Why is PFAS-containing AFFF being phased out of fire suppression systems?

AFFF (Aqueous Film Forming Foam) contains per- and polyfluoroalkyl substances (PFAS), which are persistent environmental contaminants that pollute soil and groundwater at fire training facilities and airports. US EPA regulations set maximum contaminant levels for PFOA and PFOS at 4 parts per trillion under the 2024 NPDWR, and military-use AFFF phase-out was mandated by the NDAA 2020. NFPA 11 (2021) now includes Chapter 10 covering fluorine-free foam (F3) with equivalency testing per FM Approval Standard 5130 or UL 162.

What is the design application rate for AFFF on a cone roof storage tank?

NFPA 11 Table 5.2.3.2.2 sets the minimum application rate for AFFF (3% or 6%) at 0.10 gpm/ft² of liquid surface area with a 55-minute application time for cone roof (fixed roof) tanks using Type II top pour. For example, a 100-ft diameter tank has a liquid surface area of 7,854 ft², producing a design flow of 785.4 gpm of foam solution and requiring 1,296 gallons of AFFF concentrate at 3% proportioning.

What are the three foam expansion ratio categories under NFPA 11?

NFPA 11 classifies foam by expansion ratio: low expansion is ≤20:1 (typically 6:1 to 12:1), used for spill fires and tank protection via most Type II foam chambers; medium expansion is 20:1 to 200:1 (typically 30:1 to 100:1), used for LNG spill protection; and high expansion is 200:1 to 1,000:1 (commonly 500:1 to 750:1), used to flood large enclosed spaces like aircraft hangars, ship holds, and coal mines.

What is the difference between top pour and subsurface injection foam application?

Top pour (Type I or Type II discharge) applies foam to the liquid surface from above — Type I discharges gently onto the inside tank shell while Type II uses a foam maker chamber on the shell, and is most common for cone roof tanks. Subsurface injection instead injects foam solution at the tank bottom through the product pipe, letting it rise through the fuel and spread across the top; it avoids application through flames but requires AFFF or FFFP that is fuel-resistant enough to survive the ascent without breaking down.

How is foam application only applied to the seal area on floating roof tanks?

On floating roof tanks, foam is applied only to the annular seal area — the rim seal between the floating roof and the tank shell — because a floating roof fire (typically a seal fire) is more common than a full surface fire. NFPA 11 sets the design rate at 0.20 gpm/ft² of seal area for AFFF, and foam dams (bunding) are used to contain the foam within the annular space.

Engineering·14 min read·June 17, 2026

🧯 Foam Fire Suppression Systems per NFPA 11: Class B Hazard Protection and PFAS Transition

Comprehensive engineering guide to foam fire suppression systems under NFPA 11, covering foam types, expansion ratios, application methods, tank protection design, and the critical transition away from PFAS-containing AFFF to fluorine-free alternatives.

📖Fire Suppression Systems Reference (Full Access)

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Class B Fire Hazard and Foam Suppression Principle

Class B fires involve flammable and combustible liquids and gases: petroleum products (gasoline, diesel, jet fuel, crude oil), solvents, alcohols, and other organic liquids. Water alone is ineffective or counterproductive for most Class B fires — it sinks below the fuel surface (for hydrocarbons less dense than water), spreads burning fuel, and can steam-explode if applied to hot fuel. Foam suppression works by generating a stable, cohesive foam blanket on the fuel surface that: (1) excludes oxygen, (2) prevents fuel vapor generation, (3) cools the surface, and (4) provides a water seal that persists after initial application to prevent re-ignition.

Topics covered

NFPA 11foam suppressionAFFFfluorine-free foamPFASClass B fireflammable liquidtank protectionfoam expansion ratio