Fully-sealed Turboexpander Generator for Natural Gas Pressure Letdown Station Driven on PM-biased Homopolar Active Magnetic Bearings
Baker Hughes, one of the world's largest oilfield services companies, selected Calnetix Technologies, LLC. to provide design and manufacturing services for a turboexpander generator to be utilized at a natural gas PLD station. The turboexpander, coupled to a high-speed generator, would allow kinetic energy to be converted into electrical energy in natural gas pipelines while still reducing the gas pressure.
The turboexpander generator co-developed, tested and commissioned by Baker Hughes in conjunction with Calnetix consists of a 300 kW radial inflow turbine equipped with a fully integrated and sealed high-speed generator. Calnetix was responsible for designing and building the Magnaforce™ permanent magnet (PM) synchronous generator supported by PM-biased homopolar active magnetic bearings (AMBs). The rated speed of the generator is 30,000 RPM, the maximum continuous speed is 31,500 RPM and the overspeed limit of the generator is 36,300 RPM. The PLD expander generator was also provided with a Calnetix Vericycle™ Variable Speed Drive (VSD) and an Insight™ Magnetic Bearing Controller (MBC). A touchdown bearing system was implemented using rolling element bearings with soft mount supports.
One of the main challenges was to keep an approximate distance of 100 feet between the machine and the controller due to the potentially explosive nature of the environment, thus making it a long cable application.
The backup bearings selected for this application consist of two separate rows of balls, contained by two separate outer rings and a single inner ring construction. A series of drop tests were performed on the PLD expander to test the reliability of the backup bearings. During the drop testing, it was observed that the inner ring was heating up on acceleration, thereby increasing the load on the balls and moving them in the axial direction. A retainer spring was used to provide external axial load to keep the bearing in place.
Compared to conventional expanders, the 300 kW AMB-supported turboexpander generator provided the following advantages:
The turboexpander generator was commissioned at the customer site in April of 2019 and has been operating in parallel with a Joule-Thomson valve. The PLD turboexpander is producing 1,600,000 kWh per year. It is expected that several additional units will be commissioned in next 24 months at the customer’s sites.
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