Upload
lukman
View
32
Download
0
Tags:
Embed Size (px)
DESCRIPTION
Understanding Class B Fire-Fighting Foam and Application
Citation preview
1
Understanding Class B Fire-Fighting Foam and Application
Rita Silbernagel
In an emergency flammable liquid fire situation, reliance on safety personnel,
local police and fire departments, and foam performance are required for a successful fire
knockdown. Training is commonplace for safety officials but foam testing is overlooked
in many fire departments across the country. As specified by NFPA 11: Standard for
Low-, Medium-, and High Expansion Foam, fire-fighting foam should be tested at least
annually by the foam manufacturer or an independent laboratory. Poor performance of
fire-fighting foam may be caused by water dilution, tank corrosion or failure to follow the
manufacturer’s tank and sample requirements. For these reasons, it is recommended that
safety personnel fully understand the basics of fire-fighting foam and that foam testing
should be conducted at least annually, to ensure the best performance when it is needed
most.
Definition and Varieties of Class B Foams
Class B fires involve flammable or combustible liquids, flammable gases, greases
and similar materials, some rubber and plastic materials.1 Some common flammable
liquids are acetone, benzene, ethanol, and gasoline. Fire-fighting foam extinguishes
flammable liquid fires in a number of ways: foam smothers the fire by preventing air
from mixing with flammable vapors; foam suppresses flammable vapors from being
released; foam separates the flames from the fuel surface; and foam cools the fuel and
product surface.2
There are several different varieties of fire-fighting foam including low-
expansion: aqueous film-forming foam (AFFF); protein-based foams, fluoroprotein
foams, film-forming fluoroprotein foam (FFFP), alcohol-resistant foam concentrate; and
high expansion foams. See Table 1 for a detailed list of foam types.
Table 1 – Periodic Testing Requirements for Fire-Fighting Foam
1 OSHA Standard for Fire Protection #1910.155 Class B: 1910.155(c)(9) http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9809 2 Woodworth, Steven P., Frank, John A. Fighting Fire with Foam. Van Nostrand Reinhold Publishing 1994. Page 18.
2
Low Expansion Foam Types
Foam Type Typical Concentrations* Synthetic or
Protein-based?
Aqueous Film Forming Foam (AFFF) 1%, 3%, 6% Synthetic
Alcohol-Resistant Aqueous Film Forming Foam (AR-AFFF) 1x3%, 1x6%, 3x3%, 3x6% Synthetic
Military Specification Aqueous Film Forming Foam (AFFF-MS) 1%, 3%, 6% Synthetic
Fluoroprotein Foam 3%, 6% Protein-based
Standard Protein-based Foam 3%, 6% Protein-based
Film Forming Fluoroprotein Foam (FFFP) 3x3%, 3x6% Protein-based
Alcohol-Resistant Film Forming Fluoroprotein Foam (AR-FFFP) 3x3%, 3x6% Protein-based
High Expansion Foam Types
High Expansion Foam (Hi-Ex) 1%, 2%, 2.2%, 2.75%, 3% Synthetic
* - Due to space restrictions only the most common concentrations are listed.
.
An AFFF is defined by NFPA 11 as a concentrate based on fluorinated surfactants
plus foam stabilizers to produce a fluid aqueous film for suppressing hydrocarbon fuel
vapors.3 The requirement of an AFFF is that when applied to a hydrocarbon liquid, such
as gasoline, the sample will form an aqueous film over the liquid, thereby, blocking the
release of hydrocarbon vapors. Photo 1 demonstrates how film-forming foam spreads
over a hydrocarbon liquid.
Photo 1: How AFFF/FFFP and AR-(AFFF/FFFP) apply a film over a hydrocarbon liquid.
