Velocity, hydraulic gradient, and head loss for water supply pipes — with C-value presets for DI, MS, HDPE, PVC, and aged cast iron.
📘 Read the full CPHEEO Chapter →The Hazen-Williams equation is the workhorse for Indian water supply pipe design. It's simpler than Darcy-Weisbach, calibrated for water at ambient temperature, and every state PWD, NHAI, and AMRUT consultant uses it.
The formula: V = 0.849 × C × R^0.63 × S^0.54 (metric), where C is the material-specific roughness coefficient, R is hydraulic radius, and S is the friction slope. From this you get velocity, head loss per km, and total head loss over the pipe length — the three numbers that govern transmission main and pumping design.
The C value is where most errors happen. A new DI main has C = 130-140, but after 20 years of service it drops to 100-110. HDPE keeps C = 150 for its full 80-100 year life. Plugging in the wrong C under-predicts head loss and under-sizes the pump — a mistake that shows up as low residual pressure 5 years after commissioning.
Based on the CPHEEO Manual on Water Supply and Treatment, published by the Central Public Health and Environmental Engineering Organisation, Ministry of Housing and Urban Affairs, Government of India.
Compute velocity and head loss for a given pipe using the Hazen-Williams equation. Standard Indian design basis for water supply transmission mains.
Velocity outside 0.6-2.5 m/s is a red flag: below 0.6 silt will deposit inside the pipe and reduce capacity over time; above 2.5 you'll see internal erosion, severe water hammer at valve closures, and excessive pumping energy. The economic sweet spot is 1.0-1.5 m/s — design toward that range by adjusting the diameter.
Head loss per km above 5 m/km typically indicates an undersized pipe. For a 15 km main, 5 m/km × 15 = 75 m of head loss — that's a huge pumping cost. Upsize one pipe diameter step and the loss drops roughly to a third (friction loss varies with D^4.87 in Hazen-Williams).