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Spontaneous Ignition ofHydrogen Leaks:
A Review of Postulated Mechanisms
S.J. Hawksworth and G.R. Astbury
Health & Safety Laboratory
Buxton, U.K.
Introduction
• Hydrogen has reputation for spontaneous ignition
• Major Hazard Incident Database Service (MHIDAS) searched
• 81 incidents reported – 4 delayed ignition– 86% no ignition source identified
• Compare with non-hydrogen releases– 65% no ignition source identified
• Zero non-ignitions not significant – – no reports?
Introduction
• Frequency of occurrences of ignition sources:
Ignition Hydrogen Non-hydrogensource incidents incidents
Number % Number %Arson 0 0 37 2.6Collision 2 2.5 121 8.4Flame 3 3.7 113 7.9Hot Surface 2 2.5 56 3.9Electric 2 2.5 114 7.9Friction Spark 2 2.5 33 2.3Not identified 70 86.3 942 65.5Non-ignition 0 0 21 1.5Total 81 100 1437 100
Specific Incidents
• 1922 – Work by Nusselt – Germany
• 1926 – Fenning & Cotton – U.K.
• 1930 – Fenning & Cotton – U.K.
• 1991 – Bond – U.K.
• 1991 – Bond – U.K.
• 1964 – Reider, Otway & Knight – U.S.A.
• 2004 – Work at HSL Buxton – not yet reported
Nusselt - 1922
• Spontaneous ignitions occurred – releases at 21 bar
• Test work releasing hydrogen through different nozzles – no ignitions
• Cylinders contained rust although apparently dry
• Potential for electrostatic charging• No ignitions using many fine powders• Only fine iron oxide and manganese oxide
caused ignitions• Rust then thought to catalyse oxidation
Nusselt Experiments
• Hydrogen and oxygen mixtures stored at
• 11 bar – pressure fell but no explosions– 24 hrs at 100°C
– 9 hrs at 200°C
– 1 hrs at 380°C
• Subsequent trials in dark revealed corona discharge – fine rust present
• Tapping equipment caused ignitions – disturbed rust?
• Corona discharge probable cause
Fenning & Cotton - 1930
• First ignition 1926
• Only reported after second occurred in 1930
• Fine dust present
• Thought to be electrostatic ignition
• Charging of dust due to high velocity
• Many experiments – no ignitions at all
• Review suggested electrostatic ignition
• Inconclusive – probably electrostatic ignition
Fenning & Cotton - 1926
• First ignition 1926 – reviewed
• Fine spray of mercury at atmospheric pressure
• Thought to be electrostatic ignition
• Now known mechanism of bursting bubbles and sprays igniting hydrocarbon/oxygen mixture
• Hydrocarbon/oxygen ignition energy similar to hydrogen/air
• Sufficient charge to ignite sensitive atmospheres
Bond - 1991
• First release– 110 bar release from flange
– Ignition reported to occur on second strike of hammer wrench by fitter
– Not apparent whether impact spark or diffusion ignition
• Second release– “Snifting” gas cylinder (230 bar)
– Attributed to diffusion ignition
Reider, Otway & Knight - 1964
• Release at 230 bar through nozzle
• After 10 seconds, valve closed• 3 seconds after starting to closing valve, ignition
occurred• System cleaned prior to test to eliminate static
generation from loose dust• After event: velocity far higher than previous
runs• Bar across nozzle detached at one end –
possible ignition source
Health & Safety Laboratory
• Releases from storage at 150 bar
• Various nozzles from 1 mm to 12 mm
• No ignitions occurred
• Attempts to induce ignition by entraining dust in the jet (externally) did not produce ignition.
