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Aviation Fuels with Improved Fire Safety: A Proceedings II PRESENTED PAPERS FUEL AND ADDITIVE TECHNOLOGIES
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Aviation Fuels with Improved Fire Safety: A Proceedings This page in the original is blank.
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Aviation Fuels with Improved Fire Safety: A Proceedings 4 Potential Surfactant Additives: The Search for the Oxymoron Paul Becher Paul Becher Associates Ltd. According to the program, I am expected to discuss potential surfactant additives. This is a difficult problem, because prior to hearing the talks at this meeting, it was impossible to determine what these additives were, in fact, to be added to. I may say that this problem has not become much easier, but I shall return to that point a little later. Let me concentrate instead on the subtitle of my presentation, namely, ''The Search for the Oxymoron." Since this is an educated audience, I will assume that everyone knows what an oxymoron is. However, on the statistically small chance that some member of the audience has not come across this word, I give you the following definition (American Heritage Dictionary, 1996): ox·y·mo·ron () n., pl.ox·y·mo·ra () or ox·y·mo·rons. A rhetorical figure in which incongruous or contradictory terms are combined, as in a deafening silence and a mournful optimist. [Greek , from neuter of , pointedly foolish: oxus, sharp; see OXYGEN + , foolish, dull.] —ox´y·mo·ron 'ic () adj.—ox´y·mo·ron'i·cal·ly adv. However, the oxymoron for which we are searching is (as you might guess) "nonflammable fuel." You will at once assure me that what we are seeking is actually "reduced flammability," and I will agree. But the further target is, at least, worth thinking about. The oxymoron in question is not particularly new. I recall using it in, I believe, 1945. At that time, I was serving as an enlisted research chemist in the Air Force laboratories at Wright Field, Dayton, Ohio. My general area of activity was work on petroleum products used in military aircraft, including fuels, lubricants, and hydraulic fluids. A principal target was the development of a nonflammable hydraulic fluid. We did, in fact, succeed in developing a usable fluid, and I share in a patent (U.S. Patent 2,470,792) which, to the best of my knowledge, has never actually been tested, much less used, by the Air Force (or for that matter, anyone else). I am not too unhappy about this result because I have since developed some doubts about the toxicity of the principal ingredient. Nonetheless, I may perhaps be able to claim priority in this area over most attendees to this workshop. I should point out that even in 1945 we recognized that the term nonflammable fuel was an oxymoron. However, we also understood that it was, in principle, obtainable and, perhaps, important. While teaching in the Georgia University system, I did a small amount of research into the synthesis of the principal ingredient of our patented hydraulic fluid. This resulted in my one and only citation in the Beilstein Handbook of Organic Chemistry. However, subsequent employment by the Colgate-Palmolive Company and the Atlas Powder Company, which I followed through its various name changes finally to ICI Americas, shifted my interest to surface-active materials. So I allowed this particular oxymoron to fall into disuse. Although my research interests have changed, my fate seems to be involved in this problem. About 20 years ago, I was invited to participate in a study by a Bureau of Ships committee to consider the problem of the flammability of hydraulic fluids in submarines. Flammability, as anyone who has ever been inside a submarine knows, is an even greater hazard in a submarine than in aircraft. Nowhere in a submarine is one more than a few feet from a hydraulic line. In addition, one has the option of parachuting from an airplane, but the same option does not present itself in a submerged vessel. The committee met here in Washington, D.C. to consider the problem. Although the Navy's not unreasonable lack of interest in retrofitting the entire submarine fleet limited our options rather severely, a report was issued. As far as I know, none of the recommendations was ever implemented. My final experience with this problem actually completed my tour of the military services, as well as resurrecting, at last, the oxymoron. I was called to join a committee at the U.S. Army Fuels & Lubricants Laboratory in San Antonio, Texas, to consider problems involving fuel fires in military vehicles. This is a substantial problem because many military vehicles, tanks and the Bradley vehicle, for example, are run by diesel engines where the fuel doubles as engine coolant. Thus, at any time there may be as many as 400 gallons of diesel fuel sloshing about the vehicle.
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Aviation Fuels with Improved Fire Safety: A Proceedings The Army also proved to be unwilling to retrofit, and other restrictions seriously reduced the possible solutions to the problem. Nonetheless, a report was issued. To the best of my knowledge, the recommendations for further study were not implemented. It will be noted that I have qualified the results of my tour of the services by saying in each case, "to the best of my knowledge" or "as far as I know." The reason for the qualification is obviously the possibility of "military secrecy" (another oxymoron?). The fact that we are here today suggests, however, that no great measure of success has been achieved in the intervening years. Now, to return to my principal theme: in what way can surface-active agents, new or old, contribute to progress in this area? The obvious answer is as stabilizers in multiphase systems. Some form of emulsion seems an obvious approach. The limitation that no water be included in the formulation has been imposed. It should be noted that hydrocarbon-water systems have been used extensively as fuels, notably in diesel engines. Generally, these have been water-in-oil systems and have been investigated with a view to reducing emissions and costs, rather than to reducing flammability. An interesting review of some research in this area was recently given by Thompson (Thompson, 1990). In addition, some work has been done on using oil-in-water emulsions as heating or power-plant fuel, namely using emulsions of heavy crude, as exemplified by Orimulsion fuel (Briceno et al., 1990; Marcano et al., 1991). These studies reveal that fuel emulsions can indeed burn. Unfortunately, I have not been able to determine if studies on the prevention or limitation of burning have been carried out. I have identified a suggestive paper by Kitamura and coworkers dealing with water-in-oil emulsions (Kitamura et al, 1991). This paper suggests that the flash point decreases with increasing surface area per unit volume of the emulsion. Since the utility of the emulsion as a fuel depends on its ability to burn, it is obvious that this clue must be employed with some caution. All of the papers I consulted in preparing this presentation on the use of surface-active agents have included very conventional surfactants in the formulations. This is not surprising because there are literally thousands of surface-active materials available to the investigator or formulator. The laws of thermodynamics being what they are, one may expect that all of these materials will act in pretty much the same way. For any given system, these thousands of surfactants may shrink to a few suitable ones. The methods of determining the appropriate ones are well known and are described in a number of places, including in my own work. In conclusion, new approaches to this problem may be rather limited. Although it may be that no current product meets the needs of a new system subjected to, for example, environmental extremes, the synthesizer of new materials will be guided by the wealth of previous experience, as well as by the laws of thermodynamics. REFERENCES American Heritage Dictionary (3rd ed.). 1996. CD-ROM version. Becher, P., and W. Schelsinger. 1949. U.S. Patent 2,470,792. Becher, P. In press. Emulsions: Theory and Practice (3rd ed.). American Chemical Society (ACS) Monograph. Washington, D.C.: ACS. Briceno, M.I., M.L. Chiroinos, I. Layrisse, G. Martinez, G. Nunez, A. Padron, L. Quintero, and H. Rivas. 1990. Emulsion technology for the production and handling of extra heavy crude oils and bitumens. Revista Tecnica Intevep 10(1):5-14. Kitamura, Y., Q. Huang, Y. Oka, and A Williams. 1991. Flashing of superheated water-in-oil emulsions. Journal of Chemical Engineering of Japan 23:711–715. Marcano, N., M. Pourkashanian, and A. Williams. 1991. The combustion of bitumen-in-water emulsions . Fuel 70(8):917–923. Thompson, R.V. 1990. To fly to float: a half life of research and development. Transactions of the Institute of Marine Engineering 102(1):1–16.
