Basis for Updated Bulk Liquid Hydrogen Separation Distances in NFPA 2

NFPA 2 Hydrogen Technologies Code currently contains separation distances for bulk liquid hydrogen storage systems from various exposures based on the storage volume of the system. While the original historical distances of the 1960’s have served the industry well, experience and modern modeling tools can improve on the values and provide scientific justification. In this work, we present a risk-informed process and calculations that led to updated separation distances for the next release of NFPA 2. A characteristic leak size for liquid hydrogen storage systems was determined, informed by a sensitivity study on a quantitative risk assessment of a representative system. Simulations using Hydrogen Plus Other Alternative Fuels Risk Assessment Models (HyRAM+) with consequence-based criteria of different concentration contours, overpressures, and heat flux levels led to separation distances based on internal pressure and pipe size, rather than storage volume. Similar to the bulk gaseous separation distance table, individual exposures (e.g., ignition sources, air intakes) have been placed into three groups based on the consequence criteria. Confirmatory modeling built on prior assessments provides justification of reduction credits for the use of fire-rated walls, since walls reduce the consequences of leaks/flames. Finally, a characteristic fueling station with bulk liquid hydrogen storage using the updated separation distances is shown relative to a station sited using the current separation distances to provide an example of the impact of these revised distances. The reduced footprint enables refueling stations to be sited in more locations.

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