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Ground-Based Inerting of Commercial Transport Aircraft Fuel Tanks. William M Cavage Lead Engineer - Fuel Tank Inerting FAA AAR-440, Fire Safety R&D Branch. North Atlantic Treaty Organization AVT Panel on Fire Safety and Survivability Aalborg, Denmark September 23-26, 2002. Outline. - PowerPoint PPT Presentation
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North Atlantic Treaty OrganizationAVT Panel on Fire Safety and Survivability
Aalborg, DenmarkSeptember 23-26, 2002
Ground-Based Inerting ofCommercial Transport Aircraft
Fuel Tanks
William M CavageLead Engineer - Fuel Tank Inerting
FAA AAR-440, Fire Safety R&D Branch
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
Outline
• Background• Previous Research• Preliminary Research• Experimental Equipment• Analysis and Modeling• Results• Summary
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
• Three Accidents in Recent History– All Accidents had Explosions in Center Wing-Tank
– Explosions Occurred During/Just After Long Ground Operations on Hot Days with Empty (Residual Fuel) Center Wing Tanks
– Exact Ignition Source Not Found During Any Investigation
• FAA/Industry Seeking More Cost-Effective Inerting Methodologies– GBI is Inerting on Ground and then Continuing to Operate
– ARAC Committee Stated Could be Cost-Effective if Focused on Center-Wing Tanks (CWTs) Only
Background
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
• Macdonald and Wyeth - Fire and Explosion Protection of Fuel Tank Ullage, Circa 1970s
– Compares Different Methods of Reducing Flammability including Nitrogen Inerting
• Klueg, McAdoo, and Neese - Performance of a DC-9 Aircraft Liquid Nitrogen Fuel Tank Inerting System, 1972
– FAA Developed and Tested an Inerting System Using Stored LN2
• Stewart and Starkman - Inerting Conditions for Aircraft Fuel Tanks, 1955
– Comprehensive Work on Flammability Limits and [O2] Requirements
• Kuchta - Oxygen Dilution Requirements for Inerting Aircraft Fuel Tanks, 1970
Previous Research - Methods
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
• Ullage Washing Experiments (Inerting)– Quantitatively Determined Amount of Nitrogen Enriched Air
(NEA) Required to Inert a Fuel Tank Test Article• Simple Rectangular Tank with Single Deposit Nozzle and Single Vent
• Examined Different NEA % (Residual O2 Concentration) and Flow Rates as well as Examined Effects of Fuel Vapor and Temperature
• Ullage Washing is Term Describing Inerting by Ventilation
– Developed Nondimensional Relationships, Empirical Equation, and a Theoretical “Perfect Mixing” Solution
– Data Illustrated that a VTE of 1.5 to 1.6 is Needed to Inert an Ullage to 8% Oxygen by Volume with 95% NEA (5% [O2])
– Published Report DOT/FAA/AR-01/6
Preliminary Research
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
0
5
10
15
20
25
0 0.5 1 1.5 2 2.5 3
Volumetric Tank Exchage
Tan
k O
xyg
en C
on
cen
trat
ion
(%
)
88% NEA
90% NEA
92% NEA
94% NEA
96% NEA
98% NEA
88 Cubic Ft Tank, 6 CFM Flow Rate
Rectangular Fuel Tank Inerting Data
VolumeTankFuel
RateFlowVolumeTimeExchangeTankVolumetric
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
• Fuel Effects on an Inert Ullage– Quantitatively Determined the Effect an Unscrubbed Fuel Load Can
Have on an Adjacent Inert Ullage• Simple Rectangular Tank with Single Deposit Nozzle and Single Vent
• Examined Fuel Loads of 20, 40, 60, and 80 Percent Full at Sea Level and Two Altitudes; Inerted to 6, 8, and 10 Percent
• Measured the Increase in Ullage [O2] due to Air Evolving from Fuel
– Circulated the Ullage Through the Fuel to Bring Tank to Equilibrium Quickly - Represents Maximum Fuel Effect on Ullage
– Study of Time Effects Showed Unless You Stimulate the Fuel, The Excess Air in Fuel had Small Effect on Ullage Compared to Air Entering Vent System (Fuel Burn)
– Report Pending Publication
Preliminary Research
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________Inert Ullage Fuel Effects Data
0
2
4
6
8
10
12
0 5000 10000 15000 20000 25000 30000 35000
Altitude (ft)
Oxy
gen
Co
nce
ntr
atio
n I
ncr
ease
(%
Vo
l) 20 % Fuel Load
40 % Fuel Load
60% fuel Load
80% Fuel Load
Ullage Recirculation at 3 Altitudes Inert to 8% Oxygen Concentration
Max Increase in Ullage [O2]Due to Adjacent Fuel Load
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
Preliminary Research• Lower Oxygen Content Study
– Performed Ignition Experiments with Model Fuel Tank in Pressure Chamber with both Propane and JP-8 at Reduced Oxygen Concentrations
• Tank Instrumented with Thermocouples and Sample Lines for Hydrocarbon and Oxygen Concentration Measurement.
