Predicting, Improving and Visualizing Cavitation Characteristics of First-Stage Impellers in High-Speed, High-Energy Pumps

This paper describes the development and verification of a new first-stage impeller using an automated and parameterized process of reshaping the vane leading edge. By utilizing Computational Fluid Dynamics (CFD), several design iterations can be performed in a short period of time driven by the numerical NPSHi curve — which is an indication of the NPSH at which the first cavitation bubbles will start to form [1]. Based on the original (parent) impeller hydraulic, five designs were created with a variety of vane leading edge shapes. To ensure the Best Cavitation Point (BCP) is located near rated condition and the NPSH3 requirement is met at maximum operating flow, the most promising design was selected and manufactured at full scale using rapid prototyping. During full scale, reduced speed, flow visualization testing in a dedicated test loop of a reworked and a non-reworked original impeller and the new impeller design, it was confirmed that the results were in line with the numerical predictions; the field impeller showed cavitation formation that matched the observed damage pattern and the new impeller design demonstrated a significant decrease in cavity lengths. For the condition of field NPSHA at 80 and 100 percent rated flow, cavitation formation was not observed anymore for the new design. The lesson learned from this study is that the NPSH3 requirement alone is an inappropriate criterion for high-speed, high-energy centrifugal pumps. It is advised to map the development of cavitation for all high-energy pumps having high eye peripheral speeds. Besides this, it can be concluded that CFD is a viable tool in assessing incipient cavitation behavior and can be considered an alternative to flow visualization testing.