Structural Analysis of Inclined Pressure vessel Using FEM

Structural Analysis of Inclined Pressure vessel Using FEM

Imran M.Jamadara, Prof.R.M.Tayadeb, Mr.Vinay Patilc

aPG Student Mechanical Engineering Depart ment, Veermata Jijabai Technological Institute(VJTI),

Matunga,Mumbai, Maharashtra, Mumba i,India

bProf.R.M.Tayade, Mechanical Engineering Depart ment, Vee rmata Jijaba i Technological Institute(VJTI),

Matunga, Mumba i ,Maharashtra,India

cMr.Vinay Patil, FEA Consultant, VAFTSY CAE, Hadapsar, Pune, Maharashtra,India

Abstrac t: Inclined pressure vessel (IPV) study using finite ele ment analysis using ANSYS to find out stresses in the vessel for its structural stability is done in this paper. Inclined pressure vessel is used for production of nitrous oxide by ammoniu m nitrate pyrolysis reaction by passing the steam at around 2000 C and 1.37895 Mpa over the ammon iu m nit rate contained in the cylindrica l vessel. Here design challenge is inclined nature of vessel as, ASME code enables design of Horizontal or a Vertica l vessel but there is no provision fo r an Inclined Vessel in it.

Ke ywor ds-IPV, stress anal ysis, structural stability.

1. Intr oducti on

   Vessels, tanks, and pipelines that carry, store, or receive flu ids are called pressure vessels. A pressure vessel is defined as a container with a pressure differentia l between inside and outside. The inside pressure is usually higher than the outside, except for some isolated situations. The flu id inside the vessel may undergo a change in state as in the case of steam boile rs, or may combine with other reagents as in the case of a chemical reactor. Pressure vessels often have a combination of high pressures together with high temperatures, and in some cases fla mmable fluids or highly radio- act ive materia ls. Because of such hazards it is imperative that the design be such that no leakage can occur.

   Designing, thus, involves estimation of stresses and

   deformations of the components at different critica l points of a component for the specified loads and boundary conditions, so as to satisfy operational constraints .Design is associated with the calculation of dimensions of a component to withstand the applied loads and perform the desired function. Analysis is associated with the estimat ion of displace ments or stresses in a component of assumed dimensions so that adequacy of assumed dimensions is validated.

2. Material selection

   We are doing analysis of inclined pressure vessel for which ASM E codes are not applicable. To

   overcome this firstly we have done analysis of horizontal vessel and its results are utilized to do further inclined vessel analysis. This section discusses the prima ry factors that influence materia l selection for pressure vessels and the ma ximu m a llo wable materia l stresses specified by the ASME Code [9]. The mechanica l design of a pressure vessel can proceed only after the materia ls have been specified. The ASME Code does not state what materia ls must be used in each application. It specifies what materials may be used for ASM E Code vessels, plus rules and limitations on their use. But, it is up to the end user to specify the appropriate materials fo r each applicat ion considering various material selection factors in conjunction with ASME Code require ments. Accordingly, structural steel is selected as a materia l for vessel the properties of which are tabulated.

3. Modeling

   The detailed 3D modeling of pressure vessel was done using CATIA V5R17.

4. Meshing

   The accuracy of the FE model is highly dependent on the mesh employed, especially if h igher order (cubic, quadratic etc.) ele ments are not used. In general, a finer mesh will produce more accurate results than a coarser mesh. At some point, one reaches a point of diminishing returns, where the increased mesh density fails to produce a significant change in the results. At this point the mesh is said to be converged. This process of refining the mesh and evaluating the results is norma lly re ferred to as a mesh convergence study or analysis. Although many FE codes contain error estimates of one sort or another, mesh convergence remains the most reliab le means of judging model accuracy. Coarse meshes almost always under-report the stresses in a model. It is not uncommon to have ma ximu m reported stresses on the order of less than 50% of the converged

   stresses on a coarsely meshed model. Thus, without consideration of mesh convergence, gross errors in stress estimates are quite possible. If higher order ele ments are used, good results can be obtained with fe wer ele ments. Either mesh convergence analysis or a reliable error estimate is absolutely necessary to quantify the analysis results. Typically, an increase of less than 5% in the stress levels after a doubling of mesh density or an error estimate of less than 0.05 will ensure that the indicated stresses are within 5-10% of the converged values [2]. So me FE codes employ an adaptive process to automatically refine the mesh and/or increase the order of the elements to reach the desired degree of accuracy. When available, these processes can save a lot of manual effort. They do not, however, completely relieve the engineer of the need to check the results.

