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Síle Brennan , Vladimir Molkov

ICHS 4 San Francisco 12-14 2011. Safety Assessment of Unignited Hydrogen Discharge from Onboard Storage in Garages with Low Levels of Natural Ventilation. Síle Brennan , Vladimir Molkov. Hydrogen Safety Engineering and Research Centre (HySAFER). Outline. Motivation for the work

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Síle Brennan , Vladimir Molkov

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  1. ICHS 4 San Francisco 12-14 2011 Safety Assessment of Unignited Hydrogen Discharge from Onboard Storage in Garages with Low Levels of Natural Ventilation Síle Brennan, Vladimir Molkov Hydrogen Safety Engineering and Research Centre (HySAFER)

  2. Outline • Motivation for the work • Pressure peaking • Description of the problem • Methodology • Results • Conclusions 12th September 2011

  3. Motivation • Safety levels in H2technologies need to be at least the same as those in existing fossil fuel applications • Necessary to consider indoor use e.g. forklifts, vehicles, equipment in a garage etc. • Build on overlooked safety issue of “pressure peaking” to understand requirements for safe blow-down of on-board hydrogen storageindoors 12th September 2011

  4. On-board H2 storage • H2 as compressed gas (350 - 700 bar) • Tanks equipped with pressure relief devices • Composite tanks rupture in < 6.5 min in fire • Current venting area of the PRD releases hydrogen quickly from the tank before its catastrophic failure • However, even if unignited, the release of hydrogen has been shown to result in unacceptable overpressures within the garage capable of destroying the structure 12th September 2011

  5. Pressure peaking (1/3) Example • Release in 30.4 m3garage from 350 bar onboard storage • PRD with typical diameter of 5 mm • Steady mass flow rate release: 390 g/s of • Garage has single vent (area ~ 1 brick) • What is the overpressure in the garage? • Simple methods predict max 18 kPa 12th September 2011

  6. Pressure peaking (2/3) Garage destroyed in seconds 12th September 2011 30.4 m3 garage, “brick” vent, mass flow rate 390 g/s (350 bar, 5 mm orifice)

  7. Pressure peaking (3/3) H2 only! 10-20 kPa – safety limit for civil structures 12th September 2011 30.4 m3 garage, “brick” vent, mass flow rate 390 g/s (350 bar, 5 mm orifice)

  8. Problem description (1/2) • Used phenomenological model to investigate releases indoors e.g. garage • Model based on a known volume, vent area and release rate • Characterise garage by Air Change per Hour (ACH) • Consider range of scenarios involving a release from onboard storage through a PRD in a vented garage 12th September 2011

  9. Problem description (2/2) • Pressure: Onboard storage tanks @ 350 & 700 bar • Mass of H2: “Typical” inventories of 1, 5 and 13 kg • Garage volume:Free volumes in range 18-46 m3 • Ventilation: All natural ventilation, assume flow out: • ACH values(0.03 – 1) • Release parameters: the unignited hydrogen is released into the enclosure through PRDs with different areas 12th September 2011

  10. Methodology • Step 1: Relate ACH to garage volume & vent size • Step 2: UU blown-down model to calculate dynamics of H2 mass flow rate from storage tankInput to phenomenological model • Step 3: For each scenario use phenomenological model to iteratively find PRD area such that: Pgarage< 20 kPa i.e. a “safe” level • Step 4:Find blow-down time, through PRD with “safe diameter” to tank over-pressures of 100, 50, 20, 1 & 0.1 bar 12th September 2011

  11. ACH & Vent Size (1/2) • Air changes per hour (ACH) is a measure of how many times the air within a defined space (e.g. a garage) is replaced. • ACH = Qhr/VQhr = air flow rate (m3/hr), V = volume (m3) • Uncertainty in the literature in how to relate ACH to volume and vent area • Bernoulli: Qs= air flow rate (m3/s) A = vent area • C = coefficient of discharge = 0.6 • ∆P = pressure differential between garage & atmosphere 12th September 2011

  12. ACH & Vent Size (2/2) • Bernoulli: • Fix volume and ACH > find Q (per hr & per s) • Fix ∆P to find vent area, A • BUT - what do we take as “∆P” ?? • 50 Pa commonly used in building applications “N50” • Bigger ∆Pused, smaller the vent for a given volume • Thus vent size and hence “peak-pressure” sensitive to ∆P chosen 12th September 2011

  13. Effect of ∆P on Vent Area 12th September 2011

  14. “Current” & “safe” PRD Pressure dynamics in 30m3 garage, ACH-0.18, 5 kg hydrogen @ 350 barsPRD diameters of 5 mm and 0.5 mm

  15. Nomogram: 5 Kg H2 @ 350 bar Volume > ACH f(P) ACH > diameter diameter > time 12th September 2011

  16. Nomogram: 5 Kg H2 @ 700 bar Volume > ACH f(P) ACH > diameter diameter > time 12th September 2011

  17. Conclusions • Garages characterised by ACH and volume • Pressure-peaking model for unignited released used to calculate “safe” PRD diameters and corresponding blow-down time from on-board storage in vented enclosures • This phenomenon should be accounted for in indoor use of HFC systems and must be reflected in RCS. • Work raises questions about current approaches to fire resistance of onboard storage and PRD parameters • Further research is needed to develop safety strategies and engineering solutions. 12th September 2011

  18. Thank you for your attention Any questions? sl.brennan@ulster.ac.uk 12th September 2011

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