Fatigue life prediction of mistuned steam turbine blades subjected to deviations in blade geometry

The blades of the steam turbine are subjected to bending of the steam flow, centrifugal loading, vibration response, and structural mistuning. These factors mentioned contribute significantly to the fatigue failure of steam turbine blades. Low pressure (LP) steam turbines experience premature blade and disk failures due to the stress concentrations in the root location of the blade of its bladed disk. This study of mistuned steam turbine blades subjected to variation in blade geometry will be of great significance to the electricity generation industry. A simplified, mistuned, scaled-down steam turbine bladed disk model was developed using ABAQUS finite element analysis (FEA) software. The acquisition of the vibration characteristics and steady-state stress response of the disk models was carried out through FEA. Such studies are very limited. Subsequently, numerical stress distributions were acquired and the model was subsequently exported to Fe-Safe software for fatigue life calculations based on centrifugal and harmonic sinusoidal pressure loading. Vibration characteristics and response of the variation of the geometric blade of the steam turbine were investigated. Natural FEA frequencies compared well with the published literature of real steam turbines, indicating the reliability of the developed FEA model. The study found that fatigue life is most sensitive to changes in blade length, followed by width and then thickness, in this order. Analytical life cycles and Fe-Safe software show a percentage difference of less than 4.86%. This concludes that the numerical methodology developed can be used for real-life mistuned steam turbine blades subjected to variations in blade geometry

Transient Analysis of Thermal Bending and Vibration of Steam Turbine Rotor

Rotor-bearing systems often exhibit nonlinear behavior due to hydrodynamic effects and external forces. Finite element methods based on linear analysis are commonly used for rotor dynamic analyses, where nonlinear bearing/damping forces are linearized into equivalent stiffness and damping coefficients. However, this method may not accurately describe strongly nonlinear systems. Engineers use transient analysis and nonlinear models to improve rotor behavior analysis. This study investigates the effects of transient-thermal bending and vibration on a high-pressure steam turbine rotor using the finite element method. A scaled rotor-shaft was used to study thermal bending and vibrations caused by steam heat. The design of the shaft was based on an existing power station high-pressure turbine rotor. Numerical modal analyses were performed using ANSYS software to obtain a partial level of integrity between the numerical model and the analytical model. Natural frequencies were compared between the experimental, numerical, and analytical results, which showed good correlations

Fatigue life prediction of turbine blades with geometric imperfections made of stainless steel

This research addresses critical challenges faced by steam turbine blades, particularly in low-pressure (LP) turbines, where premature failures are common due to stress concentrations at the blade root area. The study introduces a numerical methodology aimed at predicting the life of mistuned steam turbine blades, with a focus on variations in blade geometry which have received limited exploration in existing literature. A simplified, scaled-down mistuned steam turbine bladed disc model was developed using Abaqus finite element software. Acquisition of steady-state stress response of the disc models was performed through finite element analysis (FEA). Thereafter, numerical stress distributions were obtained. Subsequently, within Companion software, a Monte Carlo simulation-based probabilistic approach was applied to evaluate and quantify uncertainties in fatigue life for 17 cases. This analysis considered an accepted manufacturing percentage scatter of ± 5 % for the steam turbine bladed disc. That was conducted by selecting mistuning (geometry variation) percentages as the random variables. The methodology demonstrated reliability, correlating well with literature-based and discrete fatigue life results. This study establishes the potential for accurately predicting the fatigue life of mistuned steam turbine blades using the developed methodology

Determination of the dominant failure mechanism of P92 steam piping subjected to daily operational cycle using finite element (FE) technique

In a bid to minimise the cost of electrical energy production through the reduction in the quantity of coal usage, the power generation companies resolved to operate different daily cycles characterized by peak and off-peak energy demand periods. These daily operational cycles have left the power plant components operating in a possible creep-fatigue regime. As such, earlier failure of the plant’s components such as the steam pipes due to creep, fatigue, or the combination of both failure modes became inevitable. This study employed finite element (FE) technique to determine the dominant failure mechanism and useful life of P92 power generation steam piping subjected to one of the daily operational cycles. The outcome of the study showed that the failure of the piping when subjected to the daily cycle is creep dominated, and failure due to fatigue or possible creep-fatigue interaction was impossible since the daily cycle was insufficient to induce the extensive plastic strain required for the initiation and propagation of fatigue failure. Hence, the best form of operating steam pipe/piping is steady-state, since subjecting them to daily cycles significantly reduce their useful life.Tshwane University of Technology; University of Pretoria and Eskom Power Plant Engineering Institute (Republic of South Africa).http://www.satnt.ac.za/index.php/satntMechanical and Aeronautical Engineerin

