Computational fluid dynamics analysis of liquefied natural gas dispersion for risk assessment strategies

Computational fluid dynamics (CFD) simulations have been conducted for dense gas dispersion of liquefied natural gas (LNG). The simulations have taken into account the effects of gravity, time-dependent downwind and crosswind dispersion, and terrain. Experimental data from the Burro series field tests, and results from integral model (DEGADIS) have been used to assess the validity of simulation results, which were found to compare better with experimental data than the commonly used integral model DEGADIS. The average relative error in maximum downwind gas concentration between CFD predictions and experimental data was 19.62%.The validated CFD model was then used to perform risk assessment for most-likely-spill scenario at LNG stations as described in the standard of NFPA 59A (2009) “Standard for the Production, Storage and Handling of Liquefied Natural Gas”. Simulations were conducted to calculate the gas dispersion behaviour in the presence of obstacles (dikes walls). Interestingly for spill at a higher elevation, e.g., tank top, the effect of impounding dikes on the affected area was minimal. However, the impoundment zone did affect the wind velocity field in general, and generated a swirl inside it, which then played an important function in confining the dispersion cloud inside the dike. For most cases, almost 75% of the dispersed vapour was retained inside the impoundment zone. The finding and analysis presented here will provide an important tool for designing LNG plant layout and site selection.     

Numerical simulation of LNG dispersion under two-phase release conditions

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

Numerical simulation of evaporation of volatile liquids

The paper presents the results of the validation of the developed pool evaporation model using literature and our own experimental data. The proposed model was used to examine the effect of wind velocity and pool sizes on the evaporation rate of volatile liquid (hexane). Contrary to the semi-empirical evaporation model widely used in hazard assessment, stronger dependence of evaporation rate on pool size at low wind speeds is obtained.     

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.     

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.     

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.     

Modeling the vapor source term associated with the spill of LNG into a sump or impoundment area

The recent publication of evaluation protocols for vapor source term models and vapor dispersion models have influenced the modeling approaches that can be used for approval of new and expansion projects at LNG receiving terminals. In the past few years the scientific basis of integral vapor source term models has been questioned with growing concerns regarding their validity. In this paper, the shallow water equations (SWEs) were solved to study the characteristics of the evaporating LNG pool associated with a constant flow rate spill of LNG into a concrete sump. In the early stages of pool spreading, the leading edge thickness profile of the SWE model scales with the square root of the distance from the leading edge as the pool spreads. After the edge of the pool reaches the wall, the reflected wave forms a hydraulic jump that travels back towards the center of the pool at a speed that is considerably slower than the initial spreading of the pool. Once the hydraulic jump reaches the center, the pool assumes a nearly flat free surface for the rest of the spill. The pool spreading and the rate of evaporation from the SWEs were then compared to the solution provided by the integral model, PHAST. The two approaches were found to agree well with one another. The SWE model was also used to demonstrate the influence of an elevated spill source. With an elevated source, the LNG pool spreads faster, significantly increasing the initial rate of vaporization and peak vaporization rate. This increase in the initial rate of vaporization could lead to an increase in the vapor cloud hazard distance. The SWE model was also used to demonstrate the influence of an inclined sump floor in the shape of an inverted cone where the spilling LNG accumulates in the low vertex of the cone. Inclined sump floors can be used to significantly reduce the cumulative evaporation, making them attractive as a possible mitigation approach in cases where a containment sump is located close to a property boundary.     

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.     

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.     

大型原油储罐泄漏隔堤池火灾后果研究

为了研究储罐大孔泄漏后可能产生的隔堤局部面状液池火灾,以10万立方大型原油储罐为例,采用计算流体力学软件FLUENT和火灾模拟软件FDS计算储罐在真实泄漏场景下的液池区域,模拟发生隔堤池火的分布特征及对临罐热辐射影响。研究结果表明:储罐原油泄漏后将在隔堤内形成相对稳定面积的液池,在储罐不同方位处泄漏形成的液池面积与储罐壁距雨水收集槽长度相关;储罐正下方的隔堤池火对储罐造成的热辐射极大;风对临罐受到的池火热辐射强度影响明显,指向罐组中心方向的来风对临罐热辐射强度影响较大。     

Methodology for consequence analysis of LNG releases at deepwater port facilities

A methodology to perform consequence analysis associated with liquefied natural gas (LNG) for a deepwater port (DWP) facility has been presented. Analytical models used to describe the unconfined spill dynamics of LNG are discussed. How to determine the thermal hazard associated with a potential pool fire involving spilled LNG is also presented. Another hazard associated with potential releases of LNG is the dispersion of the LNG vapor. An approach using computational fluid dynamics tools (CFD) is presented. The CFD dispersion methodology is benchmarked against available test data. Using the proposed analysis approach provides estimates of hazard zones associated with newly proposed LNG deepwater ports and their potential impact to the public.     

