8.5 Foam Fire Suppression Systems (NFPA 11)

Foam Fire Suppression Systems (NFPA 11)

Foam fire suppression systems, governed by NFPA 11 (Standard for Low-, Medium-, and High-Expansion Foam), are specialized fire protection installations designed to combat fires involving Class B flammable and combustible liquids. Water alone is often ineffective—and can be highly dangerous—when applied to Class B fires. Because water is denser than most hydrocarbons, applying water causes it to sink beneath the fuel. This action can splash the fuel and spread the fire. Foam solves this hazard by combining water, foam concentrate, and air to create a stable, low-density aqueous blanket that floats on top of the liquid fuel.

The Extinguishing Mechanism

Foam suppresses and extinguishes fires through four primary, concurrent mechanisms:

1. Smothering: The foam blanket forms a physical barrier that prevents oxygen from reaching the fuel surface, disrupting the chemical chain reaction of fire.
2. Separating: The foam layer physically separates the liquid fuel source from the heat and flames of the combustion zone above.
3. Cooling: Because foam contains a high percentage of water, it cools both the liquid fuel and any adjacent hot metal surfaces (such as tank walls), reducing the rate of fuel vaporization.
4. Suppressing Vapors: The foam blanket seals in flammable vapors, preventing them from mixing with air and forming an ignitable mixture.

Foam Expansion Categories and Ratios

Foam is classified by its expansion ratio, which is the ratio of the volume of finished foam produced to the volume of the foam solution (water plus concentrate) before air is introduced. NFPA 11 divides foam into three expansion ranges:

Low-Expansion Foam

Low-expansion foam is highly cohesive and has excellent flowability, allowing it to spread rapidly across the surface of a burning liquid. It contains a high water content, making it heavy and resistant to wind, which is critical for outdoor applications like oil refinery storage tanks. However, because of its low expansion, it is not suitable for filling large volumes.

Medium-Expansion Foam

Medium-expansion foam provides a transition between the wetness of low-expansion foam and the high volume of high-expansion foam. It is frequently utilized for vapor suppression over toxic or flammable chemical spills because it is light enough to cover the liquid quickly but dense enough to resist moderate wind currents. It is also applied in enclosed areas where rapid coverage of a horizontal surface is required.

High-Expansion Foam

High-expansion foam is designed for three-dimensional total flooding applications. It uses very little water to produce enormous volumes of foam, which can rapidly fill large rooms or basements. When applied to a fire, high-expansion foam fills the entire space, engulfing the fire. As the foam contacts the heat of the fire, the water inside the bubbles evaporates into steam, which reduces the oxygen concentration in the immediate area to dilute the combustion process while simultaneously cooling the fuel. It is highly susceptible to wind and draft, so it is strictly used indoors or in highly shielded spaces.

Foam Concentrates

Foam concentrates are formulated for specific fuel hazards. Applying the wrong type of concentrate can lead to rapid foam breakdown and system failure.

• Aqueous Film-Forming Foam (AFFF): Formulated from synthetic surfactants. When AFFF solution is discharged, it drains a thin aqueous film that floats rapidly across the surface of hydrocarbon fuels. This film provides rapid knockdown of flames, even if the foam blanket itself is disrupted. AFFF is only effective on hydrocarbons (e.g., gasoline, diesel, jet fuel).
• Alcohol-Resistant AFFF (AR-AFFF): Polar solvents (such as alcohols, ketones, ethers, and esters) are water-miscible and will rapidly extract water from standard AFFF, destroying the foam blanket. AR-AFFF contains a polysaccharide polymer that precipitates upon contact with a polar solvent, forming a tough physical barrier (membrane) between the fuel and the foam. AR-AFFF is a multipurpose concentrate that can be used on both hydrocarbon fuels (typically at 3% concentration) and polar solvent fuels (typically at 3% or 6% concentration).
• Fluoroprotein Foam (FP): Formulated with natural protein bases and fluorinated surfactants. FP has excellent heat resistance, fuel tolerance, and burnback resistance (the ability of the foam to resist ignition when exposed to open flame). Unlike AFFF, it does not produce a film, but its heavy blanket is highly durable.
• Film-Forming Fluoroprotein Foam (FFFP): Combines the rapid knockdown of AFFF with the heat resistance and burnback resistance of fluoroprotein foam.
• Synthetic Fluorine-Free Foam (SFFF): Driven by environmental regulations concerning persistent PFAS (per- and polyfluoroalkyl substances) chemicals, which are present in AFFF, FP, and FFFP, the industry is transitioning to SFFF. SFFFs use non-fluorinated surfactants. For the CFPS exam, note that SFFFs have different physical properties: they typically exhibit higher viscosity, produce a different bubble structure, and do not form an aqueous film. Consequently, SFFFs often require higher application rates, longer discharge times, and specific discharge devices to achieve the same extinguishing performance as AFFF. Retrofitting an existing system from AFFF to SFFF requires complete system recalculation, and potentially changing proportioners and nozzles.

