Development of a realistic hydrogen flammable atmosphere inside a 4-m3 enclosure

To define a strategy of mitigation for containerized hydrogen systems (fuel cells for example) against explosion, the main characteristics of flammable atmosphere (size, concentration, turbulence…) shall be well-known. This article presents an experimental study on accidental hydrogen releases and dispersion into an enclosure of 4 m3 (2 m x 2 m x 1 m). Different release points are studied: two circular releases of 1 and 3 mm and a system to create ring-shaped releases. The releases are operated with a pressure between 10 and 40 bars in order to be close to the process conditions. Different positions of the release inside the enclosure i.e. centred on the floor or along a wall are also studied. A specific effort is made to characterize the turbulence in the enclosure during the releases. The objectives of the experimental study are to understand and quantify the mechanisms of formation of the explosive atmosphere taking into account the geometry and position of the release point and the confinement. Those experimental data are analyzed and compared with existing models and could bring some new elements to improve them

Ingénierie de la sécurité appliquée à des applications hydrogène-énergie

Since a few years, hydrogen appears as a credible energy-vector. AREVA Stockage d’Energie develop in this framework energy-storage solutions allowing to transform electrical energy into chemical energy (here hydrogen) to store it. However, hydrogen applications are still considered dangerous, so that is feared a hazardous event like explosion which could occur if a hydrogen leakage happened. And it should be recognized that hydrogen leaks can produce extended explosive clouds because of the broad flammability range, and that hydrogen-air mixtures ignite extremely easily and burn/explode fast and violently. As compared to other fuels (ex: hydrocarbons), the flammability range is 5-10 times larger, the minimum ignition energy is 5-10 times smaller and the maximum burning velocity is also about 5-10 times larger. However, due to its physical properties, hydrogen may offer some appreciable advantages in terms of mitigation: because the flammable mixtures are much lighter than air, they disperse rapidly. Design engineers have to face this reality and the logical consequence is that the safety demonstration has to be very strong and clearly understandable. Safety is the essential issue for the introduction of hydrogen applications on the market. If the risk is unacceptable (too near to important issues, sensitive environment…), Design engineers should be able to change the process and them to reassess the risk. The objective of this study is to establish a tool that permit to do that. In a way, a safety engineering tool is aimed to be created. The main points of this study are : - The design of a risk matrix: she must include both the probability and the severity of the accident. The specific case of AREVA Stockage d’Energie is presented that takes into consideration the geometry of the application studied and probabilities that reflects the real risks (here: ATEX risk). But the matrix could be formed with different criteria. - Identification of the “Critical events” or EC (Laurent, 2011): for hydrogen applications, the critical events are principally leaks and secondarily bursting. The EC is the ultimate event of a succession of events that are linked together by cause-and-effect relations. Once the EC occurs the series of following events is judged inexorable, automatic and with cause-and effect relation between the events but those events follow a temporal sequence. The probability of the accident is the one of the EC ; - Risk assessment: For each EC, a fault tree and an event tree are associated. The fault tree permit to calculate the EC probability if the probabilities of the initial events are known. In the framework of the major risk where the precision required is average, generic database are used. Those database, rather old and related to hydrocarbon technologies, are not adapted to the sharpness of the tool developed and neither to hydrogen objects. Since the feedback is also limited, an approach based on the extension through the upstream of the fault tree have been developed; however, some keys probabilities are still missing. Otherwise the methods of consequences evaluation used for major accidents are also not adapted. To correctly assess the consequences of an accident and the sizing of the safety barriers (vents, flowrate limiter…), the characteristics of the flammable cloud must be known precisely. Experiments are realized and will allow the development of the specific tool box for the evaluation of the consequences.Depuis quelques années, l’hydrogène apparaît comme un vecteur d’énergie crédible. AREVA Stockage d’Energie développe, dans ce cadre, des solutions de stockage d’énergie, permettant de transformer l’énergie électrique en énergie chimique (de l’hydrogène) afin de la stocker. Cependant les applications hydrogène sont toujours considérées dangereuses, tant est redouté un évènement dangereux tel qu’une explosion qui pourrait avoir lieu si une fuite d’hydrogène se produisait. Il faut reconnaitre que les fuites d’hydrogène peuvent produire de vastes nuages inflammables à cause de sa plage d’inflammabilité étendue et qu’un mélange hydrogène-air peut s’enflammer extrêmement facilement et brûler rapidement et violement. En comparant l’hydrogène aux autres carburants usuels, sa plage d’inflammabilité est 5 à 10 fois plus étendue, son énergie minimale d’inflammation est 5 à 10 fois plus faible et sa vitesse maximale de propagation est aussi 5 à 10 fois plus importante. Cependant, grâce à ses propriétés physiques, l’hydrogène peut offrir des avantages appréciables en termes de sécurité/mitigation, tels que sa densité plus faible que l’air entraînant une dispersion rapide. Les ingénieurs de conception doivent faire face à cette réalité et la conséquence logique est que la démonstration de la sécurité doit être solide et clairement compréhensible. La sécurité est donc l’enjeu essentiel pour l’introduction des objets hydrogène sur le marché. Dans l’hypothèse où ce risque serait inacceptable pour l’usager (trop grande proximité avec des enjeux forts, un environnement sensible…), il faut être capable de faire évoluer le procédé proposé puis de requalifier le risque. L’objectif de ce travail est de constituer l’outil qui permette de faire cela. On vise en quelque sorte un outil d’ingénierie de la sécurité. L’objectif de cette communication est de présenter le travail accompli et l’outil. Les principaux points sur lequel ce travail a porté sont les suivants : - Conception d’une matrice d’acceptabilité : elle comporte une dimension probabilité de l’accident et gravité de l’accident. Le cas spécifique d’AREVA Stockage d’Energie est présenté mais la matrice pourrait être constituée sur la base de critères différents ; - Identification des « évènements redoutés centraux » ou ERC (Laurent, 2011) : dans le cas des objets hydrogène, il s’agit principalement de fuites et accessoirement d’éclatements de capacité. L’ERC est l’évènement ultime d’une série d’évènements reliés les uns aux autres par des relations de cause à effet. Sur cet enchaînement, des calculs de probabilité sont possibles. Dès lors que l’ERC s’est produit la séquence d’évènements qui suit est jugée inexorable, automatique sans relation de cause à effet entre les évènements mais suivant une séquence temporelle. La probabilité de l’accident est donc celle de l’ERC ; - Calcul du risque : pour chaque ERC, un arbre de défaillances est construit en amont de l’ERC et un arbre d’évènements en aval. L’architecture de l’arbre de défaillances permet de calculer la probabilité de l’ERC si la probabilité des évènements les plus en amont est connue. Dans le cadre du risque majeur, où la précision des estimations requise est moyenne, on utilise des bases de données génériques relatives aux accidents majeurs. On a montré que ces bases de données, plutôt anciennes et relatives aux technologies des hydrocarbures, ne sont pas adaptées ni à la finesse de l’outil ni aux objets hydrogène. Comme par ailleurs le retour d’expérience sur ces objets est très limité, on a développé une approche nouvelle basée sur le prolongement vers l’amont des arbres de défaillances. Par ailleurs les méthodes d’estimation des conséquences des accidents habituellement employées pour les accidents majeurs ne sont pas adaptées. Il faut être capable notamment d’estimer assez précisément les caractéristiques d’un nuage inflammable d’une part pour prédire les effets de l’explosion mais aussi pour dimensionner les barrières (évents, limiteurs de débit...). Une boite à outil spécifique a été développée pour cela

