Cryogenic Hydrogen Storage Tanks Exposed to Fires: a CFD study

Federico Ustolin, Giordano Emrys Scarponi, Tommaso Iannaccone, Valerio Cozzani and Nicola Paltrinieri

Abstract: Hydrogen is one of the most suitable candidates in replacing heavy hydrocarbons. Liquefaction of fuels is one of the most effective processes to increase their low density. This is critical especially in large-scale or mobile applications such as in the maritime or aeronautical fields. A potential loss of integrity of the cryogenic storage equipment might lead to severe consequences due to the properties of these substances (e.g. high flammability). For this reason, this critical event must be avoided. The aim of this study is to analyse the behaviour of the cryogenic vessel and its lading when it is exposed to a fire and understand how to prevent a catastrophic rupture of the tank during this accident scenario. A two-dimensional computational fluid dynamic (CFD) analysis is carried out on a cryogenic liquid hydrogen (LH₂) vessel to investigate its thermal response when engulfed in a fire. The model accounts for the evaporation and condensation of the substance and can predict the tank pressurization rate and temperature distribution. It is assumed that the vessel is completely engulfed in the fire (worst-case scenario). The CFD model is validated with the outcomes of a small-scale fire test of an LH₂ tank. Critical indications on the dynamic response of the cryogenic tank involved in a worst-case accident scenario are provided. Tank pressurisation and temperature distributions of the case study can be exploited to provide conservative estimations of the time to failure (TTF) of the vessel. These outcomes represent useful information to support the emergency response to this type of accident scenario and can aid the selection of appropriate and effective safety barriers to prevent the complete destruction of the tank.

Chemical Engineering Transactions, 90, 535-540, 2022.

Experimental investigation into the consequences of release of liquefied hydrogen onto and under water

Kees van Wingerden, Martin Kluge, Abdel Karim Habib, Hans L. Skarsvåg, Federico Ustolin, Nicola Paltrinieri and Lars H. Odsæter

Abstract: Large-scale experiments have been performed to investigate the possible consequences of realistic amounts of liquified hydrogen (LH₂) encountering water. The experiments aimed at simulating an accidental release of LH₂ onto water, for instance during the fuelling of a ship. For liquified natural gas (LNG), it has been demonstrated that physical explosions may occur when it is spilled onto water. These phenomena are referred as rapid phase transitions (RPTs). It cannot be excluded that RPTs are also possible in the case of LH₂. The tests were performed at the Test Site Technical Safety of the Bundesanstalt für Materialforschung und –prüfung (BAM) in Horstwalde, Germany. The tests were performed in a 10 m x 10 x 1.5 m basin filled with water. LH₂ releases of up to about 1 kg/s were established releasing directly from a trailer carrying LH₂. The releases occurred from a height of 50 cm above the water surface pointing downwards, 30 cm under the water surface pointing downwards and 30 cm under the water surface pointed along the water surface. All release configurations resulted in a very chaotic LH₂-water mixing zone, causing considerable evaporation and resulting in minor over pressures. No RPTs were observed. The main phenomenon to be observed is, however, an ignition of the released gas cloud resulting in significant blast wave overpressures and heat radiation to the surroundings. The ignition occurred in all under-water releases and in about 90 % of the releases above the water surface.

Chemical Engineering Transactions, 90, 541-546, 2022.

Medium-scale tests to investigate the possibility and effects of BLEVEs of storage vessels containing liquefied hydrogen

Kees van Wingerden, Martin Kluge, Abdel Karim Habib, Federico Ustolin and Nicola Paltrinieri

Abstract: Experiments have been performed to determine the consequences of a storage vessel containing liquified hydrogen (LH2) is engulfed by a fire. The tests were performed at the Test Site Technical Safety of the Bundesanstalt für Materialforschung und –prüfung (BAM) in Germany within a research cooperation between BAM and Gexcon as part of the SH2IFT program. Three tests were performed using double-walled vacuum insulated vessels of 1 m³ volume varying the orientation of the vessel and the effect of the insulation material used (perlite or multi-layer insulation (MLI)). The degree of filling of the vessel was approximately 35 % in each of the tests performed. The fire load was provided by a propane fed burner positioned under the storage vessel and designed to give a homogeneous fire load. In one of the tests a rupture of the storage vessel occurred causing a blast, a fireball and fragments. Apart from measuring these consequences, the conditions in the vessel (e.g. temperatures and pressure) during the heating process were monitored in all three tests. The work described was undertaken as part of the project Safe Hydrogen fuel handling and Use for Efficient Implementation (SH2IFT).

