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--> Appendix A Halon Use by the Navy Why Halon? A Historical Overview Extinguishing fires at sea has always been a matter of priority throughout maritime history, particularly for navies, which are faced with the dual challenge of putting out fires caused by both accidental ignition and enemy action. And crews of ships and aircraft share a common threat from fires—the potential for loss of life. Understandably then, fire prevention and fire fighting readiness are major concerns of captains and pilots as well as senior commanders. The Navy follows the general practice of categorizing fires by type of fuel—Class A for paper, wood, and general combustibles, B for flammable liquids, C for electrical, and D for metals. The frequency of occurrence and seriousness of a class of fire and the impact on occupants vary considerably between ships and aircraft. A Class B fire, for example, ranks at the top of the critical list and can lead very quickly to loss of an aircraft if unchecked; on a ship the possibility of immediate loss of the vessel is less, but loss of life can be considerable if such a fire is not brought under control in short order. In recent years the need to prevent loss of aircraft damaged by enemy guns and missiles has been a driver in efforts to develop and install fire extinguishing systems in aircraft. In ships, on the other hand, the requirement to cope with accidental fires, particularly in machinery spaces, has led the way to developing more effective fire extinguishing systems. Over the years the Navy has sought ways to improve fire fighting capabilities as ships have been required to handle increasing quantities of munitions and volatile aircraft fuels, and have been equipped with propulsion systems requiring high-pressure, easily atomized fuels. Continuing this tradition of evolutionary improvement to meet changing needs, halon was introduced into the Navy as a principal fire extinguishing agent in recognition of its extraordinary fire extinguishing capabilities. Ships Fires aboard ship in World War II were fought by damage control teams applying water and protein foam. Hand-held CO2 extinguishers were widely used, and overhead water sprinklers were also employed in confined spaces and aircraft carrier hangar decks. Steam smothering systems were available in some ships to handle engine room bilge fires. The emphasis in fire fighting was to attack fires directly with men manning hoses dispensing solid water streams or fog, and this remained the accepted approach during the immediate postwar years. In the late 1960s, a series of aircraft carrier fires incident to Vietnam War operations triggered a search for more effective ways to fight massive flight deck fires. Fighting fires aboard carriers, while always challenging, had become more so in the age of jet aircraft, which carried ten times as much fuel and ten times the weight of explosives as had predecessor aircraft in World War II. As a result, aqueous film-forming foam (AFFF) was introduced as an effective replacement for protein foam, and it became the primary agent in fighting flight deck fires. Flush deck nozzles were installed to dispense AFFF on demand to deal with pooled area fuel fires. Airfield-style fire trucks were put aboard carriers, each carrying AFFF and dry powder (PKP—potassium bicarbonate powder). Additionally, small twin-agent flight deck tractors were equipped with small amounts of AFFF and PKP for quick-reaction fire suppression. Twin-agent hoses also began to appear in ship machinery spaces mounted on reels for ready access by engine room personnel. Despite these advances, a flammable fuel fire with vertical dimensions remained a fire fighting challenge, as did the pressure-fed engine room fuel spray fire. And in the late 1960s, machinery space
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--> fires on board several ships induced a review of both equipment and fire fighting tactics. A variety of technical approaches were considered that would make possible quick evacuation of a machinery space, followed by remote actuation of the fire extinguishing system. After a rigorous selection process and substantial testing in the early 1970s, halon 1301 was chosen as the optimum total flooding agent for ''abandon-the-machinery-space'' fires. The first halon systems were installed in aircraft carriers and mine craft in 1978. A policy was established calling for installation of halon in new-construction ships, such as the FFG-7 class frigates, and selective retrofitting into older vessels began on an age-selective basis. The original twin-agent AFFF/PKP reels were retained in engine rooms for small-fire application, but with the PKP side deactivated. Engine rooms also were equipped with AFFF bilge flooding systems. And although the principal reason for acquiring halon 1301 systems was to fight machinery space fires, halon 1301's attraction as a very effective, non-toxic agent resulted in its being substituted for CO2 in other spaces where flammable liquids were stored. This introduction of halon to the Navy followed its earlier acceptance for total space flooding applications in the civil community. Thus, because of a confluence of events—availability and civil acceptance of halon 1301, an urgent Navy need for a better agent, and top-level support—halon became the agent of choice for coping with fuel spray fires in confined spaces. Use of halon