Fire extinguisher

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Image:FireExtinguisherABC.jpg A fire extinguisher is an Active fire protection device to put out fires, often in an emergency situation. They consist of a pressurized container of chemicals that when discharged can put out a fire. It is important to familiarize yourself with the use of fire extinguishers in your vicinity, as improper or untimely use may be counterproductive.

History

The first version of the modern fire extinguisher was invented in the United Kingdom by Captain George William 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 vial containing concentrated sulfuric acid. The vial of acid was broken by one of two means depending on the type of extinguisher. One means involved the use of a plunger that broke the acid vial, while the second involved the release of a lead bung that held the vial closed. Once the acid was mixed with the bicarbonate solution, carbon dioxide gas would be expelled and this would in turn pressurize the water. The pressurized water was forced from the canister 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.

Construction

General

Image:Fire extinguisher section.png A fire extinguisher is usually made using the following components :

One or more bottles A, containing either the extingushing agent or the propelling gas, or both;
A valve B, used to block and/or control the flow of the extinguishing agent;
A hose C, which is a flexible pipe allowing easy aiming of the jet of the extinguishing agents towards the fire (this may be avoided for masses up to 3 kg);
An extinguishing agent D which, sprayed or sparged or in any other way put in contact with the burning substances, interacts with these to obtain the extinguishment or the limitation of the fire;
A propellant gas, used for the expulsion of the extinguishing agent.
A label E showing information as defined by the applicable regulations.

One of the most common types of fire extinguishers, a portable stored pressure type, is shown in the picture. The shape, proportions and details may vary according to the model, but the components are as above.

Bottle

Image:Fire extinguisher bottle.png

Bottle characteristics vary according to the use of the extinguisher. Basically, there are two classes of bottles :

Low pressure bottles, used for dry powder (dry chemical), water, AFFF and Halon extinguishers. These bottles have a design pressure ranging between 1 and 3 MPa. The most common material used is carbon steel, deep drawing grade. Austenitic stainless steel, usually UNS 30403 (AISI 304 L), is often used for water-base fire extinguishers. Recently, deep-drawn aluminium bottles (UNS A93003, or Alloy 3003) have been used. Stainless steel and aluminium bottles are used in specialised Navy vessels to avoid detection by MAD detectors. A typical low pressure steel bottle is shown in fig. 2.
High pressure bottles, used almost exclusively for carbon dioxide extinguishers, have design pressures exceeding 25 MPa. These bottles are single-piece, usually extruded at high temperatures, either from low-alloy steel or specialty aluminium alloys such as UNS A97075 or UNS A96061 with aluminium being used mainly in larger units to reduce the extinguisher mass.

Classification

Image:German fire extinguisher.JPG

Europe

Three classes are defined according to the European Standard (EN 3):

Class A: Fires involving common solid combustibles such as wood, grass, paper, plastics and rubber.
Class B: Fires involving flammable liquids such as kerosene, diesel fuel, oil and petrol.
Class C: Fires involving flammable gases like LPG, natural gas, and acetylene.

In addition to this, some other categories appear from time to time

D: Fires involving flammable metals such as magnesium.
E: Sometimes refer to electrical fires.
F: UK class for fires involving preheated fats and oils, e.g. in a deep fat fryer. Its use is currently very uncommon and mostly restricted to a small number of liquid chemical-based extinguishers.

Classes A and B are designated by a number, expressing the capability of that particular model to extinguish a specified fire, thus defining a fire class as :

for class A, the capacity of the extinguisher to put out a fire generated by a stack of wood of a specified nature, having a specified cross section and having a length (in decimetres) equal to that number;
for class B, the capacity of the extinguisher to put out a fire generated by a volume of a standard flammable liquid (n-heptane) expressed by that number (in litres), contained in a tank having standard dimensions.

The standard rating for class A and B range from 5 to 233 according to the Fibonacci sequence : 5, 8, 13, 21, 34, 55, 89, (113), 144, (168), 233. Values 113 (= 89 + 34) and 168 (= 113 + 55) are not part of the Fibonacci sequence, but were used in the past, and remain valid. Ratings 5 and 8 are used only for class A fires.

Thus, a 13A 168B fire class extinguisher, if properly used by a professional, will put out fires both from a wood stack 1.3 metre long and from a circular basin containing 168 litre of liquid, water below and n-heptane floating on water.

Class C has no numerical rating; according to EN3 standards, the fulfilment of the gas extinguishing test is shown by a C after the designation of fire classes A and B; in the above example, a 13A 168 B C extinguisher will also put out a flame created by LPG in standard test conditions. (The LPG flame, mind you, is put out, but the LPG flow is not discontinued, so putting out the flame may be more dangerous than letting it burn).

Class D is not covered by EN3.

Class E has been deleted in EN3; a dielectric test is performed if required by the manufacturer, and the capability of operating on live electrical equipment without electrocuting hazard is now shown by a pictogram.

A good 6 kg dry powder extinguisher will rate 21A 233B C fire class; a 5 kg carbon dioxide will rate 89B. A 1 kg extinguisher for cars or small leisure boats rates 34B C or, if multi-purpose, 8A 34B C (passing C-class test is dubious for small extinguishers).

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. The symbol is a circle around the letter C.
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 specialized 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.

United States ratings

In the US, extinguishers also receive ratings for Class A and B fires.

