Abstract
The rapid increasing interest in the usage of Hydrogen has obviously triggered a lot of safety related questions. Apart from non-trivial questions about failure frequencies and ignition probabilities, the consequence modelling of potential events also contains significant uncertainties. Something to be aware of is, that even when assuming the straightforward scenario of direct ignition of the Hydrogen release, the resulting phenomena ”jet fire” (continuous event) or “fireball” (instantaneous event) are still being modelled using traditional and potentially misleading methods. While the Hydrogen jet fire is known to have a very small impact zone around the flame itself, the commonly applied “Chamberlain” approach would result in a flame Surface Emissive Power (SEP) which is highly unrealistic for Hydrogen. Furthermore, for the instantaneous release of compressed Hydrogen, the fireball phenomenon is often modelled using typical BLEVE models. The relations in these BLEVE models correlate a radiative fraction to a vapor pressure, which is irrelevant for situations where non-Pressurized Liquefied Gases (PLG) are being studied. Both approaches result in a very high flame emissive power, while the BLEVE fireball growing and rising behaviour is also based upon experiments with liquefied gas flashing, which is a different phenomenon than the compressed gas expansion situation. Other methods that correlate fireball diameter to the expansion to the Upper Flammability Limit (UFL), are non-conservative for Hydrogen due to its very high UFL.
Because of the big uncertainty and unrealistic approaches in the current modelling, Gexcon started applying a dedicated gas fireball model in its consequence modelling tool EFFECTS. This model differs from the commonly applied BLEVE fireball approaches. While experimental data about compressed Hydrogen fireballs is still scarce, the gas fireball model is based upon relations from available literature, focussing on non PLG fireball data and available experiments providing Hydrogen flame radiation fluxes. The selected relations for fireball diameter and lift-off are similar to those for the BLEVE fireball model, but the rising and growing velocity is different, because it does not include flashing liquid behaviour. During the research of an appropriate model to simulate gas fireballs, it has been encountered that there is very little information in literature that suggests how to correlate the SEP of the fireball to the chemical properties of the substance. This radiative behaviour is highly influenced by the flame’s temperature, gas composition and potential soot formation. Because usage of a “soot fraction” would be unrealistic for substances like Hydrogen, experimentally derived values have been applied for the fireball radiative flux. Apart from the heat radiation effect, the overpressure phenomenon (blast) is also being derived using equations that differ from expanding vapour explosions.
