Which pump type is typically associated with high Ns?

• A. Radial or centrifugal
• B. Axial-flow
• C. Positive displacement
• D. Piston

Answer: (B)

Specific speed (Ns) is a way to relate a pump’s speed, flow, and head to categorize the type of pump. A high Ns value means the pump is designed to move a large amount of water at relatively low head. Axial-flow pumps are built like propellers and are optimized for very high flow with low head, which pushes their Ns into the high range. That combination—high flow and low head, achieved at higher speed and with a large flow area—is what gives axial-flow pumps their high Ns.

Radial (centrifugal) pumps, in contrast, are designed to generate higher head at lower flow, yielding lower Ns. Positive displacement and piston pumps don’t follow the same dynamic-pump Ns pattern; they are driven by stroking or fixed displacement and aren’t characterized by high Ns in the same sense. So the type typically associated with high Ns is the axial-flow pump.