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Steam Turbine Thermal Modeling for Improved Transient Operation
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology. (Concentrating Solar Power)ORCID iD: 0000-0002-8888-4474
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The growing shares of renewable energy sources in the market and solar thermal power applications have set higher requirements on steam turbine operation.These requirements are related to flexibility during transients. A key aspect sought of such flexibility is the capability for fast starts. Due to the varying temperature gradients during start-up, the speed at which the turbine can start is constrained by thermal stresses and differential expansion. These phenomena either consume component lifetime or may result in machine failure if not carefully controlled. In order to accomplish faster starts while ensuring that lifing requirements are preserved, it is important to analyze the thermal behavior of the machine. For this, a transient thermal model was developed with a focus on adaptability to different turbine sizes and geometries. The model allows for simple and fast prediction of thermo-mechanical properties within the turbine metal, more importantly, of the temperature distribution and the associated thermal expansion. The next step of this work was to validate the assumptions and simplifications of the model. This was done through the study and comparison of two turbines against measured operational data from their respective power plants. Furthermore,validation studies also included comparisons concerning the geometric detail level of the model. Overall, comparison results showed a large degree of agreement with respect to the measured data and between the geometric detail levels. The validated model was then implemented in studies related to reducing start-up times and peak differential expansion. For this, the potential effects of turbine temperature maintaining modifications were investigated and quantified.The modifications studied included: increasing gland steam pressure, increasing back pressure and increasing barring speed. Results yielded significant improvements starting from 9.5% in the start-up times and 7% in the differential expansion.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , 63 p.
Series
TRITA-KRV, ISSN 1100-7990 ; 14:06
Keyword [en]
steam turbines, transients, start-up, finite element model, heat transfer
National Category
Mechanical Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-156196ISBN: 978-91-7595-367-0 (print)OAI: oai:DiVA.org:kth-156196DiVA: diva2:765657
Presentation
2014-12-05, Sal Learning Theater (M235), Brinellvägen 68, KTH, Stockholm, 11:00 (English)
Opponent
Supervisors
Note

QC 20141128

Available from: 2014-11-28 Created: 2014-11-24 Last updated: 2014-11-28Bibliographically approved
List of papers
1. Geometric Modularity in the Thermal Modeling of Solar Steam Turbines
Open this publication in new window or tab >>Geometric Modularity in the Thermal Modeling of Solar Steam Turbines
2014 (English)In: Proceedings of the SolarPACES 2013 International Conference, Elsevier, 2014, Vol. 49, 1737-1746 p.Conference paper, Published paper (Refereed)
Abstract [en]

To optimize the start-up schedules of steam turbines operating in concentrating solar power plants, accurate predictions of the temperatures within the turbine are required. In previous work by the authors, thermal models of steam turbines have been developed and validated for parabolic trough solar power plant applications. Building on these results, there is an interest to increase the adaptability of the models with respect to different turbine geometries due to the growing trend of having larger steam turbines in parabolic trough and solar tower power plants. In this work, a modular geometric approach has been developed and compared against both the previous modeling approach and 96h of measured data from an operational parabolic trough power plant. Results show a large degree of agreement with respect to the measured data in spite of the different detail levels. The new model allows for simple and fast prediction of the thermal behavior of different steam turbine sizes and geometries, which is expected to be of significant importance for future concentrating solar power plants.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
steam turbine, thermal stresses, start-up, finite element method
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-126931 (URN)10.1016/j.egypro.2014.03.184 (DOI)000340733700178 ()2-s2.0-84902271058 (Scopus ID)
Conference
International Conference on Solar Power and Chemical Energy Systems, SolarPACES 2013, Las Vegas, NV, United States, 17 September 2013 through 20 September 2013
Funder
Swedish Energy Agency
Note

QC 20140922. QC 20160129

Available from: 2013-08-22 Created: 2013-08-22 Last updated: 2017-08-11Bibliographically approved
2. Operational Improvements for Startup Time Reduction in Solar Steam Turbines
Open this publication in new window or tab >>Operational Improvements for Startup Time Reduction in Solar Steam Turbines
Show others...
2015 (English)In: Journal of engineering for gas turbines and power, ISSN 0742-4795, E-ISSN 1528-8919, Vol. 137, no 4, 042604Article in journal (Refereed) Published
Abstract [en]

Solar steam turbines are subject to high thermal stresses as a result of temperature gradients during transient operation, which occurs more frequently due to the variability of the solar resource. In order to increase the flexibility of the turbines while preserving lifting requirements, several operational modifications for maintaining turbine temperatures during offline periods are proposed and investigated. The modifications were implemented in a dynamic thermal turbine model and the potential improvements were quantified. The modifications studied included: increasing the gland steam pressure injected to the end-seals, increasing the back pressure and increasing the barring speed. These last two take advantage of the ventilation and friction work. The effects of the modifications were studied both individually as well as in different combinations. The temperatures obtained when applying the combined modifications were compared to regular turbine cool-down (CD) temperatures and showed significant improvements on the startup times of the turbine.

Place, publisher, year, edition, pages
ASME Press, 2015
Keyword
Steam Turbines, Start-up time
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-148147 (URN)10.1115/1.4028661 (DOI)000350145500024 ()2-s2.0-84940473273 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20150408

Available from: 2014-07-30 Created: 2014-07-30 Last updated: 2017-08-11Bibliographically approved
3. Differential Expansion Sensitivity Studies during Steam Turbine Startup
Open this publication in new window or tab >>Differential Expansion Sensitivity Studies during Steam Turbine Startup
2015 (English)In: Journal of engineering for gas turbines and power, ISSN 0742-4795, E-ISSN 1528-8919, Vol. 138, no 6, GTP-15-1419Article in journal (Refereed) Published
Abstract [en]

In order to improve the startup flexibility of steam turbines, it becomes relevant to analyze their dynamic thermal behavior. In this work, the relative expansion between rotor and casing was studied during cold-start conditions. This is an important property to monitor during startup given that clearances between rotating and stationary components must be controlled in order to avoid rubbing. The investigation was performed using a turbine thermal simplified model from previous work by the authors. The first step during the investigation was to extend and refine the modeling tool in order to include thermomechanical properties. Then, the range of applicability of the model was validated by a twofold comparison with a higher order finite element (FE) numerical model and measured data of a cold start from an installed turbine. Finally, sensitivity studies were conducted with the aim of identifying the modeling assumptions that have the largest influence in capturing the correct thermal behavior of the turbine. It was found that the assumptions for the bearing oil and intercasing cavity temperatures have a large influence ranging between ±25% from the measured values. In addition, the sensitivity studies also involved increasing the initial temperature of the casing in order to reduce the peak of differential expansion. Improvements of up to 30% were accounted to this measure. The studies performed serve as a base toward further understanding the differential expansion during start and establishing future clearance control strategies during turbine transient operation.

Place, publisher, year, edition, pages
American Society of Mechanical Engineers (ASME), 2015
Keyword
Thermal expansion, Steam Turbines
National Category
Mechanical Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-156419 (URN)10.1115/1.4031643 (DOI)000374713500011 ()2-s2.0-84947466310 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20160122

Available from: 2014-11-28 Created: 2014-11-28 Last updated: 2017-08-14Bibliographically approved

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