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--> Executive Summary Halons are recognized as ideal fire extinguishing agents, particularly for fighting fires caused by flammable liquids and explosive gases. They are highly effective in extinguishing fires in minimal time, are non-corrosive, and when deployed at recommended volume densities are non-toxic. Because of these characteristics, halons are widely used on board Navy ships and in aircraft. Unfortunately, halons contribute to the depletion of Earth's stratospheric ozone layer, which results in higher levels of ultraviolet radiation at Earth's surface and thus gives rise to serious health effects. Halons are chemically related to chlorofluorocarbons (CFCs) that were earlier shown to degrade the ozone layer and consequently were banned. Although as compared with CFCs the quantities of halons released are much smaller, the halons are more active in destroying ozone and their effect is significant. Under an Executive Order effective in January 1994, halons may no longer be manufactured in the United States. But because of the difficulty of finding a suitable fire extinguishing substitute for halons 1301 and 1211, the military services are permitted to use these chemicals for mission-critical purposes, such as fire fighting, in existing platforms (ships, aircraft, weapons, vehicles) until the current halon supply or ''bank'' is exhausted. This study was undertaken by the Committee on Assessment of Fire Suppression Substitutes and Alternatives to Halon to assess research in the science and engineering relevant to identification and evaluation of alternative agents that could be developed to replace halons, including the likelihood of the existence of virtually identical agents. For alternative agents that have been identified, the committee also assessed efforts to quantify the penalties in added weight and volume of storage and distribution systems required. The study also addressed non-halon-like extinguishing systems as possible alternatives. Approach and Scope of Study The study involved a review of the extensive research and engineering literature covering the physical and chemical processes active in flames and involved in flame extinguishment. Information on test methodology, including agent toxicology, storage stability, and extinguishment effectiveness, was collected. Synthetic chemical schemes for the manufacture of halon alternatives were reviewed. Published lists of prospective halon replacements were evaluated. Atmospheric chemistry relevant to ozone depletion was examined in the light of recent work, and the possibility of secondary environmental effects was studied. Ozone depletion potential (ODP), a useful metric found in regulatory legislation, was described and tabulated for alternative agent candidates. Global warming potential (GWP), a measure of agent effect on climate, was treated similarly. The history of halon regulation and the likelihood of important modifications were also discussed. In addition, information concerning Navy-specific aspects of halon replacement was assembled. The nature of Navy fires, existing extinguishing systems, and hardware configurations were reviewed, with Navy guidance, for ships and aircraft. Alternatives to halon that are under consideration for new-construction platforms were examined (HFC-227ea for non-machinery spaces on ships and HFC-125 for engine bays on aircraft). Non-halon-like methods were also reviewed (water mist systems for shipboard machinery spaces and inert gas generators for aircraft dry bay and engine bay applications). The potential for retrofit of those four alternatives on existing platforms was examined. Finally, the projected evolution of the Navy halon inventory was studied. With these various aspects of the study carried through, the committee was able to take positions on the first two elements of the statement of task (see preface, p. vii) and to comment knowledgeably on the third.
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--> Key Findings After reviewing research, development, toxicology, and engineering activities directed toward finding alternative and replacement agents for halon 1301 and halon 1211, the committee finds that in this context, the relevant aspects of the problem are being studied effectively and a comprehensive body of scientific and engineering knowledge is being developed, and the committee has identified no obvious gaps in these important efforts. It is unlikely that a drop-in replacement agent will be discovered that will exhibit all of the beneficial properties of halon 1301 and not also exhibit a significant environmental impact. Effective alternative chemical agents have been identified by the Navy and are currently being incorporated into the design of new ships and aircraft. There is a weight and volume penalty associated with these agents relative to halons, but the impact can be minimized if use of these agents is incorporated into initial platform design. Further, retrofit of these agents into existing naval platforms is technically feasible in most cases. In addition to the chemical replacement agents, promising alternative fire extinguishing systems such as water mist systems and inert gas generators are under consideration by the Navy for some applications. These systems are being incorporated into new-design naval platforms. Contents of This Report This study addresses various science and engineering aspects of the agents and methods that are being considered as substitutes for halons and halon systems used as fire suppressants by the U.S. Navy in ships and aircraft. Chapter 1 points out the Navy's ongoing need for effective fire suppression and outlines the set of extensive requirements that candidate replacements for halons must satisfy. Chapter 2 describes research on flames, mechanisms of flame suppression, methodology of agent evaluation including suppression efficiency and toxicology, manufacturability, and aspects of the search for alternative agents. Atmospheric chemistry phenomena relating to an alternative agent's capacity to deplete ozone, contribute to global warming, or give rise to surface pollution through the accumulation of noxious reaction products are covered in Chapter 3. Chapter 4 describes Navy-specific aspects of halon replacement in ships and aircraft, suggests possible courses of action, and provides the committee's preferred alternative. The appendixes give details as appropriate.
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