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Aviation Fuels with Improved Fire Safety: A Proceedings 5 Fire Safety in Military Aircraft Fuel Systems Robert G. Clodfelter AFP Associates Inc. INTRODUCTION Because of the large quantity and dispersed storage of fuel on board aircraft, there is a high probability that aircraft accidents will involve fire in one way or another. The probability of fire is increased by the aircraft design requirement for lightweight structures for fuel containment. Fire could result from a fuel explosion in a fuel tank, fuel leakage into dry bays, cabin areas, cargo bays, or engine compartments, or from a fuel release on impact. Whether the fire or explosion is caused by the initial event or by a secondary cause, the result is often catastrophic. One way to reduce the aircraft fire problem is to develop a fuel that is less susceptible to combustion in locations other than the engine. This is a challenging technical and economic goal, especially considering the requirements of aircraft operation and fuel availability. Candidate "fire-safe" fuels should have the availability, reasonable cost, and suitable physical and chemical properties for direct utilization in operational aircraft without extensive fuel system modifications or serious degradation of aircraft performance. Because of these stringent requirements, most of the successful efforts to reduce the aircraft fire hazard have been improvement in prevention and protection areas rather than development of fire-safe fuel. This paper uses the term fire-safe in the broad sense, i.e., any change in fuel that improves fire safety. The Air Force's recent conversion from JP-4 (which has Jet B as its commercial equivalent) to JP-8 fuel is a step in the right direction from the standpoint of fire safety. Army aircraft use the same fuel as the Air Force. The Navy has used JP-5 fuel for many years for fire safety reasons. Properties of these military fuels and commercial fuels (Jet A and Jet A-1) are given in Table 5-1. The production of these fuels is illustrated in Figure 5-1 as a percentage obtained by simple distillation of an average crude oil. By using refinery techniques, such as hydrocracking, hydrotreating, and reforming, these percentages can be adjusted on demand. However, fewer added refinery costs are associated with converting heavy bottom fuels to high vapor pressure fuels (low flash point temperature) than vice versa. The use of JP-5 and JP-8 was implemented to reduce the availability of fuel vapors for ignition and to reduce the amount of fuel vapor which could support a reaction. The use of antistatic additives in aircraft fuels (a specification requirement for JP-4 and JP-8 but not for Jet A or Jet A-1) is helpful for minimizing static as a potential ignition source. The lack of an antistatic additive may have been a contributing factor in the recent TWA 800 accident (17 July 1996). Other TABLE 5-1 Comparison of the Properties of Aviation Fuels JP-4 JP-8 Jet A-1 Jet A JP-5 Minimum Flash Point, °F -20 100 100 100 100 Maximum Freeze Point, °F -72 -53 -53 -40 -51 Lbs/gal 6.3 6.7 6.7 6.8 6.8 BTUs/gal 118,900 124,500 124,500 125,700 126,000 Vapor Pressure, psi 2-3 Viscosity, cSt@-4°F 2.4 4.2 4.2 5.5 5.5 Acid NR, mg KOH/g 0.015a 0.015a 0.1 0.1 0.015a a Allow up to 0.022.
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Aviation Fuels with Improved Fire Safety: A Proceedings FIGURE 5-1 Availability of distillate fuels. attempts to make fuels more fire safe have included developing modified fuels, such as gels, emulsions, and antimisting kerosene (AMK). Tests on modified fuels have demonstrated potential reductions in ignition and flame propagation by the mists and sprays normally associated with "neat" fuels in the event of a crash landing and structural failure. All of the currently known modified fuels, however, have serious operational and logistical problems mainly associated with fuel transfer, storage, and engine performance. Many of the proposed solutions for these problems have inherent negative safety factors and present new design challenges. Needless to say, modified fuels have only been used in test programs and not operational service. The search for a less hazardous fuel has taken two fundamental approaches. The first entails reducing the availability of fuel vapors for ignition and limiting their availability for feeding the combustion process. This may be accomplished by increasing the flash point temperature of the fuel (lowering the vapor pressure), a fuel property that can be easily measured. Fuels with a wide range of flash point temperatures are currently available. Gasoline has a flash point temperature of about -60°F, whereas aviation jet fuels have flash points in the 100¹ to 150°F range (with the exception of JP-4 [Jet B] which has a flash point near 0°F). Unfortunately, increasing the flash point temperature of a fuel has negative operational and performance effects, such as increased viscosity, increased freeze point temperature, and problems with engine starting and engine relight at altitude. These problems can be minimized by refinery techniques and design or procedural changes with some increase in fuel cost and operational inconvenience. The second approach to the search for fire-safe fuel involves modifying the fuel to reduce the ignition and flame propagation of mists and sprays normally associated with neat fuels in a crash situation. This approach has few fire safety benefits for neat fuels with high vapor pressures, i.e., for low flash point fuels (Gandee and Clodfelter, 1974). This is one reason the Air Force showed limited interest in the many modified fuel programs that were being conducted in the U.S. and United Kingdom in the 1960s, 1970s, and into the 1980s. The Air Force was using JP-4, a high vapor pressure fuel, during this time and had many other concerns to address associated with their planned conversion from JP-4 to JP-8. This conversion was completed, at all but a few locations, in 1995. Since the changeover to JP-8 fuel, the Air Force and commercial airlines are using fuel with the same vapor pressure. The Air Force fuel (JP-8), however, contains several additives, some of which are not required in the commercial fuel (Jet A and Jet A-1). See Table 5-2. Based on the experience of the Air Force during their conversion to JP-8, operational fuel changes are very complex and time-consuming in addition to being expensive. In 1967, when the Air Force initiated efforts to evaluate the combat benefits of JP-8, commercial airlines had more than 15 years of operational experience with Jet A and Jet A-1, the commercial equivalent of JP-8. Even with this long-term experience, the first JP-8 conversions at NATO bases didn't occur until 1978. A study team of the Coordinating Research Council (CRC) reviewed the JP-8 (Jet A and Jet A-1) and JP-4 (Jet B) accident experience from 1952 to 1974 (CRC, 1975). The study team reported that: TABLE 5-2 Characteristics of Current Military Fuel Additives Fuel Additive Comments Antioxidants Reduces or prevents formation of gums and peroxides Key ingredient in JP-8+100 Metal Deactivator Deactivates trace metals Maintains thermal oxidate stability Corrosion Inhibitors/Lubricity Enhancers Originally required to protect pipelines Removal from fuel caused significant wear problems Minimum adjusted to protect F-111 NATO, ASCC investigating potential of lowering minimums FSII Water in fuel caused crashes Prevents biological growth Allows A/C and ground storage sumps to be drained in winter Static Dissipator 1960s USAF experienced 1 or 2 static-initiated fires per year Rash of fires when A/C equipped with foam A/C currently equipped with conductive foam With conversion to JP-8, need to continue is being evaluated Source: Personal communication from C.L. Delaney and W.E. Harrison III, USAF Wright Laboratories, November 1996.
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Aviation Fuels with Improved Fire Safety: A Proceedings In 200 survivable accidents with spilled fuel that were reviewed, there were 7 to 19 percent fewer fires with JP-8 than with JP-4 at the 95 percent confidence level. In 13 in-flight accidents involving fuel releases or fuel tank explosions that were studied, there were more fires with JP-4 (100 percent) than with JP-8 (44 percent). Ground accidents with JP-4 resulted in more aircraft destroyed (78 percent) than ground accidents with JP-8 (0 percent). 13 events were analyzed. Even after these CRC fire-safety results had been released, and combat benefits of using JP-8 had been identified (Beery et al, 1975), the Air Force conversion was not completed until 1995. Considering that fire-safe fuel has greater pay offs in terms of flight safety and combat survivability for the military than for commercial aviation, and considering that the military uses only one-tenth of the fuel used by U.S. airlines (see Figure 5-2), the fuel change of this type should have been easier to justify and less difficult to implement for the military then for commercial aviation. The total time for the military's conversion to JP-8 was 28 years. The number of aircraft involved in commercial aviation, combined with the number of airports and airlines and their associated infrastructures, suggest that a commercial fuel change would involve a significant phase-in time. Operational compatibility between the old and new fuels will therefore be essential. FIGURE 5-2 Demand for kerosene jet fuel in the United States. Source: Personal communication from C.L. Delaney and W.E. Harrision III, USAF Wright Laboratories, November 1996. Despite these concerns, it is appropriate to address periodically opportunities for the development of a fire-safe fuel. Technical people tend to be optimistic about achieving the short-term goals but pessimistic about the long-term possibilities. It is possible that an AMK-type fire-safe fuel could be developed in the distant future. Against this backdrop, a brief discussion of jet fuels, fuel flammability, the fire problem (including fire prevention and protection techniques), cost/benefit analysis, and future aircraft trends follows. JET FUELS Jet fuels in the United States have been evolving since the 1940s (Martel, 1987). JP-1, the first jet fuel specified in the United States (1944), was a kerosene with a freeze point temperature of -77 °F and a minimum flash point temperature of 109°F. The availability of JP-1 was limited to about 3 percent of the average crude oil. JP-2 (1945) was an experimental fuel that was found to be unsuitable because of its viscosity and flammability. JP-3 (1947 to 1951), the second operational fuel, had a high vapor pressure similar to aviation gasoline. Because of its high vapor pressure and because jet aircraft tend to fly at higher altitudes than reciprocating engine-powered aircraft, losses from fuel boil-off and vapor lock were problems. JP-4 fuel (1951 to 1995), also designated as NATO F-40 and Jet B, a blend of gasoline and kerosene with a Reid vapor pressure restriction of 2 to 3 psi has fewer problems with boil-off and vapor lock. JP-4 has a freeze point of -77°F and a flash point temperature of about 0°F (not a specification requirement). JP-4 was the primary jet fuel used by the U.S.
