funded by fch ju (grant agreement no. 256823) 1 © hyfacts project 2012/13 confidential – not for...

Funded by FCH JU (Grant agreement No. 256823)

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© HyFacts Project 2012/13CONFIDENTIAL – NOT FOR PUBLIC USE

Chapter F:Hydrogen fires

Compiled by Vladimir Molkov, University of Ulster

Welcome to the HyFacts Short Course


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Introductory statements

Safety is often mistakenly called a “non-technical” barrier to the hydrogen economy. In fact, the hydrogen safety is a challenging area of science and engineering, technological development and innovation. Unresolved issues include the reduction of jet flame length from current 10-15 m from onboard storage to allow self-evacuation of passengers and their safeguarding by first responders.

Another unresolved safety issue to be addressed is the increase of fire resistance of onboard storage tanks from present 1-7 minutes for type 4 vessels to 1-2 hours to allow longer time for blow-down of tanks. This in turn would prevent destruction of civil structures like garages during accidental release, and exclude formation of large hydrogen-air clouds in tunnels able to make fatalities throughout the whole length of the tunnel. Higher fire resistance rating of storage tanks would permit safe evacuation from the accident scene, etc.


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Hydrogen fires

Range from micro-flames (10-9 kg/s) to high debit flames (100 kg/s) Laminar diffusion and turbulent non-premixed flames Buoyancy-controlled and momentum-dominated jets Subsonic, sonic and highly under-expanded supersonic jet fires Fireballs during storage tank failure Liquefied hydrogen (LH2) fires (little knowledge)

Combustion terminology is applied: Laminar diffusion flame Turbulent non-premixed flame


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Hindenburg fire (public perception)

The 1937 Hindenburg dirigible disaster

No explosion


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Hydrogen vehicle fires (state-of-the-art) Fire was initiated on the instrumentation panel ashtrays. The PRD was actuated after 14

min 36 s (upward scenario). Upward release from PRD. Vehicle equipped with two 34 L capacity cylinders at 350 bar and “normal” PRD (5 mm).

(Watanabe et al., 2007) Do we accept 10-15 m flame from a car? No harm separation distance is about 50 m (public perception!) Flame jet of hydrocarbons longer in momentum controlled regime (methane 130%:

propane 200%, Hestestad ,1999)

Car back view Car side view


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Hydrogen vehicle fires (state-of-the-art)

The PRD was actuated after 16 min and 16 s (downward scenario). Blowdown less than 5 min (no tank failure, but…).

(Watanabe et al., 2007)…what if car is indoor (public perception!)?

Car side viewCar back view


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Fireballs (storage tank failure) Type 4 (stand-along tank 72.4 L, 343 bar, 1.64 kg): fireball diameter of 7.7 m

(45 ms after tank rupture). Fireball is lifted in 1 s (Zalosh 2007). Type 3 (tank under vehicle 88 L, 318 bar): fireball diameter of 24 m.

The simple correlation gives 9.4 m for 1.64 kg of hydrogen (Zalosh 2007). Fireball duration is about 4.5 s. The correlation gives 0.6 s duration Heat flux

(Type 3) at distance 15.2 m in peak spikes was 210-300 kW/m2 (flux 35 kW/m2 during 10 s -1% fatality).

Pressure: Type 4: 41 kPa-6.5 m (15% fatality); Type 3: 12 kPa-15 m (people knocked down).

