Ground Based Fuel Tank Inerting

1
The Effect of Fuel on an Inert Ullage in
Commercial Airplane Fuel Tanks
William CavageAAR-440 Fire Safety BranchWm. J.
Hughes Technical CenterFederal 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

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

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

5
Block Diagram of Experiment Configuration
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

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
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

9
Increase in O2 Over Time with Different
Stimulations
10
Resulting Maximum Increase in Ullage O2 due to
Fuel
11
Benefit of Inerting Through Fuel
Note Inerting through manifold required more 5
NEA for the same ullage volume
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

13
Change in O2 Increase due to Altitude
14
Resulting Max Increase in Ullage O2 due to
Altitude
Initial Oxygen Concentration 8 Stimulated with
Ullage Recirc
15
Comparison of Stimulation Methods at Altitude
16
Comparison of Lab Simulation with Flight Test Data
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