Gas Turbine Enclosures: Determining Ventilation Safety Criteria using Hydrogen Explosion Modelling

Dilution ventilation is the current basis of safety following a flammable gas leak within a gas turbine enclosure, and compliance requirements are defined for methane fuels in ISO 21789. These requirements currently define a safety criteria of a maximum flammable gas cloud size within an enclosure. The requirements are based on methane explosion tests conducted during a HSE Joint Industry Project, which identified typical pressures associated with a range of gas cloud sizes. The industry standard approach is to assess the ventilation performance of specific enclosure designs against these requirements using CFD modelling. Gas turbine manufacturers are increasingly considering introducing hydrogen/methane fuel mixtures and looking towards operating with hydrogen alone. It is therefore important to review the applicability of current safety standards for these new fuels, as the pressure resulting from a hydrogen explosion is expected to be significantly higher than that from a methane explosion. In this paper, we replicate the previous methane explosion tests for hydrogen and hydrogen/methane fuel mixtures, using the explosion modelling tool FLACS CFD. The results are used to propose updated limiting safety criteria for hydrogen fuels to support ventilation CFD analysis for specific enclosure designs. It is found that significantly smaller gas cloud sizes are likely to be acceptable for gas turbines fueled by hydrogen, however, significantly more hydrogen than methane is required per unit volume to generate a stoichiometric cloud (as hydrogen has a lower stoichiometric air fuel ratio than methane). This effect results in the total quantity of gas in the enclosure (and as such, detectability of the gas) being broadly similar when operating gas turbines on hydrogen when compared to methane.