Concept reference

Velocity Head

C.1Plain-language definition

Velocity head is the height equivalent of a stream’s kinetic energy per unit weight. It is the vertical gap between the energy grade line and hydraulic grade line in a uniform section. Because it varies with velocity squared, a smaller pipe makes both this gap and most fitting losses grow rapidly.

C.2Governing relation or decision logic

hv = αV²/(2g), in metres (ft), with V = Q/A. Most routine turbulent-pipe work takes α = 1. A local loss uses the same scale: hm = K V²/(2g), but the reference velocity must be the one used to define that particular K.

C.3The failure to name

Using upstream velocity with a coefficient tabulated on downstream velocity can materially change a reducer or expander loss. Another common error is to add outlet velocity head after already charging a full exit loss into a large reservoir.

C.4Field review checklist

• Compute velocity from the actual internal flow area, not nominal diameter.
• State the section and kinetic-energy coefficient used.
• Confirm the reference velocity behind every K value.
• At each boundary, decide explicitly whether residual kinetic energy remains or is dissipated.

C.5Where it fits in the course

See §2.2 for EGL/HGL and §4.3 for transitions and exits.