This paper presents an experimental investigation of flow instabilities in a centrifugal pump impeller at low flow rates. The measurements of pump hydraulic performance and flow field in the impeller passages were made with a hydraulic test rig. Analysis of Q-∆P-η data and flow structures in the impeller passages were performed. In the present work, the effect of various flowrates on centrifugal pump impeller performance was analyzed based on measured parameters. To improve the pump performance the impeller’s geometry was modified with positioning a curved spacer at the impeller suction side. This research aims to investigate the effects of each inlet curved spacer model on pump performance improvement. The hydraulic performance and cavitation in the pump have been tested experimentally. The flow field inside a centrifugal pump is known to be fully turbulent, three dimensional and unsteady with recirculation flows and separation at its inlet and exit. The effectiveness of various curved disk models has been analyzed numerically using a three-dimensional Navier-Stokes code with a standard k-ε turbulence model (Hermez et al. 2016 ASME International Mechanical Engineering Congress and Exposition). The successful curved spacer model was then manufactured, and positioned at the inlet section of reference impeller for testing and analysis in the present study. Based on the numerical simulation and experimental tests, the following conclusions can be drawn: (1) The experimental results provided evidence for the influence of flow instabilities on pump hydraulic performance. (2) The impeller inlet geometry had important influence on improving the performance of a centrifugal pump; e.g. performance improvement was achieved by separating the inlet flow region into two lanes. (3) Improvement was observed for inlet static and total pressure values, mostly at low flowrates trials. (4) A uniform flow upstream of the impeller inlet was helpful for improving cavitation conditions in the pump.