• Heaters Placed Underneath the Tank Control the Liquid Fuel Temperature (Flammability).
• Used Single High Power Spark to Ensure a Reaction if Probable
• Study at Sea Level and Altitude to 38K Feet
– Follow on Tests Validated Previous Testing that the Critical Oxygen Concentration is Approximately 12% at Sea Level
– Report Pending Publication
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
0.0%
5.0%
10.0%
15.0%
20.0%
25.0%
0 10 20 30 40
Altitude (kft)
Oxy
gen
Co
nce
ntr
atio
n (
% V
olu
me)
Ignition
Non-Ignition
Lower Oxygen Content Study Data
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
• GBI Proof of Concept Ground/Flight Testing– Joint Project with Boeing that Evaluated the Concept of Ground-
Based Inerting
– Examined the Effects of Wind and Flight Conditions on Ullage Oxygen Concentration of a Model 737-700
• Tank Instrumented with Gas Sample Ports
• NEA Distribution Manifold Installed in Tank by Boeing
– Inerted Tank with Ground-Supplied NEA to Approximately 8%
– Measured Oxygen Concentration at 8 Locations in CWT During “Normal” Ground and Flight Conditions
• Examined Effects of Quiescent and Simulated Wind Conditions on Ground and Representative Flight Conditions in Air
Experimental Equipment/Procedures
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________Boeing 737-700 Test Article
1
2
3
4
5
6
7
8
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
• Boeing 747SP Test Article– Decommissioned from Airline Service and Purchased by the
FAA for Ground Testing Only• All Major Systems Fully Operational
• Has Independent Power for Test Equipment and Instrumentation
– Center-Wing Tank Fully Instrumented• Gas Sample Tubing for Oxygen and Total Hydrocarbon Analysis
• 32 Thermocouples in Tank (Ullage, Fuel, Walls, Floor, and Ceiling)
• Additional Thermocouples and User Specified Oxygen Analysis Channels
• Installed NEA Deposit Nozzle in Bay 3
– Inerted Tank with One Deposit Scheme Under a Variety of Conditions
Experimental Equipment/Procedures
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
747SP Center Wing Tank
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
• Scale Boeing 747SP Center Wing Tank – Quarter-Scale Model of Boeing 747SP CWT was Built from
Three Quarter Inch Plywood By Scaling Drawings from Shepherd Report (24% Length Scale)
• Variable Deposit Manifold Allowed for NEA to be Deposited at Any Rate in any Bay(s) Desired
• Measured Oxygen Concentration in Each of the 6 Bays
– Examined Various Distribution and Inert Gas Flow Scenarios• Varied Flow and Purity to Validate existing Nondimensional and
Empirical Relationships
• Examined Depositing at Different Locations with Different Venting Schemes
Experimental Equipment/Procedures
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
747SP Scale Center Wing Tank Model
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
Analysis• Volumetric Tank Exchange is the Ratio of the Volume of
Deposited Gas to the Volume of the Tank
• Average Tank Oxygen Concentration for 747SP
• Equation for Oxygen Concentration Given Inerting Efficiency k
VolumeTankFuel
RateFlowVolumeTimeExchangeTankVolumetric
]6[13.0]5[13.0]4[10.0
]3[10.0]2[23.0]1[31.][
222
2222
BayOBayOBayO
BayOBayOBayOOAverage
)]1)([()( *2222
VTEkeOOOtONEAAmbAmb
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
Inert Gas Distribution Engineering Model• Model Calculates Inert Gas Distribution in 6 Bay Tank,
in terms of Oxygen Concentration Evolution, Given NEA Purity and Bay Deposit Flow Rates– Based on Original Single Bay Inerting Model, by FAA CSTA for Fuel
Systems, which Tracks Oxygen In and Out of Each Bay Assuming Perfect Mixing During the Time Step
– Assumes an “Outward” Flow Pattern and Splits Flow into a Bay to Adjacent Bays Using Out Flow Area Relationships
• Basic Formula for Volume of Oxygen in a Bay:
Bay
OBay V
tVtO
)()]([ 2
2
BayOsumBaynnBayOnBayNEAOO VtVtQVtVtQIGOFtQtVtV /)1(/)1()1()(2222
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
Results - 737 Proof of Concept Flight Test• CWT Was Rendered Inert from External Source with
Few Problems – Slightly more NEA Required then Predicted
• Oxygen Concentration of the CWT Rose Very Little over Time Provided Cross-Venting was Eliminated
• CWT Stayed Inert During Flights with Small Fuel Loads and Provided Some Protection with Medium to High Fuel Loads
