Steam turbine technology took a leap in June as Siemens revealed a 10-MW prototype that uses magnetic force to suspend a rotor weighing several tons. The innovation means that instead of needing hundreds of liters of oil for the bearings, the first-of-its-kind steam turbine only needs about three liters of oil (for the valve actuators that control the steam supply). And by eliminating the entire oil management system, it also removes the need for oil tanks, lines, pumps, disposal systems, and safety precautions against fires and environmental damage.
The SST-600 (Figure 2), as it is dubbed by Siemens, was developed in cooperation with the University of Zittau/Görlitz. The turbine with magnetic bearings has since seen a successful trial run at Vattenfall’s lignite-fired 3-GW Jänschwalde steam power plant in the German state of Brandenburg. It has run reliably in regular full-load operation as one of a dozen 10-MW drive turbines for the power plant’s feedwater pumps since February 2015, at speeds of between 3,600 and 5,700 rpm, under main steam temperatures of up to 535C and pressure of 36 bar. This June, Siemens officially handed the model SST-600 steam turbine to Vattenfall.
|2. Magical magnets. Siemens and the University of Zittau/Görlitz in June handed over the first SST-600 steam turbine, a model that uses magnetic bearings and only needs about three liters of oil (for the valve actuators). Courtesy: Siemens|
Siemens noted that active magnetic bearings that suspend rotating components with electromagnets are currently used in other machines such as compressors and electric motors. They have never been used in steam turbines, however, because of high operational temperatures that often exceed 500C. To overcome this hurdle, Siemens developed a patented air cooling system. Engineers also reduced the oil needed by the valve actuators with the use of a compact hydraulic system.
In Siemens’ new design, the position of the rotor is registered by sensors and controlled by a system that adjusts the magnetic field. Then, a Siemens SIMOTICS system compensates for all the weights and process forces acting on the rotor. “Because of this technology, active magnetic bearings also open up the possibility of monitoring rotors online,” says Siemens.
In general, the magnetic bearing technology is appropriate for Siemens steam turbines with rotor weights of up to ten metric tons, which corresponds to an output of between 45 kW and 40 MW. Oil-free steam turbines also operate more efficiently, because there is almost no rotational friction, Siemens says.
“Depending on the design of the turbine, efficiency gains of up to 1% are realistic,” the announcement noted.
—Sonal Patel is a POWER associate editor.