Closing the gap

Engineers at Siemens have used a simple trick to increase the efficiency of gas turbines in power plants.

The engineers developed a hydraulic system that reduces the ‘radial clearance,’ the gap between a turbine’s rotor blades and its housing. Secondary flows that pass by the rotor blades unused are reduced, which increases the turbine’s efficiency.

The hydraulic optimisation of a gas turbine’s clearance improves its efficiency by about 0.2 percentage points. With a 100-megawatt turbine, that represents annual fuel cost savings of about 60,000 Euros. Siemens says that existing turbines can be retrofitted with the new system at any time. One such system has already been installed at the Mainz-Wiesbaden power plant in Germany.

Gas turbines are used in power plants to generate electricity. The turbines work by means of a compressor that draws air from the environment, compresses it and then channels it into a combustion chamber, where a pressurised mixture of air and fuel, usually natural gas, is burned.

This process produces combustion gases at temperatures as high as 1,500 degrees Celsius. These combustion gases drive the turbine by flowing into it at high speed. The combustion gas expands in the turbine, converting pressure energy into mechanical energy. At this point, the larger the radial clearance in a turbine, the lower is its energy efficiency.

When starting up a turbine in a hot-start, a defined clearance between the rotor blades and housing is necessary because of the different expansion characteristics of the various components in the turbine. As soon as all the components are at stationary operating temperature, however, the hydraulic system can reduce this gap by shifting the entire rotor about three millimetres closer to the compressor.

Siemens turbines have a conical housing, so the axial shifting of the rotor reduces the gap between the rotor blade tips and the housing by about one millimetre.

If unexpected problems occur while the turbine is in operation, the shaft automatically returns to its starting position, which prevents the rotor blades from touching the housing.