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
0
2
4
6
8
10
12
14
16
18
0 20 40 60 80 100 120 140 160
Time (minutes)
Oxy
gen
Co
nce
ntr
atio
n (
% v
ol)
Vent Modified for No X Flow
No Vent Modification
Climb and Cruise, 0 Fuel Takeoff at Time 0
737NG CWT Oxygen Concentration
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________
737NG CWT Oxygen Concentration Increase
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
0 20 40 60 80 100 120 140 160
Time (minutes)
Oxy
gen
Co
nce
ntr
atio
n C
han
ge
(% v
ol)
80% Fuel Load
40 % Fuel Load
20% Fuel Load
Zero Fuel
Climb and Cruise with No X VentingTakeoff at Time 0
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________Results - Full Scale GBI
• Single Deposit of Inert Gas in the Compartmentalized Tank Worked Well– Provided Efficient Distribution of Inert Gas with Fair Bay-Bay
Mixing
• Quiescent Inerting of CWT Could Be Problematic– Vertical Mixing Problems Caused Inefficient Inerting in Some
Cases
– More Work Needed to Determine if Vertical Mixing is Easily Stimulated
– Calculated and Compared Efficiency Factors
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________747SP Inerting Data with Single Deposit
0
5
10
15
20
25
0 5 10 15 20 25 30 35 40
Time (minutes)
Oxy
gen
Co
nce
ntr
atio
n (
% V
ol)
Bay 1 Bay 2 Bay 3
Bay 4 Bay 5 Bay 6
Vent
Boeing 747SP Inerting DataSingle Deposit; 140 CFM of 95% NEA
8% Line
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________747SP Inerting Data with Different Vertical Mixing
0
5
10
15
20
25
0 10 20 30 40 50 60 70 80 90 100
Time (mins)
Ave
rag
e O
xyg
en C
on
cen
trat
ion
(%
Vo
l) APU Off - Poor Mixing
APU Off - Fair Mixing
APU On - Good Mixing
Boeing 747SP Inerting DataMixing Comparison, 95% NEA, ~ 20 Min Inerting
Start Inerting
Stop Inerting
Start ACMs
Tank Mixing
Well Mixed
Well Mixed
Start ACMs Tank Mixing Well Mixed
8% Line
ACMs Running
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________747SP Calculated Inerting Curves with Different Vertical Mixing
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
VTE
Ave
rag
e O
xyg
en C
on
cen
trat
ion
(%
Vo
l) Expected Even Distribution
Quiescent Inerting - Bad Mixing
Quiescent Inerting - Fair Mixing
APU On - Good Mixing
Boeing 747SP Inerting DataCalculated Curves Using Full-Scale Data
8% Line
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________Results - Inert Gas Distribution Modeling
• Engineering Model Data Modeled Full-Scale Test Article Oxygen Concentration Trends Well but Magnitudes Off Somewhat
• Scale Tank Data Modeled Full-Scale Test Article Very Well– Bay 4 Results Skewed in All Methods
• CFD Data Modeled Fairly Well, with Some Changes to Model
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________Engineering Model Data Comparison
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Overall VTE
Oxy
gen
Co
nce
ntr
atio
n (
% v
ol)
Bay 1 Bay 1
Bay 2 Bay 2
Bay 3 Bay 3
Bay 4 Bay 4
Bay 5 Bay 5
Bay 6 Bay 6
747SP CWT Inerting, Single Bay DepositComparison with Engineering Model Data
747SP Data Engineering Model
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________Scale Plywood CWT Model Data Comparison
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Overall VTE
Oxy
gen
Co
nce
ntr
atio
n (
% v
ol)
Bay 1 Bay 1 Bay 2 Bay 2 Bay 3 Bay 3 Bay 4 Bay 4 Bay 5 Bay 5 Bay 6 Bay 6
747SP CWT Inerting, Single Bay DepositComparison with Scale Tank Data
747SP Data Scale Tank
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________CFD Model Data Comparison
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Overall VTE
Oxy
gen
Co
nce
ntr
atio
n (
% v
ol)
Bay 1 Bay 1 Bay 2 Bay 2 Bay 3 Bay 3 Bay 4 Bay 4 Bay 5 Bay 5 Bay 6 Bay 6
747SP CWT Inerting, Single Bay DepositComparison with CFD Data
747SP Data CFD Data
AAR-440 Fire Safety R&D
Ground-Based Inerting Research___________________________________Summary
• Inerting a Fuel Tank from a Ground Source of NEA is Simple and Practical Method of Providing Protection to Heated CWTs When Needed Most
• Limited Distribution of Inert Gas is Required to Inert a Compartmentalized Tank Provided Modeling is Performed and Vertical Mixing is Examined
• Modeling of Inert Gas Distribution in a Compartmentalized Tank Can be Achieved with a Limited Amount of Resources