The purpose of this paper is to examine the failure problems encountered during the operation of the rainwater pump impellers installed in a wastewater station. The rainwater pumps are specially designed to operate in wastewater plants. The fibbers and wastes are discharged together with rainwater during storms with this type of pumps to avoid the flood of the wastewater station. Several problems have occurred in service due to the fibbers clog the gap between the impeller blades and the pump casing. The analysis of the catastrophic events associated to the rainwater pumps installed in a wastewater station is performed. As a result, it has been identified that the catastrophic events are due to the brittle fracture of the fasten bolts connecting the impeller blades and the pump hub, respectively. Analytical and numerical analysis were perform in order to determine the maximum shear stress on the fasten bolts. As a conclusion the partial clogging of the gap between one impeller blade and pump casing, inducing the lock of the blade and leading to catastrophic failure.
The pump inlet casing deflects the fluid flow from the inlet pipe, mainly arranged normally to the axis, into the axial direction. The pump inlet casing can take a large variety of geometrical shapes from curved pipes to three-dimensional elbows. The hydrodynamic field induced by symmetrical suction elbow (SSE) at the pump inlet is experimentally investigated in order to quantify it effects at the pump inlet. The pump test rig and the experimental setup are detailed. A SSE model is installed at the pump inlet. Laser Doppler Velocimetry (LDV) measurements are performed on the annular cross section located at the pump inlet. As a result, the map of the velocity field is determined quantifying the non-uniformities induced by SSE. Next, the full 3D turbulent numerical investigation of the flow in the SSE is performed. The numerical results on the annular cross section are qualitatively and quantitatively validated against LDV data. A good agreement between numerical results and experimental data is obtained. The hydrodynamic flow structure with several pairs of vortices is identified examining the vorticity field. However, two pairs of vortices with largest contribution to the flow non-uniformity are examined. Three parameters are considered to quantify the evolution of each vortex center: two geometrical quantities (e.g. the radial and angular coordinates) and one hydrodynamic (the magnitude of vorticity). The largest values of the vorticity magnitude are identified in the center of both vortices located behind the shaft. The 3D distribution of the vortex core filaments is visualized. As a result, the 3D geometry of the SSE and the pump shaft are identified as the main sources of the flow non-uniformity at the pump inlet. This deep analysis of the 3D flow field induced by the SSE paves the way towards an improved geometry with practical applications to real pump and pump-turbines.
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