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### Fire Hydrant Rated Capacity Calculator

Fluids Flow Design and Engineering

Fire Hydrant Rated Capacity Calculator

Typically, residual pressures in municipalities are in excess of 50 psi, especially in newly developed parts of town. Fire flow, however is measured consistently at 20 psi. In order to get the fire flow in gallons per minute (gpm) at 20 psi, the equations below will need to be performed.

The following equations are used to determine fire flow based on the static, residual (flowing), and pitot pressures:

Eq.1

$Q}_{r}=29.83\cdot {c}_{d}\cdot {D}^{2}\cdot \sqrt{{P}_{p}$

Eq. 2

$Q}_{f}={Q}_{r}{\left(\frac{{P}_{s}-20}{{P}_{s}-{P}_{r}}\right)}^{0.54$

Where

Q_{r} = is the residual flow at the pitot pressure
measured in gpm

c_{d} =
is the friction loss coefficient (usually 0.9
for a smooth 2½” opening)

D =
is the diameter of the opening in inches

P_{p} =
is the pitot pressure in psi

Q_{F} = is the FIRE FLOW in gpm at 20 psi

P_{s} = is the static pressure in psi

P_{r} = is the residual pressure in psi

Reference:

NFPA Standard 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants.

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