Design and optimization_of_saddle_for_ho

www.ierjournal.org International Engineering Research Journal (IERJ) Special Issue 2 Page 4201-4204, 2015, ISSN 2395-1621
© 2015, IERJ All Rights Reserved Page 1
ISSN 2395-1621 Design and Optimization of Saddle
For Horizontal Pressure Vessel
#1
Mr. Amarnath Y. Zore, #2
Prof. Mudassar G. Qaimi
1
amarnathzore@gmail.com
2
q_mudassar@rediffmail.com
#1
Mechanical Engineering, DYPIET, Ambi, Pune, India.
#2
Professor, Mechanical Engineering, DYPIET, Ambi, Pune, India.
ABSTRACT ARTICLE INFO
It is imperative for an engineer to design and analyse the pressure vessel that will
provide safety, durability and serviceability to the end user. Accomplishing this task will
require a very good knowledge of design parameters, the most important being,
geometry of pressure vessel that must be analysed to comply design standards. The
design of saddle support for the pressure vessel is also a part of this analysis. The most
common method adopted by American Society of Mechanical Engineers (ASME) Boiler
and Pressure Vessel Code for design of saddle support was developed by L. P. Zick in
1951. Zick’s analysis was based on the assumption that the supports are rigid and not
connected to the vessel shell. In reality, most vessels have flexible supports that are
welded onto the vessel shell. In the present paper, the saddle design is optimized for a
material quantity and in turn for the cost by reducing number of gusset plates and
thickness thereof. The task is accomplished by analysis of pressure vessel and saddle by
finite element analysis (FEA) based software PV Elite. The result of this analysis shows
a measurable 20 percent reduction in the material quantity post optimization.
Keywords— pressure vessel supports, saddle design, Zick’s analysis, pressure vessel
design, optimization of saddle parameters.
Article History
Received :18th
November
2015
Received in revised form :
19th
November 2015
Accepted : 21st
November ,
2015
Published online :
22nd
November 2015
I. INTRODUCTION
A Pressure vessel is a closed cylindrical vessel widely used in
industries like process, power, oil and gas for the storage of
fluid or gaseous products. These are of two types, horizontal
and vertical. Pressure vessels are subjected to pressure loading
i.e. internal or external operating pressure different from
ambient pressure. For horizontal vessel the saddle supporting
plays an important role in the performance of the equipment.
A proper saddle supporting improves safety and durability.
Horizontal pressure vessels are usually supported on two
vertical cradles called saddles. The use of more than two
saddles is unnecessary and should be avoided. The reason
behind not using more than two saddles is to avoid an
indeterminate structure, both theoretically and practically.
With two saddles, there is a high tolerance for soil settlement
with no change in shell stresses or loading. Even where soil
settlement is not an issue, it is difficult to ensure that the load
is uniformly distributed. ASME Code does not have specific
procedures for the design of saddles or the induced stresses in
the vessel. While the ASME Code does have allowable
maximum stresses for the stresses in the vessel shell, the code
does not specifically address the support components
themselves. The purpose of this paper is to help understand
the extent to which the saddle parameters like number of
gusset plates and their thickness can be practically optimized.
The optimized designs parameters reduce the direct material
cost and indirect cost such as transportation and construction.
I. LITERATURE REVIEW
A methodology for the determination of the stresses in the
shell and heads of a horizontal vessel supported on saddles
was first published in 1951 by L. P. Zick [1] [9]. This
procedure has been used, with certain refinements since that
time, and is often called Zick’s analysis, or the stresses are
referred to as Zick’s stresses. 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
[3]. 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, therefore saddle support is critical while designing the
horizontal type pressure vessel. 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 study of stress behaviour of the pressure vessel with
different configurations of saddle supports shows that the
location of saddle at a distance less than the one fourth of the
vessel length is the most effective one [4]. The maximum load
on a saddle as given by Megyesy [2] may be conservative or
liberal, depending upon the value of the ratio A/L used.
