Impeller Classification and Specific Speed:

In general, centrifugal pumps are divided into three categories according to their specific speed. (Impeller Classification)

1. Radial-Vane
2. Mixed-flow
3. Axial-flow

Specific speed (Ns) is calculated using the following formula:

N: Rotational speed of the pump in RPM unit

Q: Volumetric flow rate at the best point of operation of the pump (BEP) per unit m3 / hour. In double suction pumps, the flow rate must be divided by 2.

H: The pump head (at each stage a impeller) at the point of best pump operation (BEP) per unit m. In multi-stage pumps, the pump head is divided by the number of stages.

Pump impeller classification according to specific speed

Example: Get the specific speed of a pump with a head of 30.5 m and a flow rate of 1135.7 m3 / hr at 500 rpm.

• Depending on the specific speed obtained, the pump impeller is selected from the radial flow type.
• In constant head and flow, higher specific speeds lead to higher rotational speeds, resulting in smaller impeller diameters.
• At constant head and rotational speeds, higher specific speeds lead to increased pump flow rate.

Reference: Exxon Mobil