Design and Optimization of a Centrifugal Pump Stage

Introduction

The project involves the design and optimization of a high specific speed centrifugal pump stage. The optimized stage design should deliver the required head with all the stage components and their roughness included while meeting the required suction performance (NPSHR) and axial thrust criteria.

Hence the scope of optimization covers all the stage components such as impeller, volute, pump out vanes (POV) and balance holes (BH). Water is used as the working fluid.

Deliverables

The final optimized design of all the stage components including optimized impeller, volute POV and BH along with a performance that exceeds the best of state-of-the art and yet meets the strict NPSHr and axial thrust requirements.

Impacts

A high efficiency stage that exceeds state-of the-art performance but meets stringent NPSHr and axial thrust requirements.

This project carried out by ADT is for a leading pump manufacturer. The main aim is to produce a new pump design that can deliver class leading efficiency. In addition to meeting the efficiency targets, there are requirements on suction performance and the axial thrust. The optimization process is required to cover all the stage components such as impeller, volute with a splitter/cut-water, pump out vanes (POV) and balance holes (BH) to maximize the performance.

For all the design tasks ADT’s own software package TURBOdesign Suite is used along with the ANSYS CFX for CFD analysis.

Initial Designs

Based on the geometric constraints given by the customer, a meridional geometry of the impeller is defined by using TURBOdesign Pre and the 3D blade is designed by using ADT’s inverse design software TURBOdesign1 (TD1) after defining an appropriate blade loading. A volute is designed using TURBOdesign Volute using exit flow conditions of the impeller as inputs. Standard customer designs of the POV and BH design are used and the whole stage assembly is simulated. With roughness included, the initial stage design obtained from TURBOdesign Suite gave an efficiency of about ~2% points short of target as per CFD predictions. The requirements on NPSHR and axial thrusts are met by the initial design at design and off design operating points. Figure 1 shows the initial Impeller with Balance holes and POV. The next phase involves the detailed optimization of the entire stage design to maximize the performance.

Fig.1: Initial Impeller with Balance Holes and POV

The new centrifugal pump stage would deliver class leading performance. Extensive studies on POVs and BH could be used as reference for future work in this area.

Optimisation of Pump Stage

The first step is the optimization of the impeller, ADT’s optimization software TURBOdesign Optima is used. TD1 performance parameters such as secondary flow factor and profile losses are used as objectives, while cavitation and diffusion parameters are used as constraints. An optimized impeller design with a modified blade loading, meridional geometry and trailing edge stacking is obtained. This optimization that relies on speed of computations in TURBOdesign1 and yet accuracy of surface static pressure can explore a large design space in less than an hour on a single core processor. The TD1 blade loading comparison is shown in Figure 2 where it can be seen that the new design has a fore-loaded shroud and aft-loaded hub. This helps in reducing the secondary flow losses as well as profile losses in the impeller, consequently improving the efficiency. A matching volute is designed and the stage is simulated using CFD. These modifications together produced an efficiency gain of ~2% points. This is followed by a study of the effect of volute splitter thickness.

Three different balance hole sizes are simulated to study the effect on efficiency and axial thrust. Novel POV shape designs were created by using TURBOdesign1, which helped to improve POV performance. A balance hole size study is also performed in combination with POV. From the different studies mentioned above, a POV and BH combination which satisfies the axial thrust criteria with minimal drop in stage efficiency. A simulation is also performed with roughness to assess its impact on efficiency. A multi-phase simulation ensured that the NPSHR targets are satisfied. The final design is predicted to satisfy the performance, NPSHR and axial thrust requirements. Figure 2 shows the final design assembly and Figure 3 shows the velocity streamlines at the design flow rate.

Fig.3. Velocity Streamlines at Design Flow Rate

Conclusion

The detailed optimization of a centrifugal pump stage is carried out and the final design satisfies all the performance targets.