Portable Fire Extinguishers - Principle of Operation
A portable fire extinguisher by definition is an item of equipment for the purpose of extinguishing a fire. The reality is however that a portable fire extinguisher is effective only for the type and size of a fire that it is rated for.
Portable fire extinguishers are generally provided as "first attack" units in fire fighting and should be used only in early stages of fire before the fire grows to a stage that is beyond the capacity of the extinguisher. There are broadly six types of fire extinguisher; Water, Foam, Wet Chemical, Dry Chemical Powder, Vaporising Liquid and Carbon Dioxide. The selection of an extinguisher must be made with the class of fire in mind.
The first record of a fire extinguisher was patent lodged in 1723 by English scientist Ambrose Godfrey#. In the following 300 years there have been literally hundreds of iterations and variations on the basic design for a fire extinguisher.1
If you distil all of the variations down you will find five components common to almost every type of extinguisher manufactured throughout history;
- a storage vessel
- an extinguishing agent
- a propellant (or expellant)
- a valve
- and a directional nozzle
Modern fire extinguishers continue to retain these common components, however improvements in technology have resulted in improvements to the performance (capacity and rating) of an extinguisher.
Earlier in this series, we outlined the foundation for fire chemistry. Combustion (fire) is a sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the by-products of combustion being; heat, smoke & electromagnetic radiation (light).
There are four elements that contribute to provide the correct environment for a fire to start and be sustained. These four elements are; Fuel, Heat, Oxidant, and the Chemical Reaction (Oxidation).
The effective removal of any one of these essential elements will result in the fire being extinguished. In simple terms, a fire extinguisher works by influencing, eliminating or the ongoing effect of one or more of these four elements.
Classification of fires
Fires are classified according to the type of fuel and if live electrical equipment is present. The classification of a fire is important, as it influences the selection and use of the correct extinguisher required to extinguish a fire. The six classes of fire are:
- Class A - Ordinary Combustibles (such as paper, wood, plastics, etc.)
- Class B - Flammable and combustible liquids
- Class C - Flammable gases
- Class D - Combustible metals
- Class E - Electrically energised equipment
- Class F - Cooking oils and fats
These classes of fire are also illustrated by a pictogram to assist people quickly identify the type of fire.
Types of Fire Extinguisher
As we stated earlier, there are broadly six types of extinguisher; Water, Foam, Wet Chemical, Dry Chemical Powder, Vaporising Liquid and Carbon Dioxide. The selection of an fire extinguisher for a given classification of a fire is assisted by the colour coding of extinguishers according to their type.
Portable fire extinguishers are distinguishable by their labels and their colouring. In 1997 the standard colour of portable fire extinguishers changed. From this date, extinguishers supplied to the market are required to be painted red or be polished stainless steel. As this change was relatively recent it is still common to find extinguishers using both Pre 1997 and Post 1997 extinguisher colour schemes.
There is no one type of fire extinguisher that is universally acceptable for all classes of fire. Careful consideration needs to be given to the selection of the most suitable type of fire extinguisher, or combination of fire extinguishers for each application.
Water Extinguishers (W)
A water based extinguisher also referred to as a Stored Pressure Air-Water fire extinguisher is an extinguisher that is filled with water which is stored under pressure (normally by air). These extinguishers are only appropriate for use on Class A fires.
A water extinguisher is effective because it cools the fire, interrupting the exothermic reaction of a self-sustaining fire.
A water extinguisher operates when water, stored under pressure and contained within the extinguisher container is expelled after the valve, operated by a hand-held trigger is depressed.
Dry Chemical Powder (DCP)
There are several types of dry chemical powder fire extinguisher, the two main types are;
- ABE Type - Effective on Class, A, B, C & E fires.
- BE Type - Effective on Class B, E & F fires.
Note: Special powders are available to extinguish fires involving Class D combustible metals.
Dry chemical powder fire extinguishers are effective because they interrupt the oxidation process.
Dry-chemical powder is stored under pressure inside the extinguisher cylinder and is expelled when the hand-held trigger valve is depressed.
An air-foam extinguisher is applied to either a class A or B fire as either an aspirated (mixed & expanded with air in a branch pipe) or non aspirated form to establish a foam blanket or seal over the fuel, preventing oxygen reaching it.
Wet Chemical (WC)
Wet Chemical extinguishers are typically installed in commercial kitchens for the protection from Class F fires where the fuel is cooking oils or fats. A wet chemical extinguisher is applied to a fire by forming a soapy foam blanket over the burning oil, smothering it and by cooling the oil below its ignition temperature. The chemical is expelled as a fine mist that does not cause grease to splash onto other surfaces. A Wet Chemical extinguisher may also be safely used in “A” class fires.
Carbon Dioxide (CO2)
Carbon dioxide fire extinguishers are useful in protecting against fire when an inert, electrically nonconductive, gas is desirable and where clean up from the agent must be minimal. Carbon dioxide extinguishers contain liquid CO2 that is expelled as a gas. Carbon dioxide blankets a fire, because of its heaviness, relative to air. It acts by preventing oxygen from getting to the fire, the result, the burning fuel deprived of the oxygen.
Carbon dioxide extinguishers are effective against Class “B” and “C” fires. Unlike other chemicals, CO2 does not leave a harmful residue and does not adversely affect the environment. It also poses very little danger to electronics and is effectively employed in laboratories, computer rooms, and other areas with sensitive electrical and electronic equipment.
