Aqueous film-forming foam - Revision history

Jan: Remove links to pages that do not exist; retain displayed text (wiki-wide link audit).

2026-11-02T21:32:46Z

Remove links to pages that do not exist; retain displayed text (wiki-wide link audit).

← Older revision		Revision as of 22:32, 2 November 2026
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	AFFF shares the general limits of foam. Low-expansion foams are effective on two-dimensional pool fires, but not on three-dimensional fires, especially those of low-flashpoint fuels.<ref name="Sch-4-88" /> Where running fuel or a pressurised spray is expected, an auxiliary agent such as dry chemical is used together with the foam.<ref name="Sch-4-88" /> Foam is an unstable mixture of air and water that can be broken down by mechanical forces, certain chemicals and some other extinguishing agents.<ref name="IS-B-58" /> Rupture of the foam blanket can lead to burnback, which may put firefighters at risk.<ref name="IS-B-60" />		AFFF shares the general limits of foam. Low-expansion foams are effective on two-dimensional pool fires, but not on three-dimensional fires, especially those of low-flashpoint fuels.<ref name="Sch-4-88" /> Where running fuel or a pressurised spray is expected, an auxiliary agent such as dry chemical is used together with the foam.<ref name="Sch-4-88" /> Foam is an unstable mixture of air and water that can be broken down by mechanical forces, certain chemicals and some other extinguishing agents.<ref name="IS-B-58" /> Rupture of the foam blanket can lead to burnback, which may put firefighters at risk.<ref name="IS-B-60" />

	Hot liquids are a particular hazard. If water penetrates an oil or fat that is well above the boiling point of water, it turns to steam below the surface and can cause an eruption of burning oil, known as [[boilover]].<ref name="IS-B-58" /> Because foam is water-based, its use on a liquid at 100 °C or above can cause a four-fold expansion of the foam, with boilover of the burning liquid.<ref name="IS-B-60" /> Foam should only be used on a horizontal liquid surface below its boiling point, and, like water, it is unsuitable for live electrical equipment or substances that react with water.<ref name="IS-B-60" /><ref name="IS-B-61">Iqbal and Salley (2004), p. B-61.</ref>		Hot liquids are a particular hazard. If water penetrates an oil or fat that is well above the boiling point of water, it turns to steam below the surface and can cause an eruption of burning oil, known as boilover.<ref name="IS-B-58" /> Because foam is water-based, its use on a liquid at 100 °C or above can cause a four-fold expansion of the foam, with boilover of the burning liquid.<ref name="IS-B-60" /> Foam should only be used on a horizontal liquid surface below its boiling point, and, like water, it is unsuitable for live electrical equipment or substances that react with water.<ref name="IS-B-60" /><ref name="IS-B-61">Iqbal and Salley (2004), p. B-61.</ref>

	Engineering knowledge of AFFF is also limited. According to Scheffey, modelling of foam extinguishment cannot yet be performed, because too many parameters, such as foam loss rates and foam viscosity, have not been quantified.<ref name="Sch-4-92">Scheffey (2002), p. 4-92.</ref> Design therefore relies on rates from fire tests with added safety factors.<ref name="Sch-4-100" /><ref name="Sch-4-112" /> Environmental impact, together with reliability and false discharges of foam-water deluge systems, was among the reasons why the US Navy re-evaluated its approach to hangar fire protection.<ref name="Sch-4-110">Scheffey (2002), p. 4-110.</ref>		Engineering knowledge of AFFF is also limited. According to Scheffey, modelling of foam extinguishment cannot yet be performed, because too many parameters, such as foam loss rates and foam viscosity, have not been quantified.<ref name="Sch-4-92">Scheffey (2002), p. 4-92.</ref> Design therefore relies on rates from fire tests with added safety factors.<ref name="Sch-4-100" /><ref name="Sch-4-112" /> Environmental impact, together with reliability and false discharges of foam-water deluge systems, was among the reasons why the US Navy re-evaluated its approach to hangar fire protection.<ref name="Sch-4-110">Scheffey (2002), p. 4-110.</ref>

Lisa: Add Intermediate SVG figures with Fire Safety Academy watermark at planned locations; preserve article text

