Indian Journal of Science and Technology, Vol 9(21), DOI: 10.17485/ijst/2016/v9i21/90567, June 2016
ISSN (Print) : 0974-6846
ISSN (Online) : 0974-5645
Design and Crack Analysis of Pressure Vessel Saddles
Using Finite Element Method
A. M. Senthil Anbazhagan and M. Dev Anand
Department of Mechanical Engineering, Noorul Islam Centre for Higher Education, Kumaracoil - 629 180,
­Thuckalay, Kanyakumari District, Tamilnadu State, India; ams097@yahoo.com, anandpmt@gmail.com
Abstract
Background/Objectives: Saddles are one of the important parts of vessel as they support the vessel. In this study, how
Mode II type crack propagated inside the junction of saddle and vessel connection would spoil the entire setup of the
­vessel was investigated. The main intention of this work was to avoid the repeated failures of saddles during operation in
­energy ­development industries and wherever it is used. Methods/Statistical Analysis: Two different types of saddles were
­considered and fabricated using IS2060 Grade B material. The saddle parts were welded as per the code rule of API. Findings:
Normally, welding in inclined saddle is difficult in comparison with straight saddle. This may be reason; the ­failure rate of
inclined saddle is high in comparison with straight saddles during operation and loading conditions. The other ­possibility
of failure is the gap formation inside the weld during joining the plates. This is due to non-deposition of weld ­materials.
The gap would grow during operation and loaded conditions. To avoid these types of failures, external and internal crack
­inspections were done. Once the inspection was done, it was examined the load carrying of the fabricated saddles using
FEM. Then the solid works software was used to simulate the solid model which it’s developed similar like original saddle
fabricated for estimating the load carrying capacities. The obtained results of NDT and FEM were presented and the design
­recommendations based on investigation and study are also suggested. Applications/Improvements: The obtained results
of NDT and FEM were presented and the design recommendations based on investigation and study are also suggested.
Keywords: Pressure Vessels, Saddles Finite Elements, Inspection, Support Structure.
1. Introduction to Pressure
Vessels and Saddles
A pressure vessel is a container which is designed closed
for holding gases or liquids at a pressure considerably
dissimilar from the ambient pressure. Classification of
pressure vessels are of various types like Cylindrical,
Rectangular, and Spherical. Cylindrical and Rectangular
types can be further classified into Horizontal and Vertical
type vessels. Horizontal vessels normally placed on saddle supports and vertical vessels placed on skirt support.
Since these types are based on process requirements,
sometime in these vessels would place inclined position.
Placing inclined vessel is a difficult task as we require
inclined saddle. As we mentioned in synopsis, this project, we considered only horizontal and inclined saddles
for ­cylindrical type vessels for design and ­development.
*Author for correspondence
Saddles are supports which hold the vessels. The
­bottom portion of the saddle is normally welded or bolded
to the ground or wherever it is mounted. As far as horizontal vessels are concerned, straight and inclined saddles are
used for placing the vessels. Normally, horizontal vessels
are supported using two numbers of saddles. However,
the no of saddles can be vary based on the length of the
cylinder. Sometimes, if the vessel length is too long we
may require three to four numbers of saddles for placing
the pressure vessels.
2. Design of Saddles
2.1 Design of Inclined and Horizontal
Saddles
Saddles contain Base plate, Web, Ribs and Wear Plate.
The wear plate is used to carry the pressure vessel and has
Design and Crack Analysis of Pressure Vessel Saddles Using Finite Element Method
the internal diameter same as that the outer ­diameter of
vessel. The total weight of the pressure vessel is directly
distributed to wear plate and then transferred to the based
plate through web and rip plates. The web and ribs are
designed to carry or support the wear plate. They are
located bottom of the wear plate and above the base plate.
Wear, Web and Ribs are welded on the base plate. This
base plate is fixed on the bottom. Please refer Figure 1
shown for reference.
2.2 Design Methodology of Horizontal and
Inclined Saddles
Our saddle designs are based on the assumption of the
cylindrical vessel diameters 16” and 12” as shown I
Figure 2. It is assumed 16” diameter vessel is sitting on
the inclined saddle and 12” diameter vessel is sitting on
the horizontal saddle which designed and manufactured.
The methodology of design calculations are as follows.
