Structural and Fatigue Life Evaluation of High Pressure
Stage Steam Turbine Blade and Disc
N S Akash
Department of Computer Science and Engineering
University of Visvesvaraya College of Engineering (UVCE), Bangalore, India
Email: nsakash752003@gmail.com
Abstract - This paper presents a comprehensive safety are paramount. Among the various components of a
structural and fatigue analysis of a high- steam turbine, the high-pressure (HP) stage blade and disc
are critical for ensuring smooth energy conversion from
pressure stage steam turbine blade and disc thermal to mechanical form.
operating under varying thermal and
mechanical loads. Using finite element These components are subject to complex loading
methods, the study evaluates stress distribution, conditions involving high rotational speeds, thermal
deformation behaviour, and potential failure gradients, and dynamic forces, making them susceptible to
zones within the turbine assembly. The blade structural fatigue and failure over time. The combination
of centrifugal forces and cyclic thermal stresses often leads
and disc models are constructed with detailed to low-cycle fatigue, which can significantly reduce the life
material properties and subjected to both expectancy of the turbine components if not properly
operational and overspeed conditions to identify accounted for during the design and analysis phase.
critical stress concentrations. Modal analysis is
conducted to determine natural frequencies and The study uses finite element analysis to evaluate the
prevent resonance. Fatigue life estimation is structural integrity and fatigue behaviour HP stage blades
and discs. By simulating real-world operational conditions,
performed to assess the impact of cyclic loading the objective is to identify stress-prone regions and assess
using both stress-based and strain-based their influence on component durability. The insights from
approaches. The results indicate that specific this work contribute to improving the design methodology
regions of the blade root and disc exhibit higher and maintenance strategies for enhancing turbine
stress accumulation, which may lead to efficiency and safety.
potential fatigue crack initiation.
LITERATURE REVIEW
Recommendations are provided for improving
component reliability through optimised design
Understanding the structural behaviour and fatigue
and material selection. The findings contribute mechanisms of turbine components has been a significant
to enhancing the safety, performance, and area of research in turbomachinery design. Early studies
durability of steam turbine components used in have emphasised the combined impact of centrifugal loads,
thermal power systems. vibratory stresses, and thermal effects on turbine blade
failure. Researchers have employed analytical and
numerical methods to predict stress distribution and fatigue
Index Terms - Fatigue analysis, finite element life under cyclic operating conditions, highlighting the
method, high-pressure turbine, modal analysis, importance of accurate modelling for reliable performance.
steam turbine blade.
Advanced computational techniques, such as finite element
INTRODUCTION analysis (FEA), have enabled detailed simulation of blade
and disc assemblies. The methods provide insights into
The global demand for efficient and reliable energy critical stress locations, mode shapes, and natural
generation has intensified the development of advanced frequencies that can influence resonance conditions.
technologies in thermal power systems. Steam turbines Various investigations demonstrated that failure often
play pivotal role in this domain, especially in large-scale originates from stress concentrations at the blade root or
power plants where high performance and operational
disc periphery, especially under low-cycle fatigue 4. To conduct modal analysis and determine the
conditions. natural frequencies of the blade-disc assembly.
5. To generate a Campbell diagram and assess the
Material selection has been a focus area, with high-strength risk of resonant vibration.
alloys such as chromium-vanadium and nickel-based 6. To estimate fatigue life using a local stress-strain
materials being studied for their resistance to high- approach under cyclic loading.
temperature fatigue and creep. Manufacturing techniques, 7. To compare results from stress-based and strain-
blade geometry, and attachment methods have been found based fatigue life prediction methods.
to significantly influence the structural integrity and
fatigue performance of turbine assemblies. These objectives support the development of more robust
and reliable steam turbine components through improved
J. S. Rao [1]: Investigated the estimation of a turbine analysis, material selection, and design optimization.
blade's life that considers the combined effects of
temperature, vibratory, and centrifugal loads. The stresses I. Finite Element Analysis
are computed by accounting for the rotor acceleration. The
aerodynamic excitation force acting on the turbine blades Finite Element Analysis (FEA) serves as the primary
under incompressible flow is derived from the theory of computational tool employed in this study to evaluate
thin-cambered aerofoils. structural integrity and fatigue life of high-pressure (HP)
W. J. Kearton [2] described a steam turbine as a heat stage steam turbine blade and disc. The FEA methodology
engine, which enables the pressure and temperature energy provides a detailed, numerical representation of the blade-
of steam to be transformed into useful work. This disc assembly, allowing for the simulation of complex
transformation is achieved by expanding the steam through loading conditions, stress distributions, and potential
nozzles, giving a high kinetic energy, which is converted failure modes that would be difficult to replicate through
into useful work by impinging on moving blades mounted physical testing.
on the rotor. A turbine containing two or more stages is
known as a multi-stage turbine.
