FINITE ELEMENT ANALYSIS: PRESSURE VESSEL WITH · PDF fileFINITE ELEMENT ANALYSIS: PRESSURE VESSEL WITH ANGULAR LEG SUPPORTS ... we are having leg supports for our pressure vessel and

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Int. J. Mech. Eng. & Rob. Res. 2014 Amit Patil and Amol Kolhe, 2014

ISSN 2278 – 0149 www.ijmerr.com

Vol. 3, No. 3, July, 2014

© 2014 IJMERR. All Rights Reserved

Research Paper

FINITE ELEMENT ANALYSIS: PRESSURE VESSEL

WITH ANGULAR LEG SUPPORTS

Amit Patil*1 and Amol Kolhe2

*Corresponding Author: Amit Patil � [email protected]

The objective of the pressure vessel is to have production of phenol and acetone. Cumeneprocess is an industrial process of producing phenol (C6H5-OH) and acetone (CH3-CO-CH3)from benzene (C6H6) and propene (C3H6). The term stems from isopropyl benzene or cumene(C6H5-CH (CH3)2), the intermediate material during the process. With the help of this processtwo relatively cheap materials, benzene and propene are converted into two more valuableproducts such as phenol and acetone. For this process other required reactants are oxygenfrom air and small amounts of a free radical initiator. The pressure vessel is being designed toimplement the cumene process. The process is extremely sensitive to pressure and temperatureconditions and requires a lot of control systems to monitor it. These control systems are to beplaced below the vessel for effective monitoring.

Keywords: Non-linear, Supports, Stability, Vessel, FEA

INTRODUCTION

Type of support used depends on theorientation and pressure of the pressurevessel. Support from the pressure vesselmust be capable of withstanding heavy loadsfrom the pressure vessel, wind loads andseismic loads. Pressure on pressure vesseldesign is not a consideration in designingsupport. Temperature can be a considerationin designing the support from the standpointof material selection for the different thermalexpansion. The current range of PressureVessels in the market of AZ’series, comeeither in skirt support or supported by 8 legs

1 Asst. Professor Department of Mechanical Engineering, Pad. DR D Y Patil Institute Engineering & Technology, Pimpri, Pune, India.

equidistance from each other. However, acustom made pressure vessel has beenordered for the cumene process. The custommade vessel has to have a lot of controls forthe cumene process; hence 8 legs are notfeasible. Six legs support with a non-symmetric distribution was tried out initially.However the current requirement is to havemore floor space.

OBJECTIVE OF WORK

Typically most of the pressure vessels areeither skirt or leg supported and observationis that legs are primarily vertical. However in

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case vertical expansion of the vessel eitherdue to thermal expansion or due to verticalloading the legs are susceptible to buckling.Hence, objective of this work is to determinewhether creating an angle in the legs incombination with unsymmetrical distributionaffects the structural stability of the system.

LITERATURE SURVEY

After going through various literatures onpressure vessel supports some of theselected papers are cited below and whichare considered as basis for developingstructural analysis methodology for verticalpressure vessels

Flexible Saddle Support of

Horizontal Cylindrical Pressure Vessel

K Magnucki et al. (2003) have reported thatthe pressure vessel was treated as anintegrated system, including the deformablesupport with stiffness adjusted to minimizethe stress concentration in the vessel shell.The support should be of appropriate shape.Simple design and suitable thickness relativeto thickness of vessel shell. The use ofsupports of high stiffness was certainlyunfavourable taking into account the strengthof vessels.

Stress Distributions in a Horizontal

Pressure Vessel and Saddle Supports

Shafique M A Khan (2010) has mentionedthat the analysis of stress distribution inhorizontal pressure vessels and saddlesupports. A quarter of pressure vessels havebeen modelled with realistic details of saddlesupport. Physical reasons for favouring ofparticular value of ratio of the distance of

support from end of the vessel to the lengthof vessel have been outlined. This paper hasbeen a basis for deciding the support lengthand total length of vessel.

Three Dimensional Finite Element

Analysis of Saddle Supported

Pressure Vessels

N EI-Abbasi et al. (2000). A three-dimensionalfinite element analysis is made of a pressurevessel resting on flexible saddle supports.The analysis is carried out using a newlydeveloped thick shell element and accountsfor frictional contact between the support andthe vessel. The seven-parameter shellelement is capable of describing the variationof the stress and the strain fields through itsthickness. A variable inequalities basedformulation is utilised for the accuratedescription of this class of frictional contactproblems. Different pressure vesselconfigurations are considered and theresulting contact stresses are examined. Theeffect of saddle radius, saddle width, plateextension, and support overhang on theresulting stress field in both the vessel andsupport are evaluated and discussed. Sincewe are having leg supports for our pressurevessel and for constant single pressurevessel analysis is done.

