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Thursday, July 25, 2013

Firefighting Foam Demand Calculations

To begin with let us define some terms common to firefighting foam.
 
Aqueous Film Forming Foam (AFFF)
A concentrate based on fluorinated surfactants plus foam stabilizers to produce a fluid aqueous film for suppressing hydrocarbon fuel vapors and usually diluted with water to a 1 percent, 3 percent, or 6 percent solution.
 
Alcohol Resistant -  Aqueous Film Forming Foam (AR-AFFF)
A specially formulated foam concentrate for use on fires from alcohols and other polar solvents.
 
Class B Fire
A fire in flammable liquids, combustible liquids, petroleum greases, tars, oils, oil-based paints, solvents, lacquers, alcohols, and flammable gases.
 
Flammable Liquid
A liquid that has a closed-cup flash point that is below 37.8°C (100°F) and a maximum vapor pressure of 2068.6 mm Hg (40 psia) at 37.8°C (100°F).
 
Foam
A stable aggregation of bubbles of lower density than oil or water.
 
Compressed Air Foam (CAF)
A homogenous foam produced by the combination of water, foam concentrate, and air or nitrogen under pressure.
 
Foam Concentrate
A concentrated liquid foaming agent as received from the manufacturer.
 
Minimum or Critical Application Rate (CAR)
It is the minimum flow of finished foam per square foot to extinguish a flammable liquid fire. The CAR was found for different fuels through extensive testing by the National Fire Protection Association (NFPA). The CAR for hydrocarbon fuels has been calculated to be 4.1 L/min*m2 (0.1 gpm/ft2) and the CAR for polar liquids, like alcohols, has been calculated to be 8.1 L/min*m2 (0.2 gpm/ft2).
 
Chemical Foam
When two or more chemicals are added the foam generates due to chemical reaction.  The most common ingredients used for chemical foam are sodium bicarbonate and aluminum sulphate with stabilizer. Chemical foam is generally used in portable fire extinguishers.
 
Mechanical Foam
It is produced by mechanically mixing a gas or air to a solution of foam compound (concentrate) in water. Various types of foam concentrates are used for generating foam, depending on the requirement and suitability.  Each concentrate has its own advantage and limitations. Mechanical foam can further be classified as LowMedium and High Expansion Foam.
 
Low Expansion Foam
Foam expansion ratio may be upto 50 to 1, but usually between 5:1 to 15:1 as typically produced by self aspirating foam branch pipes. The low expansion foam contains more water and has better resistance to fire. It is suitable for hydrocarbon liquid fires and is widely used in oil refinery, oil platforms, petrochemical and other chemical industries.
 
Medium Expansion Foam
Foam expansion ratio vary from 51:1 to 500:1 as typically produced by self aspirating foam branch pipes  with nets. This foam has limited use in controlling hydrocarbon liquid fire because of it's limitations  w. r. t. poor cooling & poor resistance to hot surfaces/radiant heat.
 
High Expansion Foam
Foam expansion ratio vary from 501:1 to 1500:1, usually between 750:1 to 1000:1 as typically produced by foam generators with air fans. This foam has also very limited use in controlling hydrocarbon liquid fire because of its limitations w. r. t. poor cooling and poor resistance to hot surfaces/radiant heat. It is used for protection of hydrocarbon gases stored under cryogenic conditions and for warehouse protection.
 
Class B fires generally require foam application in addition to firewater for quick and effective extinguishment of fire. Hydrocarbon storage tanks are also provided with the provision of sub-surface injection of foam to effectively stop fires by forming a impervious barrier to prevent fire propagation inside a tank which is a very big source of the fuel for fire propagation. Additionally, foams can also be used on liquid pool fires to smother the surface of the pool with foam, thereby starving the fire for oxygen and providing quick extinguishment.
 
When trying to fight a liquid pool fire of hydrocarbons not miscible with water such as Gasoline, Diesel, JP4. heptane and kerosene a minimum application rate of 4.1 L/min*m2 (0.1 gpm/ft2) of water-foam solution should be used. For liquid pool fire due to polar solvents such as Ketones, Esters, Alcohols, MTBE, Amine which are water miscible or will mix with water, a minimum foam-water application rate of 8.1 L/min*m(0.2 gpm/ft2) is recommended.
 
 
For sub-surface injection of foam-water solution in Fixed-roof (Cone) storage tanks NFPA 11 gives the following application rates and minimum discharge times:
 
Minimum Discharge Times & Application Rates for Type II Fixed Foam Discharge Outlets on Fixed-Roof (Cone) Storage Tanks Containing Hydrocarbons
 
NFPA 11 (Table 5.2.5.2.2)
 
Attached Image 

Besides the NFPA 11 standard related to sub-surface injection of foam in hydrocarbon storage tanks, NFPA 11 also provides recommendations for supplemental hose stream requirements for foam spraying. Following are the recommendations:
 
5.9.2.1: Approved foam hose stream equipment shall be provided in addition to tank foam installations as supplementary protection for small spill fires.
 
5.9.2.2: The minimum number of fixed or portable hose streams required shall be as specified in Table 5.9.2.2 and shall provide protection of the area.
 
NFPA 11 (Table 5.9.2.2)
 
Attached Image 
 
5.9.2.3:  The equipment for producing each foam stream shall have a solution application rate of at least 189 L/min (50 gpm), with the minimum number of hose streams shown in Table 5.9.2.2.
 
5.9.2.4: Additional foam-producing materials shall be provided to allow operation of the hose stream equipment simultaneously with tank foam installations as specified in Table 5.9.2.4.
 
NFPA 11 (Table 5.9.2.4)
 
Attached Image 
 
Let us do some actual calculations for finding out the foam requirement of some storage tanks:
 
Problem Statement:
 
Two Tanks storing Aviation Turbine Fuel (ATF) are provided in a common diked area. The capacity of each tank is 8895 m3. The tank dimensions are 30 m D X 12.2 m H. The containment dike for this pair of tanks is 65.5 m W X 104.5 m L. Calculate the foam-water solution requirement, 3% foam concentrate requirement & storage requirement for foam concentrate.
 
Solution
 
Inputs
 
No of Tanks: 2
Tank Diameter (each): 30.5 m
Tank Height (each): 12.2 m
Dike Width: 65.5 m
Dike Length: 104.5 m
Foam concentrate: 3%
Application Time for tanks: 30 minutes
Application Time for Dike: 20 minutes
Foam Concentrate Storage Buffer: 200%
 
Calculations
 
Tank C/S area: 730.6 m2
Gross Dike Area: 6845 m2
Net Dike Area: 5383.5 m2 (Gross Dike Area - 2*Tank C/S Area)
Foam Solution Rate for Tanks: 5991.1 LPM (4.1*Tank C/S Area*2)
Foam Soln Rate for Dike: 22072.4 LPM (4.1*Net Dike Area)
Foam concentrate Rate for tanks: 179.7 LPM (Foam Soln Rate for Tanks*3%)
Foam concentrate rate for Dike: 662.2 LPM (Foam Soln Rate for Dike*3%)
Foam concentrate qty reqd for tanks: 5392 Liters (Foam conc. rate for tanks*Application time for tank)
Foam concentrate qty reqd for Dike: 13243 Liters (Foam conc. rate for dike*Application time for Dike) 
Total Foam concentrate qty: 18635 Liters (5392 + 13243)
Foam concentrate storage capacity: 37270 Liters or 37.3 m3 (18635*Foam concentrate storage buffer)

A process flow scheme for fixed sub-surface foam system is shown as an attachement to this blog entry.
 
Attached Image 

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