Inerting system

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.

Inerting gas systems in aircraft

Fuel tanks for combat aircraft have long been inerted, as well as self-sealing, 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.

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.

Other methods of inerting fuel tanks

Two other methods in current use to inert fuel tanks are a foam suppressant system and a ullage system. The FAA has decided that the added weight of both alternatives make them impractical for implementation in the aviation field [8]. Some US Military aircraft still use Nitrogen based foam inerting systems, and some companies will ship containers of fuel with an ullage system across train routes.

See also

• Dilution (equation)
• TWA Flight 800

References

1. ^ "The F-16 Halon Tank Inerting System" (PDF). http://pdf.aiaa.org/preview/1981/PV1981_1638.pdf. Retrieved November 17, 2005. 
2. ^ "US proposes fuel safety rule for commercial planes". Reuters. http://today.reuters.com/investing/financeArticle.aspx?type=governmentFilingsNews&storyID=URI:urn:newsml:reuters.com:20051114:MTFH44489_2005-11-14_23-19-12_N14432707:1. Retrieved November 16, 2005. 
3. ^ Isidore, Chris; Senior, /Money (September 14, 2005). "Delta and Northwest airlines both file for bankruptcy". CNN. http://money.cnn.com/2005/09/14/news/fortune500/bankruptcy_airlines/. Retrieved November 17, 2005. 
4. "FAA to Order Long-Delayed Fixes To Cut Airliner Fuel-Tank Danger", Wall Street Journal, November 15, 2005, page D5
5. "FAA Proposes Rule to Reduce Fuel Tank Explosion Risk (FAA press release)". http://www.faa.gov/news/press_releases/news_story.cfm?newsId=5785. Retrieved 18 January 2007. 
6. "FAA proposed flammability rule (PDF file)" (PDF). Archived from the original on September 27, 2006. http://web.archive.org/web/20060927083349/http://www.faa.gov/regulations_policies/rulemaking/historical_documents/2005/oct_dec/media/flammability_rule.pdf. Retrieved November 18, 2005. 
7. "The FAA is not wholly INERT on OBIGGS". http://www.iasa.com.au/obiggs.htm. Retrieved December 2, 2009. 
8. "Fuel Tank Inerting, Aviation Rulemaking Advisory Committee, 28 June 1998" (PDF). http://www.fire.tc.faa.gov/pdf/TG3.pdf. Retrieved December 2, 2009. 

External links

• Hollow Fiber Gas Separation