Firefighting foam

Firefighting foam is a mass of air-filled bubbles made from water and a foam concentrate. It is applied to a burning surface or used to fill a space. On a suitable liquid pool, the foam forms a floating blanket that limits vapour release, reduces heat feedback and separates the fuel from air.

Foam is not simply soapy water or one standard chemical product. Different formulations have different flow, drainage, fuel resistance and film-forming behaviour. Concentrate must be mixed in the intended proportion, aerated and delivered in a way that creates and preserves the required blanket.

The principal use discussed in the sources is control and extinction of flammable-liquid pool fires. Foam can also be used for particular ordinary-combustible and space-filling applications. Its suitability is limited by fuel chemistry, surface shape, heat, water incompatibility and destruction of the blanket.

Formation and physical form

Producing foam starts with proportioning concentrate into water. Scheffey describes typical proportions of 1, 3 or 6 percent concentrate for the applications in his chapter, not as universal values for every product. The solution is then aerated to create bubbles.

Foam production ends with a blanket over the fuel surface. Based on Scheffey (2002), p. 4-88.

Expansion describes the increase from solution volume to foam volume. Low-expansion foams are relatively fluid and can spread across a liquid surface. Medium- and high-expansion foams can provide large bubble volumes for filling spaces, but these forms create different application and occupancy concerns.

A useful blanket combines expansion, cohesion, stability and fluidity. Bubbles must adhere sufficiently to form a layer, while the foam must still flow around obstructions. It must also resist fuel pickup and thermal damage so that the blanket does not itself become contaminated and burn.

Drainage is the loss of liquid from the bubble structure. NUREG-1805 describes a 25 percent drainage measure as the time for one quarter of the liquid content to drain under standard conditions. The cited typical range of 2 to 5 minutes is a foam property under that test, not an extinction time.

Film formation is distinct from bubble coverage. Some solutions draining from foam spread as a thin aqueous film over hydrocarbon fuel. That film can help seal vapour, but it does not occur for every formulation or fuel.

Expansion and a worked flow example

Expansion ratio compares the volume of finished foam with the liquid solution used to make it. Both volumes refer to the same sample or to matching flow intervals. The ratio describes added air volume, rather than an increase in the amount of water or concentrate.

$$E=\frac{V_f}{V_s},{\dot{V}}_f=E{\dot{V}}_s$$

Symbol	Description	Unit
$E$	Foam expansion ratio	1
$V_f,V_s$	Finished foam and solution volumes	m³
${\dot{V}}_f,{\dot{V}}_s$	Finished foam and solution volume flow rates	L/min

The example in NUREG-1805 uses an expansion ratio of 8:1. Its solution application rate converts to approximately 4.07 L/(m² min). Multiplying by eight gives approximately 32.6 L/(m² min) of expanded foam, before accounting for losses between generation and useful coverage.

$$8\times 4.07\approx 32.6\mathrm{L}/\mathrm{(}{\mathrm{m}}^{\mathrm{2}}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{)}$$

This is a reproduction of the source's volume calculation, not a prescribed application rate for another fuel. The finished foam contains the same liquid quantity as the original solution, distributed through a larger bubble structure. Heat, drainage and mechanical damage then change the volume and condition that remain over the fuel.

Expansion and drainage are therefore reported together. Scheffey describes an early aqueous film-forming foam (AFFF) formulation with expansion of 8:1 and a 25% drainage time of 6 min. That historical result differs from the 2–5 min discussion in NUREG-1805 because the passages describe different foam evidence, rather than one universal pass criterion.

Extinguishing action

The Extinguishing mechanisms of foam include blanketing, cooling and reduced vaporisation. A complete surface layer restricts air contact and interrupts the transfer of radiant heat that supports fuel evaporation. Less vapour then reaches the combustion zone.

Water draining from the foam provides additional cooling. If the liquid is water soluble, that water can also dilute the fuel and reduce its vapour pressure. Deep pools can require substantial dilution, so formulation and application remain important even for soluble fuels.

Foam is lighter than the aqueous solution draining from it and lighter than the hydrocarbon liquids described by Scheffey. It therefore floats and spreads over the fuel. Low-expansion foam is especially associated with two-dimensional Pool fire conditions rather than fuel running from an elevated leak.

