Unignited High-Pressure Methane Jet Impinging a Pipe Rack: Practical Tools for Risk Assessment

Although the diffusion of its storage and transport under liquefied conditions, nowadays it is common to have methane in gaseous form in several industrial applications. This leads to safety implications to be considered: hazards are linked to both the high-pressure at which the gas is kept and to its flammability. Scenarios where flammable jets impact an obstacle are of paramount importance because of their possible occurrence. Following a numerical approach, literature shows up that their assessment can be reliably performed by means of only Computational Fluid Dynamics tools. However, despite the improvements of computing power, Computational Fluid Dynamics costs still limit its use in daily risk analysts’ activities. Therefore, considering an accidental jet-obstacle scenario of industrial interest, the present work investigates how a pipe rack can influence the development of a high-pressure methane jet. Based on a Computational Fluid Dynamics analysis, main achievements of this work are a simple criterion able to identify the situations where the pipe rack does not influence the high-pressure methane jet behavior, therefore allowing to identify the scenarios where simpler models can be used (i.e., analytical correlations known for the free jet situation), and, if present, a simple analytical relationship that roughly predicts the influence of the pipe rack without the need of performing complex Computational Fluid Dynamics simulations.     

Development of inherent safety and health index for formulated product design

The numerous formulated products which are introduced to the market consist of chemical ingredients that may cause various safety and health hazards to the consumers. Therefore, it is extremely important to practice a systematic methodology to formulate products with acceptable safety and health performances. This work presents an index-based methodology to assess the safety and health hazards of the ingredients during the early formulation stage of product design. Hence, new inherent safety and health sub-indexes are introduced to improve the current safety and health hazards that are needed in formulated product design. The inherent safety and health sub-indexes are assigned with scores based on the degree of potential hazards. A higher score indicates a higher safety risk or severe health effect, and vice versa. The proposed methodology will greatly assist the users to identify the adverse safety and health effects caused by the ingredients. Hence, it is pivotal to eliminate or reduce the safety and health impacts from product usage. A case study on common ingredients used in the formulation of paint is presented on this study to describe the proposed method.     

An improved grey wolf optimizer algorithm for identification and location of gas emission

Identification of the leakage of hazardous gases plays an important role in the environment protection, human health and safety of industry production. However, lots of current optimization algorithms, such as particle swarm optimization (PSO) and Grey Wolf Optimizer (GWO), suffer from poor global optimization capability and estimation accuracy. In this work, a hybrid differential evolutionary and GWO (DE-GWO) algorithm is proposed. Tested by simulation cases and Prairie Grass emission experimental data, DE-GWO shows higher estimation accuracy than GWO. Compared with the other four optimization algorithms, DE-GWO exhibits finer robust stability under different population sizes, fewer iterations, as well as higher estimation accuracy with fewer search agents. Importantly, simulation results demonstrate that DE-GWO is more suitable to apply in the scene with a small number of sensors. Therefore, the proposed in this paper outperforms other optimization algorithms for the gas emission inverse problem. DE-GWO can provide reliable estimation towards gas emission identification and positioning, which shows huge potential as the data analysis module of real-time monitoring and early warning system.     

Resilience-based approach to safety barrier performance assessment in process facilities

The performance assessment of safety barriers is essential to find vulnerable elements in a safety barrier system. Traditional performance assessment approaches mainly focus on using several static indicators for quantifying the performance of safety barriers. However, with the increasing complexity of the system, emerging hazards are highly uncertain, making it challenging for the static indicators to assess the performance of safety barriers. This paper proposes a resilience−based performance assessment method for safety barriers to overcome this problem. Safety barriers are classified according to their functions first. The dynamic Bayesian network (DBN) is then introduced to calculate the availability function under normal and disruption conditions. The ratio of the system's availability, when affected by the disruption, to the initial availability, is used to determine the absorption capacity of the system. The ratio of the quantity of availability recovery to the total quantity of system represents the adaptation and restoration capacity of the system. The system's resilience is represented by the sum of absorption, adaptation, and restoration capacities. The wax oil hydrogenation process is used to demonstrate the applicability of the proposed methodology.     

Study on accidental fire at a large-scale floating-roof gasoline storage tank

For an accident involving a large-scale internal floating-roof tank with 28.4 m diameter and filled with 4600 m³ gasoline, the actual behavior of the gasoline fire and the fire-fighting strategies that were applied to it were analyzed in terms of the heat release rate, burning rate, and regression rate. During the accident, the initial fire suppression strategy failed and the gasoline was moved to an external tank. A total of 2800 m³ gasoline was burned for 17 h with a resulting heat release rate of 1475 MW. The long duration of the fire burning was attributed to the burning surface of the gasoline, which was not covered with foam at the beginning of the fire using the active foam fire-extinguishing system due to damage to one of the foam chambers. The average regression rate of the gasoline was 0.16 m/h after 8 h of burning and 0.35 m/h when the fire was completely suppressed.     

