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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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

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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

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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

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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.

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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

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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.