Safety

Hydrogen is widely recognized as an attractive energy carrier due to its low-level air pollution and its high mass-related energy density. However its wide flammability range and high burning velocity present a potentially significant hazard. A significant fraction of hydrogen is stored and transported as a cryogenic liquid (liquid hydrogen or LH2) as it requires much less volume compared to gaseous hydrogen. In order to exist as a liquid H2 must be cooled to a very low temperature 20.28 K. LH2 is a common liquid fuel for rocket applications. It can also be used as the fuel storage in an internal combustion engine or fuel cell for transport applications. Models for handling liquid releases both two-phase flashing jets and pool spills have been developed in the CFD-model FLACS. The very low normal boiling point of hydrogen (20 K) leads to particular challenges as this is significantly lower than the boiling points of oxygen (90 K) and nitrogen (77 K). Therefore a release of LH2 in the atmosphere may induce partial condensation or even freezing of the oxygen and nitrogen present in the air. A pool model within the CFD software FLACS is used to compute the spreading and vaporization of the liquid hydrogen depositing on the ground where the partial condensation or freezing of the oxygen and nitrogen is also taken into account. In our computations of two-phase jets the dispersed and continuous phases are assumed to be in thermodynamic and kinematic equilibrium. Simulations with the new models are compared against selected experiments performed at the Health and Safety Laboratory (HSL).

The EC 6th framework research project HyApproval will draft a Handbook which will describe all relevant issues to get approval to construct and operate a Hydrogen Refuelling Station (HRS) for hydrogen vehicles. In WP3 of the HyApproval project it is under investigation which safety information competent authorities require to give a licence to construct an operate an HRS. The paper describes the applied methodology to collect the information from the authorities in 5 EC countries and the USA. The results of the interviews and recommendations for the information to include in the Handbook are presented.

The development of hydrogen as energy carrier is promoted by the increasing in energy demand depletion of fossil resources and the global warming. However this issue relies primarily on the safety aspect which requires the knowledge in the case of gas release of the quantities such as the flammable cloud size release path and the location of the lower flammability limit of the mixture. The integral models for predicting the atmospheric dispersion were extensively used in previous works for low pressure releases such as pollutant and  flammable gas transport. In the present investigation this approach is extended to the high pressure gas releases. The model is developed in the non-Boussinesq approximation and is based on Gaussian profiles for buoyant variable density jet or plume in stratified atmosphere with a crossflow. Validations have been performed on a broad range of hydrogen methane and air dispersion cases including vertical or horizontal jets or plumes into a quiescent atmosphere or with crosswind.

Hydrogen jet flames resulting from ignition of unintended releases can be extensive in length and pose significant radiation and impingement hazards. One possible mitigation strategy to reduce exposure to jet flames is to incorporate barriers around hydrogen storage and delivery equipment. While reducing the extent of unacceptable consequences the walls may introduce other hazards if not properly configured. This paper describes experiments carried out to characterize the effectiveness of different barrier wall configurations at reducing the hazards created by jet fires. The hazards that are evaluated are the generation of overpressure during ignition the thermal radiation produced by the jet flame and the effectiveness of the wall at deflecting the flame.<br/>The tests were conducted against a vertical wall (1-wall configuration) and two “3-wall” configurations that consisted of the same vertical wall with two side walls of the same dimensions angled at 135° and 90°. The hydrogen jet impinged on the center of the central wall in all cases. In terms of reducing the radiation heat flux behind the wall the 1-wall configuration performed best followed by the 3-wall 135° configuration and the 3-wall 90°. The reduced shielding efficiency of the three-wall configurations was probably due to the additional confinement created by the side walls that limited the escape of hot gases to the sides of the wall and forced the hot gases to travel over the top of the wall.<br/>The 3-wall barrier with 135° side walls exhibited the best overall performance. Overpressures produced on the release side of the wall were similar to those produced in the 1-wall configuration. The attenuation of overpressure and impulse behind the wall was comparable to that of the three-wall configuration with 90° side walls. The 3-wall 135° configuration’s ability to shield the back side of the wall from the heat flux emitted from the jet flame was comparable to the 1-wall and better than the 3-wall 90° configuration. The ratio of peak overpressure (from in front of the wall and from behind the wall) showed that the 3-wall 135° configuration and the 3-wall 90° configuration had a similar effectiveness. In terms of the pressure mitigation the 3-wall configurations performed significantly better than the 1-wall configuration

Clarification of questions of safety represents a decisive contribution to the successful introduction of vehicles fuelled by hydrogen. At the moment the safety of hydrogen is being discussed and investigated by various bodies. The primary focus is on fuel-cell vehicles with hydrogen stored in gaseous form. This paper looks at the safety of hydrogen-fuelled vehicles with an internal combustion engine and liquefied hydrogen storage. The safety concept of BMW’s hydrogen vehicles is described and the specific aspects of the propulsion and storage concepts discussed. The main discussion emphasis is on the utilization of boil-off parking of the vehicles in an enclosed space and their crash behaviour. Theoretical safety observations are complemented by the latest experimental and test results. Finally reference is made to the topic-areas in the field of hydrogen safety in which cooperative research work could make a valuable contribution to the future of the hydrogen-powered vehicle.

The first part of the present work is to validate a detailed kinetic mechanism for the oxidation of hydrogen – methane – air mixtures in a detonation waves. A series of experiments on auto-ignition delay times have been performed by shock tube technique coupled with emission spectrometry for H2 / CH4 / O2 mixtures highly diluted in argon. The CH4/H2 ratio was varied from 0 to 4 and the equivalence ratio from 0.4 up to 1. The temperature range was from 1250 K to 2000 K and the pressure behind reflected shock waves was between 0.15 and 1.6 MPa. A correlation was proposed between temperature (K) concentration of chemical species (mol m-3) and ignition delay times. The experimental auto-ignition delay times were compared to the modelled ones using four different mechanisms from the literature: GRI [22] Marinov et al. [23] Hughes et al. [24] Konnov [25]. A large discrepancy was generally found between the different models. The Konnov’s model that predicted auto-ignition delay times close to the measured ones has been selected to calculate the ignition delay time in the detonation waves. The second part of the study concerned the experimental determination of the detonation properties namely the detonation velocity and the cell size. The effect of the initial composition hydrogen to methane ratio and the amount of oxygen in the mixture as well as the initial pressure on the detonation velocity and on the cell size were investigated. The ratio of methane / (methane + hydrogen) varied between 0 and 0.6 for 2 different equivalence ratio (0.75 and 1) while the initial pressure was fixed to 10 kPa. A correlation was established between the characteristic cell size and the ignition delay time behind the leading shock of the detonation. It was clearly showed that methane has an important inhibitor effect on the detonation of these combustible mixtures.

