|
【范围】
|
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/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 Suppression 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 the
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 concentration) 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 light
as the criteria for determining whether a combustion reaction had taken place. More recent testing, including 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 limit 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 results
[2, 3, 4, 5]. The tests show that a visible reaction can occur at oxygen 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 larger 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, 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 applications; including
9 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 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 temperature (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 protection, 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).
A ballistic penetration can, in theory, increase the flammability of an ullage by creating a fuel spray [2, 6]; however the test
data 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 pressure rise
and 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 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 sufficient 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 History 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 explosion.
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 systems 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 stored 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 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-Enriched 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 P-8A airplanes all were designed with a continuous flow PM OBIGGS.
1.2.2 Commercial Applications
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 center 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 investigation of on-board and ground-based inerting and pack bay ventilation
[10].
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 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 military 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 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 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 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 systems 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 mandate 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 membrane 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 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 lower 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 contains 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 oxygen 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 (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 Leakage. 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.
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 potential fuel sources and that it is quickly dispersed into the ambient air.
Anaerobic Microbes. Anaerobic microbes are very corrosive. These microbes only multiply in the complete absence
of oxygen and, therefore, have not been a problem in any inerting systems fielded to this point.
Constant venting at altitude. A continuous flow of NEA into the fuel tanks during cruise creates a continuous flow of
ullage gas out of the fuel tank vents. This ullage gas may contain unburned hydrocarbons which may have ozone
depletion and/or greenhouse gas implications.
Connection of engine bleed source to the fuel tanks. Many OBIGGS use conditioned engine bleed air as a source of
pressurized air for the ASMs. The bleed air connection creates a new potential fuel tank heat source, especially
during failure modes, which must be addressed in the system design. Similarly, provisions must be made to prevent
fuel or fuel vapors from back-flowing into the system or air supply source.strRefField
|