DESIGN AND ANALYSIS OF STEAM TURBINE ROTOR

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 6, Issue 11, Nov 2015, pp. 195-201, Article ID: IJMET_06_11_022
Available online at
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=11
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
DESIGN AND ANALYSIS OF STEAM
TURBINE ROTOR
M. Chandra Sekhar Reddy
Department of Mechanical Engineering
University College of Engineering
Osmania University
Hyderabad – 500 007, India
ABSTRACT
Rotor is an very important part in the machines, especially in the rotating
machines like gas and steam turbines. In this paper steam turbine rotor is
analysed by using finite elements. In the complex systems, many of the
engineering problems, it is difficult to solve the problem by closed form or
exact solution method. Then we have to go for some numerical/approximate
method for solving the problem. There are lot of numerical/approximate
methods available. Finite element technique is an numerical method used for
many engineering applications very widely. We have analyzed the rotors acted
by different mechanical & thermo-mechanical loads, and analysed to find out
the behaviour of the rotors. In the analysis results it is seen that the solid rotor
is better than the hollow rotor.
Cite this Article: M. Chandra Sekhar Reddy. Design and Analysis of Steam
Turbine Rotor. International Journal of Mechanical Engineering and
Technology, 6(11), 2015, pp. 195-201.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=11
1. INTRODUCTION
Steam Turbine is a very sophisticated machinery. Its each and every component is
designed by doing a lot of calculations & experiments. The main component of a
steam turbine is its rotor, it is the part which is both under thermal & mechanical
stresses. The shafts used in steam turbines can be both hollow or solid. Previously
shafts were made hollow due to forging defects. During forging all the impurities
collected in the core of the shaft, due to this there was chance of crack formation &
failure of shaft. But nowdays due to improvement in forging techniques & fault
detection methods the impurities in the core can be made very little ie within tolerable
limit.
It has reduced a lot of issues such as machining cost & time, scrap, stiffness &
stress in the rotor shaft. So in the current times industries started using solid forged
shafts for their turbine rotors & in this paper we will compare the two shafts on
various aspects using finite element analysis. With the firm establishment of the

principles of finite element analysis, it is found that the development of element
characteristics will follow a prescribed path once the shape functions have been
chosen. For instance in the analysis of plane stress or strain once the functions
describing the displacements within the element in terms of the nodal values are
known, standard expressions can be used and the element properties are uniquely
defined. The possibilities of improvement of approximation are thus confined to
devising alternative element configurations and developing new shape functions.
The difficulty in analysis of stress and strain in structural engineering depends on
the structure involved. As the structure grows in complexity, so does the analysis.
Many of the more commonly used structures in engineering have simplified
calculations to approximate stress and strain. However, these calculations often
provide solutions only for the maximum stress and strain at certain points in the
structure. Furthermore, these calculations are usually only applicable given specific
conditions applied to the structure.
As FE models should meet the quality criteria stipulated[1-3], element quality
checks ensure least model errors and sanity checks verify the robustness or integrity
of structural models. Babu et al.[4], studied the determination of Stress Concentration
Factors of a Steam Turbine Rotor. Thermal gradients developed during thermal
transients[5] are the key source of stress generation in the rotor. Under such
conditions there is the probability of failure of turbine rotor if the turbine rotor is not
designed taking into consideration the transient effect. There are many Finite element
packages available for conducting the transient thermal analysis. Kolhe[6] presented
how to vary the ambient temperature with time, vary the convective heat transfer
coefficients and heat flux with time/temperature. The temperature gradients that can
be established in the transient state are generally higher than those that occur in the
steady-state and hencethermal shock is important factor to be considered relative to
ordinary thermal stress. The “heart” of these versatile machines is made by the blades
and vanes , which are subjected during operation to very high thermal and mechanical
stresses (combined effects of centrifugal force and thermal gradient), in aggressive
environment. The turbine rotor is subjected to temperature variations in short periods
of time due to the start and stop cycles of the turbine.
2. METHODOLOGY
To replace the existing hollow rotor shaft with a solid shaft & find out the advantage/
disadvantage of doing so. We have to perform various calculations on centrifugal
loads, torsional loads Thermal loads &core defects for the comparison. When a Steam
turbine rotor rotates at 3000 rpm the blades exert a centrifugal pull on the rotor discs.
If the discs are integrated to the shaft of rotor then there is a reduction in length of the
rotor & expansion in the rotor discs. Now the turbine is a very sophisticated machine
in which the clearances between the blades & casing is very low to avoid steam
leakage & efficiency loss. In order to properly design such fine clearances the
mechanical as well as thermal expansion of the blades & rotor must be calculated
accurately. Theoritical calculations can give a value for expansion/contraction, We
have to verify this value by FEM analysis in which all the real constraints are defined.
The result of the analysis must give the results within acceptable range of theoretical
result. Since the rotor is symmetric about its rotational axis we use axisymmetric
modelling technique to create the model. After modelling meshing has to be done
with proper element shapes &size to get the desired results. The FE model of IP Rotor
of Steam turbine is shown in Fig. 1.