Creep-fatigue interaction of P91 steam piping subjected to typical start-up and shutdown cycles

The possibility of creep–fatigue interaction occurring in steam pipes of power generation plants during operation has been on the front burner for years. Often, failure of the pipes during operation is attributed to either creep alone or fatigue alone. Of recent, some failures are speculated to be due to simultaneously coupled interaction between creep and fatigue, especially when the failure occurs earlier than anticipated. The literature shows that such studies are very limited and indeed under-researched. Thus, there is a dire need to systematically investigate this coupled creep–fatigue phenomenon and provide clarity as the failure of high-pressure steam piping has consequential and very significant effects on the suppliers and end users. In this work, a special Fortran user subroutine script of a phenomenological modified hyperbolic sine creep model was developed and implemented in Abaqus CAE/2019 finite element code to initially determine the creep behavior of a P91 steam piping network subjected to a typical daily start-up and shutdown cycle. Subsequently, fe-safe/TURBOlife software was employed to investigate whether the failure induced by the start-up and shutdown cycles was due to fatigue alone, creep alone or due to creep–fatigue interaction. Interestingly, the study showed that the failure of the piping network under the specified operating conditions is specifically due to creep alone. Furthermore, the intrados of the elbow in the piping network was identified as the region most prone to failure, and the piping network will only survive a total of 7.1 and 7.7 years under these operating conditions for both machined and fine-machined surfaces, respectively. These results were thereafter analytically validated, and it showed a strong correlation with the numerically determined creep rate.https://www.springer.com/journal/11668hj2021Mechanical and Aeronautical Engineerin

Prediction and comparison of creep behavior of X20 steam plant piping network with different phenomenological creep models

In service, steam pipes are subjected to high temperature close to 0.4 Tm (melting temperature) or higher and pressure; thus, making them prone to failure due to creep. Often, the design methods for these steam pipes usually do not provide their specific in-service life; hence, some type of service fitness tests are performed, and data obtained from the tests are used to inform the routine inspections. Choosing a creep model that favorably describe the creep behavior of components in service is paramount to engineers as well as the plant operators. Reports have shown that there are several creep models available and they all behave differently with different materials, and operating conditions. In this study, the creep behavior of X20 (12Cr-1MoVNi) steam piping network subjected to three phenomenological creep models (conventional hyperbolic sine creep, modified hyperbolic sine creep and constitutive creep model) was investigated. Fortran user subroutine scripts were developed for the three models and implemented in finite element (FE) code, Abaqus to determine the creep stress and strain rate, while the useful creep life and creep damage was determined using fe-safe/TURBOlife software. The results show that the modified hyperbolic sine creep model is more suitable for estimating the creep behavior of X20 steam piping under the specified operating conditions because of its more conservative prediction.Tshwane University of Technology, the University of Pretoria, South Africa and Eskom Power Plant Engineering Institute (Republic of South Africa).http://link.springer.com/journal/116652021-10-28hj2021Mechanical and Aeronautical Engineerin

Comparative evaluation of creep response of X20 and P91 steam piping networks in operation

