Impeller Manufacturing: Design for Machining

Today, there are 3 primary methods Solar Turbines, Inc. uses for manufacturing shrouded centrifugal impellers for the Oil and Gas industry. Impellers can be made by an investment casting process, single piece integrally-machined using point milling, or fabricated using a flank milled, open faced impeller and brazing a shroud in place. Investment cast impellers give the aero designer the greatest flexibility in the design, since the designer doesn’t need to be concerned about tool access or other manufacturing constraints. It is common to use this process for any impeller. Single piece integrally-machined impellers are relatively straightforward for high flow coefficient impellers, as the wide flow-path provides plenty of room for tool access. For lower flow coefficients, brazed shrouded impellers can be made very precisely, but at a substantially higher cost than the other methods. Each of these manufacturing methods also has a cost and aerodynamic performance associated with them. With the advent of high-speed machining and better cutting tools, integrally-machined impellers can offer an alternative manufacturing process over cast or fabricated impellers. However, determining the ability to integrally-machine an impeller historically is done late or even after the detailed design process. This can lead to costly redesigns to make sure the impeller can be machined, with flow-path and blade adjustments done after the aerodynamic design. Sometimes these adjustments are unsuccessful and the manufacturing process is abandoned. In this paper, 2 medium flow coefficient impellers are redesigned. These impellers are used in pipeline applications for the transmission of natural gas. The original designs were cast. By incorporating highly customized commercial software written exclusively for defining tool paths of integrally-machined shrouded impellers into the aerodynamic design process, new impeller geometry was defined that was able to be integrally-machined, while meeting or exceeding existing impeller performance and improved design cycle time. This paper will discuss how the machining software was used concurrently with CFD and FEA analysis during the design process. Test results from rig tests will be presented, showing measured results from both the original and redesigned stages.