,水幕的动量越大,阻挡稀释效果越好,水幕流量适中时效果最好,流量过大或过小阻挡稀释效果都要差一些。因此,合理设置相关参数有利于提高水幕性能,更加有效地降低氯气泄漏事故的后果。     

Underwater LNG release test findings: Experimental data and model results

An underwater LNG release test was conducted to understand the phenomena that occur when LNG is released underwater and to determine the characteristic of the vapor emanating from the water surface. Another objective of the test was to determine if an LNG liquid pool formed on the water surface, spread and evaporated in a manner similar to that from an on-the-surface release of LNG.A pit of dimensions 10.06 m × 6.4 m and 1.22 m depth filled with water to 1.14 m depth was used. A vertically upward shooting LNG jet was released from a pipe of 2.54 cm diameter at a depth of 0.71 m below the water surface. LNG was released over 5.5-min duration, with a flow rate of 0.675 ± 0.223 L/s. The wind speed varied between 2 m/s and 4 m/s during the test.Data were collected as a function of time at a number of locations. These data included LNG flow rate, meteorological conditions, temperatures at a number of locations within the water column, and vapor temperatures and concentrations in air at different downwind locations and heights. Concentration measurements were made with instruments on poles located at 3.05 m, 6.1 m and 9.14 m from the downwind edge of the pit and at heights 0.46 m, 1.22 m, and 2.13 m. The phenomena occurring underwater were recorded with an underwater video camera. Water surface and in-air phenomena including the dispersion of the vapor emanating from the water surface were captured on three land-based video cameras.The lowest temperature recorded for the vapor emanating from the water surface was −1 °C indicating that the vapor emitted into air was buoyant. In general the maximum concentration observed at each instrument pole was progressively at higher and higher elevations as one traveled downwind, indicating that the vapor cloud was rising. These findings from the instrument recorded data were supported by the visual record showing the “white” cloud rising, more or less vertically, in air. No LNG pool was observed on the surface of water. Discussions are provided on the test findings and comparison with predictions from a previously published theoretical model.     

Highlights of LNG risk technology

The authors have recently undertaken a major review of LNG consequence modeling, compiling a wide range of historical information with more recent experiments and modeling approaches in a book entitled “LNG Risk-Based Safety: Modeling and Consequence Analysis”. All the main consequence routes were reviewed – discharge, evaporation, pool and jet fire, vapor cloud explosions, rollover, and Rapid Phase Transitions (RPT’s). In the book, experimental data bases are assembled for tests on pool spread and evaporation, burn rates, dispersion, fire and radiation and effects on personnel and structures. The current paper presents selected highlights of interest: lessons learned from historical development and experience, comparison of predictions by various models, varying mechanisms for LNG spread of water, a modeling protocol to enable acceptance of newer models, and unresolved technical issues such as cascading failures, fire engulfment of a carrier, the circumstances for a possible LNG BLEVE, and accelerated evaporation by LNG penetration into water.     

Underwater LNG release: Does a pool form on the water surface? What are the characteristics of the vapor released?

One of the scenarios of concern in assessing the safety issues related to transportation of LNG in a marine environment (ship or underwater pipeline) is the release of LNG underwater. This scenario has not been given the same level of scientific attention in the literature compared to surface releases and assessment of consequences therefrom. This paper addresses questions like, (1) does an LNG spill underwater form a pool on the water surface and subsequently evaporate like an LNG spill “on the surface” producing cold, heavier than air vapors?, and (2) what is the range of expected temperatures of the vapor, generated by LNG release due to heat transfer within the water column, when it emanates from the water surface?Very limited data from two field tests of LNG underwater release are reviewed. Also presented are the results from tests conducted in other related industries (metal casting, nuclear fission and fusion, chemical processing, and alternative fuel vehicles) where a hot (or cold) liquid is injected into a bulk cold (or hot) liquid at different depths.A mathematical model is described which calculates the temperature of vapor emanating at the water surface, and the liquid fraction of released LNG that surfaces, if any, to form a pool on the water surface. The model includes such variables as the LNG release rate, diameter of the jet at release, depth of release and water body temperature. Results obtained from the model for postulated release conditions are presented. Comparison of predicted results with available LNG underwater release test data is also provided.     

Gas screening effect caused by steam curtains

Steam curtain equipment is used to prevent leaking flammable gases from reaching ignition sources, such as furnaces. However, steam curtains are sometimes designed badly and are ineffective for preventing the leaking gas from spreading in the windward direction. The leaked gas may pass on either side of the stream curtain, it may pass between the nozzles, or it may pass on the upper side of the stream curtain. At present, data to design the best stream curtains are quite rare. It is necessary to obtain fundamental data on the entrainment of air by steam jet and on the inclination of the steam curtain caused by the strong wind. Therefore, a series of experiments to investigate the entrained air and the inclination of the steam curtain were performed, in addition to further theoretical considerations. Thus, fundamental data useful for the design of the steam curtain equipment have been obtained.     

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.     

Mutual effects between dynamic leakage behavior and the pressure/temperature in a LNG tank with external heat fluxes

The objective of this work is to investigate and model the mutual effects between the dynamic pressure/temperature in the LNG tank and the leakage behavior with external heat fluxes. The results suggest that the pressure and temperature in tank during leakage change with the comparison results between the heat flux consumed in liquid boil-off and the external heat flux supplied. At the liquid leakage stage, when the external heat flux is not very high, the pressure in tank tends to increase significantly, even results in tank explosion. It increases more and more heavily with higher and higher external heat fluxes. At the vapor leakage stage, large amount of vapor spray out, which results in a high generation rate of vapor by the liquid boil-off. The pressure in tank normally decreases to be low, which is unfavorable for the LNG tank explosion. Therefore, at this vapor leakage stage, blocking the leakage hole as soon as possible is not always a right choice for fire fighters. Finally, it is suggested that reducing the heat flux into the tank, either at the liquid leakage stage or in vapor one, is key to the tank safety.     

Numerical simulation of ammonia dispersion around a water curtain

Water curtains have been suggested as a way of limiting the spread of ammonia in the event of an accidental release. Several field experiments have already been performed to investigate the interaction between a water curtain and a cloud of ammonia, most recently by Bara and Dusserre (1997, J. Loss Prev. Ind., 10(3), 179–183). Those experiments have been modelled numerically, using the computational code Mercure. The calculated velocities and concentrations agree reasonably well with the measurements.     

受限空间水幕隔热阻烟性能的试验研

为了研究消防水幕衰减火灾热辐射和阻隔火灾烟气蔓延的隔热阻烟特性,基于搭建的3 m× 3 m × 2.5 m（长×宽×高）小尺寸试验平台,开展大量实体火灾试验,研究水幕作用下空间温度分布特性以及烟气运动规律,并重点分析了喷头压力和喷头类型2种参数对水幕隔热阻烟性能的影响。结果表明:实施水幕分隔对火灾上层高温烟气具有明显的降温作用;实施水幕能够有效阻止火灾烟气的前期扩散;喷头压力越大,水幕的隔热效率越高,当喷头压力增大至0.6 MPa时,水幕的隔热效率可达70%以上;使用ZSTWC撞击式水雾喷头比ZSTM-AT式扇形水幕喷头更有利于形成均匀分布的水幕断面。     

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

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