However the design of the saddle structure may be optimized
by redesigning selectively [5]. The selection of saddle radius
also affects the stresses in the pressure vessel and can benefit
up to 50% reduction in shell stresses if it is designed 1 to 2 %
larger than vessel diameter [6]. The use of supports of high
stiffness like concrete is unfavourable taking into account the
strength of the vessel. The supports should be located near the
vessel ends, thus taking full advantage of the increased
stiffness of the head, due to both its shape and increased
thickness relative to the vessel shell [7]. Finite element
analysis of Pressure vessel by David Heckman also advocates
the use of computer programs instead of hand calculations for
analysing the high stress areas and different end connections;
Researchers have determined the optimal radius of the support
with a preliminary clearance between the vessel and saddle. In
the area of the vessel-saddle contact constant distribution of
the contact pressure along the vessel, but varying
circumferentially is assumed. A parametric analysis is
performed to reduce the stress concentration at the saddle horn.
Mr. Tooth analytically and experimentally determined the
stresses in real supports of multilayered GRP vessel Banks
presented the approximate solution of the strain state [8].
II. CONVENTIONAL DESIGN APPROACH
The design parameters of pressure vessel (here it’s a flash
drum) under study are as per Table I. The manual calculation
is carried out as per procedure given by Dennis Moss [1] for
the given input parameters. The schematic diagram of the
vessel is depicted in figure 1. Accordingly saddle is located so
as to limit all the calculated stresses (S1 to S14) within the
allowable limits. The output results thus obtained by manual
calculation and the dimensional details in fabrication drawing
are listed in Table II. As per Dennis Moss Saddle properties
like number of gusset plate n = [W/24 +1] is empirically
found. This has effect on width of wear plate as per following
equation.
𝐺
Where FL & Fb are maximum longitudinal force & allowable
bending stress respectively.
TABLE I
INPUT PARAMETERS
Parameter Notation Dimension Unit
Distance
from tangent
line of head
to the centre
of saddle
A 2100 mm
Length of
the vessel
L 9500 mm
Inside
diameter of
the vessel
D 3200 mm
Thickness of
the shell
ts 17 mm
Width of the
saddle
H 316 mm
Thickness of
the head
T 17 mm
Contact
angle of
saddle
θ 150 deg
Outside
Diameter
Do 3234 mm
Design
Pressure
P 11.3 Bar g
Vessel
Empty
Weight
We 43281 kg
Vessel
Operating
weight
Wo 110827 kg
Maximum
wind speed
V 58.9 m/sec
Importance
Factor
1.25 -
Snow Load 0.7 kN/m2
Insulation
thickness
100 mm
Base
Elevation
30275 mm
PROPOSED DESIGN APPROACH
In order to optimize the gusset thickness and number of
gussets the FEA based software tool like PV Elite can be used
that will prove saving in material cost. Using PV Elite all the
input design parameters as per Table I are entered on the
appropriate fields carefully. In The next step the number of
gusset plate are reduced one by one. For further optimization
the thickness of the gusset plates is reduced by unit quantity
each time running the analysis and checking for calculated
stresses. In doing so the results obtained are listed in Table II.
Further optimization is possible by using more sophisticated
FEA tools Like ANSYS that would lead cost saving in the
form of direct material, its handling and construction. FEA is
a numerical technique for finding approximate solutions to
boundary value problems. FEA is used to solve numerically
the governing equations for stress within the wall material.

The solution provides a complete stress distribution in the
saddle supports. FEA is 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. FEA 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.
Complete solid models of the pressure vessel and saddle
support can be created with the pressure vessel filled with
liquid and subjected to the operating weight (Figure 1).
Meshing is required to be selected and limited to study the
stresses for saddle support region under defined loading
conditions. Appropriate extents of support saddles located at
sliding and fixed side can be considered for application of
displacement boundary condition.
Fig. 1 Schematic Diagram
III. OBSERVATION
The conventional approach corresponds to design by
experience used down many years whereas finite element
analysis corresponds to design by analysis. The stress
distribution of various geometric parameters of gussets and
number of gussets of saddle can be observed to select the
optimal size of saddle. The Table II shows the effect of
number of gussets on the value of calculated stresses obtained
from the analysis results. From the Table II it is understood
that for 10 mm gusset thickness with three gussets, the
obtained stress result value falls below the stress limit without
compromising safety of equipment.