2026-11-02T17:14:45Z

Add Intermediate SVG figures with Fire Safety Academy watermark at planned locations; preserve article text

← Older revision		Revision as of 18:14, 2 November 2026
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	The SFPE Handbook notes that the mechanisms of foam extinguishment of pool fires have not been developed from first principles.<ref name="Sch-4-89" /> Extinguishment is usually described as the stopping of fuel vaporisation at the fuel surface.<ref name="Sch-4-89" /> As the vapour supply falls, the combustion zone shrinks, and once the whole area is covered the fire goes out.<ref name="Sch-4-89" /> The floating foam forms an air-excluding layer of aqueous agent that suppresses combustion by halting vaporisation.<ref name="Sch-4-88" /> With AFFF, the solution that drains from the foam adds a film that spreads rapidly and is highly stable on the fuel.<ref name="Sch-4-89" /> NUREG-1805 adds that this kind of film re-forms rapidly if it is disturbed.<ref name="IS-B-63">Iqbal and Salley (2004), p. B-63.</ref>		The SFPE Handbook notes that the mechanisms of foam extinguishment of pool fires have not been developed from first principles.<ref name="Sch-4-89" /> Extinguishment is usually described as the stopping of fuel vaporisation at the fuel surface.<ref name="Sch-4-89" /> As the vapour supply falls, the combustion zone shrinks, and once the whole area is covered the fire goes out.<ref name="Sch-4-89" /> The floating foam forms an air-excluding layer of aqueous agent that suppresses combustion by halting vaporisation.<ref name="Sch-4-88" /> With AFFF, the solution that drains from the foam adds a film that spreads rapidly and is highly stable on the fuel.<ref name="Sch-4-89" /> NUREG-1805 adds that this kind of film re-forms rapidly if it is disturbed.<ref name="IS-B-63">Iqbal and Salley (2004), p. B-63.</ref>

			[[File:fse-aqueous-film-forming-foam-v1.svg\|thumb\|upright=1.5\|alt=Cross-section of a fuel pool: left part covered by a foam blanket over a thin aqueous film, right part still burning with flames above.\|AFFF on a pool fire: the foam blanket and the aqueous film draining from it seal the fuel surface, while radiant heat and drainage consume the blanket. Based on Scheffey (2002), pp. 4-88 to 4-91.]]

	The foam blanket is continuously lost while it works. Radiant heat from the fire evaporates water from the foam, and liquid drains out of the bubbles.<ref name="Sch-4-90">Scheffey (2002), p. 4-90.</ref> For radiant fluxes of 45 to 185 kW/m² from large pool fires, a simple heat balance gives evaporation rates of 18 to 72 g/(m²·s).<ref name="Sch-4-90" /> In laboratory tests on one AFFF, heat began to penetrate the blanket when it had thinned to about 50 mm.<ref name="Sch-4-91">Scheffey (2002), p. 4-91.</ref> When the fuel below reignited, almost all the foam had been lost, with less than 0.8 kg/m² remaining.<ref name="Sch-4-91" /> Foam must therefore be applied in enough volume and at a high enough rate to make up for these losses and keep a residual layer over the extinguished area.<ref name="IS-B-59">Iqbal and Salley (2004), p. B-59.</ref>		The foam blanket is continuously lost while it works. Radiant heat from the fire evaporates water from the foam, and liquid drains out of the bubbles.<ref name="Sch-4-90">Scheffey (2002), p. 4-90.</ref> For radiant fluxes of 45 to 185 kW/m² from large pool fires, a simple heat balance gives evaporation rates of 18 to 72 g/(m²·s).<ref name="Sch-4-90" /> In laboratory tests on one AFFF, heat began to penetrate the blanket when it had thinned to about 50 mm.<ref name="Sch-4-91">Scheffey (2002), p. 4-91.</ref> When the fuel below reignited, almost all the foam had been lost, with less than 0.8 kg/m² remaining.<ref name="Sch-4-91" /> Foam must therefore be applied in enough volume and at a high enough rate to make up for these losses and keep a residual layer over the extinguished area.<ref name="IS-B-59">Iqbal and Salley (2004), p. B-59.</ref>
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	== Spreading coefficient ==		== Spreading coefficient ==
	A film-forming foam is defined by the ability of the solution draining from it to spread spontaneously across a hydrocarbon fuel.<ref name="Sch-4-93" /> Whether this happens depends on three tensions: the surface tension of the fuel, the surface tension of the foam solution, and the interfacial tension between the two liquids.<ref name="Sch-4-93" /> The fundamental relation is the spreading coefficient:<ref name="Sch-4-93" />		A film-forming foam is defined by the ability of the solution draining from it to spread spontaneously across a hydrocarbon fuel.<ref name="Sch-4-93" /> Whether this happens depends on three tensions: the surface tension of the fuel, the surface tension of the foam solution, and the interfacial tension between the two liquids.<ref name="Sch-4-93" /> The fundamental relation is the spreading coefficient:<ref name="Sch-4-93" />

