Fire Extinguishers
The Fire Triangle
In order to understand how fire extinguishers work, you
first need to know a little bit about fire.
Four things must be present at the same time in order to
produce fire:
- 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, exothermic reaction
that is fire.

Oxygen, heat, and fuel are frequently referred to as the
"fire triangle." Add in the fourth element, the
chemical reaction, and you actually have a fire "tetrahedron."
The important thing to remember is: take any of these four
things away, and you will not have a fire or the fire
will be extinguished.
Essentially, fire extinguishers put out fire by taking
away one or more elements of the fire triangle/tetrahedron.
Fire safety, at its most basic, is based upon the
principle of keeping fuel sources and ignition sources separate.
Basic types of fire extinguishers
The two most common types of extinguishers in the chemistry
laboratory are pressurized dry chemical (Type BC or ABC, left)
and carbon dioxide (CO2 ,right) extinguishers:

Water-filled extinguishers are not acceptable for
chemistry laboratory use. If you have a water-filled extinguisher,
have it replaced immediately by contacting EH&IS. If
you are not familiar with fire extinguishers and have not
been trained in their use, DO NOT attempt to use them! If
you work in a laboratory you should have received this training
(it is usually a state or federal OSHA
requirement); if not contact your supervisor immediately (TODAY....no
kidding).
Which kind of extinguisher should I use?

Not all fuels are the same, and if you use the wrong type of fire
extinguisher on the wrong type of fuel, you can, in fact,
make matters worse. It is therefore very important to understand
the four different classifications of fuel.
First recognize that there are four different kinds of fires:
- Class A fires are ordinary materials like burning
paper, lumber, cardboard, plastics etc.
- Class B fires involve flammable
or combustible
liquids such as gasoline, kerosene, and common organic
solvents used in the
laboratory.
- Class C fires involve energized electrical equipment,
such as appliances, switches, panel boxes, power tools,
hot plates and stirrers. Water is a particularly dangerous
extinguishing medium for class C fires because of the
risk of electrical shock.
- Class D fires involve combustible metals, such
as magnesium, titanium, potassium and sodium as well as
pyrophoric
organometallic reagents such as alkyllithiums, Grignards
and diethylzinc. These materials burn at high temperatures
and will react violently with water, air, and/or other
chemicals. Handle with care!!
Some fires may be a combination of these! Your fire extinguishers
should have ABC ratings on them. These ratings will often
have numbers on them that look something like "3-A:40-B:C".
Higher numbers mean more firefighting power. In this example,
the extinguisher has a good firefighting capacity for Class
A, B and C fires.
Most fire extinguishers will have a pictograph label telling
you which fuels the extinguisher is designed to fight. For
example, a simple water extinguisher might have a label like
the one below, indicating that it should only be used on Class
A fuels.

Here are typical uses for common extinguishers:
- Water extinguishers (not pictured and not found
in laboratories) are suitable for class A (paper etc.)
fires, but not for class B, C and D such as burning liquids,
electrical fires or reactive metal fires. In these cases,
the flames will be spread or the hazard made greater!
- Dry chemical extinguishers are useful for class
ABC fires and are your best all around choice. They have
an advantage over CO2 extinguishers in that
they leave a blanket of non-flammable material on the
extinguished material which reduces the likelihood of
re-ignition. They also make a terrible mess -- but if
the choice is a fire or a mess, take the mess! Note that
there are two kinds of dry chemical extinguishers!
- Type BC fire extinguishers contain sodium or
potassium bicarbonate.
- Type ABC fire extinguishers contain ammonium
phosphate.
- CO2 (carbon dioxide) extinguishers
are for class B and C fires. They don't work very well
on class A fires because the material usually re-ignites.
CO2 extinguishers have an advantage over dry
chemical in that they leave behind no harmful residue
-- a good choice for an electrical fire on a computer
or other delicate instrument. Note that CO2
is a bad choice for a flammable metal fires because CO2
reacts with these materials. CO2 extinguishers
are not approved for class D fires!
- Metal/Sand Extinguishers are for flammable metals
(class D fires) and work by simply smothering the fire.
You should have an approved class D unit if you are working
with flammable metals.
Check out the potential fire hazards in your area. Is there
an extinguisher available? Do you know how to operate it?
Are your extinguishers suitable for the fires you may encounter?
If not, you'll want to contact EH&IS (x7233).
Typical small lab fires (in a hood or on a bench) can easily
be controlled by a dry chemical (ABC) or CO2 extinguisher
provided that you are properly trained.
Source: Oklahoma State University