enabled the Navy to adopt a casualty-reducing tactic of (1) taking the man out of the loop initially by abandoning the fire scene, (2) remotely actuating the halon 1301 flooding system, and (3) reentering the space when the fire was extinguished to deal with any minor residual flare-ups. Halon 1211 has only limited application aboard ship. It replaced PKP in fire trucks aboard aviation ships in the late 1980s for fighting three-dimensional fires. The agent is also used in mine craft (MSCs) and air cushion landing craft (LCAC), and there are a few hand-held bottles to be found in certain other ship classes. Aircraft Fires in aircraft have been a major concern since the inception of powered flight in the early 1900s. The very nature of aircraft—being airborne, carrying large amounts of flammable liquids, containing potential ignition sources—makes them inherently vulnerable to loss if fire should break out. Hence, fire prevention is a major consideration in aircraft design, as are fire extinguishing systems tailored for the specific plane and its anticipated operating environment. Since most fires start in inaccessible areas, particularly in military tactical aircraft, extinguishing them must depend on automatic or remote activation of extinguishing systems. And as mentioned above, combat aircraft have the additional challenge of coping with damage that may be inflicted by enemy antiaircraft artillery and missiles. Early combat loss experience in World War II highlighted the vulnerability of tactical aircraft to loss by fire and explosion. Self-sealing fuel tanks were installed to reduce the probability of leakage if hit, with the resultant fumes causing explosions in void (dry bay) areas. Additionally, attention was paid to placement of fuel lines and shielding components. CO2 fire extinguishing systems were installed in the nacelles of multiengine aircraft, as they were in civil airliners of the time. The introduction of jet aircraft into the Navy in the 1950s was accompanied by a change in strategic emphasis toward nuclear warfare. Attack aircraft were designed to fly long ranges, while designers tried to exact maximum speed and altitude performance from fighters. In the quest for performance, the vulnerability of planes to combat damage, including fire and explosion, was accorded low priority during aircraft design. Even in the case of rotor craft that fly slowly at low altitude, little attention was paid to measures that might reduce vulnerability to loss if the helicopter was struck by enemy projectiles or small missiles. During the Vietnam War the United States suffered combat losses totaling 5000 aircraft—2500 fixed-wing jets and 2500 helicopters. As losses mounted during the course of the conflict, studies were initiated to see what might be done to lower loss rates, an effort that continued after the war. The analysis revealed that fuel fires and explosions accounted for 50% of the losses and that half of these
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--> were attributable to fuel explosions in voids or so-called dry bays. As a result, the military services joined in an effort to improve the survivability of jet aircraft and helicopters. This led to development and experimentation with a variety of approaches that addressed fire extinguishing in several areas, including engine nacelles/bays, dry bays, fuel tanks, occupied spaces, and ground and ship flight decks. Technologies considered were (1) solid foams, powders, and inert gas generators for dry bays; (2) solid foams and inert gases for fuel tank ullage areas; (3) halon 1301 for engine nacelles and bays; (4) portable halon bottles, principally 1301, for occupied areas; and (5) AFFF and halon 1211 for crash fire fighting and small fires incident to engine start. The adoption of halons 1301 and 1211 was the culmination of fleeting military involvement with halons over the years. In the 1920s non-fluorinated halon agents were tried experimentally in engine nacelle extinguishers, but their use was abandoned by the U.S. military in favor of the non-corrosive CO2. Despite their relatively high inhalation toxicity, systems using halon 104, 1001, and 1011 were developed during World War II and employed by the British and Germans in military aircraft. The use of these agents expanded into the civil sector after the war. In the United States, however, it was only after development of fluorinated halons (1301, 1211) that CO2 was replaced in Air Force and Navy aircraft by these new highly effective, less toxic, and non-corrosive agents. And since they had already gained some acceptance in U.S. civil aviation as well as in various civil ground applications, the military quickly adopted them to meet the variety of needs cited above. Ship Fire Extinguishing Systems Fire at sea has always posed a special danger. In warships, the fire hazard is exacerbated by the threat of explosive weapon warheads and propellants. Throughout its history the Navy has dealt with fire protection challenges by exacting the most from existing fire fighting systems through organization and training as well as by exploiting new technologies. The exploitation of dry chemical powders and aqueous film-forming foam as well as the introduction of specialized naval fire fighting systems are examples of the constant improvement sought by the Navy in the safety and survivability of its vessels, aircraft, and crews. Employing halons for machinery space and