Analysis to Support Revised Distances between Bulk Liquid Hydrogen Systems and Exposures

The minimum distances between exposures and bulk liquid hydrogen listed in the National Fire Protection Agency’s Hydrogen Technology Code, NFPA 2, are based on historical consensus without a documented scientific analysis. This work follows a similar analysis as the scientific justification provided in NFPA 2 for exposure distances from bulk gaseous hydrogen storage systems, but for liquid hydrogen. Validated physical models from Sandia’s HyRAM software are used to calculate distances to a flammable concentration for an unignited release, the distance to critical heat flux values and the visible flame length for an ignited release, and the overpressure that would occur for a delayed ignition of a liquid hydrogen leak. Revised exposure distances for bulk liquid hydrogen systems are calculated. These distances are related to the maximum allowable working pressure of the tank and the line size as compared to the current exposure distances, which are based on system volume. For most systems, the exposure distances calculated are smaller than the current distances for Group 1, they are similar for Group 2, while they increase for some Group 3 exposures. These distances could enable smaller footprints for infrastructure that includes bulk liquid hydrogen storage tanks, especially when using firewalls to mitigate Group 3 hazards and exposure distances. This analysis is being refined as additional information on leak frequencies is incorporated and changes have been proposed to the 2023 edition of NFPA 2.