A Review Of NSS Limitations: New Opportunities

Abstract

Specifications of some pump users limit the suction specific speed (Nss) with the objective of selecting more reliable centrifugal pumps. This limitation is based on a statistical evaluation of pump failures published by Hallam (1982) that seemed to show a relationship between the value of Nss and the probability of failure. It was stated that intensive suction recirculation had caused the failures of the pumps with high suction specific speed, but the root causes of these failures were not examined at that time. Investigations presented by Bunjes and op de Woerd (1984), Breugelmanns and Sen (1987), and Guelich and Egger (1992) have since shown that suction recirculation depends on a variety of parameters. Experimental investigation by Stoffel and Jaeger (1996) with high and medium Nss-impellers showed that no significant differences of vibrations, shaft deflection, and other dynamic loadings were detected. Therefore the suction specific speed cannot be used as the one and only parameter for the prediction of the reliability of a centrifugal pump. The effect of preventing the use of high suction specific speed pumps can result in having to use a less efficient and more expensive pump, without seeing any measurable benefits in pump reliability.