Magnetic bearings & steam turbines: A look at the SST-600
The Siemens SST-600. This is the world’s first oil-free steam turbine with levitating rotor. Today’s turbine rotors use oil to keep their bearings from overheating. However, the high temperatures involved mean that oil must be continually pumped through the bearings. So what if the rotor, which weighs several tons were to be suspended using magnetic forces? Instead of requiring hundreds of litres of oil for the bearings, this system uses only about three litres of oil for the valve actuators that control the steam supply. This makes the design an attractive option for optimal fire protection, lower frictional resistance resulting in higher efficiency, and lower maintenance expenditure due to wear-free bearing.
The main advantage of an oil-free steam turbine becomes evident when the machinery powered by it (e.g., a generator or compressor) also uses magnetic bearings. An oil-free (or drastically reduced use of oil) system eliminates the need for oil tanks, lines, pumps, disposal systems, and safety precautions against fires and environmental damage.
Potential areas of application for oil-free steam turbines:
- Oil & gas industry, e.g. oil and natural gas pumping and processing
- Offshore applications, e.g. drilling platforms
- Industries with high fire hazards, such as the chemical industry, petrochemical industry, metallurgy, mining, pulp and paper industry, refineries
- Industries with stringent environmental requirements, such as near drinking water catchment areas
Active magnetic bearings that generate their force through controlled electromagnets are currently used in machines such as compressors and electric motors but were never implemented in steam turbines until now. One reason for this is the steam which flows into a turbine often exceeds 500 degrees Celsius. This resulted in Siemens developing a patented air cooling system where the magnetic bearings are water and air-cooled. On the one hand the air flow cools the airgap between stator and rotor (airgap cooling) of the electromagnetic actuator. On the other hand the rotor itself is cooled (bushing cooling) by air. The bearings are protected by limiting the heat flow coming from the process environment.
The setback of this design is that cooling of the sleeves is currently very demanding – compressors are necessary. The tube cooling applied reduces the efficiency of the magnetic bearing which is high compared with hydrostatic friction bearing. Moreover, the additional peripheral effort has a negative effect for investment and maintenance in cost accounting.
The successful commissioning of the oil-free industrial steam turbine SST-600 was in 2015. The first experiences in the power plant Janschwalde show a well operating system. Since then, more experimental investigations have been determining which state of the art materials and construction principles must be used and applied to reduce to demand for cooling to further the technology.
 Siemens hands over first oil-free steam turbine to Vattenfall, Siemens Global Website (online), available from [https://www.siemens.com/press/ PR2015060247PGEN ]
 Haje, D. and Grund, C., Oil free Siemens industrial steam turbine – Initial operating experience, 10th Workshop of Magnetic Bearing Technology Zittau-Chemnitz, Zittau, Germany, 2015 (in German)