aviation fire extinguishing applications is an example of adopting new technology to improve fire protection. Machinery Space Fires The principal fire threats in machinery spaces are the combustible liquid pool and pressurized spray fire. The most hazardous type of incident, and one that absolutely requires a gas-phase fire suppressant, is the three-dimensional spray or cascading fire. These fires arise from fuel or lubricant pipe or fitting leaks, human error, or mechanical damage. Leaks can vary in scale from less than 1 to greater than 50 gallons per minute. Pressurized spray fires generally occur in fuel, lubricating oil, or hydraulic fluid system piping. Pressures range from 10 to 1000 psi. Non-pressurized cascading fuel fires often involve sounding tubes, gravity storage tanks, and fuel piping that transit the space servicing other areas such as aviation fuel systems. In general, a spray or cascading fuel fire will also produce a pool fire. A release of fuel or lubricating oil can be quickly ignited by hot surfaces (steam pipes or boiler fronts), electrical arcing or shorts, welding operations, and mechanical sources (friction, sparking, and so on, related to equipment failure). The intensity of these fires can easily approach 50-MW power equivalent. The fire growth time scale is on the order of several seconds, so that very large fires, high temperatures, and fatal concentrations of carbon monoxide (CO) can occur in 30 seconds or less. Since there is insufficient oxygen to maintain a large fire, the power level will decrease with time, and higher CO production will occur. The size and growth rate of these three-dimensional fires preclude safe reliance on manual firefighting in closed spaces. Clearly, manual fire fighting against a large machinery space fire is not the approach of choice because of the rapidity with which the space becomes untenable due to heat, smoke,
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--> and toxic combustion by-products. Indeed, these hazardous conditions are the very reason halon 1301 was introduced in the Navy some 20 years ago. Reignition is also a key consideration in fighting machinery space fires. Three sources of reignition in ship machinery spaces—hot surfaces, electrical sources, and smoldering solid combustibles—form the basis for the required agent hold times and reentry procedures. Each source of reignition is described briefly below. Hot surfaces—These result from normal power generation in a ship and include steam piping where lagging is breached, diesel engine exhaust manifolds, gas turbine casing and exhaust stacks, and boiler fronts. Some hot surfaces, particularly steam piping, can remain above the fuel autoignition temperature for hours. Unless there is an exceptionally long (> 5 to 10 minutes) preburn time associated with a fire, metal surfaces heated to above the fuel autoignition temperature will cool relatively quickly after the fire has been put out. Accordingly, after a typical 20- to 30-minute agent hold/cool down period, fire-heated surfaces will usually not be a hazard. Electrical sources—These arise from fire damage to electrical equipment and cabling. Shorts between cables or to ground may result in arcing or resistance heating. These reignition sources, if in proximity to a fuel surface or fuel vapor, are energetic enough to cause reignition and, in extreme cases, deflagration of a fuel vapor cloud. These reignition sources will remain a threat until all the power to the space is secured. While it is a relatively straightforward exercise to secure power serving a space, it is difficult in some ships to secure power in all cables that transit the space. Smoldering fires—Some transient combustibles, insulation, and lagging materials are susceptible to smoldering combustion. Typical design concentrations of halon 1301 (or replacements) are not sufficient to extinguish the condensed-phase, oxidation process, particularly in cellulosic materials. Thus, once the agent concentration has decayed sufficiently, a smoldering fire may reignite. Extinguishing Systems Three systems are employed to control or extinguish fires in machinery spaces. The total flooding halon 1301 system is the key element of a three-element overall system of fire protection. It is capable of extinguishing any flammable or combustible liquid fire, pool or spray, as well as solid combustibles ignited as a result of the liquid pool or spray fire. Halon 1301 is used when a fire is too large to suppress manually, which is usually the case with pressurized spray fires. Additional fixed protection in machinery spaces is offered by the AFFF bilge foam system. It is designed to extinguish pool fires caused by fuel or lubricating oil leaks and to prevent reignition. The system may be used in conjunction with halon 1301 or independently to extinguish and secure fuel spills in the bilges. A third means of suppression is to fight a fire manually with portable extinguishers and hose streams. Hose streams include AFFF hand lines as well as regular water-only hoses available throughout the ship. Manual fire fighting may be employed in machinery spaces (1) when a fire is small or localized and can be readily extinguished by watch standers without protective equipment and (2) when a space is reentered after a halon system has been activated in order to extinguish any residual fire or, less likely, if the application of halon has not been completely effective.
