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FOAM BASICS
This article is intended to be only an introduction into foam
concentrates to aid the buyer in acquiring the correct foam for their
needs. For a look at the foam we stock and the associated application
equipment, click here.
To properly understand foam basics some definitions are in
order:
FOAM: A fire fighting foam is simply a stable mass of
small air-filled bubbles, which have a lower density than oil, gasoline or
water. Foam is made up of three ingredients - water, foam concentrate and
air. When mixed in the correct proportions, these three ingredients form a
homogeneous foam blanket.
FOAM SOLUTION: This is a solution of water and foam
concentrate after they have been mixed together in the correct proportions.
FOAM CONCENTRATE: This liquid concentrate is supplied
from the manufacturer which when mixed with water in the correct proportion
forms a foam solution.
FINISHED FOAM: Foam solution as it exits a discharge
device, having been aerated.
DRAINAGE RATE: This is the rate at which the foam
solution will drain from the expanded foam mass or how long it will take for 25%
of the solution to drain from the foam. This is often called the quarter
life or 25% drain time. Foam that has a fast drain time is normally very
fluid and mobile, spreading across the fuel surface very quickly. While
foams with longer drain times are normally less mobile, they move across the
fuel surface slowly.
EXPANSION RATE: Volume of finished foam divided by the
volume of foam solution used to create the finished forma; i.e., a ration of 5
to 1 would mean that one gallon of foam solution after aeration would fill an
empty 5-gallon container with the expanded foam mass.
LOW EXPANSION FOAM: Foam aerated to an expansion ratio
of between 2 to 1 and 20 to 1.
MEDIUM EXPANSION FOAM: Expansion ratio between 20 to 1
and 200 to 1.
HIGH EXPANSION FOAM: Expansion ratio above 200 to 1.
DILUTION RATE, MIXING RATE, OR PROPORTIONING RATE (Correct
amount of foam concentrate to be mixed with water): The amount is normally shown
on the pail or drum of concentrate. If the container of foam concentrate
has 3% shown, it means that for every 100 gallons of foam solution required, 3
gallons of the foam concentrate must be used in the solution with the balance
being 97 gallons of water.
SEAWATER COMPATIBLE: Some foam concentrates can be used
successfully with either sea, fresh or brackish water.
The types of foam used by our customers can be basically
broken into two categories, Class A and Class B. These are the same
classes as used in identifying fire extinguishers.
- Class A foam is used on common combustibles
such as
paper, wood, and textiles.
- Class B foam is used on flammable liquids.
Class "A" Foam
This is a biodegradable mixture of foaming and wetting agents.
When mixed in correct proportions with water, it can change two properties of
the water. Class "A" foam will increase wetting effectiveness, which
allows for greater penetration into Class "A" fuels. It also gives water a
foaming ability, which allows water to remain and cling to vertical and
horizontal surfaces without run off. This allows water to absorb more
heat. By adding a small quantity of a Class "A" foam concentrate into a
water stream, the effectiveness of the water can be increased up to 5 times.
Class "A" foam concentrates are generally mixed with water in
ratios from 0.1% to 1.0%. They can be added to the fire stream by
premixing in the tank, mechanical eductors, or electronic injection systems.
As a rule, the electronic injection systems will be the most accurate means of
mixing the concentrate with water but mechanical methods are most commonly used.
It is often said that adding a little dish soap is the same
thing as using Class "A" foam. That simply is not true. Yes, it will
make the water foam somewhat but that is all. Foam concentrates are much
more complex than simple dish soap. The results will look like fire
fighting foam but will not perform like foam.
Class "B" Foam
This is the foam that is designed to apply to flammable liquids.
There are many types of Class "B" foam. Each type has been designed for a
specific use. Most of these types are highly specialized and are used in
fixed installations where a known hazard will be present. Different
liquids will affect the foam blanket in different ways. The same foam
used to extinguish one flammable liquid might be totally ineffective against
another due to chemical composition. Since our customers do not have the
luxury of a fixed installation that deals with known hazards, many of the
specialty foams simply would not be appropriate for their use.
