Some twelve years after the event, the US Federal Aviation Administration (FAA) has finally laid to rest all the conspiracy theories that surrounded the fate of flight TWA 800 on 17 July 1996. The New York-to-Paris Boeing 747 flight exploded in mid-air off the coast of Long Island shortly after takeoff, killing all 230 people on board and giving rise to years of speculation as to what really happened.
In July this year the FAA announced a new rule: ‘Reduction of Fuel Tank Flammability in Transport Category Airplanes’. This requires all aircraft coming under its scope to be fitted with technology that reduces the risk of fuel tank explosions - now officially accepted as the cause of the downing of TWA 800.
The preferred technology, which will have to be designed into all new Boeing and Airbus passenger aircraft with heated centre-wing tanks after 2009 and retrofitted to those built after 1991, is a fuel tank inerting system, known as OBIGGS (On Board Inert Gas Generator System). In this system, nitrogen is used as a blanket above the fuel in the tank to prevent explosive mixtures of air and aviation fuel vapour from building up during flight.
In principle this is the same technology used throughout the oil and gas and process industries to ensure the safety of large-scale storage installations of flammable liquids. Such installations, however, generally have access to large volumes of nitrogen, either generated on site in cryogenic air separation plants or stored in bulk cylinders supplied from off-site. The first option is clearly not open to aircraft manufacturers and operators, and the second is increasingly uneconomic for an industry where weight is such a critical factor.
Instead, manufacturers such as Boeing and Airbus are turning to membrane technology developed by the likes of Parker Filtration and Separation. Already widely used in the process and packaging industries, Parker’s nitrogen generators have now been ordered by Airbus for its new extra wide-body A350 passenger plane due in service next year. This follows several years of development during which Parker has also been supplying OBIGGS systems for US military aircraft.
The system
An OBIGGS comprises a series of cylindrical nitrogen-generating modules, each containing a bundle of hollow fibre permeable membranes. As pumped air flows down the inside of the fibres, oxygen, water vapour and helium selectively diffuse through the fibre walls, leaving a nitrogen-rich stream to be used as the blanketing medium.
One of the few manufacturers of hollow-fibre membranes in the world for these air separation applications, Parker has invested significantly in their development and production. Most membranes on the market are manufactured in two stages - a porous substrate is first made, which then has to be coated with the very thin gas separation layer.
Parker’s process produces its membranes in one step, with the outer coating being formed by carefully controlled crystallisation of the substrate polymer. The result is not just a faster production process, but also a far thinner separation layer.
The membranes used in Parker’s nitrogen generators now have a separation layer that is just 40nm (0.04um) thick. The significance of this can be seen in the fact that a halving of the thickness of the separation layer can double the capacity of the nitrogen-generator module.
The maximum nitrogen concentration that can be produced by the modules is 98%, though this can be boosted to 99.5% for extreme applications. For most fire protection applications, however, it is only necessary to reduce the oxygen content of air from the normal level of approximately 21% to around 12%. For a fire to start anywhere there have to be three components present - a flammable material, a source of ignition and a supply of oxygen to feed the fire. This is known as the fire triangle. Take away one of the sides of the triangle and a fire can’t start.
Sources of ignition on board an aircraft have been designed out to a great extent, but not totally eliminated (it is now thought that an electrical short circuit in the fuel tank heating mechanism was to blame for the TWA 800 disaster). And aircraft will always need flammable aviation fuel to fly, which leaves the level of oxygen as the one variable that can be changed to improve safety.
In the Parker OBIGGS system the small lightweight membrane modules continually pump nitrogen-enriched and low-oxygen air into the fuel tank, removing the possibility of an explosive mixture of vapour building up above the liquid fuel in the tank. The oxygen levels in the nitrogen-enriched air generated by the modules are generally maintained at less than 12%, while the efficiency of the thin gas separation layers of the membranes ensures that the maximum amount of nitrogen is produced at 1.8bar pressure.
This ability to function at relatively low pressures is one of the main advantages that the Parker membranes claim over competitive products. The higher permeability of these membranes means that less power is required for air compression. In aviation applications engine bleed air generally provides the feed to the nitrogen-generator modules, but in the process and packaging industries cutting the cost of air compression is an important issue.
Other applications
As mentioned earlier, the non-aviation applications for membrane-produced nitrogen are already many and varied. The oil and gas industry, for example, uses inert gas systems in many ways - gas-seal compression, purging pipelines, blanketing storage tanks, purging flare systems, and many other applications where safety is paramount.
In the food process and packaging sector, for example, nitrogen-enriched air takes on a different, though just as vital role. The shelf lives of many bagged, bottled or carton-filled products are extended by replacing the ‘head space’ air above the product with a nitrogen-enriched, depleted-oxygen atmosphere.
From the rather mundane business of inflating vehicle tyres (because nitrogen diffuses through tyre walls far more slowly than oxygen), to providing a protective atmosphere for the Hungarian crown jewels, Parker’s membrane generators have proved themselves to be an economical and efficient alternative to all other sources of nitrogen.
David Wright is sales and marketing manager of Parker Filtration and Separation