Rotordynamic Validation of an Ultra High Speed Multistage Centrifugal Compressor Stacked Rotor

Abstract

The author’s company is currently developing some centrifugal compressor prototypes where the traditional rotor design based on shrunk on wheels and spacers is not suitable due to the specific service requirements (e.g. high peripheral speed or high temperature). For instance the development of a new centrifugal compressor technology aimed to reduce the number of compressor units needed to fulfill a given service is ongoing. In order to accomplish this challenging target a very high rotational speed is required together with special “high pressure ratio” centrifugal stages. The rotor mechanical configuration which has been selected here is a stacked configuration where each centrifugal wheel is integral with the relevant shaft portion. The several shaft portions are mated together through high precision toothed connections (Hirth couplings) and the assembly is secured through a pre-stretched tie-rod. This rotor stacked configuration is not typical for the most of the industrial centrifugal compressors (a solid shaft with shrink fitted impellers is the common solution, as anticipated) but it is allowed by API standards [1], and it is referenced in Turbomachinery technical literature [2]. The rotordynamics of this special prototype is very challenging since it deals with a seven piece stacked rotor running supercritical. An extensive validation program was required in addition to careful design. This is the specific subject of the present paper which will cover the main following items: validation of the “rotor alone” rotordynamic modelization through comparison with the relevant ping test results, selection of special high speed journal bearings, and overview of the low and high speed balancing process. All these steps together finally allowed the author’s company to fully demonstrate the soundness of this stacked rotor technology for application in High Pressure Ratio Compression service.