The blanket can protect exposed surfaces that are not yet burning. Low surface tension can also help foam enter crevices. These benefits depend on reaching the surface and maintaining coverage under fire and wind conditions.

Visible flame knockdown does not guarantee lasting extinction. If the blanket is disrupted, vapour can again reach an ignition source and burnback can occur. The residual layer and continued control of the fuel surface are therefore central to performance.

Foam types and fuel compatibility

Protein foam uses hydrolysed protein materials and forms a blanket without the aqueous surface film associated with AFFF. Fluoroprotein products add surface-active agents to alter fuel shedding and flow. These are separate formulations, not interchangeable descriptions of one concentrate.

Aqueous film-forming foam (AFFF) contains surface-active compounds that allow drained solution to form a film on suitable hydrocarbon fuels. The foam bubbles and draining film both contribute to vapour suppression. Excessive stiffness and slow drainage can hinder flow and film formation.

Alcohol-resistant concentrates are intended for water-miscible fuels such as the alcohols identified in the source. Scheffey describes a floating polymeric skin that protects the foam from polar-solvent breakdown. An ordinary hydrocarbon-fuel foam should not be assumed to survive on those fuels.

Fire classes provide selection categories, but their definitions depend on the national system. In the US table reproduced in NUREG-1805, foam is listed for ordinary combustibles and flammable-liquid fires. This does not imply that every foam is suitable for every fuel within those broad groups.

Foam remains principally water based. It can share water's incompatibility with reactive substances and energised electrical equipment. The chosen foam must be checked against the actual liquid, equipment and discharge arrangement rather than selected solely by its fire-performance label.

Application and system design

Foam delivery has three stages: proportioning, foam generation and distribution. Air aspiration is a basic generation method. Fixed spray systems, monitors and mobile hose equipment can apply foam, but the final blanket depends on the operation of the complete chain.

Low-expansion foam is commonly associated with storage-tank tops, bunds and spills. Medium- and high-expansion foam can fill spaces such as basements. These applications differ in distribution, agent quantity and hazards to people within the foam-filled space.

Application must compensate for foam lost through heating, vaporisation and physical disruption. NUREG-1805 explains that a residual layer must remain over already extinguished portions while coverage advances. A nominal flow at the nozzle is therefore not the same as effective coverage at the fuel.

A foam blanket needs continued protection against losses. Based on Iqbal and Salley (2004), pp. B-58–B-60.

Tank application can use foam chambers, internal distributors or subsurface injection, depending on tank form. Top application can be hindered by rising gases or damage from an initial explosion. Floating roofs introduce different distribution problems, so one tank arrangement cannot be transferred uncritically to another.

The fixed Extinguishing agent supply must include enough concentrate and water for the intended duration. Large tank fires can require substantial inventories and logistical arrangements. Disposal and containment of the resulting foam solution are part of the operational problem.

Fire geometry and application method

A horizontal, immiscible hydrocarbon pool offers a surface on which foam can spread. Water alone may pass beneath the lighter liquid while the fuel continues to burn. The foam blanket addresses that geometry by remaining above the liquid and limiting vapour production.

Running fuel and pressurised sprays do not provide the same continuous horizontal surface. Scheffey describes these as three-dimensional fires for which low-expansion foam is less effective. The source discusses supplementary agents in such cases, subject to compatibility between the chosen products.

Fuel geometry changes the usefulness of a foam blanket. Based on Scheffey (2002), p. 4-88.

Water-soluble fuels alter the blanket's survival. Their ability to mix with the foam's liquid can destroy an unsuitable formulation, while drainage also changes fuel concentration. Alcohol-resistant products address this specific interaction rather than merely increasing the quantity of generic foam.

Tank-top application must overcome the size and conditions of the fuel surface. Strong upward gas flow can divert light foam, and explosion damage can impair fixed equipment. Distribution should therefore be assessed for the fire condition, not only for a calm commissioning demonstration.

Space-filling foam follows a different objective from pool coverage. Large bubble volumes inundate surfaces and cavities, making distribution through the volume central to the design. This introduces restricted visibility and movement, which are less prominent in a thin surface blanket.

