Hazen-Williams C for Plastic Pipe

Hazen-Williams C runs the opposite way to absolute roughness — higher C means smoother pipe — and it is an empirical coefficient, not a physical property. C = 150 for plastic, 140 for new steel or cement-lined ductile iron, 130 for new cast iron, 100 for aged cast iron, 120 for concrete. Many water utilities design plastic at 140 rather than 150 to leave margin for biofilm, fittings and future degradation.

Its limits deserve respect. The Hazen-Williams equation was fitted to water at ordinary temperatures in turbulent flow at velocities of about 0.9–3 m/s in pipes 50 mm and larger, and it has no viscosity term at all — which means it is simply wrong for glycol, oil, hot water outside its calibration range, or any laminar flow. Darcy-Weisbach with a Colebrook friction factor is physically correct everywhere and is what any general-purpose calculation should use. Hazen-Williams survives because fire-protection and water-distribution codes are written around it and because the arithmetic is easy, not because it is better.