3D numerical simulations with mass transfer cavitation model are performed in a centrifugal pump (n_q=26 min^-1). Simulation results are compared to measurements of NPSH characteristics and optical measurements of void structures. Transient single bubble cavitation is observed in the experiments with hydraulically smooth blades, whereas in the simulations, the cavitation region appears as stationary sheet cavitation for the entire operation range. Local cavitation structures in the impeller are spatially integrated for measurements and simulation results. The resulting cavitation intensity provides additional information on the extent of cavitation zones at NPSH levels well above NPSH_3%, where the head is yet unaffected by cavitation. The NPSH_3% curve as well as cavitation intensity are well predicted, although local void structures show a mismatch to measurements.