AFFF is considered to be a low-expansion foam type. Typical AFFF foam is
diluted with water for standard proportioning at 1 percent, 3 percent or 6 percent. AFFF
foams are synthetic, meaning that they do not come from organic sources and were first
3 Ibid. NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam 2010 Edition. Page 11-8. Portion: 3.3.12.2.
3
developed in the 1960s by the military in association with industry. Standard AFFF is
usually a thin liquid that is comprised of 65-90% water, foamer surfactants and detergents
that cause foaming 5-30%, and fluorochemical surfactants that cause film formation 0.5-
3%. These ranges exist because each foam manufacturer has a different formulation.
Typical shelf life of AFFF foams are at least 20 years. Standard AFFF are only effective
on hydrocarbon liquid fires and are not effective on polar solvents, otherwise known as
water-soluble solvents such as ethanol, methanol, isopropanol and acetone.
Synthetic foams, like AFFF have been around for some time but the original
foams used before 1960s were protein-based. Protein-based foams have been in use since
the 1930s and were proportioned at 6% or higher. The first 3% protein foam was
developed in 1947.4 Protein foam concentrate is described as a concentrate consisting
primarily of products from a protein hydrolysate (a product of hydrolysis, which is a
chemical process of decomposition or alteration by water), plus stabilizing additives and
inhibitors… to otherwise ensure readiness for use under emergency conditions.5
Standard protein foams usually contain 25-60% water, 35-50% protein materials, 10-30%
stabilizers, and 1-5% salts. Protein foam is available for proportioning at 3% or 6% and
is only valid on hydrocarbon liquids. The lifetime of protein foams are can be anywhere
from 5-15 years. Protein-based foams usually appear as dark brown, thin liquids and have
a characteristic smell.
Another subset of protein-based foam is fluoroprotein foam. Fluoroprotein foam
is protein-based but also contains fluorinated surfactants. The only major difference in
the composition of basic protein-based and fluoroprotein foams would be an additional
0.5-2% of a fluorochemical surfactant. Even with the fluorochemical, fluoroprotein
foams do not form a film over a hydrocarbon liquid like an AFFF. Fluoroprotein foams
typically have a longer shelf life than standard protein-based foams and may have a shelf
life of 10-25 years, if stored under the proper conditions. Fluoroprotein foams are
proportioned at 3% or 6% and are typically valid on only hydrocarbon liquids and
possibly some weak polar solvents. Check with the foam product manufacturer for a
detailed list. Fluoroprotein foams were developed in 1965.6
4 Ibid. Fire Suppression and Detection Systems. Pages 20-21. 5 Ibid. NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam 2010 Edition. Page 11-8. Portion: 3.3.12.7. 6 Hughes Associates. Table 2-4: FAA Burn-back Resistance Tests. 1990.
4
A film forming fluoroprotein (FFFP) is essentially a fluoroprotein foam with an
elevated level of fluorochemical, enabling the product to form a film on hydrocarbon
fuels in a similar manner to AFFFs. FFFP was developed in the early 1980s and is
available in 3% and 6% concentrations. FFFP foams are known for rapid knockdown and
good burn-back resistance. Shelf life may extend from 5-15 years, depending on storage
conditions. FFFP foams, like AFFF are only usable on hydrocarbon liquid fires. For
extinguishing alcohols or other polar solvents, an alcohol-resistant foam concentrate must
be used.
Alcohol-resistant foams are described as a concentrate used for fighting fires on
water-soluble materials and other fuels destructive to regular AFFF, Fluoroprotein or
FFFP foams.7 Alcohol type concentrates are valid for polar solvents. Examples of
flammable liquids that require alcohol-resistant foam are ethanol, methanol, acetone, and
methyl tertiary butyl ether (MTBE). The alcohol-resistant foams are usually thicker than
standard foam because the alcohol-resistance is enabled by a polymeric compound within
the foam. Photo 2 shows some appearance differences between alcohol-resistant, standard
AFFF, and protein-based foams. See Photo 3 for the polymeric appearance of AR-AFFF
foam.
Photo 2: (Left to right) 1 & 2 alcohol-resistant foam, 3 standard aqueous film-forming
foam (AFFF), and 4 protein-based foam.
