ON-BOARD INERT GAS GENERATING SYSTEMS: A BETTER ALTERNATIVE Justin Bright Advisor: Margaret Browning Hampton University Abstract The world is more environmentally conscious than ever before. This is clearly indicated by how much government funding is invested in environmental research, conservation efforts, pollution reduction, and so forth. For aviation to maintain its place as a leading industry, airlines and manufacturers, alike, must make environmental concerns a priority. Currently, the Boeing Company and the Honeywell Company have produced On-Board Inert Gas Generation Systems (OBIGGS) that help prevent the probability of fuel tank explosions by use of nitrogen. However, there yet exists an OBIGGS system that actually extinguishes a fire. Halon 1211 extinguishers are still used to put out fires that might occur within the aircraft cabin or flight deck, and Halon 1301extinguishers are still used for hold fires. These extinguishers can be broken down into harmful, corrosive by-products when in contact with fire and thus have ozone depleting and global warming potential. As an ecological alternative, an OBIGGS can be paired with a Water Mist System (WMS). An FAA study proved this combination to be much more efficient than Halon 1301 and Halon 1211 extinguishers. This project will therefore focus on finding the proper placement of this OBIGGS/WMS combination system to provide for a safer flight environment. Background Halon Extinguishers Halon is a liquefied, compressed gas that is able to stop the spread of fire by chemically disrupting combustion. 1 Instead of removing one of the three ingredients that creates a fire: fuel, oxygen, ignition source, Halon breaks the chain reaction by keeping those three ingredients from interacting. 1 Research conducted by the Purdue Research Foundation and U.S. Army in 1947 resulted in the discovery of Halon 1211 and Halon 1301. 1 Halon 1211 is considered a liquid streaming agent and Halon 1301 is considered a gaseous flooding agent. 1 In terms of aircraft use, Halon 1211 is utilized within the flight deck and cabin for its versatility and Halon 1301 is utilized within the hold. 2 Halon 1211 has a combination of characteristics from both carbon dioxide extinguishers and water glycol extinguishers 2 and thus is able to extinguish Class A, Class B, and Class C fires. 1 Class A fires include ordinary combustibles such as cloth, rubber, and plastics; Class B fires include flammable liquids, oils, greases, and flammable gases; and Class C fires include electrical equipment. 2 Environmental Impact Halon was used throughout the 20 th century until the Montreal Protocol of 1987. 1 The Montreal Protocol identified Halon as one of many chlorofluorocarbon (CFC) compounds that deplete the ozone layer and require limitations of use and production. 1 The ozone layer is the outer edge of Earth s atmosphere that is made of naturally occurring gas which protects life on Earth from the harmful UV rays of the sun. 1 About 90% of ozone is located in the stratosphere which extends from 5 to 11 miles above the Earth s surface to 30 miles above the Earth s surface. 1 When chemicals from CFCs escape into the stratosphere those chemicals are broken down by solar radiation and subsequently release chlorine and bromine atoms that destroy ozone molecules. 1 By this occurring more Bright 1
frequently than natural ozone replenishment, the ozone layer is allowed to thin. 1 Halon 1211 is more apt to release Chlorine where as Halon1301 more so releases Bromine. 1 A relative scale known as Ozone Depletion Potential (ODP) was created by scientists to compare the environmental detriment of compounds. 1 Halon 1301had an ODP value between 10 and 16. 1 This meant that Halon 1301 was 10 to 16 times more likely to destroy ozone. 1 Although Halon is used significantly less worldwide as compared to other CFCs, Halon is estimated to account for upwards of 20% of all ozone depletion. 1 With this knowledge, the EPA began encouraging non-ozone depleting alternatives. 1 In 1990, the Significant New Alternatives Policy (SNAP) was established to evaluate new chemicals and technologies for the replacement of ozone depleting substances. 1 Also, an amendment to the Montreal Protocol known as the Clean Air Act was enacted on January 1, 1994 to cease further production of Halon. 1 This meant that the existing supply of Halon would need to be recycled and reused. 1 However, there is no cost effective means of safely and effectively disposing of Halon. 1 On March 5, 1998, the EPA issued a final ruling on Halon. 