Health & Safety Laboratory
Postulated Mechanisms
• Reverse Joule-Thomson Effect
• Electrostatic ignition– Spark discharges from isolated conductors
– Brush discharges
– Corona discharges
• Diffusion ignition
• Sudden adiabatic compression
• Hot surface ignition
Reverse Joule-Thomson Effect
• Joule-Thomson inversion temperature 193 K
• Above inversion temperature, temperature rises on expansion (opposite to air at ambient)
• Known data partly experimental, part calculation
• Isenthalpic lines very non-linear at very high pressures
• At 2500 bar, coefficient is 0.53 K MPa-1
• Maximum temperature rise typically only 132 K
• Unlikely to cause ignition – AIT is 560°C
Electrostatic Discharge Types
• Three possible types of discharge:
– Spark discharges from isolated conductors• Discrete plasma channel
– Brush discharges• Typically from plastics and insulators
– Corona discharges• Continuous discharge with no plasma channel
Spark Discharge
• Energy calculated from:
• E = ½ C V2
• Typical hydrocarbon (propane) E is 0.29 mJ
• For 100 pF person, voltage required is ~ 2kV
• Breakdown of air is 30 kV cm-1, so gap is 0.8 mm
• Quenching gap typically 2 – 3 mm
• Gap of 2 mm needed for ignition gives required voltage as 6 kV – not easy to achieve
Spark Discharge
• For hydrogen E is 0.017 mJ
• Breakdown strength for hydrogen 17.5 kV cm-1
• Assuming linearity, breakdown is 26.25 kV• Quenching gap is 0.69 mm• For 100 pF person, voltage required is 1810
volts• Easy to reach 2 kV on person• Cannot feel such small energy discharges• Higher risk of ignition of hydrogen than petrol
vapour
Brush Discharges
• Typically discharge from insulating plastic – cannot measure energy as capacitance cannot be measured
• Gibson and Harper determined “incendivity” using flat polyethylene sheets
• Brush discharge equivalent to about 4 mJ
• New work by Ackroyd shows “incendivity” greater than Gibson's work
• Higher incendivities with fluorinated polymers and thin layers on metal substrate
Corona Discharges
• Silent – usually continuous
• Tip radius determines corona or spark discharge
• Small tip radius gives corona rather than spark• Incendive to hydrogen – air mixtures• Atmospheric electrical activity:
– high field strength – starts corona from sharp edges
• Hydrogen vents known to ignite during frosty weather, rain, sleet and falling snow
• Assume hydrogen vents will always ignite
Diffusion Ignition
• Theory postulated by Wolański and Wójcicki
• High pressure ignition in shock tube
• Confirmed theory with confined shock tube
• No experimental work for open ignition
• Initial conditions high temperature for experiments
• No indication that atmospheric releases would be ignited by diffusion ignition
Sudden Adiabatic Compression
• Temperature rise when gas compressed adiabatically
• For compression volume ratio 10:1– theory pressure rise ratio 25.7– theory temperature rise 428 K
Adiabatic Compression
• Work by Cain indicates compression ignition occurs at about 1050K for H
2/O
2/He mixtures
• Relatively constant ignition temperature irrespective of pressure rise ratio starting at 300K
• Ratio of 80 needed in theory for adiabatic temperature rise from 300K to 1050K
• Much lower ratio needed by Cain ≈ 35 to 70
• Suggests another mechanism present
Hot Surface Ignition
• At high temperature– Oxidation generates heat
– Heat lost to surroundings
– If less lost than generated, chain reaction occurs
• Under turbulent conditions ignition occurs at lower temperatures
• Also ignition occurs at lower temperatures under shock conditions
Turbulence
• Neer suggests ignition speed rather than temperature – ignition under shock conditions needs lower
temperature than classical stationary conditions
• Bulewicz showed position and mode of heating affected ignition temperature
• Heated surface down – longer delay
• Impulsively heated plate – higher temperature
Discussion - 1
• No one mechanism explains all ignitions
• Potential for electrostatic ignition to occur– Demonstrated by some incidents
• Confined heated surfaces act as ignition sources– Unconfined hot surfaces – not well understood
• Joule-Thomson Effect needs high initial temperature
• Diffusion ignition only demonstrated in shock-tube apparatus
Discussion - 2
• Shock-tube theory and experiments for diffusion ignition appear non-specific to hydrogen– But, no other gases appear to exhibit spontaneous
ignition on release from high pressure
• Adiabatic compression requires confinement
• Difficult to separate diffusion ignition from adiabatic compression – both unlikely with discharge direct to atmosphere
Discussion - 3
• Electrostatic charging of pure gases negligible
• Particulates present can charge
• Corona known to be able to ignite hydrogen
• Possible erosion of metal of pipes – particles then able to charge
• Expansion increases temperature – lowers ignition energy
• Potential for corona to ignite more sensitive atmosphere
Conclusions - 1
• Hydrogen does not necessarily ignite when released at high pressure
• Compression ignition, Joule – Thomson expansion and diffusion ignition unlikely mechanisms for releases at ambient temperature
• Possible electrostatic charging is part of mechanism of ignition of high pressure releases
Conclusions - 2
• Mechanisms postulated in literature do not account for all ignitions and non-ignitions
• Possibility that ignitions of hydrogen are a combination of two or more postulated mechanisms
• Further work is required to establish conditions under which hydrogen release ignites – particularly electrostatic phenomena
• Stuart.Hawksworth@hls.gov.ukTel: +44 (0)1298218139Fax: +44 (0)1298218160
• Graham.Astbury@hsl.gov.uk Tel: +44(0)1298218145 Fax: +44(0)1298218160
Further Information
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