Fire departments generally equip themselves with a type of
foam that will handle multiple flammable liquid types that are commonly
encountered. Generally that would be AR-AFFF (Alcohol Resistant, Aqueous
Film Forming Foam.) AR-AFFF is a multi-use, multi-purpose foam
concentrate. This type of foam can be used in low concentrations (3%) to fight a
common gasoline or diesel fire at an accident scene. At higher percentages
(6%) the same foam can be used to fight fires that contain polar solvents such
as alcohol. (Polar solvents are compounds that readily mix with water
which will cause a foam blanket to dissolve rapidly.)
FOAM TYPES
We stock Class "A" and AR-AFFF Class "B" foam but, there are
many more out there you will probably hear about. If you have special
needs for any of these foams we can readily get them for you. Following
are the general foam types and their descriptions. The wording is straight
from Chemguard where we buy our foam so it is technical in nature but we are
supplying it for reference purposes.
Aqueous Film Forming Concentrate (AFFF)
Available in either a 1%, 3%, or 6% type concentrate. These
concentrates are manufactured from synthetic type materials such as: Synthetic
foaming agents (hydrocarbon surfactants); Solvents (i.e., viscosity leveler,
freezing point depressant, foam booster); Fluoro Chemical surfactants; Small
amounts of salts; Foam stabilizers (slow drainage, increases fire resistance).
AFFF generated foams extinguish hydrocarbon flammable liquid
fires the same way as the protein or fluoroprotein foams; however, there is an
additional feature. An aqueous film is formed on the surface of the
flammable liquid by the foam solution as it drains from the foam blanket.
This film is very fluid and floats on the surface of most hydrocarbon fuels.
This gives the AFFF unequaled speed in fire control and knockdown when used on a
typical hydrocarbon spill fire. In certain circumstances, it is
possible to notice the fire being extinguished by the "invisible" film before
there is complete foam blanket coverage over the surface of the fuel.
AFFF foam solutions can be applied to a flammable liquid fire
using either aspirating or non-aspirating discharge devices. The
difference between the two is that the air-aspirating device entrains air and
causes it to mix with the foam solution within the device. The non
air-aspirating device is incapable of this process.
- The AFFF/Water solution requires relatively low energy
input to expand the foam solution into an expanded foam mass.
- AFFF foam solutions are unique in that in addition to
forming an expanded foam mass, the liquid that drains from the blanket has a
low surface tension, which gives it the ability to form the aqueous film that
floats on the fuel surface.
When flow rates and pressures are similar, AFFF solutions used
with a non air-aspirating discharge device will generally discharge/throw the
foam a greater distance than the foam that is discharged from the air-aspirating
discharge device. A non-aspirating AFFF will generally extinguish a low
vapor pressure fuel spill fire slightly faster than the foam discharged from an
air-aspirating device. This is because the non-aspirated nozzle generated
foam has a lower expansion and will be more fluid; therefore, it will move
faster across the fuel surface.
When using AFFF foams, application technique is not as
critical as with Proteins or Fluoroproteins. AFFF foam can also be used
successfully with the sub-surface injection method. NOTE: The
sub-surface method of discharging foam into a storage tank can only be used with
tanks that contain standard hydrocarbon fuels, NOT polar solvent/alcohol type
fuels.
The recommended application rate for AFFF 3%-6% generated foam
solution on a hydrocarbon spill fire with low water solubility is .10 GPM/Sq.
Ft. Remember the protein and the fluoroprotein foam solutions require an
application rate of .16 GPM/Sq. Ft.
AFFF is suitable for use in a premix state and is suitable for
use with dry chemical extinguishing agents. (In major tank farm fires it is
common to use dry chemical and foam simultaneously for extinguishment.)
Alcohol Resistant-Aqueous Film Forming Foam (AR-AFFF)
AR-AFFF's are available in a 3%/6% type or 3%/3% type concentrate.
