Safety

The properties of hydrogen compared to conventional fuels such as gasoline and diesel are substantially different requiring adaptations to the design and layout of test cells for hydrogen fuelled engines and vehicles. A comparison of hydrogen fuel properties versus conventional fuels in this paper provides identification of requirements that need to be adapted to design a safe test cell. Design examples of actual test cells are provided to showcase the differences in overall layout and ventilation safety features fuel supply and metering and emissions measurements. Details include requirements for ventilation patterns the necessity for engine fume hoods as well as hydrogen specific intake and exhaust design. The unique properties of hydrogen in particular the wide flammability limits and nonvisible flames also require additional safety features such as hydrogen sensors and flame cameras. A properly designed and implemented fuel supply system adds to the safety of the test cell by minimizing the amount of hydrogen that can be released. Apart from this the properties of hydrogen also require different fuel consumption measurement systems pressure levels of the fuel supply system additional ventilation lines strategically placed safety solenoids combined with appropriate operational procedures. The emissions measurement for hydrogen application has to be expanded to include the amount of unburned hydrogen in the exhaust as a measurement of completeness of combustion. This measurement can also be used as a safety feature to avoid creation of ignitable hydrogen-air mixtures in the engine exhaust. The considerations provided in this paper lead to the conclusion that hydrogen IC engines can be safely tested however properly designed test cell and safety features have to be included to mitigate the additional hazards related to the change in fuel characteristics.

Fault trees and event trees have for decades been the most commonly applied modelling tools in both risk analysis in general and the risk analysis of hydrogen applications including infrastructure in particular. It is sometimes found challenging to make traditional Quantitative Risk Analyses sufficiently transparent and it is frequently challenging for outsiders to verify the probabilistic modelling. Bayesian Networks (BN) are a graphical representation of uncertain quantities and decisions that explicitly reveal the probabilistic dependence between the variables and the related information flow. It has been suggested that BN represent a modelling tool that is superior to both fault trees and event trees with respect to the structuring and modelling of large complex systems. This paper gives an introduction to BN and utilises a case study as a basis for discussing and demonstrating the suitability of BN for modelling the risks associated with the introduction of hydrogen as an energy carrier. In this study we explore the benefits of modelling a hydrogen refuelling station using BN. The study takes its point of departure in input from a traditional detailed Quantitative Risk Analysis conducted by DNV during the HyApproval project. We compare and discuss the two analyses with respect to their advantages and disadvantages. We especially focus on a comparison of transparency and the results that may be extracted from the two alternative procedures.

Experimental data obtained for hydrogen mixtures in a room-size enclosure are presented and compared with data for propane and methane mixtures. This set of data was also used to develop a three-dimensional gas dynamic model for the simulation of gaseous combustion in vented enclosures. The experiments were performed in a 64 m3 chamber with dimensions of 4.6 × 4.6 × 3.0 m and a vent opening on one side and vent areas of either 2.7 or 5.4 m2 were used. Tests were performed for three ignition locations at the wall opposite the vent at the center of the chamber or at the center of the wall containing the vent. Hydrogen–air mixtures with concentrations close 18% vol. were compared with stoichiometric propane–air and methane–air mixtures. Pressure data as function of time and flame time-of-arrival data were obtained both inside and outside the chamber near the vent. Modelling was based on a Large Eddy Simulation (LES) solver created using the OpenFOAM CFD toolbox using sub-grid turbulence and flame wrinkling models. A comparison of these simulations with experimental data is discussed.

Performing research in the interest of providing relevant safety requirements is a valuable and essential endeavor but translating research results into enforceable requirements adopted into codes and standards a process sometimes referred to as codification can be a separate and challenging task. This paper discusses the process utilized to successfully translate research results related to bulk gaseous hydrogen storage separation (or stand-off) distances into code requirements in NFPA 55:Storage Use and Handling of Compressed Gases and Cryogenic Fluids in Portable and StationaryContainers Cylinders and Tanks and NFPA 2: Hydrogen Technologies. The process utilized can besummarized as follows: First the technical committees for the documents to be revised were engaged to confirm that the codification process was endorsed by the committee. Then a sub-committee referred to as a task group was formed. A chair must be elected or appointed. The chair should be a generalist with code enforcement or application experience. The task group was populated with several voting members of each technical committee. By having voting members as part of the task group the group becomes empowered and uniquely different from any other code proposal generating body. The task group was also populated with technical experts as needed but primarily the experts needed are the researchers involved. Once properly populated and empowered the task group must actively engage its members. The researchers must educate the code makers on the methods and limitations of their work and the code makers must take the research results and fill the gaps as needed to build consensus and create enforceable code language and generate a code change proposal that will be accepted. While this process seems simple there are pitfalls along the way that can impede or nullify the desired end result – changes to codes and standards. A few of these pitfalls include: wrong task group membership task group not empowered task group not supported in-person meetings not possible consensus not achieved. This paper focuses on the process used and how pitfalls can be avoided for future efforts.

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.

