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Federal Aviation Administration. Limiting Oxygen Concentration of Aviation Fuels. Steve Summer Project Engineer Federal Aviation Administration Fire Safety Branch. Objectives.

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Federal Aviation

Administration

Limiting Oxygen Concentration of Aviation Fuels

Steve Summer Project Engineer Federal Aviation AdministrationFire Safety Branch


Objectives
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 article1
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 article2
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 parameters
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 parameters1
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 msecond) 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
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





Test results ignition source variance
Test Results – Ignition Source Variance Relief Mechanism

  • 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
Comparison with Previously Published Data Relief Mechanism

  • 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

  • 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)


Comparison with previously published data1
Comparison with Previously Published Data Relief Mechanism

  • 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


Comparison with previously published data2
Comparison with Previously Published Data Relief Mechanism

  • 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


Comparison with previously published data3
Comparison with Previously Published Data Relief Mechanism

  • 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
Conclusions Relief Mechanism

  • 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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