3 EPA Rule 40 CFR Part 82 Subpart H states that the disposal of Halon containing equipment must be conducted by a Halon manufacturer, Halon system manufacturer, fire equipment distributer, or Halon recycler and requires technician training relevant to Halon emissions be provided. The final ruling also bans the manufacture of any Halon blend with the exception of those Halon blends manufactured solely for the purpose of aviation fire protection and exempts the release of Halon during testing of fire extinguishing systems or equipment provided 4 criteria are met: systems employing suitable alternatives are not available, system testing is essential to demonstrate system functionality, failure of system would pose great risk to human safety and the environment, and a simulate agent cannot be used for testing purposes. 3 Introduction FAA Study In September 2000, the Halon Options Task Group, working under the International Aircraft Systems Fire Protection Working Group submitted a report on Aircraft Cargo Compartment Fire Protection to the IASFPWG Chairman. 4 The report was a review of six fire extinguishing and suppression system options for potential use in Aircraft engines and cargo compartments. 4 After further review, the team recommended, by consensus, two systems for FAA tests: water mist and inert gas system, and pentafluoroethane (HFC-125). 4 The FAA Technical Center Fire Safety Section, per their Halon Replacement Program, took up the recommendations and evaluated the fire suppression alternatives. 4 Their primary objective was to design and develop a water mist, nitrogen system that was capable of meeting the Minimum Performance Standard (MPS) acceptance criteria for Aircraft Cargo Compartment Gaseous Fire Suppression Systems. 4 Tests and Results The four tests conducted were the bulk-load test, containerized test, surface burn test, and exploding aerosol can test. 4 Each test had specific requirements (Table 1) and was repeated 5 times. 4 Table 1- Test Requirements Tests Average Compartment Peak Temperature Average Temperature- Time Area Bulk-load 582 F 10452 F-min Containerized 612 F 14102 F-min Surface Burn 1125 F 2964 F-min Aerosol 582 F Retrieved from Report No. DOT/FAA/AR-01/121 Bright 2
The bulk-load test and containerized test were deep-seated fires fueled by shredded paper loosely packed inside cardboard boxes to simulate Class A fires. 4 In the bulk-load test boxes were placed directly onto the cargo compartment floor and in the containerized test boxes were stacked inside an LD-3 container. 4 The surface burning test simulated Class B fires by placing.5 gallons of Jet A fuel in a 2 x2 steel pan. 4 Class C fires were replicated by the exploding aerosol can test; a simulator released a flammable, explosive mixture of propane, alcohol and water into an arc from a sparking electrode. 4 Results showed that the OBIGGS-WMS system surpassed the MPS for each of the tests (Table 2) using proportionate amounts of nitrogen (Table 3). Table 2- Test Results Test Average Compartment Peak Temperature Average Temperature- Time Area Bulk-load 387 F 4744 Containerized 313 F 5518 Surface Burn 438 F 1054 Aerosol 533 F 3810 Retrieved from Report No. DOT/FAA/AR-01/121 Table 3- Nitrogen Consumed Test Nitrogen Consumption Bulk-load 2325 ft 3 Containerized 2321 ft 3 Surface Burn 111.2 ft 3 Aerosol 2930 ft 3 Retrieved from Report No. DOT/FAA/AR-01/121 Advantages of Nitrogen There are several advantages to using nitrogen as a fire suppressant. First off, nitrogen is readily available and easily accessible. 5 Nitrogen naturally occurs within the atmosphere and makes up 78% of the air we breathe. 5 Also, nitrogen is non-toxic in any concentration and there is limited effect on people provided that minimum oxygen levels are applied. 5 In a normal air environment in which the nitrogen level is increased there is no threat to safety as the body is conditioned to breathe high levels of nitrogen, so therefore the low oxygen content in environments associated with inert gas fire extinguishing systems will automatically lead to accelerated breathing and increased oxygen intake. 5 OBIGGS In order to acquire the necessary nitrogen an On-board Inert Gas Generation System must be used. The purpose of an OBIGGS is to provide an inert gas to the ullages, portion of the fuel tank above the liquid, or to any compartments for combustion prevention. 6 An OBIGGS includes three main components: inlet, heat exchanger, gas separation module. 6 The inlet allows for an influx of bleed air from the turbine engine to enter the system. 6 A heat exchanger is located downstream from the inlet and while working in fluid communication with the inlet cools the bleed air received. 6 Downstream from the heat exchanger is the gas separation module. 6 While in fluid communication with the heat exchanger, the gas separation module separates the bleed air received into a nitrogen-enriched gas flow (NEA) and an oxygen-enriched gas flow (OEA). 6 The NEA is then either delivered directly to the fuel tank or indirectly to the fuel tank via the fuel tank vent. 6 Also, there is an environmental control system that is mounted on the airframe for receiving and conditioning bleed air for delivery to passenger and crew compartments. 6 The premise behind an OBIGGS is to have the NEA remain flowing and to not stagnate. 6 The gas separation module will thus generate the NEA at a multiplicity of flow rates to ensure that the fuel tank is inert during any operational condition. 6 As a result, there should not be a need to store NEA for future use or a need to charge the system with a predetermined amount of NEA. 6 However, there are instances when OBIGGS may not Bright 3