Since shortfalls of design code guidelines, so necessary to
follow L.P. Zicks1 method to design the saddles. L.P. Zick
method is very much suitable for our horizontal saddles.
But applied his method for both horizontal and inclined
saddles. The method of calculation to fix the Web, Rib,
Base and Wear ticks are as follows as based on the2-15.
2.3 Design of Inclined Saddle
A TA2885A-0000-C-00116 is the formal written ­document
describing mechanical calculation procedures, which
provides direction to the pressure vessels designer for
making sound and quality production components as
per the code requirements. The purpose of the document
is to guide designer to the accepted procedures so that
repeatable and trusted techniques are used. A TA2885A0000-C-001 is developed for each material alloy and for
each type used. Specific codes and/or engineering societies are often the driving force behind the development
of a company’s TA2885A-0000-C-001. Based on that, it is
designed for vertical and inclined saddle and the ­summary
of entire things are indicated in Table 2 to 10.
Table 1.
Inclined Saddle Results
Actual
Allowable
Long. Stress at Top of
Midspan
51.77
137.90 N/mm²
Long. Stress at Bottom of
Midspan
53.52
137.90 N/mm²
Long. Stress at Top of
Saddles
52.71
137.90 N/mm²
Long. Stress at Bottom of
Saddles
52.60
137.90 N/mm²
Tangential Shear in Shell
1.46
110.32 N/mm²
Circ. Stress at Horn of
Saddle
0.81
172.38 N/mm²
Circ. Compressive Stress
in Shell
0.07
137.90 N/mm²
Table 2.
Figure 1. Horizontal saddle support.
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Summary of Loads
Vertical Load (Including Saddle
Weight)
802.36 Kgf
Transverse Shear Load Saddle
37.93 Kgf
Longitudinal Shear Load Saddle
7.62 Kgf
Table 3.
Figure 2. Horizontal inclined saddle support.
Inclined saddle results
Summary of Dimensions
Base Plate Length
Bplen
420.0000 mm
Base Plate Thickness
Bpthk
10.0000 mm
Base Plate Width
Bpwid
180.0000 mm
Number of Ribs (Inclined
Outside Ribs)
Nribs
2
Rib Thickness
Ribtk
10.0000 mm
Web Thickness
Webtk
10.0000 mm
Web Location
Webloc
Center
Indian Journal of Science and Technology
A. M. Senthil Anbazhagan and M. Dev Anand
Table 4.
Results for Saddle Ribs, Web and Base Plate
Y
A
AY
Table 9.
Moment of Inertia of Saddle
Io
Y
A
AY
Io
Shell
13
92
124855
225
Shell
13
89
120051
216
Wear Plate
32
23.
75200
243.
Wear Plate
32
23
75200
243
Web
166
26.
431235.
8628
Web
148
22
331155
5842
Base Plate
301
18.
541800
16310.
Total
513
160
1173090.
25405.
Base Plate
265
18
477000
12642
Totals
459
153
1003406
18942
Table 5.
Table 10.
Moment of Inertia of Saddle
Y
A
AY
AY2
Io
Rib
102.0
16.9
172686.0
0.0
480.4
Web
102.0
19.7
201246.0
0.0
3.3
Values
102.0
36.7
373931.9
0.0
483.6
Table 6.
Y
A
AY
AY2
Io
102.0
16.9
172686.0
0.0
480.4
Web
102.0
19.7
201246.0
0.0
3.3
Values
102.0
36.7
373931.9
0.0
483.6
A
AY
AY2
Io
Rib
102.0
16.9
172686.0
0.0
480.4
Web
102.0
17.5
178296.0
0.0
2.9
Values
102.0
34.4
350982.0
0.0
483.3
Straight Saddle Design Results
Results
Actual
Allowable
Long. Stress at Top of Midspan
40.10
137.90 N/mm²
Long. Stress at Bottom of Midspan
42.68
137.90 N/mm²
Long. Stress at Top of Saddles
41.61
137.90 N/mm²
Long. Stress at Bottom of Saddles
41.27
137.90 N/mm²
Tangential Shear in Shell
1.71
110.32 N/mm²
Circ. Stress at Horn of Saddle
0.97
172.38 N/mm²
Circ. Compressive Stress in Shell
0.06
137.90 N/mm²
Table 8.
Y
Outside Rib Inertia of Saddles
Rib
Table 7.
