Integral Shroud Research Articles

Study on the collision dynamics of integral shroud blade for high-pressure turbine in different integral shroud clearance distance

In steam turbine, turbine blades are prone to vibrate during operation, resulting in steam turbine accidents. The most common method for reducing the vibration of steam turbine blades is to design an integral shroud for blade which is termed as integral shroud blade. Most previous studies simplified straight integral blades into cantilever beam and used harmonic response analysis method to simulate the vibration response of blades. This method is suitable for simulating straight blade vibration under harmonic force conditions. For twisted blades, accurate results are hard to acquire and the specific collision process cannot be simulated. In order to observe the collision process on a microscopic scale and explore its collision damping mechanism, this study evaluated the collision process of twisted blades with different integral shroud clearance distance based on LS-DYNA software. The collision process for a two-blade system and a three-blade system with integral shroud clearance distance from 0.1 mm to 0.5 mm has been simulated. The results indicated that integral shroud clearance distance have opposite vibration damping effect when the blade under the condition of forced vibration and free vibration. For the two-blade system, the optimal integral shroud clearance distance is 0.4 mm for forced vibration condition and 0.1 mm for free vibration condition. For the three-blade system, the optimal integral shroud clearance distance is 0.1 mm for forced vibration condition and 0.5 mm for free vibration condition.

Thermo-mechanical analysis and estimation of turbine blade tip clearance of a small gas turbine engine under transient operating conditions

Turbine blade tip clearance is one of the significant factors that influence turbine efficiency, Specific Fuel Consumption (SFC), Exhaust Gas Temperature (EGT), and emissions. Controlling these parameters in a small gas turbine engine (SGT) is a challenging task due to small blade height and viscous working environment. SGT are subjected to high-temperature gradients at the combustor outlet, which affects the turbine blade tip clearance. This paper presents the thermo-mechanical analysis of a typical SGT engine to study the blade tip clearance influenced by the deformation of turbine stage components (turbine rotor, nozzle guide vane (NGV) with integral blade shroud) during transient phases. ANSYS Workbench is used to perform transient thermal and structural analyses. The structural analysis is performed taking the material properties to be temperature-dependent. The SGT engine under consideration operates at a design speed of 45,000 rpm. Initially, steady-state thermal analysis and static structural analysis were carried out to understand the structural behaviour of the system under a thermal and centrifugal loading environment. Since different components of the engine assembly operate at different temperatures, the effects of convection and conduction at the interfaces influence the radial clearances between the static and rotating parts of the engine. A one-way coupled transient thermal-structural analysis was performed on a three-dimensional model to capture the actual behaviour of the tip clearance during transient operating conditions. Significant growth of blade and rotor was observed relative to the casing resulting in minimal clearances during these transient operations. Hence, it is important to estimate desired cold clearance, considering transient phenomena, to avoid mechanical blade rub with the shroud. It is observed that high-temperature gradients contribute primarily to the stresses and radial displacement of the rotor compared to centrifugal effects. The turbine rotor takes more time (t = 600 s) to reach steady-state temperatures compared to NGV (t = 120 s) due to the solid mass of the disc. The location and magnitude of maximum and minimum equivalent stress changes with time in NGV and rotor, and they experience maximum stress at the initial time steps compared to steady-state.

Calibration of blade tip-timing sensor for shrouded 40″ last stage blade

Abstract Today’s power generation market requests steam turbines of high efficiency, reliability and wide operation range. The last stage moving blades are designed with integral shroud, mid-span, and fir-tree dovetail. The blades are continuously coupled by the blade untwist due to the centrifugal force, and thus the vibration control and increased structural damping are provided. Unfortunately this shroud complicates the blade vibration measurement using contactless methods. This paper describes in detail the Blade Tip-Timing measurement and calibration provided in test rig and in steam power station where the new eddy-current and optical sensors were used together to measure blade vibration in various conditions. Detail calibration procedure for Blade Tip-Timing sensors will be presented as well as the results of the measurements and comparison to FEM Maxwell calculation and to FEM stress analysis.

The Radial Displacement of the HPT Blade under the Effects of the Temperature Field and the Centrifugal

The blade temperature field is computed using the finite element software of ANSYS and Hypermesh. The radial displacement response under the temperature field and centrifugal is analyzed. And a new method of plotting the grid and loading the boundary for complicated three-dimensional model in the finite element software is advanced. The heat transfer coefficients at different parts of the blade are introduced. The results show that for the temperature circumference distribution of the blade profile, the temperature is the highest at the leading and trailing edges and low at the middle. For the radial distribution the temperature is low at the blade root, and high at the blade profile and the integral tip shroud. The effect of temperature field is the main reason of the blade radial displacement while the centrifugal effect accounts for little.