0.07 s 0.17 s0.045 s 1 s

Stand-alone Type 4Type 3 (under vehicle)


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Bonfire Test of Hydrogen and CNG tank

Hydrogen tank CNG Tank

Catastrophic failure of stand alone storage tanks subjected to bonfire testing Stephenson (2005)


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16% of H2 (1 car) damage 28.8% of H2 (no car) damage

Indoor combustion (NIST GCR 10-929)

NIST Hydrogen release and combustion measurements in a full scale garage (2010)


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Dimensionless numbers (for reference)

Froude number (U - velocity, D – characteristic size, g – acceleration of gravity) is a ratio of inertial to gravity force (when multiplied by the product of density by area rA)

Reynolds number (U velocity, D – characteristic size, r – density, m – viscosity) is a ratio of inertial to viscous force

Mach number (U - velocity, C – speed of sound) is a ratio of inertial force to inertial force at sonic flow

The speed of sound in gas isM

RTC

gD

UFr

2

UD

Re

C

UM


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Hydrogen jet flames: laminar and turbulent

Hottel and Hawthorne, Proceedings of Combustion Institute, 4, 1949. Transition from laminar flame to non-premixed turbulent flame at Reynolds number Re≈2000.

?


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Hydrogen jet flames: dependence on Re and Fr

Baev and Yasakov (1974) showed theoretically that depending on Froude number (Fr) there will be a characteristic peak in the LF (Re) function or not. Confirmed experimentally by Shevyakov and Komov (1977). Expanded jets.

?


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Jet flame length: diameter and flow rates

m – mass flow rate, D – burner diameter. Flame length increases with D (m is fixed), and m (D is fixed). Data converges when a new group (mD) is used to correlate experiments.

Lf=f(m) Lf=f(m, D)

Kalghatgi (1984)


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The dimensional correlation

+20%

+50%

Best fit (nomogram) Conservative347.0)(76 DmLF 347.0)(116 DmLF


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The nomogram (hydrogen flame length) The nomogram is based on the best fit

curve of the dimensional correlation (conservative estimate 50% longer).

Example: 3 mm orifice, storage 350 atm will produce 5 m flame (best fit).

Conservative estimate of flame length is 7.5 m. Thus, “no harm” separation distance (x3.5 of flame length – see later) is more than 26 m.

The nomogram incorporates “No flame area”: no stable flame was observed for D=0.1-0.2 mm as the flame blew off although the pressures were as high as 40 MPa. D=3 mm

P=350 bar

Flame LF=5 m

No flame


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The flame blow-off phenomenon The blow-off means extinction as soon as the pilot burner is switched-off. Left: blow-off area in “P-D” coordinates (<0.1 mm no flame up to 400 atm). Right: blow-off as a function of P and D (only 2 mm orifices have no blow-off).

Mogi and Horiguchi, 2009


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The dimensionless correlation

FrC

g

C

UX

NS

N

N

N

S

N

Re

3

3

Buoyancy-controlled

Momentum-

Unde

r-exp

ande

d

Three jet fire regimes: Buoyancy-controlled

(only expanded) Momentum-dominated

(expanded jets) Momentum-dominated

(under-expanded jets)Validation:

P =0.1-90 MPa D=0.4-51.7 mm Flow: L/T; SS/S/SS


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(+) Hawthorn et al., 1949: Concentration fluctuations in turbulent flame or local “unmixedness” produce a statistical smearing of reaction zone and a consequent lengthening beyond the point where the mean composition of mixture is stoichiometric.

(-) Sunavala, Hulse, Thring, 1957: “Calculated flame length may be obtained by substitution the concentration corresponding to the stoichiometric mixture in equation of axial concentration decay for non-reacting jet”.

(-) Bilger and Beck, 1975: flame length is defined “for convenience” as the length on the axis to the point having a mean composition which is stoichiometric (hydrogen concentration is twice that of oxygen).

(-) Bilger, 1976: the calculated flame length may be obtained by substitution the concentration corresponding to the stoichiometric mixture in the equation of axial concentration decay for a non-reacting jet.

Contradictory statements: flame tip location


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Where is a jet flame tip location?

Flame tip location: from 8% to 16% in unignited jet (average – 11%).

Flame is longer than the distance to axial concentration 29.5% in unignited jet (stoichiometric hydrogen-air mixture) by 2.2 times (16%) to 4.7 times (8%)!

