Fire Hazards Of Small Hydrogen Leaks - HySafe

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Third European Summer School on Hydrogen SafetyJuly 21 – 30, 2008Fire Hazards of Small Hydrogen LeaksPeter B. SunderlandDept. of Fire Protection EngineeringUniversity of MarylandCollege Park MD, U.S.1/28

Acknowledgments Third European Summer Schoolon Hydrogen Safety. NIST (Grant 60NANB5D1209)under the technical managementof J. Yang. R.L. Axelbaum(WashingtonUniv.) and B.H. Chao (Univ. ofHawaii). Students: M.S. Butler,Moran, N.R. Morton.C.W.2/28

Unique Fire Hazards of H2 Steel embrittlement Lowest MW of any fuel, thus requiring the highest storage pressure Highest volumetric leak propensity of any fuel Permeation leaks Smallest ignition energy of any fuel in air (0.028 mJ) Lowest autoignition temperature of any fuel ignited by a heated air jet(640 C) Widest flammability limits of any fuel in air (4 – 75% by volume) Highest laminar burning velocity of any fuel in air (2.91 m/s) Smallest quenching distance of any fuel premixed with air (0.51 mm) Dimmest flames of any fuel in air3/28

Present Fire Scenario¾ A small leak develops in a H2 system, e.g., a H2vehicle.¾ The leak could arise from H2 embrittlement, H2permeation, impact, equipment failure, orimproper repair.¾ The leak ignites from static discharge or heat.¾ The leak burns undetected for a long period,damaging the containment system and providingan ignition source for a subsequent largerelease.4/28

Background¾ Swain and Swain (1992) modeled and measuredH2, CH4, and C3H8 leak rates.¾ Quenching and blowoff of CH4 and C3H8 flameswere measured and modeled by Matta et al.(2002) and Cheng et al. (2006).¾ Khan et al. (2002) considered the effects of heaton carbon fabric composites.¾ No codes or standards exist for permissible H2leak rates.5/28

Objectives¾ Measure quenching and blowoff limits forH2, CH4 and C3H8 on small round burners.¾ Measure quenchingcompression fittings.limitsforleaky¾ Examine material degradation arising fromexposure to H2 and CH4 flames.6/28

Experimental¾ Quenching and blowoff limits Fuels: H2, CH4, and C3H8 Diameters: 8 μm – 3.2 mm Leaky compression fittingsPinhole BurnerCurved-wall BurnerTube Burner¾ Materials degradation Fuels: H2 and CH4 Materials: aluminum alloy 1100,galvanized steel, stainless steel, SiC Test times: up to 300 hours7/28

Quenching ScalingFlame length:Lf / d a Re a ρ u0 d / μLength at quenching: Lf Lq / 2Equating these:mfuel π Lq μ / ( 8 a )FuelaLq[mm]SL[cm/s]μ[g/m-s]H2CH4C 3H -27.95e-3mfuel[mg/s]predicted0.0080.0850.0638/28

H2 Pinhole Quenching Limit0.356 mm A H2 flame at itsquenching limit isshown. This flame is notvisible without aidand required 30 scamera exposures. Stand-off height isabout 0.25 mm. Thermocouples wereusedtoidentifyflaming conditions.9/28

Tube Burner Limits1000 Quenching limits arenearly independent ofd.Blowoff LimitsMass Flow Rate (mg/s)10010H2CH4CH3 H2 has the lowestquenching limit andthe highest blowofflimit.8This workMattaChengKalghatgi10.10.0056 mg/sQuenching Limits0.010.00100.511.522.53 CH4 and C3H8 havesimilar quenching andblowoff limits.Tube Diameter (mm)10/28

H2 Quenching Limits0.016Pinhole burner6.35 mm curved-wall pinhole burner1.59 mm curved-wall pinhole burnerTube burnerQuenching Mass Flow Rate 2.5Burner Diameter (mm)3 Three burner typesare shown. For large d thelimits converge. Heat losses aregreatestforpinholes, least fortube burners. Limits increase atthe smallest d. This plot helpedidentify the world’s3.5weakestflame(0.25 W).11/28

