Steam Turbine Stage Research Articles

Influence of valve governing mode on solid particle erosion and efficiency in governing stage of steam turbine

Solid particle erosion (SPE) in governing stage is one of the concerns for large capacity steam turbine. Based on Euler-Lagrange method and Finnie erosion model, the influence of valve governing mode on SPE and efficiency of governing stage were numerically investigated. Results show that the erosion rate density of different circumferential nozzles changes greatly, and the erosion rate density of the most serious erosion nozzle is about 4 × 103 times that of the lightest erosion nozzle. The max erosion rate density of nozzle under sequential valve governing mode (SEVGM) is higher than that under single valve governing mode (SIVGM), in which the particle impact velocity plays a decisive role. With the increase of roughness, the governing stage efficiency decreases under two types of valve governing modes. Moreover, the governing stage efficiency under SEVGM decreases more obviously than that under SIVGM.

Optimization volumetric heating in condensing steam flow by a novel method

In the last stages of steam turbines, the formation of liquid droplets causes losses and reduces turbine efficiency. One of the major problems of wet steam flow is the presence of the liquid phase, which volumetric heating reduces the wetness. One of the disadvantages of adding volumetric heating is increasing entropy generation and cost. Considering that three important parameters, including entropy losses, economic cost, and wetness play, an important role in the turbine blade flow. Reducing wetness can be accompanied by increasing heat and, on the other hand, adding heat leads to increasing cost and entropy. Therefore, as a first, a novel entropy, economic, and wetness (EEW) method is introduced to achieve optimal volumetric heating. Numerical simulation of the turbulent steam flow, SST k–ω model is used. Results are validated with experimental and numerical previous studies for two different geometries. It was also found that when the volumetric heating is optimal, these three parameters in EEW method have the lowest value. Moreover, EEW method is recommended for future research, and it is more optimized compared to the other types of methods. Therefore, corrosion and shock condensation can be decreased by using EEW method.

Optimization of stator blade profile design for last stages of steam turbines based on the features of coarse droplets movement in inter-blade channels

Results of the investigation of coarse droplets movement in steam turbine blade passages are presented in the paper. The structure of generated streams of secondary droplets was considered. The analysis of the effect of these streams on the liquid film formation on the blade surfaces was performed. On the basis of obtained data, several recommendations for blade design operating in wet steam conditions were formulated. They were used to optimize the baseline profile geometry in order to control the coarse droplets streams and provide the increasing of liquid film suction effectiveness. The numerical and experimental comparison of the baseline and optimized geometries was performed.

An Experimental Investigation into Nozzle Vane Heating to Prevent Erosion in the Last Stages of Steam Turbines

The paper is devoted to an experimental investigation of nozzle vane heating in the last stage of a wet steam turbine. The investigation was performed at a steam-dynamic test facility whose circuit was connected to an operating cogeneration power station (TETs). The working fluid was steam from a heat extraction assembly of a steam turbine. To prepare a polydisperse wet steam flow specific for the last stages of steam turbines, a three-stage wetting system based on feedwater injection into the flow was used. The object of investigation is a flat vane cascade consisting of hollow vanes. The vane cavities are fed with superheated steam to provide heating. Experimental data are presented demonstrating that it is advisable to heat nozzle vanes: first, the economic characteristics are improved, and, second, erosion of the blades in the last stages decreases. This method can be used as an alternative to the internal moisture separation from the steam path. The experimental investigation yielded the effect of the temperature difference between the main flow and the heating steam on the amount of heat transferred to the main flow and on the steam conditions at the outlet from the heating chamber. It was found that condensation in the heating chamber decreased its outlet pressure. This should be considered in designing a similar active system to control erosion of rotor blades. The measured profile losses of kinetic energy downstream of the vane cascade for different temperature difference values are presented. The kinematic characteristics of the liquid phase downstream of the vane trailing edge are determined. Large temperature differences are found to have a negative effect on droplets and can reduce the erosion resistance of the blades as compared with profiles without any active erosion control methods.

