Abstract

Over the last years, the increasing demand in the oil and gas compressor market for performance, efficiency, compactness and reliability, has led Turbomachinery OEMs to increase the “power density” level. In most cases, power density entails high-speed which may introduce rotordynamic technical challanges (e.g. rotor passing through several critical speeds). Tiling pad journal bearings (TPJBs) are commonly adopted in high-speed application for their inherent stability characteristics. In this context, Flexure Pivot® journal bearings (FPJBs) present some distinct advantages for high-speed application compared to standard TPJBs. Integral squeeze film damper (ISFD®) are sometimes added in series to the FPJB to reduce stiffness and increase damping [1]. Originally introduced in the aircraft gas turbines, squeeze film dampers represent an efficient solution to ensure stable operation for supercritical rotors. ISFD are not so often adopted by OEMs and in many cases they are selected only as a retrofit solution when high level of synchronous/sub-synchronous vibrations are experienced in the field [2]. ISFD can be locked with the insertion of dedicated pins which block the ISFD and the journal bearing can operate stand-alone. If introduced by OEMs in the design phase, ISFD can represent a bearing-integrated back-up solution.

The centrifugal compressor subject of this study is a high-speed, high-power density unit absorbing about 2.4MW at 27000rpm. It is a single stage machine compressing CO2 starting from about 80bar, 34°C up to 250bar. The rotor is monolithic in order to have the most rigid design as possible and improve the rotordynamic behaviour which is challenging since the rotor is expected to cross the first two damped critical speeds and finally run close to the third one. The rotor is supported with a pair of FPJBs equipped with ISFD. The FPJB is a four-pads, load-between-pad configuration with 60mm bore diameter. ISFD consists of several EDM made springs and dampers according to a design already investigated by the Authors [3]. In order to validate the rotor and journal bearings behaviour prior to perform the contractual mechanical running test the high-speed balancing activity is used as a system level rotordynamic test. For this reason, the test facility was equipped with special instrumentation in order to monitor journal bearing flow, pad temperatures and rotor vibrations.

References

[1] J. Agnew and D. Childs, "Rotordynamic Characteristics of a Flexure Pivot Pad Bearing With an Active and Locked Integral Squeeze Film Damper", Proceedings of the ASME Turbo Expo 2012: Turbine Technical Conference and Exposition. Volume 7: Structures and Dynamics, Parts A and B. Copenhagen, Denmark. June 11–15, 2012. pp. 551-561.

[2] R. Ferraro, M. Catanzaro, J. Kim, M. Massini, D. Betti and R. Livermore-Hardy, "Suppression of Subsynchronous Vibrations in a 11 MW Steam Turbine Using Integral Squeeze Film Damper Technology at the Exhaust Side Bearing", in Proceedings of the ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition. Volume 7A: Structures and Dynamics, Seoul, South Korea. June 13-17, 2016.

[3] G. Vannini, “Rotordynamic Validation of an Ultra High Speed Multistage Centrifugal Compressor Stacked Rotor”, Proceedings of ASME IGTI Turbo Expo (GT2014-27339), Dusseldorf, 2014.