Burning hydrogen emits thermal radiation in UV NIR and IR spectral range. Especially in the case of large cloud explosion the risk of heat radiation is commonly underestimated due to the non-visible flame of hydrogen-air combustion. In the case of a real explosion accident organic substances or inert dust might be entrained from outer sources to produce soot or heated solids to substantially increase the heat release by continuum radiation. To investigate the corresponding combustion phenomena different hydrogen-air mixtures were ignited in a closed vessel and the combustion was observed with fast scanning spectrometers using a sampling rate up to 1000 spectra/s. In some experiments to take into account the influence of organic co-combustion a spray of a liquid glycol-ester and milk powder was added to the mixture. The spectra evaluation uses the BAM code of ICT to model bands of reaction products and thus to get the temperatures. The code calculates NIR/IR-spectra (1 - 10 μm) of non-homogenous gas mixtures of H2O CO2 CO NO and HCl taking into consideration also emission of soot particles. It is based on a single line group model and makes also use of tabulated data of H2O and CO2 and a Least Squares Fit of calculated spectra to experimental ones enables the estimation of flame temperatures. During hydrogen combustion OH emits an intense spectrum at 306 nm. This intermediary radical allows monitoring the reaction progress. Intense water band systems between 1.2 and 3 μm emit remarkable amounts of heat radiation according to a measured flame temperature of 2000 K. At this temperature broad optically-thick water bands between 4.5 μm and 10 μm contribute only scarcely to the total heat output. In case of co-combustion of organic materials additional emission bands of CO and CO2 as well as a continuum radiation of soot and other particles occur and particularly increase the total thermal output drastically.

An essential problem for the operation of high pressure water electrolyzers and fuel cells is the permissible contamination of hydrogen and oxygen. This contamination can create malfunction and in the worst case explosions in the apparatus and gas cylinders. In order to avoid dangerous conditions the exact knowledge of the explosion characteristics of hydrogen/air and hydrogen/oxygen mixtures is necessary. The common databases e.g. the CHEMSAFE® database published by DECHEMA BAM and PTB contains even a large number of evaluated safety related properties among other things explosion limits which however are mainly measured according to standard procedures under atmospheric conditions.<br/>Within the framework of the European research project “SAFEKINEX” and other research projects the explosion limits explosion pressures and rates of pressure rise (KG values) of H2/air and H2/O2 mixtures were measured at elevated conditions of initial pressures and temperatures by the Federal Institute of Materials Research and Testing (BAM). Empirical equations of the temperature influence could be deduced from the experimental values. An anomaly was found at the pressure influence on the upper explosion limits of H2/O2 and H2/air mixtures in the range of 20 bars. In addition explosion pressures and also rates of pressure rises have been measured for different hydrogen concentrations inside the explosion range. Such data are important for constructive explosion protection measures. Furthermore the mainly used standards for the determination of explosion limits have been compared. Therefore it was interesting to have a look at the systematic differences between the new EN 1839 tube and bomb method ASTM E 681-01 and German DIN 51649-1.

Hydrogen is not more dangerous than current fossil energy carriers but it behaves differently. Therefore hydrogen specific analyses and countermeasures will be needed to support the development of safe hydrogen technologies. A systematic step-by-step procedure for the mechanistic analysis of hydrogen behaviour and mitigation in accidents is presented. The procedure can be subdivided into four main parts:<br/>1) 3D modelling of the H2-air mixture generation<br/>2) hazard evaluation for this mixture based on specifically developed criteria for flammability flame acceleration and detonation on-set<br/>3) numerical simulation of the appropriate combustion regime using verified 3D-CFD codes and<br/>4) consequence analysis based on the calculated pressure and temperature loads.

This paper presents a compilation of the results supplied by HySafe partners participating in the Standard Benchmark Exercise Problem (SBEP) V2 which is based on an experiment on hydrogen combustion that is first described. A list of the results requested from participants is also included. The main characteristics of the models used for the calculations are compared in a very succinct way by using tables. The comparison between results together with the experimental data when available is made through a series of graphs. The results show quite good agreement with the experimental data. The calculations have demonstrated to be sensitive to computational domain size and far field boundary condition.

All vehicular high-pressure hydrogen tanks are equipped with thermally-activated pressure relief devices (TPRDs) required by Global Technical Regulation. This safety device significantly reduces the risk of tank catastrophic rupture by venting the hydrogen pressure outside. However the released flammable hydrogen raises additional safety problems. Japan Automobile Research Institute has demonstrated that in the vehicle fire event once the TPRD opens the hydrogen fires will engulf the whole vehicle making it difficult for the drivers and passenger to evacuate from the vehicle. This paper designs a new safety system to solve the evacuation problem. The safety system includes a rotatable pressure relief device with a motor a sensory system that consists of infrared sensors ultrasonic radar and temperature sensors a central control unit and an alarm device. The new design of the pressure relief device allows the system actively adjusting the release direction towards void open space outside the vehicle to minimize the risks of hydrogen fires. The infrared sensors located at the roof of the vehicles collect info inside the vehicle and the ultrasonic radar detect the region outside the vehicle. Temperature sensors tell when to trigger the alarm and set the motor in standby mode and the central control unit determines where to rotate based on the info from the infrared sensors and ultrasonic radars. A control strategy is also proposed to operate the safety system in an appropriate way. The cost-benefit analysis show that the new safety system can significantly reduce the risks of intended hydrogen releases from onboard pressure relief devices with total cost increases by less than 1% of the vehicle cost making it a good cost-effective engineering solution.

