Suction Specific Speed (Nss) Definition

What is Suction Specific Speed (Nss)?

Suction Specific Speed (Nss) is an index of impeller design that describes the relationship between the rotating speed of the impeller, the head generated by the impeller, and the NPSHr of the impeller.

Suction Specific Speed (Nss) is closely related to Specific Speed (Ns). While Ns describes an impeller design relative to head and flow, Nss describes an impeller design relative to head and NPSHr.

Nss is calculated based on flow and NPSHr at BEP for a given impeller design. The following formula, courtesy of Wikipedia, is used to calculate Nss.

According to the Hydraulic Institute, Nss should be maintained below 8500 for most applications, and some customers may set hard limits on the maximum Nss values they will accept. Other standards and design approaches consider Nss values as high as 12000 to be acceptable.

High Nss values may be indicative of an impeller eye design that is oversized. An oversized impeller eye can have a negative impact on impeller efficiency and result in pumps that are prone to cavitation.

Jacques Chaurette’s excellent article Centrifugal Pump Specific Speed Primer and the Affinity Laws provides a great deal more detail on this topic.