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Halon Fire Extinguishers

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Category: Fire Smoke and Fumes Fire Smoke and Fumes
Content source: SKYbrary About SKYbrary
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Definition

Halons are fire extinguishing agents which are gaseous when discharged in the aircraft environment and are electrically non-conducting. Halons are in almost universal use in aircraft fire extinguishers, both portable and fixed. They exist in two forms Halon 1211, BromoChlorodiFluoromethane (CBrClF2)also sometimes known as BCF, and Halon 1301, Bromotrifluoromethane (CBrF3).

  • Halon 1211 is used only in portable extinguishers and is a streaming agent
  • Halon 1301 is used only in fixed extinguisher installations and is a total flooding agent.

Description

Halons are electrically non-conducting and are acknowledged, by a considerable margin, to be the most effective universal extinguishing agent for aircraft use. They work mainly by chemically interrupting the reaction described as the 'Fire Triangle'(Fuel-Oxygen-Heat) which must be sustained for a fire to continue. They do not produce residues and therefore do not cause secondary damage. However, the fumes are toxic if inhaled and all practicable precautions should be taken when they are used.

The chemical constituents in Halon gases, and the products of the reactions they induce when discharged on fires, have been identified as causing damage to the Ozone layer. As a result, their manufacture and use have been banned for many years in most countries and non-essential uses have been eliminated. Nevertheless, they remain widely in use on board aircraft for most applications because of the difficulty in finding alternatives.

How Halon Extinguishers Work

Both Halon variants work by a combination of chemical and physical effects. The chemical effects, which are dominant in their overall effect, are achieved by the atoms in the gas directly inhibiting combustion in two different ways:

  • Bromine, Iodine and Chlorine atoms act catalytically so that each atom participates repeatedly in the scavenging of important free radicals from the combustion gases.
  • Fluorine atoms react with free radicals and form strong chemical bonds which neutralise combustion but can only do so once and are then “consumed.” The physical effects are both temperature reduction and dilution.

Temperature reduction occurs, whenever a non-reactive gas is added to a flammable gas, because the heat liberated by the reaction of oxygen molecules with a fuel source must be distributed into the overall environment. The rate of the combustive chemical reaction decreases rapidly with reductions in temperature and, if the concentration of added inert gas is high enough, the flame chemistry fails altogether.

Halon gas mixtures are not only inert but of low temperature when released from their pressurised state. Dilution is a simple matter of reducing the collision frequency of the oxygen and fuel source so that there is a reduction in chemical reaction rates. The magnitude of this effect, however, is relatively small compared to chemical inhibition and thermal effects, the former of these being the predominant one.

The toxicity of the Halon gases, especially the combination which makes up Halon 1211, is such that use in confined spaces requires care to minimize any inhalation of the discharged gases. Where a portable Halon extinguisher is used by cabin crew, it is usually recommended to consider donning a smoke hood before discharge to eliminate this risk, but for flight crew use on the flight deck, this will not be an option and risk awareness is the only defence.

Alternatives to Halon

The search for Halon alternatives for use on aircraft has proved particularly challenging. Extinguishing agents in use outside aviation are either formulations based on sodium bicarbonate, suitable only for fires involving flammable liquids and gases, or those known as ‘ABC’ which are usually based upon ammonium phosphate and are suitable for use on fires of all origins. Both types may be used safely on fires where electrical circuits are present. However, although these extinguishing agents are the most effective fire extinguishing agents in use, they both leave a very fine, powdery residue after discharge which consists of particles ranging down to sub-micron diameter which is difficult if not impossible to clean up. This means they are unsuitable for use on or in the vicinity of complex and often sensitive aircraft electrical systems. It is understandable that the ‘option’ of returning to the more complex and, overall, far less effective dual extinguisher-type solution of Carbon Dioxide and Water Glycol has not found favour either. Although Carbon Dioxide, as an extinguishing agent, is both ‘clean’ and much cheaper than both Halon or potential Halon replacements, its overall effectiveness on a unit stored volume basis is nowhere near that of Halon. In addition, it cannot be used on solid material fires and the method of achieving effective discharge on other fires is also more complicated - tube to horn delivery rather than nozzle discharge.

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