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1Limiting Oxygen Concentration of Aviation Fuels Federal AviationAdministration 1

Limiting Oxygen Concentration of Aviation Fuels

Steve Summer Project Engineer Federal Aviation AdministrationFire Safety Branch

Federal AviationAdministration


Objectives

• Determine the Limiting Oxygen Concentration (LOC) of Jet Fuel at altitudes ranging from 0 – 40 kft.

• Compare these results with previously published literature.


Test Article


Test Article

• 353 ft3 Pressure Vessel• Working pressure of 650 psi• Attached Vacuum pump used to evacuate

chamber to reduced pressures seen at altitude

• Not capable of simulating temperatures seen at altitude


Test Article

• 9 ft3 simulated fuel tank placed inside of vessel equipped with:• Bottom surface heaters.• 12 thermocouples.• 2 piezoresistive pressure transducers mounted

behind sintered porous metal discs.• Interchangeable pressure relief mechanism.

• ¼-in. aluminum plate.• Foil diaphragm.

• Gas Sampling• Oxygen• Total Hydrocarbon (THC)


Test Article

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L eft Tes t Track

1 0 m P re ssu re Ves s e l

3 Ana lyze r Byp a ss

Sa m p le Line O A n aly ze r

2

H e aterH e ater

S p a rk S o u rce

F an

= T h e rm o co u p le F eed th ro u gh

N L in e s2

H o u se A ir L in e


Test Article

R ig h t T /C Tre eL e ft T /C T re e

L iq u id JP -8 F u e lF u e l P a n

= T h e rm o c o u p le B e a d6.

0"6.

0"


Test Parameters

• Mass Loading ~ 4.5 kg/m3 (wt of fuel/vol. of tank)• Tests conducted at or near stoichiometric levels• Ambient pressure corresponding to altitudes of 0, 10,

20, 30 and 38 kft• Ullage oxygen concentrations ranging from 21% to

below the determined LOC• Tests conducted with two different pressure relief

mechanisms• ¼-in. aluminum plate

• Ignition = movement of plate

• Foil diaphragm • Ignition = rupture of foil


Test Parameters

• Ignition Sources• 10 kV Oil burner transformer operating at ~30 mA

provided both long (~1 second) and short (~0.1 second) arcs

• J-57 Engine spark igniter provided a very short (~175 second) spark

• 3" x 6" x 1" metal block heated by two cartridge heaters to temperature in excess of 1400°F

• 400 cycle, 120 V hard short to ground provided high energy/current, short duration spark


Test Parameters – Measured Spark/Arc Energies

• Energy Measurements• Voltage and current traces were taken using a HV

and current probe at the spark gap connected to oscilloscope


Sample Oil Burner Transformer Arc


Sample Spark from 400 Cycle Short


Test Results – 1s Oil Burner Transformer, Foil Pressure Relief Mechanism


Test Results – Ignition Source Variance

• Little effect seen on sea level LOC due to ignition source

• Long duration arc: 12%

• High powered spark: 12.3%

• Short duration arc: 12.9%

• HSVI: 12.1

• 400-cycle short: 11.9%


Comparison with Previously Published Data

• LOC values of pure hydrocarbons though are readily available and their range is rather small, with only 3 falling outside of 11-12% O2

Ref. Kuchta (1986)

• Standardized test method for determination of LOC values of gases and vapors controlled by ASTM E 2079-00

• This test method is difficult to apply to jet fuels due to the complex nature of its composition and variance of composition from batch to batch



• In 1971, a literature search of experimental data pertaining to aircraft fuel tank inerting requirements was performed. (Report FAA-RD-72-112)

• Work dating back as far as 1946 was examined• Data was obtained by:

• Boeing Aircraft Company• Bureau of Mines• University of California• Wright Aernautical Development Center (WADC)• Convair Aircraft Company• Wright Patterson Air Force Base

• All but one arrived at the conclusion that an O2 concentration of 11-12% was sufficient to render an aircraft fuel tank inert



• WADC research resulted in an LOC of 9.8% at sea level• Data utilized flame propagation not pressure rise as the

ignition/non-ignition criteria• It is noted in their report that at times, flame propagation

occurred with little or no resulting pressure rise• This disparity in ignition criteria does not allow for direct

comparison to other data sets

• Bureau of Mines research suggested a safety factor of 20% be added on to their determined LOC of 12%• This appears to be the origin of the military’s use of a 9%

design target



• More recently, the Naval Weapons Center performed research looking at Nitrogen inerting effectiveness against 30-mm high explosive incendiary projectiles in 1991. (Report JTCG/AS-90-T-004)

• Their findings concluded that:

“…at oxygen concentrations of 12%, a large reduction in the overpressure resulting from a fuel-vapor explosion initiated by the

30-mm HEI was achieved. Oxygen concentrations of 9% were found to provide very little improvement…when compared to

oxygen concentrations of 12%.”


Conclusions

• LOC at 0 and 10 kft is 12% increasing linearly to approximately 14.5% at ~40 kft

• Little effect seen on the LOC at sea level due to ignition source

• Previous experimental data shows excellent agreement with current data set

• All reported levels that were lower than 11 – 12% O2 are attributable to either:• A difference in ignition criteria• Excessive safety factors added on to experimental values

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