A portable fire extinguisher is an active fire protection device to extinguish or control a fire in emergency situations. A typically portable fire extinguisher consists of a pressure vessel containing an agent that when discharged, can extinguish a fire. In order to use a fire extinguisher, you must first make sure it is suitable to the type of fire. If it is not a suitable type, it may not be effective or it may cause additional dangers. For example, water on a kitchen chip pan fire will sink into
the hot fat and be rapidly converted to steam erupting the flaming fat over a wide area. Water on an active electrical fire might create an electric shock danger.Over the years many very effective fire extinguishing agents have been discovered but they went out of favour because of the side effects and the main one was toxicity. Others have been superceded by more effective extinguishing agents with fewer side effects consequently in the following page there are many extinguishing agents that have not been used for many years or banned.
In about 200 BC, Ctesibius of Alexandria invented a hand pump able to deliver water to a fire and it is known that the Romans used bucket chains, buckets passed hand-to-hand to deliver water to the fire. Then, in the middle Ages, ‘squirts’ began to be used to apply jets of water to fires. The squirt worked rather like a bicycle pump. The nozzle was dipped into water and about one litre was sucked up by pulling out the plunger. The charged squirt was then directed at the fire and the plunger pushed home to eject the water. Squirts were used on the 1666 Great Fire of London. The first version of the modern portable fire extinguisher was invented by Captain George William Manby in 1819, consisting of a copper vessel of 3 gallons (13.6 litres) of pearl ash (potassium carbonate) solution under compressed air pressure.
Around 1912 Pyrene pioneered the carbon tetrachloride or CTC extinguisher, where the liquid was expelled from a brass or chrome container by hand pump, onto a fire. The sizes were 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) sizes. The CTC vapourised and extinguished the flames by interfering with the 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.
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 sulphuric 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 short length of hose and a nozzle. The acid was neutralised by the sodium bicarbonate.
Foam extinguisher consisted of the main body of the extinguisher filled with foam producing chemical and a second container filled with another chemical which reacts when it came into contact with the solution in the main cylinder. To operate you turned the extinguisher upside down and allowed the two solutions to mix, then hold your finger over the discharge nozzle and shake the extinguisher to ensure the solution was properly mixed then direct it at the fire.
Middle of the twentieth century the modern type of extinguisher appeared using different extinguishing agents. Manufacturers of extinguishers generally use some type of pressurized vessel to store and discharge the extinguishing agent.
First type of fire extinguishers (Fig1) are pressurized with air to approximately 10 bar, five times a car tyre pressure, from a compressor. A squeeze-grip handle operates a spring-loaded valve threaded into the pressure cylinder. Inside, a pipe or dip tube extends to the bottom of the extinguisher so that in the upright position, the opening of the tube is submerged. The extinguishing agent is released as a steady stream through a hose and nozzle, pushed out by the stored pressure above it.
The second type of fire extinguishers (Fig2) are the “gas cartridge” type operate in the same manner, but the pressure source is a small cartridge of carbon dioxide gas (CO2) at 130 bars, rather than air. A squeeze-grip handle operates a spring-loaded device causing a pointed spike to pierce the disc that held back the pressure, releasing the gas into the pressure vessel. The released CO2 expands several hundred times its original volume, filling the gas space above the extinguishing agent. This pressurises the cylinder and forces the extinguishing agent up through a dip-pipe, out through a hose and nozzle to be directed upon the fire. This design proved to be less prone to leak down, loss of pressure over time, than simply pressurizing the entire cylinder.
For water, foam, dry powder and wet chemical extinguishers, the extinguishing agents can either be put under stored pressure, or a gas cartridge expeller, however the stored-pressure type is more widely used. Dry powder generally use the CO2 cartridge method to prevent the powder being affected by moist air used for the stored pressure method.In carbon dioxide extinguishers, the CO2 is retained in liquid form under 50 to 60 bar and is self-expelling, meaning that no other element is needed to force the CO2 out of the extinguisher. In halon units, the chemical is also retained in liquid form under pressure, but a gas booster, usually nitrogen, is generally added to the vessel.The nozzles are the main difference; water has a circular nozzle which forms a solid jet which can penetrate to the heart of the fire. Foam has a miniature foam nozzle that aerates the foam solution and forms a floppy stream of foam that can be gently applied to the surface of the liquid. Dry Powder has an elliptical nozzle which spreads the powder and prevents air being entrained. Wet Chemical has a nozzle that creates a fine spray which allows you to apply the extinguishing agent gently. Carbon dioxide has a discharge horn which slows down the jet of gas and prevents air being entrained.
