Evaluating the potential for overpressures from the ignition of an LNG vapor cloud during offloading

Ignition of natural gas (composed primarily of methane) is generally not considered to pose explosion hazards when in unconfined and low- or medium-congested areas, as most of the areas within LNG regasification facilities can typically be classified. However, as the degrees of confinement and/or congestion increase, the potential exists for the ignition of a methane cloud to result in damaging overpressures (as demonstrated by the recurring residential explosions due to natural gas leaks). Therefore, it is prudent to examine a proposed facility’s design to identify areas where vapor cloud explosions (VCEs) may cause damage, particularly if the damage may extend off site.An area of potential interest for VCEs is the dock, while an LNG carrier is being offloaded: the vessel hull provides one degree of confinement and the shoreline may provide another; some degree of congestion is provided by the dock and associated equipment.In this paper, the computational fluid dynamics (CFD) software FLACS is used to evaluate the consequences of the ignition of a flammable vapor cloud from an LNG spill during the LNG carrier offloading process. The simulations will demonstrate different approaches that can be taken to evaluate a vapor cloud explosion scenario in a partially confined and partially congested geometry.     

The multi-energy critical separation distance

Devastating vapour cloud explosions can only develop under appropriate (boundary) conditions. The record of vapour cloud explosion incidents from the past demonstrates that these conditions are readily met by the congestion by process equipment at (petro-) chemical plant sites. Therefore, the possibility of an accidental release of a flammable and a subsequent vapour cloud explosion is a major hazardous scenario considered in any risk assessment with regard to the process industries.If an extended flammable vapour cloud at a chemical plant site extends over more than one process unit, which are separated by lanes of sufficient width, the vapour cloud explosion on ignition develops the same number of separate blasts. If, on the other hand, the separation between the units is insufficient, the vapour cloud explosion develops one big blast. The critical separation distance (SD) is the criterion that allows discriminating in this matter for blast modelling purposes.This paper summarises some major results of an experimental research programme with the objective to develop practical guidelines with regard to the critical SD. To this end, a series of small-scale explosion experiments have been performed with vapour clouds containing two separate configurations of obstacles. Blast overpressures at various stations around have been recorded while the SD between the two configurations of obstacles was varied.The experimental programme resulted in some clear indications for the extent of the critical SD between separate areas of congestion. On the basis of safety and conservatism, these indications have been rendered into a concrete guideline. Application of this guideline would allow a greater accuracy in the modelling of blast from vapour cloud explosions.     

Un-ignited and ignited high pressure hydrogen releases: Concentration – turbulence mapping and overpressure effects

Safety studies for production and use of hydrogen reveal the importance of accurate prediction of the overpressure effects generated by delayed explosions of accidental high pressure hydrogen releases. Analysis of previous experimental work demonstrates the lack of measurements of turbulent intensities and lengthscales in the flammable envelope as well as the scarceness of accurate experimental data for explosion overpressures and flame speeds. AIR LIQUIDE, AREVA STOCKAGE ENERGIE and INERIS join in a collaborative project to study un-ignited and ignited high pressure releases of hydrogen.The purpose of this work is to map hydrogen flammable envelopes in terms of concentration, velocity and turbulence, and to characterize the flame behaviour and the associated overpressure. These experimental results (dispersion and explosion) are also compared with blind FLACS modelling.     

Application of the TNO multi-energy and Baker-Strehlow-Tang methods to predict hydrogen explosion effects from small-scale experiments

Analytical models or abacus are of importance to predict explosion effects in open and congested areas for industrial safety reasons. The goal of this work is to compare overpressure and flame speed values of small-scale deflagration experiments to predicted values from the TNO multi-energy (TNO ME) method and the Baker-Strehlow-Tang (BST) method. Experiments were performed in cylindrical congested volumes of hydrogen – air mixtures varying from 1.77 L to 7.07 L. The reactivity was controlled by the equivalence ratio of hydrogen-air mixtures, ranging from 0.5 to 2.5. The congestion was realized with varying numbers of grid layers and configurations. The influence of the obstacle density and the importance of the mixture reactivity to choose the strength index in order to predict the effects of an explosion has been highlighted for the TNO ME method. Predictive flame speed values from the BST method are in accordance with almost half of the experimental results and the method is conservative in most tested configurations. The use of the TNO ME method has been validated on a small-scale experiment to predict maximal overpressures generated by the deflagration of medium and large-scale H₂/air clouds.     