Results outside Ribs
Results Ribs, Webs and Base Plate
Base Plate Length
Bplen
375.0000 mm
Base Plate Thickness
Bpthk
10.0000 mm
Base Plate Width
Bpwid
180.0000 mm
Number of Ribs
(InclinedOutside Ribs)
Nribs
2
Rib Thickness
Ribtk
10.0000 mm
Web Thickness
Webtk
10.0000 mm
Web Location
Webloc
Center
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Figure 3. Horizontal straight saddle support.
2.4 Design of Straight Saddle
• No external loads and moments considered for this
saddle design.
• This saddle design is only based on the internal
­pressure of the vessel.
• Developed stresses are within the allowable limit,
so saddle is safe for the considered 150bar design
­pressure.
3. Fabrication of Saddles
3.1 Fabrication of Saddles Inclined and
Straight Types
This section explains how the saddles were fabricated and
tested. The saddle consists of a base plate, web plate, two
ribs and a wear plate in top. These plates were cut from
Indian Journal of Science and Technology
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Design and Crack Analysis of Pressure Vessel Saddles Using Finite Element Method
Table 11.
Straight Saddle Dimensions
Base Plate
375 X 180
Rib Plate
488 X 150
Web Plate
370 X 488
Wear Plate
545 X 235
the raw material and welded as per the ­manufacturing
­drawing which we prepared. The API code rules for
­welding were followed to join the plates.
3.2 Fabrication of Straight Saddle
The straight was fabricated based on the dimensions
­mentioned in the Table 11. These dimensions were
obtained from our baseline line calculation. Dimension
of all the straight saddle plate are given Table 11.
The above mentioned plate dimensions were cut
from a standard plate which we bought it from market. They were cut by oxy- acetylene welding. During
cutting, iron oxides formed on the edges of the work
pieces, those are called as slag. Slag formation was
because of the reaction between the atmospheric air
and iron. Normally, at an elevated temperature, oxygen in atmosphere air would start react with ferrous
material. As a result, they form as iron oxide and got
deposited on the edges of welding. While cutting the
material by oxy- acetylene welding, the material reach
melting point and separated into two halves. Due to
this, the edges would not be in a proper shape and it
was rough. Hence, the edges were machined with 5
mm clearance. The machining was done in the Vertical
Milling Machine and the subsequent machined materials were removed layer by layer with the help of rotary
cutters. In every pass 1-2 mm was removed with respect
to the sizes required. Similarly, all plates were machined
and the required dimensions were acquired.
Wear plate was taken in to roller machine and rolled
for required diameter. Cold rolling was selected and
done under the re-crystallization temperature. Cold rolling consists of three rollers and they were arranged in to
two rows. One roller was located at upper row and the
other two rollers were in bottom rows. The plate (Wear)
was inserted in between these rollers. By reducing the gap
between the rollers, accurate diameter could obtain in the
plate. Suitable template was made with the radius of 210
mm. The pre stated radius is the outer radius of the vessel.
The plate was inserted in to the rollers often getting the
template dimension which we made. There were several
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insertion were done for obtaining the exact shape. And
the process was repeated and attained the required shape.
The base plate and the ribs were also ready after completing the machining process. The web was cut at the top,
to the outer diameter of the wear plate. Hence the wear
plate was kept on the web and positioned accordingly.
With the help of scriber the portion was marked and
punched. Then the portion was cut by oxy-fuel welding
then machined with the help of Hand grinding machine.
The (height wise) edges were cut to 2 degrees to make the
ribs inclined.
Once all the machining processes were over, the
plates were joined by welding. The required grooves were
taken in the plate. Bottom portion of web and ribs were
provided with 2 mm double V groove and the ribs was
provided with 4 mm single V groove at the top. Grooving
was done by using Vertical Milling machine with the
help of angle cutter. Then the plates were taken to welding shop for welding. The type of weld was chosen based
on the advantage, availability, cost, and strength and
slag formation. Arc welding and Gas welding were usually form slag on the weld surfaces and meantime it will
reduce the strength of the weld but it is very economic.
TIG welding is costlier than arc and gas welding but it
won’t produce slag on the surface and the strength vise it
is good. Hence TIG welding was chosen. For Mild steel
material, Carbon steel (T-70S2) were chosen as filler rod.
