THICKNESS OPTIMIZATION OF INCLINED PRESSURE VESSEL USING FEA

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 3, Issue 3, September - December (2012), pp. 682-689
IJMET
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“THICKNESS OPTIMIZATION OF INCLINED PRESSURE VESSELE
USING NON LINEAR FINITE ELEMENT ANALYSIS USING DESIGN
BY ANALYSIS APPROACH”
I.M.Jamadar1, S.M.Patil2, S.S.Chavan3, G.B.Pawar4, G.N.Rakate5
1,2
Assistant Professor, Department of Automobile Engineering, Annasaheb Dange College
of Engineering and Technology, Ashta-416301, Maharashtra, India. E-mail:
imranjamadar2@gmail.com
3,4,5
Assistant Professor, Department of Mechanical Engineering, Annasaheb Dange College
of Engineering and Technology, Ashta-416301, Maharashtra, India.

ABSTRACT

Nitrous oxide (N2O) has been produced and distributed by the industrial, gas
industries for many years. It is mainly used for medical purposes (anesthesia). It is also used
in the food (whipped cream) and electronic industries. Severe accidents such as violent
decomposition of N2O and rupture of N2O tanks have occurred at production, storage and
distribution facilities. A major cause of N2O accidents has been insufficient attention to the
specific properties of N2O when designing equipment and developing operating procedures.
On this basis, the principles and relevant details of safe production, storage and distribution
of N2O are considered. The Objective of the Inclined Pressure Vessel (IPV) is to have large
scale production of Nitrous Oxide. The rate of the reaction and its temperature is controlled
by the inclination of the vessel. This investigation primarily deals with the probable causes of
in-service damage of IPV with approximate estimation of stresses using Finite element
analysis (FEA).

Keywords: IPV-Inclined Pressure Vessel.FEA-Finite element analysis.

I. INTRODUCTION

Specifically Nitrous Oxide is obtained by “ammonium nitrate pyrolysis synthesis”. It
is exothermic reaction occurring at around 200 deg C. Ammonium nitrate is a moderately
sensitive explosive and a very powerful oxidizer. Above 240 deg C, the nitrate can even
detonate. Hence, it is imperative to maintain temp below 240 deg C .The rate of the reaction
and its temperature is controlled by the inclination of the vessel .At lower inclinations (Closer

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to horizontal) the reaction will progress rapidly, as the steam will spread more and expose to
more surface area of ammonium nitrate. As the inclination will increase, the steam will rise
rapidly and escape to top chamber, causing the rate of reaction to reduce, thus in effect the
inclination will control the rate of reaction, which is an exothermic reaction. This in turn will
control the temperature of the reaction. Hence the temperature will be mostly maintained
around 200oC. This reduces the cost of control, plus rate of reaction can be controlled without
hampering the process.

II. CONSTRUCTION

It consists of oblique elongated inclined reactor. The vessel is closed at both the ends
by conventional heads. Lower end is provided with furnace to supply steam which is
circulated around the ammonium nitrate through steam pipe. Ammonium Nitrate receives the
heat from steam pipe and undergoes pyrolysis, forming water vapors and nitrous oxide gas
which are collected and separated out from upper end.

III. DESIGN CHALLENGES

From a design point of view, we can categorize the challenges as temperatures are to
be maintained at 200oC, can cause considerable thermal stresses and Inclined nature of vessel
(ASME code enables design of Horizontal or a Vertical vessel .No provision for an inclined
vessel in it.)In Horizontal Vessels, the key challenge is the bending that will occur at the
center. In such a case the vessel, behaves more like a beam supported at two ends with central
bending. In Vertical Vessels, the key challenges are the bending loads that will occur at base
due to wind load. In such a case the vessel will behave more like a cantilever beam supported
at the base. In inclined vessels both wind deflection and central deflection has to be
considered, plus we need to account for the temperature based stresses. In addition the
internal weights in the system will be a function of the angle of inclination which will have to
be considered.

IV. DESIGN BY ANALYSIS (DBA)

Design by analysis uses stress analysis directly. The maximum allowable load for the
design is determined by performing a detailed stress analysis and checking against specified
design criteria. Design by analysis can also be used for calculating the component thicknesses
for pressure vessel components [2]. In the early days of DBA, the analysis methods were
focused on linear elastic stress analysis. This is mainly so because inelastic analysis required
considerable computer resources which at the time were not present. However as computers
became more powerful inelastic analysis has become more popular. The DBA procedures
were developed with the assumption that shell discontinuity analysis would be used for the
calculations. Today the Finite Element Method (FEM) is the most popular approach for using
DBA.