Previous studies have demonstrated that while blowdown pressure is reproduced well by both adiabatic and isothermal analytical models the dynamics of temperature cannot be predicted well by either model. The reason for the last is heat transfer to cooling during expansion gas from the vessel wall. Moreover when exposed to an external fire the temperature inside the vessel increases i.e. when a thermally activated pressure relief device (TPRD) is still closed with subsequent pressure increase that may lead to a catastrophic rupture of the vessel. The choice of a TPRD exit orifice size and design strategy are challenges: to provide sufficient internal pressure drop in a fire when the orifice size is too small; to avoid flame blow off expected with the decrease of pressure during the blowdown; to decrease flame length of subsequent jet fire as much as possible by the decrease of the orifice size under condition of sufficient fire resistance provisions to avoid pressure peaking phenomenon etc. The adiabatic model of blowdown [1] was developed using the Abel-Nobel equation of state and the original theory of underexpanded jet [2]. According to experimental observations e.g. [3] heat transfer plays a significant role during the blowdown. Thus this study aims to modify the adiabatic blowdown model to include the heat transfer to non-ideal gas. The model accounts for a change of gas temperature inside the vessel due to two “competing” processes: the decrease of temperature due to gas expansion and the increase of temperature due to heat transfer from the surroundings e.g. ambience or fire through the vessel wall. This is taken into account in the system of equations of adiabatic blowdown model through the change of energy conservation equation that accounts for heat from outside. There is a need to know the convective heat transfer coefficient between the vessel wall and the surroundings and wall size and properties to define heat flux to the gas inside the vessel. The non-adiabatic model is validated against available experimental data. The model can be applied as a new engineering tool for the inherently safer design of hydrogen tank-TPRD system.

This paper describes a methodology for simulating the structural response of vented enclosures during hydrogen deflagrations. The paper also summarises experimental results for the structural response of 20-foot ISO (International Organization for Standardization) containers in a series of vented hydrogen deflagration experiments. The study is part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA). The project is funded by the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 671461. The HySEA project focuses on vented hydrogen deflagrations in containers and smaller enclosures with internal congestion representative of industrial applications. The structural response modelling involves one-way coupling of pressure loads taken either directly from experiments or from simulations with the computational fluid dynamics (CFD) tool FLACS to the non-linear finite element (FE) IMPETUS Afea Solver. The performance of the FE model is evaluated for a range of experiments from the HySEA project in both small-scale enclosures and 20-foot ISO containers. The paper investigates the sensitivity of results from the FE model to the specific properties of the geometry model. The performance of FLACS is evaluated for a selected set of experiments from the HySEA project. Furthermore the paper discusses uncertainties associated with the combined modelling approach.

Composite cylinders with metal liner are used for the storage of compressed hydrogen in automotive application. These hybrid pressure cylinders are designed for a nominal working pressure of up to 70 MPa. They also have to withstand a temperature range between -40°C and +85°C according GRPE draft [1] and for short periods up to a maximum temperature of 140°C during filling (fast filling) [2]. In order to exploit the material properties efficiently with a high degree of lightweight optimization and a high level of safety on the same time a better understanding of the structural behavior of hybrid designs is necessary. Work on this topic has been carried out in the frame of a work package on safety aspects and regulation (Subproject SAR) of the European IP StorHy (www.storhy.net). The temperature influence on the composite layers is distinctive due to there typical polymer material behavior. The stiffness of the composite layer is a function of temperature which influences global strains and stress levels (residual stresses) in operation. In order to do an accurate fatigue assessment of composite hybrid cylinders a realistic modeling of a representative temperature load is needed. For this climate data has been evaluated which were collected in Europe over a period of 30 years [3]. Assuming that the temperature follows a Gaussian (normal) distribution within the assessed period of 30 years it is possible to generate a frequency distribution for different temperature classes for the cold extreme and the hot extreme. Combining these distributions leads to the overall temperature range distribution (frequency over temperature classes). The climatic temperature influence the filling temperature and the pressure load have to be considered in combination with the operation profile of the storage cylinder to derive a complete load vector for an accurate assessment of the lifetime and safety level.

As a part of the experimental work of the EC-funded project HYPER Pro-Science GmbH performed experiments to evaluate the hazard potential of a severe hydrogen leakage inside a fuel cell cabinet. During this study hydrogen distribution and combustion experiments were performed using a generic enclosure model with the dimensions of the fuel cell "Penta H2" provided by ARCOTRONICS (now EXERGY Fuel Cells) to the project partner UNIPI for their experiments on small foreseeable leaks. Hydrogen amounts of 1.5 to 15 g H2 were released within one second into the enclosure through a nozzle with an internal diameter of 8 mm. In the distribution experiments the effects of different venting characteristics and different amounts of internal enclosure obstruction on the hydrogen concentrations measured at fixed positions in- and outside the model were investigated. Based on the results of these experiments combustion experiments with ignition positions in- and outside the enclosure and two different ignition times were performed. BOS (Background-Oriented-Schlieren) observation combined with pressure and light emission measurements were performed to describe the characteristics and the hazard potential of the induced hydrogen combustions. The experiments provide new experimental data on the distribution and combustion behaviour of hydrogen that is released into a partly vented and partly obstructed enclosure with different venting characteristics.

The effect of surfaces on the extent of high pressure horizontal unignited jets of hydrogen and methane is studied using CFD numerical simulations performed with FLACS Hydrogen. Results for constant flow rate through a 6.35 mm PRD from 100 barg and 700 barg storage units are presented for horizontal hydrogen and methane jets. To quantify the effect of a horizontal surface on the jet the jet exit is positioned at various heights above the ground ranging from 0.1 m to 10 m. Free jet simulations are performed for comparison purposes.

The possibility of using a risk based approach for the safe installation and siting of stationary fuel cell systems depends upon the availability of normative data and guidance on potential hazards and the probabilities of their occurrence. Such guidance data is readily available for most common hydrocarbon fuels. For hydrogen however data is still required on the hazards associated with different release scenarios. This data can then be related to the probability of different types of scenarios from historical fault data to allow safety distances to be defined and controlled using different techniques. Some data on releases has started to appear but this data generally relates to hydrogen vehicle refuelling systems that are designed for larger throughput higher pressures and the general use of larger pipe diameters than are likely to be used for small fuel cell systems.

High pressure hydrogen leak is one of the top safety issues presently. This study elucidates the physics and mechanism of high pressure hydrogen jet ignition when the hydrogen suddenly spouts into the air. The experimental work was done elsewhere while we did the numerical work on this high pressure hydrogen leak problem. The direct numerical simulation based on the compressible fluid dynamics considering viscous effect was carried out with the two-dimensional axisymmetric coordinate system A detailed model of hydrogen reaction is applied and a narrow tube attached to a high pressure reservoir is assumed in the numerical simulation. The exit of the tube is opened in the atmosphere. When high pressure hydrogen is passing through the tube filled by atmospheric air a strong shock wave is formed and heats up hydrogen behind the shock wave by compression effect. The leading shock wave is expanded widely after the exit hydrogen then mixed with air by several vortices generated around the exit of the tube. As a result a couple of auto-ignitions of hydrogen occur. It is found that there is a certain relationship between the auto-ignition and tube length. When the tube becomes longer the tendency of auto-ignition is increased. Additionally other type of auto-ignitions is predicted. An explosion is also occurred in the tube under a certain condition. Vortex is generated behind the shock wave in the long tube. There is a possibility of an auto-ignition induced by vortices.