			[[File:fse-aqueous-film-forming-foam-v2.svg\|thumb\|upright=1.5\|alt=Edge of a thin solution layer on fuel with three arrows: fuel surface tension pulling outward, solution surface and interfacial tension pulling back.\|The spreading coefficient balances the surface tension of the fuel against the surface tension of the AFFF solution and the interfacial tension; a positive value allows a film to spread. Based on Scheffey (2002), p. 4-93.]]

	<math display="block">S_{a/b} = \gamma_b - \gamma_a - \gamma_i</math>		<math display="block">S_{a/b} = \gamma_b - \gamma_a - \gamma_i</math>
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	As a worked example, take the AFFF solution FC-194 tested by the US Naval Research Laboratory, with a surface tension of 15.5 dynes/cm at 25 °C.<ref name="Sch-4-94" /> On cyclohexane, with a surface tension of 24.2 dynes/cm and an interfacial tension of 4.3 dynes/cm, the spreading coefficient is 24.2 − 15.5 − 4.3 = 4.4 dynes/cm, and a film formed.<ref name="Sch-4-93" /><ref name="Sch-4-94" /> On certified n-heptane, with 19.8 and 5.5 dynes/cm, it is 19.8 − 15.5 − 5.5 = −1.2 dynes/cm, and no film formed.<ref name="Sch-4-93" /><ref name="Sch-4-94" /> On JP-5 jet fuel, the result is +5.2 dynes/cm.<ref name="Sch-4-94" /> Fuels with a low surface tension, such as avgas and n-heptane at 19 to 20 dynes/cm, are therefore the hardest for a film to cover.<ref name="Sch-4-93" />		As a worked example, take the AFFF solution FC-194 tested by the US Naval Research Laboratory, with a surface tension of 15.5 dynes/cm at 25 °C.<ref name="Sch-4-94" /> On cyclohexane, with a surface tension of 24.2 dynes/cm and an interfacial tension of 4.3 dynes/cm, the spreading coefficient is 24.2 − 15.5 − 4.3 = 4.4 dynes/cm, and a film formed.<ref name="Sch-4-93" /><ref name="Sch-4-94" /> On certified n-heptane, with 19.8 and 5.5 dynes/cm, it is 19.8 − 15.5 − 5.5 = −1.2 dynes/cm, and no film formed.<ref name="Sch-4-93" /><ref name="Sch-4-94" /> On JP-5 jet fuel, the result is +5.2 dynes/cm.<ref name="Sch-4-94" /> Fuels with a low surface tension, such as avgas and n-heptane at 19 to 20 dynes/cm, are therefore the hardest for a film to cover.<ref name="Sch-4-93" />

			[[File:fse-aqueous-film-forming-foam-v3.svg\|thumb\|upright=1.5\|alt=Bar chart of spreading coefficients of one AFFF solution on seven fuels, from −1.2 for n-heptane to +5.2 for JP-5.\|Spreading coefficient of AFFF solution FC-194 (15.5 dynes/cm) on seven hydrocarbon liquids at 25 °C; negative values gave no film. Based on Scheffey (2002), pp. 4-93 to 4-94.]]

