Numerical Simulations of a Large Hydrogen Release in a Process Plant

This paper describes a series of numerical simulations with release and ignition of hydrogen. The objective of this work was to re-investigate the accidental explosion in an ammonia plant which happened in Norway in 1985 with modern CFD tools. The severe hydrogen-air explosion led to two fatalities and complete destruction of the factory building where the explosion occurred. A case history of the accident was presented at the 1.st ICHS in Pisa, 2005.
The numerical simulations have been performed with FLACS, a commercial CFD simulation tool for gas dispersion and gas explosions. The code has in the recent years been validated in the area of hydrogen dispersion and explosions.
The factory building was 100 m long, 10 m wide and 7 m high. A blown-out gasket in a water pump led to release of hydrogen from a large reservoir storing gaseous hydrogen at 3.0 MPa. The accident report estimated a total mass of released hydrogen between 10 and 20 kg. The location of the faulty gasket is known but the direction of the accidental release is not well known and has been one of the topics of our investigations. Several simulations have been performed to investigate the mixing process of hydrogen-air clouds and the development of a flammable gas cloud inside the factory building, resulting in a simulation matrix with dispersions in all axis directions. Simulations of ignition of the different gas clouds were carried out and resulting pressure examined. These results have been compared with the damages observed during the accident investigation.
We have also performed FLACS simulations to study the effect of natural venting and level of congestion. The height of the longitudinal walls has been varied, leading to different vent openings at floor level, at the ceiling and a combination of the two. This was done to investigate the effects of congestion with regards to gas cloud formation.
The base case simulation appears to be in good accordance to the observed damages from the accident. The simulations also show that the build up of the gas cloud strongly depends on the direction of the jet and degree of ventilation. The CFD study has given new insights to the accident and the results are a clear reminder of the importance of natural venting in hydrogen safety.