TL11 - Numerical and Experimental Analysis of Flow-induced Subsynchronous Vibrations in Integrally Geared Turboexpanders

Abstract

Turboexpanders are widely used as energy recovery systems, in plants where a stream of pressurized gas is available as output of the main process. The train is typically composed of a single or multi-stage radial inflow turbine, coupled with an electrical generator by means of a gearbox. When a turboexpander is operated in off-design conditions at low load, the tangential velocity of the gas at expander discharge is high with respect to its axial velocity. In this condition the decelerated, swirled flow may generate a helical vortex in the diffuser, resulting in a pressure gradient of the core flow rotating around the diffuser axis. This rotating pressure gradient is a potential source of radial vibrations for the expander rotor. The present study starts with an analytical description of the phenomenon and a review of dedicated numerical simulations and experimental tests results. The characterization of this flow-induced vibration is followed by the description of mitigating actions that can reduce the risk of high vibrations. Finally, the analysis of three case studies provides evidence of how the vibration can be reduced through such mitigating actions, and allows to draw some general conclusions and recommendations for the design and operation of turboexpanders.