Field note / closed-conduit hydraulics
The C value is part of the result.
The Hazen-Williams coefficient represents the assumed hydraulic condition of the pipe. Material, age, internal lining, water quality, and the chosen reference can all affect the value. Keep it beside the result so another engineer can review the basis.
Diameter and flow dominate.
In the SI relation, flow appears to the 1.852 power while diameter appears to the 4.87 power. A small diameter change can therefore have a large effect on head loss, which is why commercial pipe selection and velocity limits still need a separate check.
What still needs a system check
- Use a pipe length and internal diameter that represent the actual hydraulic boundary.
- Add fittings, valves, entrances, exits, static elevation, pump performance, and required residual pressure separately.
- Compare with Darcy-Weisbach when fluid properties, Reynolds number, roughness, or local losses need a more explicit basis.