The geometrical increase in the demand for electrical energy has posed serious pressure on the power generation components such as the steam pipes due to the consequential increase in the operating parameters such as temperature and pressure. This increment in operating parameters tends to limit the useful life of these pipes. Thus, high creep resistant materials such as X20 CrMoV12-1 and P91 (9Cr-1Mo) are used to manufacture steam pipes. In this paper, the creep behaviour of X20 CrMoV12-1 and P91 (9Cr-1Mo) steam piping network subjected to typical operating condition was determined via a finite element analysis code, Abaqus CAE/2017 alongside fe-safe/Turbolife software, and their results were compared. The maximum creep stress, strain rate, creep damage and worst creep life in both piping materials were developed on the intrados of the elbow, with P91 steam pipe having higher useful creep life. Furthermore, a good correlation was achieved between the result of the analytically calculated and numerically simulated creep rate at the straight section of the piping networks.Tshwane University of Technology, the University of Pretoria, South Africa and Eskom Power Plant Engineering Institute (Republic of South Africa).http://link.springer.com/journal/1702021-07-24hj2020Mechanical and Aeronautical Engineerin

A critical review of alternative erection methods for overhead line towers

The construction of overhead power lines in the EHV (Extra High Voltage) category is a costly exercise. For the erection of the towers that supports the conductors, mobile cranes have become the dominant piece of equipment to erect these towers. Although convenient and relatively quick, these mobile cranes do come at a considerable cost and hence, alternative erection methods will be economically beneficial. This paper takes a critical review of alternative options to erect overhead power line towers thereby eliminating mobile cranes. The work presented here is part of a larger study to develop a numerical safety tool for the safe erection of guyed V-towers without cranes. It proposes the use of novel methods like air cushions and a degree of automation to lift these guyed V-towers autonomously. This paper deals with the first part of the study and analyses the different lifting configurations using gin poles and winches and selects the most suitable method to achieve this.http://www.irphouse.comam2020Mechanical and Aeronautical Engineerin

Comparison of infrared thermography and miniature Deltatron accelerometer sensors in the measurement of structural vibration characteristics

This paper investigates the comparison of infrared thermography (IRT) and miniature Deltatron accelerometer sensors in measuring structural vibration characteristics in terms of frequency and displacement, given that of age IRT has fully grown for temperature condition monitoring. In addition, IRT has been extensively applied in non-destructive techniques for evaluation of surface cracks through the observation of thermal imaging of vibration-induced crack frictional heat generation. Therefore, in order to conduct this study, both single and dual cantilever beam-like structures (AISI 304 steel) coupled with a slipping frictional rod (lacing wire) were subjected to forced excitations with an infrared camera capturing the thermal profile emanating from beam-lacing wire frictional interface. Concurrently, miniature Deltatron accelerometer sensors were attached to the beam surface next to the frictional interface focused by IR camera. The thermally analyst vibration characteristics parameters were compared against those acquired by accelerometers. The comparison of results exhibited a maximum relative difference of 0.28% and 14.88% for frequencies and displacements, respectively. This shows that IRT is more reliable in measuring structural vibration frequency than displacements. The finding is particularly useful in overcoming many limitations inherent in some of the current vibration measuring techniques such strain gauges failure due to fatigue.The Eskom Power Plant Engineering Institute (Republic of South Africa), the University of Pretoria and the Tshwane University of Technology.http://www.tandfonline.com/toc/rajs202018-12-20hj2017Mechanical and Aeronautical Engineerin

Infrared thermography applied to the prediction of structural vibration behaviour

This paper concerns the development of methodology for use of Infrared thermography (IRT) for online prediction of mechanical structural vibration behaviour; given that it has extensively been applied in non-destructive technique for evaluation of surface cracks through the observation of thermal imaging of the vibration-induced crack heat generation. To achieve this, AISI 304 steel cantilever beam coupled with a slipping friction rod was subjected to a forced excitations with an infrared camera capturing the thermal profile at the friction interface. The analysis of thermal image data recorded (radiometric) for the frictional temperature time domain waveform using a MATLAB FFT algorithm in conjunction to IR camera frequency resolution of 120 Hz and the use of the heat conduction equation with the help of a finite difference approach successfully identified the structural vibration characteristics in terms of frequency and displacement, the maximum relative errors being 0.09% and 5.85% for frequencies and displacements, respectively. These findings are particularly useful in overcoming many limitations inherent in some of the current vibration measuring techniques in harsh and remote environments.Eskom Power Plant Engineering Institute (South Africa), University of Pretoria and Tshwane University of Technology.http://www.elsevier.com/locate/aejam2020Mechanical and Aeronautical Engineerin