Limiting Oxygen Concentration of Aviation Fuels

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Limiting Oxygen Concentration of Aviation Fuels
Steve Summer
Project Engineer
Federal Aviation AdministrationFire Safety
Branch
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Objectives

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

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Test Article
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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

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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)

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Test Article
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Test Article
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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

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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 1400F
• 400 cycle, 120 V hard short to ground provided
  high energy/current, short duration spark

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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

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Sample Oil Burner Transformer Arc
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Sample Spark from 400 Cycle Short
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Test Results 1s Oil Burner Transformer, Foil
Pressure Relief Mechanism
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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

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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)
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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

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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 militarys
  use of a 9 design target

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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 improvementwhen
  compared to oxygen concentrations of 12.

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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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