generate NEA at a flow rate sufficient to ensure the compartment is incombustible. 6 Often OBIGGS are unable to generate enough NEA during short flights and so crews will have to charge storage tanks with proper NEA levels before flight, which can take several hours. 6 Enhanced versions of OBIGGS exist to increase the NEA flow by back pressuring OEA into the NEA line, but an increased NEA flow results in increased oxygen concentration and thus a decrease in nitrogen purity. 7 This project will therefore adhere to the concept that OBIGGS are not always able to generate sufficient NEA. An alternative to charging storage tanks with NEA from an external source is to compress and store excess NEA generated during periods of flight with low demand. 6 In addition to finding the proper placement for an OBIGGS-WMS system, this project will also attempt to utilize NEA storage tanks for the purpose of extinguishing fire in the flight cabin, flight deck, and hold. Analysis Detailed Description of OBIGGS Operation An OBIGGS system operates as follows (Figure 1). A system shut-off is adjoined to the inlet for selectively controlling operation of the system. 6 Located between the inlet and the heat exchanger is a compressor. 6 The compressor is composed of a turbine, heat exchanger, outlet, and regulator. 6 A portion of the gas from the inlet is delivered to the turbine to drive operation of the compressor before exiting the turbine through the outlet as waste gas. 6 The heat exchanger is connected to the outlet and pre-conditions the air that is to be compressed. 6 Further downstream is a pressure sensor that measures the pressure of the compressor and heat exchanger. 6 Connecting the pressure sensor to the compressor regulator is a processor. 6 This processor works in conjunction with the pressure sensor to determine when the must open and close for additional or less air. 6 Also, a check is used by the compressor to prevent air downstream from flowing upstream. 6 Adjacent to the compressor check is a bypass check. 6 This bypass check allows air to avoid having to go through the compressor. 6 Just past the bypass check is the heat exchanger that cools air received from the inlet. 6 The heat exchanger can either be a component of the environmental control system and may use a cooling medium other than air, or be an entity separate from the system. 6 A temperature sensor downstream from the heat exchanger measures the temperature of the air that is past the sensor itself. 6 Connecting the temperature sensor to the heat exchanger bypass, adjacent to the heat exchanger, is a processor. 6 The temperature sensor and heat exchanger bypass work in synchronization via processor to control the temperature of air by allowing at least a portion of the air to bypass the heat exchanger and mix with the air exiting the heat exchanger. 6 There are two ground connection ports: one between the heat exchanger and gas separation module, and one past the oxygen sensor. 6 The purpose of the ground connection ports is to introduce air from a pre-conditioned source external to the aircraft. 6 They allow the OBIGGS to operate without electrical power and they can allow for the removal of some air to drive the operation of pressure intensifiers. 6 Also, past the ground connection port and between the heat exchanger and gas separation module is a filter that entrains moisture and particulate contaminants. 6 The gas separation module assembly is located past the temperature sensor and can have upwards of four modules. 6 Each module has its own shut-off in front and check in back. 6 The shut-off controls the air flow into the module, allowing each Bright 4
module to operate individually, and the check prevents air from flowing upstream back into the module. 6 The assembly separates the air into an NEA flow and an OEA flow. 6 The OEA is discharged through an outlet as waste gas or can be provided as breathable air to passenger and flight crew compartments. 6 Gas separation modules can either use pressure swing absorption or preamble membranes to separate NEA and OEA. 6 The OBIGGS that this project references utilizes preamble membranes. 6 The membranes are formed from polyetherimide by extruding a hollow fiber using a core liquid, quenching the extruded fiber in dry air to promote loss of solvent and non-solvent, and drying the fiber. 8 Also, the membranes are stable during temperatures up to 160 C. 8 Rapid Descent 40 9.8 Warming 26 16 Figure 1- OBIGGS. Retrieved from Patent No. US 7,402,868 B2 A flow rate sensor and an oxygen sensor are located opposite each other downstream from the gas separation module assembly. 