Suction specific speed
Suction specific speed nss as defined in EN 12723 is used to classify the suction characteristics of centrifugal pumps. It is derived from the similarity conditions for cavitation and calculated on the same formal basis as the specific speed ns. The SI unit for suction specific speed is rpm, its numerical number is established using the equation below. Suction specific speed is:
n in rpm Pump speed
Qopt in m3/s Flow rate at ηmax
NPSH3opt in m Required NPSH at Qopt for a 3 % head drop in the first pump stage and max. i impeller diameter
On double-suction pumps, the flow rate at BEP (Qopt) is inserted for one impeller half.
In the USA the suction specific speed is denoted by the symbol Nss:
The suction specific speed enables an independent evaluation and comparison of centrifugal pumps' suction characteristics independent of the individual design data (rotational speed, flow rate, pump size). In general, the following applies: the higher the suction specific speed, the better the suction characteristics of a pump.
While suction specific speeds of around 240 rpm can be reached with carefully designed and manufactured impellers (not specifically designed as suction impellers), higher values can be achieved with pumps designed with specific suction impellers and even higher values with pumps equipped with inducers.
When pumps are designed with very high suction specific speeds for use in systems with very low NPSHa (available net positive suction head), it must be taken into account that compromises with regard to the hydraulic properties, e.g. efficiency, may be required under certain circumstances.
An explicit dependency between suction specific speed and specific speed ns does not exist for centrifugal pumps with radial impellers.
Taking an average value of approx. 200 rpm for the suction specific speed as can be achieved by a standard pump without any special features designed to improve its suction characteristics, it is possible as early as at the planning stage to estimate the system's minimum NPSHa level at a given flow rate Qopt for a chosen pump speed or the maximum possible pump speed for a defined NPSHa.
