Prediction of fatigue life of mistuned steam turbine blades subjected to variations in blade geometry

Abstract

There is a large number of power stations suffering from fatigue failures of the steam turbine blades. The steam turbine blades are also subjected to steam flow bending, centrifugal loading, vibration response, and structural mistuning. These mentioned factors significantly contribute to the fatigue failure of the steam turbine blades. Low-Pressure (LP) steam turbines experience premature blade and disk failures due to the stress concentrations at the blade root area of its bladed disk. Driven by the problems encountered by the steam power plant electricity generating utilities with regards to steam turbine blades fatigue failure, this study of the mistuned steam turbine blades subjected to variation in blade geometry will be of great significance to the electricity generation industry. A simplified, scaled-down mistuned steam turbine bladed disk model was developed using ABAQUS finite element analysis (FEA) software. Acquisition of the vibration characteristics and steady-state stress response of the disk models was performed through FEA. Thereafter, 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. The vibration characteristics and the response of the variation steam turbine geometric blade was conducted. The FEA natural frequencies compared well with published literature of the real steam turbines indicating reliability of the developed FEA model. The study found that the fatigue life is most sensitive to changes in blade length, followed by the width, and then the thickness, in this order. The analytical life cycles and Fe-Safe software shows the percentage difference of less than 4.86%. This concludes that the developed numerical methodology can be used for real-life mistuned steam turbine blades subjected to variations in blade geometry.

Prediction of fatigue life of geometrically deviated steam turbine blades under thermo-mechanical conditions

“…These areas are considered to be the stress concentration areas, with high possible fatigue failure. These results are consistent with those of the recent studies on HCF damage illustrated in Mashiachidi and Desai (2023).…”

“…The FE model boundary condition was initially assigned by applying the rotational constraints to the center hub surface to prevent axial movements within the simulation. Therefore, a rotational body speed of 314.159 rad/s was applied on the entire bladed-disk model that rotates around the z-direction (as shown in Figure 3) acting along the center hub (Mashiachidi and Desai, 2023;Rani and Agrawal, 2023). Figure 4 illustrates the applied rotational body forces acting in an axial direction propagating from the base state.…”

“…The maximum modal-based von Mises stress found to occur at the two opposing blade roots is 155.6 MPa, as shown in Figure 11. The von Mises stress is equivalent to the mean stress (Mashiachidi and Desai, 2023). Mashiachidi and Desai (2023) established the distinction between HCF and LCF by comparing both simulated stress and analytical centrifugal force with the yield strength.…”

“…Prediction of the life of steam turbine blades subjected to varying blade geometries attached to a common disk is a great benefit in fossil fuel power plants (Booysen et al, 2015). While the turbine is in operation, the blades experience bending due to high-pressure flows, endure centrifugal stresses, and respond to significant vibrations caused by alternating stress conditions (Mashiachidi and Desai, 2023).…”

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

“…In Figure 3 the maximum FE simulation mean stress at blade roots is anticipated as 137.3 MPa. The mathematical centrifugal stress was calculated to be 121.8 MPa by Mashiachidi and Desai [15]. Both stresses are compared to the material yield strength of 205 MPa; thus it can be concluded that no yielding takes place as both the analytical and FE results fall below the maximum yield strength of the material.…”