Firefighting Foam has been widely used as a fire extinguisher for flammable and combustible liquids for many years. It is consisted of premix solution (a ratio of water and foam concentrate) mixed with air. It forms a stable form blanket which spreads above the fuels and adheres to surfaces.
Foam forms a film above the fuel surface and seals vapor. Fire fighting foam concentrate prevents fuel contact with oxygen, resulting in fire suppression. It also has a high cooling effect and will avoid reignition.
Before we jump into how to calculate firefighting foam demand, it is better to understand the type of flammable and combustible liquids. The are two types of flammable and combustible liquids in terms of its miscibility with water:
- Hydrocarbon : non miscible with water
- Polar solvent : water miscible
Fire caused by hydrocarbon liquids can be extinguished by Aqueous Film Forming Foam (AFFF) or Alcohol Resistant-Aqueous Film Forming Foam (AR-AFFF). On the other hand, fire caused by polar solvents can be extinguished by AR-AFFF only.
In addition to AFFF and AR-AFFF, there are several type of foam concentrates that are widely used, such as synthetic foam concentrate, class A foam concentrate, wetting agent, fluoroprotein, protein, and film forming fluoroprotein.
In order to estimate foam concentrate demand for cone roof storage tank, the following steps should be followed.
Table below shows number of Foam Chambers required for the protection of a flammable liquid stored in a vertical cone roof atmospheric storage tank, where the discharge device is attached to the tank. Where two or more outlets are required, the outlets are to be equally spaced around the tank periphery and each outlet is to be sized to deliver foam at approximately the same rate.
It is suggested that for tanks above 200 ft (60 m) in diameter at least one additional discharge outlet be added for each additional 5000 ft2 (465 m2) of liquid surface or fractional part.
As per NFPA 11, approved foam hose stream equipment shall be provided in addition to tank foam installations as supplementary protection for small spills. Minimum number of hose streams required is a function of diameter. Table below shows supplemental foam hose stream requirements diameter of the largest tank.
The equipment for producing each foam stream shall have a solution application rate of at least 189 L/min (50 gpm).
Table below shows minimum discharge time and application rate for Type II fixed foam discharge devices on cone roof storage tanks. Based on NFPA 11, Type II discharge outlet is an approved discharge outlet that does not deliver foam gently onto the liquid surface but is designed to lessen submergence of the foam and agitation of the surface. Example of this type of device are Foam Chambers and Foam Makers.
liquid
Flammable liquids having a boiling point of less than 100ºF might require higher rates of application. These should be determined by test.
The following table indicates the minimum operating time of the supplementary hose stream(s) for various size tanks.
Since I love making calculator so much, in this post I want to share calculator of foam concentrate demand for cone roof storage tank. In the next post, I will share for floating roof type. I hope you find this post useful.
References:
- NFPA 11 Standard for Low-, Medium-, and High Expansion Foam
- How do firefighting foams work in fire suppression?
- Fixed or Semi-fixed Fire Protection Systems for Storage Tanks