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Aviation Fuels with Improved Fire Safety: A Proceedings Air Force from 1951 to 1995. In the mid 1980s an antistatic additive was added to JP-4 for fire-safety reasons. JP-5 fuel (1952 to the present), also called NATO F-44, has a 140°F minimum flash point temperature. This kerosene fuel is currently the primary fuel used by the U.S. Navy and was developed mainly in response to fire-safety concerns on ships. JP-5 fuel has a freeze point temperature of -51°F and does not contain an antistatic additive. JPTS (1956), developed for the U-2, is a highly refined kerosene with a low freeze point of -64°F and a thermal stability additive package (CJFA-5). Its minimum flash point temperature is 109°F. JP-6 (1956), developed for the XB-70, is similar to JP-5 but has a lower freeze point (-66°F) and more thermal stability. The flash point temperature is not a specification requirement. JP-7 (1960s), developed for the SR-71, has a low vapor pressure and excellent thermal stability for high altitude and mach 3+ operations. It has a freeze point of -47°F and a minimum flash point temperature of 140°F. Jet A and Jet A-1 (1950s to the present) are the two fuels used by commercial airlines. Both fuels have a 100°F minimum flash point temperature for fire-safety reasons. Jet A has a freeze point of -40°F and Jet A-1 has a freeze point of -53°F. Because of its lower freezepoint, Jet A is more widely available and is, therefore, more widely used. Commercial fuels in the United States are not required to contain antistatic additives and generally do not. Finally, JP-8, also known as NATO F-34, was first introduced at NATO bases in 1978 and is currently the primary fuel used by the U.S. Air Force. JP-8 is very similar to Jet A-1, but it contains an icing inhibitor, a corrosion/lubricity enhancer, and an antistatic additive (see Table 5-2). The U.S. Air Force conversion to JP-8 was virtually complete in 1995 and was undertaken for fire-safety and combat survivability reasons. FUEL FLAMMABILITY The classic fire pyramid or fire tetrahedron (shown in Figure 5-3) illustrates the four elements necessary for a sustained hydrocarbon fire: heat for ignition, fuel, oxygen, and free radicals. The first three elements must be in the proper range and state for ignition and chain branching reactions must occur to maintain the reaction process. The first three elements must also be available in the proper balance for a sufficient period of time for ignition to occur. The required time increases near the flammability and ignition energy limits. All fire prevention, fire protection, or fire-safe fuel must negate at least one of these elements in some way. By this criterion, the two basic fuel properties used for first order assessments of a fuel's hazardous nature were established. FIGURE 5-3 The fire pyramid. FIGURE 5-4 Autoignition temperature (AIT) and flash point temperature measurement apparatus. The first and most important fuel property in many fire-safety scenarios is flash point temperature. Figure 5-4 is a simplified sketch of the equipment used to determine the flash point temperature of a fuel. The test fuel is placed in a container, the liquid fuel is slowly and uniformly heated and its temperature monitored. Periodically, as the temperature is increased, a large ignition source is inserted into the fuel container, until the lowest temperature at which a flash occurs is identified. This temperature is called the flash point and is
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Aviation Fuels with Improved Fire Safety: A Proceedings a measure of the availability of fuel vapors for ignition. In relation to the fire pyramid, the flash point temperature is a good estimate of the minimum fuel-air ratio necessary for ignition. The flash point temperature is close to, but slightly higher (5 to 10°F) than, the lower flammability limit. The flash point test involves downward flame propagation whereas the lower flammability limit test involves upward flame propagation. Because upward flame propagation is easier to achieve than downward flame propagation, it requires fewer fuel vapors at a lower temperature. The apparatus used to determine the minimum autoignition temperature (AIT), or spontaneous ignition temperature, of a fuel is also illustrated in Figure 5-4. A 500-ml flask is slowly and uniformly heated. At a known temperature, a small amount of fuel is injected. This is followed by a 10-minute period during which any flash or light emitted from the flask is noted. This procedure is repeated a number of times with varying amounts of injected fuel at various temperatures to determine the lowest temperature at which an indication of ignition is observed. In terms of the fire pyramid, the AIT of a fuel can be viewed as the lower limit at which a hot surface can cause ignition. Unfortunately from a fire-safety point of view, these two properties (flash point temperature and AIT) relate to each other in different directions for most fuels. For example, aviation gasoline (Grade-100) has a flash point temperature of -50°F (bad) and an AIT of 824°F (good). JP-8 has a flash point temperature of 100°F (good) and an AIT of 435°F (bad). In most real world situations, however, JP-8 is considered much less hazardous than aviation gasoline (see Figure 5-5 for the fuel hazard classification used by the National Fire Protection Association [NFPA] and in the Hazardous Substances Act). The relative fire hazard of a fuel can be better estimated from the flash point temperature than the AIT. However, many fuel combustion characteristics must be evaluated under real world conditions and then assessed against operational problems to evaluate potential fire hazard benefits as a function of fuel properties. Can the number of aviation fatalities and injuries be reduced in a cost-effective manner by using a fuel with a higher flash point temperature, a lower probability of ignition in a crash situation, or some combination of other fuel properties? A commercial aviation switch to a higher flash point fuel (similar to JP-5) seems possible with reasonable effort and would result in some fire-safety benefits. An AMK-type fuel may have more potential fire-safety benefits, but a conversion of operational aircraft may be impractical in the final analysis. Before a conversion in initiated, detailed study of the aircraft fire problem covering these two fire-safe fuel approaches and their potential operational problems must be done. Some fuel characteristics that should be assessed when evaluating the fire-safety benefits of a fuel are: volatility/atomization flash point autoignition temperature (AIT) flame spread rate over a pool burning velocity (vapor-air mixture) fuel-air mixture, including equilibrium and nonequilibrium (venting, sloshing, vibration, spray, mists, foams) conditions electrical characteristics (static accumulation) extinguishing characteristics Some environments that should be considered when assessing the fire-safety benefits of a fuel are: FIGURE 5-5 Flammable liquids classification from the National Fire Protection Association (NFPA) and Hazardous Substances Act as related to flash point.
