Suction specific speed, also known as cavitation specific speed, is an index related to the suction performance of centrifugal pump. The suction performance of centrifugal pump is a very important factor in the optimal design of the pump. The suction performance not only directly affects the investment cost of the user, but also has a significant impact on the long-term safe and stable operation of the centrifugal pump unit.
Generally, the definition and calculation formula of suction specific speed are only given in the centrifugal pump design manual or relevant standards and specifications at home and abroad. Some manuals also give the approximate relationship between the value of suction specific speed in different ranges and the anti cavitation performance and efficiency of the centrifugal pump. As for the impact of the value on the performance and safety reliability of the centrifugal pump in the full flow range, it is not involved. However, in engineering practice, the value of suction specific speed actually affects the performance and safety reliability of the centrifugal pump.
For this reason, many domestic and foreign engineering companies or design institutes will judge whether the design of a pump is reasonable according to the limit value specified in UOP code  or SH / t3139  or SH / t3140  and other standards for the suction specific speed of the pump (when exceeding the limit value range in the above codes and standards, it is considered that the design of the centrifugal pump is unreasonable). Such understanding and operation mode is too simple. The author thinks it is necessary to interpret this and study its influence on the performance and safety reliability of centrifugal pump.
Suction specific speed s is calculated by the flow rate at the best efficiency point under the maximum impeller diameter and given speed. It is an index related to suction performance of centrifugal pump. Suction specific speed is a measure of the sensitivity of a centrifugal pump to internal backflow
N = speed in R / min;
Q = flow, m3 / S (US system: gallon / min); for single suction impeller, q is the total flow, for double suction impeller of centrifugal pump , q is half of the total flow;
Npsh3 = required NPSH in M (FT).
Note: multiplying the specific suction speed derived from metric units by a factor of 51.64 is equal to the specific suction speed in American units, which are usually represented by the symbol NSS.
Q in the formula is the flow at the maximum impeller diameter, given speed and the best efficiency point. When it is a single suction impeller, q is the total flow; when it is a double suction impeller, q is half of the total flow.
Npsh3 in the formula is the necessary NPSH margin corresponding to the maximum impeller diameter (3% head drop) under the given speed and the optimal efficiency point flow. In engineering practice, for low specific speed centrifugal pump, the necessary NPSH of the corresponding pump is different under the same speed and different impeller diameter. The maximum impeller diameter corresponds to the minimum NPSH, while the minimum impeller diameter corresponds to the maximum NPSH.