Three-dimensional (3D) unsteady Reynolds averaged Navier-Stokes (URANS) flow simulations are conductedto investigate the highly unsteady flow field at part-load operation of a centrifugal pump. By the availability of unsteady flow field measurement data in the impeller wake region, a thorough validation of the simulation method is performed. Grid independence of the results is ensured. Unsteady characteristics in terms of head and shaft power as well as transient blade loads are evaluated to assess the unsteady pump performance. Significant mis-loading of the blading is revealed when one blade passes the volute tongue and associated to the strong unsteady and 3D flow field in the impeller-volute tongue region. Negative radial velocity in the tongue region is the origin of a vortex at the blade pressure side and a subsequent pressure drop that leads to even temporally negative blade loading. The results provide a detailed insight in the complex part load flow field that might be utilized for an improved pump design. As a valuable secondary outcome, a comparison of results obtained by two widely used computational fluid dynamics (CFD) codes for pump flow simulation is provided, i.e. the commercial code ANSYS CFX and the branch foam-extend of the open source software OpenFOAM. It is found that the results of both methods in terms of unsteady characteristics as well as local ensemble-averaged velocity field are consistent.