7 Ibid. NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam 2010 Edition. Page 11-8. Portion: 3.3.12.1.
5
Photo 3: A zoomed in view on the apparent thickness of alcohol-resistant foam.
The polymer membrane works by acting as a separator from the polar solvent liquid and
the foam blanket and forms when in contact with polar solvents.8 Most alcohol-resistant
foams produced today are effective on both polar and non-polar fuels but may require a
higher concentration when used on polar solvents. For example, common alcohol-
resistant foams may be used at 3% on non-polar or hydrocarbon fuels, but must be used
at 6% on polar solvent fuels.
All of the previously discussed foam types have been classified as low expansion
foams, where the foam to solution (expansion) ratio is anywhere from 1:1 to 20:1. High
expansion foams have an expansion ratio of 200:1+.9 High expansion foams are generally
used in closed locations and typically fill the entire enclosed space, causing a fire
knockdown. High expansion (Hi-Ex) foams are typically used in airplane hangars and
engine rooms. Low and medium expansion foams are typically used for specific fire
locations and do not expand as readily as high-expansion foams. The large amount of
foam generated is highlighted in Photo 4 with a high-expansion foam generator.
8 Ibid. Fire Suppression and Detection Systems Page 33. 9 Ibid. NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam 2010 Edition. Page 11-8. Portion: 3.3.12.6.
6
Photo 4: A high-expansion foam generator as referenced in NFPA 11.
Hi-Ex foams are synthetic foams that are frequently proportioned anywhere from 1-3%,
depending on the manufacturer. They appear as thin liquids. High expansion foams
consist of 35-60% water, 15-35% synthetic detergents, 15-30% foam stabilizers, and 0.5-
1% corrosion inhibitors. High expansion foams extinguish fires by: reducing the flow of
air into the area; liquid in foam will produce steam to dilute the oxygen concentration; it
will cool the fuel surface areas; and it acts as an insulating barrier for exposure
protection.10 High-expansion foam has also been shown to be effective in fires where
extinguishment is necessary and minimal runoff is desirable.11 Shelf life for high
expansion foams can be anywhere from 10-20 years, when stored under proper
conditions.
10 Ibid. Fire Suppression and Detection Systems Pages 36-41. 11 Carlson, Gene. Hazardous Materials. American Fire Journal. 41 (January). Page 12.
7
Premixed Foam Solutions
Foam concentrates are sold in their raw, undiluted or concentrated state, meaning
that the user must dilute them with water to the proper proportioning rate as listed by the
manufacturer. A premix is described as a water solution mixture that is stored for later
usage. For example, a 3% AFFF for a premixed solution would have 3 gallons of AFFF
concentrate mixed with 97 gallons of water for a combined total of 100 gallons of
premix. Storage of premixes is generally not recommended for longer than two years,
due to bacterial accumulation and breakdown of the solution. Premixes are not
recommended for protein-based foams.
Often times foam is applied to a fire through an automatic system that proportions
the foam concentrate into a water stream and then applies it to the fire hazard through
sprinklers, foam chambers, nozzles or other application device. The proportioning
concentration of these automatic systems should be verified prior to putting a system into
service and periodically thereafter. The proportioning can be verified by a refractometer
or conductivity meter (as seen in photo 5) to ensure that the system is properly mixing the
foam concentrate and water.
8
Photo 5 – an example of a handheld conductivity meter.
NFPA 11 requires that the proportioning be no more than 30 percent above the
rated concentrate, or 1 percentage point above the rated concentration (which ever is
less).12 This means that foam specified as 3% must have a concentration between 3.0-
3.9%, 1% should be 1.0-1.3% and 6% should be 6.0-7.0% to be within the acceptable
limits. Lower levels are set to ensure the foam concentrate will extinguish the fire
effectively and upper level limits are set to ensure the foam concentrate supply will meet
the minimum application time requirements set by NFPA. An independent laboratory or
12 Ibid. NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam 2010 Edition. Page 11-8. Portion: 11.6.4 (2).
9
onsite foam technician can verify the concentration as long as the foam concentrate,
premixed solution, and water used for mixing, are provided.