V. FINITE ELEMENT ANALYSIS FOR PRESSURE VESSEL DESIGN

Design engineers must use their experience and the latest design tools to maintain
reasonable safety levels while providing the most cost effective design. One tool being used
on an ever increasing basis is Finite Element (FE) analysis software [1]. The current
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capabilities of FE software on desktop computers provide pressure vessel design engineers
with the ability to employ FE analysis on a nearly routine basis. Pressure vessel design engineers must
have a reasonable understanding of FE fundamentals to adequately use this design tool. The
engineer must determine the most appropriate modeling approach; select the proper elements
and solution technique to assure a reasonable analysis. The engineer must also determine if
the model is reacting correctly and presenting reasonable results.

VI. STRESS ANALYSIS OF IPV
In dealing with the various modes of failure, the designer must have at his disposal a
picture of the state of stress in the various parts. It is against these failure modes that the
designer must compare and interpret stress values. But setting allowable stresses is not
enough! For elastic instability one must consider geometry, stiffness, and the properties of the
material. Material selection is a major consideration when related to the type of service.
Design details and fabrication methods are as important as “allowable stress” in design of
vessels for cyclic service. The designer and all those persons who ultimately affect the design
must have a clear picture of the conditions under which the vessel will operate. This
investigation primarily deals with the probable causes of in-service damage of IPV with
approximate estimation of stresses [11]. The design temperature and pressure of vessel are
148.880C and 1.38795Mpa, respectively. There were four numbers of openings, Viz.entry
and exit of steam, Exit of Nitrous oxide and drain. The vessel thickness was around 9.6mm,
length 1275mm; inner diameter304.8mm.Stress analysis was carried out by finite element
method using ANSYS 13.0 code. Both the ASME (2007) code and the EN13445-3 (2002)
code regulate that the safety coefficient is 2.4 and thus the design stress strength is Sm=min
(460/2.4, 250/1.5)=166.66MPa.
Material Selection: Usually material in pressure vessel technology are ductile, the plastic
flow does not necessarily restricts the usability. Limited plastic flow in testing and in normal
operating load cases is admissible, even if it may occur repeatedly; it is taken into account in
constitutive laws of material models. Because of plastic flow DBA is restricted to
sufficiently ductile materials at operating temperature below creep region.

Properties
Density 7.85e-006 kg mm^-3
Isotropic Secant Coefficient of Thermal Expansion 1.2e-005 C^-1
Specific Heat 4.34e+005 mJ kg^-1 C^-1
Tensile Yield Strength MPa 250
Tensile Ultimate Strength MPa 460
Reference Temperature C 22
0
Design Temperature in C 148.88
Young's Modulus MPa 2.e+005
Poisson's Ratio 0.3
Bulk Modulus MPa 1.6667e+005
Design Pressure, MPa 1.37895
Allowable Stress, MPa 166.67

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Model Geometry: In evaluating the geometry, there are several prime considerations. In
addition to the necessity to accurately represent the actual geometry of the vessel or
component of the vessel, one must consider the loading and support (boundary) conditions
and the mesh to be employed. The extent of the vessel or component modeled is also of prime
concern when the decision is made to model only part of an overall system. Modeling of the
pressure vessel was done using CATIA V5R17 software. Later on to model was imported to
ANSYS 13 where symmetric model was prepared, and then accordingly vessel was tilted to
required inclinations.

Figure.1 Full Model in CATIAV5R15 Figure.2 Mehing with higher order brick element

Element Selection and Meshing: Once the geometry of the object to be analyzed is defined,
the first task is to select the type of element that is to be employed. For most pressure vessel
analyses, the element selection is made from three categories of elements: axisymmetric solid
elements, shell/plate elements and 3-D brick elements. Although nearly all problems can be
solved using 3-D brick elements, the other two types offer significant reductions in the
solution time and effort where they are applicable. Often, this reduction in solution effort is
significant enough to make the use of FE analysis feasible where it might not be with 3-D
bricks. The higher order hexahedron element was used for meshing. The element is defined
by twenty nodes.
Boundary Conditions: The whole vessel is supported on two saddle supports. One saddle is
a fixed saddle while the other is a sliding type saddle. The upper saddle was fixed while to
the lower saddle cylindrical support was provided. All degrees of freedoms of are constrained
for fixed saddle while sliding saddle provides free sliding along axis of vessel.
Loadings: The vessel was analyzed for internal pressure 1.38 MPa, plus Thermal loads from
steam at 148.880C plus Self Weight.