A numerical approach to investigate the maximum permissible nozzle diameter in explosion by hot turbulent jets

The ignition of a combustible environment by hot jets is a safety concern in many industries. In explosion protection concepts, for a protection of the type “flameproof enclosures” a maximum permissible gap is of major importance. In this work a numerical framework is described to investigate the ignition processes by a hot turbulent jet which flows out from such gaps. A Probability Density Function (PDF) method in conjunction with a reaction-diffusion manifold (REDIM) technique is used to model the turbulent reactive flow. In this paper the ignition of a stoichiometric mixture of hydrogen/air gas by a hot exhaust turbulent jet is examined. The impact of the nozzle diameter on the ignition delay time is investigated, too. The method is used to explore the maximum nozzle diameter for specific boundary conditions for which there is no ignition.     

The effect of vent size and congestion in large-scale vented natural gas/air explosions

A typical building consists of a number of rooms; often with windows of different size and failure pressure and obstructions in the form of furniture and décor, separated by partition walls with interconnecting doorways. Consequently, the maximum pressure developed in a gas explosion would be dependent upon the individual characteristics of the building. In this research, a large-scale experimental programme has been undertaken at the DNV GL Spadeadam Test Site to determine the effects of vent size and congestion on vented gas explosions. Thirty-eight stoichiometric natural gas/air explosions were carried out in a 182 m³ explosion chamber of L/D = 2 and K_A = 1, 2, 4 and 9. Congestion was varied by placing a number of 180 mm diameter polyethylene pipes within the explosion chamber, providing a volume congestion between 0 and 5% and cross-sectional area blockages ranging between 0 and 40%. The series of tests produced peak explosion overpressures of between 70 mbar and 3.7 bar with corresponding maximum flame speeds in the range 35–395 m/s at a distance of 7 m from the ignition point. The experiments demonstrated that it is possible to generate overpressures greater than 200 mbar with volume blockages of as little as 0.57%, if there is not sufficient outflow through the inadvertent venting process. The size and failure pressure of potential vent openings, and the degree of congestion within a building, are key factors in whether or not a building will sustain structural damage following a gas explosion. Given that the average volume blockage in a room in a UK inhabited building is in the order of 17%, it is clear that without the use of large windows of low failure pressure, buildings will continue to be susceptible to significant structural damage during an accidental gas explosion.     

A method for simulation of vapour cloud explosions based on computational fluid dynamics (CFD)

The effectiveness of the application of CFD to vapour cloud explosion (VCE) modelling depends on the accuracy with which geometrical details of the obstacles likely to be encountered by the vapour cloud are represented and the correctness with which turbulence is predicted. This is because the severity of a VCE strongly depends on the types of obstacles encountered by the cloud undergoing combustion; the turbulence generated by the obstacles influences flame speed and feeds the process of explosion through enhanced mixing of fuel and oxidant. In this paper a CFD-based method is proposed on the basis of the author’s finding that among the various models available for assessing turbulence, the realizable k-? model yields results closer to experimental findings than the other, more frequently used, turbulence models if used in conjunction with the eddy-dissipation model. The applicability of the method has been demonstrated in simulating the dispersion and ignition of a typical vapour cloud formed as a result of a spill from a liquid petroleum gas (LPG) tank situated in a refinery. The simulation made it possible to assess the overpressures resulting from the combustion of the flammable vapour cloud. The phenomenon of flame acceleration, which is a characteristic of combustion enhanced in the presence of obstacles, was clearly observed. Comparison of the results with an oft-used commercial software reveals that the present CFD-based method achieves a more realistic simulation of the VCE phenomena.     

基于20 L球罐的多相混合物扩散模拟

为更好地探索多相混合物的爆炸特性，以铝粉、乙醚、空气为研究对象，基于20 L球型爆炸罐建立三维计算模型，对气固两相和气液固三相混合物的分散过程进行数值模拟，以分析不同多相混合物分散过程的差异，并为测量多相混合物爆炸下限时的点火延迟时间设定提供参考。监测分析铝粉浓度粒子分布、流场内部湍流动能以及液相体积百分数等的演化过程，讨论混合物分散效果的差异，并确定测量爆炸下限的点火延迟时间。研究结果表明:实验工况下，液相的存在会降低粉尘云团的湍流动能、降低其扩散速度，并使粉尘云内部浓度更均匀。测量多相混合物爆炸下限时，三相混合物的最佳点火延迟时间早于气固两相混合物10~20 ms。     