Flutter Analysis of a Transonic Steam Turbine Blade with Frequency and Time-Domain Solvers

The aim of this study was to assess the capabilities of different simulation approaches to predict the flutter stability of a steam turbine rotor. The focus here was on linear and nonlinear frequency domain solvers in combination with the energy method, which is widely used for the prediction of flutter onset. Whereas a GMRES solver was used for the linear problem, the nonlinear methods employed a time-marching procedure. The solvers were applied to the flutter analysis of the first rotor bending mode of the open Durham Steam Turbine test case. This test case is representative of the last stage of modern industrial steam turbines. We compared our results to those published by other researchers in terms of aerodynamic damping and local work per cycle coefficients. Time-domain, harmonic balance, and time-linearised methods were compared to each other in terms of CPU efficiency and accuracy.

Influence of the Tip Clearance on the Aeroelastic Characteristics of a Last Stage Steam Turbine

In this paper, the tip clearance effects on the aeroelastic stability of a last-stage steam turbine model are investigated. Most of the unsteady aerodynamic work contributing to flutter of the long blades of the last-stage of a steam turbine is done near the tip of the blade. The flow in this region is transonic and sensitive to geometric parameters such as the tip clearance height. The KTH Steam Turbine Flutter Test Case was chosen as the test case, which is an open geometry with similar parameters to modern free-standing last-stage steam turbines. The energy method based on 3D URANS simulation was applied to investigate the flutter characteristics of the rotor blade with five tip gap height varying from 0–5% of the chord length. The numerical results show that the global aerodynamic damping for the least stable inter-blade phase angle (IBPA) increases with the tip gap height. Three physical mechanisms are found to cause this phenomenon. The primary cause of the variation in total aerodynamic damping is the interaction between tip clearance vortex and the trailing edge shock from the adjacent blade. Another mechanism is the acceleration of the flow near the aft side of the suction surface in the tip region due to the well-developed tip leakage vortex when the tip clearance height is greater than 2.5% of chord. This causes a stabilizing effect at the least stable IBPA. The third mechanism is the oscillation of the tip leakage vortex due to the blade vibration. This has a negative influence on the aeroelastic stability.

A generic low-pressure steam turbine test case for aeromechanics and condensation

At the Institute of Thermal Turbomachinery and Machniery Laboratory (ITSM) a generic test case was designed to investigate aeromechanical phenomena and condensation in low-pressure steam turbines. This test case, referred to as Steam turbine Test case for Aeromechanics and Condensation (STAC) consists of the two last stages of a low-pressure steam turbine and is representative for a modern steam turbine design. STAC is intended to serve as a numerical test case to allow studying the fields of aerodynamic damping and spontaneous condensation in low-pressure steam turbine last stages. The geometry of the turbine is made available online at www.itsm.uni-stuttgart.de/research/test-cases/ .

Dielectric properties of wet steam based on a double relaxation time model.

The last stages of most steam turbines operate in wet steam, resulting in water erosion of the rotor blades and the reduction of turbine efficiency. Accurate measurement of steam wetness is the key to ensure an efficient and stable operation of steam turbines. The equivalent complex permittivity model of wet steam was established by Maxwell-Wagner non-homogeneous dielectric theory, and the complex permittivity distribution of frequency and temperature changes of saturated water, dry saturated steam, and wet steam was derived. The measurement experiments verified the above properties of dry saturated steam and wet steam. The complex permittivity of the wet steam is similar to that for the dry saturated steam. The real part increases with increasing frequency and temperature. When the frequency is large or the temperature is low, the real part approaches 1. The imaginary part increases first and then decreases with the increase of frequency. In addition, with the increase of temperature, the imaginary part becomes larger. When the temperature is low, the imaginary part is close to 0, which is independent of the frequency.

An Automated Flow Traversing Measurement System in the Last Stages of Large-Capacity Steam Turbines

The article presents the results from putting a new automated flow traversing measurement system (AFTMS) in an experimental turbine intended for testing steam paths of the low-pressure cylinders (LPC) of large-capacity steam turbines into use. Modern computerized facilities have been used in developing the system. Such an approach has been implemented for the first time in the full-scale test bench available at PJSC Power Machines in carrying out experimental investigations of an LPC double stage compartment (on a 1 : 1 scale) equipped with a new, ultimately long last stage blade (LSB) developed for a steam turbine plant for ultrasupercritical steam conditions. The investigations were aimed at confirming the effectiveness of the developed system in measuring the flow parameters in the last LPC stage. The necessity of making a shift from nonautomated and semiautomatic systems to the use of automated measurement systems is substantiated. The article describes a system that makes it possible, with automated setting of the gas dynamic probe head position on the traversing line, to determine the steam flow direction; to measure the stagnation pressure, static pressure, and stagnation temperature; and to record the measures indicators with exactly assigned time tags. The results from measurements of flow parameters upstream of the last LPC stage in a partial load mode are given. A comparison between the experimental and calculated flow parameter distribution data over the stage radius in the control section is shown.

