1. The Effect of Fuel on an Inert Ullage in Commercial Airplane Fuel Tanks
William Cavage AAR-440 Fire Safety Branch Wm. J. Hughes Technical Center Federal Aviation Administration
International Systems Fire Protection Working Group Tropicana Casino and Resort Atlantic City, NJ November 1-2, 2005

2. Outline Background Test Article Test Methods Calculations Results
Summary
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3. Background
FAA developed a proof of concept inerting system to illustrate the feasibility of fuel tank inerting
FAA intends to make a rule requiring flammability control of some or all CWTS with an emphasis on inerting system technologies
The effect of adjacent fuel loads on an inert ullage has not been studied thoroughly
Air in fuel can evolve and spoil the inert atmosphere in the ullage
Military work indicates “fuel scrubbing” is necessary to prevent large increases in ullage oxygen concentrations with high fuel loads
Need to know what considerations need be made to account for adjacent fuel loads (more NEA required?)
Commercial airlines have no intention of scrubbing fuel
Fuel tanks effected by rule tend to have low fuel loads
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4. Test Article
Used a 3x3x2 ft rectangular tank made for fuel tank flammability and inerting research
Instrumentation panel installed to allow for gas samples, thermocouples, and inerting agent to pass through
Used lab oxygen analyer for sea level work and single channel altitude analyzer (similar to OBOAS) for altitude work
Could deposit nitrogen, air, or NEA into the tank depending upon the needs of the experiment
Had manifold installed in the bottom of tank to allow for gases to be passed through the fuel
Selector valve allowed for air, ullage gas, or NEA to be passed through the manifold to scrub fuel, revive fuel, or equalize the ullage gases with the fuel gases
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5. Block Diagram of Experiment Configuration
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6. Test Methods – 2 Primary Areas
Sea level testing focused on the change in [O2] due to fuel load when tank is brought to equilibrium
Looked at how to bring fuel/ullage to stable state
Quantified the change in oxygen concentration due to fuel load
Examined the benefit of inerting the ullage through the fuel (rudimentary scrubbing)
Altitude testing focused on quantifying the altitude effects for both equilibrium state and potentially what would be seen in a commercial transport fuel tank
Validated measured sea level changes and quantified altitude effects
Examined what stimulates oxygen evolution from fuel
Simulated two flight tests to determine modeling capability
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7. Calculations – Two Ways Ullage [O2] Increases
Tank air entry due to fuel consumption
Tanks normally vented to atmospheric pressure
Use inerting equation with fuel consumption is VTE and inerting gas is air (20.9% oxygen concentration)
Change in [O2] due to air evolving from fuel
Solve a series of equations that equalize the partial pressure of oxygen and nitrogen across the fuel given the Ostwald Coefficient
Mass of O2 in system is constant
and partial pressure of O2 in
Ullage and fuel equal at state 2
calculate mass of oxygen at state 2
given conditions at state 1
calculate partial pressure O2
with equation of state
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8. Results – Sea Level Testing
Stimulation Methods Studied
Best method by far was ullage recirculation which highlights the fact that “oxygen evolution” is a misnomer, the process is an exchange of gases to bring partial pressures of fuel/ullage gases to equilibrium
Resulting increase in oxygen concentration due to adjacent fuel (maximum increase) was measured/calculated
Calculations match measured numbers fairly well
The benefits of inerting through fuel (rudimentary scrubbing)
Illustrated some benefit by depositing inert gas at the bottom of a fuel tank, allowing the inert gas to displace some O2
Requires more inert gas per volume of ullage to inert in this manner, but still less inert gas then required to inert empty tank
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9. Increase in [O2] Over Time with Different Stimulations
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10. Resulting Maximum Increase in Ullage [O2] due to Fuel
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11. Benefit of Inerting Through Fuel
Note: Inerting through manifold required
more 5% NEA for the same ullage volume
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12. Results – Altitude Testing
Effect of altitude on ullage oxygen concentration quantified
After ullage is at equilibrium with fuel at sea level, increase in altitude (decrease in ullage pressure) causes partial pressure imbalance
Used ullage recirc at three altitudes and illustrated consistent results with poor agreement to calculations
Altitude stimulation work increase examined qualitatively
Besides ullage recirc, examined fuel pumping and altitude change only as potential methods of balancing the ullage/fuel gas partial pressures
Simulation of Boeing GBI flight tests compared fair
Simulation was a performed with no fuel/ullage stimulation (altitude only) and compared with GBI flight tests (not the descent portion)
Results illustrated qualitatively that altitude stimulation was closest studied to flight test data but more work is needed to optimize results
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13. Change in [O2] Increase due to Altitude
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14. Resulting Max Increase in Ullage [O2] due to Altitude
Initial Oxygen Concentration 8%
Stimulated with Ullage Recirc
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15. Comparison of Stimulation Methods at Altitude
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16. Comparison of Lab Simulation with Flight Test Data
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17. Summary
Oxygen evolving from fuel is a misnomer, changes in ullage oxygen concentration due to adjacent fuel are a result of the equalization of the partial pressures of gases at the fuel/ullage interface and is difficult to get without mixing fuel/ullage together
Measured sea level increases in ullage oxygen concentration match well with calculations
Some benefit can be garnered from remedial fuel scrubbing by inerting through fuel but more NEA / ullage volume is required
Changes in ullage altitude cause additional increases in ullage oxygen concentration from fuel with calculations agreeing poorly
Lab experiments can simulate flight test results with some accuracy with very little stimulation needed to match results
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