Concept reference

Head — Energy per Unit Weight

C.1Plain-language definition

Head is energy divided by fluid weight. That choice lets elevation, pressure, velocity, pump input and losses share one vertical unit and be added directly. A pump produces head; the corresponding pressure depends on the liquid density. This is why a water pump curve in metres does not need to be redrawn merely because the site elevation changes.

C.2Governing relation or decision logic

Pressure head is hp = p/(ρg), elevation head is z, and velocity head is V²/(2g); all are metres of fluid (ft of fluid). Conversely p = ρgh in Pa. For water near ordinary ambient conditions, 1 m of water is about 9.8 kPa, but final conversions should use the design density and stated gravity.

C.3The failure to name

Treating head as pressure causes errors when the fluid density changes, while mixing metres of water with pressure from a different liquid hides the same mistake. Gauge and absolute pressure are also not interchangeable when vapour pressure or NPSH is involved.

C.4Field review checklist

• Label every energy term as head, pressure or energy and convert before adding.
• State the fluid density or specific gravity used for pressure conversion.
• Use one elevation datum and one pressure reference.
• Reserve absolute pressure for calculations, such as NPSH, that physically require it.

C.5Where it fits in the course

§2.1 builds the energy bookkeeping; §2.4 treats pressure and unit conversions.