Williams Hazens Equation of Pressure Drop

The Hazen-Williams equation can be used to calculate pressure drop (psi) in pipes or tubes due to friction

The Hazen-Williams formula for calculating head loss in pipes and tubes due to friction can be expressed as:

P_d = 4.52  q^1.85 / (c^1.85 d_h^4.8655) (1)

where

P_d = pressure drop (psi/ft pipe)

c = design coefficient determined for the type of pipe or tube - the higher the factor, the smoother the pipe or tube

q = flow rate (gpm)

d_h = inside hydraulic diameter (inch)

Note! The Hazen-Williams formula gives accurate head loss due to friction for fluids with kinematic viscosity of approximately 1.1 cSt. More about fluids and kinematic viscosity.

The results for the formula is acceptable for cold water at 60 ^oF (15.6 ^oC) with kinematic viscosity 1.13 cSt. For hot water with a lower kinematic viscosity (0.55 cSt at 130 ^oF (54.4 ^oC)) the error will be significant.

Since the Hazen Williams method is only valid for water flowing at ordinary temperatures between 40 to 75 ^oF, the Darcy Weisbach method should be used for other liquids or gases.

Online Hazens-Williams Calculator

The calculator below can used to calculate the head loss:

l - pipe or tube length (ft)

c - design coefficient determined for the type of pipe or tube

q - flow rate (gal/min)

d_h - hydraulic diameter (inch)

The Design Factor - c

The design factor is determined for the type of pipe or tube used:

• The c-value for cast iron and wrought iron pipes or tubes ranges from 80 to 150, with average value 130 and design value 100.
• The c-value for copper, glass or brass pipes or tubes ranges from 120 to 150, with average value 140 and design value 140.
• The c-value for cement lined steel or iron pipes has average value of 150 and design value 140.
• The c-value for epoxy and vinyl ester pipes can be set to 150.

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