Flammable liquids that readily mix with water are a more difficult fire to
extinguish as opposed to a hydrocarbon fire. Polar solvent/alcohol liquids
destroy any foam blanket that has been generated using standard AFFF or
fluoroprotein type concentrates. Water in the generated foam blanket mixes
with alcohol causing the foam blanket to collapse and disappear until the fuel
surface is completely exposed again. To overcome this problem, AR-AFFF
type concentrates were developed. Using plain AFFF concentrate as a base
material, a high molecular weight polymer is added during the manufacturing
process. When AR-AFFF is used on a polar solvent fuel fire, the polar
solvent fuel tries to absorb water from the foam blanket. A polymer
precipitates out forming a physical membrane/barrier between the fuel surface
and foam blanket. This barrier now protects the generated foam blanket
from destruction by the alcohol fuel.
AR-AFFF concentrates are very viscous. Initial
impression of this type of foam concentrate my lead one to believe that the
concentrate has "gelled" and somehow gone bad. However, a thick, gel-like
appearance is normal. This appearance is caused by the presence of
polymers, which are the main components required for polar solvent applications.
Modern AR-AFFF concentrates are designed to work through proportioning equipment
such as in-line eductors, bladder tanks, and balanced pressure pump systems.
AR-AFFF 3%/6% type of concentrate is designed to be used at
the 3% application rate when used on a standard hydrocarbon fuel fire and 6%
when used on a polar solvent/alcohol fuel. Current 3% AR-AFFF type
concentrate is designed for 3% application on either type group, i.e. 3% on
hydrocarbons and 3% on polar solvent fuels.
When AR-AFFF is used at the correct proportioning rate on
hydrocarbon fuel, fire fighting performance and application rate are the same as
for standard AFFF agents. An "invisible" film is formed, the speed of
covering a fuel spill with the foam blanket is similar and the application
technique using either air-aspirating or non air-aspirating nozzles can be used.
When used on an alcohol fire, an air-aspirating nozzle will give a better
performance over the non air-aspirating nozzle. The increased expanded
foam mass generated by the air-aspirating nozzle will give a more gentle
application onto the surface of the alcohol liquid fire than will the
non-aspirating nozzle. The intensity of the fire, distance the foam must
be thrown, and the application rate also play an important part in determining
the type of nozzle and method of extinguishment. The application technique
and performance factors are the same for both the 3% and the 3%-6% types of
AR-AFFF concentrates.
Synthetic/Detergent (High Expansion) Foam Concentrate
Normally used at a concentrate rate between 1.5% to 2.5%, this type of foam
concentrate is manufactured from a combination of hydrocarbon surfactants and
solvents. High expansion foam solution is normally used through devices
that give high expansion ratios such as the medium or high expansion type foam
generators.
In areas such as a basement, mine shaft or a ship's hold where
volume fire control is required, a high expansion foam generator can be used to
fill an entire room with large amounts of very light expanded foam bubbles.
Depending on the generator being used, high expansion ratios of 400 to 1 up to
1,000 to 1 can be achieved.
Fire control and extinction is achieved by rapid smothering
and cooling. Fires involving solid material as well as flammable liquids
can be controlled and extinguished using high expansion foam. It also has
a special value for dealing with spillages of liquefied natural gas (LNG).
A deep layer of 500 to 1 expanded foam will provide a thermal insulation barrier
around the LNG spill, which reduces the heat intake, and therefore the rate of
evaporation is decreased. Because of the high expansion ratios being
achieved, there is very little water used; even with large discharges of the
high expansion foam. High expansion foam has little water content within
the bubble wall making it very light and not suitable for outdoor use.
Medium expansion foam normally has an expansion of around 50-60 to 1. This
foam is denser and can be used outdoors but is still affected by weather
conditions.
