Fire extinguisher

A fire extinguisher is a device used to put out a fire, often in an emergency situation. They consist of a pressurised container of chemicals that when discharged can put out a fire. It is important to familiarise yourself with the use of fire extinguishers in your vicinity, as improper or untimely use may be counterproductive.

Classification

In the United States, the National Fire Protection Agency has created five classes of fire extinguishers, A, B, C, D, and K.

• Class A extinguishers are used for putting out fires of general combustibles such as wood or paper. These usually contain a foam-like mixture of water and carbon dioxide. The symbol is a triangle around the letter A.
• Class B extinguishers are intended for use on liquids or grease and are distinguished by a square around the letter B.
• Class C extinguishers are used on electrical fires. Their mixture is non-conducting to prevent zapping the user.
• Class D extinguishers are used on fires of flammable metals and certain other exotic chemicals. They are designated with a star around the letter D. These are rarely encountered and much more expensive than class A, B, or C extinguishers. Class D extinguishers have to specialised to the particular hazard and require special operator training.
• Class K extinguishers are used in Kitchens and environments with the high probability of grease fires. These are rarer because the category has only existed since 1998, so many kitchens are still equipped with Class B extinguishers.

Fire extinguishers generally are designed for one or more classes. Commonly available are A (water), BC (carbon dioxide), and ABC (dry powder).

Other countries may have broadly similar systems which differ in detail. For example in Australia, the classes are:

• Class A: Same as US class A.
• Class B: Same as US class B.
• Class C: Flammable gases.
• Class D: Same as US class D.
• Class E: Same as US class C.
• Class F: Same as US class K.

The Australian markings are:

Type	pre-1997	current
Water	Solid red
Foam	Solid blue	Red with a blue band
Powder	Red with a white band
Carbon dioxide	Red with a black band
Vapourising liquid (not halon)	Red with a yellow band
Halon	Solid yellow	—
Wet chemical	Solid oatmeal	Red with an oatmeal band

Chemistries

A fire extinguisher may emit a solid, liquid, or gaseous chemical.

Water

Water is the most common chemical for class A fires and is quite effective as one would imagine. Most water based extinguishers also contain traces of other chemicals to prevent the extinguisher rusting. Some also contain wetting agents which help the water penetrate deep into the burning material and cling better to steep surfaces. Water works to extinguish a fire by simply cooling it below the ignition point, although large amounts of water can also exclude oxygen. However, water will merely exacerbate other fire classes. For instance, water sprayed over burning liquid petroleum merely spreads the flames around.

Similarly water sprayed on an electrical fire may cause the operator to receive an electric shock. (However, if the power can be reliably disconnected and a carbon dioxide or halon extinguisher is not available, clean water will actually cause less damage to electrical equipment than will either foam or dry powders.)

Foams

Foams are commonly used on class B fires, and are also effective on class A fires. These are mainly water based, with a foaming agent so that the foam can float on top of the burning liquid to exclude oxygen. Ordinary foams are designed to work on nonpolar flammable liquids such as petrol (gasoline), but may break down too quickly in polar liquids such as alcohol or glycol. Facilities which handle large amounts of flammable polar liquids use a specialised "alcohol foam" instead. Alcohol foams must be gently "poured" across the burning liquid. If the fire cannot be approached closely enough to do this, they should be sprayed onto an adjacent solid surface so that they run gently onto the burning liquid. Ordinary foams work better if "poured" but it is not critical.

Dry Powder

For classes B and C, a dry powder is used. There are two main dry powder chemistries in use:

• BC powder is either sodium bicarbonate or potassium bicarbonate and calcium carbonate, finely powdered and propelled by carbon dioxide or nitrogen. Similarly to halon (see below) - but less effectively - these powders actually interrupt the chemical processes of the fire. They also provide some cooling and exclusion of oxygen, although this effect is relatively weak. These powders thus provide rapid knockdown of flame fronts, but may not keep the fire suppressed. Consequently, they are often used in conjunction with foam for attacking large class B fires. BC extinguishers are often kept in small vehicles since they provide good knockdown of a rapidly flaring class B fire, from a small package.
• ABC powder is monoammonium phosphate and/or ammonium sulfate. As well as suppressing the flame in the air, it also melts at a low temperature to form a layer of slag which excludes oxygen from the fuel. For this reason it can also be effective against class A fires. ABC powder is usually the best agent for fires involving multiple classes. However it is not effective against three dimensional class A fires, or those with a complex or porous structure. Foams or water are better in those cases.

