Fires
2. Extinguishing Agents
Water:
Extinguishes the fire by cooling and may be used in the form of a jet or finely pulverised. Water in a jet should only be used in Class A fires. Pulverised water can be use in Class A fires and in Class B fires when these involve so-called “heavy” combustible liquids, such as fuel-oil gas-oil, etc.
Carbon dioxide (“dry ice” or CO2):
This is an inert gas and hence is used as an element to suffocate fires. It is effective for fires produced by inflammable liquids and in electrical fires because it is not a conductor and it does not leave any residue.
Dry chemical powder:
This is generally a chemical compound made up of sodium bicarbonate and a water-resistant agent. It acts by suffocating and paralysing the chain reaction. Two types of dry chemical powder are chiefly employed nowadays: normal and polyvalent (or anti-re-ignition) dry chemical powder. The latter cools the fuel much more and so is more effective than normal dry chemical powder for Class A fires. There also exist a series of special dry chemical formulations for Class D fuels.
Normal dry chemical powder is effective against Class B and C fires and against fires in the presence of electric voltage. It can be used in Class A fires, but water will then have to be used to avoid re-ignition.
Chemical foam:
This is formed by mixing an acid solution in another, basic solution. When closely mixed together, the two solutions react producing carbon dioxide (CO2), with the subsequent increase in pressure that discharges the extinguishing foam.
This type of foam presents the drawbacks of attacking metals, being a conductor of electricity and dissolving alcohols and so is not presently used.
Physical foam:
This is a mass of bubbles joined together by a stabiliser that is applied like a blanket over liquids in combustion, thus impeding or putting out the fire by suffocation. This foam dissolves into water-soluble hydrocarbons, such as alcohols, acetone, etc., and so cannot be used to fight fires of this type.
It should never be used jointly with water, as the latter breaks up the foam blanket. It is effective for fighting Class B fires with the aforementioned limitations and in Class A fires, maintaining the blanket that is formed in place for quite some time. Due to the drawbacks it presents, physical foam is now used less and less.
Halon substitutes:
These are extinguishing agents that act in fire extinction by paralysing the chain reaction. These compounds are very effective against electrical fires and are acceptable for Class A and B fires.
They substitute the so-called halons, since these, being CFC’s (chlorofluorocarbons), are harmful to the ozone layer and also contain bromine, which, as has been shown, also contributes to the reduction of ozone in the atmosphere.
Accordingly, the use of halons has been banned and other products have recently been employed to replace them such as FM200, NAF SIII, INERGEN, etc., which have a similar extinguishing efficiency but do not present the drawbacks of halons.
The following table shows the choice of extinguishing agent depending on the class of fire.
| EXTINGUISHING AGENT | CLASS OF FIRE | ||||
|---|---|---|---|---|---|
| A | B | C | D | E | |
| Fires involving Solids that leave Embers | Fires involving Liquids or Liquefiable Solids | Fires involving Gases | Fires Involving Metals | Fires in the Presence of Electric Voltage Above 25 V. | |
| Pulverised Water | Excellent | Acceptable for liquid fuels that are not water soluble (gas-oil, oil, etc.) | Null | Null | Hazardous |
| Water Jet | Good | Null | Null | Null | Highly dangerous |
| Carbon dioxide (CO2) | Acceptable May be used for small fires Does not put out embers |
Acceptable May be used for small fires |
Null | Null | Good |
| Physical Foam | Good | Good Do not use in water-soluble liquids |
Null | Null | Hazardous |
| Dry Chemical Normal (BC) | Acceptable May be used for small fires Does not put out embers |
Good | Good | Null | Good |
| Polyvalent Dry Chemical (anti-re-ignition) (ABC) | Good | Good | Good | Null | Good for voltages below 1,000 V. Do not use above this voltage. |
| Special Dry Chemical for Metals | Null | Null | Null | Good | Null |
| Halon Substitutes: (FM200-NAFSIII-INERGEN, etc.) | Acceptable May be used for small fires |
Acceptable May be used for small fires |
Null | Null | Good |