Some issues concerning the CFD modelling of confined hydrogen releases

In SUSANA E.U. project a rather broad CFD benchmarking exercise was performed encompassing a number of CFD codes, a diversity of turbulence models... It is concluded that the global agreement is good. But in this particular situation, the experimental data to compare with were known to the modelers. In performing, this exercise, the present authors explored the influence of some modeling choices which may have a significant impact on the results (apart from the traditional convergence testing and mass conservation) especially in the situation where little relevant data are available. The configuration investigated is geometrically simple: a vertical round hydrogen jet in a square box. Nevertheless, modeling aspects like the representation of the source and of the boundary conditions have a rather strong influence on the final results as illustrated in this communication. In other words, the difficulties may not be so much in the intrinsic capabilities of the code (which SUSANA tends to show) but more in the physical representation the modelers have. Even in the specific situation addressed in this communication, although looking simple, it may not be so obvious to grasp correctly the leading physical processes

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

Best practice guidelines in numerical simulations and CFD benchmarking for hydrogen safety applications

Correct use of Computational Fluid Dynamics (CFD) tools is essential in order to have confidence in the results. A comprehensive set of Best Practice Guidelines (BPG) in numerical simulations for Fuel Cells and Hydrogen applications has been one of the main outputs of the SUSANA project. These BPG focus on the practical needs of engineers in consultancies and industry undertaking CFD simulations or evaluating CFD simulation results in support of hazard/risk assessments of hydrogen facilities, as well as on the needs of regulatory authorities. This contribution presents a summary of the BPG document. All crucial aspects of numerical simulations are addressed, such as selection of the physical models, domain design, meshing, boundary conditions and selection of numerical parameters. BPG cover all hydrogen safety relative phenomena, i.e. release and dispersion, ignition, jet fire, deflagration and detonation. A series of CFD benchmarking exercises are also presented serving as examples of appropriate modelling strategies.JRC.C.1-Energy Storag