1. Model Development: The first step in FEA
Christoph-Hermann Richter [3] has been able to provide
process involves creating a detailed geometric
an overview of the structural design of modern steam
model of blade and disc assembly. The model is
turbine blades at Siemens Power Generation using the
constructed using high-precision CAD software,
finite element code ADINA. Geometry and loading of the
ensuring that critical features, including cooling
various types of blades are explained in depth. It is
channels, blade fillets, and disc interfaces, are
demonstrated that the modular building block approach to
accurately represented. The geometry is then
modelling is crucial.
meshed into finite elements, with finer mesh areas
focused on regions expected to experience high-
OBJECTIVES OF THE PRESENT WORK
stress concentrations, such as blade roots and the
disc-blade attachment points.
The primary aim of this research is to investigate the 2. Material Properties and Boundary Conditions:
structural behaviour and fatigue life of a high-pressure For an accurate simulation, material properties
stage steam turbine blade and disc under realistic operating corresponding to the high-strength alloys used in
and overspeed conditions. Utilising finite element analysis, turbine blade and disc construction are assigned
the study focuses on evaluating stress concentrations, to the model. These materials typically
deformation behaviour, and potential failure points within characterised by tensile strength, yield strength,
the blade-disc assembly. and fatigue resistance. Boundary conditions
representing the actual operational environment,
The specific objectives of the present work are as follows: including thermal gradients, centrifugal forces,
and boundary constraints at the blade-disc
1. To perform structural analysis of the HP stage interface, are applied to replicate realistic loading
blade-disc sector at standard and overspeed scenarios.
conditions. 3. Static Structural Analysis: The first phase of FEA
2. To identify high-stress zones and evaluate their involves performing a static structural analysis,
impact on structural durability. which evaluates the deformation and stress
3. To compute safety margins against centrifugal distribution within the blade-disc assembly under
forces, which are primary contributors to fatigue standard operating conditions. This analysis
failure. determines regions of high stress, including those
potentially subject to fatigue failure, allowing for
an initial assessment of the structural integrity of transferring its kinetic energy to the turbine rotor.
the components. The blades are mounted on the rotor, which is a
4. Dynamic Analysis: In addition to static analysis, rotating component of turbine. The nozzles, on
dynamic analysis is conducted to account for the the other hand, are typically fixed to the stationary
impact of fluctuating loads and dynamic forces part of turbine, often referred to as the stator,
encountered during turbine operation. This casing, or cylinder.
includes investigating the influence of rotational
speed, temperature changes, and transient forces
on the blade and disc assembly. Modal analysis is
performed to determine the natural frequencies of
the system and identify any potential resonant
vibrations that may exacerbate fatigue failure.
5. Overspeed Conditions: The turbine blades and
discs are subjected to overspeed conditions during
abnormal operation, and behaviour under this
circumstances is critical to understanding the
safety margins of turbine components. The FEA
model simulates these overspeed scenarios to
assess the impact on the stress and deformation
characteristics, providing insights into the
system's stability during extreme operational
conditions.
6. Fatigue Life Prediction: Using the results from the
static and dynamic analyses, a fatigue life
estimation is performed based on both stress-
based and strain-based fatigue prediction models.
These models consider the cyclic loading Figure 2.1 illustrates a Simple Impulse Steam Turbine.
conditions, material properties, and stress The diagram consists of a longitudinal section through the
concentrations to estimate number of cycles to upper half of the turbine, with the development of the
failure for different regions of the blade-disc nozzles and blades depicted in the middle section.
assembly. The results are then used to evaluate the
overall fatigue life and provide recommendations
for design modifications to enhance component
longevity.
II. Major Components of Steam Turbine
The processes of steam direction-changing and expansion
can vary, depending on the design and operational
requirements, and may occur sequentially in certain
configurations. The steam is directed through the turbine’s
moving components, also known as blades, in such a
manner that the pressure at the inlet and outlet of the blades
remains equal. This type of turbine is generally referred to
as an impulse turbine.