	The spreading coefficient test is used worldwide as a standard indicator of film-forming foams.<ref name="Sch-4-93" /> It does not, however, predict fire performance. Studies have shown no direct correlation between the spreading coefficient and fire extinguishment, so the coefficient alone cannot rank foams.<ref name="Sch-4-93" /> Formulators must also keep a balance: adding more fluorinated surfactant lowers the interfacial tension but can reduce the foam's ability to shed fuel.<ref name="Sch-4-93" />		The spreading coefficient test is used worldwide as a standard indicator of film-forming foams.<ref name="Sch-4-93" /> It does not, however, predict fire performance. Studies have shown no direct correlation between the spreading coefficient and fire extinguishment, so the coefficient alone cannot rank foams.<ref name="Sch-4-93" /> Formulators must also keep a balance: adding more fluorinated surfactant lowers the interfacial tension but can reduce the foam's ability to shed fuel.<ref name="Sch-4-93" />
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	Aviation fire protection is built on these results. An aluminium fuselage exposed to a fuel spill fire burns through in about 1 minute, so rescue vehicles must control the fire within 1 minute of arriving.<ref name="Sch-4-103">Scheffey (2002), p. 4-103.</ref> The resulting design application rates are 5.5 L/(min·m²) for AFFF, 7.5 L/(min·m²) for fluoroprotein foam and 8.2 L/(min·m²) for protein foam.<ref name="Sch-4-103" /> Foam quality is also specified: under NFPA 412, air-aspirated AFFF must have an expansion ratio of at least 5:1 and a 25% drainage time of at least 2.25 minutes.<ref name="Sch-4-103" /> Longer drainage times generally give longer burnback times, but no direct correlation has been found between expansion, drainage and extinguishing performance.<ref name="Sch-4-104">Scheffey (2002), p. 4-104.</ref>		Aviation fire protection is built on these results. An aluminium fuselage exposed to a fuel spill fire burns through in about 1 minute, so rescue vehicles must control the fire within 1 minute of arriving.<ref name="Sch-4-103">Scheffey (2002), p. 4-103.</ref> The resulting design application rates are 5.5 L/(min·m²) for AFFF, 7.5 L/(min·m²) for fluoroprotein foam and 8.2 L/(min·m²) for protein foam.<ref name="Sch-4-103" /> Foam quality is also specified: under NFPA 412, air-aspirated AFFF must have an expansion ratio of at least 5:1 and a 25% drainage time of at least 2.25 minutes.<ref name="Sch-4-103" /> Longer drainage times generally give longer burnback times, but no direct correlation has been found between expansion, drainage and extinguishing performance.<ref name="Sch-4-104">Scheffey (2002), p. 4-104.</ref>

			[[File:fse-aqueous-film-forming-foam-v4.svg\|thumb\|upright=1.5\|alt=Bar chart of aviation design application rates: AFFF 5.5, fluoroprotein 7.5 and protein foam 8.2 litres per minute per square metre.\|Design application rates for aircraft rescue and fire fighting by foam type. Based on Scheffey (2002), p. 4-103.]]

	== Fixed systems and aircraft hangars ==		== Fixed systems and aircraft hangars ==

Daan: New FSE wiki article: Aqueous film-forming foam (written by Claude Opus 5.5)

2026-11-01T22:53:25Z

New FSE wiki article: Aqueous film-forming foam (written by Claude Opus 5.5)

New page

'''Aqueous film-forming foam''' ('''AFFF''') is a synthetic [[firefighting foam]] whose solution forms a thin watery film that spreads across the surface of a burning hydrocarbon fuel.<ref name="Sch-4-89">Scheffey (2002), p. 4-89.</ref> The concentrate combines fluorine-free hydrocarbon foaming compounds with highly fluorinated surfactants.<ref name="Sch-4-89" /> Mixed with water and aerated, it produces a foam that extinguishes a fire by covering the fuel and stopping it from vaporising, like other foams.<ref name="Sch-4-88">Scheffey (2002), p. 4-88.</ref><ref name="Sch-4-89" /> In addition, the liquid that drains from the bubbles forms a stable film on the fuel, and this film formation is the feature that sets AFFF apart.<ref name="Sch-4-89" />

