1. S.J. Hawksworth and G.R. Astbury Health & Safety Laboratory
Spontaneous Ignition of Hydrogen Leaks: A Review of Postulated Mechanisms
S.J. Hawksworth and G.R. Astbury
Health & Safety Laboratory
Buxton, U.K.

2. 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?

3. Introduction
Frequency of occurrences of ignition sources:

4. Specific Incidents 1922 – Work by Nusselt – Germany
1926 – Fenning & Cotton – U.K.
1930 – Fenning & Cotton – U.K.
1991 – Bond – U.K.
1964 – Reider, Otway & Knight – U.S.A.
2004 – Work at HSL Buxton – not yet reported

5. 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

6. 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

7. 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

8. 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

9. Bond - 1991 First release Second 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

10. 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

11. 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.

12. Health & Safety Laboratory

13. 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

14. 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

15. 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

16. 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

17. Spark Discharge For hydrogen E is 0.017 mJ
Breakdown strength for hydrogen 17.5 kV cm-1
Assuming linearity, breakdown is kV
Quenching gap is 0.69 mm
For 100 pF person, voltage required is volts
Easy to reach 2 kV on person
Cannot feel such small energy discharges
Higher risk of ignition of hydrogen than petrol vapour

18. 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

19. 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

20. 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

21. 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

22. Adiabatic Compression
Work by Cain indicates compression ignition occurs at about 1050K for H2/O2/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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. Further Information Stuart.Hawksworth@hls.gov.uk
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