11%

8%

16%


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The nomogram for flame length (8%-16%)

D

N

The nomogram developed originally for unignited releases, e.g. separation to 1%, 2%, 4%, etc.

Due to knowledge of flame tip location (8%-16%, average 11% in unignited release) it can be now applied to calculate flame length.


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Pressure 205 bar, ignition delay 800 ms. Attached jets – release 0.11 m above the ground (horizontal). Free (unattached jets) – release 1.2 m above the ground (horizontal).

Explanation: change in entrainment (dilution by air), and momentum “killing”. Conclusion: release along the ground, wall, ceiling or other surface can

increase flame length (the same is valid for unignited releases).

Jet flame elongation due to the attachment

Orifice diameter, mm

Flame length, mAttached jets

Flame length, mFree jets

Flame length increase, times

1.5 5.5 3 x1.83

3.2 9 6 x1.50

6.4 11 9 x1.22

9.5 13 11 x1.18


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Outdoor hydrogen jet fire experiments by HSL: Storage pressure: 205 bar (two 50 litre cylinders). Stainless steel tubing ID=11.9 mm, a series of ball valves with internal bore

of 9.5 mm, or restrictors of 2 mm length and diameter: 1.5, 3.2, 6.4 mm. The release point is 1.2 m above the ground. Ignition by a match head with small amount of pyrotechnic material. Ignition 1.2 m above the ground. Ignition point is located 2-10 m from the release point. Pressure transducers pointed out upwards (except for wall mounted).

Transducers are located at axial distance 2.8 m from the nozzle, 1.5 m (then +1.1 m and +1.1 m) perpendicular to the axis, at height 0.5 m.

260 ms to fully open the valve, 140 ms for hydrogen to reach 2 m, i.e. 400 ms is shortest ignition delay.

Pressure effects of jet flames (1/5)


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Free jet fire: 9.5 mm, 800 ms, visible (16.5 kPa)



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Free jet fire: 9.5 mm, 800 ms, infrared 4.1-5.3 microns (16.5 kPa)



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Effect of orifice diameter on overpressure DP


Orifice diameter, mm Ignition delay, ms Max overpressure, kPa

1.5 800 Not recordable

1.5 400 Not recordable

3.2 800 3.5

3.2 400 2.1

6.4 800 15.2

6.4 400 2.7-3.7

9.5 800 16.5

9.5 400 3.3-5.4

Conclusion: reduce the release orifice diameter ALARP (as low as reasonably practicable) to reduce overpressure following ignition of jet.


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Effect of ignition location on overpressure DP (orifice D=6.4 mm, fixed ignition delay 800 ms.


Ignition position, m Max overpressure, kPa2 15.23 5.04 2.15 2.16 Not recordable8 Not recordable10 No ignition


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Barrier 90o: 9.5 mm, 800 ms (42 kPa). Free jet DP=16.5 kPa.

Pressure effects of jet flames: barriers (1/3)


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Barrier 60o: 9.5 mm, 800 ms (57 kPa). Free jet DP=16.5 kPa.



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Dynamics: release, ignition, deflagration, jet fire (free jet DP=16.5 kPa)


42 kPa 57 kPa


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Hazards: a small leak burns undetected for a long period, damaging the containment system and providing an ignition source for a subsequent large release.

Left: hydrogen flowing downward into air (mass flow rate 3.9 mg/s, power 0.46 W).

Right: hydrogen flowing downward into oxygen (2.1 mg/s, 0.25 W).

The tube inside/outside diameters are 0.15/0.30 mm. The exposure time 30 s.

SAE J2600 permits hydrogen leak rates below 200 mL/hr (0.46 mg/s) – no flame!

Microflames: hazards and SAE J2600 limit


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Tube burner is used. Quenching limits are

nearly independent of diameter.

Hydrogen has the lowest quenching limit and the highest blow-off limit (here it is compared to methane CH4, and propane C3H8).

Quenching limit for tube burner is 3.9 mg/s.

