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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

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