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Aviation Fuels with Improved Fire Safety: A Proceedings FIGURE 5-6 Rate of flammability volume buildup. in-flight operations ground operations combat threats (projectiles, fragments, high explosive incendiary, etc.) terrorist activities FIGURE 5-7 Flammable zone between leaking fuel-rich vapors and ambient air. Figure 5-6 illustrates the relative rate of fuel-vapor generation and the flammability volume associated with a fuel spill at ambient conditions for kerosene, JP-4, and gasoline. Fuels with a flash point temperature above the ambient temperature have no flammable zone. Figure 5-7 illustrates the flammable zone between a leaking fuel rich mixture and ambient air. Leaking fuels with a flash point temperature above the ambient temperature have no flammable zone, assuming that no sprays or mists are present. Flammable regions for JP-4 under equilibrium conditions are shown in Figure 5-8, including the important flammable zone below the flash point temperature when sprays and mists may be present. The rates of flame spread across a spill of JP-4, JP-8, and JP-5 are shown in Figure 8. The flame spread rate is strongly dependent on flash point temperature. The flammability properties of several common aircraft fluids are given in Table 5-3, where the safe hot surface temperature is the temperature at which the probability of ignition is about 10-3 (Clodfelter, 1990). THE FIRE PROBLEM The general fire problem is the same for military and commercial aircraft, except for the combat threat to military
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Aviation Fuels with Improved Fire Safety: A Proceedings FIGURE 5-8 Flammable regions for JP-4. aircraft (the projectile threat is illustrated in Figure 5-10). Some fire prevention, detection, and control measures are given in Table 5-4. In addition to the measures in Table 5-4, several active and passive fuel system and dry bay fire protection techniques have been studied over the years. Some of these techniques are listed below: FIGURE 5-9 Flame spread across a jet fuel spill. flame arrestor foam inerting (nitrogen enriched air [NEA], N2, CO2, halon 1301 [CF3Br]) systems explosion suppression systems self sealing fuel tanks crash resistance fuel tanks dry bay protection (void filler materials, extinguishing systems, purge mats, powder packs) fire proofing, fire resistance, fire hardening gelled/emulsified fuels antimisting kerosene (AMK) fuel fogging All but the last three protection techniques have been successfully used on operational aircraft. Flame arrestor foams have been used very successfully on many military fighter aircraftto prevent fuel tank explosions. Inerting systems are currently used to protect the fuel tanks of SR-71, C-5, OV-22, C-17, AH-64, and F-22 aircraft. As a result of the recent TWA 800 accident, fuel tank inerting systems will be conscientiously reconsidered for commercial aircraft. COST/BENEFIT ANALYSIS Changing fuels or adding fire protection hardware to an aircraft will require cost/benefit analysis prior to approval and
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Aviation Fuels with Improved Fire Safety: A Proceedings TABLE 7-1 Examples of Some Jet Fuel Specifications Characteristic Value Volatility Flash Point Distillation (10% Rec) Final Boiling Point Density Min 38°C Max 205°C Max 300°C 775 to 840 kg/m3 Direct Compositional Total aromatics Naphthalenes Total sulfur Mercaptan sulfur Acidity Max 25.0 vol pct Max 3.0 vol pct Max 3,000 ppm S Max 30 ppm S Max .015 mg KOH/g Indirect Compositional Freeze Point Smoke Point Viscosity @ -20°C Thermal Stability Test Copper Strip Corrosion Test Net Heat of Combustion Max -47°C Min 18 mm Max 8 mm2/s Pass @ 260°C Pass Min 42.8 MJ/kg Additives Only listed additives permitted stringent requirements of ASTM D 1655, Defense Standard 91/91, and IATA Guidance Material called the "check list" is used. In recent years efforts have been made to harmonize Western specifications and to encourage non-Western countries to move in this direction. Some typical specification requirements for a Jet A-1 type fuel are shown in Table 7-1. The specification puts many limits on what is acceptable as jet fuel. These include direct limits on volatility, which controls the size of the compounds in the fuel, and on the type of chemical compounds that can be used. There are also indirect restrictions on composition imposed via requirements that fuels meet various performance tests (e.g., thermal stability) and physical properties tests (e.g., freezing point). Additives in aviation fuels are also strictly controlled. Unlike other transportation fuels, aviation fuel specification requirements are deemed absolute in ASTM D 1655. Every batch of jet fuel is tested to prove its conformance to all specification requirements, and test results are not subject FIGURE 7-1 Two examples of chemical processing sequences used to produce jet fuel blend stocks. to correction for tolerance of test methods. In many cases, refineries impose even more stringent internal requirements to ensure conformance statistically. PRODUCTION METHODS AND THEIR EFFECT ON CHEMICAL COMPOSITION Aviation turbine fuels are generally blended from straight-run kerosene fractions that have been subjected to some form of additional processing (Dukek, 1992). In some cases, jet fuel blending stocks are produced by hydrocracking heavier fractions. Fractions obtained from Tar Sand bitumen that have been coked and hydrotreated may also be used. All processing sequences employ distillation at some point. The initial boiling points are generally controlled to produce a product that meets flash-point requirements. The final boiling points are generally set to meet other requirements, such as freezing point, smoking point, or naphthalene content. Fuels with a lower freezing point, such as Jet A-1, require reducing the boiling point of the back end (the heavier boiling part of the fuel). Kerosene jet fuels generally contain hydrocarbon compounds from the C8/C9 carbon number range up to the C15/C16 range. The sequence of processing and finishing steps used in the production of jet fuel blending stocks can vary widely depending on factors such as crude oil type, the complexity of the refinery, and concerns about specification limits. Detailed processing sequences are generally regarded as proprietary. However, processing sequences used to produce JP-5 have been surveyed for the U.S. Navy (Lieberman and Taylor, 1980; Varga, 1985). Examples of jet fuel processing sequences reported in this survey are discussed below. Crude oils that produce kerosene fractions with low total sulfur can be chemically processed to reduce mercaptan sulfur (thiol) or organic acid content to meet specification requirements (Dukek, 1992; Taylor, 1992). Two examples of chemical processing sequences are shown in Figure 7-1. In the first example, a kerosene with a high acid content but low mercaptan sulfur and total sulfur is first treated with a caustic followed by water washing. Treating the fuel blending stock with sodium hydroxide converts the carboxylic acids, which are soluble in the hydrocarbon phase but not soluble in the
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Aviation Fuels with Improved Fire Safety: A Proceedings aqueous phase, to sodium carboxylic salts, which are insoluble in the hydrocarbon phase but soluble in the aqueous phase, where they are removed. Phenols and hydrogen sulfide can also be removed, but jet fuel range mercaptans and other sulfur compounds are left unconverted in the fuel. Clay treatment using attapulgus clay is employed to remove trace levels of sodium carboxylate salts left in the fuel and/or other trace polar compounds. If the fuel blending stock contains excess odorous mercaptan sulfur compounds but acceptable total sulfur levels, it can be treated by a sweetening process (example 2). The sweetening process converts mercaptans into odor-free disulfides, which are left in the fuel. Older processes (e.g., Doctor or Bender sweetening) involve the oxidation of mercaptans using elemental sulfur, which can produce unwanted polysulfides. Newer processes (e.g., Merox) involve the catalytic oxidation of mercaptans with air over a fixed bed (Duval, 1968; Scheumann, 1956; Brown, 1973; Verachtert et al, 1985). The Merox oxidation step is preceded by a caustic wash to reduce the carboxylic acid level and followed by a water wash to remove carryover caustic and/or sodium carboxylate salts. A salt drier may be used to lower free-water levels prior to clay treatment. Catalytic treatment in the presence of hydrogen can also be used to manufacture jet fuel blending stocks, for example, when higher sulfur levels mandate sulfur removal (Figure 7-2). Sulfur removal can involve either hydrotreating kerosene fractions or hydrocracking heavier fractions down to the boiling range of kerosene. The nature and magnitude of specific changes in compounds is a function of the type of hydrotreating process and its operational severity (e.g., catalyst type, pressure, temperature, hydrogen consumption, and space velocity). Hydrotreating usually removes sulfur from compounds such as mercaptans, sulfides, disulfides, and condensed thiophenes, removes oxygen from compounds such as carboxylic acids, peroxides, and phenols and removes nitrogen from nitrogen compounds such as indoles, carbazoles, and quinolines. In the hydrotreating process, organically bound sulfur is converted to H2S, oxygen is converted to H2O, and nitrogen is converted to NH3. Non-heteroatom compounds are left in the fuel. Hydrotreating also removes olefins FIGURE 7-2 Two examples of catalytic treatments in the presence of hydrogen used to manufacture jet fuel blend stocks. by adding hydrogen to olefin molecules, thus converting olefin to a saturated compound. Hydrocracking generally is done under high severity conditions. In addition to affecting boiling range conversion, hydrocracking also removes most heteroatoms and olefins (Mohantry et al, 1990). ADDITIVES Additives can have a significant impact on both the chemical composition and the chemical reactivity of jet fuel. Additives are strictly regulated by the specification to which the fuel is being manufactured. The approval process for a jet fuel additive is lengthy and involves extensive laboratory testing for material compatibility as well as testing for compatibility with other additives already approved for use in jet fuel. Specific approval by major engine and airframe manufacturers is also required. In general, military fuels require more additives than civil fuels. Most of the Jet A civil fuel used in the U.S. contains no additives. Outside the U.S., civil jet fuel, Jet A-1, contains up to 5 ppm of an additive to increase electrical conductivity, which increases the rate of dissipation of any electrostatic charge that may have built up in the fuel as a result of microfiltration. The additive in use today, Stadis 450, is a mixture of polysulfones, polyamines, and dinonylnaphthyl sulfonic acid (Henry, 1975). Antioxidants may also be added (up to 24 ppm) to prevent the formation of hydroperoxide and gum from free radical reactions involving dissolved molecular oxygen, which is present in all fuel exposed to air. A number of different compounds from the family of sterically-hindered alkyl phenols are permitted or mandated, particularly in hydroprocessed jet fuel. ASTM D 1655 permits the use of N, N-diisopropylparaphenylene diamine, but hindered alkyl phenols are generally the antioxidant of choice in a clear product like jet fuel. A metal deactivator is permitted (up to 5.7 ppm) to chelate dissolved metals, such as copper. Chelation eliminates the deleterious catalytic effects of dissolved metals on fuel stability. ASTM D 1655 and Defense Standard 91/91 only allow one metal deactivator, N, N disalicylidene-1, 2-propane diamine.
