30120130405016

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME
139
DESIGN AND ANALYSIS OF SADDLE SUPPORT: A CASE STUDY IN
VESSEL DESIGN AND CONSULTING INDUSTRY
Pallavi J.Pudke1
, Dr. S.B. Rane2
, Mr. Yashwant T. Naik3
1
Post Graduation Student- Department of Mechanical Engineering, Sardar Patel College of
Engineering, Mumbai
2
Associate Professor, Department of Mechanical Engineering, Sardar Patel College of Engineering,
Mumbai 400058
3
Design Engineer, Process Equipment Division, Engineering Department, Zamil Industry, Dammam
31421, Kingdom of Saudi Arabia
ABSTRACT
In order to investigate the impact of different geometrical parameters like number of gussets,
and gusset thickness, on the design of saddle support, the horizontal pressure vessel is designed and
analyzed by Finite Element Method. The saddle support of the horizontal pressure vessel is designed
as per Handbook of the Pressure Vessel by Megyesy. The solid model of pressure vessel with
realistic details of two saddle supports is analyzed by using FEA software package ANSYS APDL.
The purpose of this paper is to optimize the geometrical parameters of the saddle and to find whether
saddle support can sustain the horizontal pressure vessel under loading conditions.
We found 28 % reduction in material requirement which results reduction in costs.
Keywords: ANSYS, APDL Technology, Finite Element Method, Pressure Vessel, Saddle Supports
1. INTRODUCTION
Saddle supports are commonly used to support Horizontal pressure vessels. A Pressure vessel
is a closed cylindrical vessel, widely used in process industry, power, oil and gas industries, and also
for the storage of fluid or gaseous products. Pressure vessels are subjected to pressure loading i.e.
internal or external operating pressure different from ambient pressure. The pressure vessels are of
horizontal or vertical type. For horizontal vessel the saddle supporting system plays an important role
in the performance of the equipment. A proper saddle supporting system improves safety and
facilitate to operate the pressure vessel at higher pressure conditions which finally leads to higher
efficiency. The optimized designs parameters reduce the material cost.
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 4, Issue 5, September - October (2013), pp. 139-149
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2013): 5.7731 (Calculated by GISI)
www.jifactor.com
IJMET
© I A E M E

140
2. LITERATURE SURVEY
Generally, horizontal cylindrical pressure vessels are supported by two symmetrically placed
saddle supports, which cause stresses in the pressure vessel in addition to the stresses generated by
the internal pressure in the vessel [1]. More than two supports would result in static indeterminacy
and cause difficulty in predicting the load distribution in the event of foundation settlement [2] . One
saddle is fixed to the foundation, while the other saddle is free to move axially. This incorporates an
element of axial flexibility and prevents induced stresses due to overall temperature variation. The
presence of supports has two distinct effects on the vessel. Firstly it interferes with the normal
expansion of the vessel due to internal pressure or temperature change; secondly the concentrated
support reaction induces highly localized stresses in the support region[2].A rigid support will give
rise to greater stress concentration compared to a flexible one. The main cause of stress concentration
is the abrupt transition of structural rigidity between the support and the vessel [3].The saddle
structure itself is stressed, as all forces acting on the vessel are ultimately transferred to the support.
The saddle support will have subsidiary stress and the internal stress of the pressure vessel,
So the design of the saddle support is critical when designing the horizontal type pressure vessel [4].
Therefore the design of the saddle and determination of the stresses induced in it are important steps
during the design of horizontal pressure vessel. The semi-empirical method [5-8], is based on the
beam theory. It has assumption that the vessel cross-section remains circular under loadto simplify
the problem. However, Zick’s analysis is better judged on the performance it has demonstrated, since
it was first published, and therefore, it is also the basis of saddle design guidelines given in pressure
vessel design handbook by E.Megyesy. A parametric study is presented to determine the peak
circumferential stress at the saddle support of an un-stiffened horizontal cylindrical vessel [9-10]
Wilson and Tooth were the "first to study flexible saddle supports using the "Finite Element
Method [11].They used cylindrical shell theory to model the pressure vessel and a two-dimensional
"finite element program to simulate the flexible saddle. The solution of the shell problem was
obtained using a Double Fourier Series Expansion.However, moreaccurate analyses, based on
cylindrical shell theory, are available [12-14].El-Abbasi performed a 3D finite element analysis of a
flexible and loose fitting saddle supported pressure vessels using a newly developed finite element
that accounts for the contact stresses between the vessel and the saddle supports. They concluded that
a saddle radius 1-2 % larger than that of the vessel leads to a 50% reduction in the stresses and an
overhang of 5-10% leads to25 – 40 % reduction. Widera also performed 3D FEA with welded
support[15-16].Nash and Banks [17-18] used a standard penalty based approach to account for
contact effects in sling-supported composite pressure vessels. Their solution was highly sensitive to
the choice of the user-defined penalty parameter. A more accurate contact formulation was presented
by Bisbos et al. [19] for horizontal pipes loosely resting on saddle supports. The unilateral contact
conditions and Coulomb's friction law were used to formulate two linear complementary problems in
the normal and tangential directions. Finite element analysis of Pressure vessel by David Heckman
[23] also advocates the use of computer programs instead of hand calculations for analyzing the high
stress areas and different end connections, Ong LS has also given a computer program for cylindrical
shell analysis[24].
Ong and Lu[20] determined the optimal radius of the support with a preliminary clearance
between the vessel and saddle. In the area of the vessel–saddle contact they assumed a constant
distribution of the contact pressure along the vessel, but varying circumferentially. They performed a
parametric analysis aimed at reducing the stress concentration at the saddle horn. Tooth analytically
and experimentally determined the stresses in real supports of multilayered GRP vessel Banks
presented the approximate solutionof the strain state. [21-22].Boutros discussed the results of
parametric analysis of deformable saddle supports of circular cylindrical vessels of large diameter.

