The instabilities, occurring in pumps operating at partial flow rates, may directly cause several negative effects, such as vibration, noise, mechanical damage. The present study is focused on the investigation of the instabilities developing in the vaneless diffuser of a radial flow pump. The prediction of this kind of complex phenomena by numerical simulation usually involves complex (3D unsteady) and expensive calculations. The main goal of this paper is to propose and evaluate some “low cost” numerical or analytical approaches to predict the arising and the characteristics of rotating stall instabilities. For that purpose, two-dimensional numerical unsteady calculations were used. The boundary condition for the diffuser inlet was based on the flow field at the impeller outlet and static pressure was set at the diffuser outlet. The simulations have been managed for several flow rates conditions. The results of the numerical simulation are discussed and compared to i/ an already existing database (including PIV and pressure measurements), ii/ results of a 2D linear stability analysis.