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--> Halons Halon 1301 and halon 1211 are employed on board ships to meet unique fire fighting challenges—where their fire extinguishing effectiveness, low toxicity, minimum space and weight requirements, and lack of agent-induced damage are required characteristics. Halon 1301 is used in machinery spaces and flammable liquid storage and issue rooms because of its effectiveness against those fires requiring total gas flooding, low toxicity, lack of agent-induced damage, and a relatively low system space and weight impact. This agent is suitable for flammable gas, liquid, and typical solid combustible fires. It extinguishes fires in enclosed spaces by employing the principle of gas-phase catalytic interruption of combustion reactions (see Table 2.1), when used in concentrations of 5 to 6% by volume. A properly designed system will distribute a uniform concentration throughout the space, thereby providing extinguishment at all locations. Once a uniform mixture of halon in air is generated, the extinguishing concentration must be maintained for a period of time to preclude possible reignition. Halon 1211 is used for so-called streaming or local applications where it is desirable to direct a stream of liquid agent to a localized fire. In Navy applications, halon 1211 is always manually applied and directed at a specific fire location. On board aviation ships, it is used to extinguish small fires in engines that result from the pooling of fuel when an aircraft engine fails to start. It is also employed in fighting large three-dimensional cascading flight deck fires. Halon 1211 is applied from portable extinguishers, wheeled bottle carts, and crash vehicle hose lines. Finally, halon 1211 is employed in LCAC engine compartments for fire suppression. Aqueous Film-Forming Foam Aqueous film-forming foam is a mixture that is 6% AFFF concentrate (primarily fluorosurfactants and solvents) and 94% water, either seawater or fresh water. It is used to extinguish pool fires and to prevent the fuel vaporization and subsequent reignition. All foams, including AFFF, are only effective in suppressing two-dimensional flat pools of fuel and are generally ineffective on spray fires or cascading fuel fires. The primary shipboard uses of AFFF are in machinery spaces, aircraft hangars, fueled vehicle stowage areas, and on flight decks. In machinery spaces, an AFFF spraying system is located in the bilge to extinguish fires and secure fuel spills to prevent ignition. AFFF hose lines are also provided for manual application. Flight deck applications include an AFFF spraying system with flush mounted nozzles located in the flight deck. AFFF hoses are also provided on the flight deck and on crash/fire vehicles. While AFFF is a primary flight deck extinguishing agent, halon 1211 is used to attack cascading or spray fires which may occur as a result of a crash or catastrophic failure, and for small engine or "wet start" fires. In most cases, AFFF is used to extinguish and secure the pool fire resulting from a crash or large wet-start, and halon 1211 is used to extinguish localized three-dimensional fuel fires. Other Fire Fighting Measures In addition to the special-hazard extinguishing systems and agents described above, the Navy uses traditional fire protection systems as appropriate. On new-construction ships, automatic sprinkler systems are installed in storage and berthing spaces as well as in areas surrounding vital electronic equipment, and water deluge systems are provided for ordnance magazine cooling. With respect to manual fire fighting equipment, seawater fire mains run throughout a ship, with hose stations localized so as to provide coverage in all areas. Portable extinguishers, filled with CO2, AFFF, or PKP are distributed throughout the ship for first-response fire fighting.
