Inerting system
Encyclopedia
An inerting system increases the safety of a fuel tank
, ball mill
, or other sealed or closed-in tank that contains highly flammable material. Inert in scientific terminology means ‘not readily reactive with other elements; forming no chemical compounds or something that is not chemically reactive.’ An inert fuel tank is non-combustible. The inerted space may be on land, or aboard ship, or airborne.
A fire requires three elements
: heat (ignition source), fuel and oxygen (or air) to initiate and sustain. A fire can be prevented by removing any one of the three elements. If presence of an ignition source can not be prevented in a fuel tank then a fuel tank can be made inert by (1) reducing the oxygen content of the ullage
(space above the fuel that contains air and fuel vapors) below the threshold required for combustion, or (2) by reducing the air-fuel ratio
of the ullage below the minimum threshold (Lower Flammability Limit) required for combustion, or (3) increasing the fuel air ratio above the maximum threshold (Upper Flammability Limit) that can support combustion.
At present, fuel tanks are rendered inert by adulterating the ullage with an inert gas
such as nitrogen
, nitrogen enriched air, steam or carbon dioxide
. This reduces the oxygen content of the ullage below combustion threshold. Without sufficient oxygen in the tank, the fuel vapors in the ullage cannot ignite, and an explosion does not occur. Alternate methods based on reducing the ullage fuel air ratio below Lower flammable limit
(LFL) or increasing the fuel air ratio above the Upper flammable limit (UFL) have also been proposed.
, but those for transport planes, both military and civil, have not, due to considerations of cost and weight.
Cleve Kimmel first pitched an inerting system to passenger airlines in the early 1960s. His proposed system for passenger aircraft would have used nitrogen. However, the Federal Aviation Administration
refused to consider Kimmel's system after the airlines complained it was impractical. Indeed, early versions of Kimmel's system weighed 2,000 pounds—which would have probably made an aircraft too heavy to fly with passengers on it. However, the FAA did almost no research into making fuel tanks inert for 40 years, even in the face of several catastrophic fuel tank explosions. Instead, the FAA focused on keeping ignition sources out of the fuel tanks.
The FAA did not consider lightweight inerting systems for commercial jets until the 1996 crash of TWA Flight 800
. The crash was blamed on an explosion in the center wing fuel tank of the Boeing 747
used in the flight. This tank is normally used only on very long flights, and little fuel was present in the tank at the time of the explosion. A small amount of fuel in a tank is more critical than a large amount, since heat entering the fuel tank with residual fuel causes the fuel to increase in temperature faster and evaporate. This causes the ullage fuel air ratio to increase rapidly and the ullage fuel air ratio to exceed the lower flammability limit. Large quantity of fuel (high mass loading) in the fuel tank retains the heat energy and slows down the fuel evaporation rate. Explosion of Thai Airways International
Boeing 737
in 2001 and Philippine Airlines 737 in 1990
also occurred in a tank that had residual fuel. All the above three explosions occurred on a warm day, in the Center Wing tank (CWT) that is within the contours of the fuselage. These fuel tanks are located in the vicinity of external equipment that heats the fuel tanks. The National Transportation Safety Board's (NTSB) final report on the crash of TWA 747 concluded “The fuel air vapor in the ullage of the TWA flight 800 CWT was flammable at the time of the accident.” NTSB identified “Elimination of Explosive Mixture in Fuel tanks in Transport Category Aircraft” as Number 1 item on its Most Wanted List in 1997.
After the Flight 800 crash, a 2001 report by an FAA committee stated that U.S. airlines would have to spend US$35 billion to retrofit their existing aircraft fleets with inerting systems that might prevent future such explosions. However, another FAA group developed a nitrogen enriched air (NEA) based inerting system prototype that operated on compressed air supplied by the aircraft’s propulsive engines. Also, the FAA determined that the fuel tank could be rendered inert by reducing the ullage oxygen concentration to 12% rather than previously accepted threshold of 9-10%. Boeing commenced testing a derivative system of their own, performing successful test flights in 2003 with several 747 aircraft.
The new, simplified inerting system was originally suggested to the FAA through public comment. It uses a hollow fiber membrane material that separates supplied air
into nitrogen-enriched air (NEA) and oxygen enriched air (OEA).[1] This technology is extensively used for generating oxygen-enriched air
for medical purposes. It uses a membrane that preferentially allows the nitrogen molecule (molecular weight 28) to pass through it and not the oxygen molecule (molecular weight 32).
Unlike the inerting systems on military aircraft, this inerting system would run continuously to reduce fuel vapor flammability whenever the aircraft's engines are running; and its goal is to reduce oxygen content within the fuel tank to 12%, lower than normal atmospheric oxygen content of 21%, but higher than that of inerted military aircraft fuel tanks, which is a target of 9% oxygen. This is accomplished by ventilating fuel vapor laden ullage gas out of the tank and into the atmosphere.
