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Limitations on the use of water as an extinguishing agent

Despite the enormous advantages of water as an extinguishing agent caused primarily by their physical properties, has other properties that limit their application when used in a fire. Here are some of these disadvantages:

Electrical conductivity

Impurities and salts which generally have the water make great conductor of electricity, which makes their use very dangerous especially in high voltage electrical installations. Being in fact the amount of current through the body responsible for the electrical shock, the danger is not great for a person who directs a jet of water on a line with voltage, provided it is below 600 volts.

The danger increases and higher if the person is in a pool of water and makes contact with a portion of the electrical circuit, because the grounding will occur through the individual. Sometimes the rubber boots because of their high content of carbon in its composition have not sufficiently insulating to provide protection in high voltage lines because they allow the passage of current.

Is experimentally shown that as people get to 4 or 5 milli-Ampere feel unpleasant sensations that can endure more or less, but spending 20 to 30 milli-A can be fatal. So the fact of being conductive water limits its indiscriminate application.

Solidification temperature and antifreeze additives

Water freezes at 0 º C, which limits its use in places where this temperature can be achieved commonly (southern Argentina), as the driving valves and pipes are often clogged and burst.

In practice often ignored to some extent this problem through the plumbing system empty (dry pipe) instead of pipes or heating water tank supplier and / or antifreeze additive uses.

Generally CaCl2 is often used as antifreeze and anticorrosive additives added effect until 49 º C approximately. In automatic sprinkler systems, you can use different compounds of glycerine or glycols, although the latter have certain toxicological properties that prevent their use when water service is connected to the potable water system.

Surface Tension and Wetting

The relatively high surface tension of water slows their ability to penetrate fuels burned, and prevents their diffusion through compacted materials packaged or stacked. When a fire originates or enters into a mass of combustible material is necessary or dismantle this mass, or use a wetting agent to lower the surface tension of water. Many chemicals can be used as wetting agents, but few as good extinguishing agents because they are toxic, corrosive or unstable when mixed with water. Wetting agents are effective because they reduce the surface tension of water, thereby increasing the free surface available for absorption of heat. There is less water slide and thus increases their effectiveness.

Viscosity and thickener

The relatively low viscosity of water causes it to rapidly slide surfaces and limits its ability to put out the fire by forming a barrier on the surface of combustible materials. Additives to increase the viscosity of water (thick) increase their effectiveness on certain types of fires.

Two thickening agents currently used in forest fires are the CMC (sodium carboxymethylcellulose) and Gelgard (trade name product of Dow Chemical).

Additives that change the characteristics of water flow

Friction losses in hoses are always a problem for fire fighting. The greater the length of the hose and pump more water, the greater the loss. With good quality hoses, most of the pressure loss is caused by friction generated between the particles of water due to turbulence in the flow.

Until 1948 it was thought that little could be done to reduce friction losses. At that time it was discovered that small amounts of certain polymers reduced friction losses in turbulent flows. Most of the researchers, said that the linear polymers (chemical straight unbranched chains) are most effective in reducing friction losses due to turbulence and of these, the polyoxyethylene is the most effective. The effectiveness of the friction reducing effect is a direct function of the linearity of the polymer chain. Compatible with all fire-fighting equipment and can be used fresh or salt water.

Although water is a universal extinguishing agent, other prohibitions and precautions to be observed when applied manually on some burning materials that either chemically react, or explode on contact with water. In other cases, the mechanical action of water should be applied carefully controlled so as not to create conditions that intensify risk control. Here are some items on which attention should be given with reference to extinction with water.

Chemical fires

No water should be used in materials such as carbides, peroxides, etc., Because, in reacting, and flammable gases can be spent with heat. When they are wet, some materials such as lime, warmed spontaneously for some time if it can not dissipate heat due to storage conditions.

Combustible metals

Do not use water on fires involving combustible metals such as magnesium, titanium, sodium metal, hafnium, or metals that are combustible under certain conditions, such as calcium, zinc and aluminum.

Gas fires

The water used on gas fire emergencies is usually used to control the heat of the fire while trying to hack, or stop the flow of gas emitted. The water spray applied from hose or from the control nozzles or sprinkler systems fixed, is used continuously to dilute concentrations of flammable gases.

Flammable liquid fires and fuel

Heavy oil, lubricating oil, asphalt and other liquids with high combustion point, flammable vapors will not unless they are heated. Once you have entered into ignition, the heat of vaporization sufficient cause fire to produce a continuous combustion. If you apply water spray to the surface of liquids with high fire point found burning, cooling will reduce the degree of vaporization enough to extinguish the fire. If water is applied to liquids with high combustion point that are burning, extinction can be obtained by emulsification via a coarse spray.

The capacity of the water without additives to stop a fire, flammable liquid is limited in low point of combustion, such as Class I flammable liquids If water reaches the surface of a flammable liquid with a low point of combustion, which is burning in a tank, in all probability will go to the bottom may cause the overflow tank. For fires of discharges, the water will probably cause the fire to spread.



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