Suction Specific Speed (N_ss) Definition

What is Suction Specific Speed (N_ss)?

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

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

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

High N_ss 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.