The FAA issued the final rule on July 21, 2008. The rule amends regulations applicable to the design of new airplanes (14CFR§25.981), and introduces new regulations for continued safety (14CFR§26.31-39), Operating Requirements for Domestic Operations (14CFR§121.117) and Operating Requirements for Foreign Air Carriers (14CFR§129.117). The regulations apply to airplanes certificated after January 1, 1958 of passenger capacity of 30 or more or payload capacity of greater than 7500 pounds. The regulations are performance based and do not require the implementation of a particular method.
The proposed rule would affect all future fixed-wing aircraft designs (passenger capacity greater than 30) , and require a retrofit of more than 3,200 Airbus and Boeing aircraft with center wing fuel tanks, over nine years. The FAA had initially planned to also order installation on cargo aircraft, but this was removed from the order by the Bush administration. Additionally, regional jets and smaller commuter planes would not be subject to the rule, because the FAA does not consider them at high risk for a fuel-tank explosion.
The FAA estimated the cost of the program at US$808 million over the next 49 years, including US$313 million to retrofit the existing fleet. It compared this cost to an estimated US$1.2 billion "cost to society" from a large airliner exploding in mid-air. The proposed rule comes at a time when nearly half of the U.S. airlines' capacity is on carriers that are in bankruptcy.[2]
The order affects aircraft whose air conditioning units have a possibility of heating up the center wing fuel tank. Some Airbus A320 and Boeing 747 aircraft are slated for "early action". Regarding new aircraft designs, the Airbus A380 does not have a center wing fuel tank and is therefore exempt, and the Boeing 787 has a fuel tank safety system that already complies with the proposed rule.
The FAA has stated that there have been four fuel tank explosions in the previous 16 years—two on the ground, and two in the air—and that based on this statistic and on the FAA's estimate that one such explosion would happen every 60 million hours of flight time, about 9 such explosions will probably occur in the next 50 years. The inerting systems will probably prevent 8 of those 9 probable explosions, the FAA said.
Before the inerting system rule was proposed, Boeing stated that it would install its own inerting system on airliners it manufactures beginning in 2005. Airbus had argued that its planes' electrical wiring made the inerting system an unnecessary expense.
As of December 2, 2009, the FAA has a pending rule to increase the standards of on board inerting systems again. New technologies are being developed by others to provide fuel tank inerting:
(1) The OBIGGS system, tested in 2004 by the FAA and NASA, with an opinion written by the FAA in 2005 [7]. This system is currently in use by many military aircraft types, including the C-17. This system provides the level of safety that the proposed increase in standards by the proposed FAA rules has been written around. Critics of this system cite the high maintenance cost reported by the military.
(2) Three independent research and development firms have proposed new technologies in response to Research & Development grants by the FAA and SBA. The focus of these grants is to develop a system that is superior to OBIGGS that can replace classic inerting methods. None of these approaches has been validated in the general scientific community, nor have these efforts produced commercially available products. All the firms have issued press releases or given non-peer reviewed talks.
Pressure vessel
A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...
, ball mill
Ball mill
A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...
, or other sealed or closed-in tank that contains highly flammable material. Inert in scientific terminology means ‘not readily reactive with other elements; forming no chemical compounds or something that is not chemically reactive.’ An inert fuel tank is non-combustible. The inerted space may be on land, or aboard ship, or airborne.
A fire requires three elements
Fire triangle
The fire triangle or combustion triangle is a simple model for understanding the ingredients necessary for most fires.The triangle illustrates a fire requires three elements: heat, fuel, and an oxidizing agent . The fire is prevented or extinguished by removing any one of them...
: heat (ignition source), fuel and oxygen (or air) to initiate and sustain. A fire can be prevented by removing any one of the three elements. If presence of an ignition source can not be prevented in a fuel tank then a fuel tank can be made inert by (1) reducing the oxygen content of the ullage
Ullage
Ullage or Headspace refers to the unfilled space in a container, particularly with a liquid.-Etymology:The word comes ultimately from the Latin oculus, “eye”, which was used in a figurative sense by the Romans for the bung hole of a barrel...
(space above the fuel that contains air and fuel vapors) below the threshold required for combustion, or (2) by reducing the air-fuel ratio
Air-fuel ratio
Air–fuel ratio is the mass ratio of air to fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich...
of the ullage below the minimum threshold (Lower Flammability Limit) required for combustion, or (3) increasing the fuel air ratio above the maximum threshold (Upper Flammability Limit) that can support combustion.