141
He indicated the influence of proportions between vessel dimensions and support locations of the
stresses occurring in the structure [25-26].
Therefore we have identified the following research gaps.
1. The stress analysis of saddle support by varying the gusset thickness is the area of research
attention.
2. The stress analysis of saddle support by varying the number of gusset is theanother area of
research attention.
3. The solid modeling is done in ANSYS Parametric Design Language for Analysis of saddle
support.
Therefore we have formulated the following problem statement taking into consideration the standard
nomenclature.
3. NOMENCLATURE AS PER MEGYESY DESIGN CODE
Q = Load on one saddle.
R = Radius of shell
ts = Wall thickness of shell
th = Wall thickness of head.
K = Constant values
Ѳ = Constant angle of saddle degree
R = Radius of the pressure vessel
A = The distance from the tangent line to the saddle centre.
H = The distance from the tangent line to the end of the head.
L = The length from tangent to tangent line of pressure vessel.
b = Width of saddle.
P = Design pressure
T = Design temperature
Ww = Width of wear plate
Wb = Web plate thickness
F = Force
Tw = Wear plate thickness
4. DEVELOPMENT OF PROBLEM STATEMENT
4.1 Material Properties
Material used for shell and head, SA 516 Gr.70, maximum allowable stress value=133.6 N/mm².
Material used for saddle support parts, SA 283 C, Maximum Allowable stress value = 105.2 N/mm².
4.2 Assumptions: The wall thickness of shell is assumed to be constant everywhere.
4.3 A sketch of the vessel is shown in Fig. 1.

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Fig1. Pressure vessel and saddle structure
4.4. Geometric Parameters
Table 1. Input data
Inputs Geometry Parameters Values
Geometric
Shell Thickness 25 mm
Shell Radius 2900 mm
Shell Length 37350 mm
Wear Plate Width 850 mm
Base Plate Length 4000 mm
Head Thickness 25 mm
Wear Plate (Pad) Thickness 25 mm
Contact Angle 160 ˚
Gusset Thickness 25 mm
Material
Saddle SA 283 C
Heads SA 516 Gr.7
Shell SA 516 Gr.70
Loading Input
Load On One Saddle 6012549 N
Design Temp (Int.) 315° C
Design Pressure (Int) 3.5 Kg/cm²
Problem statement: To design and optimize the saddle support by using ANSYS APDL technology
for above said horizontal pressure vessel resting on two saddle support where one is fixed and
another is flexible for above said material and parameters for above said boundary conditions.
5. FINITE ELEMENT ANALYSIS
FEA is now an extremely sophisticated tool for solving numerous engineering problems and
is widely accepted in many branches of industry. It is a numerical technique for finding approximate
solutions to boundary value problems. Finite Element Analysis is used to solve numerically the
governing equations for stress within the wall material. The solutions provide a complete stress
distribution in the saddle supports.

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5.1 Finite Element Model
It operates under the circumstances of ANSYS APDL Technology. We created the complete
solid models of the pressure vessel and saddle support. The pressure vessel is filled with liquid and is
subjected to the operating weight. Map meshing is done to the complete solid model. The scope of
analysis is limited to study the stresses for saddle support region under defined loading conditions.
Appropriate extents of support saddles located at sliding and fixed side are considered for application
of displacement boundary condition.
Fig.2. FEA model of Pressure Vessel on Saddle Support
5.2 Boundary Conditions
The boundary conditions of the sliding support are free and constrained. The movements of
saddle in Z-axis directions are not constrained:Uz≠0. The other movements of saddle are fixed:
Uy = Ux = 0
While the boundary conditions of the fixed support are fixed. The movements around the
three axes are constrained : Ux = Uy = Uz = 0
Fig. 3. FEA model with Boundary Conditions applied

