Concept reference

The Hydraulic Profile (HGL and EGL)

C.1Plain-language definition

A hydraulic profile is the energy equation drawn along distance. The hydraulic grade line (HGL) is the height water would rise in a piezometer; the energy grade line (EGL) sits above it by velocity head. Friction slopes both lines downward with flow, a pump steps them upward, and diameter changes alter their separation.

C.2Governing relation or decision logic

HGL = z + p/(ρg) and EGL = HGL + αV²/(2g), both as elevation in metres or feet on the drawing datum. For a known pipe centreline elevation z, gauge pressure is p = ρg(HGL − z). A negative gauge value is possible, but acceptable pressure and vacuum limits belong to the pipe, equipment and governing criteria.

C.3The failure to name

A profile at only one flow or level can hide maximum pressure, minimum pressure and air-pocket conditions. Vertical exaggeration can also make a gentle energy slope look dramatic, while an unlabelled datum makes every elevation impossible to reconcile.

C.4Field review checklist

• Show ground, pipe crown/invert or centreline, HGL and EGL with a named datum.
• Label flow, level condition, pipe diameter/material and loss method.
• Run the combinations that govern maximum and minimum pressure, not only average duty.
• Inspect every summit, outlet, valve and pump transition for its specific failure mode.

C.5Where it fits in the course

§5.1 teaches the profile as a proof; §5.5 reviews a complete station profile.