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Aviation Fuels with Improved Fire Safety: A Proceedings A number of additives have been approved by the US Air Force and Navy for use in JP-8 and JP-5 as corrosion inhibitor and lubricity improving additives at concentrations ranging from 9 to 23 ppm. These additives are organic fatty acids, typically a C36 dicarboxylic acid. They were used initially to prevent rust pickup in the fuel from pipelines and tanks, but they were subsequently found to improve the lubricity properties of fuel that had been severely hydrotreated. Hydroprocessing tends to remove the trace impurity lubricity-enhancing compounds that are naturally present in the fuel. Although individual specifications vary, it is common to require a purchaser agreement for the use of a metal deactivator or corrosion inhibitor/lubricity improver in civil fuels. Military aircraft fuel systems do not include fuel filter heaters, which are used in civil aircraft (Dukek, 1992). As a result, military fuels require up to 0.15 vol pct of a fuel system icing inhibitor. Diethylene glycol monomethyl ether is approved for this purpose, replacing ethylene glycol monomethyl ether, which is being phased out because of concerns about toxicity. The U.S. Air Force is currently developing and introducing into service an improved thermal stability JP-8 jet fuel called JP-8+100, which contains proprietary detergent/dispersant additives in conjunction with the sterically-hindered phenolic antioxidant, 2,6 di-tertiary butyl-4-methyl phenol, and a metal deactivator. CHEMICAL COMPOSITION AND REACTIVITY The overwhelming majority of chemical compounds that make up jet fuel are hydrocarbons, including normal and branched paraffins, single and multi-ring cycloparaffins, single and multi-ring aromatics, hydroaromatics, and olefins. The relative proportions of hydrocarbon compound types can vary considerably, depending on the type of crude oil (see Table 7-2). Specifications limit the total of aromatics as measured by ASTM D 1319, "Hydrocarbon Types In Liquid Petroleum Products by Fluorescent Indicator Adsorption," to a maximum of 25.0 vol pct. This adsorption method reports alkyl benzenes, polycyclic aromatics, and aromatic olefins as aromatics. Naphthalenes are generally limited to 3.0 vol pct. Older specifications limited olefins by ASTM D 1319 to a maximum of 5.0 vol pct. Surveys indicate that the actual vol pct of olefins, in general, is much lower (Dickson, 1995). Trace levels of heteroatom compounds are also present in jet fuel. Sulfur compounds are restricted to a maximum of 30 ppm from mercaptan (thiol) compounds and 3,000 ppm from all sulfur compounds. NIPER surveys indicate that the actual total sulfur levels are generally much lower (Dickson, 1995). In addition to thiols, sulfur compounds include sulfides, disulfides, and condensed thiophene-type compounds. Carboxylic acids (which are typically alkyl cyclo paraffinic acids) and phenols are also present and are limited by the acid number specification. A typical acid number specification limit of 0.015 mg KOH/g by ASTM D 3242 suggests an upper limit of approximately 60 ppm monocarboxylic acids in the jet fuel boiling range. Nitrogen compounds are not controlled by any specification, but analyses indicate that straight-run kerosenes typically have very low total nitrogen levels, e.g., less than 5 ppm. Nitrogen compounds are predominately alkyl indoles, carbazoles, pyridines, and quinolines (Snyder, 1970). At ambient conditions, the majority of hydrocarbon compounds present in jet fuel are unreactive toward one another. Because jet fuels are normally saturated with air, there is always the potential for a reaction of the hydrocarbon/heteroatom mixture with the dissolved molecular oxygen. However, autooxidation reactions are typically very slow at ambient temperatures because jet fuels normally contain either natural or added inhibitors and in addition do not contain a strong initiation source for free radicals. Thus, under normal circumstances, jet fuel is by and large chemically unreactive at ambient conditions. However, the fuel can undergo autooxidative thermal stability-type reactions leading to the formation of deleterious surface deposits as the fuel is exposed to higher aircraft and engine temperatures as it is being delivered to the combustor (Taylor, 1992). Hereroatom compounds also tend to be more susceptible to thermal stability autooxidation reactions than hydrocarbons. Trace impurities and additive compounds can potentially be reactive, although such occurrences are rare. For example, naturally occurring acids or acids present in some additives, such as corrosion inhibitors, have been known to become involved in acid-base reactions to form an adduct or reactions with metal cations to form soaps (carboxylic acid salts), which can have surfactant properties (Hazlett et al, 1991, 1993). TABLE 7-2 Variations in Kerosene Hydrocarbon Compounds Hydrocarbon Typea Variations in 150/280°C Kerosenes Paraffins Normal and branched paraffins 10–65 % Cycloparaffins Single-ring cycloparaffins Multi-ring cycloparaffins 10–35 % 5–35 % Aromatics Single-ring aromatics Multi-ring aromatics 12–20 % 1–3 % Olefins 0.5–5.0 % a By mass spectroscopy except for olefins which are by fluorescence indicator adsorption
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Aviation Fuels with Improved Fire Safety: A Proceedings FIGURE 7-3 Typical aviation fuel distribution system. TRANSPORTATION AND STORAGE The shipment of jet fuel from the refinery to an intermediate terminal to airport storage facility can involve pipelines, tankers, barges, railcars, and/or trucks. In the U.S., pipeline shipments often involve common carriers, fungible pipelines where fuel from multiple shippers is commingled. Fueling planes at larger airports generally involves using an airport hydrant system or a servicer (Waite, 1989; ATA, 1986). This complex storage and delivery system (illustrated in Figure 7-3) is designed to eliminate or control excess free water (i.e., water suspended in the fuel above the fuel saturation point), particulates such as rust and dirt, microbial contamination, and contamination from other products and their additives. A highly redundant system is employed with a number of different approaches that reduce fuel contaminants. Controlled tank settling to remove water and particulates involves quarantining the tank, removing settled water and dirt from the bottom of the tank, and withdrawing clean fuel from the top of the tank via a floating suction take-off. Filters can also be used to remove particulates. Units called filter/separators, which combine the functions of filtration to remove dirt, coalescence to remove excess water, and a hydrophobic barrier or separator that rejects any free water in the effluent fuel are also used. In addition to controlled tank settling and filter/separators, water-adsorbing media elements (called monitors) are often employed for filtration into and out of airport storage and as the final step before the fuel is loaded into an aircraft. Monitors stop fuel flow in the presence of excess water and filter out dirt. Clay treaters are also employed in the distribution system to remove surfactants, particularly in the U.S. where a large amount of jet fuel is shipped through multi-product pipelines. Surfactants can stabilize water droplets against natural settling and can also disarm coalescers. Removing water from the distribution system is the most effective way of preventing microbial contamination in jet fuel (Swift, 1988). Because of the importance of the distribution system for ensuring that clean, dry, uncontaminated fuel is loaded into aircraft, the effects of any new fuel additive on the integrity of the distribution system must be considered.
