What is the specific speed and what is it used for?
The specific speed Ns is a dimensionless coefficient following from similarity considerations that describes all operating conditions (Q, N, H) that can be obtained by similar flow conditions in geometrically similar pumps. Consequently, a combination of operating conditions of varying specific speeds cannot be achieved using a geometrically similar pump with similar flow conditions. A change in flow conditions or the geometry of the pump is then required. The main advantage of using the specific speed is the possibility of comparing various pumps (sizes) under varying operating conditions.
For each geometrical pump design a large variety of operation conditions can be covered, by either changing the rotational speed or size of the rotor. However, one geometrical design is not sufficient in order to cover all possible operating conditions. Different types of pumps e.g. radial or axial with similar rotor size D yield different performance curves. For each design, the pump efficiency is maximized for one specific capacity Q and is referred to as the Best Efficiency Point (BEP). It might not be clear what type of pump is suitable for a given capacity. However, another coefficient which is independent of the rotor diameter can be obtained by combining the head coefficient and the flow coefficient. The rotor diameter D is eliminated by raising the flow and head coefficient to the powers 1/2 and 3/4 respectively and dividing the former expression with the latter.
Ns is the non-dimensional specific speed and describes all operating conditions that can be covered by similar flow in geometrically similar pumps (Q, N, H). Thus, any change in specific speed requires a corresponding change in pump geometry or a change in flow conditions. Operating conditions of different specific speeds cannot be covered by similar machines with similar flow conditions. A pump with a certain geometric design does not only satisfy one particular specific speed, but rather a range of specific speeds when one (or multiple) operating conditions are changed. However, this implies that the flow conditions also change and this may not always result in high performing pumps.
Pump specific speed overview.