The web was kept on the base plate and adjusted according to the markings made. And the tri-square was used
to keep the web vertically and the welding was made on
3 to 4 points. This was done to set the working pieces
in accordance with the position required. Then the ribs
were kept on its places and welded. Similarly the wear
plate was also joined with the ribs and web. The required
saddles were fabricated fully as per the drawing as shown
in Figures 4-13. The weld joints were cooled after the
­fabrication was done.
3.3 Fabrication of Inclined Saddle
Inclined saddle needs of 5 plates with 10 mm thickness.
Dimension of all the plates are given Table 12.
Table 12.
Straight Saddle Dimensions
Base Plate
420 X 180
Rib plate
517 X 150
Web plate
386 X 488
Wear plate
615 X 235
Indian Journal of Science and Technology
A. M. Senthil Anbazhagan and M. Dev Anand
Figure 4. Ribs Plates of Inclined Saddle.
Figure 9. Wear Plates after Rolling.
Figure 5. Ribs Plates of Inclined Saddle.
Figure 10. Grooving of Plates in Vertical Milling Machine.
Figure 6. Web Plate of Straight Saddles.
Figure 11. Fabricated Saddles (Front View).
Figure 7. Web Plate of Inclined Saddles.
Figure 12. Fabricated Saddles (Side View).
Figure 8. Wear Plates before Rolling.
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Figure 13. Fabricated Saddles (Inclined and Straight).
Indian Journal of Science and Technology
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Design and Crack Analysis of Pressure Vessel Saddles Using Finite Element Method
As stated previously, the plates were cut from a
s­ tandard dimensioned plate by oxy-acetylene welding
and machined with the help of milling machine. A template was made to the radius of 243 mm. The pre stated
radius is the outer radius of the vessel. And the rolling
was done on the plate according to the template made.
The difference between straight saddle and inclined saddle is that the wear plate of the inclined saddle will be
kept 5 degrees from the axis whereas the wear plate of the
straight saddle is kept straight and coincides with the axis.
To make this inclination the ribs were cut for 5 degrees
widthwise. Cutting was done by oxy-fuel welding and
machined by using Vertical Milling machine. Web plate
was also grinded a bit to accommodate the wear plate.
Once it was done then the plates were grooved as stated
above and then welded properly. At this instant, both
straight and inclined saddles were made according to
the procedure and API weld guidelines. After fabricating
the saddles, they must be checked for cracks, defects, etc.
These defects might have occurred during operation or
heavy loaded conditions. For testing these saddles, NDT
methods were chosen to do the surface and internal crack
checking’s.
4. NDT
Non-Destructive Test (NDT) is defined as the method
of testing the materials for surface cracks and internal
cracks without damaging the parent material. The terms
Nondestructive Examination (NDE), Nondestructive
Inspection (NDI) and Nondestructive Evaluation (NDE)
are also commonly used to describe the Non-Destructive
Testing. NDT tests are widely used by the quality departments by sampling basis for a batch of products or 100%
checking of the materials for defects occurred while manufacturing. This system is very helpful for production quality
control system to monitor the deviations in products that
occur due to improper manufacturing techniques. In a
manufacturing unit products are manufactured as a batch
and the batches may contain defected products. As per the
acceptance sampling method or by other sampling methods the batch will be checked for defects by taking some
samples. To conduct this sampling test, NDT method will
be used. It also widely used by the maintenance department in each mechanical company for detecting internal
and external cracks. Hairline cracks and internal cracks
occurred in a machine part cannot be found by naked eyes
and it can be detected only by NDT methods.
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4.1 Applications of NDT Inspection
• Raw materials which are used in the construction of
the product.
• Fabrication processes which are used to manufacture
the product.
• Finished product before it is put into service.
4.2 Methods of NDT Inspection
The NDT technique uses various principles and different
methods. There is no single procedure depending upon
which the result are produced. The various methods of
NDT are given below.
•
•
•
•
•
•
•
•
•
•
Dye Penetrant Test.
Magnetic Particle Inspection.
Ultra Sonic Testing.
Visual Inspection.
Oil and Chalk Process.
Eddy-Current Method.
Radiography Method.
Acoustic Emission Testing.
Leak Testing.
Infrared and Thermal Testing.
From the above methods, dye penetrant and magnetic
particle inspection were chosen to find the surface cracks
and ultra-sonic and radiography tests were chosen to find
internal cracks of saddles.