Foam Compatibility
It should be noted that synthetic and protein samples, and even synthetic high
expansion, AFFF and AR-AFFF (alcohol-resistant) samples should never be mixed.
Ideally, it is best not to mix foams from different manufacturers as well. Only military
specification foams – specifically indicated by the foam manufacturer are allowed to be
mixed (but only based on their composition, example 3% Milspec AFFF with another 3%
Milspec AFFF).
Foam Lifetime and Breakdown
Overtime, fire-fighting foam tends to break down with age. Protein-based fire-
fighting foam has a lifetime anywhere from 5-25 years, where as, synthetic foams should
last 10-25 years. A few leading causes of foam degradation is water dilution or rust
corrosion and breakdown of the foam tank. Water dilution can occur if the foam isolation
valve is left open, the bladder tank lining is torn, or if improper tank operations that
deviate from the manufacturer’s specification are undertaken. Rust corrosion is typically
caused from the dilapidation of the tank. Rust and particulates tend to lower the
expansion of the foam, resulting in poor performance. The best way to avoid corrosion is
to store the foam in accordance with the foam manufacturer recommendations.
Generally, it is best to leave fire-fighting foam in a constant temperature-controlled room.
Leaving foam tanks exposed to harsh sunlight, and extreme hot and cold temperatures
can degrade the foam more quickly. Some foam manufacturers offer freeze-protected
foam, which is recommended for outdoor storage, especially in colder climate areas. Due
to dilution, corrosion effects, and environmental concerns, it is best to test fire-fighting
foam on an annual basis.
Understanding Fire-Fighting Foam Testing
Table 2 – Periodic Testing Requirements for Fire-Fighting Foam
Standard National Fire Protection Association
Standard on Foam NFPA 11
United States Coast Guard 49
CFR
International Maritime
Organization
Frequency Annually Annually Three Years after Installation and
10
Annually Thereafter Tests Required Quality Condition
Testing “specific gravity, pH, percentage of
water dilution, solid content and
certification as a suitable firefighting
foam”
Sedimentation - <0.25%
pH Value – 6.0 – 9.5
Volumetric mass, expansion, 25%
drain time – within acceptable range of
type approval. Water Not Specified Not Specified Sea Water
Table 2 is a list of some common specifications that may be required for
evaluating the effectiveness of fire-fighting foam that is stored in the field. Typically, the
overall evaluation is a combination of physical property tests run on the concentrate in
addition to performance tests run on the foam mixed with water at its nominal
concentration. Each test for fire-fighting foam is run to predict the performance in a fire-
fighting situation.
The physical properties of a foam concentrate are specific to the particular brand
and model of the foam and are typically set by the manufacturer. For instance, when the
refractive index or density of a foam sample is below the specification, as specified by
the foam manufacturer, it is an indication that the foam has been inadvertently diluted by
water. The pH of a foam concentrate, unless otherwise specified, is considered
acceptable if the reading is between 6.0 and 9.5. When the pH is outside of this range, it
indicates that corrosion is either occurring or the foam itself might be breaking down.
Even the appearance can provide information on the likelihood of foam to work in a fire
situation. A rusty color can indicate that corrosion is taking place within the foam tank;
while large particles in a concentrate can cause concern about the ability of the foam to
flow through a proportioning device. The viscosity of the foam is an additional physical
property that is tested to ensure that the concentrate is not too viscous to proportion
properly. The viscosity is particularly important for alcohol-resistant foam concentrates.
Perhaps the most critical tests are the performance tests measured after the foam
concentrate is mixed with water. The following is a summary of laboratory scale
performance properties that can be measured on fire-fighting foam to ensure
effectiveness.
