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.

The Effect of Fuel on an Inert Ullage in Commercial Airplane Fuel Tanks

William CavageAAR-440 Fire Safety Branch

Wm. J. Hughes Technical CenterFederal Aviation Administration

International Systems Fire Protection Working Group

Tropicana Casino and Resort Atlantic City, NJ

November 1-2, 2005

AAR-440 Fire Safety R&D

Fuel Effects on an Inert Ullage____________________________________

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



Altitude Chamber

Air

Cle

aner

OxygenAnalyzer

DAS

Nitr

ogen

NEA Generator

Com

pute

r

T

Altitude O2

Analyzer

PressureTransducer

Sample Return

Fuel TankTest Article

ASM

Gas Sample

T

T

T

CompressedAir

SelectorValve

FuelPump

SelectorValve

Ull

age

Pum

p

Gas Sample/Pressure LineInstrumentation Wire

LegendNEA/Air DepositFuel Line

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)

)]1)(9.20[()( %222

FLStartStart eOOtO

• 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

22 FU OO PP )1(

)(11

2

OUF

FU

OUF

FUFOUO

U

CTV

TVCTV

TVmm

Om

U

OUUO

U V

RTm

OP2

2

Mass of O2 in system is constantand 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

Static

UO

P

PO 2][ 2

1122 FUFU OOOO mmmm



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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80 90 100

Time (Minutes)

Oxy

gen

Co

nce

ntr

atio

n I

ncr

ease

(%

Vo

l) Ullage Recirculation

Side-to-Side Slosh

Radiant Under Heat

40% Fuel Load, Inert to 8%: Different Stimulation Methods


Fuel Effects on an Inert Ullage____________________________________Resulting Maximum Increase in Ullage [O2] due to Fuel

0%

2%

4%

6%

8%

10%

12%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Fuel Load (% Full)

Oxy

gen

Co

nce

ntr

atio

n I

ncr

ease

(%

Vo

l)

6% Inert Inert to 6%



Comparison of Test Data with O2 Mass Balance Calculationsat Sea Level

Calculations Measured



Benefit of Inerting Through Fuel

-1

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90

Fuel Load (% full)

Oxy

gen

Co

nce

ntr

atio

n I

ncr

ease

(%

vo

l) Inerting Through Manifold

Standard Ullage Washing

Total Increase in Ullage Oxygen Concentration from 8% at Sea Level

Note: Inerting through manifold requiredmore 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

0

2

4

6

8

10

12

14

0 50 100 150 200 250

Time (mins)

Oxy

gen

Co

nce

ntr

atio

n (

% v

ol)

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

Pre

ssu

re A

ltit

ud

e (f

t)

Oxygen Concentration

Pressure Altitude

Ullage Recirculation at 3 Altitudes

60% Fuel Load



Resulting Max Increase in Ullage [O2] due to Altitude

0

2

4

6

8

10

12

14

16

0 5000 10000 15000 20000 25000 30000 35000

Altitude (ft)

Oxy

gen

Co

nce

ntr

atio

n I

ncr

ease

(%

vo

l)

20 % Fuel 20% Fuel

40 % Fuel 40% Fuel

60% Fuel 60% Fuel

80% Fuel 80% Fuel

Measured Calculated

Comparison of Test Data with O2 Mass Balance Calculations at Sea Level

Initial Oxygen Concentration 8%Stimulated with Ullage Recirc



Comparison of Stimulation Methods at Altitude

0

2

4

6

8

10

12

0 50 100 150 200 250

Time (mins)

Ox

yg

en

Co

nc

en

tra

tio

n (

% v

ol)

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

Alt

itu

de

(ft

)

Ullage Recirc Fuel Pump Quiescent Pressure Altitude

40% Fuel Load



Comparison of Lab Simulation with Flight Test Data

0

5

10

15

20

25

0 20 40 60 80 100 120 140 160

Test Time (mins)

Oxy

gen

Co

nc

en

tra

tio

n (

%V

ol)

40% Fuel Lab Test 40% Fuel Boeing Test

80% Fuel Lab Test 80% Fuel Boeing Test

Flight and Lab Test Results Comparedwith Calculation Bands

Boeing 80% Fuel Test CalculatedBand with Air Evolution

Boeing 40% Fuel TestCalculated Band with Air Evolution

Note: Calculation Band Represents Maximum and Minimum Resulting Oxygen Concentration Given 0% and 100% Air Evolution From Fuel



• 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

Summary

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.

Documents

fuel effects

fuel scrubbing

effect of fuel

fuel tank flammability

fuel gases slide

inert ullage

high fuel loads

fuel consumption tanks