Analysis and Measurement of Damping Characteristics of Integral Shroud Blade for Steam Turbine

To improve the reliability and the thermal efficiency of LP Low pressure end blades of steam turbine, new standard series of LP end blades have been developed. The new LP end blades are characterized by the ISB (Integral Shroud Blade) structure. In the ISB structure, blades are continuously coupled by blade untwist due to centrifugal force when the blades rotate at high speed. This paper, first, explains the effect of the contact condition of the shroud on the vibration characteristics of the ISB structure, using the analysis method developed by authors. Second, the calculated natural frequency and blade damping are compared with the measured ones. From these results, the effect of the shroud contact condition on the vibration characteristics of the ISB structure is clarified, and the validity of the analysis method is confirmed.

Analysis of Fretting Fatigue Strength of Integral Shroud Blade for Steam Turbine

To improve the reliability and the thermal efficiency of LP (Low Pressure) end blades of steam turbine, new standard series of LP end blades have been developed. The new LP end blades are characterized by the ISB (Integral Shroud Blade) structure. In the ISB structure, blades are continuously coupled by blade untwist due to centrifugal force when the blades rotate at high speed. One of the probable failure modes of the ISB structure seems to be fretting fatigue, because the ISB utilizes friction damping between adjacent shrouds and stubs. Therefore, in order to design a blade with high reliability, the design procedure for evaluating the fretting fatigue strength was established by the model test and the nonlinear contact analysis. This paper presents the practical design method for predicting the fretting fatigue strength of the ISB structure, and the some applications are explained.

Vibration of Steam Turbine Blade Caused by Shock Load due to Partial Admission

Partial admission has been applied to the control stage of steam turbine because it produces high turbine efficiency at low loads. The shock load caused by partial admission generates excitation force on a blade, and the vibratory stress of the blade may become a cause of HCF (High Cycle Fatigue). Therefore, in designing a control stage blade of steam turbine, the vibratory stress caused by partial admission should be exactly predicted. Recently, ISB (Integral Shroud Blade) structure has been applied to not only the low-pressure end blade but also the control stage blade of steam turbine. Because the traditional analysis method based on experiments can predict only the vibratory stress of the grouped blade structure, it is indispensable to develop the general analysis method of vibratory stress caused by shock load, which can be applied to both of the grouped blade and ISB structure. This paper, first, presents the analysis method for the grouped blade and ISB structure to calculate the vibratory stress caused by partial admission. Second, the vibration response characteristics to shock load is explained, using some calculated results of simplified blade models. Finally, the response analysis is carried out for the actual grouped blade and ISB structure, and the results are evaluated from the viewpoint of blade reliability.

Development and Verification of 3000 Rpm 48 Inch Integral Shroud Blade for Steam Turbine

The 3000rpm 48inch blade for steam turbine was developed as one of the new standard series of LP end blades. The new LP end blades are characterized by the ISB (Integral Shroud Blade) structure. In the ISB structure, blades are continuously coupled by blade untwist due to centrifugal force when the blades rotate at high speed. Therefore, the number of the resonant vibration modes can be reduced by virtue of the vibration characteristics of the circumferentially continuous blades, and the resonant stress can be decreased due to the additional friction damping generated at shrouds and stubs. In order to develop the 3000rpm 48inch blade, the latest analysis methods to predict the vibration characteristics of the ISB structure were applied, after confirming their validity to the blade design. Moreover, the verification tests such as rotational vibration tests and model turbine tests were carried out in the shop to confirm the reliability of the developed blade. As the final verification test, the field test of the actual steam turbine was carried out in the site during the trial operation, and the vibration stress of the 3000rpm 48inch blade was measured by use of telemetry system. In the field test, the vibratory stress of the blade was measured under various operating conditions for more than one month. This paper first presents the up-to-date design technology applied to the design of the 3000rpm 48inch blade. In the second place, the results of the various verification tests carried out in the shop are presented as well as their procedure. Lastly, the results of the final verification tests of 3000rpm 48inch blade carried out in the site are presented.

Failure analysis of gas turbine last stage bucket made of Udimet 500 superalloy

ABSTRACT This study presents the failure analysis of the 37.5 MW gas turbine third stage buckets made of Udimet 500 (U-500) superalloy. The buckets experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely: the commensurate loss of a load bearing cross-sectional area of the bucket tip shroud by the conversion of metal to the corrosion product (scale) and related increase in stress; the influence of the tip shroud microstructure, e.g. a presence of equiaxed and columnar grains, their distribution and orientation; evidence of the transgranular fracture initiation and intergranular creep mechanism fracture propagation. Finally, the most probable mechanism for bucket damage, conclusions and recommendations to prevent failures of the buckets are presented.

