Numerical assessment of the performance of a wastewater pump under off-design operating conditions

Abstract

Pumps are often key elements of industrial processes, and are commonly used to move water in municipal water schemes and in wastewater treatment of industrial effluents, representing a substantial energy cost. In the wastewater industry, pumps often have a single blade impeller with non-clog centrifugal designs, as the main design criterion is to allow the passage of solid objects through the vane without blockage. Due to the asymmetric geometry of the vane, the vibrations induced by the impeller radial thrust may be severe, significantly shortening the life of bearings and seals. This increases leakage through the pump components, and thus affects the device performance. In addition, the implementation of more stringent energy optimization strategies to reduce the operational costs, such as demand-side management, including load shifting to off-peak periods and adoption of variable speed drives, will require industrial machinery to work at part load more often. As a result, it is likely that pumping systems will be run at off-design conditions, with significant effects on the energy consumption. The scope of this thesis is to assess the performance of a wastewater pump at off-design conditions. A high fidelity three-dimensional computational model has been developed from a commercially available version of a single-blade wastewater pump (Sulzer XFP PE-2 150E CB 1.1). Numerical simulations are run using unsteady Reynolds-Averaged Navier-Stokes simulations with ANSYS Fluent and results are compared to experimental and published data to provide confidence in the modelling approach. The transient behaviour of the wastewater pump is also assessed varying the back-pressure and the rotational speed of the pump in a one-dimensional pipeline system. While it is found that the transient behaviour does not have a significant impact on the overall performance of the pump, the transition strategy between two operating conditions has a significant impact on the peak power and the total energy input. The effect of the axial gap between the impeller and the casing is investigated and non-dimensional coefficients are defined to estimate the performance drop with the gap size. It is found that the efficiency can drop up to 13.5%/mm. The outcomes emphasize the need to include specific maintenance protocols to ensure axial clearance is kept close to design targets.