Which pump type is typically associated with low Ns?

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

Answer: (A)

Specific speed describes how a pump’s performance relates to its impeller geometry, speed, flow, and head. Low values of this parameter point to radial (centrifugal) impellers, which are designed to develop higher head at lower to moderate flow. That’s why radial or centrifugal pumps are typically associated with low Ns. Axial-flow pumps, in contrast, move large volumes with relatively low head and have higher Ns, while positive-displacement pumps don’t follow the same Ns framework because their flow is more a function of displacement and speed rather than head-driven performance. So, when you hear low Ns, think radial/centrifugal pumps.