Concept reference

Specific Speed

C.1Plain-language definition

Specific speed places geometrically similar pump designs on a continuum from high-head, low-flow radial machines toward low-head, high-flow axial machines. Despite its name, common engineering forms are not a shaft speed and may be dimensional. It is a selection and interpretation tool, not a standalone acceptance criterion.

C.2Governing relation or decision logic

One pump form is Ns = N√Q/H3/4, using rotational speed N, flow Q and head per stage H at BEP. Numerical values change drastically with rpm versus rev/s, gpm versus m³/s, metres versus feet, and single- versus double-suction flow conventions. Always state the complete convention.

C.3The failure to name

Comparing two published values without unit convention can make identical hydraulic designs appear unrelated. Using total multistage head instead of head per stage, or total double-suction flow when a reference expects half, shifts the classification.

C.4Field review checklist

• Record the formula, unit convention and suction/stage convention.
• Use BEP quantities from the same tested pump curve.
• Compare only values calculated on identical bases.
• Use specific speed alongside solids passage, NPSH, efficiency and service constraints.

C.5Where it fits in the course

See §6.4 for convention conversion and what pump geometry it predicts.