Tube Burner Orientation EffectsQuenching Mass Flow Rate (mg/s)0.008 H2 quenching limitsgenerally decreasefor small burnersowingtoheatlosses.0.0070.0060.005 Inverted limits arelowest, attributed tofuel preheating andflame 11.5Tube Diameter (mm)22.512/28

Pinhole Burner Orientation EffectsQuenching Mass Flow Rate (mg/s)0.0120.01 No significant effectof orientation isseen.0.0080.006 Choked flow is dVertical0.002000.511.522.533.5Pinhole Burner Diameter (mm)13/28

Upstream Pressure Effects60 Upstream pressurerequired for 8 μg/sH2isentropicchokedflowisshown. Viscous effects areneglected here. This predicts thatvery small pinholescan support flamesin high pressure H2systems.Hole Area (μm 2)5040302010002468Pressure (bar)101214/28

Leaky Fittings Tests Leakpathshownobtained with loosefittings. Flow rates were measureddownstream of the leaks.15/28

16/2813 mm tube6.4 mm tube3.2 mm tube

Leaky Fittings0.63.1 mm Fitting0.56.3 mm FittingMinimum Flowrate (mg/s)12.6 mm Fitting0. thanePropane Previousslideshowsflaming leak quenchinglimitsforcompressionfittings (vertical orientation). H2 flame is smallest here,attributed to quenchingdistance. H2 mass flow rate is anorder of magnitude lowerthan CH4 or C3H8. Leaks large enough to burnproduce bubbles whensoap water is applied.17/28

Effects of Upstream PressureMinimum Flowrate (mg/s)0.40.378 mg/s0.30.336 mg/sHydrogenMethanePropanehmpLinear (h)Linear (m)Linear (p) mg/s0110100Pressure (bar)1000 Quenching limitsfor a 6 mmcompression fitting are shown. H2 limits are thelowest. Limitsareindependentofpressure. Resultsshouldguide future codesand standards.18/28

Orientation Effects0.4 Quenching limits for 6mm compression fittings are shown. Orientation has aweak effect. Invertedorientationhas the lowest heatloss rates.Minimum Flowrate MethanePropane19/28

Materials Degradation20/28

Al Degradation10 mmAluminum / H21 – hr exposure21/28

Al Degradation¾Aluminum failed in H2 flame at 8 hours.10 mm10 mmH2CH422/28

304 SS Degradation¾ Corrosion after prolonged H2 flame exposure.23/28

SiC Degradation¾ SiC filaments failed at 12 minutes in the H2flame, and at 356 minutes in the CH4 flame.5 mm5 mmH2 FlameCH4 Flame24/28

Al Degradation Microscopy Control specimen is shown.25/28

Al Degradation Microscopy Images following exposure to H2 flame.26/28

Possible Mitigation Strategies¾ Apply intumescent paints.¾ Apply steel wool or ceramic blankets.¾ Consider novel flame detectors:- Cable heat detectors- UV and IR detectors27/28

Conclusions¾ Stable H2 flames were observed on roundburners and leaky compression fittings at flowrates down to 4 and 28 μg/s, respectively.¾ Fuel mass flow rate at quenching is largelyindependent of burner diameter.¾ H2 has a lower mass flow rate at quenchingand a higher mass flow rate at blowoff thaneither CH4 or C3H8.¾ H2 flames caused much faster corrosion thanCH4 flames to aluminum and SiC fibers.28/28

Tube burner 1.59 mm curved-wall pinhole burner Pinhole burner 6.35 mm curved-wall pinhole burner Burner Diameter (mm) Quenching Mass Flow Rate (mg/s) H 2 Quenching Limits Three burner types are shown. For large d the limits converge. Heat losses are greatest for pinholes, lea

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