The EC funded Naturalhy project is assessing the potential for using the existing gas infrastructure for conveying hydrogen as a mixture with natural gas (methane). The hydrogen could then be removed at a point of use or the natural gas/hydrogen mixture could be burned in gas-fired appliances thereby providing reduced carbon emissions compared to natural gas. As part of the project the impact on the safety of the gas system resulting from the addition of hydrogen is being assessed. A release of a natural gas/hydrogen mixture within a vented enclosure (such as an industrial housing of plant and equipment) could result in a flammable mixture being formed and ignited. Due to the different properties of hydrogen the resulting explosion may be more severe for natural gas/hydrogen mixtures compared to natural gas. Therefore a series of large scale explosion experiments involving methane/hydrogen mixtures has been conducted in a 69.3 m3 enclosure in order to assess the effect of different hydrogen concentrations on the resulting explosion overpressures. The results showed that adding up to 20% by volume of hydrogen to the methane resulted in a small increase in explosion flame speeds and overpressures. However a significant increase was observed when 50% hydrogen was added. For the vented confined explosions studied it was also observed that the addition of obstacles within the enclosure representing congestion caused by equipment and pipework etc. increased flame speeds and overpressures above the levels measured in an empty enclosure. Predictions of the explosion overpressure and flame speed were also made using a modified version of the Shell Global Solutions model SCOPE. The modifications included changes to the burning velocity and other physical properties of methane/hydrogen mixtures. Comparisons with the experimental data showed generally good agreement.

A hydrogen fuel is expected to expand its demand in the future. However hydrogen has to be treated with enough caution because of wide combustible conditions and easiness to ignite. Detonation accidents are caused in hydrogen gas such as the explosion accident in Fukushima first nuclear plant (2011). Therefore it is necessary to comprehend initiation conditions of detonation to prevent its detonation explosion. In the present study cylindrical detonation induced by direct initiation is simulated to understand the dependency of equivalence ratios in hydrogen-oxygen mixture. The several detailed kinetic models are compared to select the most appropriate model for detonation in a wide range of equivalence ratios. The Petersen-Hanson model is used in the present study due to the best agreement among the other models. In the numerical results of cylindrical detonation induced by direct initiation a cellular structure which is similar to the experimental smoked foil record is observed. The local pressure is up to 12 MPa under the condition at the standard state. The ignition process of cylindrical detonation has two stages. At the first stage the normalized cell width /L1/2 at each equivalence ratio increases linearly. At the second stage cell bifurcations appear due to a generation of new transverse waves. It is observed that a transverse wave transforms to a transverse detonation at the end of the first stage and after that some disturbance is developed to be a new transverse wave at the beginning of the second stage.

Buoyant round vertical jet had been investigated using Large Eddy Simulations at low Mach number. For the purpose of comparison with in-house experimental data in the present work helium has been used as a substitute for hydrogen. The influence of the transient concentration fields on the volume of gas with concentration within flammability limits has been investigated and their evolution and relation with average fields ad been characterized. Transient concentration fields created during initial jet development had been considered. Numerical results have been compared with in-house experiments and data published in the literature.

The objectives of the presented experimental work were investigation of hydrogen release distribution and combustion modelling possible emergency situation at industry scale. Results of large scale experiments on distribution and combustion in an open and congested area are presented. The mass of hydrogen in experiments varied from 50g to 1000g with release rate from 180 to 220 g/s. Qualitative characteristics of high momentum hydrogen jet releases distribution and subsequent combustion were obtained. It is shown that obstacles slow down jet speed promote combustible mixture formation in a large volume and accelerate combustion process. The maximum overpressure in experiments with additional congested area reached ΔР = 0.4 atm. Using partial confinement of congested area turbulent combustion regime with the maximum overpressure more than 10 atm. was obtained.

Hydrogen energy based vehicles or power generators are expected to come into widespread use in the near future. Safety information is of major importance to support the successful public acceptance of hydrogen as an energy carrier. One of the most important issues in terms of safety is the use of such system in closed area such as a private garage in which a fuel cell car may be parked. This kind of situation leads to the fundamental problem of the dispersion of hydrogen due to a simple vertical source in an enclosure. Many numerical and experimental studies have already been conducted on this problem showing the formation of a stably stratified distribution of concentration. Most of them consider the cases of accidental situation in which the flow rate is relatively important (of the order of 10Nl/min to 100Nl/min). We present a set of experiments conducted on a full scale facility of the size of a typical private garage with helium as a model gas for hydrogen. In this study we focus on the low flow rates that can be characteristic of chronic leaks that may not be detected by security devices of the system (of the order of 0.1Nl/min to 10Nl/min). The facility allows changing natural ventilation conditions and experiments have been conducted from the tightest which is less than 0.01ACH to that typical of a real garage say of the order of 0.1ACH.

Results are given of experimental study of propagation of explosion waves in hydrogen-air mixtures of different compositions under conditions of cumulation. The investigations are performed in a setup consisting of two parts namely the upper part in the form of a metal cone and the lower part in the form of a rubber envelope hermetically attached to the cone. The upper and lower parts of the experimental setup are separated by a thin rubber film and may be filled with hydrogen-air mixtures of different compositions.

Discrete Event Simulation (DES) environments are rapidly developing and they appear to be promising tools for developing reliability and risk analysis models of safety-critical systems. DES models are an alternative to the conventional methods such as fault and event trees Bayesian networks and cause-consequence diagrams that could be used to assess the reliability of fuel supply. DES models can rather easily account for the dynamic dimensions and other important features that can hardly be captured by the conventional models. The paper describes a novel approach to estimate gas supply security and the reliability/safety of gas installations and argues that this approach can be transferred to estimate future hydrogen supply reliability. The core of the approach is a DES model of gas or other fuel propulsion through a pipeline to the customers and failures of the components of the pipeline. We will argue in the paper that the experience gained in the modelling of gas supply reliability is very relevant to the security and safety of a future hydrogen supply and worth being employed in this area.