Chemical Engineering Transactions, 90, 547-552, 2022.

On the Mechanical Energy Involved in the Catastrophic Rupture of Liquid Hydrogen Tanks

Federico Ustolin, Leonardo Giannini, Gianmaria Pio, Ernesto Salzano and Nicola Paltrinieri

Abstract: Hydrogen can play a central role in the energy transition thanks to its unique properties. However, its low density is one of the main drawbacks. The liquefaction process can drastically increase its density up to virtually 71 kg m^-3 at atmospheric pressure and -253°C (NIST, 2019). The safety knowledge gap on physical explosions is still broad in the case of liquid hydrogen (LH₂). For instance, it is unclear what are the consequences yields as well as the probabilities of a catastrophic rupture of an LH₂ tank. A boiling liquid expanding vapour explosion (BLEVE) might arise after this top event. In this case, the expansion of the compressed gaseous phase is followed by the flashing of a fraction of the liquid. Moreover, combustion may occur for hydrogen since it is highly flammable. This complex phenomenon was not widely explored for LH₂ yet. This study focused on the physical explosion by also considering the combustion process. Many integral models were adopted to estimate the mechanical energy developed by the explosion. The tank pressure prior to the rupture was considered below the critical one (1.298 MPa (NIST, 2019)). It was assumed that both liquid and gaseous phases are present inside the tank. The influences of the filling degree of the tank (liquid level) and the temperatures of the liquid and gaseous phases on the explosion energy were analysed. The results were compared with the ones of a previous study where similar models were employed to estimate the mechanical energy of an LH₂ tank with different initial conditions (Ustolin et al., 2020a). In particular, the effect of the combustion process on the explosion energy and shock wave overpressure was not accounted for. The aim of this study is to conduct a comparison between different models and assess which are the most and the least conservative. The outcomes of this work provide critical suggestions on the consequence analysis of cryogenic liquefied gas vessels explosions.

Chemical Engineering Transactions, 91, 421-426, 2022.

Liquid Hydrgen Spills on Water - Risk and Consequences of Rapid Phase Transition

Lars H. Odsæter, Hans L- Skarsvåg, Eskil Aursand, Federico Ustolin, Gunhild A. Reigstad and Nicola Paltrinieri

Abstract: Liquid hydrogen (LH₂) spills share many of the characteristics of liquefied natural gas (LNG) spills. LNG spills on water sometimes result in localized vapor explosions known as rapid phase transitions (RPTs), and are a concern in the LNG industry. LH₂ RPT is not well understood, and its relevance to hydrogen safety is to be determined. Based on established theory from LNG research, we present a theoretical assessment of an accidental spill of a cryogen on water, including models for pool spreading, RPT triggering, and consequence quantification. The triggering model is built upon film-boiling theory, and predicts that the mechanism for RPT is a collapse of the gas film separating the two liquids (cryogen and water). The consequence model is based on thermodynamical analysis of the physical processes following a film-boiling collapse, and is able to predict peak pressure and energy yield. The models are applied both to LNG and LH₂, and the results reveal that (i) an LNG pool will be larger than an LH₂ pool given similar sized constant rate spills, (ii) triggering of an LH₂ RPT event as a consequence of a spill on water is very unlikely or even impossible, and (iii) the consequences of a hypothetical LH₂ RPT are small compared to LNG RPT. Hence, we conclude that LH₂ RPT seems to be an issue of only minor concern.

Energies 2021, 14(16), 4789.

Modelling of Accident Scenarios from Liquid Hydrogen Transport and Use

Federico Ustolin

Abstract: Hydrogen is one of the most suitable candidates to replace hydrocarbons and reduce the environmental pollution and CO₂ emissions. Hydrogen is valuable energy carrier, potentially clean and renewable thanks to its peculiar properties. However, hydrogen has a few characteristics, such as high flammability and low density that must be taken into account when stored or handled, especially in relation to the associated safety. For this reason, this PhD study aims to increase the knowledge on safety of hydrogen technologies.