In most steam turbines, two primary components (or sets
of components) are present:
The steam turbine is divided into three stages: high-
1. The Nozzle: This component facilitates the pressure (HP), intermediate-pressure (IP), and low-
expansion of steam, transitioning it from a pressure (LP). These stages affect the energy-converting
relatively high-pressure, stationary state to a high- blades and influence material selection and design. The HP
velocity moving state. stage involves high pressure and temperature entering the
2. The Blade or Deflector: These components alter turbine, requiring blades to handle extreme conditions. The
the direction and momentum of the steam flow, IP stage involves partial expansion and pressure reduction,
designed for moderate pressures and temperatures. The LP Finite Element Model
stage expands the steam further, requiring blades to resist
corrosion and erosion from steam flow. Each stage The finite element (FE) model of the HP stage blade-disc
optimizes performance under specific operating sector is created using appropriate elements from the
conditions. Figure 2.2 provides an overview of stages. element library. A solid brick element with 8 nodes is
selected to discretize the geometry. To ensure
convergence, the aspect ratio of the blade fillet and disc
fillet regions is kept below 5%, while the aspect ratio of the
III. Methodology disc is maintained below 10%.
Geometric Modelling of Blade and Disc Assembly For the contact between the blade and disc, a contact 173
element is used for the blade region, and a target 170
element is used for the disc region in this analysis. The
For this analysis, the HP stage row blades are considered,
typically tip-shrouded to resist the significant bending model consists of 40,757 nodes and 10,943 elements to
forces from steam loads. The blade-disc sector geometry is achieve convergence. The blade and disc at the T-root are
discretized with similar element divisions to maintain
modeled using CatiaV5, a commercially available solid
modeling software. consistency, with a fine mesh applied in this zone to ensure
continuity.
The aerofoil surface is generated from these coordinates,
and the T-root of the blade is designed with a fillet to avoid The FE model geometry shown in Fig. 3.2, Fig. 3.3, and
sharp corners, ensuring smooth analysis. The geometric Fig. 3.4.
model of the HP stage blade-disc assembly shown in Fig.
3.1.
IV. Material Properties
Chromium-Vanadium alloy materials are considered for
the present analysis. The chromium vanadium steel with
different propositions considered to blade and disc
respectively. The mechanical properties of blade and disc
at room temperature 270chave been listed in the Table 4.1
Model Description:
• Blade height: 25.4 mm
• Leading edge thickness: 2 to 4 mm
• Trailing edge thickness: 0.19 to 0.2 mm V. Results and Discussions
• Number of blades: 60
• Sector angle: 6° Displacement Analysis
• Mean diameter: 250 to 350 mm
The displacement of the blade at 100% speed ranges from
0 mm to 0.18568 mm, while at overspeed conditions, it
ranges from 0 mm to 0.20942 mm. These results show that
the blade does not experience significant radial growth
under normal or overspeed conditions, preventing tip
rubbing. Fig. 5.1 shows the radial growth of the blade at
6000 rpm.
Stress Analysis Fatigue Life Analysis
1. Blade Stresses
Peak tensile stresses in the HP blade-disc sector are critical,
The von-Mises stress in the blade ranges from 0.8338 MPa as they can initiate cracks that may lead to failure during
at the tip to 581.93 MPa at the root (Fig. 5.2). Tensile operation. This study evaluates low-cycle fatigue life for
stresses, particularly at the root (571.3 MPa), influence the an expected 4000 cycles, representing 30–40 years of
low-cycle fatigue life of the blade. At 100% speed, the turbine use.
average von-Mises stress at the blade neck is 101.04 MPa
(Fig. 5.3), and 146.07 MPa at 120% speed. These stresses Using the local stress-strain method and S-N curves (Fig.
are within the blade's yield stress limit of 590 MPa, 5.5, 5.6), the blade and disc are found to have an infinite
indicating no risk of failure. fatigue life at both 100% and 120% operating speeds.
Applying the Zero-Maximum-Zero stress approach, the
estimated fatigue life is 1.81×10⁵ cycles for the disc and
1.91×10⁵ cycles for the blade at full speed. The factor of
safety ranges from 2.3 to 15, indicating the assembly
remains structurally safe under fatigue loading.
2. Disc Stresses
In the disc, stresses range from 27.218 MPa to 596.05 MPa,
with peak stress occurring at the blade-disc interface (Fig.
5.4). The average stress at the disc’s minimum cross-
section is 125.54 MPa at 100% speed, and 180.77 MPa at
120% speed. With a yield stress of 540 MPa, these stresses
are within safe limits.
CONCLUSION under dynamic loading by avoiding resonance conditions,
preventing high-cycle fatigue failure.
The analysis of the HP stage blade-disc assembly in this
study led to several key findings:
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