As Europe is gradually moving towards a hydrogen based society it has been acknowledged that adding certain amount of hydrogen as a clean energy carrier to the existing natural gas pipeline will help reduce the CO2 emissions which contribute to the greenhouse effect. On the other hand hydrogen has been demonstrated to be able to change the behaviour of the pipeline steel such as lower toughness and faster crack growth due to hydrogen embrittlement. Therefore it is necessary that the risks associated with the failure of the pipeline carrying mixtures of natural gas and hydrogen be assessed.<br/>The study reported in this paper is part of European NATURALHY project whose aim is to investigate the possibility of using the existing natural gas transmission pipelines to convey natural gas/hydrogen mixtures. According to the EGIG database the most common cause of failure for the existing natural gas pipelines is third party damage which mainly refers to a gouge a dent/gouge combination of known geometry. Among third party damage failures 90% are the result of immediate failure i.e. leakage or rupture of the pipeline and only 10% of them are the result of delayed failure. While its not expected that hydrogen will impact the immediate failure it could increase the vulnerability of the pipe to delayed failure through the initiation or activation of crack like defects.<br/>This paper will present a methodology to predict the probability of increased failures and describe a software tool that has been developed to perform the calculations.

Any experimental study of catalysts and catalytic recombining devices for removal of hydrogen gas from industrial environments is known to carry a risk of ignition of hydrogen. Experiments conducted in an atmosphere with a high concentration of hydrogen present a particular danger. Here a technique is reported that allows conducting such experiments with relative safety. This technique has been developed and applied by the company ‘Russian Energy Technologies’ for the last five years without any significant incident.<br/>A “Gas stream method” for testing and analysis of the characteristics of a catalyst for hydrogen/oxygen recombination is proposed. Tests with a variety of catalysts in a passive recombining device were carried out in a climatic chamber (86 l in volume) with a hydrogen/air mixture containing up to 20% (v/v) hydrogen flowing through it. The balance equation for hydrogen and oxygen flows entering reacting and exiting the chamber led to a formula for calculating the efficiency of a catalyst or a catalytic device under stationary conditions.<br/>Fluctuations in local temperatures of the catalyst and other parts of the chamber along with variation in the concentration of hydrogen gave the authors an insight into the thermal regime of an active catalyst. This enabled them to develop new catalysts for removal of hydrogen from the environment using industrial recombining devices.

The lack of experimental data on hydrogen dispersion led to the experimental project DRIVE (Experimental Data for Hydrogen Automotive Risks Assessment for the validation of numerical tools and for the Edition of guidelines) that involves the CEA (French Atomic Energy Commission) the National Institute of Industrial Environment and Risks (INERIS) the French car manufacturer PSA PEUGEOT CITROËN and the Research Institute on Out of Equilibrium Phenomena (IRPHE). The CEA has developed an experimental setup named GARAGE in order to analyze the condition of formation of an explosive atmosphere in an enclosure. This is a full scale facility in which a real car can be parked. Hydrogen releases were simulated with helium which volume fraction was measured with mini-katharometers. These thermal conductivity probes allow spatial and time volume fraction variations measurements. We present experimental results on the dispersion of helium in the enclosure due to releases in a typical car. The tested parameters are the location of the source (engine bottom of the car storage) and the flow rate. Emphasis is put on the influence of these parameters on the time evolution of the volume fraction in the enclosure as well as on the vertical distribution of helium.

One of the main benefits sought by including hydrogen in the alternative fuels mix is emissions reduction – eventually by 100%. However in the near term there is a very significant cost differential between fossil fuels and hydrogen. Hythane (a blend of hydrogen and natural gas) can act as a viable next step on the path to an ultimate hydrogen economy as a fuel blend consisting of 8−30 % hydrogen in methane can reduce emissions while not requiring significant changes in existing infrastructure. This work seeks to evaluate whether hythane may be safer than both hydrogen and methane under certain conditions. This is due to the fact hythane combines the positive safety properties of hydrogen (strong buoyancy high diffusivity) and methane (much lower flame speeds and narrower flammability limits as compared to hydrogen). For this purpose several different mixture compositions (e.g. 8 % 20 % and 30 % hydrogen) are considered. The evaluation of (a) dispersion characteristics (which are more positive than for methane) (b) combustion characteristics (which are closer to methane than hydrogen) and (c) Combined dispersion + explosion risk is performed. This risk is expected to be comparable to that of pure methane possibly lower in some situations and definitely lower than for pure hydrogen. The work is performed using the CFD software FLACS that has been well-validated for safety studies of both natural gas/methane and hydrogen systems. The first part of the work will involve validating the flame speeds and flammability limits predicted by FLACS against values available in literature. The next part of the work involves validating the overpressures predicted by the CFD tool for combustion of premixed mixtures of methane and hydrogen with air against available experimental data. In the end practical systems such as vehicular tunnels garages etc. is used to demonstrate positive safety benefits of hythane with comparisons to similar simulations for both hydrogen and methane.

This paper demonstrates experimental and numerical study on spontaneous ignition of H2–N2 mixtures during high-pressure release into air through the tubes of various diameters and lengths. The mixtures included 5% and 10% (vol.) N2 addition to hydrogen being at initial pressure in range of 4.3–15.9 MPa. As a point of reference pure hydrogen release experiments were performed with use of the same experimental stand experimental procedure and extension tubes. The results showed that N2 addition may increase the initial pressure necessary to self-ignite the mixture as much as 2.12 or 2.85 – times for 5% and 10% N2 addition respectively. Additionally simulations were performed with use of Cantera code (0-D) based on the ideal shock tube assumption and with the modified KIVA3V code (2-D) to establish the main factors responsible for ignition and sustained combustion during the release.

INERIS has set up large-scale fully instrumented experiments to study the formation of flammable clouds resulting from a finite duration spillage of hydrogen in a quiescent room (80 m3 chamber). Concentration temperature and mass flow measurements were monitored during the release period and several hours after. Experiments were carried out for mass flow rates ranging from 02 g/s to 1 g/s. The instrumentation allowed the observation and quantification of rich hydrogen layers stratification effects. This paper presents both the experimental facility and the test results. These experimental results can be used to assess and benchmark CFD tools capabilities.

Explosion relief panels or doors are often used in industrial buildings to reduce damages caused by gas explosions. Decades of research have contributed to the understanding of the phenomena involved in gas explosions in order to establish an effective method to predict reliably the explosion overpressure. All the methods predict a monotonic increase of the overpressure with the concentration of the gas in the range from the lower explosion limit to the stoichiometric one. Nevertheless in few cases a non-monotonic behaviour of the maximum developed pressure as a function of hydrogen concentration was reported in the literature. The non-monotonic behaviour was also observed during experimental tests performed at the Scalbatraio laboratory at the University of Pisa in a 25 m3 vented combustion test facility with a vent area of 112 m2. This paper presents the results obtained during the tests and investigates the possible explanations of the phenomena.

