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ΔP is the pressure difference across pipe length L, kN/m 2.Q is the water flow rate in the pipe, m 3 /hr,.ΔP is the pressure difference across pipe length L, psi.Q is the water flow rate in the pipe, gpm,.In terms of frictional pressure drop, ΔP instead of frictional head loss, h L, the Hazen Williams equation is: units: Q = 0.278 C D 2.63 S 0.54, where the parameters are as defined above with Q in m 3 /s and D in meters. Q is the water flow rate in the pipe, gal/min (gpm).Substituting these equations into those for velocity give the following for the Hazen Williams equation: units: V = 0.85 C R 0.633 S 0.54, where the parameters are as defined above with V in m/s and R in meters.Īs an equation for water flow rate for pipe that is circular, the hydraulic radius is R = A/P = (πD 2 /4)/(πD) = D/4 and Q = VA = V(πD 2 /4). S is the slope of the energy grade line, dimensionless (S = head loss/pipe length = h L /L).R is the hydraulic radius, ft (R = cross-sectional area of flow/wetted perimeter).C is the Hazen Williams coefficient, dimensionless (depends on pipe material and age).units.Īs an equation for Velocity, the traditional form of the Hazen Williams equation is: Finally, the Hazen Williams equation can be written for U.S. It also can be expressed in terms of the pipe head loss or the frictional pressure drop. The Hazen Williams equation is sometimes expressed as an equation for velocity in the pipe and sometimes as an equation for pipe flow rate. ” Forms of the Hazen Williams Equation for Water Flow Rate for Pipe Sizes For more information about this alternative, see the post, “ Friction Factor/Pipe Flow Calculations with Excel Spreadsheets. For calculation of flow rates for pipes with a fluid other than water or for water at a temperature that is far above or far below 60 o F, the Darcy Weisbach equation is an alternative to the Hazen Williams equation. The Hazen Williams formula works best for water temperature that isn’t too far above or below 60 o F. The turbulent flow requirement isn’t normally a problem, because most practical transport of water in pipes is turbulent flow. The Hazen Williams equation is intended for turbulent water flow rate in pipes at normal ambient temperatures. units will be used in the Hazen Williams spreadsheets and calculations discussed in this article. Read on for information about the Hazen Williams equation and its use to calculate water flow rate for pipe sizes.Įxcel spreadsheets are very convenient to calculate water flow rate for pipe sizes with the Hazen Williams equation. Where to Find Excel Spreadsheets to Calculate Water Flow Rate for Pipe Sizesįor Excel spreadsheets to calculate water flow rate for pipe sizes using the Hazen Williams equation, click here to visit our spreadsheet store.