A 1-A rating is given to an extinguisher equivalent to 1 1/4 gallons of water. A 2-A rating is 2 1/2 gallons (twice a 1-A). The highest rating given to a Class A extinguisher is 40-A, equal to 50 gallons of water.
A 1-B rating is given to an extinguisher which can extinguish 1 square foot of a flammable liquid fire. A 2-B is 2 square feet. The highest rating given to a Class B extinguisher is 640-B, 640 square feet.

Fire extinguishers are often designed to combat more than one class of fire. Commonly available are A (water), BC (carbon dioxide), and ABC (dry powder).

Markings

Australia

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

United Kingdom

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

Continental Europe (CEN countries other than UK)

No colour coding imposed : All extinguishers shall be signal red (RAL 3000). Recently, some voluntary color coding has appeared, however different from the UK National Addendum to EN 3.

United States

Fire extinguishers in the US are not color coded, though most Class D extinguishers are colored yellow. Extinguishers are marked with symbols, formerly with colored geometric symbols, though pictograms are now used. Some extinguishers use both the old symbols and modern pictograms. No official pictogram exists for Class D extinguishers, though training manuals sometimes show a drill press with shavings burning underneath.

Type	Geometric	Pictogram
Class A	Green Triangle	Garbage can and wood pile burning
Class B	Red Square	Gasoline can with a burning puddle
Class C	Blue Circle	Electric plug with a burning outlet
Class D	Yellow Star	N/A
Class K	Black Hexagon	Pan burning

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. Water has a great effect on cooling the fuel surfaces and thereby reducing the pyrolysis rate of the fuel. The gaseous effect is minor for extinguishers, but water fog nozzles used by fire brigades creates water droplets small enough to be able to extinguish flaming gases as well. The smaller the droplets, the bigger the gaseous effect.

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 may or may not help extinguishing class B fires. It depends on whether or not the liquid's molecules are polar molecules. If the liquid that is burning is polar (such as alcohol), there won't be any problem. If the liquid is nonpolar (such as large hydrocarbons, like petroleum), the water will merely spread the flames around.

Similarly, water sprayed on an electrical fire (US: Class C, UK: Class E) will probably 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). Special spray nozzles, equipped with tiny rotating devices called spiracles will replace the continuous water jet with a succession of droplets, greatly increasing the resistivity of the jet. These shall however be used by skilled personnel, since improper handling of the nozzle may restore continuity of the water jet.

Foams

Image:Extincteur-essence-3.jpg 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 and break the interaction between the flames and the fuel surface. 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 ("Dry Chemical" in the US)

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, finely powdered and propelled by carbon dioxide or nitrogen. Similarly to almost all extinguishing agents the powders acts as a thermal ballast making the flames too cool for the chemical reactions to continue. Some powders also provide a minor chemical inhibition, 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.

BC Powder has a slight saponification effect on cooking oils & fats due to its alkalinity & sometimes used to be specified for kitchens prior to the invention of Wet Chemical extinguishers. Where an extremely fast knockdown is required potassium bicarbonate (Purple K) extinguishers are used. A particular blend also containing urea (Monnex) decrepitates upon exposure to heat increasing the surface area of the powder particles and providing very rapid knockdown.

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 the gas and heat transfer at the fuel surface. 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 less 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 electrical 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

Image:CO extinguisher 2.PNG Carbon dioxide (CO₂) also works on classes B and C/E and works by suffocating the fire. Carbon dioxide will not burn and displaces air. Carbon dioxide can be used 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.) Carbon dioxide extinguishers have a horn on the end of the hose. Due to the extreme cold of the carbon dioxide that is expelled from an extinguisher, it should not be touched.

Halons

Image:Fire extinguisher.jpg

Halons are very versatile extinguishers. They will extinguish most types of fire except class D & K/F and are highly effective even at quite low concentrations (less than 5%). Halon is a poor extinguisher for Class A fires, a nine pound Halon extinguisher only receives a 1-A rating and tends to be easily deflected by the wind. They are the only fire extinguishing agents that are quite suitable for discharge in aircraft (as other materials pose a corrosion hazard to the aircraft). The major extinguishing effect is by disturbing the thermal balance of the flame, and to a small extent by inhibiting the chemical reaction of the fire. Halons are chlorofluorocarbons causing damage to the ozone layer and are being phased out for more environmentally-friendly alternatives. Halon fire extinguishers may cost upwards of 800 US dollars due to production and import restrictions.

Halon extinguishers used to be widely used in vehicles and computer suites. It is mildly toxic in confined spaces, but to a far less extent than its predecessors such as carbon tetrachloride, chlorobromomethane and methyl bromide.

Since 1992 the sale and service of Halon extinguishers has been made illegal in Canada except for in a few rare cases.

In the UK and Europe Halons were made illegal at the end of 2003, except for certain specific aircraft and law enforcement uses. This appears to be at least partially in response to the Montreal Protocol and effort by the United Nations Environment Programme (UNEP) to combat release of quantities of harmful chemicals into the atmosphere.

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 (table 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.

There is no all-encompassing fire code in the United States. Generally, most municipalities (by adoption of the International Fire Code) require inspections every 30 days to ensure the unit is pressurized and unobstructed (done by an employee of the facility) and an annual inspection by a qualified technician. Also, a 5 yearly hydraulic pressure testing for all types of extinguisher is required. Through electronic monitoring of fire extinguishers the 30 day inspection can be eliminated.

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 gauge) & 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.

Recommended locations in houses

It is recommended that houses have a fire extinguisher accessible on every floor, especially in the kitchen. For each room it important to choose a fire extinguisher of the type appropriate to the fire risk.

See also

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Categories: Firefighting equipment | Safety

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