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5.3 Mesh Sensitivity Analysis
A mesh sensitivity analysis is performed on the FEA model of pressure vessel on saddle
support, to ensure the optimum mesh size for proper convergence and accurate numerical results.
The value of maximum Von Mises stress occurring in the structure is used as a convergence
criterion. The figure 4. shows that from the element number 2136061 the mesh become precise. It
also shows that as the number of element reaches 2626264, themeshing in the model of pressure
vessel on saddle support receives enough sensitivity.
Fig. 4. Graph of Mesh Sensitive Analysis
Fig.5. Meshed model of Saddle Support
6. RESULTS AND DISCUSSION
6.1 FEA Results
In this section, the results gave a detailed distribution of local stresses in the saddle support
area.It is seen that the maximum stress is located near the horn of the saddle.
The effects of different design parameters on the stress values are investigated. The
Reinforcement Pad [RF] width and thickness has been kept constant as they are suitable for large
vessel.

145
Fig. 6. Stress pattern of saddle support
Figure 6. illustrates the Stress pattern of saddle support which shows the location of maximum
and minimum stress in saddle.
Fig. 7. Impact of no. of gussets for saddle on primary stress
The Figure 7 shows the effect of number of gussets on the value of primary stresses obtained
from the analysis results. From the graph it is understood that for 18 mm gusset thickness with 6
gussets, the obtained Primary stress result value falls below the Primary stress limit. So design is
safe.

146
Fig.8. Variation of primary stress values and secondary stress values for gusset thickness
The Figure 8. shows the variation of primary stress values and the secondary stress values
with respect to gusset thickness obtained from the analysis results. Primary and Secondary ASME
allowable values are plotted to check the design. From the graph it is seen that the obtained
secondary stress values falls below the Secondary stress limit for the gusset thickness of 18 mm and
for 10 gussets.
6.2 FEA and Megyesy Stress Results
Table 2
Table 2: shows the stress results obtained by the FEA method and design calculations by
MEGYESY.

147
Fig. 9. Graphical presentation of FEA and MEGESSY results
The stress results obtained in the Megessy are the average stresses acting on the area of the
Gusset plate in (N/mm²). From the results of design calculations and analysis it is observed that
analysis results are closed to that design value which is acceptable.
7. CONCLUSION
The empirical approach corresponds to design by rule and finite element analysis corresponds
to design by analysis method are adopted and calculations were made according to Megyesy,
(Pressure Vessel Handbook).The stress distribution of various geometric parameters of gussets and
number of gussets of saddle is observed to select the optimal size of saddle. This shows that the
design by analysis is the most desirable method to evaluate and predict the behaviour ofdifferent
configurations of saddle supports. The comparison of these results helps to provide the most
optimized design with an ability to meet the requirements.
After analysing the stress behaviour of the pressure vessel with different geometrical
parameters of saddle supports, it is concluded that the saddle support of given Horizontal pressure
vessel is safe according to the both the results. The stress values obtain by empirical method and
analysis stresses are below allowable limits which is acceptable.
The recommended gusset thickness was 25mm in industry whereas the optimal value is
18mm. Therefore 28 % of thickness reduction of the gusset is achieved, which leads to saving
material and cost associated with it.

148
8. REFERENCES
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supports, Int. J. Pres. Ves. & Piping, Vol 87 March(2010). pp 239-244.
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loadings, Int. J. Pres. Ves. & Piping ,Vol.35 March (1988).pp423-427.
[3] L. S. Ongand G.Lu ,Stress reduction factor associated with saddle support with extended top
plate ,Int. J. Pres. Ves. & Piping, Vol.62 March (1994).pp 205-208.
[4] RenHaidong, PengErbao ,A Study on the stress distribution of the pressure vessel and saddle
support, International Conference on Electronic & Mechanical Engineering and Information
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[8] L. P. Zick and C. E. Carlson., Strain gage technique employed in studying propane tank
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[10] L.S. Ong ,J.S.T. Cheung ,H.W. Ng , A.S. Tooth, Parametric equations for maximum stresses
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[11] Wilson JD, Tooth AS., The support of unstiffened cylindrical vessels. Second International
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[12] Ong Lin Seng, Effectiveness of wear plate at the saddle support. Journal of Pressure Vessel
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[20] Ong LS, Lu G. Optimal support radius of loose-fitting saddle support. International Journal of
Pressure Vessel Piping 1993;54:465-79.
[21] Tooth AS, Banks WM, Seah CP, Tolson BA, The twin-saddle support of horizontal multi-
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International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 4,
2013, pp. 105 - 117, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.
Books:
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