Innovative impeller design for double suction centrifugal pumps

World’s energy consumption is increasing year by year driven by the industrial and the tertiary sector, where most of the electrical energy is consumed by electrical motors (65%). The latter drive pumps (22%), most of which are centrifugal pumps (73% - world market = 20 billion euro per year). Due to their wide use, a great effort has been spent in the last decades in trying to improve the selection and the efficiency of pump systems with the aim to reduce their energy consumption. Besides, it has been found out that among all the rotating devices in a process plant, centrifugal pumps typically have the best overall potential for energy savings. In this work, a variety of activities has been carried out in order to introduce innovation in the field of the hydraulic turbomachinery. Firstly, an innovative impeller for double suction centrifugal pumps will be presented and the techniques and methodology adopted to make this geometry feasible will be described. The novel geometry embodiment includes an innovative arrangement of the impeller vanes. The novel impeller has been designed with the purpose of retrofitting the impeller of a conventional pump, preserving its initial size and specific speed. Comparing the baseline and the novel geometry, the latter shows an improvement of the hydraulic efficiency (+1.2%) and a remarkable reduction of the slip phenomenon at the outlet of the impeller (sigma- slip factor +8.5%). Thus, it is possible to provide a larger pressure rise with the same size of the pump or alternatively to choose a more efficient and compact system. Moreover, two different prototypes with different number of blades have been designed and their performance has been studied numerically, by means of 3D URANS simulations. Thereafter, they have been manufactured using the lost-foam casting technique, and tested experimentally thanks to the collaboration with the Hydraulic test rig of Nuovo Pignone, Bari (Italy). The novel design is now protected by a patent application. Furthermore, given the increasing use of centrifugal pumps as turbines (PaT) in water distribution systems, the baseline and the novel geometry have been simulated as turbines with the aim to investigate their performance in reverse mode. This allowed a description of the slip phenomenon at the outlet of centripetal runners. This parameter has proved to be helpful for a 1D performance prediction model based on loss analysis, which works starting from the machine geometrical information; indeed, it allows to minimize the error in predicting the characteristic curve at part-load (-8\% error compared to Busemann and Stodola formulation) and at the best efficiency point (-6\% error compared to models which neglect this phenomenon). Finally, in order to enlarge the operative range of hydraulic pumps and turbines working under cavitating conditions, a passive cavitation control system (PCS) has been proposed. The aim is to guarantee a high performance level, comparable to the single-phase conditions, for these machines in the regions where they usually are affected by performance degradation. The system proposed has been applied to a symmetric 2D hydrofoil which is an archetype of the leading edge of a blade. Even though it appears to induce a loss of performance in terms of lift and drag, it is shown to be helpful for reducing the vapour volume cavity up to 93%. Thus, the introduction of this system to a real impeller (e.g., a centrifugal pump or a turbine) shall be positive to enlarge their operative field towards low suction pressure providing at the same time acceptable level of losses.