Class "A" Foam Concentrate
This is a biodegradable mixture of foaming and wetting agents. When
mixed in correct proportions with water, it can change two properties of the
water. Class "A" foam will increase wetting effectiveness, which allows
for greater penetration into Class "A" fuels. It also gives water a
foaming ability, which allows water to remain and cling to vertical and
horizontal surfaces without run off. This allows water to absorb more
heat. By adding a small quantity of a Class "A" foam concentrate into a
water stream, the effectiveness of the water can be increased up to 5 times.
Wetting Agent
This type of agent is very similar to Class "A" foam with regard to
increasing wetting effectiveness of the water but does not have the foaming
abilities.
Fluoroprotein Foam Concentrate
Available in either a 3% or 6% type of concentrate. This product is
manufactured using the same method as Protein but with the addition of
fluorocarbon surfactants. The addition of these surfactants in the
concentrate improves the performance of fluoroprotein foam over protein foam in
two areas.
It makes the fluoroprotein foam more resistant to fuel
contamination/pickup and makes the foam blanket more mobile when discharged onto
the flammable liquid. Because the fluoroprotein foam is more resistant to
fuel contamination, it allows the discharging foam to be applied directly to the
fuel surface and the foam blanket will not become as saturated by fuel vapor.
This type of foam can be used with a high back pressure foam maker by utilizing
the sub-surface method of forcing expanded foam into the base of a cone roof
storage tank containing a hydrocarbon fuel. The expanded foam enters the
base of the storage tank then floats up through the flammable liquid to the
surface where it covers the surface with a foam blanket. Fluoroprotein
foam is sometimes used in the hydrocarbon processing industry for storage tank
fire fighting. It is necessary to use with air-aspirating discharge
devices. The recommended foam solution application rate on hydrocarbon
spills is .16 gpm/sq.ft.
Film Forming Fluoro-Protein (FFFP)
FFFP is a derivative of AFFF and fluoroprotein. These concentrates are
based on fluoroprotein formulations to which an increased quantity of
fluorocarbon surfactants has been added. FFFP concentrates were developed
to obtain the quick knockdown of AFFF with the added burn back resistance of
standard fluoroprotein foam. It appears that the FFFP concentrate performance
factor lies somewhere between AFFF and fluoroprotein. FFFP concentrates do
not have the quick knockdown of the AFFF's when used on a spill fire such as an
aircraft crash or a highway spill. When used on fuel in depth fires they
do not have the burn back resistance of fluoroprotein. FFFP foam can be
generated with either air-aspirating or non air-aspirating nozzles. When
used through a non air-aspirated nozzle they do not provide expansion ratios as
good as AFFF when used through the same type of nozzle. The application
rate is .10 gpm/sq. ft. when used on a hydrocarbon spill fire.
Protein Foam Concentrate
Available in either a 3% or 6% type concentrate. This type of
concentrate is based on hydrolyzed protein, foam stabilizers and preservatives.
It will produce highly stabilized air foam. Protein foam must always be
used with an air aspirating type discharge device. Protein foam can become
contaminated with fuel if plunged directly on to the fuel surface; therefore,
the application technique for Protein foam is quite critical. The foam
should be applied as gently as possible to the flammable liquid surface.
The application rate for Protein foam solution on a
hydrocarbon spill fire having low water solubility is .16 gpm/sq.ft.
Protein foam, because of its stability, is relatively slow moving when used to
cover the surface of a flammable liquid.
Shelf Life
Shelf life is the term used to describe the length of time which foam
concentrates remain stable and usable without a significant change in their
performance characteristics. The shelf life depends upon the composition
of the concentrate, the ambient temperature storage range encountered, the
container materials and if the concentrate is stored in its original container.
A shelf life of 20-25 years is possible if the standard AFFF, AR-AFFF and other
synthetic type agents are stored within the manufacturer's guidelines,
temperature limits and in the original shipping container. Protein based
foam concentrates are not totally synthetic and have a naturally occurring
product in their formulations. If these products are stored within the
manufacturer's guidelines including temperature limits. and in their original
shipping container, a shelf life of 10 years or more can be expected.
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