Specific Speed (ns) - Specific speed is an index of pump performance at the pump's best efficiency point (BEP) rate of flow, with the maximum diameter impeller, and at a given rotative speed. Specific speed is used for many design purposes including efficiency prediction and classifying the relative discharge of an impeller as radial, mixed or axial and expressed by the following equation: Specific Speed Equation.
Radial flow – Pumps of this type with single inlet impellers usually have a specific speed below approximately 90 (4500) and, with double suction impellers, a specific speed below approximately 135 (7000). This type is commonly referred to as a centrifugal pump. In In centrifugal pumps the liquid enters the impeller at the hub and flows radially to the periphery, exiting perpendicular to the rotating shaft. - Figure 18.104.22.168
Mixed flow - This type of pump has a single inlet impeller where the flow enters axially and discharges in a mixed axial and radial direction. Pumps of this type usually have a specific speed from approximately 90 (4500) to 200 (10,000). - Figure 22.214.171.124
Axial flow - A pump of this type, sometimes called a propeller pump, has a single inlet impeller with the flow entering axially and discharging nearly axially. Pumps of this type usually have a specific speed above approximately 200 (10,000). - Figure 126.96.36.199