Fire extinguisher
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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.
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History
The first version of the modern fire extinguisher was invented in the United Kingdom by Captain George Manby in 1816, consisting of a copper vessel of 3 gallons (13.6 litres) of pearl ash (potassium carbonate)solution under compressed air pressure.
The late 19th century saw the invention of the Soda-Acid extinguisher where a cylinder contained 1 or 2 gallons of water that had sodium bicarbonate mixed in it. Suspended in the cylinder was a phial containing concentrated sulphuric acid. Activating the extinguisher by striking a plunger to break the phial, or inverting the extinguisher to release a lead bung from the phial (depending on type), released the acid which mixed with the bicarbonate solution to generate carbon dioxide gas that expelled the water under pressure through a nozzle or short length of hose.
Around 1912 Pyrene pioneered the carbon tetrachloride or CTC extinguisher, where the liquid was expelled from a brass or chrome container by handpump, usually of 1 imperial quart (1.1 L) or 1 imperial pint (0.6 L) capacity but also made in up to 2 imperial gallon (9 L) size, onto a fire. The CTC vapourised and extinguished the flames by chemical reaction. This extinguisher was suitable for liquid and electrical fires and was popular in motor vehicles for the next 60 years. The vapour and combustion by-products were highly toxic and deaths did occur from using these extinguishers in confined spaces.
Classification
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In UK, Europe and Australia, the classes are:
- Class A: Fires involving common combustibles such as wood, grass, paper, plastic's and rubber.
- Class B: Fires involving flammable liquids such as kerosene, diesel fuel, oil and petrol.
- Class C: Fires involving flammable gasses like LPG, Natural gas, and acetylene.
- Class D: Fires involving flammable metals such as magnesium.
- Class E: This is not a class of fire per se but is used to refer to electrical fires.
- Class F: Fires involving preheated fats and oils such as fires in a deep fat fryer.
In the United States, the National Fire Protection Association 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 mixture of water and compressed gas. 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 ensure the user is not electrocuted.
- 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 are specialised for a particular hazard, and require additional 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).
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 |
Before 1997, UK Fire Extinguishers were also colour coded but slightly differently:
| Type | pre-1997 | current |
|---|---|---|
| Water | Signal Red | |
| Foam | Cream | Red with a Cream panel above the operating instructions |
| Powder | French Blue | Red with a Blue panel above the operating instructions |
| Carbon Dioxide CO2 | Black | Red with a Black panel above the operating instructions |
| Halon | Emerald Green | No longer produced - illegal in the UK |
| Wet Chemical | Not in use | Red with a Canary Yellow panel above the operating instructions |
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.
A "protein foam" was used for fire suppression in aviation crashes until the 1960s development of "light water", also known as "Aqueous Film-Forming Foam" (or AFFF). Carbon dioxide (later sodium bicarbonate) extinguishers were used to knock down the flames and foam used to prevent re-ignition of the fuel fumes. "Foaming the runway" can reduce friction and sparks in a crash landing, and protein foam continued to be used for that purpose, although FAA regulations prohibited reliance upon its use for suppression.
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 ('Wet Chemical')
Most class F (class K in the US) 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 F extinguishers cannot be used for class B fires. The misting also helps to prevent splashing the blazing oil.
Carbon dioxide
Carbon dioxide (CO2) 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.
In the UK and Europe Halons were made illegal at the end of 2003, except for certain specific aircraft and law enforcement uses.
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.
Maintenance
To operate safely and effectively, extinguishers should be subject to regular maintenance by a competent person and most countries in the world require this maintenance as part of fire safety legislation.
Lack of maintenance can lead to an extinguisher not discharging when required, or worse still, rupturing when pressurised. Deaths have occurred, even in recent times, from corroded extinguishers exploding.
In the UK, three types of maintenance are required:
- Basic Service:-- All types of extinguisher require a basic inspection annually to check weight, correct pressure (using a special tool, not just looking at the guage) & for signs of damage or corrosion;
- Extended Service:-- Water, Wet Chemical, Foam & Powder extinguishers require every 5 years a more detailed examination including a test discharge of the extinguisher & recharging if satisfactory;
- Overhaul:-- CO2 extinguishers, due to their high operating pressure, are subject to pressure vessel safety legislation and must be hydraulic pressure tested & date stamped every 10 years.
In the US, maintenance requirements can be more stringent with 6 monthly servicing and 5 yearly hydraulic pressure testing for all types of extinguisher.