Experimental studies on the ignition behavior of pure silane released into air

Silane is a well known pyrophoric gas which normally ignites upon contact with air. However, a silane release from a pressure source may not always lead to prompt ignition and frequently the ignition occurs when the release is shutoff. In a confined space, significant quantities of silane can accumulate prior to autoignition leading to an explosion, causing significant damage. To date, the mechanism and condition of pure silane ignition upon release into air has not been completely explained. Thus there is a need for additional experimental investigations covering a wide range of release conditions to enable a full understanding of silane ignition and explosion characteristics.This work presents a series of tests that aims to uncover the precise condition for pure silane ignition upon release into air. Tests were performed for releases at controlled and steady velocities. Steady flow of silane to a burn box and nitrogen to the desired vent stub are first established through mass flow controllers. An electrically controlled four-way switching valve is used to switch the silane and nitrogen flow such that steady silane flow without acceleration to the vent stub can be established. With careful control of vent conditions, we are able to find a reproducible critical exit velocity for prompt ignition of steady silane releases for different vent diameters. If the releases are reduced to below the critical exit velocity, prompt ignition of silane release is ensured. Above this critical exit velocity, silane can be released indefinitely into air without any ignition. The critical exit velocity is found to vary with the vent diameter. This relationship between the critical exit velocity and the vent diameter suggests that the silane release without prompt ignition was most likely caused by quench of the reactive kernel from the scalar dissipation between the release gas and the ambient air. Analysis of locations where prompt ignition occurred from the clips from high speed video camera found that the most reactive ignition kernel occurs in the range when the ratio of volumetric flow rate of entrained air to the silane flow reaches 0.322 ± 0.076, which is equivalent to the most reactive silane concentration of 75.6% in air. The implications from these results are discussed with emphasis on the safety of silane supply systems and operation practices.     

Jet Fires Involving Releases of Crude Oil,Gas and Water

A Joint Industry Project was undertaken to study releases representative of those that could occur on offshore installations involving well-stream fluids either at the wellhead or during processing. The project involved a programme of large-scale jet fire experiments to investigate releases involving mixtures of crude oil and gas (simulating ‘live’ crude) and water. Seven oil and gas companies and the UK Health and Safety Executive sponsored the work that was undertaken by Advantica and Shell Global Solutions at the Spadeadam Test Site, Cumbria, UK. The results demonstrate that the presence of water modifies, significantly, the size, shape, thermal characteristics and stability of a jet fire. For an equivalent release of fuel, the distance from the release point within which flame impingement could occur is slightly increased. However, the overall size of the flame, the temperatures within the flame and the overall heat loading to objects within the flame are reduced. The results also indicate that the flame is extinguished if the water content of the well-stream fluid reaches about 55% by mass. Some well-stream fluids contain a lot of water, in some cases up to 80% by mass, and these will not produce jet fires.     

Assessment of an accidental vapour cloud explosion: Lessons from the Indian Oil Corporation Ltd. accident at Jaipur,India

On 29 October 2009, at 19:30 IST, a devastating vapour cloud explosion occurred in a large fuel storage area at the Indian Oil Corporation (IOC) Depot in Jaipur, India, generating significant blast pressure. As a consequence of this explosion, the entire installation was destroyed, buildings in the immediate vicinity were heavily damaged, and windowpane breakages were found up to 2 km from the terminal. The IOC estimated that the total loss from the fire and explosion was approximately INR 2800 million.Ironically, as a storage site, the Jaipur terminal was not highly congested, and thus was not considered to have adequate potential for a vapour cloud explosion (VCE). Nevertheless, the prima facie evidences indicate that this was a case of VCE. Therefore, the main objective of this study is to quantify the potential overpressures due to vapour cloud explosions (VCEs) using the Process Hazard Analysis DNV Norway based PHAST 6.51 Software. The results are validated by the extent of the damage that had occurred. The estimation of the VCE shows that a maximum 1.0 bar overpressure was generated in the surrounding area. The initial assessment of the accident data roughly estimates the release mode, time, and amount of vaporized fuel. A more accurate estimate has been obtained by modelling the dispersion of vapour clouds in the surrounding atmosphere, which reveals trends and relationships for the occurrence of vapour cloud explosions.     

