In this work, we have studied how gas accumulates in an industrial centrifugal pump under various steady-state two-phase flow conditions. Thereby, we considered both horizontal and vertical pump installation positions. Phase fractions within the impeller region of the pump have been quantitatively disclosed using high-resolution gamma-ray computed tomography (HireCT) and applying time-averaged rotation-synchronized CT scanning technique. The study was made for inlet volumetric gas flow rates between 0% and 5%. To account for different inlet flow conditions, which are assumed to occur during unwanted gas entrainment by hollow vortices, we produced disperse and swirling gas–liquid inlet flows. In this way, the influence of inlet flow boundary conditions on the pump performance as well as gas fraction distributions and gas holdup within the impeller wheel region could be successfully analyzed and compared with respect to the impeller alignment. It was shown that the installation position offers only a minor effect on the pump performance in comparison to the inlet flow conditions. In addition, for the first time, thin gas films at the pressure side of the impeller wheel blades could be visualized in an industrial centrifugal pump.