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.
NFPA 11 (2021 edition) is the governing standard for low-, medium-, and high-expansion foam systems. It references NFPA 16 for foam-water sprinkler and deluge systems and NFPA 30 for flammable liquid storage requirements.
Foam Concentrate Types
Foam concentrates differ in their chemistry, proportioning rate, and application:
| Type | Proportioning Rate | Fuel Compatibility | PFAS Content |
|---|---|---|---|
| AFFF (Aqueous Film Forming Foam) | 3% or 6% | Hydrocarbons only | Yes (PFAS-containing) |
| AR-AFFF (Alcohol-Resistant AFFF) | 3%×3% or 6%×6% | Hydrocarbons AND polar solvents | Yes (PFAS-containing) |
| Protein Foam (P) | 3% or 6% | Hydrocarbons | No |
| Fluoroprotein (FP) | 3% or 6% | Hydrocarbons | Yes |
| Film-Forming Fluoroprotein (FFFP) | 3% or 6% | Hydrocarbons | Yes |
| Fluorine-Free Foam (F3 / C6-F3) | 1–3% | Hydrocarbons and some polars | No |
AFFF was the gold standard for aviation and petroleum hazard suppression due to its rapid knockdown (aqueous film forms ahead of the foam blanket, sealing fuel vapor). However, AFFF contains per- and polyfluoroalkyl substances (PFAS) — persistent environmental contaminants now subject to regulatory phase-out worldwide.
Expansion Ratios: Low, Medium, and High
Foam expansion ratio = (volume of foam produced) / (volume of foam solution used):
- Low Expansion (≤ 20:1): Typical 6:1 to 12:1. Dense, stable foam that flows well over fuel surfaces. Used for spill fires, tank protection, flammable liquid storage areas. Most Type II tank foam chambers use low-expansion foam.
- Medium Expansion (20:1 to 200:1): 30:1 to 100:1 typical. Used for LNG spill protection and some industrial applications. Less common than low or high expansion.
- High Expansion (200:1 to 1,000:1): Produced by forcing solution through a net mesh exposed to ambient or induced air. Used to flood large enclosed spaces (aircraft hangars, ship holds, coal mines, basements). Very lightweight, fills volumes rapidly, but susceptible to wind and water spray disruption. Expansion ratios of 500:1 to 750:1 are typical for NFPA 11 high-expansion systems.
Application Methods
NFPA 11 §5 defines application methods for flammable liquid storage tanks:
- Top Pour (Type I or Type II discharge): Foam applied to the liquid surface from above. Type I devices gently discharge onto the inside tank shell, allowing foam to flow down the shell without disturbing the fuel. Type II (foam maker, vapor seal) discharges foam through a chamber on the shell. Most common for cone roof tanks. Top pour onto the burning fuel surface can cause fuel pick-up and foam breakdown if not applied gently.
- Subsurface Injection: Foam solution injected at the tank bottom through the product pipe or dedicated subsurface foam injection inlet. The foam rises through the fuel and spreads across the top. Avoids application through flames; requires AFFF or FFFP (must be fuel-resistant to prevent breakdown during ascent). Used for fixed-roof atmospheric storage tanks with product compatible with the foam type.
- Foam-Water Sprinklers/Foam-Water Deluge: NFPA 16 systems using foam-water solution through sprinkler heads or open deluge heads. Used for aircraft hangars (FAA AC 150/5210-6), loading racks, dip tanks. The foam concentrate is proportioned into the water supply via bladder tank, balanced pressure pump, or inline inductor.
- Mobile/Portable: Foam monitors, handlines, and apparatus — not covered by NFPA 11 fixed system requirements but must use compatible concentrate.
Tank Protection Design
NFPA 11 §5.4 specifies application rates and foam quantities for different tank types:
- Cone roof (fixed roof) tanks: Type II (top pour) design rate: 0.10 gpm/ft² of liquid surface area for AFFF; higher rates (0.16–0.24 gpm/ft²) for protein foam. Design quantity: application rate × liquid surface area × 55 minutes (application time) + 200% reserve in proportioning equipment. Required application time varies by foam type and agent per Table 5.2.3.2.2.
- Floating roof tanks: Foam applied only to the annular seal area (rim seal area between the floating roof and the tank shell). Foam dams (bunding) contain the foam. Design rate: 0.20 gpm/ft² of seal area for AFFF. Floating roof fire is more common than full surface fire.
- Internal floating roof tanks: Similar to open-top floating roof but with a fixed roof above the internal float. More complex access for foam application — often requires supplemental fixed discharge chambers.
Worked example — cone roof tank: 100-ft diameter tank, AFFF at 3%, application rate 0.10 gpm/ft², application time 55 min. Liquid surface area = π/4 × 100² = 7,854 ft². Design flow = 7,854 × 0.10 = 785.4 gpm foam solution. AFFF concentrate volume = 785.4 × 0.03 × 55 min = 1,296 gallons of AFFF concentrate minimum in storage.
Environmental Concerns and the PFAS Phase-Out
PFAS (per- and polyfluoroalkyl substances) in AFFF, AR-AFFF, FP, and FFFP foam concentrates are classified as persistent environmental pollutants. PFAS contaminate soil and groundwater at fire training facilities and airports globally. Regulatory actions:
- US EPA PFAS regulations: MCLs for PFOA and PFOS at 4 parts per trillion (2024 NPDWR). Military-use AFFF phase-out mandated by NDAA 2020.
- NFPA 11 (2021): Now includes Chapter 10 on fluorine-free foam (F3) with equivalency testing requirements — performance must be demonstrated per FM Approval Standard 5130 or UL 162 alternative protocols using F3-specific test fires.
- Fluorine-Free Alternatives (C6-F3, F3): Silicone-based, hydrocarbon surfactant-based. Current F3 concentrates typically require 1–3% proportioning. Equivalent or near-equivalent knockdown on hydrocarbon fires in FM 5130 Class B testing; some formulations show slower knockdown than AFFF on JP-8 aircraft fuel fires.
- Transition challenges: F3 concentrates may not be compatible with existing proportioning equipment, bladder tanks, or storage materials. Full system flushing and component replacement may be required. Existing AFFF inventory must be disposed of as hazardous waste — not discharged to the environment.
Engineers specifying foam systems today must consider the 20–30 year system life and the regulatory trajectory — new systems should be designed for F3 from the outset unless the specific application (military aviation, certain international jurisdictions) still permits AFFF.
Application Rates and Design Density Summary
NFPA 11 Table 5.2.3.2.2 minimum application rates (low expansion, top pour):
| Foam Type | Minimum Rate (gpm/ft²) | Application Time (min) |
|---|---|---|
| AFFF 3% | 0.10 | 55 |
| AFFF 6% | 0.10 | 55 |
| AR-AFFF on polars | 0.10 | 55 |
| Protein 3% | 0.16 | 55 |
| Protein 6% | 0.16 | 55 |
| Fluoroprotein | 0.10 | 55 |
| F3 (Fluorine-free) | Per FM 5130/UL 162 listing | Per listing |
Hose stream supplement per NFPA 11 §5.2.3.5: add minimum 50 gpm for tanks ≤ 65 ft diameter, 95 gpm for tanks 65–120 ft, and proportionally more for larger tanks. The complete design must also address cooling water demand for tank shell and adjacent exposed tanks per NFPA 15 water spray standards.