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--> Figure A.1 Typical distributed (banked) halon system. How a Shipboard Halon 1301 System Works Halon 1301 systems installed in shipboard machinery spaces are designed to discharge sufficient halon into a space to develop a uniform 5% (by volume) concentration of halon 1301 in air throughout the space. Two types of systems are employed—manifold and modular. Manifold systems, also called "banked" or "distributed," consist of a bank of cylinders connected through a manifold or distribution piping (Figure A. 1). Piping from the bank runs throughout the space to nozzles located on the overhead and beneath the intermediate deck levels. Manifold systems were, and still are being, installed in new ships built after 1980. Modular systems feature individual cylinders of halon distributed throughout the protected space, connected via short lengths of pipe to one or two nozzles nearby (Figure A.2). These systems were used principally to retrofit existing ships with a halon 1301 system at the time the decision was taken to install halon fire extinguishing systems in naval vessels.
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--> Figure A.2 Typical modular halon system. The halon 1301 agent is in liquid form in cylinders, pressurized by nitrogen at 600 psi. The nitrogen is required to provide adequate cylinder pressure to discharge the halon in 10 seconds and to provide sufficient nozzle pressure to ensure proper distribution of the agent in the compartment. All machinery space halon systems are manually actuated using pneumatic actuation lines. Actuation stations are located inside the space, at least one of which is near the normal exit. Additionally, another station is outside the space, often on the main deck above proximate to the path leading to and from the space. Actuation of a station trisects release of CO2 from a small cylinder. This release in turn actuates a pressure switch that shuts down the ventilation fans to the space and closes dampers if installed. The CO2 flow also passes through a time-delay device. Warning horns are sounded during the 30- to 60-second time delay, signaling that the affected space is to be evacuated prior to halon discharge. After approximately 30 to 60 seconds, CO2 is permitted to flow to the halon cylinder actuation valves, thus initiating halon release. For manifold systems, a single, small CO2 cylinder opens valves for all cylinders coupled to the manifold. Modular systems, on the other hand, require pneumatic actuation piping to each cylinder for halon release. In either case, halon is discharged from the cylinders within approximately 10 seconds after the valves are opened.
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--> Once the halon has been released, fire fighting doctrine calls for monitoring the space for 20 to 30 minutes. In certain manifold system ships, a second halon discharge is available if needed—the so-called "two-shot" system. After 30 minutes, a fire party reenters the space to extinguish residual fires and provide cooling with AFFF hoses if necessary. Shipboard Spaces Where Halon 1301 Systems Are Installed Tables A.1 to A.5 list the location of halon 1301 extinguishing systems in five representative ship classes—two amphibious aviation ships (LHA, LHD), two surface combatants (FFG-7, DDG-51), and an aircraft carrier (CVN-73). It can be seen that halon is installed in those spaces where flammable liquids are used for power generation, where such liquids are stored, and where aviation fuel is handled. In general, older ships have one-shot modular systems since these were easier and less costly to retrofit than the two-shot manifold (banked) installations characteristic of vessels that had not yet begun construction at the time of the decision to install halon 1301 fleetwide. Notable is that the large CVN has less halon installed than a small, 3650-ton FFG-7 frigate. This is attributable to the nuclear-powered CVN being a one-shot ship, but more importantly, to the lack of a requirement for any halon in main propulsion spaces. It also highlights the point that main machinery spaces, emergency diesel generator rooms, and aviation fuel pump rooms account for a large proportion of the halon installed in applicable ship classes. Table A.1 LHA-4 USS Nassau Class Halon 1301 Systems Space Cylinders Size (lb) Halon Quantity per System (lb) Main Machinery Room #1 19 125 2,375 Main Machinery Room #2 23 125 2,875 Auxiliary Machinery Room 8 125 1,000 Emergency Diesel Generator Room # 1 5 125 625 Emergency Diesel Generator Room #2 3 125 375 JP-5 Pump Room 2 125 250 JP-5 Pump Room 1 125 125 Fuel Pump Room 4 125 500 Quantity of halon installed on ship 8,125 On-board spares 3,250 Total halon on board 11,375 NOTE: All systems are "single shot" and "modular." This arrangement (single shot, modular) is typical of ships that received halon via backfit.