Wire-mesh inhibition of jet fire induced by explosion venting

Explosion venting is widely applied in industrial explosion-proof designs due to the convenient, economical and practical features of this method. Natural gas is usually stored in storage tanks. If the gas in the vessel is mixed with air and encounters an ignition source, explosion venting might occur, producing jet fire, generating new secondary derivative accidents and causing casualties and property losses. In this paper, a set of test platforms including wire-mesh suppression devices is established to study the inhibition of jet fire induced by explosion venting by wire mesh. The experimental research shows that a wire mesh significantly inhibits the jet fire induced by explosion venting. The flame propagation velocity and pressure clearly decrease with increasing numbers of wire-mesh layers. The wire-mesh structure significantly affects the flame propagation, and the more layers of mesh there are, the better the suppression effect is. The flame temperature gradually decreases with the addition of the wire mesh. The mesh size significantly affects the pressure propagation of explosion venting. The explosion pressure gradually decreases with the addition of the wire mesh. With increasing distance between the wire mesh and the explosion vent, the maximum temperature first increases and then decreases, and the maximum explosion pressure first decreases and then increases. In the case of single gas cloud, the flame suppression effect is the most obvious when the wire mesh is 0.2 m away from the explosion vent. In the case of double gas clouds, the flame suppression effect is the most significant when the distance between the wire mesh and the first gas cloud is 0.4 m.     

CFD modelling of hydrogen release, dispersion and combustion for automotive scenarios

The paper describes the analysis of the potential effects of releases from compressed gaseous hydrogen systems on commercial vehicles in urban and tunnel environments using computational fluid dynamics (CFD). Comparative releases from compressed natural gas systems are also included in the analysis.

This study is restricted to typical non-articulated single deck city buses. Hydrogen releases are considered from storage systems with nominal working pressures of 20, 35 and 70 MPa, and a comparative natural gas release (20 MPa). The cases investigated are based on the assumptions that either fire causes a release via a thermally activated pressure relief device(s) (PRD) and that the released gas vents without immediately igniting, or that a PRD fails. Various release strategies were taken into account. For each configuration some worst-case scenarios are considered.

By far the most critical case investigated in the urban environment, is a rapid release of the entire hydrogen or natural gas storage system such as the simultaneous opening of all PRDs. If ignition occurs, the effects could be expected to be similar to the 1983 Stockholm hydrogen accident [Venetsanos, A. G., Huld, T., Adams, P., & Bartzis, J. G. (2003). Source, dispersion and combustion modelling of an accidental release of hydrogen in an urban environment. Journal of Hazardous Materials, A105, 1–25]. In the cases where the hydrogen release is restricted, for example, by venting through a single PRD, the effects are relatively minor and localised close to the area of the flammable cloud. With increasing hydrogen storage pressure, the maximum energy available in a flammable cloud after a release increases, as do the predicted overpressures resulting from combustion. Even in the relatively confined environment considered, the effects on the combustion regime are closer to what would be expected in a more open environment, i.e. a slow deflagration should be expected.

Among the cases studied the most severe one was a rapid release of the entire hydrogen (40 kg) or natural gas (168 kg) storage system within the confines of a tunnel. In this case there was minimal difference between a release from a 20 MPa natural gas system or a 20 MPa hydrogen system, however, a similar release from a 35 MPa hydrogen system was significantly more severe and particularly in terms of predicted overpressures. The present study has also highlighted that the ignition point significantly affects the combustion regime in confined environments. The results have indicated that critical cases in tunnels may tend towards a fast deflagration, or where there are turbulence generating features, e.g. multiple obstacles, there is the possibility that the combustion regime could progress to a detonation.

When comparing the urban and tunnel environments, a similar release of hydrogen is significantly more severe in a tunnel, and the energy available in the flammable cloud is greater and remains for a longer period in tunnels. When comparing hydrogen and natural gas releases, for the cases and environments investigated and within the limits of the assumptions, it appears that hydrogen requires different mitigation measures in order that the potential effects are similar to those of natural gas in case of an accident. With respect to a PRD opening strategy, hydrogen storage systems should be designed to avoid simultaneous opening of all PRD, and that for the consequences of the released energy to be mitigated, either the number of PRDs opening should be limited or their vents to atmosphere should be restricted (the latter point would require validation by a comprehensive risk assessment).     

Acoustic analysis of blast waves produced by rapid phase transition of LNG released on water

Massive offshore and onshore storage of fuel have led the international community to raise questions about the hazards on the surrounding installations and people. Among the possible accidental scenarios when cryogenic gas as liquefied natural gas (LNG) is spilled on water at a very fast rate, the phenomenon of rapid phase transition (RPT) may occur: large amounts of energy are released during phase transition which can generate explosions. The related consequences should be added to the possible consequences of fire in terms of flash fire, fireball, pool fire, and vapour cloud explosion for confined and congested geometry surrounding the release point.In this paper, the analysis of RPT of LNG has been studied from the point of view of blast wave production, through ab initio acoustic analysis for monopole source. Maximum overpressures, as calculated at the source point and along the blast pathway are compared with results of large scale experiments. Safety distances are given for the sake of comparison with threshold distances reported in the open literature.     