Hydrogen safety is a broad topic which involves several disciplines. This PhD focusses on the modelling of atypical accident scenarios of liquid hydrogen (LH2) technologies by adopting a multidisciplinary approach. This type of accident scenarios is called atypical because they have low probability to happen but high consequences. A few times, the neglection of these scenarios by conventional risk assessment techniques led to major accidents. For this reason, the atypical accident scenario cannot be omitted during a risk assessment and must be further analysed.

An innovative and comprehensive approach for the consequence analysis of liquid hydrogen vessel explosions

Federico Ustolin, Nicola Paltrinieri and Gabriele Landucci

Abstract: Hydrogen is one of the most suitable solutions to replace hydrocarbons in the future. Hydrogen consumption is expected to grow in the next years. Hydrogen liquefaction is one of the processes that allows for increase of hydrogen density and it is suggested when a large amount of substance must be stored or transported. Despite being a clean fuel, its chemical and physical properties often arise concerns about the safety of the hydrogen technologies. A potentially critical scenario for the liquid hydrogen (LH₂) tanks is the catastrophic rupture causing a consequent boiling liquid expanding vapour explosion (BLEVE), with consequent overpressure, fragments projection and eventually a fireball. In this work, all the BLEVE consequence typologies are evaluated through theoretical and analytical models. These models are validated with the experimental results provided by the BMW care manufacturer safety tests conducted during the 1990's. After the validation, the most suitable methods are selected to perform a blind prediction study of the forthcoming LH₂ BLEVE experiments of the Safe Hydrogen fuel handling and Use for Efficient Implementation (SH₂IFT) project. The models drawbacks together with the uncertainties and the knowledge gap in LH₂ physical explosions are highlighted. Finally, future works on the modelling activity of the LH₂ BLEVE are suggested.

Journal of Loss Prevention in the Process Industries, Volume 68, November 2020, 104323.

Hydrogen Fireball Consequence Analysis

Federico Ustolin and Nicola Paltrinieri

Abstract: A fireball may occur after the catastrophic rupture of a tank containing a flammable substance such as a fuel, if an ignition source is present. The fireball is identified by the combustion of the flammable cloud created after the fuel release and composed by the mixture of the latter and air. In particular, the fuel concentration is higher at the center of the fireball compared with the external layers where the ignition takes place. After its formation, the fireball tends to rise vertically due to the buoyancy of the hot gases involved in the combustion. Moreover, the fireball emits its energy mainly through radiant heat. Hence, the fireball formation may be one of the consequences of both a liquid and a compressed gaseous hydrogen tank explosion. For instance, the fireball is a consequence of a boiling liquid expansion vapor explosion (BLEVE). A BLEVE may occur after the catastrophic rupture of a tank containing a liquid at a temperature higher than its boiling point at atmospheric pressure. The explosion is characterized by the rapid expansion of the liquid and vapor phases due to the depressurization of the vessel. The aim of this study is to model a liquid hydrogen (LH2) fireball generated subsequently the BLEVE phenomenon. Different empirical correlations were selected to estimate the fireball dimensions and duration. Moreover, the fireball radiation was estimated by means of a theoretical model. As case study, the fireball generated from the explosion of the LH2 tank with a volume of 1 m3, which will be tested during the safe hydrogen fuel handling and use for efficient implementation (SH2IFT) project, was simulated. The results achieved from the fireball numerical models can be employed to estimate the safety distance from an LH2 tank and propose appropriate safety barriers. Furthermore, these outputs can aid the writing of critical safety guidelines for hydrogen technologies. Finally, the outcome of this study will be validated with the experimental results during the SH2IFT project. 

Chemical Engineering Transactions, 82, 211-216, 2020. DOI:10.3303/CET2082036.

Loss of integrity of hydrogen technologies: A critical review

Federico Ustolin, Nicola Paltrinieri, Filippo Berto

Abstract: Hydrogen is one of the main candidates in replacing fossil fuels in the forthcoming years. However, hydrogen technologies must deal with safety aspects due to the specific substance properties. This study aims to provide an overview on the loss of integrity (LOI) of hydrogen equipment, which may lead to serious consequences, such as fires and explosions. Substantial information regarding the hydrogen lifecycle, its properties, and safety related aspects has gathered. Furthermore, focus has placed on the phenomena responsible for the LOI (e.g. hydrogen embrittlement) and material selection for hydrogen services. Moreover, a systematic review on the hydrogen LOI topic has conducted to identify and connect the most relevant and active research group within the topic. In conclusion, a significant dearth of knowledge in material behaviour of hydrogen technologies has highlighted. It is thought that is possible to bridge this gap by strengthening the collaborations between scientists from different research fields.

