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4、Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.orgAIR1903 AEROSPACEINFORMATIONREPORTIssued 2008-07 Aircraft Inerting Systems RATIONALEAerospace Information Report (AIR) 1903 provides technical information and references for developing an airplane fuel tank inerting sy
5、stem. TABLE OF CONTENTS 1. SCOPE 21.1 Inert and Flammability Limits 21.1.1 Nitrogen Inerting 21.1.2 Halon Inerting 31.2 History of Inerting System Design. 31.2.1 Military Applications 31.2.2 Commercial Applications 31.3 Other Considerations 41.3.1 Secondary Benefits. 41.3.2 Potential Risks 42. REFER
6、ENCES 52.1 List of Terms and Abbreviations . 63. INERTING TECHNIQUES 73.1 On-Board Inert Gas Generating Systems (OBIGGS) . 73.2 Stored Nitrogen Systems 73.3 Halon Systems 74. SYSTEM DESIGNS 84.1 On-Board Inert Gas Generating Systems (OBIGGS) . 84.1.1 Permeable Membrane OBIGGS . 84.1.2 Pressure-Swing
7、 Adsorption OBIGGS . 94.2 Stored Nitrogen Systems 94.3 Halon Systems 95. PRIMARY DESIGN CONSIDERATIONS . 96. OTHER INERTING APPLICATIONS 136.1 Engine Compartments 136.2 Dry Bays 136.3 Cargo Compartments 137. NOTES 13FIGURE 1 PERMEABLE MEMBRANE AIR SEPARATION MODULE 8FIGURE 2 SCHEMATIC OF TYPICAL PM
8、OR PSA OBIGGS 8FIGURE 3 PRESSURE-SWING ADSORPTION AIR SEPARATION MODULE 10FIGURE 4 SCHEMATIC OF TYPICAL STORED LIQUID NITROGEN INERTING SYSTEM . 10FIGURE 5 SCHEMATIC OF TYPICAL STORED GASEOUS NITROGEN INERTING SYSTEM . 10FIGURE 6 SCHEMATIC OF TYPICAL HALON INERTING SYSTEM . 11SAE AIR1903 - 2 -1. SCO
9、PE An airplane fuel tank inerting system provides an inert atmosphere in a fuel tank to minimize explosive ignition of fuel vapor.This AIR deals with the three methods of fuel tank inerting systems currently used in operational aircraft: (1) on-board inert gas generation systems (OBIGGS), (2) liquid
10、/gaseous nitrogen systems and (3) Halon systems. The OBIGGS and nitrogen systems generally are designed to provide full-time fuel tank fire protection; the Halon systems generally are designed to provide only on-demand or combat-specific protection. This AIR does not treat the subject of Explosion S
11、uppression Foam (ESF) that has been used for fuel tank explosion protection on a number of military aircraft. ESF is a totally passive, full-time protection system with multiple and simultaneous hit capability up to 23 mm. The primary disadvantages of foam are weight, reduction of usable fuel, and t
12、he added maintenance complexity when the foam must be removed for tank maintenance or inspection. AIR4170A is an excellent reference for the use of ESF for fuel tank explosion protection 1. 1.1 Inert and Flammability Limits 1.1.1 Nitrogen Inerting The inert limit (also called the limiting oxygen con
13、centration) is the oxygen concentration below which there is not enough oxygen present in a fuel-air mixture to sustain combustion. The earliest tests to determine the inert limit used visible lightas the criteria for determining whether a combustion reaction had taken place. More recent testing, in
14、cluding that done by the FAA, has used pressure rise as the criteria for defining whether a hazardous combustion reaction occurred. The lower and upper flammability limits are the fuel vapor concentrations below and above which the mixture is too lean or too rich to sustain combustion. The inert lim
15、it for an inerting or flammability reduction system is specified by the military customer or the regulatory agency (e.g., FAA). Different tests to verify the inert limit have been performed over the years with fairly consistent results2, 3, 4, 5. The tests show that a visible reaction can occur at o
16、xygen concentrations as low as 9.8 percent at sea level, but that the concentration has to increase to about 12 percent at sea level to produce a hazardous pressure rise. When the tank oxygen concentration increases above 12 percent, the resulting pressure rise associated with combustion becomes lar
17、ger as more of the fuel is burned and the reaction becomes more complete. The tests also show that the inert limit increases at altitudes above sea level. Additionally, lower energy ignition sources require higher oxygen concentrations for ignition. While the inert limit test results are consistent,
18、 different customers have applied different safety factors in the top-level system requirements for different aircraft platforms. The U.S. Navy has applied a 9 percent oxygen concentration by volume inert limit at all altitudes. The U.S. Air Force has specified different inert limits for different a