TABLE II
OUTOUT PARAMETERS
No. of
gusset
plates,
n
Stiffener
Rib
thickness,
tw (mm)
Compressive
Stress, Sc
(N./mm²)
Bending
stress on
outside
rib, Scao
(N./mm²)
Bending
stress on
inside
rib, Scai
(N./mm²)
5 16 5.59 147.40 153.74
5 15 5.76 147.19 153.65
5 14 5.93 146.94 153.55
5 12 6.31 146.33 153.30
5 10 6.74 145.51 152.98
4 16 6.31 146.20 154.12
4 15 6.47 146.06 154.03
4 14 6.63 145.74 153.93
4 12 6.98 144.98 153.70
4 10 7.38 143.96 153.39
3 16 7.67 153.74 153.92
3 15 7.38 143.96 154.13
3 14 7.52 143.53 154.03
3 12 7.82 142.49 153.79
3 10 8.14 141.10 153.47
The stress results obtained in the Handbook are the 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.
The plot above shows the calculated compressive stress
increases as thickness of the rib decreases.
IV. CONCLUSION
As a common engineering practice the recommended
thickness & number of gusset plates are used while designing
the saddle support for horizontal pressure vessel. Needless to
say, the same has been a proven design and the case of
overdesign too. In practice, project engineering schedules
leave least scope to act on optimizing the saddle thickness and
gusset plate quantity. This is the objective of this project work
and shall be the matter of future scope to find its practical
implementation. A 20 percent reduction in the direct material
cost and indirect cost is achieved. Further there is a scope for

significant amount of material saving as this approach can be
extended to pipe shoe support. The amount of primary support
for the pipe lies in the range of 50 to 100 tons for an average
project.
ACKNOWLEDGMENT
Thanks to Prof. Mudassar G. Qaimi for his valuable guidance.
REFERENCES
[1] Dennis R. Moss, ―Design of Vessel Supports‖, in
Pressure Vessel Design Manual, 3rd ed., Gulf
Professional Publishing, UK, 2004, pp. 109-138.
[2] Eugene Megyesy, ―Vessel Supports‖, in Pressure
Vessel Handbook, 10th ed., Pressure Vessel Publishing
Inc., pp. 86-101.
[3] Pallavi Pudke, Prof. S. B. Rane et al, ―Design and
Analysis of Saddle Support: a case study in vessel
Design and Consulting Industry‖, International Journal
of Mechanical Engineering and Technology (IJMET)
ISSN: 0976-6359, www.iaeme.com, Vol. 4, Issue 5,
Sept-Oct 2013, pp.139-149.
[4] Adithya M, M. M. M. Patnaik, ―Finite element
analysis of horizontal reactor Pressure vessel supported
on saddles‖, International Journal of Innovative
Research in Science, Engineering and Technology
(IJIRSET), ISSN: 2319-8753, www.iaeme.com, Vol. 2,
Issue 7, July 2013.
[5] Shafique M.A. Khan, ―Stress distributions in a
horizontal pressure vessel and the saddle supports‖,
International Journal of Pressure Vessels and Piping
(IJPVP) 87 (2010), pp. 239-244.
[6] N. El-Abbasi, S.A. Meguid et al, ―Three-dimensional
Finite element analysis of saddle supported pressure
vessels‖, International Journal of Mechanical Sciences
43 (2001), pp.1229-1242
[7] K. Magnucki, P. Stasiewicz et al, ―Flexible saddle
support of a horizontal cylindrical pressure vessel‖,
International Journal of Pressure Vessels and Piping 80
(2003), pp. 205–210.
[8] W.M. Banks, D.H. Nash et al, ―A simplified design
approach to determine the maximum strains in a GRP
vessel supported on twin saddles‖,International Journal
of Pressure Vessels and Piping 77(2000), pp. 837-842.
[9] L. P. Zick, ―Stresses in Large Horizontal Cylindrical
Pressure Vessels on Two Saddle Supports‖, the
Welding Journal Research Supplement, September
1951.
[10] Laurence Brundrett June 16, 2010,
Available:
http://www.pveng.com/ASME/ASME_Samples/Horizo
ntal/Horizontal.php

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