Consequences of Catastrophic Releases of Ignited and Unignited Hydrogen Jet Releases

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) linked 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 generally the use of larger pipe diameters than are likely to be used for small fuel cell systems. The aim of this paper is to report on work that is providing data for informing safety distances for high-pressure components / fuel cell systems and associated fuel storage. Using high-pressure release scenarios, the extent of the clouds, jets and, following ignition, fires, and explosions were investigated. The work was primarily focused on compressed H2 storage for stationary fuel cell systems, which may be physically separated from a fuel cell system or could be on board such a system. The flammability envelope, flame size and blast overpressure for different release geometries, pressures and dimensional envelopes were investigated. The main objective was to obtain data for realistic release scenarios based on different release levels including emergency relief operation and potential leak scenarios. This included investigating the effects of leak/release size, ignition position, ignition timing and leak orientation to establish release dimensions, jet flame size and associated radiation hazard. All experiments carried out were to simulate a leak from two 50 litre hydrogen cylinders at 200 bar, which, after discussion with fuel cell manufacturers and users was determined to be a realistic cylinder storage arrangement used with back- up power systems.