Specific Speed
What are Pump Specific Speed (Ns) and Suction Specific Speed (Nss)?
Specific speed and suction specific speed are useful indicators of pump hydraulic design. They help engineers understand the type of impeller, expected operating behavior and suction sensitivity.
What is pump specific speed Ns?
Pump specific speed describes the relationship between rotational speed, flow rate and head. In a simplified way, it indicates the hydraulic type of the impeller.
Depending on the value, a pump may behave more like a radial-flow, mixed-flow or axial-flow machine.
What is suction specific speed Nss?
Suction specific speed describes the relationship between speed, flow and required NPSH. It is used to evaluate suction performance and compare suction designs.
Why unit convention matters
Ns and Nss are not universal values unless the unit convention is clearly defined. Some calculations use Q in m³/h, others use m³/s or gpm. The result can differ by large factors.
What affects Ns and Nss?
- pump speed,
- BEP flow rate at maximum impeller diameter,
- head per stage at the same BEP,
- NPSHr at the same BEP,
- impeller eye diameter,
- single-suction vs double-suction design.
Which operating point should be used?
For both pump specific speed and suction specific speed, use the best efficiency point (BEP) at the maximum impeller diameter, not the rated duty point. For multistage pumps, use head per stage. For double-suction impellers, use one half of the total flow when calculating suction specific speed because the flow basis is per impeller eye.
Practical use
Specific speed is useful during pump selection, hydraulic review and comparison of offered pumps. Suction specific speed is especially important when evaluating low-NPSH applications and suction stability.
Engineering caution
Very high suction specific speed may look attractive because it suggests low NPSH requirement, but it can also indicate sensitivity to suction recirculation or unstable operation. Always evaluate it together with pump curve, NPSH margin and operating range.