Define the specific speed of a pump and what its value indicates about the type of pump.

• A. Ns = n√Q / H^(3/4) (dimensionless); indicates pump type (low Ns for radial/centrifugal, high Ns for axial)
• B. Ns = Q/H
• C. Ns = ρgH/ΔP
• D. Ns equals efficiency times head

Answer: (A)

Specific speed is a dimensionless grouping that combines how fast the pump is running, how much fluid it moves, and the head it develops to indicate the impeller type and duty the pump is suited for. The standard form is Ns = n√Q / H^(3/4). This expression is the best because it brings together the rotational speed (n), the flow rate (Q), and the head (H) in a way that remains dimensionless and directly reflects the pump’s geometric similarity across sizes. In practice, a low specific speed points to radial (centrifugal) impellers designed for higher head and relatively lower flow, while a high specific speed points to axial-flow impellers that handle large flow at lower head. The other options fail because they omit the essential combination of speed, flow, and head or replace it with an inappropriate relationship.