AFFF is used mainly on fires of flammable liquids, such as fuel spills, and its typical application is the two-dimensional [[pool fire]].<ref name="Sch-4-88" /> In large aviation fire tests it controlled fires at lower application rates than protein foam, and US airports were allowed a 30% reduction in water when AFFF replaced protein foam.<ref name="Sch-4-100">Scheffey (2002), p. 4-100.</ref><ref name="Sch-4-101">Scheffey (2002), p. 4-101.</ref> It can also be discharged through ordinary sprinklers and spray systems, without special foam-making devices.<ref name="IS-B-64">Iqbal and Salley (2004), p. B-64.</ref> Whether a solution can form a film on a given fuel is described by its spreading coefficient, a simple balance of surface tensions.<ref name="Sch-4-93">Scheffey (2002), p. 4-93.</ref> Its limits are those of foam in general. It is less effective on three-dimensional fires of running or sprayed fuel, needs special formulations for alcohols, and must not be used on liquids hotter than 100 °C.<ref name="Sch-4-88" /><ref name="IS-B-60">Iqbal and Salley (2004), p. B-60.</ref>

== Composition and types of foam ==
Fire-fighting foam consists of air-filled bubbles formed from an aqueous solution.<ref name="Sch-4-88" /> The solution is made by mixing a foam concentrate with water, typically at 1, 3 or 6% concentrate.<ref name="Sch-4-88" /> The solution is then aerated to form the bubbles.<ref name="Sch-4-88" /> Foam is lighter than the solution that drains from it and lighter than flammable liquids, so it floats on the fuel.<ref name="Sch-4-88" /> Low-expansion foams, with a foam volume of up to about ten times the solution volume, are the type used on pool fires.<ref name="Sch-4-88" />

There are no universally agreed definitions of foam agents, and definitions differ even between NFPA standards.<ref name="Sch-4-88" /> Scheffey, in the SFPE Handbook, describes the main families as follows. Protein foam is made from hydrolysed proteins, such as hoof and horn meal, with iron salts, and forms no film on the fuel.<ref name="Sch-4-88" /> Fluoroprotein foam adds fluorinated surfactants to a protein base; this lowers the surface tension, but the drained solution still forms no film on hydrocarbons.<ref name="Sch-4-89" /> Aqueous film-forming foam is fully synthetic and is formulated so that its solution has the surface and interfacial tensions needed to spread as a film.<ref name="Sch-4-89" /> Film-forming fluoroprotein foam (FFFP) is a protein foam with enough surfactant added that its drained solution can also spread across a hydrocarbon fuel.<ref name="Sch-4-89" />

The key ingredient of AFFF is a fluorinated surfactant. In a typical molecule, a perfluorooctyl group forms one end, which repels both water and oil, while the other end is attracted to water.<ref name="Sch-4-89" /> In solution these molecules gather at the surface with their water-repelling ends towards the air.<ref name="Sch-4-89" /> Hydrocarbon surfactants in the concentrate make the expanded foam more stable.<ref name="Sch-4-89" /> The combination gives a solution with a surface tension well below that of water and a foam that resists breakdown by heat, fuels and dry chemical agents.<ref name="Sch-4-89" />

The effect on surface tension is large. A 6% AFFF solution has a surface tension of about 15 to 16 dynes/cm at 15 to 32 °C, against 73 dynes/cm for water.<ref name="GW-2-298">Gottuk and White (2002), p. 2-298.</ref> One dyne per centimetre equals one millinewton per metre.<ref name="DiN-A-10">DiNenno et al. (2002), p. A-10.</ref> In unconfined spill tests, water and 6% AFFF solution gave spill depths that bound those of most fuels.<ref name="GW-2-298" />

Several variants exist for special conditions. Regular foams do not perform well on polar solvents such as alcohols, and alcohol-resistant AFFF has been developed for them.<ref name="IS-B-64" /> Such concentrates form a floating polymeric skin that protects the foam from breakdown by solvents such as acetone, methanol and ethanol.<ref name="Sch-4-89" /> Low-temperature foams, including AFFF types, have been developed for use at temperatures as low as −29 °C.<ref name="IS-B-64" />

== How AFFF extinguishes a fire ==
Foams are described in NUREG-1805 as a primary alternative to water, particularly for large fires.<ref name="IS-B-58">Iqbal and Salley (2004), p. B-58.</ref> As an [[extinguishing agent]] they are used mainly on flammable and combustible liquids, the Class B fuels in the [[fire classes]] used by NUREG-1805, and are also suitable for Class A fires of ordinary combustibles.<ref name="IS-B-58" /> Water alone is a poor agent on a burning liquid that is lighter than water and insoluble in it, because the liquid simply floats on the water and continues to burn.<ref name="IS-B-58" /> Foam excludes air from the fire, generates steam that dilutes the air and absorbs heat, penetrates crevices because of its low surface tension, and protects material that is not yet burning.<ref name="IS-B-58" />

