FIRE-FIGHTING AGENTS. Many materials
may be used as fire-fighting agents. The following
are the fire-fighting agents used most often aboard
Water. Water is a cooling agent, and
aboard ship the sea provides an inexhaustible
supply. If the surface temperature of a fire can
be lowered below the fuels ignition temperature,
the fire will be extinguished. Water is most
efficient when it absorbs enough heat to raise its
temperature, the water will absorb still more heat
until it changes to steam. The steam carries away
the heat, which cools the surface temperature.
A secondary method of water extinguishment
is caused by steam smothering. When water
changes into steam by absorbing heat, it expands
about 1,700 times in volume. The large quantity
of steam displaces the air from the fuel, which
smothers the fire. Steam-smothering systems are
installed in boiler casings and catapult troughs.
Aqueous Film-Forming Foam. Aqueous
film-forming foam (AFFF) is composed of
synthetically produced materials similar to liquid
detergents. These film-forming agents are capable
of forming water solution films on the surface of
flammable liquids. The Navy mixes AFFF by
volume in the following proportion: 6 parts of
AFFF concentrate to 94 parts water.
AFFF concentrate is a clear to slightly amber-
colored liquid concentrate. The AFFF solution of
water and concentrate possesses a low viscosity
and spreads quickly over a surface. AFFF
concentrate is nontoxic and biodegradable in
diluted form. AFFF concentrate may be stored
for long periods without losing its effectiveness.
The concentrate will freeze when exposed to
temperatures below 32°F (0°C) but can be
reused when thawed.
AFFF, when mixed with water, provides three
fire-extinguishing advantages. First, it forms an
aqueous film on the surface of the fuel, which
prevents the escape of the hydrocarbon fuel
vapors. Second, the layer of foam effectively
excludes oxygen from the fuel surface. Third, the
water content of the foam provides a cooling
Foam is used mainly to extinguish burning
flammable or combustible liquid spill fires
(class B). AFFF has excellent penetrating
characteristics and is superior to water in
extinguishing class A fires.
Carbon Dioxide (CO2). CO2 extinguishes
fires by smothering. Co2 is about 1.5 times
heavier than air. That makes CO2 a suitable
extinguishing agent because it tends to settle and
blanket the fire.
CO2 is a dry, noncorrosive gas that is inert
when in contact with most substances and will
not leave a residue that damages machinery or
electrical equipment. In both the gaseous state and
the finely divided solid (snow) state, it is a non-
conductor of electricity y regardless of voltage. CO2
can be safely used in fighting electrical fires.
C O2 extinguishes the fire by diluting and
displacing its oxygen supply. If gaseous CO2 is
directed into a fire so that sufficient oxygen-
supporting combustion is no longer available, the
flames will die out. Depending on what is fueling
the fire, that action will take place when the
21-percent oxygen content, normally present in
air, is diluted with CO2 below 15 percent oxygen.
Some ordinary combustible class A fires require
that the oxygen content be reduced to less than
6 percent to extinguish glowing combustion
(smoldering fire). CO2 has limited cooling
capabilities, may not cool the fuel below its
ignition temperature, and is more likely than other
extinguishing agents to allow reflash. Therefore,
the fire fighter must remember to stand by with
additional backup extinguishers. The temperature
of the burning substance and its surroundings
must be below its ignition temperature if the fire
is to remain extinguished.
CO2 is not an effective extinguishing agent
for fires in materials that produce their own
oxygen supply (as an example, aircraft parachute
flares). Fires involving reactive metals, such as
magnesium, sodium, potassium, or titanium,
cannot be extinguished with CO2. Because of the
relatively high temperatures involved, these metal
fuels decompose CO2 and continue to burn.
C O2 can produce unconsciousness and
death when present in fire-extinguishing
concentrations. The reaction in such cases
is more closely related to suffocation. A
concentration of 9 percent will cause most
people to lose consciousness within a few
minutes. Caution must be exercised when
discharging CO2 in confined spaces.