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Aviation Fuels with Improved Fire Safety: A Proceedings REFERENCES ATA (Air Transport Association of America). 1986. Specification for Airport Fuel Inspection and Testing No. 103. Washington, D.C.: ATA. Brown, K.M. 1973. Treating jet fuel to meet specs. Hydrocarbon Processing 52(2):69–74. Campbell, P.P., and D.P. Osterhout. 1967. Proceedings of Symposium on Jet Fuel held June 29, 1967, in Boston, Massachusetts. Philadelphia: ASTM, Committee D-2 Petroleum Products and Lubricants, Technical Division J on Aviation Fuels. Dickson, C.L., and G.P. Sturm. 1995. Aviation Turbine Fuels 1994. Bartlesville, Oklahoma: National Institute for Petroleum and Energy Research, Department of Energy. Dukek, W.G. 1992. Aviation and other gas turbine fuels. Kirk-Othmer Encyclopedia of Chemical Technology (4th ed.), vol. 3, p. 788, Mary Howe-Grant, ed. New York: John Wiley & Sons. Dukek, W.G., and K.H. Strauss, eds. 1979. Factors in Using Kerosene Jet Fuel of Reduced Flash Point: A Symposium. ASTM Special Technical Publication 688. Proceedings from conference held on December 3–8, 1977 in Dallas, Texas. Philadelphia: ASTM. Duval, C.A.. 1961. Treating and sweetening. Pp. 164–195 in Advances in Petroleum Chemistry and Refining, vol. 4, J. J. McKetta, Jr., ed. New York: Interscience. Dyroff, G.U. (ed.). 1993. Manual on Significance of Test for Petroleum Products (6th ed.). Philadelphia: ASTM. Hazlett, R.N., E.J. Beal, M.D. Klinkhammer, and J.A. Schreifels. 1993. Comparison of stability results for distillate fuels exposed to different stress regimes. Energy and Fuels 7(1):127–132. Hazlett, R.N., J.A. Schreifels, W.M. Stalich, R.E. Morris, and G.W. Mushrush. 1991. Distillate fuel insolubles: Formation conditions and characterization. Energy and Fuels 5(2):269–273. Henry, C. P. 1975. Composition of Olefin-Sulfur Dioxide Copolymers and Polyamines as Antistatic Additives for Hydrocarbon Fuels. U.S. Patent No. 3,917,466. Lieberman, M., and W. F. Taylor. 1980. Effect of Refining Variables on the Properties and Composition of JP-5. Final Report prepared for Naval Air Propulsion Center under contract no. N00140-78-C-1491. Linden, N.J.: Exxon Research and Engineering Company, Products Research Division . Martel, C.R. 1987. Military Jet Fuels, 1944–1987. Contract No. AFWAL-TR-87-2062. Dayton, Ohio: Air Force Aero-propulsion Laboratory. Mohanty, S., D. Kunzru, and D.N. Saraf. 1990. Hydrocracking: A review. Fuel 69(12):1467–1473. Scheumann, W.W. 1956. Chemical and hydrogen treating. Petroleum Processing 11(April):53–57. Smith, M. 1970. Aviation Fuels. Henley-on-Thames, Oxfordshire, U.K.: G.T. Foules & Co., Ltd. Snyder, L.R. 1970. Petroleum nitrogen compounds and oxygen compounds. Accounts Chemical Research 3(9):290–299. Swift, S.T. 1988. Identification and control of microbial growth in fuel handling systems. Pp. 15–26 in Distillate Fuel: Contamination, Storage and Handling, H.L. Chesneau and M.M. Dorris, eds. ASTM Special Technical Publication No. 1005. Philadelphia: ASTM. Taylor, W.F. 1988. Fuels: The Next 50 Years. Exxon Air World 40(3):29. Taylor, W.F. 1992. Effect of manufacturing processes on aviation turbine fuel thermal stability. Pp. 81–89 in Aviation Fuel: Thermal Stability Requirements, ASTM Special Technical Report No. 1138, Kirklin, P.W. and P. David, eds. Proceedings from conference held on June 26, 1991, in Toronto, Canada. Philadelphia: ASTM. Varga, G.M., M. Lieberman, and A.J. Avella. 1985. The Effects of Crude Oil and Processing on JP-5 Composition and Properties. NAPC-PE-121C. Contract No. N00140-81-C-9601. Trenton, N.J.: Naval Air Propulsion Center. Verachtert, T.A., J.R. Salazar, and B.E. Staehle. 1985. Merox Catalyst Innovation Solves Difficult Kerosene Treating Problems. Paper presented at National Petroleum Refiners Annual Meeting, San Antonio, Texas, March 24, 1985. Waite, R., ed. 1989. Manual of Aviation Fuel Quality Control Procedures (MNL5). Philadelphia: ASTM.
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Aviation Fuels with Improved Fire Safety: A Proceedings 8 Concepts for Safe-Fuel Technology Bernard R. Wright Southwest Research Institute ABSTRACT Researchers in the field of fuel safety have been on a quest to find replacements for halon and to develop new safety systems for protection against the ignition and burning of fuel in aircraft accidents. Recommended halon replacements have been announced for commercialization. New and undeveloped technologies include surface enhancement, low volatility, and self-activating powder extinguishment. These technologies may be used individually or may be synergistically combined via micro-encapsulation or specific system applications. INTRODUCTION The Montreal Protocol and the U.S. Clean Air Act stopped the production of halon at the end of 1993 because of stratospheric ozone depletion. Regulations governing the production and use of halon came about as a result of scientific assessments conducted under the auspices of the World Meteorological Organization (WMO) and the United Nations Environment Programme. The Scientific Assessment of Ozone Depletion is a collection of scientific reports and a history of those reports in relation to the international policy process (WMO, 1994). The new regulations created a need for substitute agents that could be used for fire suppression and explosives protection. To meet the new standards, researchers in the field of fuel safety have been on a quest to find new safety systems that will protect against the ignition and burning of propulsion fuel. Previous studies were based on decreasing the flammability and ignition potential of fuel on board the aircraft. The idea was to modify the fuel so it would not sustain a fire. However, fuel treatments to increase fire safety are costly and add weight to the aircraft. In addition, decreasing the flammability or ignition potential of fuel may interfere with the fuel's ability to perform regular operations. In this paper, new and unproven technologies for fuel protection will be described. These include substances that (1) are consumed with the fuel, (2) are mixed with fuel after an incident, and (3) suppress external fire. Various types of protective agents and fire suppressant agents are also described as replacements or alternatives. These include (1) surface enhancement agents, (2) low volatility agents, and (3) combined technologies, such as micro-encapsulation. The concepts presented in this paper are applicable to fuel itself and to fuel fires, such as fuel fires immediately following survivable air crashes. BACKGROUND Previous Safe-Fuel Technologies Safe-fuel technologies have been developed and implemented with varying degrees of success. Some are already in use, including fuels and hydraulic fluids with improved safety properties. Volatile JP-4 fuel has been replaced by JP-8 fuel (Jet-A/A1), which has a flash point temperature above 100°F so that flame speed is reduced from 12 feet to 1 foot per second. A ''fire-resistant-fuel," comprised of fuel, water, and a solubilizing surfactant, has also been developed and patented. A number of inerting agents and systems have been tested. The U.S. Army mixed halon with fuel for tank propulsion but could not use it because of the caustic/toxic products of combustion. Inerting of the ullage (empty space) in fuel tanks has been implemented. Halon is now used in F-16 aircraft, and nitrogen is used in the Apache helicopter and C-5A aircraft. The on-board inert gas generator (OBIGGS), which has a membrane to separate nitrogen from air, has also been developed and is used in some aircraft. The use of OBIGGS allowed the fuel tank oxygen concentration to be maintained at approximately 9 vol pct, which is too low to ignite. Foams in aircraft fuel tanks are used by the U.S. Air Force. A coannular fuel tank, with an inner space containing fuel and an outer space containing halon, has been demonstrated using halon 2402 but has not been implemented. A less vulnerable fuel has been developed using a high molecular weight polymer as an antimisting agent. This approach paralleled a Federal Aviation Administration (FAA) project that incorporated an antimisting agent with a shear thickening mechanism. Antimisting kerosene (AMK) was
