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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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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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