4. COMPARISON
4.2. Thermal parameters
This subchapter contains diagrams of steam vapor fraction throughout the turbine for both cases in accordance with specific points throughout the turbine.
Figure 37. Vapor fraction, 0.9 load level
Figure 38. Vapor fraction, 0.8 load level
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Figure 39. Vapor fraction, 0.7 load level
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4.3. Summary
Regarding energy parameters, installation of additional heat exchanger provides up to 100 extra MW of guaranteed electrical power, which is expected as additional HE provides a sizable boost to internal turbine power. Although heat consumption also increases for WHE-setup, it is still favourable, because specific heat consumption for WHE-setup is somewhat lower than WOHE-setup.
One the main benefits of sliding pressure mode with partial load operation is decrease in power usage systems including condensate, feedwater and drainage pumps, thus electrical efficiency is always more than, with throttling (constant pressure). In this case WHE-setup causes electrical efficiency to increase even more, since turbine unit provides much energy, but consumes the same amount as with WOHE-setup.
As for thermal parameters, one of the most important in this work is vapor fraction. Lower vapor fraction values cause more erosion and corrosion (Otakar J. 2008), it is also proven, that decrease in vapor fraction values correlates with decrease in efficiency of turbine (Jonas, O. 1995), thus keeping steam closer to higher vapor fraction values is advisable. Erosion is mainly dangerous for last stages of LPC and WHE-setup showed satisfactory results: an average of additional 2.2%
difference in comparison with WOHE-setup results. That bring WHE vapor fraction values to average of 90%, which is beneficial for lifetime (Trunov, N. B. et al. 2011) and reliability of turbine unit.
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5. IMPACT ON OTHER OPERATIONAL PARAMETERS
However the last concern to address regarding installation of HE, is geometry and physical size of HE itself and possible reconstruction of separator and superheaters.
To implement WHE setup, first its size should be taken in account. If it’s and it’s pipelines’ size does not exceed available space in containment building, then implementing such a method won’t be a massive problem to solve. However, it still brings a potential problem as a branch of primary circuit is a possible dangerous place for LOCA.
Based on calculation results, installation of additional heat exchanger showed possible redundancy of separator stage. This fact is supported by decrease of separator condensate mass flow rate:
• from 147 kg/s to 33 kg/s at 0.9 power mode
• from 123 kg/s to 20 kg/s at 0.8 power mode
• from 102 kg/s to 7 kg/s at 0.7 power mode.
However, having said that, SH2 with WHE-setup suffers more thermal stress due to higher temperature and needs more surface to operate, thus possible ideas of reconstruction could be implemented in further research.
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6. CONCLUSION
The objective of this thesis was to test and undertand a new and advanced deisgn of schematic diagram of secondary circuit, which includes installation of additional heat exchanger after steam generator. Main principle lies in usage of primary circuit for superheating steam during part load operation in sliding pressure mode.
The benchmark for experimental data is NPP with VVER-1000 with turbine unit K-1000-60/1500.
Utilizing standard methods schematic diagram calculations, several datasets for load level 0.7-1.0 were prepared both for WOHE and WHE setup.
The results have shown great potential, as electrical efficiency went up an average of 0.5%, electrical power values increased with an average of 100MW. In addition to that, average increase of 2% in vapor fraction provides potential benefit for reliability and lifetime of turbine machinery.
However, this work is still not enough for practical implementation yet, as more geometrical design calculation must be made to test compatibility with existing design.
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