c. Class C. Fires occur in electrical equipment such as motors, switches, transformers. It can also occur
in the electrical system of an aircraft during rapid refueling operations. These fires present an extra hazard
because of the danger of electrical shock. A smothering agent is best for use on class C fire, but the agent must
not be a conductor of electricity. Carbon dioxide is preferred for this type of fire. The first step in extinguishing
class C fires is to turn off the source of power.
d. Class D. Fires occur in combustible metals such as titanium, zirconium, sodium, and potassium. The
greatest hazard exists when these metals are in the molten state or in finely divided forms of dust trimmings, or
shavings. Ordinary extinguishing agents are ineffective on these metal fires, and they are best controlled by
covering with special dry powdered or granular materials which exclude oxygen and which will not react or
combine adversely with metal.
Types of Fire Extinguisher. The Army uses portable hand-held fire extinguishers, wheel units and skid-
mounted units at small petroleum operations and terminal operations. Hand-held extinguishers are available in
many sizes and types. They are most effective in the early sages of a fire. Wheeled and skid-mounted units have
a greater capacity than the hand-held. The fire extinguishers common to petroleum operations are described
a. Carbon Dioxide (CO2). CO2 is a gas about 1 1/2 times heavier than air. It is nonpoisonous and will
not support combustion. C02 converts into a liquid when under pressure in an individual extinguisher tank.
When discharged, the chilling effect turns about 30 percent of the charge into dry ice or snow. This occurs
because of expansion. CO2 dilutes or smothers class B fires by cutting off the oxygen. It also works well with
class C fires because it is not a conductor of electricity. This extinguisher is effective from 3 to 8 feet.
b. Dry Chemical. Fire extinguishers are of two types, consisting primarily of treated sodium
bicarbonate powder treated with additives to make it waterproof and free flowing. One type is pressurized with
150 PSI of dry nitrogen or dry air, and the other has a cartridge of pressurized carbon dioxide. The extinguisher
puts out the fire by smothering. It works well with class B and C fires.
c. Purple K. Extinguisher is a dry chemical extinguisher using the agent potassium bicarbonate
(KHCO3), commonly called purple K. Carbon dioxide gas discharges the agent in a wide stream from a low-
velocity nozzle. It works by smothering fires and is designed for use on class B and C fires. Purple K is highly
d. Fire Suppression Assembly (Twin Agent Unit). This is a self-contained dry chemical and aqueous
film forming foam fire extinguisher. The skid-mounted or wheel-mounted frame of the extinguisher holds two
independent fire-fighting systems. One is the dry chemical (potassium bicarbonate or PKP) system, and the other
is the AFFF system. Pressurized nitrogen propels the agents from their tanks. The first agent to be discharged in
a petroleum fire is the Purple K to extinguish the flames. Then the AFFF solution is applied, where the foam
forms a film over the area to prevent re-ignition. The TAU can extinguish a fire in a 1,500 square feet berm. The
extinguisher is effective from 40-50 feet and at least two (preferably three) people to operate the unit. The
(1) Dry chemical tank. The tank is made of steel and is designed to hold 450 pounds of chemical
agent at 220-240 PSI operating pressure.
(2) AFFF tank. This tank is made of steel and is designed to hold 100 gallons of AFFF mixture, (6
gallons. AFFF concentrate to 94 gallons of water), at 220-240 PSI operating pressure. An electric heater is
mounted on the outside of the tank to heat the solution when tank temperature falls below 45ƒF.