Fire extinction and suppression can be examined in terms of the above outline of the theory of fire. The gas phase combustion processes (i.e., the flame reactions) are very sensitive to chemical inhibitors. Some of the flame retardants used to improve the “fire properties” of materials rely on the fact that small amounts of inhibitor released with the fuel vapours will suppress the establishment of flame. The presence of a flame retardant cannot render a combustible material non-combustible, but it can make ignition more difficult—perhaps preventing ignition altogether provided that the ignition source is small. However, if a flame-retarded material becomes involved in an existing fire, it will burn as the high heat fluxes overwhelm the effect of the retardant.
Extinction of a fire may be achieved in a number of ways:
1. stopping the supply of fuel vapours
2. quenching the flame by chemical extinguishers (inhibiting)
3. removing the supply of air (oxygen) to the fire (smothering)
4. “blow-out”.
Controlling the flow of fuel vapours
The first method, stopping the supply of fuel vapours, is clearly applicable to a gas-jet fire in which the supply of the fuel can simply be turned off. However, it is also the most common and safest method of extinguishing a fire involving condensed fuels. In the case of a fire involving a solid, this requires the fuel surface to be cooled below the firepoint, when the flow of vapours becomes too small to support a flame. This is achieved most effectively by the application of water, either manually or by means of an automatic system (sprinklers, water spray, etc.). In general, liquid fires cannot be dealt with in this manner: liquid fuels with low firepoints simply cannot be cooled sufficiently, while in the case of a high-firepoint fuel, vigorous vaporization of water when it comes into contact with the hot liquid at the surface can lead to burning fuel being ejected from the container. This can have very serious consequences for those fighting the fire. (There are some special cases in which an automatic high-pressure water-spray system may be designed to deal with the latter type of fire, but this is not common.)
Liquid fires are commonly extinguished by the use of fire-fighting foams (Cote 1991). This is produced by aspirating a foam concentrate into a stream of water which is then directed at the fire through a special nozzle which permits air to be entrained into the flow. This produces a foam which floats on top of the liquid, reducing the rate of supply of fuel vapours by a blockage effect and by shielding the surface from heat transfer from the flames. The foam has to be applied carefully to form a “raft” which gradually increases in size to cover the liquid surface. The flames will decrease in size as the raft grows, and at the same time the foam will gradually break down, releasing water which will aid the cooling of the surface. The mechanism is in fact complex, although the net result is to control the flow of vapours.
There are a number of foam concentrates available, and it is important to choose one that is compatible with the liquids that are to be protected. The original “protein foams” were developed for hydrocarbon liquid fires, but break down rapidly if brought into contact with liquid fuels that are water soluble. A range of “synthetic foams” have been developed to tackle the entire range of liquid fires that may be encountered. One of these, aqueous film-forming foam (AFFF), is an all-purpose foam which also produces a film of water on the surface of the liquid fuel, thus increasing its effectiveness.
This method makes use of chemical suppressants to extinguish the flame. The reactions which occur in the flame involve free radicals, a highly reactive species which have only a fleeting existence but are continuously regenerated by a branched chain process that maintains high enough concentrations to allow the overall reaction (e.g., an R1 type reaction) to proceed at a fast rate. Chemical suppressants applied in sufficient quantity will cause a dramatic fall in the concentration of these radicals, effectively quenching the flame. The most common agents that operate in this way are the halons and dry powders.
Halons react in the flame to generate other intermediate species with which the flame radicals react preferentially. Relatively small amounts of the halons are required to extinguish a fire, and for this reason they were traditionally considered highly desirable; extinguishing concentrations are “breathable” (although the products generated while passing through the flame are noxious). Dry powders act in a similar fashion, but under certain circumstances are much more effective. Fine particles are dispersed into the flame and cause termination of the radical chains. It is important that the particles are small and numerous. This is achieved by the manufacturers of many proprietary brands of dry powders by selecting a powder that “decrepitates”, that is, the particles fragment into smaller particles when they are exposed to the high temperatures of the flame.
