A5. High pressure releases

The aim of this work was to provide data for informing safety distances for high-pressure components/fuel cell systems and associated fuel storage (task 5.2). Using high-pressure releases scenarios, which were defined in task 5.1, the extent of the clouds, jets and, following ignition, fires and explosions were investigated.

The work was performed at HSL and was primarily focused on compressed H2 on site storage and compression, which may be physically separated from a fuel cell system or could be on board such a system. The work was originally planned to cover a range of pressures up to 800 bar, however after discussions with fuel cell manufacturers and users, it was determined that pressures above 200 bar were impractical for current storage needs and were not necessary or being considered for future stationary fuel cell applications.

The flammability envelope, flame size and heat fluxes for different release geometries, pressures and dimensional envelopes were investigated.

The possibility of using a risk-based approach to installation and siting of stationary fuel cell systems depends upon availability of data and guidance on hazards and probabilities of occurrence. Such guidance data is readily available for most common hydrocarbon fuels. For hydrogen however data is required on the hazards associated with different release scenarios. This data can then be related 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 pressure and generally use larger pipe diameters than are likely to be used for small fuel cell systems.

The main objective of this task was to obtain data for realistic release scenarios based on different leak levels including emergency relief operation and other potential leak scenarios. This included investigating the effects of leak size, ignition position, ignition timing and leak orientation to establish release dimensions, jet flame size and radiation hazard. All experiments were carried out 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.

As a result of this work the following conclusions were made:

1. The inclusion of flow restrictors in hydrogen supply line reduces the flame lengths observed, therefore reducing safety distances required.
2. From the experiments carried out it is apparent that jets from hydrogen storage at 200 bar are predominantly momentum-driven, i.e. the cloud is relatively non-buoyant within the flammable range.
3. When a release is orientated such that attachment to a surface can occur, the jet length may be enhanced.
4. Ignition in a weak region of the jet cloud results in a relatively slow burn and hence a small overpressure.
5. Maximum overpressures were observed when the jet was ignited at a time that coincided with the area of maximum turbulence within the front portion of the jet, reaching the ignition point.