R&D projects for establishing hydrogen supply chain have already been started in Japan in collaboration among the industry government and universities. One of the important subjects of the project is development of liquefied hydrogen tankers i.e. ships carrying liquefied hydrogen in bulk. In general basic safety requirements should be determined to design ships. However the existing regulations do not specify the requirements for hydrogen tankers while requirements for ships carrying many kinds of liquefied gases are specified in “International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk” (IGC Code) issued by the International Maritime Organization i.e. a special organization under the United Nations. Therefore the basic safety requirements for hydrogen tankers should be developed. We conducted bibliographic survey on the IGC Code ISO/TR 15916:2004 “Basic considerations for the safety of hydrogen systems” and so on; in order to provide safety requirements taking into account the properties of liquid and gaseous hydrogen. In this paper we provide safety requirements for liquefied hydrogen tankers as the basis for further consideration by relevant governments.

In case of severe accidents in Pressurized Water Reactors a great amount of hydrogen can be released the resulting heterogeneous gaseous mixture (hydrogen-air-steam) can be flammable or inert and the pressure effects could alter the confinement of the reactor. Water spray systems have been designed in order to reduce overpressures in the containment but the presence of water droplets could enhance flame propagation through turbulence or generate flammable mixtures since the steam present in the vessel could condense on the droplets and could not inert the mixture anymore. However beneficial effects would be heat sinks and homogenization of mixtures. On-going work is devoted to the modelling of the interaction between fine water droplets and a hydrogen-air flame. We present in this paper an unsteady Lumped Parameter model in detail with a special focus on hydrogen-air flame propagation in the presence of water droplets. The effects of the initial concentration of droplets steam and hydrogen concentrations on flame propagation are discussed in the paper and a comparison between this model and our previous steady Lumped-Parameter model highlights the features of the unsteady approach. This physical model can serve as a validation tool for a CFD modelling. The results will be further validated against experimental data.

This paper describes work performed by a consortium led by the UK Health and Safety Laboratory(HSL)to identify the safe operating conditions for combined cycle power generating systems running on high hydrogen fuels. The work focuses on hydrogen and high hydrogen syngas and biogas waste-stream fuel mixtures which may prove hazardous in the event of a turbine or engine flame out resulting in a flammable fuel mixture entering the hot exhaust system and igniting. The paper describes the project presenting some initial results from this work including the development of large scale experimental facilities on the550 acre HSL site near Buxton Derbyshire UK. It describes the large scale experimental facility which utilises the exhaust gas from a Rolls-Royce Viper jet-engine (converted to run on butane) feeding into a 12 m long 0.60 m diameter instrumented tube at a pre-combustion velocity of 22 m/s. A variable geometry simulated heat exchanger with a 40 %2blockage ratio is present in the tube. Flammable mixtures injected into the tube close to the Viper outlet together with make-up oxygen are then ignited. Extensive optical ionisation temperature and pressure sensors are employed along the length of the tube to measure the pressures and flame speeds resulting from the combustion event. Some preliminary results from the test programme are discussed including deflagration to detonation transitions at high equivalence ratios.

Unintentional leaks at hydrogen fuelling stations have the potential to form hydrogen jet flames which pose a risk to people and infrastructure. The heat flux from these jet flames are often used to develop separation distances between hydrogen components and buildings lot-lines etc. The heat flux and visible flame length is well understood for releases from round nozzles but real unintended releases would be expected to be be higher aspect-ratio cracks. In this work we measured the visible flame length and heat-flux characteristics of cryogenic hydrogen flames from high-aspect ratio nozzles. We compare this data to flames of both cryogenic and compressed hydrogen from round nozzles. The aspect ratio of the release does not affect the flame length or heat flux significantly for a given mass flow under the range of conditions studied. The engineering correlations presented in this work that enable the prediction of flame length and heat flux can be used to assess risk at hydrogen fuelling stations with liquid hydrogen and develop science-based separation distances for these stations.

One of the focus areas in the heavy-duty transport industry globally is de-carbonization of trucks dozers shovels semi-trucks buses etc. Hydrogen fuel cells (FCs) technology is one considered solution for the industry due to its zero-emissions its MW scalability and capacity to store large amounts of energy for long duration continuous power operation. Underground deep mines is another option for deployment and operation of hydrogen FCs. Benefits include lower emissions improved health comfort and safety as well as reduced operating costs. Underground mining trucks loaders and other machines have power ratings up to 750 kW which proves difficult for battery and tethered electric energy. Hydrogen FCs have the ability to overcome these power and energy storage limitations. The risks and technologies associated with delivering storing and using hydrogen underground first need to be investigated and proven safe. This work presents the design construction and operation of a mining ventilation test facility (VTF) at the North-West University in South Africa that aims to quantify the risk of hydrogen in confined ventilated environments. Initial work has been conducted on measuring concentrations of hydrogen released in the temporary ventilation site and is discussed.

Hydrogen may play a crucial part in delivering a net zero emissions future. Currently hydrogen production storage transport and utilisation are being explored to scope opportunities and to reduce barriers to market activation. One such barrier could be negative public response to hydrogen technologies. Previous research around socio-technical risks finds that public acceptance issues are particularly challenging for emerging remote technical sensitive uncertain or unfamiliar technologies - such as hydrogen. Thus while the hydrogen value chain could offer a range of potential environmental economic and social benefits each will have perceived risks that could challenge the introduction and subsequent roll-out of hydrogen. These potential issues must be identified and managed so that the hydrogen sector can develop adapt or respond appropriately. Geological storage of hydrogen could present challenges in terms of perceived safety. Valuable lessons can be learned from international research and practice of CO2 and natural gas storage in geological formations (for carbon capture and storage CCS and for power respectively). Here we explore these learnings. We consider the similarities and differences between these technologies and how these may affect perceived risks. We also reflect on lessons for effective communication and community engagement. We draw on this to present potential risks to the perceived safety of - and public acceptability of – the geological storage of hydrogen. One of the key lessons learned from CCS and natural gas storage is that progress is most effective when risk communication and public acceptability is considered from the early stages of technology development.