At present, fuel tanks are rendered inert by adulterating the ullage with an inert gas
Inert gas
An inert gas is a non-reactive gas used during chemical synthesis, chemical analysis, or preservation of reactive materials. Inert gases are selected for specific settings for which they are functionally inert since the cost of the gas and the cost of purifying the gas are usually a consideration...
such as nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, nitrogen enriched air, steam or carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. This reduces the oxygen content of the ullage below combustion threshold. Without sufficient oxygen in the tank, the fuel vapors in the ullage cannot ignite, and an explosion does not occur. Alternate methods based on reducing the ullage fuel air ratio below Lower flammable limit
Lower flammable limit
Lower flammability limit , usually expressed in volume per cent, is the lower end of the concentration range of a flammable solvent at a given temperature and pressure for which air/vapor mixtures can ignite. The flammability range is delineated by the upper and lower flammability limit. Outside...
(LFL) or increasing the fuel air ratio above the Upper flammable limit (UFL) have also been proposed.
Inerting gas systems in aircraft
Fuel tanks for combat aircraft have long been inerted, as well as self-sealingSelf-sealing fuel tank
In aviation, self-sealing fuel tank is a fuel tank technology in wide use since World War II that prevents fuel tanks primarily on aircraft from leaking fuel and igniting after being damaged by enemy fire....
, but those for transport planes, both military and civil, have not, due to considerations of cost and weight.
Cleve Kimmel first pitched an inerting system to passenger airlines in the early 1960s. His proposed system for passenger aircraft would have used nitrogen. However, the Federal Aviation Administration
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...
refused to consider Kimmel's system after the airlines complained it was impractical. Indeed, early versions of Kimmel's system weighed 2,000 pounds—which would have probably made an aircraft too heavy to fly with passengers on it. However, the FAA did almost no research into making fuel tanks inert for 40 years, even in the face of several catastrophic fuel tank explosions. Instead, the FAA focused on keeping ignition sources out of the fuel tanks.
The FAA did not consider lightweight inerting systems for commercial jets until the 1996 crash of TWA Flight 800
TWA Flight 800
Trans World Airlines Flight 800 , a Boeing 747-131, exploded and crashed into the Atlantic Ocean near East Moriches, New York, on July 17, 1996, at about 20:31 EDT, 12 minutes after takeoff, killing all 230 persons on board. At the time, it was the second-deadliest U.S...
. The crash was blamed on an explosion in the center wing fuel tank of the Boeing 747
Boeing 747
The Boeing 747 is a wide-body commercial airliner and cargo transport, often referred to by its original nickname, Jumbo Jet, or Queen of the Skies. It is among the world's most recognizable aircraft, and was the first wide-body ever produced...
used in the flight. This tank is normally used only on very long flights, and little fuel was present in the tank at the time of the explosion. A small amount of fuel in a tank is more critical than a large amount, since heat entering the fuel tank with residual fuel causes the fuel to increase in temperature faster and evaporate. This causes the ullage fuel air ratio to increase rapidly and the ullage fuel air ratio to exceed the lower flammability limit. Large quantity of fuel (high mass loading) in the fuel tank retains the heat energy and slows down the fuel evaporation rate. Explosion of Thai Airways International
Thai Airways International
Thai Airways International Public Company Limited is the national flag carrier and largest airline of Thailand. Formed in 1988, the airline's headquarters are located in Chatuchak District, Bangkok, and operates out of Suvarnabhumi Airport. Thai is a founding member of the Star Alliance. Thai is a...
Boeing 737
Boeing 737
The Boeing 737 is a short- to medium-range, twin-engine narrow-body jet airliner. Originally developed as a shorter, lower-cost twin-engine airliner derived from Boeing's 707 and 727, the 737 has developed into a family of nine passenger models with a capacity of 85 to 215 passengers...
in 2001 and Philippine Airlines 737 in 1990
Philippine Airlines Flight 143
Philippine Airlines Flight 143 was the route designator of a domestic flight from the Manila Ninoy Aquino Airport, Metro Manila, Philippines to Mandurriao Airport, Iloilo City...
also occurred in a tank that had residual fuel. All the above three explosions occurred on a warm day, in the Center Wing tank (CWT) that is within the contours of the fuselage. These fuel tanks are located in the vicinity of external equipment that heats the fuel tanks. The National Transportation Safety Board's (NTSB) final report on the crash of TWA 747 concluded “The fuel air vapor in the ullage of the TWA flight 800 CWT was flammable at the time of the accident.” NTSB identified “Elimination of Explosive Mixture in Fuel tanks in Transport Category Aircraft” as Number 1 item on its Most Wanted List in 1997.
After the Flight 800 crash, a 2001 report by an FAA committee stated that U.S. airlines would have to spend US$35 billion to retrofit their existing aircraft fleets with inerting systems that might prevent future such explosions. However, another FAA group developed a nitrogen enriched air (NEA) based inerting system prototype that operated on compressed air supplied by the aircraft’s propulsive engines. Also, the FAA determined that the fuel tank could be rendered inert by reducing the ullage oxygen concentration to 12% rather than previously accepted threshold of 9-10%. Boeing commenced testing a derivative system of their own, performing successful test flights in 2003 with several 747 aircraft.
