The machine is a 10 MW steam turbine of the SST-600 type and is in use at Vattenfall’s Jänschwalde lignite-fuelled power plant in Germany. It is one of twelve steam turbines deployed at that 6 x 500 MW plant to drive boiler feedwater pumps, the other 11 being conventional SST-600 units with oil. The oil- free machine has been running reliably in regular full load operation at speeds of up to 5700 rpm since February 2015 says Siemens. HP steam conditions are 140 bar/535°C and the rotor weight is about 2.5t.

In a conventional steam turbine, oil is used to lubricate bearings and valve actuators. In the Siemens oil-free turbine magnetic bearings are used, which need no oil, while the amount of oil needed for valve actuators is reduced – although not entirely eliminated – by employing electrohydraulic (electric valve drives) rather than purely hydraulic valve actuation.

The oil system volume is reduced to around 6 litres rather than the several hundred litres typical of a steam turbine of this size.
Among the benefits are reduced fire hazard, less maintenance (no bearing wear), higher availability (with the potential for enhanced condition monitoring), and increased efficiency – "by up to 1%" says Siemens, arising from elimination of friction in the bearings and avoiding the need to supply heated lubricating oil to them.

The new turbine uses Siemens well established SIMOTICS Active Magnetic Bearing (AMB) technology. The position of the rotor is determined by a position sensor. This signal is evaluated in the magnetic bearing control system and the required current is determined for the magnetic bearing. The power electronics send the required signal to the magnetic bearing, holding the rotor in the desired position.

Auxiliary bearings are provided to support the rotor in the event of a control system failure. These are not designed for continuous operation but to allow the rotor to coast down safely in the event of a fault. The auxiliary bearings use double row grooved ball bearings with ceramic balls.

The challenge in using AMB technology for a steam turbine is the high temperature at the shaft journal, requiring use of a magnetic bearing cooling system patented by Siemens.

The performance of magnetic bearings (supplied by EAAT GmbH), together with the cooling system, as well as the auxiliary bearings (supplied by Schaeffler AG) has been validated by extensive testing (started in 2012) on a test bed constructed and operated by Zittau/Görlitz University of Applied Science – which has been working with Siemens on the oil-free turbine concept since 2007.

On the test bed (pictured right) an electric motor is used to drive the 2.5 t rotor up 5800 rpm, while an induction heater is used to increase the rotor temperature to about 530°C to simulate steam conditions.

The electrohydraulic valve actuators were qualified by Siemens and Bosch Rexroth on a separate test stand, with attention focused on such requirements as a closing time of less than 200 ms and reliable operation at 80°C ambient.

According to Christoph Grund, head of project development, steam turbines, Siemens Power and Gas Division, who gave a presentation on the new concept at Power- Gen Europe, Amsterdam, on 9 June, the Jänschwalde installation has demonstrated the technical feasibility of the oil-free turbine.

He suggested that, from a technical perspective, "any steam turbine with a rotor weight of up to 10t can be equipped with magnetic bearings", which, depending on application, corresponds to a steam turbine output of up to 40 MW.

The next steps depend on what customers want, with the initial target market likely to be applications in the oil and gas sector.