# Presentation: 316451

## 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

## Outline

- Background
- Test Article
- Test Methods
- Calculations
- Results
- Summary

## 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

## 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

## Block Diagram of Experiment Configuration

## 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

## 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

## 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

## Increase in [O2] Over Time with Different Stimulations

## Resulting Maximum Increase in Ullage [O2] due to Fuel

## Benefit of Inerting Through Fuel

- Note:  Inerting through manifold required
- more 5% NEA for the same ullage volume

## 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

## Change in [O2] Increase due to Altitude

## Resulting Max Increase in Ullage [O2] due to Altitude

- Initial Oxygen Concentration 8%
- Stimulated with Ullage Recirc

## Comparison of Stimulation Methods at Altitude

## Comparison of Lab Simulation with Flight Test Data

## Summary

- 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