5. Boundary conditi ons

   One of the most significant sources of errors in FE modeling is the inaccurate (or inappropriate) modeling of the loads and restraints on a model. For e xa mple , fully fixing (restrain ing) all of the nodes on the end of a pressure vessel do not represent the same condition as does a normal head. With any norma l head, the radial stiffness is not infinite, thus radial e xpansion under pressure loading will occur. This cannot be the case when the node is fixed. The symmetrica l boundary condition always restrains translations norma l to the plane of symmetry and the rotations in the plane of symmetry. In this case, we would have restrained translation in the X direction and the rotations about the Y and Z a xes. Fo rces and mo ments may be applied to a model in a number of diffe rent ways. The particular method chosen is often a function of the pre-processor used to generate the model. So me pre-processors will automatically distribute a force or mo ment a long a line or surface, such as the end of the nozzle . Even though we are using a modern computer-based tool, the sum of the forces and moments about each a xis must rema in equal to zero when the body is at rest.

6. Stress Anal ysis

Stress analysis is the determination of the relationship between external and internal forces applied to a vessel and the corresponding stress. The emphasis is not how to do stress analysis in particular, but rather how to analyze vessels and their component parts in an effort to arrive at an economical and safe design-the difference being that we analyze stresses where necessary to determine thickness of material and sizes of me mbe rs. It is not necessary to find every stress but rather to know the governing stresses and how they

relate to the vessel or its respective parts, attachments, and supports [4].

The starting place for stress analysis is to determine all design conditions for a given proble m and then determine all the related e xternal forces. We must then relate these external forces to the vessel parts which must resist them to find the corresponding stresses. By isolating the causes (loadings), the effects (stress) can be more accurately determined. The designer must also be keenly aware of the types of loads and how they relate to the vessel as a whole. Are the effectslong or short term? Do they apply to a localized portion of the vessel or are they uniform throughout? How these stresses are interpreted and combined, what significance they have to the overall safety of the vessel, and what allo wable stresses are applied will be determined by three things:

1. The strength failure theory utilized.

2. The types and categories of loadings.

3. The hazard the stress represents to the vessel The basic intent of design by analysis is to

determine the stres s conditions in a pressure vessel under load for each load condition for the vessels entire operational life. In order to meet the intent and specifications of Section VIII, Div ision 2, the designer must carry out a sufficiently detailed stress analysis of the vessel to show compliance with the stress limitations imposed by the Code for the materia l fro m which the vessel is to be fabricated. The designer may specify any manner by which to determine the stresses in the vessel under load, so long as use of the methodologies emp loyed.

This investigation primarily deals with the

probable causes of in-service damage of IPV with approximate estimat ion of stresses . The design temperature and pressure of vessel 1490C and 1.3789Mpa, respectively. There were four nu mbers of openings, Viz.entry and exit of steam, Exit of Nitrous oxide and drain. The vessel thickness was around 9.65mm. Stress analysis was carried out by fin ite ele ment method using ANSYS 13.0 code.

Structural Elements Used for the Analysis: The higher order he xahedron ele ment is used for meshing [8].

Boundary Constraints (figure 4): The whole vessel is supported on two saddle supports. The upper saddle was fixed while to the lower saddle cylindrica l support was provided. Vessel was analyzed using ANSYS code on the basis of strength for internal press ure 1.38 Mpa, plus Se lf Weight considering portions of the drum of 1275.55 mm length with inside dia meter of dru m

304.8 mm, thickness of the shell 9.65mm. Considering the symmetry of the unit, one half of the unit was analyzed for stress calculations. The fin ite ele ment model of the co mponent under investigation is shown in Fig. 1

Fig.1 3DModel Fig.2 Meshing

Fig.3 Axis-symmetric model Fig.4B oundry Conditions

Table .1 Materi al Properties

Fig.5 Equi valent Von-Mises Stress Plot Fig.6 Total De for mati on Plot

Table.2 Stress and defor mati on val ues at various angles

Gr aphs:

Fig.7 Inclination V/s Equi vale nt Stress Fig.8 Inclination V/s Linearise d Stress

7.Discussion and Conclusions

Fro m above analysis it was observed that ma ximu m equivalent stress observed in the vessel around the cylindrical support was about 117.88Mpa for vessel angle 320, it is e xceeding allo wable limits but it is less than yield strength and ultimate tensile strength .so failure will not be observed in this condition but to provide suffic ient factor of safety and reduce stress below allo wable limits some modification in this area are necessary. The deformation was observed to be ma ximu m around nozzle vessel intersection 0.17494mm for 320 inclinations. The deformation was observed to be minimu m a round fixed support. Partia lly superimposed weld heat affected zones of nozzle weld caused localized adverse non liner condition and high load that facilitated initiat ion and propagation of cracks. From fig.8 and Fig..9 it is concluded that the equivalent Von-Mises Stress and linearised me mbrane stress along vessel thickness are increasing with inclination of vessel.

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