For a person whose clothing has caught fire, a dry powder extinguisher is recognized as the best method to control flames and to protect that individual. Rapid intervention gives rapid “knockdown”, thus minimizing injury. However, the flame must be completely extinguished because the particles quickly fall to the ground and any residual flaming will quickly regain hold. Similarly, halons will only remain effective if the local concentrations are maintained. If it is applied out of doors, the halon vapour rapidly disperses, and once again the fire will rapidly re-establish itself if there is any residual flame. More significantly, the loss of the suppressant will be followed by re-ignition of the fuel if the surface temperatures are high enough. Neither halons nor dry powders have any significant cooling effect on the fuel surface.
Removing the supply of air
The following description is an oversimplification of the process. While “removing the supply of air” will certainly cause the fire to extinguish, to do this it is only necessary to reduce the oxygen concentration below a critical level. The well-known “oxygen index test” classifies combustible materials according to the minimum oxygen concentration in an oxygen/nitrogen mixture that will just support flaming. Many common materials will burn at oxygen concentrations down to approximately 14% at ambient temperatures (ca. 20 °C) and in the absence of any imposed heat transfer. The critical concentration is temperature dependent, decreasing as the temperature is increased. Thus, a fire that has been burning for some time will be capable of supporting flames at concentrations perhaps as low as 7%. A fire in a room may be held in check and may even self-extinguish if the supply of oxygen is limited by keeping doors and windows closed. Flaming may cease, but smouldering will continue at very much lower oxygen concentrations. Admission of air by opening a door or breaking a window before the room has cooled sufficiently can lead to a vigorous eruption of the fire, known as backdraught, or backdraft.
“Removal of air” is difficult to achieve. However, an atmosphere may be rendered “inert” by total flooding by means of a gas which will not support combustion, such as nitrogen, carbon dioxide or gases from a combustion process (e.g., a ship’s engines) which are low in oxygen and high in carbon dioxide. This technique can only be used in enclosed spaces as it is necessary to maintain the required concentration of the “inert gas” until either the fire has extinguished completely or fire-fighting operations can begin. Total flooding has special applications, such as for ships’ holds and rare book collections in libraries. The required minimum concentrations of the inert gases are shown in table 41.4 . These are based on the assumption that the fire is detected at an early stage and that the flooding is carried out before too much heat has accumulated in the space.
Table 41.4 Comparison of concentrations of different gases required for inerting
|
Agent |
Minimum concentration (% volume) |
|
Halon 1301 |
8.0 |
|
Halon 1211 |
8.1 |
|
Nitrogen |
approx. 58 |
|
Carbon dioxide |
approx. 35 |
“Removal of air” can be effected in the immediate vicinity of a small fire by local application of a suppressant from an extinguisher. Carbon dioxide is the only gas that is used in this way. However, as this gas quickly disperses, it is essential to extinguish all flaming during the attack on the fire; otherwise, flaming will re-establish itself. Re-ignition is also possible because carbon dioxide has little if any cooling effect. It is worth noting that a fine water spray entrained into a flame can cause extinction as the combined result of evaporation of the droplets (which cools the burning zone) and reduction of the oxygen concentration by dilution by water vapour (which acts in the same way as carbon dioxide). Fine water sprays and mists are being considered as possible replacements for halons.
It is appropriate to mention here that it is inadvisable to extinguish a gas flame unless the gas flow can be stopped immediately thereafter. Otherwise, a substantial volume of flammable gas may build up and subsequently ignite, with potentially serious consequences.
Blow-out
This method is included here for completeness. A match flame can easily be blown out by increasing the air velocity above a critical value in the vicinity of the flame. The mechanism operates by destabilizing the flame in the vicinity of the fuel. In principle, larger fires can be controlled in the same way, but explosive charges are normally required to generate sufficient velocities. Oil well fires can be extinguished in this manner. Finally, a common feature that needs to be emphasized is that the ease with which a fire can be extinguished decreases rapidly as the fire increases in size. Early detection permits extinction with minimal quantities of suppressant, with reduced losses. In choosing a suppressant system, one should take into account the potential rate of fire development and what type of detection system is available.