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Limiting Oxygen Concentration of Aviation Fuels

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

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Limiting Oxygen Concentration of Aviation Fuels

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  1. Federal Aviation Administration Limiting Oxygen Concentration of Aviation Fuels Steve Summer Project Engineer Federal Aviation AdministrationFire Safety Branch

  2. Objectives • Determine the Limiting Oxygen Concentration (LOC) of Jet Fuel at altitudes ranging from 0 – 40 kft. • Compare these results with previously published literature.

  3. Test Article

  4. 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

  5. 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)

  6. Test Article

  7. Test Article

  8. 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

  9. 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

  10. 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

  11. Sample Oil Burner Transformer Arc

  12. Sample Spark from 400 Cycle Short

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

  14. 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%

  15. 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)

  16. 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

  17. 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

  18. 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%.”

  19. 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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