Abstract
To calculate pressures resulting from gaseous deflagrations, the NFPA 68 (2023) engineering model requires various fuel-air mixture properties as input. Key properties include the fundamental burning velocity and maximum explosion pressure, for which NFPA 68 provides worst-case values for common industrial gases. Default values are provided for other properties, but their use is restricted to flammable gases with stoichiometric fuel concentrations of 5% or less. Many gases exceed this threshold (e.g., hydrogen at 29.5%), and obtaining specific mixture properties often requires specialized chemical databases or modeling tools.
This work explores three aspects of this problem: summarizing resources and methods for obtaining gas properties; performing a sensitivity study for a 20-ft ISO container geometry to determine parameter impacts; and comparing NFPA 68 pressure predictions against large-scale experimental results using both default and calculated mixture-specific properties.
The results demonstrate that using appropriate fuel-air mixture properties can significantly improve prediction accuracy. For the experimental cases investigated in this study, using calculated mixture-specific properties instead of default values reduced predicted deflagration pressures by 15% on average while maintaining conservative predictions in most scenarios. The improvements were most pronounced for hydrogen applications, with pressure reductions up to 65%, reflecting the significant impact of hydrogen's distinct gas properties. These findings suggest that while default properties can serve for preliminary design assessments, calculated mixture-specific properties should be used for final designs to ensure both standard compliance and optimal vent sizing, particularly for applications involving hydrogen.