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Aviation Fuels with Improved Fire Safety: A Proceedings developed to mitigate fuel misting thereby decreasing fuel flammability and ignition, but the fuel was not ready for propulsion combustion and had to be processed before injection. When AMK was tested at full scale, it did not prove to be cost/performance effective. Low expansion foam has been used to protect against fires in spilled liquid fuel (Class B) around the outside of the aircraft. The foam is a liquid at standard temperature and pressure and can therefore be projected to an aircraft's critical distance. Foam, which is primarily water, contains surfactants that isolate the fuel from air so that a flammable mixture of fuel and air is prevented. In other words, the lower flammable limit is not reached because of the layer of foam on the fuel surface. Another approach is using dry chemicals around an aircraft to provide for rapid knockdown of fire. A good deal of progress has been made toward reducing flammability in aircraft interiors. New materials have significantly reduced flame speeds and smoke generation, as well as retardants and intumescent coatings. New exterior materials have been tested and appropriate standards for fire protection have been defined. Halon and Halon Replacements When selecting a substance for protecting against fuel fires, the first requirement is that the agent have the desirable physical-chemical characteristics. Halon 1301 has been the primary agent for fire protection on board an aircraft because (1) it is an effective fire suppressant (through the chemical action of the bromine atom, which acts as a catalytic fire chain breaker); (2) it can be stored as compressed liquid (small volume); and (3) it can flood (in gaseous form) the engine nacelles and cargo compartments. The initiating study to find a halon replacement for the U.S. Air Force (USAF) was performed by Zallen International Associates (ZIA) (Zallen, 1992). A list of several hundred chemical agents was narrowed down to 12 potential candidates that met the criteria for a clean and effective replacement for halon 1301 fire extinguishing systems in aircraft (Bennett, 1992). Comparative analyses were performed on their physical properties and environmental parameters including boiling point, ozone depletion potential, toxicity, and fire suppression effectiveness. Iodotrifluoromethane (CF3I) was identified in the initial study as the only halocarbon containing iodine that had a sufficiently low boiling point, -23°C (CHF2I has a boiling point of +22°C, and C2F5 I has a boiling point of +12°C). Because CF3I was the only agent (Purdue University, 1950) with fire suppression effectiveness close to halon 1301 (6.8 percent suppression agent concentration for CF 3I; 6.1 percent for halon 1301 [CF3Br]) there was a great deal of interest in investigating the environmental and toxicological characteristics of CF3I data. The Purdue University study of 1950 also demonstrated that several agents were even more effective fire suppressants than halon 1301, namely C2F4I2 (5.0 percent), C2F5I (5.3 percent), and C2H5I (5.6 percent). One agent, which was proprietary at the time, can now be identified as HFC-236fa (C3H2F6). The other agents investigated had boiling points that were too high for conventional total flooding, were not efficient fire suppressants, or were too toxic. Effective agents with higher boiling points were considered for streaming, or misting, applications. The potential halon 1301 replacements were tested at laboratory scale at the National Institute of Standards and Technology (NIST, 1994), at intermediate scale by ZIA at Southwest Research Institute (Zallen, 1994), and at large scale by Wright Laboratory of Wright-Patterson Air Force Base. These investigations led to compatible conclusions, and the recommended agents (with trade names) have been announced for commercialization in the Significant New Alternatives Policy (SNAP) of the Environmental Protection Agency (EPA). DEFINING THE PROBLEM Post-crash fires cause about 40 percent of the fatalities in survivable incidents and are caused by uncontrolled fuel leaks ignited by sparks, hot surfaces, or fire elsewhere. In the critical period following a survivable crash, before ground crews arrive and gain control, immediate protection is needed against external fuel fires. The critical area around the aircraft is defined as the area beyond which heat from a fire does not affect the aircraft. Fire-safe fuel technologies must be discussed in specific contexts. The specific scenario for this discussion is a survivable aircraft crash. In this scenario, fuel near the aircraft at rest is the major threat to survivors. Fuel spilled during such a crash is not of specific concern because the aircraft is not technically involved with fuel left behind, i.e., fuel beyond the critical distance. The spilled pool of fuel near the aircraft can ignite and burn. The fuel mist near the aircraft is even more easily ignited than the fuel pool. Although the fuel mist is not a sustained hazard, it is a strong ignition hazard. However, it is not necessary to protect all of the fuel carried aboard the aircraft. Only the fuel that ends up within the critical distance from the crashed aircraft is a sustained hazard. It would therefore be efficient to treat the spilled pool of fuel and, specifically, the surface of the pool. Although the spilled pool of fuel if the greatest danger, other fire hazards must be taken into account. These include the danger of pressurized fuel being sprayed/atomized, the danger of ignition of flammable fluids in storage tanks and delivery lines, and the danger of ignition of other flammable substances, such as cargo, oxygen, metals, and hydraulic components.
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Aviation Fuels with Improved Fire Safety: A Proceedings Once the general potential hazards have been analyzed, fuel systems should be characterized to define specific hazard scenarios. Accident investigation reports and fuel systems configurations are typical ways of defining specific hazard scenarios, which can be used to hypothesize various safety concepts. Many pragmatic concerns must be addressed when evaluating potential safety concepts. After the basic performance of an agent or concept has been hypothesized and tested, the next step is analysis of a practical system's performance. Initial analyses may be done of size, weight, and costs. Cost analyses must include the number of applications and whether the system is new or retrofitted, because these factors determine how the benefits are attributed. Logistical analyses include geographic locations and transportation requirements (e.g., approval requirements in the United States, as opposed to requirements in Europe, for pressure vessels). Reliability studies include analyses of specific parts and repair time and maintenance. Once initial evaluations have been established, program plans for the safety concept must be developed. For new or undeveloped concepts, full testing plans must be developed, including tests for toxicity and potential exposure. Options and combinations of concepts should also be evaluated. For example, a fire prevention agent might be mixed with fuel by coannular fuel/agent tanks or by honeycomb compartmented agents and fuels. A fire prevention agent that may perform well in one application might not perform well in another. Perfluorohexane, for example, is a good fire suppressant in general and has no ozone depletion potential (it does have a long atmospheric lifetime). However, mixing liquid perfluorohexane with liquid fuel has a low coefficient of mass transport from liquid to gas phase and results in poor protection against open-pool ignition and spreading flames (Naegeli et al., 1993). NEW AND UNDEVELOPED SAFE-FUEL TECHNOLOGIES The following is a discussion of three technologies for making fuels safer: surface enhancement, low volatility technology, and self-activating powder extinguishment. Specific agent compositions are not presented, but recent literature abounds with descriptions of halon replacements and alternatives. Surface Enhancement Technology Halon replacements with reasonably good weight are commercially available. The effectiveness of replacements has been defined in classic tests, such as total flooding and streaming capabilities in classic halon test fire scenarios, e.g., computer room fires, Army tank engine compartment fires, USAF engine nacelle and dry bay fires, and Navy machine room fires. No halon replacement has all of the good characteristics of halon 1301. In order to get the same protection for similar weight and cost, the physical and chemical characteristics of less desirable agents must be enhanced. Because the fuel surface provides the vapor for a fire, enhancing the effectiveness of the agent at the fuel surface will enhance agent efficiency and improve its fire prevention potential. So, one idea is to provide surface-enhancement for halon replacements or alternatives. Surfactant mixtures can be used to improve the distribution of the agent at the fuel surface. The mixture must, of course, be environmentally compliant and stable. Surfactant mixtures have been used with water, which is the basis for contiguous foam. Although in principle a water/surfactant mixture works, it is neither weight nor temperature effective in many applications. But it should be possible to use surface enhancement to improve the performance of halon replacements. Low Volatility Technology The environmental problems associated with effective halons is well known. Bromine and chlorine function well against fire, but their high volatility leads to release into the atmosphere. One approach to mitigate this danger was to use hydrochlorofluorocarbons (HCFCs) whereby a substituted hydrogen atom decreases the stability of the halon molecule in the atmosphere/troposphere on its way to the stratosphere. This hydrogen substitution is also the basis of hydrofluorocarbons (HFCs). However, in general the hydrogen atom substitution implies more reactivity, which also implies more toxicity. Halon replacement studies have so far concentrated on paraffinic chemicals because the reactivity of olefinic chemicals is known and their toxicity suspect. The boiling points of higher molecular weight chemicals are too high and the vapor pressures too low for halon replacement, but larger molecules do have more bonds (i.e., degrees of freedom) to absorb ignition or fire energy. Some general (not absolute) rules follow: the boiling point of an agent rises with molecular weight; using heavier halogen atoms raises the boiling point; hydrogen substitution raises the boiling point; hydrogen substitution increases reactivity and potential toxicity; fluorine atoms provide stability; and vapor pressure is inversely proportional to the boiling point. These generalities indicate why so few chemicals can readily be identified for replacement of volatile halon 1301. Using lower volatility agents in fire prevention applications may improve effectiveness. Highly volatile agents have inherent deficiencies in that: the agent can vaporize en route to the fire; the agent may not be concentrated near the fuel surface; and the concomitant large vapor cloud is an environ
