Design and Calculations Guidelines of Foam Suppression system

Foam Suppression system

Design Applications of Foam Suppression system

 Foam/Water Sprinkler System for Warehouses/Diesel
Storage Rooms/Generator Room.
 Storage Tank Protection.
 Diked Area/Spill Protection System.
 High Expansion Foam System.
 Loading Rack Protection.

Design Procedure

Step : 1 : Determine the applicable NFPA Code or Insurance Company requirements and select the foam Agent.

a. When protecting hydrocarbon fuels, use one of the following types of foam concentrates
 Protein
 Fluoroprotein
 Aqueous film-forming foam (AFFF) ( Available in 1%, 3% and 6% )
 Film-forming fluoroprotein (FFFP)
 Alcohol-resistant (AR- AFFF) ( Available in 3% and 6% ) – Multipurpose foam

b. When protecting polar solvents
solvents with appreciable water solubility or water miscibility (e.g., methyl alcohol, ethyl alcohol, ethanol, and acetone), provide an alcohol-resistant foam concentrate. Film-forming foams will not form films over polar solvents.

Step : 2 : Determine the Application Rate of foam suppression system
Step : 3 : Determine Discharge Duration
Step : 4 : Determine the Demand Area
Step : 5 : Determine the Foam Quantity

a. Base the foam concentrate supply on the required foam concentrate injection percentage
b. Calculate the sprinkler demand with the minimum foam density/ application rate and minimum operating pressure specified for the foam-water sprinkler with the fuel and concentrate type.
c. Determine the quantity of foam concentrate (Bladder Tank Capacity)
d. Perform Hydraulic Calculation for the system

e. Calculate the pipe size carrying foam-water solution the same as carrying plain water.
f. Calculate the friction loss in piping carrying a non-alcohol resistant foam concentrate using the Darcy-Weisbach formula (also known as the Fanning formula) from the foam concentrate supply to the proportioner.
g. Consult the foam concentrate manufacturers for friction loss characteristics in pipe carrying an alcohol resistant foam concentrate (non-Newtonian fluid) from the foam concentrate supply to the proportioner.

h. Verify the selected proportioner has a flow range that meets the calculated minimum and maximum system demand.
i. Verify the minimum inlet pressure requirement of the proportioner is met.
j. Verify the maximum pressure differential for the water and foam concentrate supply of an in-line balanced pressure proportioner does not exceed the manufacturer’s specifications.

Example-1

Application : Storage Warehouse

Storage Commodity : Flammable Liquids (Hydrocarbon Liquids)
The characteristics of the stored liquid must be determined in order to select the appropriate type of foam concentrate to be used.

Type of Foam Concentrate Use : 3% AFFF
Based on the liquid being stored, an appropriate foam concentrate must be selected. Specific manufacturers will need to be consulted to determine the most appropriate foam concentrate for the hazard.

Type of System : Open Head ( Deluge)
Design Standard followed : NFPA 16
Size of the protected Area : 100 x 60 SF = 6000 SF

Application Rate : 0.16 gpm/ SF
Foam Solution Dishcharge rate : 0.16 x 6000 = 960 GPM
Duration of the Discharge = 10 Min.
Foam Solution Required = 9600 Gallon
Bladder Tank Capacity = 9600 x 0.03 = 288 Gallon
Number of Sprinkler Head required = 6000 / 100 = 60 Heads

Example-2

Application : Diesel Engine Generator Room

Storage Commodity : Combustible Liquid
Type of Foam Concentrate Use : 3% AFFF
Type of System : Foam/Water Deluge System
Design Standard followed : NFPA 16
Size of the protected area= 443 SF
Dimension of Rooom = 21’ x 21’
Application Rate = 0.16 gpm/ SF
Foam Solution Dishcharge rate : 0.16 x 443 = 70.88 GPM
Duration of the Discharge = 10 Min.
Foam Solution Required = 708.8 Gallon
Bladder Capacity required = 708.8x 0.03 = 21.2 Gallon

Hydraulic Calculations :

Design Pressure @ Nozzle : 30 psi
Coverage area per sprinkler = 100 Sft

Q = k x (P)^(1/2)

No. of Sprinklers = Total Area / Coverage area per sprinkler
No. of Sprinklers = 443 / 100
But for the maximum spacing between sprinklers not to exceed 3.7 m (NFPA 16 -7.3.7.2 Sprinkler Spacing)

No. of Sprinklers = 6 Sprinklers

Calculation for K Factor

Total requirement of foam Solution = 70.88 GPM
Discharge per Sprinkler = 70.8 /6 = 13 GPM
K = Q/(P)^1/2
K = 13/(30)^(1/2) = 2.3

Selecting Next higher available K Factor from Manufacture catalogue.

Approach-01 : Taking the design density as the governing constraint:

Discharge (Q) = k x ( P)^(1/2)
K = 3 GPM/Psi ^1/2
Q = Application Rate x Coverage Area
Q = 0.16 x 100
Q = 16 GPM

Therefore ;

P = (Q /k)^2
P = (16/3)^2
P = 28.44 < Min. Operating Pressure ( Refused)

Approach-02 : Taking Min. Operating Pressure as the governing
constraint:
P = 30 Psi
Discharge (Q) = k x ( P)^(1/2)
Discharge (Q) = 3x ( 30)^(1/2)
Discharge (Q) = k x ( P)^(1/2)
Q = 16.43 GPM

Discharge Rate will be = 0.164 > Min. required density of 0.16 GPM /SF hence Accepted.

Pipe Sizing Caculation(Refer Below Piping layout):

Maximum Liquid velocity = 10 ft/sec
For Pipe Sections (Q = 16.4 GPM)
Q = A x V
V = 0.4085 x Q (gpm) / d^2(inches)

10 = 0.4085 x 16.4/d^2
d = 0.8 inch say 1 Inch.

For Pipe Sections (Q = 32.4 GPM)
Q = A x V
V = 0.4085 x Q (gpm) / d^2(inches)

10 = 0.4085 x 32.4/d^2
d = 1.15 inch say 1.5 Inch.

For Pipe Sections (Q = 65.6 GPM)
Q = A x V
V = 0.4085 x Q (gpm) / d^2(inches)

10 = 0.4085 x 65.6/d^2
d = 1.63 inch say 2 Inch.

For Pipe Sections (Q = 98.4 GPM)
Q = A x V
V = 0.4085 x Q (gpm) / d^2(inches)

10 = 0.4085 x 98.4/d^2
d = 2.004 inch say 2 inch