Fire Extinguisher Operation
Fire extinguishers contain an agent that is expelled from the extinguisher help to try and extinguish a fire.
The agent in each of the extinguishers explained in this document is stored under pressure. The valve is operated when the hand-held trigger is depressed.
Some fire extinguishers are also fitted with a pressure gauge that provides a visual indication of the extinguishers pressurised state. Gauges may illustrate a numerical value or a colour coded pressure range where green illustrates the extinguisher is pressurised and is in a state of readiness.
Fire extinguishers complying with Australian Standards are marked with a classification and rating, determined in accordance with Australian Standard AS 1850.
Extinguishers are rated by their performance and suitability for a particular class of fire, i.e. a typical water extinguisher may be marked 2A and a typical dry chemical extinguisher marked 2A:40B:E.
The number before the letter is a measure of the relative performance within the respective class as follows:
- For Class A - Between 0 and 10
- For Class B - Between 2 and 80
- For Class F - Between 1 and 4
The higher the number , the more effective the extinguisher is for the nominated class of fire. When a fire extinguisher is rated for more than one class of fire, it is expressed in alphabetical order, i.e. 2A:40B:E.#
A common misconception is that two fire extinguishers of equal masss (Kg) or volume (Litres) have the same rating. Extinguishers are subjected to a range of standardised tests to determine their suitability and rating, in accordance with the requirements of the building Code of Australia and local authorities having jurisdiction. The rating of a fire extinguisher is required to be prominently displayed on the side of each extinguisher.
Location and Distribution
Generally the Building Code of Australia as well as various State and Territory legislation establishes the selection, location and distribution of fire extinguishers used in the community. Australian Standard AS 2444 is generally referenced in whole or in part by these legislative documents and provides further details on the selection, and location of fire extinguishers. AS 2444 is subordinate to these legislative documents and care should be taken when providing advice as to extinguisher requirements based solely on AS 2444.
Where required, fire extinguishers should be installed in a conspicuous and readily accessible position, supported by a substantial hook or bracket mounted not more than 1.2 metres above the floor (‘hip height’).
In addition, portable fire extinguishers should not be located in positions where access could present a hazard to a potential user. Where practicable, they should be located along normal paths of travel and near exits.
Most fire brigades or authorities having jurisdiction have guidelines for the selection and location of portable fire extinguishers to compliment the Building Code of Australia and Australian Standard AS2444.
There are two basic types of signage associated with portable fire extinguishers, a red, rectangular sign with a white pictorial of a fire extinguisher known as a “Location Sign” and an optional circular identification disc, specifying contents and type of fire on which the extinguisher is to be used.
The Location Sign
The location sign should be positioned directly above the fire extinguisher, the bottom edge of which is not less than 2 metres above the floor. (AS 2444:2001 clause 3.3.4).
The Identification Sign
The optional identification sign should be placed immediately above the fire extinguisher (below the location sign).
Extinguisher Selection Chart
Using a fire extinguisher
After carefully selecting the correct type of fire extinguisher or combination of extinguishers for the right class of fire, a first responder should undertake the following;
- Make sure the area is safe to access, the fire should not be either not too large or too hot, and that there are no other immediate hazards to safety.
- Contact the fire brigade.
- Choose the correct type of fire extinguisher for the class of fire.
- To use the fire extinguisher follow the acronym PASS - Pull, Aim, Squeeze, Sweep.
- Pull the pin of the fire extinguisher;
- Aim the extinguisher at the base of the fire;
- Squeeze the handle;
- Sweep it back and forth.
- After the fire has been extinguished it may reignite. If it is safe to do so, secure another fire extinguisher and watch the scene of the fire until the fire brigade arrive.
All portable fire extinguishers are subject to periodic inspection, tests and preventative maintenance activities in accordance with Australian Standard AS1851. This Standard prescribes specific intervals, criteria or events when an extinguisher should be maintained.
The frequency of these inspections is recorded on the maintenance record (a yellow tag, securely fixed to the fire extinguisher) by stamping or marking a number to represent the maintenance activity performed as follows;
1 - 6-Monthly
2 - Yearly
4 - 5-Yearly
5 - After Use
As most fire extinguishers are pressure vessels, they may also be required to be discharged, inspected and tested by an approved and licensed facility for any condition that is likely to render the vessel dangerous or unsafe.
Portable fire extinguishers may be used as the first response to most common classes of fire. In most commercial buildings, they are located throughout a building to provide occupants with the facilities to respond to a fire in its early stages.
In a domestic situation, households should be equipped with both a Dry Chemical Powder, Class AB(E) fire extinguisher and a fire blanket.
Fire extinguishers require periodic maintenance and should be maintained in accordance with Australian Standard AS1851.
More articles in this series
- Fire Alarm Systems - Principle of Operation
- Heat Detectors - Principle of Operation
- Fire Sprinkler Systems - Principle of Operation
About the author
Russ Porteous is the CEO and one of the founders of Maintenance Essentials, he has over 22 years experience in the installation and maintenance of fire and essential safety measures. Russ is an active contributor to a variety of Australian Standards including AS1851 for the Maintenance of Fire Protection Systems and Equipment. Russ speaks regularly at conferences and trade shows as a subject matter expert. You can follow Russ online via Twitter @rport or his popular fire safety blog.
- 1. http://en.wikipedia.org/wiki/Fire_extinguisher