Hazen-Williams C for Cast Iron Pipe

C is the single number that captures a water main's condition, and for unlined cast iron it decays with age: about 130 when new, 100 after a couple of decades, and 60–80 in old, aggressive-water systems where tuberculation has narrowed and roughened the bore. Fire-flow modelling is acutely sensitive to it, because available flow at a given residual pressure scales roughly with C.

Because the honest range is so wide, C for an existing system should be measured, not assumed. A two-hydrant flow test gives head loss over a known length at a known flow, and C follows directly — the standard AWWA procedure. Treat the tabulated value as a starting point for new construction only. The other trap is mixing conventions: C is defined for the Hazen-Williams equation in its specific form and units, and its numerical value is meaningless in Darcy-Weisbach; do not attempt to convert one to the other except through a fitted equivalence at a stated velocity and diameter.