Concept reference

The System Curve

C.1Plain-language definition

A system curve answers what head the piping requires at every possible flow. Its intercept is the zero-flow boundary head; its rise comes from friction and local losses. The curve belongs to a stated physical configuration and condition, so changing wet-well level, pipe roughness or valve position creates a different curve.

C.2Governing relation or decision logic

A common approximation is Hsys(Q) = Hstatic + RQ², with head in m (ft) and R carrying units consistent with the chosen flow unit. Darcy-Weisbach is not exactly quadratic because f can vary with Reynolds number; Hazen-Williams gives a Q^1.852 dependence in its calibrated use.

C.3The failure to name

Fitting one parabola through a TDH point and the origin erases static head. A more subtle failure is to create the curve with one pump operating and reuse it unchanged when common-header valve paths or parallel branches differ.

C.4Field review checklist

• Define the active flow path, boundary levels and pipe condition.
• Plot a zero-flow intercept and enough calculated points to reveal method behaviour.
• Build separate curves for governing level, age and valve configurations.
• Intersect each curve with the applicable single- or multi-pump curve.

C.5Where it fits in the course

§7.1 constructs curves; §7.2 turns them into operating envelopes.