Design and Analysis of Steam Turbine Rotor
IP ROTOR
Figure 1 IP Rotor of Steam turbine
3. SOLUTION METHOD
The preferences were chosen as structural & h-Method was used for elements with
plane 183. Plane183 is a higher order 2-D, 8-node or 6-node element. PLANE183 has
quadratic displacement behavior and is well suited to modeling irregular meshes (such
as those produced by various CAD/CAM systems). The simplest type of element has
a linear shape function. This means that the function for displacement across the
element is linear. With the h-method, the shape function of the element will usually be
linear. In an actual part, it is quite uncommon for the displacement to vary linearly.
The h-method accounts for this by increasing the number of elements. More accurate
information is obtained by increasing the number of elements. The element behaviour
is taken as axisymmetric.
Material Properties: The properties of element are defined which are the properties
of the material of the rotor. The youngs modulus is taken as-2.11x1011
& poissons
ratio is taken as 0.3. Density of material taken as 7850 kg/m3
. Length of rotor as 5.34
m
Figure 2 Keypoint diagram of the turbine rotor

Figure 3 The FE model filled with area for meshing
FEM Modelling: The rotor modelling was done by studying the manufacturing
drawing. Keypoint diagram of the turbine rotor is shown in Fig.2. Keypoints were
generated keeping the Y axis as the axis of rotation. The model was then generated by
joining the keypoints by straight lines. After the line model was made areas were
created to form the mesh. Fig. 3 shows the FE model filled with area for meshing.
Both the solid & hollow rotor were modelled separately. Inner radius of hollow rotor
was kept 60 mm & outer 300mm.
Meshing: The mesh was created by using quad elements the mesh type used was free
meshing. The size of elements was decided by global element size.The mesh was
distorted at many places with some quads not forming a perfect square,but it was due
to the shape of the rotor and the places where sudden geometry change had occoured.
But still quads were defined & mesh was acceptable
Quad Element- Refers in general to any four-sided, 2D element. Plane 183 is a
higher order 2-D, 8-node Plane183 has quadratic displacement behavior and is well
suited to modeling irregular meshes. This element is defined by 8 nodes , having two
degrees of freedom at each node: translations in the nodal x and y directions. The
elementmay be used as a plane element (plane stress, plane strain and generalized
plane strain) or as an axisymmetric element. This element has plasticity,
hyperelasticity, creep, stress stiffening, large deflection and large strain capabilities. It
also has mixed formulation capability for simulating deformations of nearly
incompressible elastoplastic materials and fully incompressible hyperelastic materials.
Initial stress import is supported.