4.2.1 Dye Penetration Test
Dye penetration test is one of the NDT methods, which
is used to find the surface cracks (Flaws which are open
to surface). This method is widely used because of its simplicity and low cost. The main principle of this method is
capillary action due to surface tension. Here a penetrant is
first applied on the material and allowed to penetrate for a
particular time. Capillary action causes this penetration.
Then a developer is applied to enhance the visibility which
shows the crack in contrast color with a light background.
Thereby the location of the crack is found. Before conducting any costly tests such as ultrasonic, radiography, dye
penetrant is used. Hence if any surface crack is found, the
material can be rejected without wasting money for high
end tests. This method can be used for ferrous and nonferrous metals and cannot be used for porous materials.
To find surface discontinuities on non- ferrous materials, magnetic particle inspection cannot be used. Hence
Indian Journal of Science and Technology
A. M. Senthil Anbazhagan and M. Dev Anand
the only possible way to find surface ­discontinuities on
­non-ferrous martial is by Dye penetrant inspection.
ASME Section 5 Article 6 SE 165.
ASTM E 165 – 02 for liquid penetrant test.
Dye penetrant test is conducted on saddles to find the
defects in welded joints. The detailed procedure of dye
penetrant test is given below.
A clean surface is essential for successful dye
­penetration inspection. Hence the welded part of the
saddle is first cleaned with the help of lint free cloth
thoroughly as sown in Figure 13. All the foreign material, dirt, welding flux, slag, etc are taken out, since they
may absorb the dye and may show a false indication of
crack. Then a cleaning solvent is applied on the parts and
wiped out with a cloth. The solvent used for cleaning is
Magno flux cleaner. Cleaning is one of the important
tasks in this test because inadequate cleaning may result
in poor sensitivity of Dye penetrant test. Now the saddle
is ready for applying penetrant. There are three methods
to apply penetrant. They are: 1. Spraying, 2. Brushing and
3. Dipping.
For small components spray method is sufficient. But
for larger components spraying will consume more time
hence they are dipped. Here the saddle is of smaller size
so spray method is chosen. The penetrant used in this
process is ORION-115 P and red in colour as shown in
Figure 14. The penetrant is sprayed at a distance of 300
mm from welded parts and moved slowly on the welded
joints. After applying penetrant thoroughly, dwell time
is given for penetration. Dwell time is the time taken by
the penetrant for capillary action to take place. This dwell
time vary with respect to the hardness of the material.
Hard materials require lengthy dwell times. The dwell
time usually vary from 10 to 30 minutes. Here the saddle is given around 30 minutes of dwell time. During the
dwell time the penetrant must have penetrated through
the cracks.
Figure 13. Surface preparation is done with the help of
cleaner and lint free cloth.
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After giving enough dwell time, the surplus dye must
be removed from the surface. Dye is removed with a
lint free cloth and all the penetrant is wiped out. This
cleaning must be done uni-directionally. Cleaning in
different directions may take out the penetrant from
cracks. Cleaning should be de done repeatedly until the
penetrant is totally wiped off from the surface. Residual
penetrant on the surface may give false or irrelevant
indications.
Developer is applied, after cleaning the surface
thoroughly. The developer used in this process is ORION115D as shown in Figure 15. This contains white powder
mixed with evaporative liquid. When it is sprayed, the
white powder is deposited on the surfaces and the liquid
is dried off. Before applying developer, the can is shook
thoroughly to make the white powder to dispense with
liquid. A fine and even coat of developer is given on
the surface and moved along the weld. Heavy coat on
the surface may give blurred indication of crack. Now
dwelling time is given for the developer to absorb the
penetrant from cracks. This dwell time varies from 7-10
minutes.
The saddle is taken out to the sun light, after sufficient
duration for developing; the saddle was inspected with
naked eyes. Complicated areas were inspected using magnifying glass is as shown in Figure 16. In case of defect
(flaw), it will appear as red colour in white background.
During inspection, no such indications of flaws were found
Figure 14. Penetrat is Applied with the Help of ORION
115P.
Figure 15. Developer is applied on the Saddles.
Indian Journal of Science and Technology
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Design and Crack Analysis of Pressure Vessel Saddles Using Finite Element Method
on the welded parts. As per the ASME standards the above
inspection on the weld parts are found satisfactory.