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.     

A modified steady-state model for evaluation of ammonia concentrations behind a water curtain

Water curtain system has been proved an effective mitigation measurement for ammonia spill dispersion. Calculating of ammonia cloud concentration with water curtain was less studied. This paper presents a steady-state calculation model to calculate open and forced ammonia spill dispersion. The formula of ammonia absorption was built and integrated into the calculation model. The calculated downwind ammonia concentrations for open and forced spill dispersion were reproduced and compared with literature using a statistical method. In addition, the relationship between ammonia concentration in water droplet and the droplet diameter was studied. The results display that the formula of ammonia absorption is suitable for calculating mass transfer process between the ammonia cloud and the water curtain. The calculation model presents good performances for open and forced ammonia spill dispersion. This study indicates that the calculation model can be satisfactory in determining the impact of open and forced ammonia spill dispersion and the design of water curtain mitigation system.     

Impact of wind on in-situ burning behavior of spilled oil on open water

Spilled oil may cause serious environmental pollution and ecological destruction. Thus, an efficient oil spill response is essential to reduce and minimize the impacts seen from an oil spill. In-situ burning (ISB) has been proven to be one of the most convenient and efficient treatment methods for oil spills on sea. When in-situ burning operation is carried out on sea, the wind is usually available. Spilled oil pool fire flame will be blown to tilt with an angle from the vertical direction and the flame length will be stretched. This effect of flame on downstream will be significantly enhanced. It may even cause the burning of spilled oil out of control or hazard the operators near site. Therefore, it is meaningful to study the burning behavior of oil pool fire on open water under wind.A small-scale oil pool fire on open water experiment system was designed, which could simulate the boundary conditions of sea surface ISB. A series of experiments were carried out to study the effects of oil pool diameter, oil layer thickness and wind speed on fire flame length and flame tilt angle. Flame characteristics and temperature of burning system were recorded. This study observes burning behavior to evolve from short pool fire to long declining phase which transient through boilover stage. Dimensionless flame length in pool fire stage increases with wind speed. As expected the flame tilt is strongly influenced by wind speed, tilt angle increase with wind speed. The observed experimental behavior is captured as improvement in the flame length and flame tilt angle models.This study helped better understanding fire behavior in evolving environmental conditions, and the most importantly developed model would help guide better assessment of fire mitigation strategies.     

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.     

Configuration predictions of large liquefied petroleum gas (LPG) pool fires using CFD method

Liquefied petroleum gas (LPG) has potential pool fire risks due to its flammability. The configuration of pool fires plays a significant role when applying the solid flame model or point source model to assess the risks from heat radiation. However, no existing correlations can precisely predict the configuration of large LPG (100% propane) pool fires. To enhance the fundamental understanding on how pool diameter and wind velocity can influence the configuration of large LPG pool fires, an experimentally validated Computational Fluid Dynamics (CFD) model is employed to simulate fires using different burning rate models. Fire temperature profiles, flame heights, and flame tilts predicted by the CFD model were compared with empirical models and experimental data. Accordingly, new correlations for flame height and flame tilt as functions of pool diameter D and wind velocity u_w have been developed. The comparisons demonstrate that the new correlations have the best overall accuracy in the prediction of flame height and tilt for large LPG pool fires under different conditions (10 m ≤ D ≤ 20 m, 0 ≤ u_w ≤ 3 m·s⁻¹).     

Evaporating liquid flow in a channel (An integral model based on shallow water flow approximation)

Liquefied Natural Gas (LNG) storage facilities generally include channels to convey potential spills of the liquid to an impoundment. There is increasing concern that dispersion of vapors generated by flow of LNG in a channel may lead to higher than limit vapor concentrations for safety at site boundary from channels that may be close to the dike walls. This issue is of recent concern to regulatory agencies, because the calculation of vapor hazard distance(s) from LNG flow in a channel is not required under existing LNG facility siting standards or regulations.An important parameter that directly affects the calculated LNG vapor dispersion distance is the source strength (i.e., the rate of vaporization of LNG flow from the wetted channel surfaces, as a function of spatial position and time). In this paper a model is presented which considers the variation of the depth of the flowing LNG with spatial location and time, and calculates the spatial and temporal dependence of the mass rate of vapor generation. Self similar profiles for the spatial variation of the thermal boundary layer in the liquid wetted wall and liquid depth variation are assumed. The variation with time of the location of the liquid spread front and the evaporation rate are calculated for the case of a constant LNG spill rate into a rectangular channel. The effects of two different channel slopes are evaluated. Details of the results and their impact on dispersion distances are 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.