An underground mining ventilation testing facility (VTF) was designed and constructed at the HySA facility at the North-West University South Africa. The purpose was to evaluate risks associated with different hydrogen storage technologies in a confined environment. The work included initial calculations of hydrogen movement in specific spaces and the development of simulation tools to compare these modelled results with experimental work. For this purpose hydrogen sensors that could accurately measure hydrogen concentrations during a controlled hydrogen leak at the VTF were required. Hazardous hydrogen sensors capable of measuring >4% hydrogen are not readily available commercially. Typically hydrogen sensors rated for hazardous environments are designed for safety actions (e.g. activating emergency measures when hydrogen is detected) at concentrations of 8%. (Measuring concentrations higher than this is not required for commercial use hence there is no market for such sensors.) At the VTF it is necessary to be able to measure hydrogen concentrations >4% in order to obtain information on the flammable hydrogen concentrations at specified distances and orientations around a controlled hydrogen leak. Initial experimental work was conducted at low pressures resulting in very low hydrogen concentrations. Commercial available original equipment manufacturer (OEM) hydrogen sensors were capable of measuring 0.2% hydrogen which for the low pressures and gas flows here proved sufficient to enable us to make sensible conclusions. However higher pressures and gas flows are essential in practical use hence higher concentrations of hydrogen need to be measured. A custom sensor was developed by HySA while commercial sensors (OEM) were investigated. This work reports on the testing and evaluation of several hydrogen sensors. Results of initial ventilation tests are presented.

Preparing for the arrival of the hydrogen society it is necessary to develop suitable sensors to use hydrogen safely. There are many methods to know the hydrogen concentration by using conventional sensors but it is difficult to know the behavior of hydrogen gas from long distance. This study measured hydrogen dispersion by using Raman scattering light. Generally some delays occur when using conventional sensors but there are almost no delays by using the new Raman sensor. In the experiments 6mm & 1mm diameter holes are used as a spout nozzle to change initial velocities. To ensure the result a special sheets are used which turns transparent when it detected hydrogen and visualized the hydrogen behaviour. As a result the behaviour of the hydrogen gas in the small container was observed. In addition measurable distance is increased by the improvement of the device.

The formation of hydrogen jets from pressurized sources and ignition has been studied by many projects also when hitting hot devices. In the paper presented at the conference 2 years ago the ignition was caused by glow plug a “point like source” at various temperatures distances of igniter and source and source pressures. In continuation of that work ignition now occurred by 1 or 3 platelets of size 45 x 18 mm at a temperatures of 1223 K. When hitting these hot platelets the resulting flame explosions and flame jets show interesting characteristics in contrast to the point like ignition where the explosions drifts downstream with the jet. Parameters of the experiments vary in initial pressure of the tubular source (10 20 and 40 MPa) distance between the nozzle and the hot surface (3 5 and 7 m) and temperature of the hot surface (1223 K). The initial explosions stabilize already at the stagnation point or the wake of the hot platelets. Furthermore flames propagate upstream and downstream depending on the pressure of the hydrogen reservoir and the distance. The achieved flame velocities vary strongly from 30 to 240 m/s. With all investigated hydrogen pressures strong reactions v > 40 m/s occur at platelet distances of 3 and 5 m. The higher values are mainly achieved with jets with 40 MPa pressure at 3 m distance. In these cases the initial explosion contours show irregular shapes. Various effects are found like explosion separation further independently initiated explosions and two parallel flame jets upstream as well as downstream.

This paper is concerned with predicting the impact on the probability of failure of adding hydrogen to the natural gas distribution network. Hydrogen has been demonstrated to change the behaviour of crack like defects which may affect the safety of pipeline or make it more expensive to operate. A tool has been developed based on a stochastic approach to assess the failure probability of the gas pipeline due to the existence of crack-lie defects including the operational aspects of the pipeline such as inspection and repair procedures. With various parameters such as crack sizes material properties internal pressure modelled as uncertainties a reliability analysis based on failure assessment diagram is performed through direct Monte Carlo simulation. Inspection and repair procedures are included in the simulation to enable realistic pipeline maintenance scenarios to be simulated. In the data preparation process the accuracy of the probabilistic definition of the uncertainties is crucial as the results are very sensitive to certain variables such as the crack depth length and crack growth rate. The failure probabilities of each defect and the whole pipeline system can be obtained during simulation. Different inspection and repair criteria are available in the Monte Carlo simulation whereby an optimal maintenance strategy can be obtained by comparing different combinations of inspection and repair procedures. The simulation provides not only data on the probability of failure but also the predicted number of repairs required over the pipeline life thus providing data suitable for economic models of the pipeline management. This tool can be also used to satisfy certain target reliability requirement. An example is presented comparing a natural gas pipeline with a pipeline containing hydrogen.

The aim of the present work is to investigate the interaction between a water spray and a laminar hydrogen-air flame in the case of steam inerted mixture. A first work is devoted to study the thermodynamics involved in the phenomena via a lumped parameter code. The flow is two- phase and reactive the gas is multi-component the water spray is polydisperse and the droplet size has certainly an influence on the flame propagation. The energy released by the reaction between hydrogen and oxygen vaporizes suspended droplets. The next step of this study will be to consider a drift-flux model for the droplets and air under hypotheses that the velocity and thermal disequilibria are weak. The multi-component feature of the gas will be further taken into account by studying a gas mixture containing hydrogen air and water vapor. A second study concerns an experimental investigation of the effect of droplets on the flame propagation using a spherical vessel. A Schlieren system is coupled to the spherical vessel in order to record the flame propagation on a digital high speed camera. Both studies will help improve our knowledge of safety relevant phenomena.

For the sake of the increasing demand of hydrogen fuel cell vehicles there are more concerns on the safety of hydrogen refueling stations. As one of the key pieces of equipment the hydrogen dispenser has drawn attention on this aspect since it involves massive manual operations and may be bothered by a high probability of failure. In this paper a numerical study is conducted to simulate the possible leakage events of the hydrogen dispenser based on a prototype in China whose working pressure is 70 MPa. The leakage accident is analyzed with respect to leakage sizes leak directions and the time to stop the leakage. It is found that due to the large mass flow rate under such high pressure the leak direction and the layout of the components inside the dispenser become insignificant and the ignitable clouds will form inside the dispenser in less than 1 s if there is a leakage of 1% size of the main tube. The ignitable clouds will form near the vent holes outside the dispenser which may dissipate quickly if the leakage is stopped. On the other hand the gas inside the dispenser will remain ignitable for a long time which asks for a design with no possible ignition source inside. The results can be useful in optimizing the design of the dispenser regarding the reaction time and sensitivity requirements of the leakage detector the size and amount of vent holes etc.

