Safety Considerations of an Unignited Hydrogen Release from Onboard Storage in a Naturally Ventilated Covered Car Park

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Unignited hydrogen release from 700 bar onboard storage in a naturally ventilated covered car park has been simulated and analysed. A typical car park with dimensions LxWxH=30x28.6x2.6 m was considered. The car park had two vents of equal area on opposing walls: front and back to facilitate crossflow
ventilation based on the British standard (BS 7346-7:2013). Each vent had an area equal to 2.5% of the car park floor area, in line with BS 7346-7:2013 and similar international standards. Releases through three different Thermally Activated Pressure Relief Devices (TPRD) diameters of 3.34, 2.00 and 0.50 mm
were compared, to understand the gas dispersion, specifically the dynamics of the flammable envelope (4% vol H2 ), and envelopes of 1% and 2% H2 as these are relevant to sensor and ventilation system activation as required by NFPA 2 standard for enclosures. Concentrations in the vicinity of the vehicle and of the vents are of particular interest. A blowdown model developed in Ulster University was applied to simulate realistic scenarios, and a comparison between an idealistic constant flow rate release and blowdown through a 3.34 mm TPRD diameter highlighted the conservative nature of a constant flow rate release. However, even accounting for the blowdown demonstrated that a release through a TPRD diameter of 3.34 mm leads to the formation of a flammable cloud throughout the majority of the carpark space in less than 20 s. Such a flammable envelope is not observed to the same extent for a TPRD diameter of 2 mm and the flammable envelope is negligible for a 0.5 mm diameter TPRD. Based on ISO/DIS 19880-1, NFPA 2 and IEC (60079-10) standards for equipment with gaseous hydrogen, the ventilation system must work to maintain hydrogen concentration under 1% of hydrogen mole fraction in the air, above this there should be ventilation sensor activation. Whilst a release through a 2 mm TPRD
diameter resulted in concentrations of 1% hydrogen along the length of the car park ceiling within 20 s, in contrast an upward release through a 0.5 mm diameter led to concentrations of 1% reaching a very limited area of the ceiling. The simulations comparing an upward and downward release through a 0.5 mm TPRD demonstrated the effect of release direction on hydrogen dispersion. However, this effect is not as pronounced as the effect of changing TPRD diameter. It can be concluded that onboard vehicle storage with a TPRD diameter of 0.5 mm appears to be inherently safer for the scenario considered, as opposed to “typical” larger diameter TPRDs which the study indicates should be carefully investigated to ensure safety in a naturally ventilated covered car park.
Original languageEnglish
Title of host publicationProceedings of the Ninth International Seminar on Fire and Explosion Hazards. Vol. 2: 21-26 April 2019
Number of pages15
ISBN (Electronic)978-5-7422-6498-9
Publication statusPublished - 19 Apr 2019
EventThe Ninth International Seminar on Fire and Explosion Hazards - Saint Petersburg, Russian Federation
Duration: 21 Apr 201926 Apr 2019
Conference number: 9


ConferenceThe Ninth International Seminar on Fire and Explosion Hazards
Abbreviated titleISFEH
Country/TerritoryRussian Federation
CitySaint Petersburg
Internet address


  • Unignited release
  • covered carpark
  • hydrogen safety
  • indoor dispersion
  • natural ventilation


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