Eliminating explosion risks
Nitrogen generation technology helps airlines comply with new safety rules
A TWA Boeing 747 exploded over the Atlantic, near Long Island in the US, killing all 230 passengers on Wednesday 17 July 1996. According to the American NTSB (National Transportation Safety Board) official report, the explosion was likely to have been caused by the ignition of fuel damps in the central fuel tank, probably the result of a spark from a short circuit in the wiring.
Following the announcement of the cause of the incident, the American Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA) introduced stringent new civil aviation regulations.
With a few exceptions, new passenger and cargo aircraft with heated centre wing tanks (HCWTS) that are manufactured from 2009 to 2017, will be required to incorporate a flammability reduction means (FRM), such as a fuel tank inerting system, or ignition mitigation means (IMM), such as polyurethane foam. An estimated 2,732 passenger aircraft manufactured before 2009 will need to have such systems retrofitted.
To achieve compliance with this new ruling, aircraft manufacturers are investing in the development of on-board inert gas generator systems (OBIGGS), which prevent the ignition of fuel tank vapours in aircraft by inerting the tank with nitrogen enriched air.
For a fire to start, three components are needed: fuel, an ignition and oxygen. Taking one of these components out of 'the fire triangle' makes it impossible for a fire to break out.
As fuel is essential for flight, and the possibility of an ignition source occurring has been reduced to a level that cannot feasibly be improved further, the one component left is oxygen. By continually pumping nitrogen enriched, and therefore low oxygen, air into the plane's fuel tank, the fuel vapours in the tank are no longer flammable, eliminating the risk of explosion.
The leading manufacturers of filtration and separation technology are now investing significant resources into the development of OBIGGS systems. For example, Parker Hannifin has a dedicated aerospace team at its Etten-Leur manufacturing facility in the Netherlands working solely on the project in collaboration with its Aerospace Division.
Parker's OBIGGS system is built upon its proven membrane technology already being used in the nitrogen generation systems in applications ranging from preventing the oxidisation of food and beverage products to the blanketing of storage tanks in the oil and gas industry.
This membrane technology is based on specially developed and patented hollow fibres, which allow the oxygen molecules in a compressed air stream to permeate through their walls but retain the nitrogen molecules. The fibres are bundled together in their thousands and enclosed inside a tube. When a compressed air stream is fed into one end of the tube, water vapour, helium and oxygen diffuse quickly through the membrane walls, producing nitrogen enriched air from the other end of the tube.
Recent advances in membrane design and production have led to the development of nitrogen generation systems that are considerably more effective and reliable than previous models. With improved performance per fibre, fewer membranes are required, reducing the weight, size and cost of the systems.
Similarly, more efficient membrane technology means that lower air pressure is required, reducing energy consumption, while robustness of the membranes extends service life and minimises maintenance requirements.
All of these improvements have contributed to making the technology increasingly viable, both financially and logistically, for use on-board aircraft, as well as in many other fire prevention applications.
In December 2006, aircraft designer Airbus signed a $500m contract with Parker to supply them with an OBIGGS system in compliance with the new FAA and EASA rulings. The fire prevention system uses the air separation membranes manufactured by Parker at Etten-Leur to produce the nitrogen enriched, oxygen depleted air that will be pumped continuously into the aeroplane fuel tanks to prevent a fire.
Since 2006, Parker's manufacturing facility has been operating five shifts instead of three to deliver the production levels needed to meet the needs of Airbus. Parker will also have invested more than €5m in expansion of the production location and quadruple its capacity to supply the OBIGGS safety systems to Airbus in 2009. The systems will be integrated in the company's aeroplanes in 2010.
With rising pressure on the air travel industry to reduce costs in the face of an increasingly gloomy financial climate, the challenge to aircraft manufacturers in meeting the FAA and EASA rulings is tough. With the cost of compliance with the rule estimated by the FAA at $1bn over 35 years, significant investment will be required to ensure the safety of passengers.
However, the latest air separation technology is going some way to making this possible, providing a compact, lightweight and, perhaps most importantly, cost effective solution to preventing another unnecessary disaster.