Modern designs for centrifugal pumps consider flow unsteadiness as a main concern. The averaged impact of blade to blade structures and impeller-tongue or impeller-diffuser interactions is considered relevant to accurately address the machine performance. In this framework, the search for a comprehensive way to explain unsteady patterns would be particularly desirable. In the present paper, an analysis of the deterministic interaction between the impeller flow and both the inlet and outlet volute tongues in a double suction centrifugal pump is carried out. The zero velocity condition imposed in both tongues gives rise to an interaction that, up to the authors’ knowledge, has not yet been studied for this type of pumps. A well-tested numerical model, developed by the authors for previous studies with the same pump, has been exploited to predict deterministic correlations in the interaction region. Therefore, the main goal of this paper becomes the evaluation of the nonuniformities induced by both the inlet and the outlet tongues over the blade to blade distributions within the impeller. As a consequence, fluctuation levels in the blade loadings, derived from deterministic nonuniformities of the inlet tongue, can be provided in the relative frame of reference. The collected data using the numerical model can be further employed to establish modeling issues for deterministic stresses. Since more and more often, it becomes a widespread practice for pump manufacturers to include the analysis of the unsteady flow patterns in the design process, the availability of methodological tools to characterize pure unsteady terms in the early stages may be extremely useful for designers. Comparison of deterministic fluctuations with unresolved turbulence intensities is also available in this study, showing the importance of unsteady mechanisms in the modeling of pumping turbomachines.