Report presents the preliminary experimental results on hydrogen subsonic leakage in a closed vessel under the well-controlled boundary/initial conditions. Formation of hydrogen-air gas mixture cloud was studied for a transient (10 min) upward hydrogen leakage which was followed by subsequent evolution (15 min) of explosive cloud. Low-intensity ( 0.46⋅10−3 m3/sec) hydrogen release was performed via circular (diameter 0.014 m) orifice located in the bottom part of a horizontal cylindrical vessel ( ≈4 m3). A spatially distributed net of the 24 hydrogen sensors and 24 temperature sensors was used to permanently track the time dependence of the hydrogen concentration and temperature fields in vessel. Analysis of the simultaneous experimental records for the different spatial points permits to delineate the basic flow patterns and stages of hydrogen subsonic release in closed vessel in contrast to hydrogen jet release in open environment. The quantitative data were obtained for the averaged speeds of explosive cloud envelop (50% fraction of the Lower Flammability Limit (LFL)) propagation in the vertical and horizontal directions. The obtained data will be used as an experimental basis for development of the guidelines for an indoors allocation of the hydrogen sensors. Data can be also used as a new benchmark case for the reactive Computational Fluid Dynamics codes validation.

An experimental study of flame propagation acceleration and transition to detonation in hydrogen-air mixture in 2 m long rectangular cross section channel filled with obstacles located at the bottom wall was performed. The initial conditions of the hydrogen-air mixture were 0.1 MPa and 293 K. Three different cases of obstacle height (blockage ratio 0.25 0.5 and 0.75) and four cases of obstacle density were studied with the channel height equal to 0.08 m. The channel width was 0.11 m in all experiments. The propagation of flame and pressure waves was monitored by four pressure transducers and four in house ion probes. The pairs of transducers and probes were placed at various locations along the channel in order to get information about the progress of the phenomena along the channel. To examine the influence of mixture composition on flame propagation and DDT the experiments were performed for the compositions of 20% 25% and 29.6% of H2 in air by volume. As a result of the experiments the deflagration and detonation regimes and velocities of flame propagation in the obstructed channel were determined.

The development of an infrastructure for the future hydrogen economy will require the simultaneous development of a set of codes and standards. As part of the U.S. Department of Energy Hydrogen Fuel Cells & Infrastructure Technologies Program Sandia National Laboratories is developing the technical basis for assessing the safety of hydrogen-based systems for use in the development/modification of relevant codes and standards. This work includes experimentation and modelling to understand the fluid mechanics and dispersion of hydrogen for different release scenarios including investigations of hydrogen combustion and subsequent heat transfer from hydrogen flames. The resulting technical information is incorporated into engineering models that are used for assessment of different hydrogen release scenarios and for input into quantitative risk assessments (QRA) of hydrogen facilities. The QRAs are used to identify and quantify scenarios for the unintended release of hydrogen and to identify the significant risk contributors at different types of hydrogen facilities. The results of the QRAs are one input into a risk-informed codes and standards development process that can also include other considerations by the code and standard developers. This paper describes an application of QRA methods to help establish one key code requirement: the minimum separation distances between a hydrogen refuelling station and other facilities and the public at large. An example application of the risk-informed approach has been performed to illustrate its utility and to identify key parameters that can influence the resulting selection of separation distances. Important parameters that were identified include the selected consequence measures and risk criteria facility operating parameters (e.g. pressure and volume) and the availability of mitigation features (e.g. automatic leak detection and isolation). The results also indicate the sensitivity of the results to key modelling assumptions and the component leakage rates used in the QRA models.

The production of hydrogen from renewable sources is essential to develop the future hydrogen economy. Biomass is an abundant clean and renewable energy source and it can be important in the production of hydrogen. The Valencian Community due to its great agricultural and forestry activities generates an important quantity of biomass residues that can be used for energy generation approximately 778 kt of wet biomass residues per year. This great quantity of biomass can be transformed into a hydrogen-rich gas by different thermochemical conversion processes. In this article the potential of production of hydrogen-rich gas is analyzed considering several factors affecting the conversion yield of these processes. As a result of this analysis it could be possible to produce 1271 MNm3 of H₂ per year considering the total biomass residues of the community and selecting the gasification processes.

We have modelled sudden hydrogen expansion from a cryogenic pressure vessel. This model considers real gas equations of state single and two-phase flow and the specific “vessel within vessel” geometry of cryogenic vessels. The model can solve sudden hydrogen expansion for initial pressures up to 1210 bar and for initial temperatures ranging from 27 to 400 K. For practical reasons our study focuses on hydrogen release from 345 bar with temperatures between 62 K and 300 K. The pressure vessel internal volume is 151 L. The results indicate that cryogenic pressure vessels may offer a safety advantage with respect to compressed hydrogen vessels because i) the vacuum jacket protects the pressure vessel from environmental damage ii) hydrogen when released discharges first into an intermediate chamber before reaching the outside environment and iii) working temperature is typically much lower and thus the hydrogen has less energy. Results indicate that key expansion parameters such as pressure rate of energy release and thrust are all considerably lower for a cryogenic vessel within vessel geometry as compared to ambient temperature compressed gas vessels. Future work will focus on taking advantage of these favourable conditions to attempt fail-safe cryogenic vessel designs that do not harm people or property even after catastrophic failure of the inner pressure vessel.

High-pressure GH2 systems are of interest for storage and distribution of hydrogen. The dynamic blow-down process of a high-pressure GH2 reservoir in case of a small leak is a complex process involving a chain of distinct flow regimes and gas states which needs to be understood for safety investigations.<br/>This paper presents models to predict the hydrogen concentration and velocity field in the vicinity of a postulated small leak. An isentropic expansion model with a real gas equation of state for normal hydrogen is used to calculate the time dependent gas state in the reservoir and at the leak position. The subsequent gas expansion to 0.1 MPa is predicted with a zero-dimensional model. The gas conditions after expansion serve as input to a newly developed integral model for a round free turbulent H2-jet into ambient air. The model chain was evaluated by jet experiments with sonic hydrogen releases from different reservoir pressures and temperatures.<br/>Predictions are made for the blow-down of hydrogen reservoirs with 10 30 and 100 MPa initial pressure. The evolution of the pressure in the reservoir and of the H2 mass flux at the orifice are presented in dimensionless form which allows scaling to other system dimensions and initial gas conditions. Computed hydrogen concentrations and masses in the jet are given for the 100 MPa case. A normalized hydrogen concentration field in the free jet is presented which allows for a given leak scenario the prediction of the axial and radial range of burnable H2-air mixtures.