Analysis of Axisymmetric Shell

Structures in Axisymmetric Loading

by the Flexible Method

Troy Alvin Smith (2008) has presented thatthe method developed for the static stressand deformation analysis of axisymmetricshells under axisymmetric loading byreduction of the shell to ring sections. The

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distribution of stresses in the meridionaldirection of the ring element is assumed tobe linear with each element. By using thederived influence coefficients, the unknownforces at the juncture of the ring elementsare found by the standard flexibility methodof indeterminate structural analysis.Subsequently, the displacements and internalstresses.

Stability of Thin Walled High

Pressure Vessels Subjected to

Uniform Corrosion

E Gutman et al. (2000) have addressed thatthe stressed state in real metal constructionchanges in the process of operation evenunder permanent external loading. It takesplace due to changes in the cross section ofthe loaded elements resulting from thesurface corrosion. This paper presented amethod for determine the critical time ofstability loss in thin walled high pressurevessels subjected to uniform shell. Since ourpressure vessel is supported with I sectionbeam and uniform shell structure resultinginto stability and safe design.

Structural Analysis of Inclined

Pressure Vessel Using FEM

R M Tayade et al. (2012) have presentedinclined pressure vessel (IPV) study usingfinite element analysis using ANSYS to findout stresses in the vessel for its structuralstability was done in this paper. Inclinedpressure vessel was used for production ofnitrous oxide by ammonium nitrate pyrolysisreaction by passing the steam at around200°C and 1.37895 MPa over the ammoniumnitrate contained in the cylindrical vessel.

Here design challenge inclined nature ofvessel as, ASME code enables design ofHorizontal or a Vertical vessel but there wasno provision for an Inclined Vessel in it. Theabove paper suggests structural analysis forinclined pressure vessel which is close withour vessel having inclined leg supports whichare under consideration.

Stresses in Large Horizontal

Cylindrical Pressure Vessels on

Two Saddle Supports

L P Zick (1951) has presented thatapproximate stresses that exist in cylindervessels. Supports on two saddles at variousexist in cylinder vessels supported on twosaddles at various locations knowing thesestresses. It has possible to determine whichvessels may design for internal pressurevessels and to design structurally adequateand economical stiffing for the vessels whichrequire it. As far as vertical pressure vesselis considered the stress at each leg and shellcontact will decided the stability of vessel.

Support Leg Stress in Pressure

Vessels Mounted on Arbitrary Legs

Subjected to Lateral Loadings

Cai Zengshen and Lin Jun (1966). A pressurevessel mounted on arbitrary leg-typesupports forms a complicated support systemwith respect to lateral loadings such as windloads and horizontal seismic motion whichdo not have a predefined direction of actionand this problem has not been solved exactly.In the present paper an exact solution is firstlyproposed which is derived from an arbitrarynumber of legs with the leg inclined at anarbitrary angle with the horizontal. Thesolution showed that: (1) in the general case

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as to the support systems, the mostvulnerable direction of the horizontal loadingwhich maximizes support leg stress is not inthe symmetrical plane of the system (2) themost vulnerable direction is independent ofthe number of the legs in the system and (3)it is not the best case for the system that theangle between the legs and the horizontaldirection should be 90°. By numerical results,when the angle is about 87° the support legstress level is near to its lowest value. Sincethis paper features about the stress inducedin the legs due to lateral loading which willgive exact idea about possible values ofstress as in case of pressure vesselsupported by angular positioning of legs.

Stress Analysis of Conical Shell

Skirt Support for High Pressure

Vessel Using Finite Element Method

K Tamil Mannan et al. (2009) Pressure vesselis a closed cylindrical vessel for storinggaseous, liquids or solid products. The storedmedium is at a particular pressure andtemperature. The cylindrical vessel is closedat both ends by means of dished head, whichmay be hemispherical, ellipsoidal. Thepressure vessels may be horizontal orvertical. The supporting system of this verticalvessel plays an important role in theperformance of the equipment. Propersupporting system gives better efficiency. Thebottom supports are critical componentssince they are to be designed with much careto avoid failure due to internal pressure withtemperature. In this analysis, skirt support forvertical vessel was analysed as per theguidelines given in the ASME (AmericanSociety of Mechanical Engineering) section

VIII division 2 and IBR (Indian BoilerRegulations) standards. The stress analysiswas carried out for this support using ageneral purpose FEM code, ANSYS macros.The coupled field (Structural and Thermal)Analysis was carried out for skirt support tofind out the stresses in the support. Theanalysis results were compared with ASMEcode allowable stress. The above paper givesanalysis for skirt support and resulting stressinduced in the support with ASME section VIIIdivision 2 and IBR (Indian Boiler Regulations)standards. With similar approach analysishas been performed on inclined leg supportswith the help of FEM method.

