Safety

In the event of a fire the TPRD (Thermally activated Pressure Relief Device) prevents the high-pressure full composite cylinder from bursting by detecting high temperatures and releasing the pressurized gas. The current safety performance of both the vessel and the TPRD is demonstrated by an engulfing bonfire test. However there is no requirement concerning the effect of the TPRD release which may produce a hazardous hydrogen flame due to the high flow-rate of the TPRD. It is necessary to understand better the behavior of an unprotected composite cylinder exposed to fire in order to design appropriate protection for it and to be able to reduce the length of any potential hydrogen flame. For that purpose a test campaign was performed on a 36 L cylinder with a design pressure of 70 MPa. The time from fire exposure to the bursting of this cylinder (the burst delay) was measured. The influence of the fire type (partial or global) and the influence of the pressure in the cylinder during the exposure were studied. It was found that the TPRD orifice diameter should be significantly reduced compared to current practice.

A large number of natural gas vehicles (NGV) with composite cylinders run in the world. In order to store hydrogen using the composite cylinder has also reached commercialization for the hydrogen fuel cell vehicle (FCV) which is been developing on ECO Energy. Under these increasing circumstances the most important issue is that makes sure of safety of the hydrogen composite cylinder. In case of the composite cylinder a standards to verify the safety of cylinders obey several country's standards. For NGV ISO 11439 has adopted as international standards but for FCV it has been still developing and there is only ISO/TS 15869 as international technical standards. In contents of international standards the fire test is the weakest part. The fire test is that the pressure relief valves (PRD) normally operate or not in order to prevent cylinders bursting when a vehicle is covered by fire. However with present standards there is no method to check the problem from vehicles in local flame. This study includes fire test results that have been performed to establish the fire-test standards.

This paper describes a large eddy simulation model of hydrogen spontaneous ignition in a T-shaped channel filled with air following an inertial flat burst disk rupture. This is the first time when 3D simulations of the phenomenon are performed and reproduced experimental results by Golub et al. (2010). The eddy dissipation concept with a full hydrogen oxidation in air scheme is applied as a sub-grid scale combustion model to enable use of a comparatively coarse grid to undertake 3D simulations. The renormalization group theory is used for sub-grid scale turbulence modelling. Simulation results are compared against test data on hydrogen release into a T-shaped channel at pressure 1.2–2.9 MPa and helped to explain experimental observations. Transitional phenomena of hydrogen ignition and self-extinction at the lower pressure limit are simulated for a range of storage pressure. It is shown that there is no ignition at storage pressure of 1.35 MPa. Sudden release at pressure 1.65 MPa and 2.43 MPa has a localised spot ignition of a hydrogen-air mixture that quickly self-extinguishes. There is an ignition and development of combustion in a flammable mixture cocoon outside the T-shaped channel only at the highest simulated pressure of 2.9 MPa. Both simulated phenomena i.e. the initiation of chemical reactions followed by the extinction and the progressive development of combustion in the T-shape channel and outside have provided an insight into interpretation of the experimental data. The model can be used as a tool for hydrogen safety engineering in particular for development of innovative pressure relief devices with controlled ignition.

Hydrogen has the potential to be used by many countries as part of decarbonising the future energy system. Hydrogen can be used as a fuel ‘vector’ to store and transport energy produced in low-carbon ways. This could be particularly important in applications such as heating and transport where other solutions for low and zero carbon emission are difficult. To enable the safe uptake of hydrogen technologies it is important to develop the international scientific evidence base on the potential risks to safety and how to control them effectively. The International Association for Hydrogen Safety (known as IA HySAFE) is leading global efforts to ensure this. HSE hosted the 2018 IA HySAFE Biennial Research Priorities Workshop. A panel of international experts presented during nine key topic sessions: (1) Industrial and National Programmes; (2) Applications; (3) Storage; (4) Accident Physics – Gas Phase; (5) Accident Physics – Liquid/ Cryogenic Behaviour; (6) Materials; (7) Mitigation Sensors Hazard Prevention and Risk Reduction; (8) Integrated Tools for Hazard and Risk Assessment; (9) General Aspects of Safety.<br/>This report gives an overview of each topic made by the session chairperson. It also gives further analysis of the totality of the evidence presented. The workshop outputs are shaping international activities on hydrogen safety. They are helping key stakeholders to identify gaps in knowledge and expertise and to understand and plan for potential safety challenges associated with the global expansion of hydrogen in the energy system.