Depending only upon the dye penetrant test, the
result cannot be concluded. Hence the saddle is checked
for defects by using magnetic particle inspection.
4.2.2 Magnetic Particle Inspection
Magnetic particle inspection is one of the Non-Destructive
testing methods, which carried out to find the surface
cracks (cracks which are open to surface). It can also be
used to detect the sub surface cracks on work pieces up
to 6 mm thickness. Here the Ferro-magnetic particle is
sprinkled over the surface and the part is magnetized.
These particles will be accumulated at the flaw and gives
an indication of crack.
The time required for conducting this test is very low
and sensitivity of this test in finding surface cracks is
higher than that of Dye penetrant test. Dye penetrant test
requires more time, since the surface has to be cleaned,
penetrant and developer should be applied and dwelling
time must be given for penetrant and developer. It consumes lot of time. But the magnetic particle test can be
conducted within a very short time. In some instances,
Dye penetrant test may give false indication. But the
probability of giving false indication in Magnetic particle
test is very low.
ASTM E - 709
ASME Section 5
This test is conducted on the saddles to find the
­surface cracks and the detailed procedure is as shown in
Figure 17.Magnetic particle inspection is depends upon
the principle of Magnetic Flux Leakage. A straight piece
of magnetic material will have poles at each ends known
as South and North Pole. As we know similar poles repel
each other and different poles attract each other (north
and south pole attracts and north and north poles repels).
Figure 16. Diagram shows the different procedures
involved in dye penetration inspection.
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Magnetic flux lines flow from South to North Pole inside
the magnet and north to south around the magnet. When
a crack begins on a surface, it will be open to the surface
and the surface will be separated into two halves. These
two separated parts will be taken as two bar magnets and
two poles are created at each ends. Now the magnetic flux
lines will get distorted due to the discontinuity and starts
flow from North to South Pole in air gap. These flux leakage will attract the Ferro-magnetic particles sprinkled
on them. Since these poles are different in nature, they
start to attract each other. Hence the strength between
these poles will be higher and they will attract the Ferromagnetic particles sprinkled over the surface. Thereby the
defect can be found easily.
Magnetic particle testing equipment consists of a
Magnetizing Yoke, Ferro-magnetic particles (Dry, Wet, and
Fluorescent) and Ultra-Violet light in case of Fluorescent
magnetic particles is as shown in Figure 18. The Yoke is
used to magnetize the material. Magnetization can be
done by either circular or longitudinal. In this ­experiment,
longitudinal magnetization is done on the saddles. The
Ferro-magnetic particle used is of ­fluorescent type.
Before testing magnetic particle inspection on
­saddles, the equipment is calibrated with a gauge known
as Pi-Gauge. The pi-gauge has a known defect on its surface, which is invisible to naked eyes. Magnetic particle
inspection will be carried out on the Pi-gauge and it has
to give the indication of crack. Thereby the equipment is
calibrated. The pi- gauge was magnetized by yoke and fluorescent magnetic particles were sprayed on the pi-gauge
as shown in Figure 19. The ultra-violet ray light is now
focused on the pi-gauge. It is found that the magnetic particles were accumulated at the defect place. The crack was
indicated by a white shiny colour in metal background.
Figure 17. Leakage fields between two pieces of a Broken
Bar Magnet (a) with Magnet Pieces Apart, and (b) with
Magnet Pieces Together (Simulating Flaw). (c) Leakage Field
at a Crack in a Bar Magnet.
Indian Journal of Science and Technology
A. M. Senthil Anbazhagan and M. Dev Anand
Hence the equipment is calibrated and the test is now can
be conducted on the saddles.
Before conducting magnetic particle test on saddles,
pre-cleaning has to be done. The saddle surface was
cleaned with cloth and all the foreign materials were
removed. Now the saddle is magnetized using yoke in
each joint and the fluorescent particles were sprayed from
Mangnaflux bottle. Since the size of the yoke is small,
magnetization cannot be done for the whole saddle at a
time. Hence saddle is divided into number of parts, and
each part is magnetized separately. Now the fluorescent
magnetic particles were sprayed on the saddle in each
part. The ultra-violet ray light was focused on each part
of the saddle and checked for cracks. No crack or defects
were found on the saddle during inspection. Hence the
weld done on the saddle was found satisfactory.
