1. Introduction:
Foam systems use Low, Medium, and High Expansion foam concentrates to discharge foam into protected areas. Finished foam is a stable mass of small bubbles of lower density than most flammable liquids and water. Foam is a blanketing and cooling agent that is produced by mixing air into a foam solution that contains water and foam concentrate. Foam extinguishes flammable or combustible liquid fires in four ways (see fig. 1 below):
a) Excludes air from the flammable vapours.
b) Eliminates vapour release from fuel surface.
c) Separates the flames from the fuel surface.
d) Cools the fuel surface and surrounding metal surfaces
Foam Tetrahedron means that before being used, Foam concentrates must be proportioned (mixed with water) and aerated (mixed with air). Four elements are necessary to produce a quality foam blanket (see fig. 2 below). These elements include: foam concentrate, water, air, and aeration (mechanical agitation).
2. Types of Foam Concentrates:
Several foam types of foam concentrates have been developed over the years, each with particular qualities:
Protein foam, one of the earliest foams, is produced by the hydrolysis of protein material such as animal hoof and horn. Stabilizers and inhibitors are added to prevent corrosion, resist bacterial decomposition, and control viscosity.
Fluoroprotein foams are formed by the addition to protein foam of special fluorochemical surfactants that reduce the surface tension of the protein-based concentrate and allow more fluid movement.
Aqueous Film-Forming Foam (AFFF) replaces protein-based foamers with synthetic foaming agents added to fluorochemical surfactants. Designed for a rapid knockdown, AFFFs sacrifice heat resistance and long-term stability.
Film-Forming Fluoroprotein Foam (FFFP) is a protein-based foam with the more advanced fluorochemical surfactants of AFFF. FFFPs combine the burnback resistance of fluoroprotein foam with the knockdown power of AFFF.
Alcohol-Resistant (AR) foam is a combination of synthetic stabilizers, foaming agents, fluorochemicals, and synthetic polymers designed for use on polar solvents. The chemical makeup of these foams prevents the polar solvents from destroying them. Today’s more modern AR foams can be used on both polar solvents and hydrocarbons.
3. Foam Characteristics
No single foam product performs the same for all classes of fires. Each foam concentrate type excels at different functions; however, performance in other areas is often diminished. Knockdown, heat resistance, fuel tolerance, vapor suppression, and alcohol tolerance are all characteristics of various foam types.
4. Expansion Ratio of Foam:
Low expansion: Low expansion nozzles expand foam solution up to 20:1. That is, for every gallon of solution that enters the base of the nozzle between 8 and 10 gallons of finished foam is produced. These nozzles draw air at the base of the nozzle; the air and the solution mix; travel up the foam tube (this is called residence time) and the properly expanded foam exits the nozzle.
Medium expansion: Medium expansion nozzles can have expansion characteristics as high as 200:1, although expansions of 50:1 are more common. They operate in much the same way as low expansion nozzles, however, the diameter of the nozzle is much larger. Medium expansion nozzles can provide tremendous benefits when you really need to bury a risk!
High expansion foam: High expansion foam nozzles or generators can expand foam in excess of 200:1, when high expansion foam concentrates are used.
5. Types of Foam Proportioning:
Below are types or methods of injecting foam concentrate at varying proportioning rate into the foam system:
(a) Eductors: Eductors are the most common form of proportioning equipment. They are used “in-line” in the hose lay or “hard piped” behind the pump panel for dedicated foam discharges and around the pump systems. Eductors work on the venturi principal. Water is introduced, under pressure, at the inlet of the eductor. The eductor reduces the orifice available for the water to pass through, so it must speed up to get through. This creates a pressure drop that, in turn, puts suction on the pick-up tube. As the foam concentrate is pulled up the tube it passes through a metering valve that allows the correct percentage to be introduced into the water stream. In most cases, the metering valve can be adjusted to select a 1, 3, or 6% foam solution.
(b) Balanced pressure foam proportioning: Balanced pressure systems are extremely versatile and accurate. Most often these systems are associated with fixed systems and specialized mobile equipment. In such systems, by means of an auxiliary pump, foam compound is injected into the water stream passing through an inductor. The resulting foam solution is then delivered to a foam maker or playpipe. The proportioner can be inserted into the line at any point between the water source and foam maker or playpipe (see fig. 3)
(c) In-Line Balanced Pressure Proportioning (ILBP): A foam proportioning system utilizing a foam concentrate pump or a bladder tank in conjunction with a listed pressure reducing valve. At all design flow rates, the constant foam concentrate pressure is greater than the maximum water pressure at the inlet to the in-line balanced pressure proportioner. A pressure balancing valve integral to the in-line balanced pressure proportioner regulates the foam concentrate pressure to be balanced to incoming water pressure
Module is used in ILBP Proportioning Systems is to provide accurate proportioning at multiple locations remote from the foam concentrate pump system and storage tank (see fig. 4).
6. Application rates of foam solution:
The application rates discussed in this section are for spill fires of shallow depth as recommended by NFPA 11. Increasing the foam application rate over the minimum recommended will generally reduce the time required for extinguishment. However, if the application rate is less than the minimum recommended, the time required to extinguish will be prolonged or, if too low, the fire may NOT be controlled.
Hydrocarbons – flammable liquids that float-on and will not mix with water (eg. gasoline, diesel, jp4, heptane, kerosene).
Example 1: An area of 2000 square feet of regular gasoline is burning. You have 3% / 6% foam available for securing the flame:
• 0.1gpm/sq. ft. x 2000 sq. ft. = 200 gpm of foam solution required.
• 0.03 x 200 gpm = 6 gallons of 3% concentrate required per minute.
• 6 gal. x 15 minutes = 90 gallons of 3% foam concentrate required to control, extinguish and initially secure a 2000 sq. ft. hydrocarbon fire.
Polar Solvents – Flammable liquids that are water miscible or will mix with water. (eg. ketones, esters, alcohol, mtbe, amine)
Example 2: An area of 1000 square feet of a known polar solvent is on fire. you have 3% / 6% foam available for securing the flame. 0.2gpm* foam solution per square foot of fire. once again, NFPA recommends a minimum run time of 15 minutes on shallow spill fires
0.2gpm* foam solution per square foot of fire. once again, NFPA recommends a minimum run
time of 15 minutes on shallow spill fires
• 0.20 gpm/sq. ft. x 1000 sq. ft. = 200 gpm foam solution required.
• 0.06 x 200 gpm = 12 gallons of 6% foam concentrate required per minute.
• 12 gal x 15 minutes = 180 gallons of 6% foam concentrate required to control, extinguish and initially secure a 1000 sq. ft. polar solvent fire.
7. Common applications for foam systems:
Foam suppression systems are typically found in large areas where there is a lot of flammable or combustible liquids. For example, some of the common uses include:
a) Aircraft hangars
b) Warehouses
c) Marine applications
d) Process areas
e) Flammable liquid storage
f) Jet engine testing facilities
g) Tank Farms
h) Racks of Bulk Plants
8. Conclusion:
PLC Fire Safety Engineering has extensive knowledge and expertise to design, review, test and commission foam systems as per NFPA 11 and NFPA 16 standards. Our Engineers can provide consultation on selection of foam concentrate and type of system, proportioning methods, performance criteria, and code compliance requirements both for new and existing fixed foam systems.
References: NFPA 11 and manufacturer chemgurd.