Ventilation theory and dispersion modelling applied to hazardous area classification

Critical formulae given in the current Explosive Atmospheres Hazardous Area Classification Standard IEC 60079-10-1 (2008) [BS EN 60079-10-1, 2009] to determine the expected gas cloud volume which is used to determine area classification do not have any scientific justification. The standard does allow the alternative use of Computational Fluid Dynamics (CFD) methods, which serve to compound the concern with these formulae: the predicted volume of the gas cloud from CFD models being several orders of magnitude smaller than that given by the formulae in question. To resolve such major discrepancies, replacement of the current formulae with a scientifically validated approach is proposed. Integral models of dispersion and ventilation have been used routinely for many years in the analysis of major hazards in the chemical industry. This paper presents an adaptation of these models to determine the expected volume of a gas cloud arising from a release of gas from a pressurised source. A very simple integral jet model is presented for outdoor dispersion, extended to the case of indoor dispersion, from which the volume of the gas cloud is derived. The single free parameter, an entrainment coefficient, is fixed by comparison with data on a free jet, and then predictions of the model are compared with CFD calculations (which themselves have been validated against experimental data) for dispersion within an enclosed volume. The results of this simple integral model are seen to agree very well with the CFD predictions. The methodology presented here is therefore proposed as a scientifically validated approach to Hazardous Area Classification.     

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.     

Analyzing effective factors on leakage-induced hydrogen fires

Currently, novel energy resources are receiving increasing attention as a response to the limitation in fossil fuels as well as their adverse effects on human health. Hydrogen, one of the most abundant elements on the earth, can be regarded as a new energy source to replace fossil fuels. Therefore, safety assessment of the relating processes is very crucial by increasing use of hydrogen as a fuel source. In this regard, consequence analysis for risk assessment and power reduction is very important. The present study aims at modeling hydrogen dispersion along with consequence analyses for such events as jet fire and flash fire. The model was validated by using the data derived from a study on hydrogen leakage in supply pipelines in the laboratory of the University of Pisa. Modeling results reveal that ambient conditions will impose a milder impact on leakage consequences if internal pressure is high in release source. The safe distance was also estimated to be 14 m. Dispersion consequence modeling was performed, followed by the evaluation of the effect of environmental (i.e., stability, ambient temperature, surface roughness, wind speed, and humidity) and process (i.e., vessel temperature and pressure, leakage diameter, and releasing point height) parameters on maximum size flammable vapor cloud and maximum level jet fire radiation on the ground. The size of flammable vapor cloud (consequence dispersion index) and the maximum flux of radiation were affected by process parameters more than ambient parameters. Leakage diameter and the vessel pressure were found to have the highest impact on the operational parameters.     

Evaluation of hazard range for the natural gas jet released from a high-pressure pipeline: A computational parametric study

In the present study, the hazard range of the natural gas (NG) jet released from a high-pressure pipeline was investigated. A one-dimensional integral model was combined with a release model to calculate the length and width (i.e., size), and the shape of NG jet release. The physical parameters affecting the jet release of NG were categorized into three types: source release, environmental and time parameters. The effects of each type of parameters on the gas jet release rate, size and shape were evaluated systematically. The results show that all of these parameters have important influence on the hazard range of NG jet release. The source release parameters, including the pipeline length, the operation pressure of the pipeline, the release hole diameter and the pipe diameter, dominate the gas release rate through a hole and therefore the length and width of gas jet release. The gas jet release rate and size are found to be highly correlative with these parameters in terms of power curve regression analysis. The environmental parameters including the atmospheric stability, the ambient wind speed and the source height, have no influence on the gas jet release rate but have influence on the hazard range of gas jet by the turbulent mixing and dilution of NG with air. The time parameters including the concentration averaged time and the valve closing time which are related to the unsteady state jet release of NG, also show the influence on the hazard range of gas jet release. The results show that the decreasing valve closing time and increasing gas concentration averaged time are in favor of reducing the length and width of gas jet release. In addition, these computational parametric studies indicate that the parameters of source release and time have no significant influence on the shape of gas jet release (i.e., jet length/width ratio, LWR) which can maintain the values between 7 and 8. However, the environmental parameters have influence on the shape of gas jet release. These comprehensive investigations provide useful database of evaluating the hazard range for NG jet released from a hole on a high-pressure pipeline and also provide the foundation of decision-making for further fire and/or explosion evaluation and people evacuation.     

