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Cold forming and welding of stainless steel sheets is becoming a leading technology in mass production of multistage, radial flow pumps. The stages of these machines are usually manufactured by resistance spot welding of cylindrical (i.e. 2D) blades on flat surfaces. Such stages commonly feature the so-called “compact design”, i.e. the maximum stage diameter is close to the impeller one. Accordingly, an annular chamber connects the impeller and the return channels that drive the flow to the stage outlet. Although competitive in terms of production cost, this technology introduces strong constraints to the fluid dynamic design. Some design methods specifically conceived for compact stages made by moulding or casting technologies are presented in the literature. However, these methods result in blade geometries that are not suited to the present manufacturing technology, due to their complex 3D shape. In the light of this, a dedicated approach is needed for the hydraulic design of compact stages for sheet metal pumps. The present work reports and discusses experimental data on performance and efficiency of several industrial machines, featuring compact stages made from sheet metal. The pumps considered in the study feature a high technological level, as assessed by comparing the measured efficiency with reference values provided by Reg. 547/2012 EC. Nevertheless, the performance measured on these machines does not meet the data suggested in the literature for conventional pumps. Consequently, a theoretical analysis of the compact stage performance is presented in order to explain the difference between traditional and compact machines. In particular, the influence of the dynamic head contribution on the total head rise and on the efficiency of the compact design stages is discussed. The theoretical and experimental results suggest that the design methods for traditional machines are not suitable for compact stages of high efficiency, sheet metal pumps.

Performance analysis of compact multi-stage pumps manufactured from sheet metal