19、pplications; including9 percent, 12 percent on ground, and the oxygen concentration vs. altitude curve for nitrogen inerting from the 1955 Stewart and Starkman report 2 that defines the inert limit as 9.8 percent at sea level increasing to 11.5 percent at 40 000 feet. The FAA defines the inert limit
20、 as 12 percent at sea level increasing linearly to 14.5 percent at 40 000 feet 5. Military inerting systems are typically sized to keep the oxygen concentrations below the inert limit throughout an entire mission, so the lower and upper flammability limits that vary with fuel composition and tempera
21、ture (typically associated with the fuel vapor content at an ambient oxygen concentration of 20.9 percent) are not relevant to a survivability analysis. Commercial systems, designed to FAA requirements to minimize (not preclude) exposure to flammable conditions as a secondary means of ignition prote
22、ction, take credit for periods when the fuel vapor content in the ullage is too lean or too rich to sustain combustion and allow periods when the tanks are not inert (for example, during some high rate descents). SAE AIR1903 - 3 -A ballistic penetration can, in theory, increase the flammability of a
23、n ullage by creating a fuel spray 2, 6; however the testdata show that there is no difference in the inert limit for the ballistic penetration of a fuel tank compared to a sufficiently-large internally-generated ignition source 2. A ballistic penetration of a full fuel tank can cause significant pre
24、ssure riseand accompanying structural damage due to hydrodynamic ram 7, even when the tank is inert. 1.1.2 Halon Inerting Halon affects combustion by displacing oxygen, but also interferes chemically with the combustion reaction. This interference is not completely understood, but elements of Halon
25、are believed to competitively react with the transient combustion products (free radicals) that are necessary for rapid and violent flame propagation. Higher energy ignition sources require higher concentrations of Halon to prevent explosion. A 6 percent by volume concentration of Halon is sufficien
26、t to protect a tank against explosion from an internally-generated spark; a 9 percent concentration is required to protect against 50-caliber Armor Piercing Incendiary (API) threats; and a 20 percent concentration is required to protect against 23-mm High Energy Incendiary (HEI) threats 8. 1.2 Histo
27、ry of Inerting System Design The earliest inerting systems were devised to protect military airplanes. Inerting (or other means of fuel tank protection) for commercial airplanes began to be considered in the late 1990s following the loss of Flight 800, which was caused by a center fuel tank explosio
28、n. 1.2.1 Military Applications Following World War II there were several proposed designs that used engine exhaust, separate combustion devices, and dry ice to produce inert gasses. Other proposed systems used reactors through which the ullage gasses were passed to remove oxygen. None of these syste
29、ms were ever operationally deployed 9. The B-57, F-86, and F-100 airplane designs used stored gaseous nitrogen systems to provide several minutes of inerting. These systems required servicing and did not have the capacity to keep the tanks inert during descent 9. The A-6 and F-16 airplanes use store
30、d Halon to provide on-demand inerting similar to the stored nitrogen systems on the B-57, F-86, and F-100 9. The XB-70, SR-71, and the C-5 airplanes use stored liquid nitrogen systems that can keep the fuel tanks inert continuously throughout the flight. The logistics and maintenance effort required
31、 to regularly service the liquid nitrogen are the main disadvantages to this approach 9. The CH-63 and AH-64 helicopters and the V-22 airplane were fielded with Pressure-Swing Adsorption (PSA) On-Board Inert Gas Generating Systems (OBIGGS). These systems generate a continuous supply of Nitrogen-Enri
32、ched Air (NEA) to the fuel tanks. The C-17 was originally designed with a PSA OBIGGS that generated more inert gas than needed during cruise and stored the surplus at high-pressure for descent. This PSA OBIGGS has been replaced by a continuous flow Permeable Membrane (PM) OBIGGS. The F-22, F-35, and
33、 P-8A airplanes all were designed with a continuous flow PM OBIGGS. 1.2.2 Commercial Applications In 1998, the FAA tasked a Fuel Tank Harmonization Working Group (FTHWG) Aviation Rule-making Advisory Committee (ARAC) to study potential fuel tank safety improvements in response to the Flight 800 cent
34、er tank explosion. The committee studied both on-board and ground based inerting systems, pack bay ventilation, explosion-suppressing foam, and higher flash-point fuels. None of the options studied were determined to be feasible for commercial use at the time, but the committee recommended further i