Microflames: quenching and blow-off


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Quenching diameter as a function of storage pressure for H2, CH4, C3H8.

Upstream pressure required for 5.6 g/s hydrogen (a bit above the quenching limit) isentropic choked flow is shown.

For hydrogen at 690 bar, any hole larger than 0.4 m will support a stable flame.

Microflames: quenching diameter

0.1

1

10

100

1 10 100 1000

Qu

ench

ing

Dia

met

er (

m)

Pressure (bar)

Methane

Propane

Hydrogen

)1(2

12

1

00 1

2

RT

MCAPm


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Quenching limits for a 6 mm compression fitting are shown.

Limits are independent of storage pressure.

Quenching limit for leaky fittings is 28 mg/s – about 10 times larger than for tube burner (3.9 mg/s).

Hydrogen limit is the lowest compared to CH4 and C3H8 (order of magnitude).

Microflames: leaky fittings

0

0.1

0.2

0.3

0.4

1 10 100 1000Pressure (bar)

Qu

ench

ing

Mas

s F

low

Rat

e (m

g/s

)

HydrogenMethanePropanehmpLinear (h)Linear (m)Linear (p)

0.028 mg/s

0.378 mg/s

0.336 mg/s


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Minimum quenching mass flow rate – H2

Minimum quenching volumetric flow rate – C3H8

Microflames: leaky fittings (quenching limits)


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There are three basic ways in which people exposed to hydrogen jet fires, may lead to incapacitation and death: hyperthermia, respiratory tract burns, and body surface burns (NFPA, 2002). Hyperthermia (heat stroke) involves prolonged exposure

(approximately 15 minutes or more) to heated environments at temperatures too low to cause burns.

Heat damage to the respiratory tract is more severe when the heated air contains steam and can cause damage deep down in the lung.

The time from the application of heat to the occurrence of body burns, of various degrees of severity, depends on the heat flux to which the skin is exposed.

Three ways how fire can incapacitate people


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x=2.LF – “death” limit (309oC, 20 s)

x=3.LF – pain limit (115oC, 5 min)

x=3.5.LF – “no harm” limit (70oC)

Three separation distances for jet fire


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Effect of heat radiation from flame on people

Radiant heat flux (kW/m2)

Effects on people

1.5 Safe for stationery personnel and members of the public2.5 Tolerable over 5 minutes; severe pain above this threshold3 Tolerable in infrequent emergency situations for 30 minutes5 Tolerable for performing emergency operations6 Tolerable to escaping personnel (evacuation)

9.5 Second degree burn after 20 seconds12.5-15 First degree burn after 10 seconds (1% fatality in 1 minute)

25 Significant injury in 10 seconds (100% fatality in 1 minute)35-37.5 1% fatality in 10 seconds

Effects of radiant heat flux on people (Lees, 1996; BS, 2004).


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Effect of heat radiation from flame on objects

Radiant heat flux (kW/m2)

Effects on structures and environment

5 Significant windows breakage

8–12 Domino effects

10 Heating of structures; increase of T and P in liquid/gas storages

10–12 Ignition of vegetation

16 Failure of structures in prolonged exposure (except concrete)

20 Concrete structures can withstand for several hours

30 Non-piloted ignition of wood occurs

38 Damages caused to process equipment and storage tanks

100 Steel weakening

200 Concrete structures to fail in several dozen of minutes


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High pressure electrolysers – internal flame


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400 bar titanium electrolyser (Japan) before and after the combustion in oxygen.

http://www.nsc.go.jp/senmon/shidai/kasai/kasai004/ssiryo4-1.pdf



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Titanium electrolyser materials (fluorine from the membrane) were dispersed into surroundings: car windshield before and after (few days) the accident.

This is only one of the knowledge gaps relevant to hydrogen fires!


funded by fch ju (grant agreement no. 256823) 1 © hyfacts project 2012/13 confidential – not for...

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