The new, simplified inerting system was originally suggested to the FAA through public comment. It uses a hollow fiber membrane material that separates supplied air
Gas separation
Gas mixtures can be effectively separated by synthetic membranes. For other methods see adsorption, absorption, cryogenic distillation.Membranes are employed in:* separation of nitrogen or oxygen out of air...
into nitrogen-enriched air (NEA) and oxygen enriched air (OEA).[1] This technology is extensively used for generating oxygen-enriched air
Oxygen concentrator
An oxygen concentrator is a device providing oxygen therapy to a patient at minimally to substantially higher concentrations than available in ambient air. They are used as a safer, less expensive, and more convenient alternative to tanks of compressed oxygen. Common models retail at around US$800...
for medical purposes. It uses a membrane that preferentially allows the nitrogen molecule (molecular weight 28) to pass through it and not the oxygen molecule (molecular weight 32).
Unlike the inerting systems on military aircraft, this inerting system would run continuously to reduce fuel vapor flammability whenever the aircraft's engines are running; and its goal is to reduce oxygen content within the fuel tank to 12%, lower than normal atmospheric oxygen content of 21%, but higher than that of inerted military aircraft fuel tanks, which is a target of 9% oxygen. This is accomplished by ventilating fuel vapor laden ullage gas out of the tank and into the atmosphere.
Current FAA Rules on inerting in aircraft
After what it said was seven years of investigation, the FAA proposed a rule in November 2005, in response to an NTSB recommendation, which would require airlines to "reduce the flammability levels of fuel tank vapors on the ground and in the air". This was a shift from the previous 40 years of policy in which the FAA focused only on reducing possible sources of ignition of fuel tank vapors.The FAA issued the final rule on July 21, 2008. The rule amends regulations applicable to the design of new airplanes (14CFR§25.981), and introduces new regulations for continued safety (14CFR§26.31-39), Operating Requirements for Domestic Operations (14CFR§121.117) and Operating Requirements for Foreign Air Carriers (14CFR§129.117). The regulations apply to airplanes certificated after January 1, 1958 of passenger capacity of 30 or more or payload capacity of greater than 7500 pounds. The regulations are performance based and do not require the implementation of a particular method.
The proposed rule would affect all future fixed-wing aircraft designs (passenger capacity greater than 30) , and require a retrofit of more than 3,200 Airbus and Boeing aircraft with center wing fuel tanks, over nine years. The FAA had initially planned to also order installation on cargo aircraft, but this was removed from the order by the Bush administration. Additionally, regional jets and smaller commuter planes would not be subject to the rule, because the FAA does not consider them at high risk for a fuel-tank explosion.
The FAA estimated the cost of the program at US$808 million over the next 49 years, including US$313 million to retrofit the existing fleet. It compared this cost to an estimated US$1.2 billion "cost to society" from a large airliner exploding in mid-air. The proposed rule comes at a time when nearly half of the U.S. airlines' capacity is on carriers that are in bankruptcy.[2]
The order affects aircraft whose air conditioning units have a possibility of heating up the center wing fuel tank. Some Airbus A320 and Boeing 747 aircraft are slated for "early action". Regarding new aircraft designs, the Airbus A380 does not have a center wing fuel tank and is therefore exempt, and the Boeing 787 has a fuel tank safety system that already complies with the proposed rule.
The FAA has stated that there have been four fuel tank explosions in the previous 16 years—two on the ground, and two in the air—and that based on this statistic and on the FAA's estimate that one such explosion would happen every 60 million hours of flight time, about 9 such explosions will probably occur in the next 50 years. The inerting systems will probably prevent 8 of those 9 probable explosions, the FAA said.
Before the inerting system rule was proposed, Boeing stated that it would install its own inerting system on airliners it manufactures beginning in 2005. Airbus had argued that its planes' electrical wiring made the inerting system an unnecessary expense.
As of December 2, 2009, the FAA has a pending rule to increase the standards of on board inerting systems again. New technologies are being developed by others to provide fuel tank inerting:
(1) The OBIGGS system, tested in 2004 by the FAA and NASA, with an opinion written by the FAA in 2005 [7]. This system is currently in use by many military aircraft types, including the C-17. This system provides the level of safety that the proposed increase in standards by the proposed FAA rules has been written around. Critics of this system cite the high maintenance cost reported by the military.
(2) Three independent research and development firms have proposed new technologies in response to Research & Development grants by the FAA and SBA. The focus of these grants is to develop a system that is superior to OBIGGS that can replace classic inerting methods. None of these approaches has been validated in the general scientific community, nor have these efforts produced commercially available products. All the firms have issued press releases or given non-peer reviewed talks.