Earlier this year, Siemens handed over its first oil-free steam turbine. We caught up with project manager Christoph Grund to hear about its development
Turbines make the world go round. Whether powering cities or platforms, steam and gas turbines are the primary means of keeping the lights on, as well as a host of other vital processes. Yet while they are an indispensable part of most major engineering operations, the concept has not seen much refinement beyond incremental gains in design, manufacturing and materials. The turbines also require regular servicing and maintenance, not to mention a healthy supply of oil for bearing lubrication.
That was until earlier this year. In June, Germany’s Siemens announced the delivery of the world’s first oil-free turbine. Unlike conventional turbines, the SST-600 uses a system of magnetic bearings to suspend a rotor weighing up to 10 tonnes. The rotor literally floats, removing the need for a film of oil between itself and the bearings – and eliminating the need for oil tanks, lines, pumps and disposal systems, and of course the hundreds of litres of oil within them.
The resulting turbine is therefore smaller and more efficient. No contact with the bearings means no loss of energy to friction, and the system is up to 1% more efficient – not an earth-shattering statistic in itself, but as Christoph Grund, Siemens’ oil-free steam turbine project manager, told InnovOil: “A higher efficiency of up to 1% is a huge development when bearing in mind that steam turbine technology itself has hardly any potential for further development anymore.”
While “oil-free” is perhaps misleading –the system contains between 3-10 litres of oil to lubricate the valve actuators which control the steam supply – the efficiency and safety gains make this an attractive investment. Installations with sensitive environmental or safety considerations, especially around fires and fire safety, may therefore find it a very prudent purchase.
The turbine has been in development since 2007 as part of a collaboration between Siemens and the Zittau/Goerlitz University of Applied Science, the latter of which provided a test bay in which to develop and refine the bearings.
Siemens added: “From a technical perspective and depending on the operating materials necessitated by operating conditions, any Siemens steam turbine with a rotor weight of up to 10 tonnes and an output of 45 kW to 40 MW can also be equipped with magnetic bearings.”
Active magnetic bearings – whose force is controlled via electromagnets – are used in a number of other technologies such as compressors and electric motor, but Siemens has been the first to deploy the system to a steam turbine. This is mainly owing to the fact that steam turbines are subjected to immense temperatures and temperature change. Overcoming this, Grund says, was a newly designed cooling system. “The challenge of a steam turbine is the heat it operates with. The average inlet steam temperature is about 500°C, thus we have a hot surrounding near the bearings. Siemens solved the challenge with a distinguished air cooling system,” he explains, the design of which Siemens has now patented.
In active magnetic bearings, the position of the rotor is registered by sensors and controlled by a system that adjusts the magnetic field. Here, a SIMOTICS system – a control system used in Siemens motors – compensates for all the weights and process forces acting on the rotor.
Because they are controlled digitally, the rotors can also be potentially monitored online, aiding elements of production efficiency and condition monitoring.
The system is also not limited to this particular model, as Grund highlights. “The magnetic bearing and electrohydraulic valve actuators which both lead to an oil-free steam turbine are not specific to one certain product line. It is applicable to steam turbines with a rotor weight up to 10 tonnes and we could adapt it to several product lines,” he added.
To help minimise the amount of oil used in the actuators, the designers also incorporated a compact hydraulic system – another way in which the team could ensure the machinery maintained a smaller footprint.
The turbine’s first commercial deployment has so far been a success. In February, Siemens delivered a 10-MW prototype unit to Vattenfall’s Jänschwalde lignite-fired power plant in Germany, where it was set up in tandem with a further 11 conventional turbines used to pump feedwater. It has since been operating at full load at a speed of up to 5,700 rpm, and in June a final model was installed and handed over to Vattenfall.
Grund says that since the installation and official handover, the response from industry has been great. “We experienced very positive feedback and interest and were congratulated as an innovative company. We appreciate the feedback very much.”