Both types of powders can also be used on class C fires, but provide a significant cleanup and corrosion problem that is likely to make the electrical equipment unsalvageable.

Wet potassium salts

Most class K extinguishers contain a solution of potassium acetate, sometimes with some potassium citrate or potassium bicarbonate. The extinguishers spray the agent out as a fine mist. The mist acts to cool the flame front, while the potassium salts saponify the surface of the burning cooking oil, producing a layer of foam over the surface. This solution thus provides a similar blanketing effect to a foam extinguisher, but with a greater cooling effect. The saponification only works on animal fats and vegetable oils, so class K extinguishers cannot be used for class B fires. The misting also helps to prevent splashing the blazing oil.

Carbon dioxide

Carbon dioxide also works on classes B and C and works by displacing the less dense oxygen. This can be problematic in enclosed occupied spaces as we need oxygen too! Although carbon dioxide is exhaled in our own breath, in the high concentrations required to extinguish deep seated fires it is one of the most toxic extinguishing agents used. Carbon dioxide is especially popular on electrical fires because, being a gas, it does not leave any residues which might further harm the damaged equipment. (Carbon dioxide can also be used on class A fires when it is important to avoid water damage, but in this application the gas concentration must usually be maintained longer than is possible with a hand-held extinguisher.)

Halons

Halons are very versatile extinguishers. They will extinguish any type of fire except class D and are highly effective even at quite low concentrations. They are the only fire extinguishing agents that are quite suitable for discharge in aircraft (carbon dioxide is too toxic in confined spaces, and other materials pose a corrosion hazard to the aircraft). They work by breaking the chemical reaction of the fire. Halons are chlorofluorocarbons and are being phased out for more environmentally-friendly alternatives. Halon fire extinguishers may cost upwards of 800 USD due to production and import restrictions.

Fluorocarbons

Recently Dupont has begun marketing several nearly saturated fluorocarbons under the trademarks FE-13, FE-25, FE-36, FE-227, and FE-241. These materials are claimed to have all the advantageous properties of halons, but lower toxicity, and zero ozone depletion potential. They require about 50% greater concentration for equivalent fire quenching.

Specialised materials for Class D

Class D fires involve extremely high temperatures and highly reactive fuels. For example, burning magnesium metal breaks water down to hydrogen gas and causes an explosion; breaks halon down to toxic phosgene and fluorophosgene and may cause a rapid phase transition explosion; and continues to burn even when completely smothered by nitrogen gas or carbon dioxide (in the latter case, also producing toxic carbon monoxide). Consequently, there is no one type of extinguisher agent that is approved for all class D fires; rather, there are several common types and a few rarer ones, and each must be compatibility approved for the particular hazard being guarded. Additionally, there are important differences in the way each one is operated, so the operators must receive special training. Some example class D chemistries include:

• METL-X, finely powdered sodium chloride (salt) propelled by carbon dioxide or argon. After a crust at least 50 mm (2 inches) thick has formed over the burning metal, it is picked up with a long handled shovel and placed in a bucket of salt or very dry sand, and additional agent poured on top. Suitable for sodium, potassium, magnesium, titanium, aluminium, and most other reactive bulk metal fires, but not lithium or finely powdered metals.
• Finely powdered graphite, applied with a long handled scoop, is preferred for fires in fine powders of reactive metals, where the blast of pressure from an extinguisher may stir up the powder and cause a dust explosion. Graphite both smothers the fire and conducts away heat.
• Finely powdered copper propelled by compressed argon is the currently preferred method for lithium fires. It smothers the fire, dilutes the fuel, and conducts away heat. It is capable of clinging to dripping molten lithium on vertical surfaces. Graphite can also be used on lithium fires but only on a level surface.
• Other materials sometimes used include powdered sodium carbonate, powdered dolomite and argon gas.
• Very dry sand may be used to smother a metal fire if nothing better is available. It should be applied with a long-handled shovel to avoid the operator receiving flash burns, then the molten mass transferred to a bucket of dry sand. Note that even the smallest trace of moisture may result in a steam explosion, spattering burning molten metal around. For this reason salt is sometimes preferred, since it is more obvious if it becomes damp.

See also

• Arson
• Fire alarm
• Fire engine
• Fire hydrant
• Fire-suppression system
• Halon