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--> Table A.2 LHD-3 USS Kearsarge Class Halon 1301 Systems Space Cylinders Size (lb) Halon Quantity per System (lb) * Main Machinery Room #1 38 125 4,750 * Main Machinery Room #2 46 125 5,750 * Auxiliary Machinery Room 20 125 2,500 Emergency Diesel Generator Room #1 5 125 625 Emergency Diesel Generator Room #2 3 125 375 JP-5 Pump Room #1 4 125 500 JP-5 Pump Room #2 3 60 180 LCAC Pump Room 3 60 180 Paint Mix & Issue Room 1 95 95 Cargo Flammable Liquid Room 6 125 750 Aviation Flammable Storeroom 2 95 190 Supply Department Flammable Storeroom 4 60 240 Ship Store Flammable Liquid Storeroom 2 10 20 Aviation Flammable Liquid Storeroom 1 15 15 Quantity of halon installed on ship 16,170 On-board spares 2,050 Total halon on board 18,220 NOTE: Entries with an asterisk (*) have "two-shot" systems. All systems are "banked." Table A.3 FFG-7 USS Perry Class Halon 1301 Systems Space Cylinders Size (lb) Halon Quantity per System (lb) * Engine Room 14 95 1,330 * Auxiliary Machine Room #1 4 95 380 * Auxiliary Machine Room #2 18 60 1,080 * Auxiliary Machine Room #3 8 60 670 * Emergency Diesel Generator Room #1 2 95 190 * Emergency Diesel Generator Room #2 2 95 190 * Emergency Diesel Generator Room #3 2 95 190 * Emergency Diesel Generator Room #4 2 95 190 * Gas Turbine Module 1A 2 60 120 * Gas Turbine Module 1B 2 60 120 Flammable Liquid Storeroom 3 10 30 Flammable Gas Cylinder Storeroom 1 95 95 Paint Mix & Issue Room 2 10 20 * TACTAS Room 2 95 190 * RAST Machinery Room 2 60 120 Quantity of halon installed on ship 4,915 On-board spares 495 Total halon on board 5,410 NOTE: Entries with an asterisk (*) have "two-shot" systems. All systems are "banked."
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--> Table A.4 DDG-51 USS Arleigh Burke Class Halon 1301 Systems Space Cylinders Size (lb) Halon Quantity per System (lb) Auxiliary Machinery Room 10 125 1,250 Engine Room #1 20 125 2,500 Engine Room #2 22 125 2,750 Generator Room 6 95 570 Gas Turbine Module - 1A/B 2 95 190 Gas Turbine Module - 2A/B 2 95 190 Ship Service Gas Turbine Generator #1 2 95 190 Ship Service Gas Turbine Generator #2 2 95 190 Ship Service Gas Turbine Generator #3 2 95 190 Flammable Liquid Storeroom 1 60 60 Flammable Liquid Issue Room 2 15 30 TACTAS Room 2 125 250 Quantity of halon installed on ship 8,360 On-board spares 635 Total halon on board 8,995 Table A.5 CVN-73 USS George Washington Class Halon 1301 Systems Space Cylinders Size (lb) Halon Quantity per System (lb) Emergency Diesel Generator Room #1 5 125 625 Emergency Diesel Generator Room #2 4 125 500 Pump Room #2 4 125 500 Pump Room #3 3 125 375 Flammable Liquid Storeroom #1 1 125 125 Flammable Liquid Storeroom #2 3 95 285 Flammable Liquid Storeroom #3 2 95 190 Aviation Flammable Storeroom 2 125 250 Paint Mix & Issue Room #1 1 60 60 Paint Mix & Issue Room #2 2 15 30 Aviation Paint Mix & Issue #1 2 15 30 Aviation Paint Mix & Issue #2 1 15 15 Bomb Hoist Room #1 2 15 30 Bomb Hoist Room #2 2 15 30 Quantity of halon installed on ship 3,045 On-board spares 1,295 Total halon on board 4,340
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--> Aircraft Fire Extinguishing Systems Halon 1301 fire extinguishing systems in aircraft are employed in ways related to the size and mission of the aircraft and the number of engines. Application areas include engine nacelles or engine bays of twin-engine tactical aircraft; dry bays—those void spaces adjacent to, or beneath, fuel tanks and fuel lines; portable, hand-held extinguishers in cockpits and cabin spaces where aircrew personnel are located; and miscellaneous uses such as inerting a fuel tank's ullage (space above fuel in a less-than-full tank) to prevent incendiary bullet-initiated explosions, and to extinguish fires in auxiliary power units. Fires Aboard Aircraft Aircraft applications involve suppression of four distinct types of fires. These include hydrocarbon/air diffusion flames characteristic of engine nacelle and bay fires; premixed fuel vapor/aerosol/air deflagrations in dry bay applications; explosions of premixed fuel/air mixtures in fuel cells; and solid combustible diffusion flames involving cable and wire