This work is driven by the need to understand the hazards resulting from the rapid ignited release of hydrogen from onboard storage tanks through a thermally activated pressure relief device (TPRD) inside a garage-like enclosure with low natural ventilation i.e. the consequences of a jet fire which has been immediately ignited. The resultant overpressure is of particular interest. Previous work [1] focused on an unignited release in a garage with minimum ventilation. This initial work demonstrated that high flow rates of unignited hydrogen through a thermally activated pressure relief device (TPRD) in ventilated enclosures with low air change per hour can generate overpressures above the limit of 10- 15 kPa which a typical civil structure like a garage could withstand. This is due to the pressure peaking phenomenon. Both numerical and phenomenological models were developed for an unignited release and this has been recently validated experimentally [2]. However it could be expected that the majority of unexpected releases through a TPRD may be ignited; leading to even greater overpressures and to date whilst there has been some work on fires in enclosures the pressure peaking phenomenon for an ignited release has yet to be studied or compared with that for an equivalent unignited release. A numerical model for ignited releases in enclosures has been developed and computational fluid dynamics has then been used to examine the pressure dynamics of an ignited hydrogen release in a real scale garage. The scenario considered involves a high mass flow rate release from an onboard hydrogen storage tank at 700 bar through a 3.34 mm diameter orifice representing the TPRD in a small garage with a single vent equivalent in area to small window. It is shown that whilst this vent size garage volume and TPRD configuration may be considered “safe” from overpressures in the event of an unignited release the overpressure resulting from an ignited release is two orders of magnitude greater and would destroy the structure. Whilst further investigation is needed the results clearly indicate the presence of a highly dangerous situation which should be accounted for in regulations codes and standards. The hazard relates to the volume of hydrogen released in a given timeframe thus the application of this work extends beyond TPRDs and is relevant where there is a rapid ignited release of hydrogen in an enclosure with limited ventilation.

In the current study a Large Eddy Simulation strategy is applied to model the dispersion of compressible turbulent hydrogen jets issuing from realistic pipe geometries. The work is novel as it explores the effect of jet densities and Reynolds numbers on vertical buoyant jets as they emerge from the outer wall of a pipe through a round orifice perpendicular to the mean flow within the pipe. An efficient Godunov solver is used and coupled with Adaptive Mesh Refinement to provide high resolution solutions only in areas of interest. The numerical results are validated against physical experiments of air and helium which allows a degree of confidence in analysing the data obtained for hydrogen releases. The results show that the jets investigated are always asymmetric. Thus significant discrepancies exist when applying conventional round jet assumptions to determine statistical properties associated with gas leaks from pipelines.

As a part of a German nuclear safety project on the combustion behaviour of hydrogen-carbon monoxide-air mixtures small scale experiments were performed to determine the lower flammability limit and the laminar burning velocity of such mixtures. The experiments were performed in a spherical explosion bomb with a free volume of 8.2 litre. The experimental set-up is equipped with a central spark ignition and quartz glass windows for optical access. Further instrumentation included pressure and temperature sensors as well as high-speed shadow-videography. A wide concentration range for both fuel gases was investigated in numerous experiments from the lower flammability limits up to the stoichiometric composition of hydrogen carbon monoxide and air (H2-CO-air) mixtures. The laminar burning velocities were determined from the initial pressure increase after the ignition and by using high-speed videos taken during the experiments.

Element One Inc. Boulder CO is developing novel hydrogen gas leak indicators to improve the safety and maintenance operations of hydrogen production and chemical processing facilities and hydrogen fueling stations. These technologies can be used to make visual gas leak indicators such as paints decals and conformal plastic films as well as RF sensors for wireless networks. The primary advantage of the Element One hydrogen gas indicators is their low cost and easy deployment which allows them to be used ubiquitously at each and every potential hydrogen leak site. They have the potential to convert safety problems into routine maintenance problems thereby improving overall safety and decreasing operational costs.

For refuelling on-board hydrogen tanks table-based or formula based protocols are commonly used. These protocols are designed to achieve a tank filling close to 100% SOC (State of Charge) in s safe way: without surpassing temperature (-40°C to 85°C) and pressure limits (125% Nominal Working Pressure NWP). The ambient temperature the initial pressure and the volume category of the (compressed hydrogen storage system CHSS are used as inputs to determine the final target pressure and the pressure ramp rate (which controls the filling duration). However abnormal out-of-spec events (e.g. misinformation of storage system status and characteristics of the storage tanks) may occur and result in a refuelling in which the safety boundaries are surpassed. In the present article the possible out of specification (out-of-spec) events in a refuelling station have been analyzed. The associated hazards when refuelling on-board hydrogen tanks have been studied. Experimental results of out-of-spec event tests performed on a type 3 tank are presented. The results show that on the type 3 tank the safety temperature limit of 85°C was only surpassed under a combination of events; e.g. an unnoticed stop of the cooling of the gas combined with a wrong input of ambient temperature at a very warm environment. On the other hand under certain events (e.g. cooling the gas below the target temperature) and in particular under cold environmental conditions the 100% SOC limit established in the fuelling protocols has been surpassed. Hydrogen safety on-board tanks refuelling protocols out-of-spec events.

In the long term the key to the development of a hydrogen economy is a full infrastructure to support it which include means for the delivery and storage of hydrogen at the point of use eg at hydrogen refuelling stations for vehicles. As an interim measure to allow the development of refuelling stations and rapid implementation of hydrogen distribution to them liquid hydrogen is considered the most efficient and cost effective means for transport and storage.

The Health and Safety Executive have commissioned the Health and Safety Laboratory to identify and address issues relating to bulk liquid hydrogen transport and storage and update/develop guidance for such facilities. This position paper the first part of the project assesses the features of the transport and storage aspects of the refuelling stations that are now being constructed in the UK compares them to existing guidance highlights gaps in the regulatory regime and identifies outstanding safety issues. The findings together with the results of experiments to improve our understanding of the behaviour of liquid hydrogen will inform the development of the guidance for refuelling facilities

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