In this paper, a deterministic stress decomposition is applied over the numerical 3D flow solution available for a single volute centrifugal pump. The numerical model has proven in previous publications its robustness to obtain the impeller to volute-tongue flow interaction and it is now used as starting point for the current research. The main objective has been oriented towards a detailed analysis of the lack of uniformity in the flow that the volute tongue promotes on the blade-to-blade axisymmetric pattern. Through this analysis, the fluctuation field may be retrieved and main interaction sources have been pinpointed. The results obtained with the deterministic analysis becomes of paramount interest to understand the different flow features found in a typical centrifugal pump as a function of the flow rate. Moreover, this post-processing tool provides an economic and easy procedure for designers to compare the different deterministic terms; also giving relevant information on the unresolved turbulence intensity scales. Complementarily, a way to model the turbulent effects in a systematic way is also presented, comparing their impact on the performance with respect to deterministic sources in a useful framework, that may be applied for similar kind of pumps.