Trimming law and mechanism of efficiency decrease in double-suction centrifugal pump with cut impeller

Abstract: Double-suction centrifugal pumps are widely used in water diversion, irrigation, drainage, and water supply engineering. In order to save energy, pump impellers often are cut a little if a double suction centrifugal pump head is much higher than the required one in a water delivery system. To investigate the influence of impeller cutting on the hydraulic performance of a double suction centrifugal pump, a general three-dimensional simulation of turbulent fluid flow was presented. The RNG κ-ε turbulence model and SIMPLEC arithmetic were adapted to analyze the external characteristics and the internal flow field of the pump equipped with a cut impeller. The impeller was trimmed seven times successively in this study, then all the cut impellers were installed into a 20SH9 type pump casing respectively. The entire computational domain was divided into 1 700 000 grid cells. According to the simulation results, it was found that with the increase of the cutting value, the pump operating point on the performance curve significantly moved towards the lower flow rate, and the best efficiency gradually decreased. The hydraulic losses induced by the impeller trimming were mainly generated inside the impeller. The vortex area in the impeller flow passage increased after impeller cutting due to the weak control of the blades over the flow. On the other hand, the increase of the tongue gap did not result in the increase of the hydraulic loss in the volute. This study also found that for a fixed cutting value, the pump head and shaft power predicted by CFD were approximately equal to the value calculated by the conventional trimming law at large flow rates. However, the pump head and shaft power were over-predicated by the conventional trimming law at low flow rates. Therefore, the impeller cutting values should not be selected simply by the trimming law if a double-suction centrifugal pump often runs at low flow conditions. The results of this study provide new references for the efficient operation of a double suction centrifugal pump in engineering applications.