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Background: Inerting History

This article provides background information on fuel tank inerting, its history, and the use of Hollow Fiber Membrane (HFM) technology for air separation in the FAA's simplified inerting system. The construction and performance of the system are also discussed.

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Background: Inerting History

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  1. Federal Aviation Administration Background: Inerting History AAR-440 Fire Safety BranchWm. J. Hughes Technical CenterFederal Aviation Administration

  2. Inerting Background • Inerting refers to rendering the ullage (air above fuel) unable to propagate a reaction given flammable conditions and ignition source • In this case Refers specifically to reducing tank oxygen concentration • Other methods of compliance possible • Fire Triangle must be satisfied to have a reaction (explosion) in the ullage of a fuel tank • Ignition Source • Correct ratio of fuel and air

  3. Fuel Tank Inerting History • Inerting has been studied since 1950s • Stored gas inerting used by military in 1970s • FAA built and tested demo cryogenic nitrogen system on DC-9 • FAA demo Post Crash effectiveness • DOD did OBIGGS research using PSA and ASMs • C-5, C-17, C-130, Fighter Aircraft • Found HFM technology made ASMs cost effective for OBIGGS

  4. Fuel Tank Inerting History • FAA research illustrated fuel tank inerting could be practical if applied in a cost effective manner • Initially focused on ASM performance for fire suppression capabilities • After second ARAC, focused on using ASMs to generate inert gas on an aircraft from bleed air during the flight cycle • FAA experiments agree with previous experiments and indicated that a tank oxygen concentration below 12% will render tank inert

  5. Hollow Fiber Membrane for Air Separation • Hollow fiber membrane (HFM) technology uses the selective permeation properties of certain materials to separate air into two streams, one nitrogen enriched air (NEA) and the other oxygen rich (relative to air) • As air is forced through the fibers, fast gases escape through the fiber wall and the nitrogen rich stream to pass through the fiber core • The separation is not direct as some oxygen passes through the core and some nitrogen ventilates from the fiber • As flow through the core is slowed (back pressure), more fast gases escape (and N2) making the core stream (product) more pure (less O2)

  6. HFM Air Separation Module Construction • HFM materials are woven into hair-sized fibers and bundled by the thousands into a canister called an air separation module (ASM) • Fibers are generally potted into an epoxy sheet at each end with some structure for strength • The epoxy sheet is machined flat, to expose the fibers to the pressurized air source • The ASM is mounted in a canister to facilitate pressurized air feed, gather the product (NEA), and collect the ventilated waste gas (OEA) Air In NEA Out OEA Vent

  7. ASM Performance • Primary Parameters used to describe ASM performance are permeability and selectivity • Permeability describes how much air flows into the ASM • Selectivity describes what percentage of that air becomes NEA • These performance parameters are a function of ASM feed pressure, OEA vent pressure, and system back pressure • Given these three parameters the ASM will flow a specific NEA volume and purity (residual O2 concentration) • This is only valid at a given operational temperature • Specific flow and purity is described by the manufacturer in terms of the above two stated parameters

  8. Simplified Inert Gas Generation System Concept • Concept utilizes ASMs in a two flow methodology • Uses low flow mode during taxi, takeoff, ascent, and cruise to deplete CWT of oxygen almost completely • Uses high flow mode during descent to offset (but not eliminate) the air entering the fuel tank vent system resulting in a net inert fuel tank oxygen concentration • Does not need to run on ground or store NEA, eliminates need for compressors or ground service equipment • System only needing to reduce the oxygen concentration below 12% (by FAA research) makes sizing more realistic

  9. FAA Simplified Inerting System Block Diagram

  10. FAA Inerting System Construction • Uses 3 ASMs based on HFM technology • Excepts 350 degree F air from aircraft bleed system through an SOV • Uses a H/x to cool air to 180˚F +/- 10˚F and a filter to condition air • Air is separated by ASMs and NEA is plumbed to output valves to control flow, OEA is dumped overboard with H/x cooling air • System flow control is accomplished with low flow orifice and high flow control valve • System controlled by control box in cabin that is connected to system with cable • Prototype built on aluminum pallet for ease of construction and to support a wide variety of installation methods

  11. CAD Rendering of FAA Inerting System Prototype

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