| Fire
and extinguisher operation |
Fire triangle
To understand how fire
extinguishers work, you need to understand a little about
fire. Fire is a very rapid chemical reaction between oxygen
and a combustible material, which results in the release
of heat, light, flames, and smoke.
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For
fire to exist, the following four elements must
be present at the same time:
- Enough oxygen
to sustain combustion,
- Enough heat
to raise the material to its ignition temperature,
- Some sort of fuel
or combustible material, and
- The chemical reaction that is fire.
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How a fire extinguisher works
Portable fire extinguishers apply an extinguishing
agent that will either cool burning fuel, displace
or remove oxygen, or stop the chemical reaction
so a fire cannot continue to burn. When the handle
of an extinguisher is compressed, it opens an inner
canister of high-pressure gas that forces the extinguishing
agent from the main cylinder through a siphon tube
and out the nozzle. A fire extinguisher works much
like a can of hair spray.
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| Types
of fire extinguishers |
Different types of fire extinguishers
are designed to fight different types of fire. The three
most common types of fire extinguishers are:
All
portable fire extinguishers must be approved by
a nationally recognized testing laboratory such
as Underwriters Laboratories, Inc. (UL) or Factory
Mutual Research (FM) to verify compliance with applicable
standards 1910.157(c)(2).
Equipment that passes the laboratory's tests are
labeled and given an alpha-numeric classification
based on the type and size of fire it will extinguish.
Let's take a look at the label pictured. The classification
is:
1-A:10-BC
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The letters (A, B, and C) represent
the type(s) of
fire for which the extinguisher has been approved.
The number in front of the A rating indicates how much
water the extinguisher is equal to and represents 1.25
gallons of water for every unit of one. For example, a
4-A rated extinguisher would be equal to five (4 x 1.25)
gallons of water.
The number in front of the B rating represents the area
in square feet of a class B fire that a non-expert user
should be able to extinguish. Using the above example,
a non-expert user should be able to put out a flammable
liquid fire that is as large as 10 square feet.
| Extinguisher
Type |
Type
of Fire |
Water |
Ordinary
Combustibles
Fires
in paper, cloth, wood, rubber, and many plastics
require a water type extinguisher labeled A.
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CO2
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Flammable
Liquids
Fires in oils, gasoline, some paints, lacquers,
grease, solvents, and other flammable liquids require
an extinguisher labeled B.
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| OR |
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Dry
Chemical |
Electrical Equipment
Fires in wiring, fuse boxes, energized electrical
equipment, computers, and other electrical sources
require an extinguisher labeled C.
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Multi-Purpose
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Ordinary Combustibles, Flammable Liquids, or Electrical
Equipment
Multi-purpose dry chemical is suitable for use on
class A, B, and C.
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| D |
Metals
Combustible metals such as magnesium and sodium
require special extinguishers labeled D.
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| Air-pressurized
water extinguishers (APW) |
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Water is one of the most commonly used extinguishing
agents for type A fires. You can recognize an
APW by its large silver container. They are filled
about two-thirds of the way with ordinary water,
then pressurized with air. In some cases, detergents
are added to the water to produce a foam. They
stand about two to three feet tall and weigh approximately
25 pounds when full.
APWs extinguish fire by cooling the surface of
the fuel to remove the "heat" element
of the fire triangle.
APWs are designed for Class
A (wood, paper, cloth, rubber, and certain plastics)
fires only.
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Important:
Never use water to extinguish flammable
liquid fires. Water is extremely ineffective at
extinguishing this type of fire and may make matters worse
by the spreading the fire.
Never use water to extinguish an electrical
fire. Water is a good conductor and may lead to
electrocution if used to extinguish an electrical fire.
Electrical equipment must be unplugged and/or de-energized
before using a water extinguisher on an electrical fire.
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| Carbon
dioxide extinguishers |
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This type of extinguisher is filled with Carbon
Dioxide (CO2), a non-flammable gas
under extreme pressure. These extinguishers put
out fires by displacing oxygen, or taking away
the oxygen element of the fire triangle. Because
of its high pressure, when you use this extinguisher
pieces of dry ice shoot from the horn, which also
has a cooling effect on the fire.
You can recognize this type of extinguisher by
its hard horn and absent pressure gauge.
CO2 cylinders are red and range in
size from five to 100 pounds or larger.
CO2 extinguishers
are designed for Class B and C (flammable liquid
and electrical) fires only.
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Important:
CO2 is not recommended for Class A fires
because they may continue to smolder and re-ignite
after the CO2 dissipates.
Never use CO2 extinguishers in a confined
space while people are present without proper respiratory
protection.
Locations:
Carbon dioxide extinguishers will frequently be
found in industrial vehicles, mechanical rooms,
offices, computer labs, and flammable liquid storage
areas.
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| Dry
chemical extinguishers |
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Dry chemical extinguishers put out fires by coating
the fuel with a thin layer of fire retardant powder,
separating the fuel from the oxygen. The powder
also works to interrupt the chemical reaction, which
makes these extinguishers extremely effective.
Dry chemical extinguishers are usually rated for
class B and C fires and may be marked multiple purpose
for use in A, B, and C fires. They contain an extinguishing
agent and use a compressed, non-flammable gas as
a propellant.
ABC fire extinguishers are red in color, and range
in size from five pounds to 20 pounds.
Dry Chemical extinguishers will
have a label indicating they may be used on class
A, B, and/or C fires.
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or
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Locations:
These extinguishers will be found in a variety of
locations including: public hallways, laboratories,
mechanical rooms, break rooms, chemical storage
areas, offices, commercial vehicles, and other areas
with flammable liquids.
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