International Journal of Hydrogen Energy, Volume 45, Issue 43, 3 September 2020, Pages 23809-23840

Theories and Mechanism of Rapid Phase Transition

Federico Ustolin, Lars H. Odsæter, Gunhild Reigstad, Hans L. Skarsvåg and Nicola Paltrinieri

Abstract: Light hydrocarbons and hydrogen can replace high-alkane fuels with the benefit of reduced CO₂ emissions. Their liquefaction to a cryogenic state is one of the most suitable solutions for storage and transport. An unexpected release of these fuels might lead to a rapid phase transition (RPT). RPT is a physical explosion well-known for liquefied natural gas (LNG), and may occur when this substance is spilled onto water. The heat provided by the water to the cryogenic fuel might lead to a sudden evaporation of the liquid, resulting in an explosion. The generated blast wave has the potential to damage equipment and personnel. The RPT phenomenon can also occur in different types of industrial applications when molten metals accidentally come in contact with water. In these cases, the water is the cold fluid which expands violently. In this study, the RPT phenomenon is investigated for cryogenic fluids (liquefied hydrocarbons, nitrogen and hydrogen) as well as for smelts (molten inorganic salts) and molten metals (aluminum). The contribution has a twofold purpose as it addresses relevant past accidents and lay the foundation for future modelling activities to simulate the cryogenic-pool formation on water, triggering of an RPT event and the RPT explosion consequences. Furthermore, the RPT theories and mechanisms comprehension is critical to qualitatively evaluate the probability for a liquid hydrogen (LH2) RPT. In particular, a comparison between liquid nitrogen (LN2) and LH2 is conducted to understand under which conditions an LH2 RPT might occur. The results of this study are to be validated through the Safe Hydrogen Fuel Handling and Use for Efficient Implementation (SH2IFT) project, in which a series of LH2 spill tests onto water will be conducted. 

Chemical Engineering Transactions, 82, 253-258, 2020. DOI:10.3303/CET208204

SH2IFT - Safe H₂ fuel handling and use for efficient implementation

Anne Steen-Hansen

Fire Safety Science News, No 43, October 2019, page 35.

The Influence of H₂ Safety Research on Relevant Risk Assessment

Federico Ustolin, Guozheng Song and Nicola Paltrinieri

Abstract: Hydrogen is a valuable option of clean fuel to keep the global temperature rise below 2°C. However, one of the main barriers in its transport and use is to ensure safety levels that are comparable with traditional fuels. In particular, potential liquid hydrogen accidents may not be fully understood (yet) and excluded by relevant risk assessment. For instance, as hydrogen is cryogenically liquefied to increase its energy density during transport, Boiling Liquid Expanding Vapor Explosions (BLEVE) is a potential and critical event that is important addressing in the hazard identification phase. Two past BLEVE accidents involving liquid hydrogen support such thesis. For this reason, results from consequence analysis of hydrogen BLEVE will not only improve the understanding of the related physical phenomenon, but also influence future risk assessment studies. This study aims to show the extent of consequence analysis influence on overall quantitative risk assessment of hydrogen technologies and propose a systematic approach for integration of posterior results. The Dynamic Procedure for Atypical Scenario Identification (DyPASI) is used for this purpose. The work specifically focuses on consequence models that are originally developed for other substances and adapted for liquid hydrogen. Particular attention is given to the parameters affecting the magnitude of the accident, as currently investigated by a number of research projects on hydrogen safety worldwide. A representative example of consequence analysis for liquid hydrogen release is used in this study. Critical conditions identified by the numerical simulation models are identified and considered for subsequent update of the overall system risk assessment. 

Chemical Engineering Transactions, 74, 1393-1398, 2019.

New research projects on alternative energy carriers in transport

Ragni Fjellgaard Mikalsen and NinaK. Reitan

Abstract: In order to reach climate goals, transformation of the transport and energy sector is necessary to replace fossil fuels. A combination of alternative propulsion technologies, e.g. electricity, fuel cells and biogas, will be required to cover the various needs for transport, both on- and offshore. RISE Fire Research is participating in two new Norwegian research projects investigating alternative energy carriers in transport: «BattMarine» and «SH2IFT», with focus on batteries and hydrogen, respectively. Both projects started in 2018 and are funded by the Norwegian Research Council.