Field Testing and Operation of a 300 kW Active Magnetic Bearing Supported Turboexpander-Generator for Natural Gas Pressure Letdown

Abstract A 300 kW turboexpander-generator was developed by Baker Hughes, in conjunction with Calnetix Technologies, for use in a natural gas pressure letdown application. A prior technical publication [1] describes the synchronous permanent magnet generator design and presented some preliminary test data from the prototype unit. In this paper, the machine design, including rotordynamics and active magnetic bearing (AMB) system design, are reviewed. Unbalance response measurements made during a standard mechanical running test at the manufacturer’s facility are shown, including comparisons to predictions made using API 617 guidelines. Turboexpander and generator operating data collected from the initial field installation in Bologna, Italy are then covered in detail. Field data presented includes aerodynamic performance data, closed-loop AMB transfer functions, and plant transfer functions at various power levels, vibration data from spin testing, backup bearing clearance measurements made using the magnetic bearings, thrust load vs. power curves, and generator efficiency at various power levels. The measurements made in the field confirm that the turboexpander-generator meets all customer requirements. Additionally, important case studies are presented, which demonstrate the magnetic bearings diagnosing real field problems. As of 2020, the machine is operating in parallel with a legacy Joule-Thomson valve. A future technical paper will discuss comparisons of predictions and measurements related to a series of shutdown tests performed on the machine under various shutdown scenarios.