4.2.3 Ultra Sonic Testing
Ultra sonic testing is one of the NDT methods, which is
used to evaluate the internal conditions of the ­material
i.e. used to find the internal cracks in the material
­particularly in sound conducting materials. It is one
of the classical method and it is been used for the past
5 decades. It can also used to find the thickness of the
material. In this method sound is used to find the defects.
The Ultrasonic instrument uses the principles of sound
propagation to detect and locate defects such as cracks,
porosity, deterioration, corrosion, lamination and foreign
Figure 18. A Yoke, Ultra-Violet Lamp and Ferro Magnetic
Particles.
Figure 19. The Pi-Gauge.
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inclusions found in material. Using this method the size
and shape of the defect could be found out which will be
very helpful in fracture mechanics. Depending upon the
size and shape of the flaw, the maximum safe stress can be
calculated. Ultra sonic testing uses the principle of sound
propagation. A short pulse of ultrasound is generated by
means of an electric charge applied to a piezo electric
crystal, which vibrates for a very short period at a particular frequency is as shown in Figure 20. This frequency
varies from 1MHz to 6MHz. These vibrations or sound
waves have ability to travel through elastic materials. The
Ultrasonic waves propagated through the material will be
reflected on reaching an interface (such as defect, flaw,
hole, back wall). These reflections will be detected by the
piezo electric crystal. These oscillations are transferred to
the CRT screen to acquire the result.
An Ultrasonic flaw detector consists of a CRT screen,
Pulse generator, and Pulse receiver is as shown in Figure
21. A pulse generator is an electronic device that can
­produce high voltage electrical pulses. These electrical
pulses are then given to the transducers. A pulse receiver
is used to receive the electrical pulses from the transducers. The CRT screen converts these electrical signals into
visible format in a digital display. The result of any test can
be obtained from the CRT screen only.
Probes are used to convert the electrical pulses into
ultrasonic waves. Different types of probes are used in
ultrasonic testing. Selection of proper probe is depending
Figure 20. Ultrasonic testing schematic setup.
Figure 21. The probe and the ultrasonic flaw detector are
shown above.
Indian Journal of Science and Technology
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Design and Crack Analysis of Pressure Vessel Saddles Using Finite Element Method
upon the thickness of the material and type of material.
The different types of probe used are:
• Straight-Beam (Single Transducer).
• Straight-Beam (Thru-Transmission).
• Angle-Beam.
Sound waves from the straight beam are transmitted
through the work piece by a transducer and reflected back
by the other side surface of the test object. These reflected
waves will be received by the same transducer. Hence the
transducer acts as both receiver and generator of ultrasonic waves. Here the transducer will have separate sound
transmitter and receiver. The ultra-sonic waves will be
transmitted by a separate transmitter and the waves are
received by a separate receiver. The angle-beam (Shear
Wave) technique is used for testing sheet, plate, pipe and
welds. This technique is used where the beam has to be
transmitted at an angle. The different angles of probes
available are 45, 60 and 70 degrees.
It is used to transfer the ultrasonic sound waves form
the probe to the work piece. It used to be in the form of
liquid or paste. Oil, Grease, etc. It can be used as a ­coupling
medium.
To inspect the saddle, Angle probe is selected. Selection
of probe angle depends upon the formulae given below.
the saddle was “undercut”. And some other echoes were
found at the thickness of 6.3-8.2. These are welding defects
caused due to insufficient filling of filler material. Since
this saddle was a structural member the welding could
not penetrate properly throughout the joints. The defects
were marked on the material and test was completed.
5. Finite Element Simulation
Finite Element Technique (FEM) is a numerical ­technique
to find fairly accurate solutions to boundary value problems which utilizes variation methods (the calculus of
variations) for minimizing an error function and create a
stable solution. Analogous to the idea that linking many
tiny straight lines could fairly accurate a larger circle,
FEM includes all the methods for linking a lot of effortless
element equations over numerous small ­sub-domains,
named finite elements, for approximating a more ­complex
equation over a superior domain. In this project, we used
FEM to find out the load carrying capacities of the saddles
which we fabricated material details are listed in Table 13
and the boundary condition are shown in Figure 24.
Probe angle = 90 – T
Where, T - Thickness of the material to be inspected.
Hence the angle for the probe selected is 70 degree. At
the fore most the Ultrasonic equipment is checked for
­calibration.