EXPERIMENTAL AND NUMERICAL RESEARCH ON FLOW IN THE LAST STAGE OF 1090MW STEAM TURBINE

The paper deals with experimental and numerical research in the last stage of real 1090MW steam turbine with the last steel blade length 1220mm placed in nuclear power station. The last stage was equipped with twelve static pressure taps. It was also possible to probe in two planes - before and behind the last stage using pneumatic or optical probes. A number of last stage flow parameters were determined at the root and tip wall for nominal turbine output. Among those parameters are static pressures, Mach and Reynolds numbers, last stage reactions and steam wetness. All the directly measured and evaluated flow parameters are taken from locally measured points and that is why even 3D CFD calculation of the whole system - last stage, diffuser and exhaust hood was implemented. Measured and calculated parameters are compared. Especially static pressures are in very good agreement; the only steam wetness has bigger difference due to different measurement position. Locally measured values are enough to estimate the flow behavior of the last stage. On the other hand, the CFD simulations with well determined boundary conditions, meshes and geometry and is effective tool to simulate even very complicated flow structures in the last stage and exhaust hood.

Influence of the temperature of the heating steam on the characteristics of the liquid phase downstream the stator blades of steam turbine

The paper presents the results of an experimental study of the efficiency of heating with the injection of steam on the surface of the stator blades of the last stages of steam turbines operating in the wet steam flow conditions. The major task of the research was to determine the main mechanisms of the heating temperature influence on the kinematic characteristics of the polydisperse flow of a liquid phase downstream an isolated nozzle blade cascade. Measurement of the characteristics of the liquid phase particles was carried out using a laser flow diagnostic system implemented on the basis of the “PIV-IT”complex and processing the obtained images using the PTV method (particle tracking velocimetry). It is established that heating leads to evaporation of the liquid film formed as a result of the deposition of liquid phase on the surfaces of the nozzle blades, as well as to the increase in the velocities of droplets, and as a consequence, a decrease in diameters and wetness.

Improvement of Steam Turbine Stage Efficiency by Controlling Rotor Shroud Leakage Flows—Part I: Design Concept and Typical Performance of a Swirl Breaker

In high and intermediate pressure (HIP) steam turbines with shrouded blades, it is well known that shroud leakage losses contribute significantly to overall losses. Shroud leakage flow with a large tangential velocity creates a significant aerodynamic loss due to mixing with the mainstream flow. In order to reduce this mixing loss, two distinct ideas for rotor shroud exit cavity geometries were investigated using computational fluid dynamics (CFD) analyses and experimental tests. One idea was an axial fin placed from the shroud downstream casing to reduce the axial cavity gap, and the other was a swirl breaker placed in the rotor shroud exit cavity to reduce the tangential velocity of the leakage flow. In addition to the conventional cavity geometry, three types of shroud exit cavity geometries were designed, manufactured, and tested using a 1.5-stage air model turbine with medium aspect ratio blading. Test results showed that the axial fin and the swirl breaker raised turbine stage efficiency by 0.2% and 0.7%, respectively. The proposed swirl breaker was judged to be an effective way to achieve highly efficient steam turbines because it not only reduces the mixing losses but also improves the incidence angle distribution onto the downstream blade row. This study is presented in two papers. The basic design concept and typical performance of the proposed swirl breaker are presented in this part I, and the effect of axial distance between a swirl breaker and rotor shroud on efficiency improvement is discussed in part II [8].

