A5. Safety barrier hydrogen release interaction

Experimental program on safety barrier and wall interaction is being carried out at Sandia National Laboratories to characterise and predict the behaviour of unintended hydrogen releases. In the case where the hydrogen leak remains un-ignited, knowledge of the concentration field and flammability envelope is an issue of importance in determining consequence distances for the safe use of hydrogen. In the case where a high-pressure leak of hydrogen is ignited, a classic turbulent jet flame forms.

Knowledge of the flame length and thermal radiation heat flux distribution is important to safety. The flow field produced by the interaction of an unintended release with a barrier is complex and engineering models previously developed for free jet flames and un-ignited jets are not suited for the analysis of barrier impingement flows. While the use of barrier walls as a mitigation strategy is an important area of current research, the number of studies currently found in the literature is relatively sparse.

Overpressures and radiative heat fluxes were measured in scenarios involving both the ignition of premixed hydrogen/air clouds and hydrogen jet releases. It was found that the overpressure produced during ignition depended on the ignition time relative to the time of release, and whether the hydrogen was premixed or non-premixed. The turbulence energy intensity also had a greater effect on explosiveness than the total amount of hydrogen leaked. The study concluded that it is not necessary to release a large volume of hydrogen to attain high overpressures and that the release of a smaller quantity of hydrogen with short ignition times in a region of high turbulence can result in significant overpressures.

The purpose of the present study was to extend the available database on barrier walls as a flame hazard mitigation strategy, and to provide technical data for hydrogen codes and standards decisions regarding barrier wall design and implementation.

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

1. The 60° barrier results in more heat flux behind the barrier (up to three times more);
2. The 90° barrier results in more heat flux in front of barrier - twice the magnitude of that for the 60° barrier;
3. The 60° barrier results in more heat flux being transmitted around the barrier, a significant reduction in overpressure compared to the 90° barrier was not observed. The only advantage in using a 60° barrier in preference to a 90° barrier is there is less heat flux reflected back to the leak source.

For more details of the experimental and modelling work performed by Sandia on barrier walls, see Chapter 6 of HYPER Report D4.3 on Releases, Fires, and Explosions and HYPER D5.4 Report on Experimental Evaluation of Barrier Walls for Risk Reduction of Unintended Releases of Hydrogen.