6 The flow rate sensor measures the flow rate and pressure of NEA downstream from the assembly, and the oxygen sensor measures oxygen concentration of NEA downstream from the assembly. 6 Based on the readings of the flow rate sensor and the oxygen sensor, the flow located past the second ground connection port will either be put into an open position for a higher NEA flow rate or a closed position for a lower NEA flow rate (Table 4). 6 There is also a two position for the fuel tank vent that controls the flow rate into the tank through the vent and a two position for the fuel tank that controls the flow rate directly into the vent (Table 4). 6 Table 4-Flow Rates. Retrieved from Patent No. US 7,402,868 B2 Operating Condition NEA Flow (lb/min) Max. Oxygen Concentration (% by vol.) Cruise 1 5 Fuel Tank Initialization 9 8.7 Climb 11 8.7 Descent 16 9.8 OBIGGS Components 50-inert gas 66-fuel tank generating system 70-inlet 72-heat exchanger 75-system shut-off 76- compressor 80-outlet 82-heat exchanger 86-compressor 88-processor regulator 92-check 94- temperature sensor 98-processor 100-ground connection port 104-filter 106-gas separation modules 110-shut-off 116-oxygen sensor 122-fuel tank 112-check 118-flow 68-fuel tank vent 74-gas separation module assembly 78-turbine 84-pressure sensor 90-bypass check 96-heat exchanger bypass 102-ground connection port 108-outlet 114-flow rate sensor 120-fuel tank vent Bright 5
Preferred Embodiment of the OBIGGS-WMS System In one embodiment, the proposed OBIGGS- WMS system generates ample amounts of NEA and stores the excess NEA in storage tanks during periods of flight with lower demand. NEA storage tanks are merged with the ground connection ports and act as a pressure intensifier in addition to being a storage device. The OBIGGS-WMS system utilizes the same configuration as a rapid descent during a climb and a regular descent to maximize the NEA flow. By opening the compressor, flow, fuel tank -vent and fuel tank, an NEA flow upwards of 40 lb/min can be reached, as well as a maximum oxygen concentration of 9.8% (Table 4). 6 A climb requires an NEA flow of 11 lb/min and a maximum oxygen concentration of 8.7% to be considered inert and a regular descent requires an NEA flow of 16 lb/min and a maximum oxygen concentration of 9.8% to be considered inert 6 ; therefore, an extra 29 lb/min of NEA with only an additional 1.1% of oxygen is accumulated during a climb and an extra 24 lb/min of NEA is accumulated during a regular descent. The exploding aerosol can test only required an oxygen volumetric concentration to be below 12% to successfully inert the compartment against ignition of the aerosol can hydrocarbon mixture. 4 Following the procedure described above, the aircraft will already have NEA ready for a succeeding flight. In another embodiment, the NEA is accessed and used simultaneously with the WMS to suppress fire in the cargo hold, flight deck, or flight cabin. NEA is transferred through pipes that are aligned throughout the cargo hold, flight deck, and flight cabin. Sprinklers are aligned throughout the cargo hold, flight deck, and flight cabin for the release of water. Temperature sensors and smoke detectors are positioned in the three locations and are synchronized with the NEA storage tanks and WMS. Upon the detection of a sufficient increase in temperature or smoke, the NEA will be released and the WMS will be activated instantly to extinguish the threat. In the event that a fire emerges within the flight deck or flight cabin, oxygen masks will be used; people need roughly 16-21% oxygen concentration and the inert requirement for oxygen concentration is below 12%. Conclusion The proposed OBIGGS-WMS system is a much better alternative to Halon 1211 and Halon 1301. Halon requires expensive handling and training, and releases chlorine and bromine atoms that are destructive to the ozone. Nitrogen and water are naturally occurring and readily available making them environmentally safe and affordable. In addition, the OBIGGS-WMS system is more efficient than Halon. The OBIGGS-WMS system surpassed all of the MPS acceptance criteria as well as the performance of Halon and thus allowing for a safer flight environment. Also, the OBIGGS-WMS system lessens the need to warm-up the OBIGGS. By following the procedure described in the preferred embodiment of the system, NEA is immediately ready for a succeeding flight. All the listed advantages equate to an improved aviation industry: aviation distinguishes itself even further as the safest mode of transportation, airlines save money, and the industry escapes the stigma of being an environmental detriment. Bright 6
Acknowledgments I would like to thank the entire Aviation department at Hampton University for their guidance and support as well as the Virginia Space Grant Consortium for giving me the opportunity to present my research. Bright 7
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