Steam Turbine Aerodynamic Improvements for Significant Efficiency Gains

The steam turbine is one of the most important power generating equipment items in use. Around half of the electricity generated worldwide comes from steam turbines. Steam turbines can be fueled by coal, nuclear energy, petroleum or natural gas, alternatively by biomass, solar energy or geothermal energy. Thus a large amount of fuel can be saved and CO2 emissions significantly reduced by optimizing key components of these widely used machines.

An important goal in steam turbine technology is to improve efficiency. The continuous flow of steam conditions is one of the commonly accepted efficiency contributor for steam power plants. The chart below shows expected improvement in thermal efficiency for USC double-reheat fossil-fuel power units compared to common supercritical-pressure ones, according to Hitachi.

Expected Improvement in Thermal efficiency for USC power units
Figure 1: Expected improvement in thermal efficiency for USC power units.

Besides steam condition elevation, other areas help the development and refinement of innovative aerodynamic flow path design approaches and the improvement of design procedures for nozzle and blades design and analysis. Continuous growth of steam conditions since the mid-1990s and some advanced steam path design for large steam turbines have brought about 5% of efficiency gain. This effect is almost the same as the transition from subcritical-pressure steam conditions to the supercritical-pressure ones with elevated steam temperatures illustrated in the figure above. Here are some practical examples of steam turbines high efficiency, achieved during the last decade by advanced aerodynamic design (source: Leizerovich, A. Sh. Steam turbines for modern fossil-fuel power plants, ©2008 by The Fairmont Press).

  1. Siemens field tests for large steam turbines demonstrated internal efficiency of 94.2% for the HP cylinder and 96.1% for the IP one.
  2. ALSTOM Power retrofitted the steam turbine’s HP-IP cylinder at the U.S. power plant J.K. Spruce, the subsequent acceptance tests showed the internal efficiency figures of 93% for the HP section and 95.7% for the IP section, whereas even for the best turbines of the 1980s these figures were around 90% and 93%, respectively.
  3. At the Siemens at the German power plant Mehrum, the HP and LP sections of the retrofitted steam turbine reached internal efficiency values of 93.6% and 89.9%, respectively, compared to the original values of 86.5% and 87.2%.
  4. The gross efficiency data for the most efficient German and Japanese steam turbines of the recent years.

 

These efficiency figures, while record-breaking, are quite characteristic for steam turbines with a modern steam path designed via advanced computation technologies.

Efficiency improvement in these turbines was achieved by many advanced design features, such as:

  1. Optimized nozzles and blades profiles with reduced profile and secondary losses
  2. 3D advanced blading airfoils with leaded and bowed geometry
  3. Reaction vs. Impulse design with increased stage count
  4. Advanced control stage design
  5. Optimized sealing technology
  6. Sustainable clearances at transient operation and etc.

The chart below (Figure 2) illustrates losses in a typical steam turbine impulse stage.

Steam Turbine stage losses (impulse design)
Figure 2 Steam Turbine stage losses (impulse design)
Shares of different energy losses for a characteristic impulse-type HP stage.
Figure 3 Shares of different energy losses for a characteristic impulse-type HP stage       (Coffer J.I., IV. “Advances in Steam Path Technology,” Transaction of the ASME.
Journal of Engineering for Gas Turbines and Power 118, April 1996: 337-352.).

Figure 3 demonstrates shares of different energy losses for a characteristic impulse-type HP stage. These shares are not absolute and the specific loss depends on the type of blading, blade profiles, and other specific design solutions. But in general, the nozzles and blades losses along with tip leakages represent the most significant portion of aerodynamic losses which is why turbine designers focus much of their attention to these areas.

Until recently, the main efforts of turbine designers were just to decrease the profile losses by experimental investigations and by use of the newest, advanced calculation methodologies.

The profile energy losses in the blade rows are conditioned by some main factors:

  1. Losses, associated with friction on the profile surface – profile losses;
  2. Losses, associated with vortices if the boundary layer separates from the surface near tip and hub walls – secondary losses;
  3. Losses with vortices downstream from the profile’s trailing edge – trailing edge losses;
  4. Wave losses under conditions of a supersonic flow through the row channel.

The original nozzle profile was developed in mid 1980s at Central Boiler and Turbine Institute and widely used for large steam turbines of LMZ production. During the modernization project of the 200MW steam turbine, the profile was upgraded by using an optimization tool, then studied in CFD, where it demonstrated high efficiency. The upgrade assumes profile shape optimization to minimize losses, but preserving near the same structural characteristics – profile area, trailing edge thickness and relative pitch. Figure 4 shows result of AxSTREAM® optimization with 0.15% profile losses reduction.

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AxSTREAM®, SoftInWay’s multidisciplinary turbomachinery design, analysis and optimization tool, was developed to tackle tasks such as those mentioned in this blog. AxSTREAM®  has a special module for blading design which includes useful features to create highly efficient nozzle and blade profiles and form advanced airfoils. Some pictures are provided below to illustrate the profiling optimization process in AxSTREAM®.

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Original and optimized profiles and velocity distribution
Figure 4 Original and optimized profiles and velocity distribution

This result was proved by 3D CFD, which demonstrated 0.158% of total nozzle losses improvement, achieved by only through profile losses reduction, because secondary losses remains the same. The next step in airfoil optimization is secondary losses reduction, which was done through advanced 3D features. Figure 5 give you a preview of this next step.  Details will be share in the next blog in this series.

Loss coefficient radial distribution for Original and optimized profiles
Figure 5 Loss coefficient radial distribution for original and optimized profiles

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