If the ‘Hydrogen Economy’ is to progress more hydrogen fuelling stations are required. In the short term and in the absence of a hydrogen distribution network these fuelling stations will have to be supplied by liquid hydrogen (LH2) road tankers. Such a development will increase the number of tanker offloading operations significantly and these may need to be performed in close proximity to the general public. LH2 was first investigated experimentally as large-scale spills of LH2 at a rate of 60 litres per minute. Measurements were made on un-ignited releases which included the concentration of hydrogen in air thermal gradients in the concrete substrate liquid pool formation and temperatures within the pool. Computational modelling on the un-ignited spills was also performed. The experimental work on ignited releases of LH2 detailed in this paper is a continuation of the work performed by Royle and Willoughby. The experimental findings presented are split into three phenomena; jet-fires in high and low wind conditions ‘burn-back’ of ignited clouds and secondary explosions post ‘burn-back’. The aim of this work was to determine the hazards and severity of a realistic ignited spill of LH2 focussing on; flammability limits of an LH2 vapour cloud flame speeds through an LH2 vapour cloud and subsequent radiative heat levels after ignition. An attempt was made to estimate the magnitude of an explosion that occurred during one of the releases. The results of these experiments will inform the wider hydrogen community and contribute to the development of more robust modelling tools. The resulting data were used to propose safety distances for LH2 offloading facilities which will help to update and develop guidance for codes and standards.

Knowledge of the concentration field and flammability envelope from small-scale leaks is important for the safe use of hydrogen. These small-scale leaks may occur from leaky fittings or o-ring seals on liquid hydrogen-based systems. The present study focuses on steady-state leaks with large amounts of pressure drop along the leak path such that hydrogen enters the atmosphere at near atmospheric pressure (i.e. Very low Mach number). A three-stage buoyant turbulent entrainment model is developed to predict the properties (trajectory hydrogen concentration and temperature) of a jet emanating from the leak. Atmospheric hydrogen properties (temperature and quality) at the leak plane depend on the storage pressure and whether the leak occurs from the saturated vapor space or saturated liquid space. In the first stage of the entrainment model ambient temperature air (295 K) mixes with the leaking hydrogen (20–30 K) over a short distance creating an ideal gas mixture at low temperature (∼65 K). During this process states of hydrogen and air are determined from equilibrium thermodynamics using models developed by NIST. In the second stage of the model (also relatively short in distance) the radial distribution of hydrogen concentration and velocity in the jet develops into a Gaussian profile characteristic of free jets. The third and by far the longest stage is the part of the jet trajectory where flow is fully developed. Results show that flammability envelopes for cold hydrogen jets are generally larger than those of ambient temperature jets. While trajectories for ambient temperature jets depend solely on the leak densimetric Froude number results from the present study show that cold jet trajectories depend on the Froude number and the initial jet density ratio. Furthermore the flammability envelope is influenced by the hydrogen concentration in the jet at the beginning of fully developed flow.

In searching for high gravimetric and volumetric density hydrogen storage systems it is inevitable that higher energy density materials will be used. In order to make safe and commercially acceptable condensed phase hydrogen storage systems it is important to understand quantitatively the hazards involved in using and handling these materials and to develop appropriate mitigation strategies to handle potential material exposure events. A crucial aspect of the development of risk identification and mitigation strategies is the development of rigorous environmental reactivity testing standards and procedures. This will allow for the identification of potential hazards and implementation of risk mitigation strategies. Modified testing procedures for shipping air and/or water sensitive materials as codified by the United Nations have been used to evaluate two potential hydrogen storage materials 2LiBH4·MgH2 and NH3BH3. The modified U.N. procedures include identification of self-reactive substances pyrophoric substances and gas-emitting substances with water contact. The results of these tests for air and water contact sensitivity will be compared to the pure material components where appropriate (e.g. LiBH4 and MgH2). The water contact tests are divided into two scenarios dependent on the hydride to water mole ratio and heat transport characteristics. Air contact tests were run to determine whether a substance will spontaneously react with air in a packed or dispersed form. Relative to 2LiBH4·MgH2 the chemical hydride NH3BH3 was observed to be less environmentally reactive.

This paper describes a combined experimental analytical and numerical modelling investigation into hydrogen jet fires in a passively ventilated enclosure. The work was funded by the EU Fuel Cells and Hydrogen Joint Undertaking project Hyindoor. It is relevant to situations where hydrogen is stored or used indoors. In such situations passive ventilation can be used to prevent the formation of a flammable atmosphere following a release of hydrogen. Whilst a significant amount of work has been reported on unignited releases in passively ventilated enclosures and on outdoor hydrogen jet fires very little is known about the behaviour of hydrogen jet fires in passively ventilated enclosures. This paper considers the effects of passive ventilation openings on the behaviour of hydrogen jet fires. A series of hydrogen jet fire experiments were carried out using a 31 m3 passively ventilated enclosure. The test programme included subsonic and chocked flow releases with varying hydrogen release rates and vent configurations. In most of the tests the hydrogen release rate was sufficiently low and the vent area sufficiently large to lead to a well-ventilated jet fire. In a limited number of tests the vent area was reduced allowing under-ventilated conditions to be investigated. The behaviour of a jet fire in a passively ventilated enclosure depends on the hydrogen release rate the vent area and the thermal properties of the enclosure. An analytical model was used to quantify the relative importance of the hydrogen release rate and vent area whilst the influence of the thermal properties of the enclosure were investigated using a CFD model. Overall the results indicate that passive ventilation openings that are sufficiently large to safely ventilate an unignited release will tend to be large enough to prevent a jet fire from becoming under-ventilated.

Refueling infrastructure for use in gaseous hydrogen powered vehicles requires extensive manifolding for delivering the hydrogen from the stationary fuel storage at the refueling station to the vehicle as well as from the mobile storage on the vehicle to the fuel cell or combustion engine. Manifolds for gas handling often use welded construction (as opposed to compression fittings) to minimize gas leaks. Therefore it is important to understand the effects of hydrogen on tubing and tubing welds. This paper provides a brief overview of on-going studies on the effects of hydrogen precharging on the tensile properties of austenitic stainless tubing and orbital tube welds of several austenitic stainless steels.