Water is the most common 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 these 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 greater the effect on flaming gases. With this in mind water spray extinguishers have bee developed, one example is the Hydro spray by Chubb. The Hydro spray Elite was developed in 1997 as a direct response to customer needs. It was felt that the while traditional 9 litre water extinguishers are very effective on class A fires made up of woods, solids or materials, a smaller, lighter version would be much easier for some people to use. Chubb developed the Hydro spray Elite. This 3 litre product is able to achieve the same fire ratings as a 9 litre water extinguisher, in a size that makes it easier for anyone to lift and manoeuvre.The unique nozzle that separates the water particles making it safe (as per the 35kV test) if it is accidentally used on an electrical fire.
In the past water based extinguishers also contain traces of other chemicals to prevent the extinguisher rusting but are now lined with plastic. Some also contain surfactants which help the water penetrate deep into the burning material and cling better to steep surfaces this is know as wet water.
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 has polar molecules, such as alcohol, there won’t be any problem. If the liquid is non polar, such as large hydrocarbons, like petroleum, the water will sink through the oil until it reaches the heat layer and then be immediately converted to steam ejecting the burning contents in a violent eruption, known as a boil over. Alternately as water being heavier than oil it will sink to the bottom and replace the oil until the flaming oil flows over the edge of the container thus spreading the fire; this is known as a slop over. This is why you should never use water on oil fires.
Similarly, water sprayed on an electrical fire will probably cause the operator to receive an electric shock. However, if the power can be reliably disconnected, 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 resistance 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 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 non polar flammable liquids such as petrol, but may break down too quickly in polar liquids such as alcohol or glycol. Facilities which handle large amounts of flammable polar liquids use specialised “alcohol resistant 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.
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.
Principally there are two types of dry powder extinguishers in use, they are BC and ABC.
Class BC dry powder is either sodium bicarbonate or potassium bicarbonate, finely powdered and propelled by carbon dioxide or nitrogen. Similarly to almost all extinguishing agents the powder acts as a thermal ballast and makes 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 Powder has a slight saponification effect on cooking oils & fats due to its alkalinity and was used 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. Sodium bicarbonate powders, unless specially treated, are not compatible with Foams. Purple-K, Monnex and ABC Powders are generally less damaging, and often are used with AFFF & FFFP, but compatibility must be borne in mind where powder and foam are used together and a higher application rate of foam allowed for.
Class ABC Powders are mixtures of ammonium phosphate and ammonium sulphate, ground to selected particle sizes and treated with flow promoting and moisture repellent additives. In addition to the particle surface extinguishing effect, ABC powders have low melting/decomposition points in the order of 150°C to 180°C. When these powders are applied to hot and smouldering surfaces, the particles fuse and swell to form a barrier which excludes oxygen and thereby completes the extinguishing process and prevents re-ignition. They are acidic in nature and are effective on Class A (flammable solids), Class B (flammable liquid/liquefiable solids) and Class C (flammable gas) fires. They are electrically non-conductive; 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. Most dry powder extinguishers in service, except aerosols, are ABC Powder. Different blends are available, the more ammonium phosphate, the more effective it is. Powder, specifically ABC powder is not permitted in or near aircraft as it can damage the metal superstructure.
Dry powders can also be used on electrical fires, but provide a significant cleanup and corrosion problem that is likely to make, especially sensitive electronics and electrical equipment unsalvageable.
Although modern powders are non-toxic, the discharge of a powder extinguisher in a confined space can cause a sudden reduction of visibility which may temporarily jeopardize escape, rescue or other emergency action. For this reason water-based extinguishers are to be preferred in hospitals, old people’s homes and hotels. Powder extinguishers are prohibited to be used in PSV’s and minibuses by UK statute law for the above reason.
When used on class B fires, the powder must extinguish the whole fire area in an uninterrupted application or flashback will occur, unlike foam there is no physical barrier, it’s all or nothing. The lack of a securing blanket means there is a re-ignition risk. Also powder has no cooling properties, one of the reasons it is ineffective against class F fires, although it can extinguish the flame, the heat of the fat will cause immediate flashback.
There were available powders designed for fires in flammable metals and three main types in use were, Sodium Chloride for fires involving alkali metals such as sodium and potassium, also zirconium, uranium and powdered aluminium which extinguished metal fire by fusing to form a crust. This excludes oxygen from the surface of the molten metal and a carbonaceous rafting agent prevents the powder from sinking into the surface of molten metal. A copper extinguishing agent specially developed by the U.S. Navy for fighting lithium and lithium alloy fires. The copper compound smothers the fire and provides an excellent heat sink for dissipating heat. Copper powder has been found to be superior to all other known fire extinguishing agents for lithium. Finally Ternary Eutectic Chloride (TEC), developed by UKAEA for uranium fires, which works similarly to Sodium Chloride but it, is extremely toxic.
Carbon dioxide extinguisher (CO2) works on classes B and C 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 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 has a discharge horn on the end of the hose which slows down the jet of gas and prevents air being entrained. Due to the carbon dioxide being expelled from an extinguisher, the horn becomes extreme cold and should not be touched.