35、nvestigation of on-board and ground-based inerting and pack bay ventilation 10.SAE AIR1903 - 4 -In response, the FAA tasked a Fuel Tank Inerting Harmonization Working Group (FTIHWG) ARAC in 2001 to further investigate fuel tank inerting. The committee sized a range of both on-board and ground-based
36、systems to reduce center fuel tank flammability exposure to the wing tank level based on a 10 percent oxygen concentration inert limit. These options could not be demonstrated to be economically feasible, but the on-board flammability reduction systems were shown to have greater potential than a mil
37、itary style OBIGGS designed to keep the tanks inert during all possible flight conditions. Ground-based inerting systems were unattractive, because of the logistic impact of servicing each airplane with NEA before every flight and the airport infrastructure costs associated with distributing NEA to
38、each gate 11. In 2001, the FAA issued Special Federal Aviation Regulation (SFAR) 88 that mandated a special safety analysis of potential fuel tank ignition sources. The same rule-making docket also revised FAR 25.981 to limit fuel tank flammability exposure to that of conventional aluminum wing tank
39、 levels for new airplane designs 12. During this same time period, the FAA was conducting laboratory tests to confirm the required inert limit. That testing confirmed that the traditional military inert limits include a generous safety margin and that high-energy ignition sources will not result in
40、a significant tank pressure rise at 12 percent oxygen concentrations at sea level, even with stoichiometric mixtures 5. With off-stoichiometric mixtures, the oxygen concentrations must be higher than 12 percent before a significant pressure rise occurs. When the on-board flammability reduction syste
41、ms were re-sized based on a 12 percent oxygen inert limit, the smaller systems were less prohibitive. The FAA installed and demonstrated prototype center tank flammability reduction systems on a Boeing 747 13 and an Airbus A320 14. In 2005, the FAA issued a Notice of Proposed Rule Making (NPRM) to m
42、andate flammability reduction for the commercial airplane fleet 15. At the time of this writing, the comment period on this proposed rule has closed, but the regulatory decision is still in the review process. All commercial airplane flammability reduction systems proposed to date use permeable memb
43、rane air separation technology to generate a continuous flow of NEA at a sufficient rate to reduce the fleet-average flammability exposure for that airplane model to a level below that of a conventional aluminum wing tank. 1.3 Other Considerations The primary benefits of an inerting or flammability
44、reduction system are to reduce the likelihood of a fuel tank explosion and to prevent fire. There are other secondary benefits and potential risks that must be mitigated in the design. 1.3.1 Secondary Benefits Reduced water in the fuel. The NEA supplied to the tanks is filtered and very dry. The low
45、er average humidity in the ullage should result in less water condensation onto the inner tank surfaces. Reduced nozzle coking. Thermal stability at the combustion fuel nozzles in the engine may be improved, although the level of improvement is difficult to quantify. In equilibrium with air, fuel co
46、ntains a significant quantity of dissolved oxygen, and this oxygen plays a role in subsequent chemical reactions that produce insoluble deposits in fuel flow passages and coking in fuel nozzles. When nitrogen inerting is employed, much of the dissolved oxygen evolves from the fuel. Also, the low oxy
47、gen concentration in the ullage minimizes the oxygen that could be dissolved in the fuel from the ullage. 1.3.2 Potential Risks Asphyxiation during maintenance. Exposure to low oxygen content inert atmospheres can lead to asphyxiation of maintenance personnel, if tanks are not purged with normal air
48、 (20.9 percent oxygen content) before entry. Existing maintenance access procedures for the fuel tanks already require purging with air to health safe limits (minimum of 19.5 percent oxygen content per OSHA), but additional placards may be used to emphasize that procedures must be followed. NEA Leak
49、age. NEA leakage into a confined space can create a hazardous condition. Even if the NEA is routed outside of the pressurized cabin, maintenance access procedures for the fuel tanks and adjacent areas must consider the risk of NEA leakage. SAE AIR1903 - 5 - OEA Leakage. The exhaust or leakage of the Oxygen-Enriched Air (OEA) waste gas must be considered to ensure that it is not exhausted near any pot