Figure 4 Meshed model with loads applied
Application of Loads: Model completely meshed with quad elements in free mesh
format and the loads applied are shown in fig. 4. The loads applied on the rotor were
as follows-
• Displacement-All the translation degrees of freedom in X &Y direction were arrested
• Inertia- Angular velocity of 314.16 rad/3000rpm was applied on the rotor along y axis
• Force- Centrifugal Loads in X direction on the rotor discs were applied.
4. RESULTS & DISCUSSIONS
Solution: After defining all the conditions the FEM analysis was done and the results
were plotted. The stress distribution in the rotor is shown in Fig. 5, and temperature
distribution is shown in Fig.6.
Figure 5 Stress distribution in the rotor

Figure 6 Nodal Temperature Distribution
The result of the FEM analysis using Quad element in free mesh for solid rotor is
0.332 and 0.346 m for hollow rotor. The effect of torsional stress on solid rotor was
found to be 1768 N-m & on hollow rotor it is 1771 N-m. After conducting Von Mises
analysis the maximum stress in both the rotors was 139 Mpa which is lower than the
yield strength of alloy steel ie in the range of 366-1793 Mpa. So the design is safe for
engineering purpose.
The thermal temperature distribution analysis on the two rotors the pattern of
distribution was same for both. The first stage of blades experiences the maximum
thermal shock as the steam impinges on it with max temperature. Thermal expansion
for both rotors in steady state was carried out and it was seen that the hollow rotor
expands by 1 mm more than solid rotor.
5. CONCLUSIONS
In this paper an attempt is made to study the effect of making ID of present shaft to
zero, ie making it into a solid shaft & study the effects of the forces which were
applied on hollow shaft. In the results till now the solid shaft is proving more
advantageous to hollow shaft.
In the solid rotor there is no stress pattern in the center of shaft so the design is
much safe than hollow rotor design.
Thermal expansion of hollow rotor is more than that of solid rotor so it will
expand quickly as compared to the casing which is relatively thick. So chances are
there that the rotor blades may get jammed. In solid rotor the rotor will expand slowly
so that the casing & rotor will expand at nearly same rate & jamming will be avoided
which will prevent damage to the seals & blades.
We have analyzed the rotors on different mechanical & thermal-mechanical
analysis to find out the behaviour of the two rotors. In the analysis result it is seen that
the solid rotor is better than the hollow rotor in both ease of manufacturing & failure
criteria. Till now we have covered steady state thermal analysis of the rotors more
analysis could be done by applying transient conditions ,as after sometime the
temperature distribution changes.

REFERENCES
[1] David Heckman (1987), “Finite Element Analysis of Pressure Vessels”,
University of California.
[2] Colin Bradley, Bernadette Currie (2005), “Advances in the Field of Reverse
Engineering”, Computer Aided Design & Applications, Vol. 2, No. 5, pp 697-
706.
[3] R.W. Edmonson, “Dimensional Changes in Steel during Heat Treatment”, Met.
Treat., Vol 20 (No 6), 1969, pp 3–19.
[4] R. Nagendra Babu, K. V. Ramana, and K. Mallikarjuna Rao (2008 ),
“Determination of Stress Concentration Factors of a Steam Turbine Rotor by
FEA” World Academy of Science, Engineering and Technology, 39.
[5] Chunlin Zhang, Niansu Hu, Jianmei Wang, Qiping, chen, Feng He,Xiaoli (2010),
“Thermal Stress Analysis for Rotor of 600MW Steam Turbine” 978-1-4244-
4813-5/10/&25.00c/2010/IEEE.
[6] Kolhe M R, A. D. Pachchhao, H.G. Nagpure (2004), “thermal stress analysis in
steam turbine rotor - a review” Computer Aided Design & Applications, Vol. 1
(4).
[7] M. Chandra Sekhar Reddy and Talluri Ravi Teja. New Approach to Casting
Defects Classification and Optimization by Magma Soft. International
Mechanical Engineering and Technology, 5(6), 2014, pp. 25-35.
[8] Prof. Nasar A, Dr. N E Jaffar and Sherin A Kochummen. Lyapunov Rule Based
Model Reference Adaptive Controller Designs For Steam Turbine Speed.
International Mechanical Engineering and Technology, 5(6), 2014, pp. 25-35

DESIGN AND ANALYSIS OF STEAM TURBINE ROTOR

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DESIGN AND ANALYSIS OF STEAM TURBINE ROTOR