The SFPE Handbook notes that the mechanisms of foam extinguishment of pool fires have not been developed from first principles.<ref name="Sch-4-89" /> Extinguishment is usually described as the stopping of fuel vaporisation at the fuel surface.<ref name="Sch-4-89" /> As the vapour supply falls, the combustion zone shrinks, and once the whole area is covered the fire goes out.<ref name="Sch-4-89" /> The floating foam forms an air-excluding layer of aqueous agent that suppresses combustion by halting vaporisation.<ref name="Sch-4-88" /> With AFFF, the solution that drains from the foam adds a film that spreads rapidly and is highly stable on the fuel.<ref name="Sch-4-89" /> NUREG-1805 adds that this kind of film re-forms rapidly if it is disturbed.<ref name="IS-B-63">Iqbal and Salley (2004), p. B-63.</ref>

The foam blanket is continuously lost while it works. Radiant heat from the fire evaporates water from the foam, and liquid drains out of the bubbles.<ref name="Sch-4-90">Scheffey (2002), p. 4-90.</ref> For radiant fluxes of 45 to 185 kW/m² from large pool fires, a simple heat balance gives evaporation rates of 18 to 72 g/(m²·s).<ref name="Sch-4-90" /> In laboratory tests on one AFFF, heat began to penetrate the blanket when it had thinned to about 50 mm.<ref name="Sch-4-91">Scheffey (2002), p. 4-91.</ref> When the fuel below reignited, almost all the foam had been lost, with less than 0.8 kg/m² remaining.<ref name="Sch-4-91" /> Foam must therefore be applied in enough volume and at a high enough rate to make up for these losses and keep a residual layer over the extinguished area.<ref name="IS-B-59">Iqbal and Salley (2004), p. B-59.</ref>

== Spreading coefficient ==
A film-forming foam is defined by the ability of the solution draining from it to spread spontaneously across a hydrocarbon fuel.<ref name="Sch-4-93" /> Whether this happens depends on three tensions: the surface tension of the fuel, the surface tension of the foam solution, and the interfacial tension between the two liquids.<ref name="Sch-4-93" /> The fundamental relation is the spreading coefficient:<ref name="Sch-4-93" />

<math display="block">S_{a/b} = \gamma_b - \gamma_a - \gamma_i</math>

{| class="wikitable"
! Symbol !! Description !! Unit
|-
| <math>S_{a/b}</math> || spreading coefficient of the foam solution (a) on the fuel (b) || dynes/cm
|-
| <math>\gamma_b</math> || surface tension of the lower liquid, the hydrocarbon fuel || dynes/cm
|-
| <math>\gamma_a</math> || surface tension of the upper liquid, the AFFF solution || dynes/cm
|-
| <math>\gamma_i</math> || interfacial tension between liquids a and b || dynes/cm
|}

A positive spreading coefficient means that a film can form; a negative value means it cannot.<ref name="Sch-4-93" /><ref name="Sch-4-94">Scheffey (2002), p. 4-94.</ref> The tensions are measured with a ring tensiometer, and reagent-grade cyclohexane is the usual reference fuel.<ref name="Sch-4-93" /> As the fuel temperature rises, the surface tensions of both fuel and solution fall, and the spreading coefficient may drop to zero or below.<ref name="Sch-4-93" /> Because the surface tensions of most AFFFs are about the same, the interfacial tension is usually the deciding factor.<ref name="Sch-4-93" />