Improvement of Steam Turbine Stage Efficiency by Controlling Rotor Shroud Leakage Flows—Part II: Effect of Axial Distance Between a Swirl Breaker and a Rotor Shroud on Efficiency Improvement

The basic principle of a distinct idea to reduce an aerodynamic mixing loss induced by the difference in tangential velocity between mainstream flow and rotor shroud leakage flow is presented in “Part I: Design Concept and Typical Performance of a Swirl Breaker.” When the swirl breaker is installed in the circulating region of leakage flow at the rotor shroud exit cavity, the axial distance between the swirl breaker and the rotor shroud is a crucial factor to trap the leakage flow into the swirl breaker cavity. In Part II, five cases of geometry with different axial distances between the swirl breaker and the rotor shroud, which covered a range for the stage axial distance of actual high and intermediate pressure (HIP) steam turbines, were investigated using a single-rotor computational fluid dynamics (CFD) analysis and verification tests in a 1.5-stage air model turbine. By decreasing the axial distance between the swirl breaker and the rotor shroud, the tangential velocity and the mixing region in the tip side which is influenced by the rotor shroud leakage flow were decreased and the stage efficiency was increased. The case of the shortest axial distance between the swirl breaker and the rotor shroud increased turbine stage efficiency by 0.7% compared to the conventional cavity geometry. In addition, the measured maximum pressure fluctuation in the swirl breaker cavity was only 0.7% of the entire flow pressure. Consequently, both performance characteristics and structural reliability of swirl breaker were verified for application to real steam turbines.

Aeroelasticity Analysis of Unsteady Rotor Blade Forces and Displacements in LP Last Stage Steam Turbine with Various Pressure Distributions the Stage Exit

Several high vibration amplitude problems have been reported regarding the slender last stage blading of commercial LP steam turbines. This paper analyses rotor blade unsteady forces and displacements in the last stage of a low pressure steam turbine with symmetrical and non-symmetrical (exhaust hood induced) pressure distributions behind the rotor blades. The FSI partially integrated method was used for the 3D ideal gas flow. 3D unsteady non-viscous flow was analysed using the Godunov-Kolgan method. Non-symmetrical pressure distributions in the stage exit were taken from 3D viscous CFX ANSYS calculations, using a CFD mesh of all the stator blades, all the rotor blades (rotating and not vibrating) and the exhaust hood. The non-uniform and uniform pressure distributions cause rotor blades to vibrate in different bending-torsion modes. Introducing blade vibration into the flow model causes the appearance of unsteady force and blade displacement harmonics that are not multiples of the rotation frequency. The unsteady rotor blade forces and blade displacements analysis shows the importance of including an exhaust hood in the CFD mesh.

Degradation of Aerodynamic Performance of an Intermediate-Pressure Steam Turbine Due to Erosion of Nozzle Guide Vanes and Rotor Blades

Deteriorations of nozzle guide vanes (NGVs) and rotor blades of a steam turbine through a long-time operation usually decrease a thermal efficiency and a power output of the turbine. In this study, influences of blade deformations due to erosion are discussed. Experiments were carried out in order to validate numerical simulations using a commercial software ANSYS-cfx. The numerical results showed acceptable agreements with experimental results. Variation of flow characteristics in the first stage of the intermediate pressure steam turbine is examined using numerical simulations. Geometries of the NGVs and the rotor blades are measured using a 3D scanner during an overhaul. The old NGVs and the rotor blades, which were used in operation, were eroded through the operation. The erosion of the NGVs leaded to increase of the throat area of the nozzle. The numerical results showed that rotor inlet velocity through the old NGVs became smaller and the flow angle of attack to the rotor blade leading edge became smaller. Consequently, the rotor power decreased significantly. Influences of the flow angle of at the rotor inlet were examined by parametric calculations and results showed that the angle of attack was an important parameter to determine the rotor performance. In addition, the influence of the deformation of the rotor blade was examined. The results showed that the degradation of the rotor performance decreased in accordance with the decrease of blade surface area.

Analytical Heat Transfer Correlation for a Multistage Steam Turbine in Warm-Keeping Operation With Air

