Specific suction speed (Nss) is a non-dimensional number or index which defines the suction characteristics of a pump.
It is calculated by the same Ns formula, replacing H by NPSHreq. In multiple stage pumps, NPSHreq is based on the first stage rotor. Specific suction speed is commonly used as a base for calculating the safe capacity operating range for a pump.
The higher the Nss, the narrower the safe operating range of its point of best efficiency BEP. Numbers vary from 3,000 to 20,000. Most users prefer that their pumps have an Nss ranging from 8,000 to 11,000, for optimum and trouble-free operation.
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