DESIGN CONSIDERATIONS

Design Pressure

A vessel must be designed to withstand themaximum pressure to which it is likely to besubjected in operation. For vessels underinternal pressure, the design pressure isnormally taken as the pressure at which therelief device is set. This will normally be 5 to10 per cent above the normal workingpressure, to avoid spurious operation duringminor process upsets. When deciding thedesign pressure, the hydrostatic pressure inthe base of the column should be added tothe operating pressure, if significant. Vesselssubjected to external pressure should bedesigned to resist the maximum differentialpressure that is likely to occur in service.Vessels likely to be subjected to vacuumshould be designed for a full negativepressure of 1 bar, unless fitted with aneffective, and reliable, vacuum breaker. Thepressure inside our vertical pressure vesselis taken as 150 Mpa.

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Design Temperature

The strength of the metal decreases withincreasing temperature so the maximumallowable design stress will depend on thematerial temperature. The designtemperature at which the design stress isevaluated should be taken as the maximumworking temperature of the material. Thetemperature inside our vertical pressurevessel is taken as 120°C.

Materials

Pressure vessels are constructed from plaincarbon steels; low and high alloy steels, otheralloys, clad plate, and reinforced plastics.Selection of a suitable material must take intoaccount the suitability of the material forfabrication (particularly welding) as well asthe compatibility of the material with theprocess environment. The pressure vesseldesign codes and standards include lists ofacceptable materials, in accordance with theappropriate material standards. The materialis selected as Structural Steel for analysispurpose.

Design Stress (Nominal Design

Strength)

For design purpose it is necessary to decidea value for the maximum allowable stress(nominal design strength) that can beaccepted in the material of construction. Thisis determined by applying a suitable factor ofsafety to the maximum stress that thematerial could be expected to withstandwithout failure under standard test conditions.The design stress factor allows for anyuncertainty in the design methods, theloading, the quality of the materials, and theworkmanship. For those materials not

subjected to high temperature the designstress is based on the yield stress (or proofstress), or the tensile strength (ultimatetensile stress) of the material at the designtemperature. For materials subjected toconditions at which the creep is likely to beconsidered, the design stress is based on thecreep characteristics of the material.

FINITE ELEMENT ANALYSIS

PROCEDURE

Selection of Material

As per the ASME code for manufacturing ofpressure vessel numbers of materials arespecified but the selection depends purelyupon nature of application. In accordancewith number of material selection factors andrules and regulation led down by ASME codespecifications material need to be decided.Since this work primarily focus on the analysisof vertical leg supports. Finally end user willdecide which material to be used. Thematerial used for this vessel is structural steeland its properties are listed below.

Geometry Creation

The vessel geometry modelling has beendone in Ansys 12.01 workbench itself.

Meshing

Meshing has been done by using the methodof Tetrahedron. The accuracy of results will

Table 1: Material Properties

Sl.No.

1

2

3

4

5

Material

Young'sModulus (Mpa)

Density (Kg/m3)

Poissions Ratio

Yield Strength (Mpa)

Tensile Strength(Mpa)

Structural Steel

2e5

7850

0.3

250

460

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largely depends upon the extent of correctmeshing. In Tetrahedron method thecomponent is been divided into small triangleon its surface which gives no of nodes andelements of that component. The meshinghas been done by changing the mesh size ofthe various component of the pressurevessel. Due to change in the density of themeshing, it results in the variation of the noof nodes and elements of the meshed parts.The result of this mesh density change affectsthe value of the stress and deformation ofthe component. For fine meshing that is forsmall mesh size the values of no of nodesand elements are high but as the elementsize is gradually increased it result in increasein the value of no of nodes and elements.For small variation in mesh size that is of 1E-03 m the values are showing small variationin no of nodes and elements and also it showsthe same amount of variation in the valuesof stress and deformation for the given meshsize. But for large variation in mesh size thevalues of no of nodes and elements are alsovary in large amount.

Steps Involved in Meshing PressureVessel Assembly

• Creating CAD model in CATIA V5R20.

• Repairing CAD geometry data

• Import geometry

• Creating simplified parts

• Meshing the part

After performing the meshing it is equallyimportant that the connectivity associatedbetween the element and each node of theelement will be smooth and it will not resultinto the formation of triangular elements. Andthus we are getting desirable mesh patternfor different element.