As a worked example, take the AFFF solution FC-194 tested by the US Naval Research Laboratory, with a surface tension of 15.5 dynes/cm at 25 °C.<ref name="Sch-4-94" /> On cyclohexane, with a surface tension of 24.2 dynes/cm and an interfacial tension of 4.3 dynes/cm, the spreading coefficient is 24.2 − 15.5 − 4.3 = 4.4 dynes/cm, and a film formed.<ref name="Sch-4-93" /><ref name="Sch-4-94" /> On certified n-heptane, with 19.8 and 5.5 dynes/cm, it is 19.8 − 15.5 − 5.5 = −1.2 dynes/cm, and no film formed.<ref name="Sch-4-93" /><ref name="Sch-4-94" /> On JP-5 jet fuel, the result is +5.2 dynes/cm.<ref name="Sch-4-94" /> Fuels with a low surface tension, such as avgas and n-heptane at 19 to 20 dynes/cm, are therefore the hardest for a film to cover.<ref name="Sch-4-93" />

The spreading coefficient test is used worldwide as a standard indicator of film-forming foams.<ref name="Sch-4-93" /> It does not, however, predict fire performance. Studies have shown no direct correlation between the spreading coefficient and fire extinguishment, so the coefficient alone cannot rank foams.<ref name="Sch-4-93" /> Formulators must also keep a balance: adding more fluorinated surfactant lowers the interfacial tension but can reduce the foam's ability to shed fuel.<ref name="Sch-4-93" />

== Fire tests and application rates ==
Since there is no theory of foam spreading, foam performance is measured with fire tests.<ref name="Sch-4-94" /> In the US standard UL 162, commercial n-heptane burns in a square pan of at least 4.6 m² for 60 s before foam is applied.<ref name="Sch-4-95">Scheffey (2002), p. 4-95.</ref> For hand-held nozzles, protein-based foams are tested at 2.5 L/(min·m²) for 5 minutes, while AFFF and FFFP are tested at 1.6 L/(min·m²) for 3 minutes.<ref name="Sch-4-95" /> After extinguishment, a burnback test checks that the foam blanket restricts a reignited area to no more than 0.9 m² for 5 minutes, or flows back over it.<ref name="Sch-4-95" />

Application rates in test standards are usually lower than those used in practice.<ref name="Sch-4-100" /> Practice adds a safety factor, while critical rates bring out the differences between agents.<ref name="Sch-4-100" /> Large aviation tests by Geyer with JP-4 pool fires of 21 to 43 m diameter illustrate this.<ref name="Sch-4-100" /> To control a fire within 60 s, AFFF needed about 1.6 to 2.4 L/(min·m²), while protein foam needed 3.3 to 4.1 L/(min·m²).<ref name="Sch-4-100" /> Above about 4.1 L/(min·m²) for AFFF and 8.2 L/(min·m²) for protein foam, higher rates hardly improved control times.<ref name="Sch-4-100" /> Below a critical application rate, a fire is not extinguished however long the agent is applied.<ref name="Sch-4-100" />

Aviation fire protection is built on these results. An aluminium fuselage exposed to a fuel spill fire burns through in about 1 minute, so rescue vehicles must control the fire within 1 minute of arriving.<ref name="Sch-4-103">Scheffey (2002), p. 4-103.</ref> The resulting design application rates are 5.5 L/(min·m²) for AFFF, 7.5 L/(min·m²) for fluoroprotein foam and 8.2 L/(min·m²) for protein foam.<ref name="Sch-4-103" /> Foam quality is also specified: under NFPA 412, air-aspirated AFFF must have an expansion ratio of at least 5:1 and a 25% drainage time of at least 2.25 minutes.<ref name="Sch-4-103" /> Longer drainage times generally give longer burnback times, but no direct correlation has been found between expansion, drainage and extinguishing performance.<ref name="Sch-4-104">Scheffey (2002), p. 4-104.</ref>

== Fixed systems and aircraft hangars ==
AFFF has a low viscosity and does not need elaborate foam-making devices, so it can be used in many water sprinkler and water spray systems.<ref name="IS-B-64" /> Overhead foam-water sprinkler systems serve two purposes: they control or suppress a fuel spill fire, and when the foam runs out they continue to cool with water.<ref name="Sch-4-112">Scheffey (2002), p. 4-112.</ref> In the US, NFPA 16 requires a minimum application rate of 6.6 L/(min·m²) for deluge and closed-head AFFF systems.<ref name="Sch-4-112" /> This rate includes a safety factor over the 4.1 L/(min·m²) at which AFFF from sprinklers is effective on pool fires.<ref name="Sch-4-112" />

