The term impeller specific speed is used to classify
impellers of various types based on their performance. Specific speed is
defined as that speed at which a geometrically similar pump would deliver one
unit of flow to one unit of head.
This value tells us something about the type of pump. Is it
a radial type pump, which provides high head and low flow or an axial or
propeller type pump, which provides high flow but low head
There are varieties of pump designs that are available for
any given task. Pump designers have needed a way to compare the efficiency of
their designs across a large range of pump model and types. Pump users also
would like to know what efficiency could be expected from a particular pump
design. For that purpose pump have been tested and compared using a number or
criteria called the specific speed (Ns), which helps to do these comparisons.
Equation below gives the value for the pump specific speed;
Specific speed, Ns = N(rpm) x sq. rt. of flow rate-Q / (Head-H)¾
Specific speed is a dimensionless quantity.
Specific speed is indicative of the shape and characteristics
of an impeller. Impeller form and proportions vary with specific speed but not
the size. It can be seen that there is a gradual change in the profiles from
radial to axial flow configuration. Studies indicate that a pump efficiency at
the best efficiency point (BEP) depends mainly on the specific speed, and a
pump with specific speed of 1500 is more efficient then the one with specific
speed of 1000.
Relation of Specific Speed to Flow, Head, and Efficiency
Beyond the simplistic correlation of impeller geometry and
pump specific speed just illustrated, some generalizations can be drawn for the
numerical values of this index number. Put succinctly, higher pump head
generation is associated with lower values of specific speed, at the expense of
lower flow rates and lower relative hydraulic efficiency. Conversely, low
pressure (head) values are experienced with higher values of specific speed,
accompanied by higher values of flow rate at correspondingly relatively higher
hydraulic efficiencies. As in most cases, there are tradeoffs that must be
weighed when selecting the impeller style, i.e., specific speed. Each
individual hydraulic service must be carefully evaluated to insure that the
proper pump, impeller style, and rotational speed, are accurately addressed