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Fire Hydrant Rated Capacity Calculator

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

Qr = is the residual flow at the pitot pressure measured in gpm
cd = is the friction loss coefficient (usually 0.9 for a smooth 2½” opening)
D = is the diameter of the opening in inches
Pp = is the pitot pressure in psi
QF = is the FIRE FLOW in gpm at 20 psi
Ps = is the static pressure in psi
Pr = is the residual pressure in psi

Reference:

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

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