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Aviation Fuels with Improved Fire Safety: A Proceedings mental hazard, may be toxic, and may not increase fire prevention. Low volatility technology (LVT) works by deriving a volatile, efficient fire suppressant from pyrolysis of the initially nonvolatile agent. LVT agents would be effective because no agent would be lost by evaporation along the delivery trajectory, the agent is deliverable because of its low volatility, harmful emissions would be minimized, and agent/surfactant control of the fuel surface would be increased. The agent would be concentrated where the fire was initiated with heat from the fire and hot surfaces releasing the fire prevention chemical. Self-Activating Powder Extinguishment Technology Dry chemical powders have been used as fire fighting agents for many years. It is known that fine particles, on the order of ten micrometers, are more effective fire extinguishers on a mass basis than larger particles, on the order of one-hundred micrometers. Large particle powders are still used, however, for two primary reasons. First, grinding powder to fine particulates, which can cake in storage, is not cost effective. Second, the short throwing (delivery) distance of fine particulate powder makes it difficult to get the agent where it is needed. Three mechanisms principally account for the fire prevention performance of dry chemicals. The first is the thermal cooling from the intrinsic thermal mass of the cold material; this is augmented by endothermic decomposition reactions. The second mechanism is chemical action, whereby flame propagating species, such as hydrogen and hydroxyl radicals, may recombine either heterogeneously on the surface of the particles or homogeneously as a result of gas phase reactions catalyzed by alkali metal atoms. Third, the water and carbon dioxide produced in the reactions act as local oxygen diluents or inertants. Two of these essential mechanisms are surface effects, so a greater surface area per unit mass of fine particulate powder speeds the mechanisms for better fire prevention. Comparative tests have shown that fine dry powder is more effective at extinguishing fires than any of the new halon replacements. But, there are some problems: limitations on the fineness of the powder that can be manufactured at reasonable cost; difficulties preventing the agglomeration and coagulation of fine powders in storage; and problems of effectively discharging particles with low momentum. The use of fine particle powders for fire prevention is currently based on propellant-generated solid aerosols. Pyrotechnic compositions, comprising a potassium-based oxidant and an organic binder, can generate a fire extinguishing aerosol. High reaction temperatures mean that the salts produced are initially gaseous and that the particulates produced by condensation from the vapor phase are extremely fine. Experiments on devices for total flood extinguishment show that suppression by fine particle powder requires significantly lower mass concentrations than halon. Pyrotechnically-generated aerosols (PGAs) use potassium-based oxidizers to maximize the known fire suppression capability of potassium salt. The fuel may consist of a binder and a metal powder as co-fuels that generate enough heat to vaporize potassium-based fire prevention additives. The oxidizer/fuel ratio is designed to generate the product quickly, absorb energy, and result in optimal composition. PGA aerosol properties give them excellent three dimensional distribution and long term suspension. They have zero ozone depleting potential, low toxicity, and show significant weight reductions compared to halon. TECHNOLOGY COMBINATIONS Known effective fire suppressing agents are limited in number and type. There might therefore be some benefit in combining individual technologies, not by the simultaneous use of two types of prevention, but rather by some synergistic combination of the scientific bases that make certain materials work. Micro-encapsulation In the development of fire resistant fuel wherein 2 to 3 percent water was used in diesel fuel, two things have been shown. First, although evaporative cooling effects are significant, they are not responsible for the self-extinguishing properties of aqueous diesel fuel. Second, when the heat transfer in the liquid surface, which causes preheating of the surface ahead of the flame, is reduced, it generates a water vapor blanket for self-extinguishment (Weatherford and Naegeli, 1984). One way to both increase the amount of fire suppressant at the fuel surface and lower the volatility of a high volatility agent is micro-encapsulation. Micro-encapsulation would reduce vapor loss from a high volatility agent until the agent is needed. The contents of the microcapsule could include any number of fire suppressants. By making them lightweight for flotation, the microcapsules could be concentrated at the fuel surface for synergistically increasing efficiency. Heavier capsules could be developed for situations where mixing is required. A urea-based capsule could potentially both release the contained substance and provide fire protection. The outer shell of the capsule might be a dry chemical powder, for example, Monnex™ powder, which provides rapid fire knock-down by cracking apart to form smaller, more efficient particles. System Applications There are many different ways to use new agents for fuel fire prevention or suppression. Insoluble compounds with fire
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Aviation Fuels with Improved Fire Safety: A Proceedings protection capacity may be added to fuel for protection in the fuel tank, but be physically separated from the fuel before combustion takes place. The stability of the suspension and the chemical composition of the compounds need to be considered in these applications. Inerting agents which are kept separate from the fuel may be released in a number of ways. The fuel and inertant could be separated by a honeycomb in an intrinsic system. The inertant would thereby be released if the fuel tank ruptured. In a separate system, the inertant could be kept in frangible tubes in the fuel, and released by fire or rupture. Fire suppressing agents could also be used as fuel additives. Mixtures of salts (acetate, citrate, carbonate) with water have long been used to protect cooking facilities. New investigations (Finnerty et al., 1996) have shown that aqueous solutions of potassium lactate or potassium acetate extinguish fuel fires much better than water alone. The improvement can be attributed to the release of solid salts upon evaporation of the water in the fire zone. This new understanding could be used for fuel fire-safety. REFERENCES Bennett, Michael. 1992. Halon replacement for aviation systems. Pp. 668–669 in Proceedings of the 1992 International CFC and Halon Alternatives Conference: Stratospheric Ozone Protection for the 90s held September 29 to October 1, 1992 at the Washington Hilton, Washington, D.C. Frederick, Md.: International CFC and Halon Alternatives Conference, 1992. Finnerty, A.E., R.L. McGill, and W.A. Slack. 1996. Water-Based Halon Replacement Sprays. ARL-TR-1138. Aberdeen Proving Ground, Md.: Army Research Laboratory. Naegeli, D.W., B.R. Wright, and D.M. Zallen. 1993. A Study of Aircraft Post-Crash Fuel Fire Mitigation. BFLRF Interim Report No. 292. San Antonio, Texas: Belvoir Fuels and Lubricants Research Facility, Southwest Research Institute. National Institute of Standards and Technology (NIST). 1994. Evaluation of Alternative In-Flight Fire Suppressants for Full-Scale Testing in Simulated Aircraft Engine Nacelles and Dry Bays. Special Publication 861. Washington, D.C.: U.S. Government Printing Office. Purdue University. 1950. Fire Extinguishing Agents. AD 654-322. Lafayette, Ind.: Department of Chemistry and Research Foundation, Purdue University. Weatherford, W.D., and D.W. Naegeli. 1984. Study of pool burning self-extinguishment mechanisms in aqueous diesel fuel microemulsions. Journal of Dispersion Science and Technology 5(2):159–177. World Meteorological Organization (WMO). 1994. Executive Summary: Scientific Assessment of Ozone Depletion. Global Ozone Research and Monitoring Project Report No. 37. Geneva, Switzerland: WMO. Zallen, D.M. 1992. Halon Replacements Study. ZIA-92-001. Wright-Patterson Air Force Base, Ohio: Aeronautical Systems Division, U.S. Air Force Material Command. Zallen, D.M. 1994. Halon Replacements Study. ZIA-94-0100. Wright-Patterson Air Force Base, Ohio: Aeronautical Systems Division, U.S. Air Force Material Command.
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