Hazen-Williams equation

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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:

P_d = pressure drop in pounds per square inch / foot

Q = 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:

Asbestos-cement - 140

Cast iron - 100

Concrete - 100

Copper - 150

Steel - 120

Galvanized steel - 120

Polyethylene - 140

Polyvinyl chloride (PVC) - 150

Fibre-reinforced plastic (FRP) - 150

d = actual inside pipe diameter in inches

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

R_h = Hydraulic Radius (in feet)

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

[edit] See also

[edit] References

Turf Irrigation Manual, 5th Ed., by James A. Watkins, published by Telsco, 1987.
Fluid Mechanics, 10th Ed., by E. John Finnemore and Joseph B Franzini, published by McGraw Hill, 2002.