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.     

Numerical study on the horizontal stretching effect of ground on high-pressure vapor jets of LNG tank leakage

The objective of this work is to investigate the horizontal stretching effect of ground on high-pressure vapor jet of LNG tank leakage near the ground. A numerical model for leakage jet was developed and several series of leakage scenarios were theoretically analyzed for different heights of the tank orifice, inner pressures and outer temperatures. The results show that the near ground plays an important role in the horizontal transportation of LNG leakage vapor. The corresponding danger distance is surprisingly lengthened because of the horizontal stretching of leakage vapor cloud by the ground nearby, especially for those cases with a lower orifice on tank. It is illustrated that there is an obvious change for the central axis track, gas concentration and velocity of the jet in the far-field during the jet is touching the ground. In addition, the dimensionless analyses on the dependence of gas concentration, velocity and gas concentration on the transportation distance indicated that there were two stages of deflection behaviors of the jet. Finally, the enlarged danger distance by the horizontal stretching for the LNG tank leakage with a low orifice indicated the more dangerous scene of those leakage close on ground. The data and revelation here about the danger area prediction can be an important guide for the emergency management during the LNG tank leakage accidents.     

Theoretical estimation of the lower flammability limit of fuel-air mixtures at elevated temperatures and pressures

We present an approach for predicting the lower flammability limits of combustible gas in air. The influence of initial pressure and temperature on lower flammability limit has been examined in this study. The lower flammability limits of methane, ethylene and propane in air are estimated numerically at the pressure from one to 100 bar and the temperature from ambient to 1200 K. It was found that the predicted LFLs of methane, ethylene and propane decrease slightly with the elevated pressure at the high temperature. The LFLs variation for methane-air mixture is 0.17, 0.18, 0.18 volume% with the initial pressure from one to 100  bar at the initial temperature of 800 K, 1000 K and 1200 K respectively, which is significantly higher than that at lower temperature. And the LFL of methane-air mixture at 1200 K and 100 bar reaches 1.03 volume% which is much lower than that at 1 bar and ambient temperature. On the other hand, the LFLs variation is 0.11–0.12 volume% for ethylene-air mixture and 0.06–0.07 volume% for propane-air mixture with the initial temperature from 800 K to 1200 K at the same range of pressure. The LFL values at high temperatures and pressures represent higher risk of explosion.     

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.     

A framework for human error risk analysis of coal mine emergency evacuation in China

Evacuation from underground coal mine in emergency as soon as possible makes the difference between life and death. Human factors have an important impact on a successful evacuation, but literature review shows that there is a lack of consideration of human error risk during coal mine emergency evacuation in China. To address the above problems, in this paper, we established a framework for human error risk analysis of coal mine emergency evacuation, consisting of scenario and task analysis, risk assessment and risk reduction. A general evacuation procedure which is applicable for different causes is detailed through the scenario and task analysis. A new method based on expert judgment, named OGI-Model, is proposed to evaluate the reliability of human safety barrier. In this new approach, human safety barrier is divided into three sub-barriers, i.e., organization safety sub-barrier (OSSB), group safety sub-barrier (GSSB), and individual safety sub-barrier (ISSB). Each sub-barrier consists of a series of concrete measures against specific evacuation actions. An example is provided in this paper to demonstrate the use of this framework and its effectiveness.     

高压天然气非恒定速率泄漏扩散数值模拟研究*

为保障天然气工业安全生产与运营,以某天然气储配厂为例,采用等效喷嘴和过程模型,利用FLACS软件对罐区高压天然气非恒定速率泄漏扩散进行数值模拟,考察环境风速及泄漏时间对气体泄漏扩散的影响。结果表明:储存压力为1.05 MPa的天然气储罐发生泄漏会产生欠膨胀射流,泄漏初期具有447.44 kJ的高动能,并在近场扩散起主导作用;在气体持续泄漏的200 s内,泄漏质量流量仅发生0.71 kg/s的变化,对泄漏扩散影响不明显,各风速条件下的泄漏会在动能稳定风场和浮力的共同作用下,使可燃气云体积及分布在一定时间内达到动态稳定状态,等效化学计量气云体积不再发生明显变化;质量流量会随着时间的增加变化会越来越明显,进行非恒定速率气体泄漏扩散的模拟,会更有利于现场情况的判断和处置;风速的增大与风向对扩散的影响成正比,与气云趋于动态稳定的时间、可燃气云分布及体积成反比。     

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

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