Due to the growing share of volatile renewable power generation, conventional power plants with a high flexibility are required. This leads to high thermal stresses inside the heavy components which reduces the lifetime. To improve the ability for fast start-ups, information about the metal temperature inside the rotor and the casing are crucial. Thus, an efficient calculation approach is required which enables the prediction of the temperature distribution in a whole multistage steam turbine. Considerable improvements of the computing power and numerical simulation tools today allow detailed investigations of the heat transfer and the flow phenomena by conjugate-heat-transfer (CHT) simulations. However, these simulations are still restricted to smaller geometries mostly by the number of elements. This leads to coarser numerical meshes for larger geometries, and thus, to a reduced accuracy. A highly accurate three-dimensional-CHT simulation of a whole multistage steam turbine can only be conducted with huge computational expense. Therefore, a simplified calculation approach is required. Heat transfer correlations are a commonly used tool for the calculation of the heat exchange between fluid and solid. Heat transfer correlations for steam turbines have been developed in a multitude of investigations. However, these investigations were based on design or to some extent on part-load operations with steam as the working fluid. The present paper deals with the theoretical investigation of steam turbine warm-keeping operation with hot air. This operation is totally different from the conventional operation conditions, due to a different working fluid with low mass flow rates and a slow rotation. Based on quasi-steady transient multistage CHT simulations, an analytical heat transfer correlation has been developed, since the commonly known calculation approaches from the literature are not suitable for this case. The presented heat transfer correlations describe the convective heat transfer separately at vane and blade as well as the seal surfaces. The correlations are based on a CHT model of three repetitive steam turbine stages. The simulations show a similar behavior of the Nusselt-number in consecutive stages. Hence, the developed area related heat transfer correlations are independent of the position of the stage. Finally, the correlations are implemented into a solid body finite element model and compared to the fluid-dynamic simulations.

Flutter of long blades in a steam turbine

Cases of flutter appearing in the last stages of LP steam turbines have recently been reported. This paper presents a numerical analysis of flutter in the last stage of LP steam turbine blades using in-house, non-viscous and viscous codes as well as the ANSYS CFX viscous code. In the case of harmonic blade oscillations, instability regions were calculated using an aeroelastic damping coefficient for various interblade phase angles in five mode shapes. The distribution of this coefficient was also calculated along the blade length. It was found that flutter appears in the first mode shape. 3D viscous and non-viscous calculations showed that flutter appeared only in the first harmonic oscillation mode. In the other four modes, flutter did not appear. A negative aerodamping coefficient for the harmonic blade oscillations is not sufficient evidence of flutter. Conclusive evidence may be obtained using the two-way fluid–structure interaction procedure without assuming harmonic oscillation. In practice, the last stage LP rotor blades are mistuned and the pressure behind them is not uniform. Analysis of such conditions has to be done.

LP Last Stage Steam Turbine Blade Vibrations Due to Mistuning

Background The self-excitation of last stage of slender blades leading to high vibration amplitudes is a problem encountered in the exploitation of LP steam turbines. This has been resolved by changing the geometry of alternate blades (mistuning).

CORROSIÓN POR HENDIDURA EN GRIETAS DEL AISI-410 USADO EN ALABES DE TURBINAS DE VAPOR

The blades of the last stage of steam turbines are subject to corrosive environment and cyclic load, which could lead to failures due to crack propagation and corrosion. Some turbine blades are made of materials like stainless steel AISI-410. In this paper the effect of temperature on corrosion crack propagation produced in a stainless steel AISI-410 type was studied. The cracked specimens were exposed in seawater during 5 days at two temperatures (25 °C and 90 °C) in static conditions. The corrosion was studied using the electrochemical noise technique (EN) and potentiodynamic polarization curves (PC). The electrochemical noise time series were used to analyze the changes in corrosion activity for AISI-410 under the mentioned experimental conditions determining the type and corrosion rate. The results showed that cracks were grown when exposed in seawater, especially at the higher temperature. Physical characterization obtained from SEM showed a great amount of corrosion products emerging from the cracks, observing a mixed corrosion type around the cracks: uniform and pitting. From the results, it is deduced that this corrosion attack may influence crevice corrosion, which eventually leads to stress corrosion cracking and corrosion fatigue.

Steam Turbine Stage Modernisation in Front of the Extraction Point

Abstract The paper presents modernisation of the steam turbine stage situated in front of the turbine extraction point, based on a 225 MW turbine LP stage as an example. The modernised design is intended to better control the steam flow in this area. In the presented design a special ring is used to drive the steam leakage flow directly to the heat exchanger. The performed experiments and numerical analyses confirmed measurable exploitation and efficiency advantages of the introduced modernisation. So far, it has been successfully applied in a number of turbines working in inland power plants, but its use can be easily extended, without need for further modification and without advantage loss, to marine turbines, especially those used as main propulsion in sea-going vessels.