Foam fire extinguishing system design Calculation

 Three important factors to be calculated mentioned below:

1. Tank capacity calculation
2. No of sprinklers
3. Minimum pressure required at the input

General assumption

The system used is an automatic sprinkler systems utilizing foam/water sprinkler heads, integrated into a complete deluge system

Design step mentioned below:

Design Criteria and Preliminary Estimation:

Discharge Criteria = 6.5 l/min.m2  = 0.16 gpm/ft2  (Refer to NFPA 16, section 7.2.2.1)

Discharge Duration  =10min (Refer to NFPA 16, section 7.2.3.1)

Foam Concentrate  = 3%

Room Area = 200m2

Min. Area per Sprinkler = 9.3m2 (Refer to NFPA 16, section 7.6.1)

Min. No. of sprinklers    = 21.50537634 = 22 Sprinklers

And hence,

Flow of foam solution = Discharge Criteria x Room Area 

Flow of foam solution = 1300 l/min.

Volume of foam solution = Flow Of Foam Solution x Discharge duration 

Volume of foam solution = 13000 l

Quantity of Foam Concentrate = Volume Of Foam Solution x Foam concentration 

Quantity of foam Conc. = 390l = 103.02708 Gallons

Foam Tank Capacity   = 103.02708 Gallons

Design Pressure at Nozzle = 30psi = 207 kpa (Refer to NFPA 16, section 4.4.1.3) 

Nozzle minimum operating pressure = 30psi (Refer to NFPA 15, section A.8.1.2)

Type of foam concentrate used is AFFF

Density of foam concentrate (ρ) = 1.011g/ml

Absolute Viscosity of foam concentrate (μ) = 1.8cps

From the sprinkler distribution:

Coverage area per sprinkler  = 9.090909091m2 = 97.75171065ft2

Since,

For open type sprinklers dedicated for foam/water deluge systems 

                                                  Q (gpm) = K √P (psi)

And,

K = 3

and since we have two approaches, one is to satisfy the density and the other is to satisfy the minimum pressure required; we'll calculate both and apply the one with the greater demand 

• Taking the design density as the governing constraint                                                

Discharge Criteria = 0.16gpm/ft2

Q = 15.6402737 gpm

P = 27.17979573 psi Rejected

• Taking the minimum operating pressure as the governing constraint 

P = 30 psi
Q = 16.43167673 gpm

Discharge Criteria = 0.168096053gpm/ft2 Accepted

Therefore,

Adding 5PSI the static pressure:

Pt = 79 psi

Assuming that the induction causes a pressure drop of 30% of the required pressure:

Pt = 102.7 psi

Based on the final Design Criteria: System Summary:

Tank Capacity = 109 Gal.

No. of Sprinklers = 22

Minimum Pressure Required at Input Point = 103 psi

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