Failure analysis of gas turbine last stage bucket made of udimet 500 superalloy

This article presents a failure analysis of 37.5 mW gas turbine third stage buckets made of Udimet 500 superalloy. The buckets experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis was carried out on elements thought to have influenced the failure process: The most probable cause of the bucket damage was the combination of increased stresses due to corrosion-induced thinning of the tip shroud and unfavorable microstructures in the tip shroud region.

Development of New Advanced Low Pressure End Blades for High Efficiency Steam Turbine.

A low pressure end blade is one of the most important elements in designing the steam turbine, because it determines the performance, the dimension, and the number of casings of the turbine. Applying the state of the art technology to the aerodynamic and mechanical design, Mitsubishi Heavy Industries, Ltd. has been conducting a development program of new advanced low pressure end blades which adopt the ISB (Integral Shroud Blade) structure. A new standard series of these low pressure end blades has already been completed for 50 Hz and 60 Hz unit application including 3 600 rpm 45 inch titanium blade, 3 000 rpm 48 inch steel blade, and 1500/1800 rpm 54 inch steel blade which have the largest exhaust annulus area among the same class blades in the world. This paper reports on the up-to-date design technology applied to the new advanced low pressure end blades and on the verification tests to ensure the high efficiency and the reliability of the developed blades

K-1107 低負荷・低真空運転時に発生する蒸気タービン長翼の振動(S18-2 圧縮機・タービン)(S18 流体機械に関する諸問題)

Under the low load and low condenser vacuum operation, the rotating end-blade of the low-pressure steam turbine is vibrated by the random excitation forces due to an inverse flow, a flow separation, and so on. These excitation forces can cause the excessive random vibration and the stall flutter to the blade. In order to research the vibratory characteristics of the integral shroud blade (ISB) structure under the low load and low condenser vacuum operation, the model turbine test is carried out and the vibratory stress of the blade is measured under various operating conditions. From the test results, it is confirmed that the vibratory stress of the ISB structure is much smaller than the conventional grouped blades and shows the excellent stability to the self-excited vibration due to the higher blade damping.

Metalurgia de alabes móviles con banda integrada de turbinas de gas y su influencia sobre el comportamiento de los alabes

The influence of the microstructure of the gas turbine blades with integral shroud on the blades performance is presented. The analysis of the solidification process of the gas turbine blades during conventionally casting process (equiaxed grains) with all elements which has influence on the mode of its solidification and variation of the microstructure is carried out. Also, the evaluation of the failure of the gas turbine blade is presented. A detailed analysis of the blade tip shroud microstructure (presence of the equiaxed and columnar grains) and its influence on the failure initiation and propagation is carried out. Finally, conclusions and some necessary improvements of the blades casting process to prevent blades failures are presented.

Feature article: Modern Diagnostics and Design of High Performance Blading for Steam and Gas Turbines

Several major developments in the technology of steam and gas turbine blading over the past decade are reviewed. Items such as the Blade Life Algorithm for Dynamic Evaluation (BLADE) Code, computerized fluid dynamics (CFD) flow codes, steam flow measuring technology, and blade vibration instrumentation are discussed. Certain practical modifications In blade groupings, i.e., long-arc shrouding, continuous tie grouping, and integral shroud lockup arrangements are considered. The significance of these developments for the design of modern blading is reviewed, and certain areas where additional research can be expected to deliver a useful return are suggested.

Vibrational Response Analysis of Mistuned Bladed Disk

When calculating the resonant response of bladed turbine disks, it has been expedient to assume that all blades on a given disk are identical. This leads to the prediction that all blades experience the same amplitude of displacement and stress when excited by forces harmonically related to the rotor speed. However, it has been shown experimentally that significant variations in these amplitudes occur for different blades on the same disk. These variations arise due to the effects of mistuning, which refers to small differences in blade characteristics. In this paper, effects of mistuning for free-standing blades and integral shroud blades are studied by means of Monte Carlo simulation and sensitivity analysis. It is found that weakly coupled systems with low damping are very sensitive to mistuning. Also, the validity of the lumped model used in this paper is examined by comparing the calculated results with experimental data.

Vibrational Response Analysis of Mistuned Bladed Disk.

When calculating the resonant response of bladed turbine disks, it has been expedient to assume that all blades on a given disk are identical. This leads to the prediction that all blades experience the same amplitude of displacement and stresses when excited by forces harmonically related to the rotor speed. However, it has been shown experimentally that significant variations in these amplitudes occur for different blades on the Same disk. These variations arise due to the effects of mistuning, wcich refers to small differences in blade characteristics. In this paper, effects of mistuning for free-standing blades and integral shroud blades are studied by using Monte Carlo simulation and the sensitivity analysis. It is found that weakly coupled systems with small damping are very sensitive to mistuning. Also, the validity of the lumped model used in this paper is examined by comparing the calculated results with experimental data.