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VII. RESULTS AND DISCUSSION

Variation of stresses with respect to angle of inclination is given below:
Equivalent Von-
Angle, Total Maximum
Mises Stress, Nodes Elements
[Degree] Deformation, [mm]
[MPa]
0 123.41 1.7265 371000 91103
4 126.12 1.7268 365123 90236
8 131.24 1.7238 364213 93125
12 150.8 1.7465 374256 94563
16 176.56 1.7524 375136 95145
20 190.8 1.7892 375812 95200
24 210.61 1.8093 371365 92428
28 225.6 1.8564 370152 90832
32 242.38 1.8916 375180 94471

ANSYS Results Plot:

Figure 3- Linearised Stress along Vessel Thickness Figure 4- Linearised Stress V/s vessel Thickness

Figure 5- Linearised Stress along Nozzle Thickness Figure 6 - Linearised Stress V/s Nozzle Thickness

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Figure 7 - Equivalent Stress Along Vessel-Nozzle Intersection with reinforcement pad

Thickness optimization results
Variation of stresses with respect to the thickness of vessel for maximum inclination angle of
320 are given below
Stress at
Membrane Bending Total
Thickness, Membrane+Bending nozzle-
Stress, Stress, Stress,
[mm] Stress, [MPa] Vessel
[MPa] [MPa] [MPa]
Intersection
11.336 16.887 13.197 27.626 28.652 39.147
9.489 22.222 15.756 34.332 34.63 61.964
7.2875 30.759 16.856 42.41 42.061 10741
6.0275 38.001 15.648 48.049 47.743 139.4
5.4671 46.859 19.968 57.824 57.81 178.45
5.6 with
33.066 24.843 53.987 57.938 87.179
RF Pad

VIII. EXPERIMENTAL TESTING

1) Ultrasonic testing: At Nozzle-vessel Intersection:Weld spot at nozzle vessel
inteersection tested with an ultrasonic probe positioned on it and transmitting sound pulses
into the weld metal, as well as the echo sequence generated on the screen display of the
ultrasonic instrument.This sound pulse is transmitted from the probe into the weld spot and
partially reflected from the interface between the probe and weld spot. This reflection appears
as interface echo at sound entry (1st indication to the farthest left) on the screen display of the
ultrasonic instrument. The continuous part of the pulse enters the weld spot and is only
reflected from its rear boundary, provided there is no flaw. This reflection is displayed as 1st
backwall echo to the right of the interface echo. The sound pulse can run several times back
and forth between the front and rear end of the weld spot, and delivers a part of the sound
pulse to the probe every time it hits the front end. This ever decreasing part of sound pulse is

687


displayed as 2nd, 3rd, 4th backwall echo at the same intervals on the screen. In this
connection, the interval between the individual backwall echoes corresponds to twice the
material thickness (round trip within the material). If there is a flaw in the weld spot, e.g. in
the form of a gas pocket, a part of the sound pulse correspoding to the size of this flaw is
additionally reflected from it. As the flaw is situated between the front and rear end of the
weld spot, the corresponding flaw echoes also occur between the backwall echoes. In the case
of major weld flaws, the flaw echoes are higher and possibly only recognizable.
2) Hydro-testing : Vessel was also tested for hydro-test pressure of 1.5 MPa and
temperature 1500C which are slightly higher than the operating values. Also at the same time
strain gauges (LC 4CI X- HBM ) are mounted at the saddle supports and at the nozzle-vessel
intersection for measuring the deformations.

IX. CONCLUSIONS

As seen from the table, the stresses in the vessel thickness are increasing with
reduction of thickness. Here, membrane and bending stresses are within allowable limits for
all cases considered. But the equivalent Von-Mises stress at nozzle-vessel intersection is
increasing abrouptly as thickness is reducing. Particularly at 5.65mm thickness the vessel will
fail at interection because stress is higher than allowable limits. So slight modification is
made in the original design i.e. provision of reinforcement pad at vessel-nozzle intersection.
Providing the reinforcement pad has reduced stress which are below allowable limits. The
results of the ANSYS were compared with experimental values which are in good agreement.

ACKNOWLEDGEMENT

We sincerely thank Mr.V.G.Patil for his continuous support in providing advances in
Pressure Vessel analysis technology and for guidance to prepare this paper. We also thank his
team of Vaftsy Engineering Services Ltd. Pune for providing testing facilities and inputs to
complete the content of this research topic.

X. REFERENCES

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THICKNESS OPTIMIZATION OF INCLINED PRESSURE VESSEL USING FEA

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