Here the probe was made to move on the calibration
block to find the known defects. First the thickness of the
material is checked and then the defects were checked for
calibration. The ultrasonic equipment was able find the
defects. Hence the equipment is calibrated is as shown in
Figure 22.
Now the saddles are checked for internal cracks. First
the coupling is poured on the surface of the saddle. The
coupling used in this test was oil. Now the probe is moved
on the surfaces nearer to the weld joints and checked for
defects on the CRT screen. The CRT screen shows the
echoes on the screen and if echoes are found then the
saddle can be declared as defected one. This procedure
is repeated for both the saddles is as shown in Figure 23.
During the test echoes were found at the thickness of
10.2-10.6. Hence it is assured that the defect available on
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Figure 22. Calibration of the ultrasonic equipment.
Figure 23. Ultrasonic inspection on saddles.
Table 13.
Materials of Vessel and Saddle
Items
Materials
Vessel Material
SA 516 Gr.70 (Assumed)
Saddle Material
IS 2026Gr-B
Indian Journal of Science and Technology
A. M. Senthil Anbazhagan and M. Dev Anand
Figure 24. Boundary condition of the saddle.
Figure 26. Strain of the inclined saddle.
5.1 Loads Considered Includes
• Internal and External design Pressures.
• Weight of the Vessel.
• Assumed Wind and Seismic Loads.
Since this analysis is only to check load bearing capacity
we didn’t consider all external loads.
5.2 Boundary Conditions
The bottom portions of the saddle were arrested in all
directions. And the top of the wear plat, we applied 150
bar uniform pressure load to check the maximum load
bearing capacity of the saddle and we obtained the following results. We checked, the obtained stresses are
within the allowable stress limit of the saddle and we
found that the obtained stresses are with the allowable
and we concluded we can use these saddles up to 30
mm thickness vessels with maximum of 150bar internal
pressure.
Figure 27. Stresses in the straight saddle.
Figure 28. Strain and displacement of straight saddle.
5.3 Allowable Limits of the Material
Allowable Stress Limit of the Saddle Material in the combined loading condition = 204MPa
5.4 Analysis Remarks
Figure 29. Strains in straight saddle.
6. Results and Discussion
Figure 25. Von-Misses and deflection of inclined saddle.
Vol 9 (21) | June 2016 | www.indjst.org
Based on the results (Figures 25 to 29), it concluded that
the developed stresses are within the allowable limit, so
Indian Journal of Science and Technology
11
Design and Crack Analysis of Pressure Vessel Saddles Using Finite Element Method
the saddle is safe up to 150 bar uniform pressure loading
condition. Also it holds up to 30 mm thickness pressure
vessel with the operating pressure load up to 150 bar. So
the load carrying capacity of the saddle is 150 bar.
7. Conclusion
Based on our study in the areas of design, fabrication,
NDT and FEM simulation, we concluded the followings
for future development. Normally Zicks method applicable only for horizontal saddle design, but this work
proved that Zick method also applicable for inclined
saddles provided inclined saddle would undergo proper
FEM simulation test.
Without FEM, the inclined saddle design based on
Zicks approach would not help all the time for make sure,
the saddle is safe.
There were no surface cracks identified during DPT,
MPT testing. This was due to the suitable welding. So it is
mandatory that quality checks need to be done on welds
prior to use the saddle after fabrication.
There were no internal cracks identified in our UT
testing. This was also due to suitable welding. Internal
crack checks need to fix as mandatory for all the weld
joints. Either UT or RT needs to be done for all the weld
corners for ensuring, the design is safe.
The FEM simulation proved that saddles are good
enough for withstanding 150bar internal pressure load.
Always check the load bearing capacity of the saddles
prior to erection using FEM.
8. References
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vessels on two saddle supports, Welding Journal Research
Supplement. 1971; 959–70.
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Vol 9 (21) | June 2016 | www.indjst.org
3. Annaratone D. Pressure Vessel Design. Berlin Heidelberg:
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4. Chattopadhyay S. Pressure Vessels Design and Practice.
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public/TA2885A-0000-C-001 Data%20Book/Databook%20
H-21002/TA2885A-0000-C-001%20-%20Mechanical
%20Calculation /TA2885A-0000-C-001%20-%20AS%20
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Indian Journal of Science and Technology