Building Design and Construction Handbook - Merritt - Ventech!

PROTECTION AGAINST HAZARDS 3.37
Air foam (mechanical foam) is made by mechanical mixing of water and a
protein-based chemical concentrate. There are several methods of combining the
components, but essentially the foam concentrate is induced into a flowing stream
of water through a metering orifice and a suitable device, such as a venturi. The
volume of foam generated is from 16 to 33 times the volume of water used. Several
kinds of mixing apparatus are available, choice depending on volume required,
availability of water, type of hazard, and characteristics of the protected area or
equipment.
Air foam can be conducted through pipes and discharged through a fixed chamber
mounted in a bulk fuel storage tank, or it can be conducted through hoses and
discharged manually through special nozzles. This foam can also be distributed
through a sprinkler system of special design to cover small equipment, such as
process vessels, or in multisystem applications, over an entire airplane hangar. The
standard for use and installation of air foam is NFPA 11, and for foam-water sprinkler
systems, NFPA 16.
High-expansion foam was developed for use in coal mines, where its extremely
high expansion rate allowed it to be generated quickly in sufficient volume to fill
mine galleries and reach inaccessible fires. This foam can be generated in volumes
of from 100 to 1000 times the volume of water used, with the latter expansion in
most general use. The foam is formed by passage of air through a screen constantly
wetted by a solution of chemical concentrate, usually with a detergent base. The
foam can be conducted to a fire area by ducts, either fixed or portable, and can be
applied manually by small portable generators. Standard for equipment and use of
high-expansion foam is NFPA 11A.
High-expansion foam is useful for extinguishing fires by totally flooding indoor
confined spaces, as well as for local application to specific areas. It extinguishes
by displacing air from the fire and by the heat-absorbing effect of converting the
foam water content into steam. The foam forms an insulating barrier for exposed
equipment or building components.
High-expansion foam is more fragile than chemical or air foam. Also, it is not
generally reliable when used outdoors where it is subject to wind currents. Highexpansion
foam is not toxic, but it has the effect of disorienting people who may
be trapped in it.
Carbon dioxide is useful as an extinguishing agent, particularly on surface fires,
such as those involving flammable liquids in confined spaces. It is nonconductive
and is effective on live electrical equipment. Because carbon dioxide requires no
clean-up, it is desirable on equipment such as gasoline or diesel engines. The gas
can be used on Class A fires. But when a fire is deep-seated, an extended discharge
period is required to avoid rekindling.
Carbon dioxide provides its own pressure for discharge and distribution and is
nonreactive with most common industrial materials. Because its density is 1 1 ⁄2 times
that of air, carbon dioxide tends to drop and to build up from the base of a fire.
Extinguishment of a fire is effected by reduction of the oxygen concentration surrounding
a fire.
Carbon dioxide may be applied to concentrated areas or machines by hand-held
equipment, either carried or wheeled. Or the gas may be used to flood totally a
room containing a hazard. The minimum concentrations for total flooding for fires
involving some commercial liquids are listed in ‘‘Standard on Carbon-Dioxide
Extinguishing Systems,’’ NFPA 12.
Carbon dioxide is not effective on fires involving burning metals, such as magnesium,
nor is it effective on oxygen-containing materials, such as nitrocellulose.