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# Hazen-Williams equation

The Hazen-Williams formula can be used to calculate the pressure loss in a length of pipe due to friction dependent on the flow. This equation is commonly used for pressure drop calculations in American fire sprinkler systems, water distribution systems, and irrigation systems.

$P_d=\frac{4.52\quad Q^{1.85}}{C^{1.85}\quad d^{4.87}}$

where:

Pd = pressure drop in pounds per square inch / foot

Q = flow in gallons per minute

C = factor (friction loss coefficient) - the higher the C factor, the smoother the pipe. Typical C factors used in design, which take into account some increase in roughness as pipe ages are as follows:

d = inside hydraulic diameter (inch)

An alternate form of the Hazen Williams Equation is:

$V=1.318\quad C\quad R_h^{0.63}\quad S^{0.54}$

where:

V = Velocity (in feet per second)

C = as above

Rh = Hydraulic Radius (in feet) (see Manning formula)

S = Energy Gradient or Friction Slope (hf/L)

## In SI units

$Q = 0,849 \; C \, A \, R_h^{0,63} \, S^{0,54}$

Q =

C = factor (friction loss coefficient)

A =

Rh =

S =