基于FLACS的海上钻探平台井喷爆炸事故后果模拟分析*

为研究海上钻探平台井喷燃爆事故后果,运用FLACS软件对某深海钻探平台井喷爆炸事故进行模拟,研究在不同事故场景下气云爆炸发展过程及平台荷载分布规律,讨论井喷速率、风向、点火位置等对爆炸超压的影响。研究结果表明:随泄漏速率增加,爆炸强度和爆炸范围均增大,爆炸严重程度不仅与井喷速率密切相关,同时也受平台结构影响;点火位置会对爆炸超压产生影响,在可燃气体与空气混合气体比例为化学理论当量比处点燃气体,生活区承受的爆炸超压最大;在设施及建构筑物分布较为密集、拥塞度较高的地方产生的爆炸超压更大。研究结果可为可为平台的阻隔防爆性能设计与应急响应提供指导。     

An integrated quantitative hazard analysis method for natural gas jet release from underground gas storage caverns in salt rock. I: Models and validation

It is very important and necessary to perform quantitative hazard analysis for possible accidental leakage from an underground gas storage cavern in salt rock. An integrated quantitative hazard analysis method for natural gas jet release from salt caverns is presented in this paper, which was constituted by a revised model for gas leakage rate calculation, a consequence analysis and a model of probability assessment for harm. The presented method was validated by comparing the analytical results with the data collected from the real accidents (including the leakage, jet fire, fireball and vapor cloud explosion). It is indicated that the proposed method was more accurate than the TNT equivalence method for vapor cloud explosion and gave more reasonable results when applied to the consequence analysis for the thermal radiation from jet fire and fireball.     

Experimental investigation on shock waves generated by pressurized gas release through a tube

An experimental investigation on the flow structures and the strength of shock waves generated by high-pressure gas release through a tube into air was conducted. The results demonstrated that a leading shock wave was generated in front of the compressed gas jet and the shock wave speed increased firstly, then decreased and finally kept constant with an increase of the propagation distance in the tube. The experimentally measured Mach numbers of shock waves were close to those calculated from the theory of ideal shock tube flow. After spouting out of the tube, the normal shock quickly developed into a hemispherical shape. The Mach disk was observed in the under-expanded jet. For high-pressure combustible gas release, the concept of theoretical critical pressure of ignition was introduced and several theoretical critical pressures of common gaseous fuels were obtained.     

A stochastic approach for risk analysis in vapor cloud explosion

A stochastic approach for evaluating the risk of vapor cloud explosions is proposed in this work. The proposed methodology aims to incorporate the effect of uncertainty into the risk analysis to produce a better overall view for the risk. Some stochastic variables are used to estimate the probability of vapor cloud explosions: frequency of the release, the probability of not having an immediate ignition, the probability of delayed ignition and the probability of a vapor cloud explosion given a delayed ignition, as well as different possible meteorological conditions. These stochastic variables are represented with probability distribution curves. Different curves for the frequencies of releases from process equipment types (steel process pipes, flanges, manual valves, actuated valves, etc.), different equipment diameters and different leak sizes are also used in this analysis. Monte Carlo simulation is performed to obtain the risk as a probability distribution using the Analytic Solver Platform. Then the risk distribution curve obtained by Monte Carlo simulation is used to estimate the probability of satisfying the risk tolerance criterion.     

Buncefield: A violent, episodic vapour cloud explosion

Damage caused by the 2005 Buncefield explosion indicates pressures in excess of 2000 mbar over all of the area covered by the vapour cloud. Such high overpressures are normally associated with high (super-sonic) rates of flame spread. On the other hand, evidence from witnesses, building damage analysis and CCTV cameras all suggest the average rate of progress of the explosion flame front was only around 150 m/s.The high overpressures in the cloud and low average rate of flame advance can be reconciled if the rate of flame advance was episodic, with periods of very rapid combustion being punctuated by pauses when the flame advanced very slowly. The widespread high overpressures were caused by the rapid phases of combustion; the low average speed of advance was caused by the pauses.Mechanisms of flame spread through radiative ignition of particulates ahead of the flame front provide possible explanations for such unusual episodic behaviour.The first part of this paper reviews a wide range of empirical evidence on average flame speed and rate of blast pressure increase.The second part explores the theoretical consequences of forward radiation and how the new theory might be developed into a practical means of assessment.