The expansion is determined by expelling a foam sample through a nozzle and
into a container and measuring the weight ratio of the foam sample to the same volume of
11
water. For example, a foam sample with an expansion of 6 to 1 means the same volume
of water will weigh six times that of the foam. A minimum expansion value ensures the
foam will sit up on top of the flammable liquid during a fire application.
The drain time is measured by how long it takes 25% or 50% of the weight of the
foam to drain from a foam blanket back into a foam solution. A minimum drain time
ensures the foam will remain foamy long enough to secure and extinguish a flammable
liquid fire.
For film-forming foams such as AFFF, FFFP, AR-AFFF, and AR-FFFP samples,
it is important to demonstrate that the foam will indeed form a film in a fire scenario.
Film formation can be measured by applying the foam over a hydrocarbon liquid (or
polar solvent in the case of an alcohol-resistant foam) and the film can be visually
observed. In addition the surface can be exposed to a flame and the hydrocarbon liquid
should not ignite if the aqueous film has completely covered the surface of hydrocarbon
liquid.
Film formation can also be determined by measuring the surface tension of the
foam solution; the surface tension of the flammable liquid; and the interfacial tension of
the foam and flammable liquid interface. In order for a solution to form a film on a
flammable liquid, the spreading coefficient must be greater than zero:
Spreading Coefficient = Surface Tension of the Flammable Liquid – Surface
Tension of the Foam Solution – Interfacial Tension.
A fire test is an additional method for determining the viability of a foam
concentrate. It is imperative however that the fire test represent the real life fire scenario.
The larger the fire test the more likely it is be representative of an emergency situation
however, it is often cost prohibitive to run a large scale fire test.
Sending in a Sample for Foam Testing
When taking a sample for foam testing, if there is reason to believe that the
sample is diluted with water, it is recommended that a top and bottom sample are
collected. Because water is less dense than foam concentrate, the water will typically sit
on the top of the foam. If the top sample appears diluted with water, it is recommended
that a portion of the top sample is drained off to remove the water. If a sample collected
12
from the bottom does not show dilution, then this foam may be saved. If water dilution is
present in both the top and bottom samples, then the foam has been completely diluted
with water and replacement is recommended.
Another possible issue with taking a sample lies with mineral oil. Some
manufacturers apply mineral oil to the top of the sample to prevent evaporation of water
and solvent from the foam concentrate. Mineral oil itself appears clear and behaves
similarly to vegetable oil in its consistency. Mineral oil is less dense than water and foam,
therefore, the oil is likely to be on top of the sample. If mineral oil is present, it is
important to collect a sample from the bottom or middle of the tank to ensure a truly
representative sample is tested. Mineral oil may be removed by centrifugation but in
some cases, the mineral oil may be entrained within the sample and could reduce the
performance properties. Photo 6 shows how mineral oil sits on top of the foam
concentrate.
13
Photo 6: Mineral oil on top of an Alcohol-Resistant-AFFF sample.
Installing Fire-Fighting Foam in a Tank
When replacing fire-fighting foam, be sure to clean and dry the tank thoroughly
beforehand. If any corrosion or degradation is seen, the tank should be replaced. This
ensures that the previous sample and/or slight tank corrosion will not affect the new
foam. In addition, make sure that the foam isolation valve is kept closed; otherwise, water
dilution will occur within the foam tank. A water-resistant label indicating the foam type,
manufacturer, lot number, and concentration should be applied to the tank so that others
can immediately identify the foam type and nominal concentration in an emergency
situation.
Reflection
14
After discussing the various types of fire-fighting foam in use today, it should be
apparent that a deeper understanding of foam can be helpful in an emergency situation.
In order to ensure that the foam concentrate or premix will work properly, it is best to test
fire-fighting foam at least annually to ensure reliance on the foam knockdown properties.
The training of personnel and testing of foam offer the best chances for an effective
emergency fire outcome.
Rita Silbernagel is the Senior Analytical Chemist of Dyne Technologies LLC, an independent compliance testing laboratory. See www.dyneusa.com for more information.