Testing, properties and records

Expansion ratio records how much foam is produced from a given solution volume, but a larger volume does not by itself establish better extinction. Flow, drainage and resistance to the fuel must remain suitable. The properties interact as the blanket spreads and ages.

Drainage influences both cooling and film formation. Liquid leaving the bubbles can spread on a suitable hydrocarbon, while excessive loss can weaken the remaining blanket. Test measurements therefore need the product, expansion condition and measurement method to be meaningful.

Foam shear properties affect its ability to flow and remain coherent. NUREG-1805 gives a torsional-viscometer example for fluidity, illustrating that foam is characterised by physical measurements as well as fire tests. Such a measurement is not a universal field-application rate.

Fire tests should represent the intended fuel's solubility, temperature and geometry. A hydrocarbon pool result cannot establish performance on a polar solvent or a pressurised discharge. The tested application rate and coverage period must also accompany any claim of extinction.

Records should preserve concentrate identity, intended proportion, compatible equipment and relevant water-quality conditions. Substitution in one part of this chain can alter the foam reaching the fuel. Test evidence belongs to the resulting formulation and application, not just to a product family name.

Losses, hazards and limitations

Heat destroys bubbles and evaporates their water. Mechanical impact, incompatible chemical agents and rising combustion gases can also divert or disrupt foam. Simultaneous use of another agent must therefore be based on demonstrated compatibility rather than assumed additive benefit.

Water quality can influence foam properties. NUREG-1805 reports deleterious effects from contaminants such as corrosion inhibitors and anti-fouling agents in a cited study. Product testing with the intended supply and equipment provides more relevant evidence than a generic claim of water compatibility.

Hot fuel creates another limit. Applying water-based foam to liquid above water's boiling temperature can generate steam, expand the foam and eject burning liquid. The source gives particular warnings at temperatures of 100 °C or above and for hot oils or fats.

Medium- and high-expansion foam can obscure visibility and restrict movement. Filling an occupied space with foam also presents an asphyxiation concern in NUREG-1805. Space-filling performance must consequently be assessed together with the safety of anyone present.

Foam performance is largely established through empirical tests. Scheffey states that there was no general international agreement on all fire tests or a complete first-principles explanation of foam extinction in the cited handbook. Product claims must therefore retain their fuel, test and application context.

Environmental acceptability is not established by fire-test performance. Historical fluorinated-agent descriptions explain chemical behaviour but do not provide current regulatory approval. The supplied engineering sources do not support a general statement that every modern foam formulation has equivalent environmental effects.

Protein-based foam generally produces a relatively thick blanket. Film-forming foam can be less viscous and spread more rapidly, allowing its drained solution to contribute to vapour sealing. Those differences affect coverage speed and resistance to disruption rather than representing a simple ranking of all products.

AFFF requires a balance between expansion and drainage. Scheffey describes early tests in which very stiff, highly expanded foam resisted flow and slowed film formation. Lower expansion improved movement and drainage while retaining enough stability to resist burnback in the reported work.

Storage-tank design also depends on roof construction. Fixed-roof tanks can use chambers, distributors or subsurface injection, while a tilted or sunken floating roof can obstruct distribution. These examples show why a foam system's hardware must match the protected geometry.

Low-expansion foam can be injected below the liquid surface in suitable tank systems. This can reduce reliance on foam falling through rising combustion gases. It does not remove the need to check distribution throughout the tank or the condition of injection equipment after an explosion.

Mobile foam equipment adds proportioning and logistics to the fireground problem. Concentrate, water, hoses and generation equipment must remain available for the intended attack. A large inventory at a remote store does not establish sufficient delivered foam at the fuel surface.

The aftermath includes contaminated fuel, drained solution and damaged blanket material. Re-ignition protection depends on the condition of the surface after active application. Assessment must therefore include the persistence of coverage as well as the initial rate of flame reduction.

See also

• Fire suppression system
• Deluge system
• Boilover

References

• Fleming, R. P. (2002). Automatic sprinkler system calculations. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-72–4-87). National Fire Protection Association.
• Iqbal, N., & Salley, M. H. (2004). Fire dynamics tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission fire protection inspection program (NUREG-1805). U.S. Nuclear Regulatory Commission.
• Scheffey, J. L. (2002). Foam agents and AFFF system design considerations. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-88–4-122). National Fire Protection Association.