Much of the test data for foam-water sprinkler systems was originally developed for aircraft hangars.<ref name="Sch-4-112" /> Hangars have traditionally been protected by deluge systems with open heads, activated by rapid-response detection.<ref name="Sch-4-106">Scheffey (2002), p. 4-106.</ref> Factory Mutual tested 83.6 m² JP-4 fires in an 18.3 m high space.<ref name="Sch-4-107">Scheffey (2002), p. 4-107.</ref> At 6.6 L/(min·m²), standard sprinklers discharging AFFF were 1.3 to 1.6 times as effective as air-aspirating foam-water sprinklers.<ref name="Sch-4-107" />

The explanation lies in the fire plume. It rose at up to 23.2 m/s, while foam particles fell at no more than about 9.1 m/s.<ref name="Sch-4-107" /> The denser droplets from standard sprinklers penetrated the plume better than the light, air-aspirated foam.<ref name="Sch-4-107" /> The US hangar standard NFPA 409 therefore allows 6.6 instead of 8.2 L/(min·m²) when standard sprinklers are used with AFFF, a 20% reduction in foam.<ref name="Sch-4-108">Scheffey (2002), p. 4-108.</ref>

Low-level systems avoid the plume problem altogether. Monitors, turret nozzles and floor or side-mounted nozzles deliver AFFF directly onto the fuel, at high local densities.<ref name="Sch-4-108" /> Such systems have been used for over two decades on US Navy ships that carry aircraft, protecting flight decks and special hazard areas.<ref name="Sch-4-108" />

== Limits and hazards ==
AFFF shares the general limits of foam. Low-expansion foams are effective on two-dimensional pool fires, but not on three-dimensional fires, especially those of low-flashpoint fuels.<ref name="Sch-4-88" /> Where running fuel or a pressurised spray is expected, an auxiliary agent such as dry chemical is used together with the foam.<ref name="Sch-4-88" /> Foam is an unstable mixture of air and water that can be broken down by mechanical forces, certain chemicals and some other extinguishing agents.<ref name="IS-B-58" /> Rupture of the foam blanket can lead to burnback, which may put firefighters at risk.<ref name="IS-B-60" />

Hot liquids are a particular hazard. If water penetrates an oil or fat that is well above the boiling point of water, it turns to steam below the surface and can cause an eruption of burning oil, known as [[boilover]].<ref name="IS-B-58" /> Because foam is water-based, its use on a liquid at 100 °C or above can cause a four-fold expansion of the foam, with boilover of the burning liquid.<ref name="IS-B-60" /> Foam should only be used on a horizontal liquid surface below its boiling point, and, like water, it is unsuitable for live electrical equipment or substances that react with water.<ref name="IS-B-60" /><ref name="IS-B-61">Iqbal and Salley (2004), p. B-61.</ref>

Engineering knowledge of AFFF is also limited. According to Scheffey, modelling of foam extinguishment cannot yet be performed, because too many parameters, such as foam loss rates and foam viscosity, have not been quantified.<ref name="Sch-4-92">Scheffey (2002), p. 4-92.</ref> Design therefore relies on rates from fire tests with added safety factors.<ref name="Sch-4-100" /><ref name="Sch-4-112" /> Environmental impact, together with reliability and false discharges of foam-water deluge systems, was among the reasons why the US Navy re-evaluated its approach to hangar fire protection.<ref name="Sch-4-110">Scheffey (2002), p. 4-110.</ref>

== See also ==
* [[Deluge system]]
* [[Extinguishing mechanisms]]
* [[Sprinkler system]]
* [[Fire suppression system]]

== Notes ==
<references />

== References ==
* DiNenno, P. J., Drysdale, D., Beyler, C. L., Walton, W. D., Custer, R. L. P., Hall, J. R., Jr., & Watts, J. M., Jr. (Eds.). (2002). ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association; Society of Fire Protection Engineers.
* Gottuk, D. T., & White, D. A. (2002). Liquid fuel fires. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-297–2-316). National Fire Protection Association; Society of Fire Protection Engineers.
* 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, Office of Nuclear Reactor Regulation.
* Scheffey, J. L. (2002). Foam agents and AFFF system design considerations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 4-88–4-122). National Fire Protection Association; Society of Fire Protection Engineers.

[[Category:Fire Suppression]]