Boundary Condition

Defining correct boundary conditions isprimary requirement for all the FEA analysis.The boundary condition applied for particularanalysis must be correct or it may causemisleading results. The nature of analysis willdecide the boundary conditions need to beapplied. As in this work the leg supports aremore important from analysis point of viewand sine pressure vessel is vertically standingon ground the following boundary conditionswere applied.

1. Wind load acting on the vessel

2. Internal pressure of the vessel

3. All the supports need to be fixed to theground

Structural Analysis

The Ansys 12.01 workbench will give theresults in terms of the maximum deformationas well as the stress subjected by thepressure vessel assembly. Here, in this workthere will be small increment in the angularpositioning of legs by 1 degree up to 30degree. This increment will result in theconsiderable amount of deformation as wellas stress in the leg supports. The nature ofthis analysis is based upon Section VIIIDivision I for the targeted compliance of thestress values in the pressure vesselassembly.

FUTURE SCOPE

The analysis is mainly focused on the amountof maximum deformation and Von MissesStress. The further study may be carried outin terms of the amount of bending stressesinduced in the leg supports for given load.

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CONCLUSION

As per the solid structural analysis performedabove it is found that the values of stress anddeformation are safe for all the values of theincrement in the angle. The analysis showsthat even though varying angle from 0 to 30degree the optimum value of stress anddeformation will be achieved for 18 degreemodel for the given boundary conditions. This18 degree model is also satisfying the projectobjective that even though creating theunsymmetrical distribution in the leg supportswill be resulting into the structural stability.Also, important consideration for this work isthe increased floor space which also can beachieved for the optimum value of stress anddeformation. So considering all above thingsthis above model is also simulated for thevarious positions of angular wind load. Thepressure vessel assembly is rotated from 15to 180 degree. This will show that the valuesof stress and deformation within this modifiedboundary condition are also within limitingvalues.

REFERENCES

1. Cai Zengshen and Lin Jun (1966),“Support Leg Stress in Pressure VesselsMounted on Arbitrary Legs Subjected toLateral Loadings”, International Journalof Pressure Vessels and Piping, Vol. 68,pp. 13-22.

2. E Gutman, J Haddad and R Bergman(2000), “Stability of thin - Walled High-Pressure Vessels Subjected to UniformCorrosion”, International Journal ofPressure Vessels and Piping, Vol. 38, pp.43-52.

3. H G Willschuttz (2003), “Simulation ofScaled Vessel Failure Experiments andInvestigation of Possible Vessel SupportAgainst Failure”, Journal of NuclearEngineering and Design, Vol. 228, pp.401-414.

4. K Magnucki, P Stasiewicz and W Szyc(2003), “Flexible Saddle Support of aHorizontal Cylindrical Pressure Vessel”,International Journal of Pressure Vesselsand Piping, Vol. 80, pp. 205-210.

5. K Tamil Mannan, Rakesh Saxena, RMurugavel and P L Sah (2009), “StressAnalysis of Conical Shell Skirt Supportfor High Pressure Vessel Using FiniteElement Method Multidiscipline Modelingin Materials and Structures”, Vol. 5, pp.355-362.

6. L P Zick (1951), “Stresses in LargeHorizontal Cylindrical Vessels PressureVessels on Two Saddle Supports”, TheWelding Journal Research Supplement,pp. 959-970.

7. N EI-Abbasi, S A Meguid and A Czekanski(2000), “Three-Dimensional FiniteAnalysis of Saddle Supported PressureVessels”, International Journal ofMechanical Sciences, Vol. 43, pp. 122-1242.

8. R M Tayade, Vinay Patil and Imran MJamadar (2012), “Structural Analysis ofInclined Pressure Vessel Using FEM”,International Journal of EngineeringResearch & Technology (IJERT), ISSN:2278-0181.

9. Shafique M A Khan (2010), “StressDistribution in Horizontal Pressure Vessel

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and the Saddle Supports”, InternationalJournal of Pressure Vessels and Piping,Vol. 87, pp. 239-244.

10. Troy Alvin Smith (2008), “Analysis ofAxisymmetric Shell Structures UnderAxisymmetric Loading by the FlexibilityMethod”, Journal of Sound and Vibration,Vol. 318, pp. 428-460.

11. W M Banks, D H Nash, A E Flaherty, WC Fok and A S Tooth (2000), “A SimplifiedDesign Approach to Determine theMaximum Strains in a GRP VesselSupported on Twin Saddles”,International Journal of Pressure Vesseland Piping, Vol. 77, pp. 837-842.

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FINITE ELEMENT ANALYSIS: PRESSURE VESSEL WITH · PDF fileFINITE ELEMENT ANALYSIS: PRESSURE VESSEL WITH ANGULAR LEG SUPPORTS ... we are having leg supports for our pressure vessel and