Abstract: A reverse running centrifugal pump is one of the attractive choices in micro-hydropower development and industrial pressure energy recovery.One of the main problems in utilizing pump as turbine (PAT) is that the performance of PAT is usually not ideal due to the impeller with the routine backward curved blades which do not match well with turbine running condition.A cost effective way for solving this problem is to redesign impeller with forward curved blades from turbine working condition while the other components do not undergo any modifications.Impeller diameter is one of the main factors in impeller design.Therefore, research on the relationship of diameter and design flow rate and the influence of impeller diameter on PAT’s performance is useful.In this paper, based on the constant velocity moment theory, calculation expression of volute constant within spiral volute was first developed and the velocity moment before impeller was acquired.Next, a relationship between impeller diameter and the critical design flow rate was deduced when the blade inlet angle was set to 90°.To perform research on how impeller diameter influenced PAT’s performance with forward curved blades, the outlet angles of three impellers with diameters of 235, 245 and 255 mm were calculated based on no-swirling outflow at the critical flow rate, respectively.And three impellers with the inlet angles of all equal to 90° were designed by using ANSYS Bladegen software.Numerical simulation and analysis of the three PATs were performed using a verified computational fluid dynamics (CFD) technique.By comparison of three performance curves obtained by CFD, we found that the trend of three performance curves was similar and the impeller diameter had an obvious effect on the performance of PAT.The flow rate, required pressure head, generated shaft power, and efficiency at best-efficiency point (BEP) increased with increasing impeller diameter.The flow rates of three PATs at BEP were about 100 , 110 and 120 m3/h, respectively as impeller diameter varied from 235 to 245 and 255 mm.Comparing the results obtained by CFD, we found that the flow rate at BEP was slightly larger than the designed flow rates predicted by using theoretical formulas.It was confirmed that the theoretical deduction was reasonable.In whole operating range, efficiency was lessened at small flow rate while increased at large flow rate in accordance with increasing impeller diameter.In operating range of flow rate lower than 90 m3/h, there was a sharp drop of efficiency in accordance with decreasing flow rate, which was primarily attributable to increase of hydraulic loss due to poor off-design performance and increase of disk friction loss due to increasing impeller diameter.The maximum overall efficiencies were obtained as 70.5%, 71.72% and 72.84% with impeller diameters varying from 235 to 245 and 255 mm, and PAT’s efficiency at BEP increased by 1.73% and 3.32%, respectively.The PAT’s efficiency with 255 mm inlet diameter maintained more than 70% at flow rates varying from 90 to 140 m3/h.The high efficiency operating range was more flat in accordance with increasing impeller diameter.This study provides a convenient way to design the special impeller with forward curved blades which can dramatically improve efficiency of PAT.