insulation or other combustibles that are typically suppressed using hand-held portable extinguishers. The most critical fires are those in engine bays and explosion/deflagration events in dry bays. Each of these situations is addressed briefly below. Halon 1301 is now used extensively to protect engine nacelles/bays. Here, the agent is discharged at a high rate through a series of nozzles to mix with the air stream through the engine and form a transient extinguishing concentration as it passes through the nacelle/bay and extinguishes the in-flight fire. The agent is discharged on command of the pilot after a positive indication of an engine fire, usually from a combination of thermal fire detection activation and/or anomalies in engine operating parameters. Thus, while the agent is discharged in only a few seconds, tens of seconds may elapse between the first indication of trouble and discharge of the suppression agent. The basic mechanism of extinguishment of engine bay fires is identical to suppressing any hydrocarbon/air diffusion flame, such as a fuel oil fire in a ship machinery space. The primary technical challenge in aircraft is to disperse a sufficiently high concentration of agent into the engine bay such that an extinguishing concentration is maintained within the very high air flow environment of the bay. Generally, this extinguishing concentration is maintained for only a few seconds. Given the high air velocities present in engine bays, the flame strain rate is much higher than what occurs in quiescent, buoyant diffusion flames. As a result, these flames can be extinguished at lower agent concentrations. The reduced partial pressure of oxygen at flight altitude also simplifies the suppression process. A complicating factor in system design is the highly obstructed nature of the engine bay, which impacts nozzle design, flow rate, and agent mixing behavior. The suppression of explosions (inertion) in dry bay applications requires sensing the presence of an explosion kernel before the flame front has expanded to a damaging size, and then rapidly applying a suppression agent in the vicinity of the ignition to quench the deflagration wave. This sequence must occur within a time scale of tens of milliseconds in order to effectively limit explosion damage, hence the need for automatic system actuation. The primary scenario for initiation of a dry bay explosion is ordnance or shrapnel penetrating a fuel cell adjacent to the bay and subsequent ignition of the resulting fuel/air mixture. Dry bay explosion suppression systems are designed primarily to ensure the survivability of aircraft in combat. An alternative, albeit much less efficient approach, is to inert bays and voids prior to combat damage. This approach has been tried with some aircraft but has not been pursued in more recent aircraft designs.
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--> Figure A.3 Typical halon aircraft engine fire extinguishing system. Typical Halon 1301 Aircraft Engine Fire Extinguishing System A typical halon 1301 aircraft engine extinguishing system is composed of fire detection sensors linked to cockpit warning lights, halon bottle(s) pressurized by nitrogen at 600 psi, tubing from bottles to strategically placed nozzles, and a pilot-actuated linkage (mechanical, electrical, pneumatic) connecting the cockpit to the halon bottle(s). See Figure A.3. It should be noted that no single-engine naval aircraft has fire extinguishers in the engine bay. This is based on the premise that, should a fire start because of battle damage or a severe fuel leak and be extinguished, it makes little sense to restart the engine after having once cut off the fuel that was feeding the fire. Dry bays or void areas alongside or beneath fuel tanks, and through which fuel lines may pass, are susceptible to explosions and fires if combat damage is suffered. Protection measures employed include solid foams, inert gas generating systems, and halon 1301. In such an application, a halon 1301 system would activate automatically in milliseconds upon detection of an explosion kernel by optical flame detectors.
Representative terms from entire chapter: