Subsynchronous Vibrations On Turboexpanders Equipped With Magnetic Bearings Assessment, Understanding And Solutions

Abstract

In most natural gas applications, turboexpanders are operating close to or on the dew point line and a mixture entering the turboexpander impeller is a combination of liquid and gas phases of the natural gas. The presence of the liquid phase may induce rotordynamic instabilities during operation. Special care should be taken in the case of magnetic bearings. This paper will present a revamp of two turboexpander-compressors equipped with active magnetic bearings (AMB). The application is liquid natural gas (LNG) rejection and gas injection on a Floating Production, Storage and Offloading (FPSO) barge. Prior to upgrade, the turboexpanders had accumulated more than 250 thousand hours in operation. The objective of the upgrade was to adapt to new process conditions and to increase the power of the turboexpanders. After the re-wheeling of turboexpanders, numerous trips due to violent subsynchronous vibrations were experienced. The root-cause analysis revealed that the presence of liquids in the front wheel labyrinth seals caused fluid-induced non-synchronous vibrations (NSV). Methods used in the redesign of the impellers and labyrinth seals that allowed the problem to be solved will be described in detail.