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Liberalized electricity market conditions and concentrating solar power technologies call for increased power plant operational flexibility. Concerning the steam turbine component, one key aspect of its flexibility is the capability for fast starts. In current practice, turbine start-up limitations are set by consideration of thermal stress and low cycle fatigue. However, the pursuit of faster starts raises the question whether other thermal phenomena can become a limiting factor to the start-up process. Differential expansion is one of such thermal properties, especially since the design of axial clearances is not included as part of start-up schedule design and because its measurement during operation is often limited or not a possibility at all.The aim of this work is to understand differential expansion behavior with respect to transient operation and to quantify the effect that such operation would have in the design and operation of axial clearances. This was accomplished through the use of a validated thermo-mechanical model that was used to compare differential expansion behavior for different operating conditions of the machine. These comparisons showed that faster starts do not necessarily imply that wider axial clearances are needed, which means that the thermal flexibility of the studied turbine is not limited by differential expansion. However, for particular locations it was also obtained that axial rubbing can indeed become a limiting factor in direct relation to start-up operation. The resulting approach presented in this work serves to avoid over-conservative limitations in both design and operation concerning axial clearances., QC 20170814

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 start-up 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 thermo mechanical properties. Then, the range of applicability of the model was validated by a two-fold comparison with a higher order finite element 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 inter-casing 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 towards further understanding the differential expansion during start and establishing future clearance control strategies during turbine transient operation., QC 20160524

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., QC 20160122

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., QC 20150408

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., QC 20140922. QC 20160129

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 lifing 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 temperatures and showed significant improvements on the start-up times of the turbine., QC 20151029. QC 20160214

Steam turbines in solar thermal power plants experience a much greater number of starts than those operating in base-load plants. By maintaining higher internal temperature during idle periods, faster start-up times can be achieved, increasing the flexibility of the plant as well as increasing net electrical production. Prior work by the authors identified a number of methods for achieving this, with strong increases in daily production predicted; only two specific start-up cases were studied however. In order to obtain a more representative evaluation of the performance increase that can be achieved through increased dispatchability of the turbine, the annual improvement needs to be studied. Building on the existing results, a dynamic system model of a parabolic trough power plant has been established and used to determine the distribution of different cool-down times experienced throughout the year, with a view to evaluating the potential annual production increase. A modification of the start-up curves allows an increase in annual electrical production between 6.4% and 2.4% depending upon the operating mode (free operation versus day-time operation). Through application of a combination of heat blankets and an increase in gland steam temperature, further annual production increases between 2.2% and 3.1% are predicted., QC 20120417

Steam turbines in solar thermal power plants experience a much greater number of starts than those operating in baseload plants. In order to preserve the lifetime of the turbine while still allowing fast starts, it is of great interest to find ways to maintain the turbine temperature during idle periods. A dynamic model of a solar steam turbine has been elaborated, simulating both the heat conduction within the body and the heat exchange with the gland steam, main steam and the environment, allowing prediction of the temperatures within the turbine during off-design operation and standby. The model has been validated against 96 h of measured data from the Andasol 1 power plant, giving an average error of 1.2% for key temperature measurements. The validated model was then used to evaluate a number of modifications that can be made to maintain the turbine temperature during idle periods. Heat blankets were shown to be the most effective measure for keeping the turbine casing warm, whereas increasing the gland steam temperature was most effective in maintaining the temperature of the rotor. By applying a combination of these measures the dispatchability of the turbine can be improved significantly: electrical output can be increased by up to 9.5% after a long cooldown and up to 9.8% after a short cooldown., QC 20111213

Steam turbines in solar thermal power plants experience a much greater number of starts than those operating in base-load plants. In order to preserve the lifetime of the turbine whilst still allowing fast starts, it is of great interest to find ways to maintain the turbine temperature during idle periods. A dynamic model of a solar steam turbine has been elaborated, simulating both the heat conduction within the body and the heat exchange with the gland steam, main steam and the environment, allowing prediction of the temperatures within the turbine during off-design operation and standby. The model has been validated against 96h of measured data from the Andasol 1 power plant, giving an average error of 1.2% for key temperature measurements. The validated model was then used to evaluate a number of modifications that can be made to maintain the turbine temperature during idle periods. Heat blankets were shown to be the most effective measure for keeping the turbine casing warm, whereas increasing the gland steam temperature was most effective in maintaining the temperature of the rotor. By applying a combination of these measures the dispatchability of the turbine can be improved significantly: electrical output can be increased by up to 9.5% after a long cool-down and up to 9.8% after a short cool-down., QC 20110628

This paper demonstrates and evaluates unique flow measurement results obtained in a high pressure test turbine, in order to analyze the effect of Low Engine Order (LEO) excitations. A stator was modified to induce LEO flow variations by either imposing a throat width variation or a blockage of the trailing edge cooling flow from some of the vanes. Laser Two Focus (L2F) and pressure probe measurements were performed at subsonic and transonic flow conditions, without and with the rotor installed and operated. Time resolved velocity data was obtained in front and inside of rotor passages covering the Low Engine Order variation period. The presented results evaluation focuses only on L2F measurements and show that the LEO variation of flow velocity and turbulence intensity is most visible in the gap between stator and rotor and at the first measurement location inside the rotor passage. The overall trend is that stator exit flow Mach numbers are higher behind the passages with smaller pitch and vice versa., QC 20150601