Proportioning Equipment

Proportioning is the mixing of foam concentrate with water in the correct ratio (usually 1%, 3%, or 6%) to create the foam solution. NFPA 11 regulates several proportioning methods:

1. Line Proportioners (Venturi Type): These devices use the Venturi effect. As water flows through a restricted orifice in the proportioner, it creates a low-pressure area (vacuum) that draws foam concentrate from an unpressurized container into the water stream. Line proportioners are simple and inexpensive but introduce a high pressure drop (typically 30% to 40% of inlet pressure) and are highly sensitive to backpressure. They are limited to lines with constant flow rates and short hose lengths.
2. Pressure Proportioners (Bladder Tanks): A bladder tank system consists of a pressure vessel containing a flexible elastomeric bladder filled with foam concentrate. Water from the main supply enters the tank shell, pressurizing the outside of the bladder, which forces the concentrate out through a metering orifice into the water stream flowing through a proportioner. Bladder tanks require no external power source, operate over a wide range of flows and pressures, and have minimal pressure drop. They are the most common choice for fixed systems in commercial and industrial facilities.
3. Balanced Pressure Proportioners: These systems use a pump to supply foam concentrate at a pressure higher than the water pressure. A balancing valve adjusts the foam concentrate pressure to match the water pressure at the proportioning inlet. This method is highly accurate and can handle extremely wide ranges of flow and pressure, making it ideal for large deluge systems, refinery systems, and foam fire trucks. They require a dedicated foam pump, which requires external power (electrical or diesel).
4. In-Line Balanced Pressure (ILBP) Proportioners: Similar to balanced pressure systems, but the proportioning assemblies are located downstream, close to the hazards, while the foam pump and atmospheric storage tank remain in a central pump house. This is used when multiple separate hazards are protected by a single foam concentrate source.

Storage Tank Hazards and Discharge Outlets

Flammable liquid storage tanks present some of the most challenging hazards in fire protection. NFPA 11 specifies fire protection designs based on tank construction:

• Fixed Roof (Cone Roof) Tanks: These tanks have a permanent, rigid roof. Foam can be applied using top-side discharge devices or subsurface injection.
  • Type I Discharge Outlets: Devices that apply foam gently to the liquid surface. An example is a foam chute that directs foam down the tank shell. (Note: These are largely obsolete but still recognized in codes).
  • Type II Discharge Outlets: Devices that deliver foam onto the liquid surface but with some potential for submergence and agitation. The most common Type II device is a foam chamber, which is mounted near the top of the tank wall and contains a vapor seal glass plate. When the system activates, foam pressure breaks the glass, and foam flows onto a deflector plate that directs it down the tank wall.
  • Subsurface Injection: Foam is pumped under pressure into the bottom of the tank through the product piping or dedicated inlets. The foam bubbles rise through the hydrocarbon liquid, forming a foam blanket at the top. Subsurface injection is only permitted for hydrocarbon fuels, never for polar solvents, because the polar solvent would destroy the foam as it rises through the liquid. High-backpressure foam makers are required to overcome the head pressure of the liquid in the tank.
• Floating Roof Tanks: These tanks have a roof that floats on top of the liquid product to minimize vapor space.
  • External Floating Roof Tanks: The primary hazard is a fire in the annular seal area between the floating roof and the tank shell. Protection is typically provided by rim seal foam systems, which use a foam dam (a steel plate attached to the roof) to confine the foam to the seal area, allowing a low application rate.
  • Internal Floating Roof Tanks: Similar to external floating roof tanks, but they have a permanent fixed roof over the floating roof. They require protection for the seal area or full top-side foam application depending on the tank diameter and construction.

Design Calculations and Foam Solution Rates

To design a foam system, fire protection specialists calculate the required foam solution flow rate ($Q$) and the total foam concentrate volume ($V_c$).

$\text{Foam Solution Flow Rate } (Q) = \text{Area } (A) \times \text{Application Rate } (R)$

$\text{Total Foam Solution Volume } (V_s) = Q \times \text{Discharge Time } (T)$

$\text{Foam Concentrate Volume } (V_c) = V_s \times \text{Proportioning Percentage } (P)$

Worked Example

Calculate the required foam concentrate volume for a fixed cone-roof storage tank containing diesel fuel (hydrocarbon). The tank diameter is $100\text{ ft}$. The system uses a Type II foam chamber discharging a 3% AR-AFFF concentrate.

1. Determine the Tank Surface Area ($A$): $A = \pi \times r^2 = 3.14159 \times (50)^2 = 7,854\text{ sq ft}$
2. Determine the Design Application Rate ($R$): According to NFPA 11, the minimum design application rate for top-side application on hydrocarbons using Type II outlets is $0.10\text{ gpm/sq ft}$.
3. Calculate the Foam Solution Flow Rate ($Q$): $Q = 7,854\text{ sq ft} \times 0.10\text{ gpm/sq ft} = 785.4\text{ gpm}$
4. Determine the Discharge Time ($T$): NFPA 11 requires a minimum discharge time of 55 minutes for cone roof tanks containing hydrocarbons.
5. Calculate the Total Foam Solution Volume ($V_s$): $V_s = 785.4\text{ gpm} \times 55\text{ min} = 43,197\text{ gallons}$
6. Calculate the Foam Concentrate Volume ($V_c$): $V_c = 43,197\text{ gallons} \times 0.03 = 1,295.91\text{ gallons}$

Note on safety margins: Designers typically add a 10% to 15% safety factor for line filling, system testing, and residual concentrate that cannot be drafted from the tank.