Experimental Investigation of Effect of Angular Positioning of Legs on Structural Stability of Pressure Vessel and by Using Non- Linear Finite Element Analysis

Experimental Investigation of Effect of Angular Positioning of Legs on Structural Stability of Pressure Vessel and by Using Non- Linear Finite Element Analysis

Sandip S. Chavan* Ramchandra G. Desavale ** Imran M. Jamadar ***

P.G. Student, Mechanical Engineering Dept., A.D.C.E.T., Ashta* Associate Professor, Mechanical Engineering Dept., A.D.C.E.T., Ashta** Assistant Professor, Automobile Engineering Dept., A.D.C.E.T., Ashta***

Abstract

The intention of the pressure vessel-reactor is to have production of phenol and acetone by cumene process which 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. The pressure vessel is being designed to implement the Cumene Process. The process is extremely sensitive to pressure and temperature conditions and requires a lot of control systems to monitor it. These control systems are to be placed below the vessel for effective monitoring. The current range of Pressure Vessels in the market of AZ series come either in skirt support or supported by 8 legs equidistance from each other. However, a custom made pressure vessel has been ordered for the cumene process. The custom made vessel has to have a lot of controls for the cumene process; hence 8 legs are not feasible. Six legs support with a non- symmetric distribution was tried out initially. In this paper, the current requirement is to have more floor space. And to simulate the structure for wind load and verify for the safe condition. Hence, it is decided to improvise on the design and introduce angular supports. It has an advantage of increased floor space to mount the controls. The results show that the effect of 6 leg support on the structural stability of pressure vessel behavior is noticeable in optimum design and minimum deformation and minimum stress will be find out for the Cumene Process of the pressure vessel.

Keywords: FEA, Cumene process, IPV, Wind load, Maximum stability, Leg supports.

1. Introduction

   With the development of the industry, high pressure vessels are widely used in the field of the petroleum

   and chemical industry, and the dimension of the vessel becomes larger and larger. A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Pressure vessels are used to store and transmit liquids, vapours, and gases under pressure. The pressure vessel is being designed to implement the Cumene Process. Cumene process 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. The pressure vessel-reactor is to have production of phenol and acetone. This process illustrates the benefit of chemical engineering in merely converting two relatively cheap starting materials, benzene and propene into two more valuable ones, phenol and acetone. Other reactants required are oxygen from air and small amounts of a free radical initiator. Most of the worldwide production of phenol and acetone are now based on this method Magnucki et al. [1, 12] have reported that the pressure a vessel was treated as an integrated system, including the deformable support with stiffness adjusted to minimize the stress concentration in the vessel shell. The support should be of appropriate shape. Simple design and suitable thickness relative to thickness of vessel shell. Shafique M.A.Khan [2, 13] has presented that analysis of stress distribution in horizontal pressure vessels and saddle supports. A quarter of pressure vessels have modeled with relastic details of saddle support. Physical reasons for favoring of particular valve of ratio of the distance of support from end of the vessels to the length of vessels have outlined. Donatello Annaratone [3] has addressed that the most comprehensive and qualified study about the behavior of a horizontal cylindrical vessel on two symmetrical saddle supports, was the total load on the support, the length of the cylinder

   measured between the tangency lines of the heads. Troy Alvin Smith [4] has presented that the method developed for the static stress and deformation analysis of axisymmetric shells under axisymmetric loading by reduction of the shell to ring sections. E.Gutman et al. [5] have addressed that the stressed state in real metal construction changes in the process of operation even under permanent external loading. This paper presented a method for determine the critical time of stability loss in thin walled high pressure vessels subjected to uniform shell. Imran Jamadar et al. [6,11] have published inclined pressure vessel (IPV) study using finite element analysis using ANSYS to find out stresses in the vessel for its structural stability was done in this paper. The custom made vessel has to have a lot of controls for the cumene process; hence 8 legs are not feasible. Six legs support with a non- symmetric distribution was tried out initially. However the current requirement is to have more floor space. It has an advantage of increased floor space to mount the controls. From above discussion research papers it is seen that there is a scope for investigation of structural stability of pressure vessel by using the finite element analysis with experimentally. This paper focuses to determine whether creating an angle in the legs in combination with unsymmetrical distribution affects the structural stability of the system. And simulate structure for the wind load from 15 to 180 degree for safety.

2. Numerical Analysis

   1. Critical Aspects of the Design.

      If supports are placed at equal angles with respect to each other, it will ensure maximum stability. However in the case of new vessel, the requirement is that to have an 80 degree opening on one side of the vessel. And to verify the structure for safe condition for wind load. If 4 supports are used then 90 degree angle can be obtained easily, but if 80 degree is to be incorporated in 6 supports, then it might result in eccentricity.

      Fig. 1 Pressure Vessel With Six Leg Vertical Supports and Design With Inclined Suppotrs

   2. Analysis Approach.

      The pressure vessel supports are designed by considering factors in mind such as wind Loads, Internal Pressure, and Self Weight. As wind load cannot be directly tested, FEA analysis becomes critical to ascertain to gauge the performance. Wind Load has to be simulated from several directions, and find weak regions in the structure. Deformation is also an important parameter, cumene process requires control to be precise, this will mean that supervisory personnel will have to regularly visit bottom of the vessel, hence safety is a major concern. In addition to unsymmetrical supports, the angle that it makes with the ground (about 0 -30 deg) will add to the complexity of the FEA. FEA simulation will be Non Linear in nature, and needs to be performed according to the standards of the client. The standards will include mesh size, number of iterations etc.

   3. Wind Load Calculation.

      Wind Load calculations are based on American Society of Civil Engineers (ASCE) 7-95 obtained from Pressure Vessel Design Manual by D. Moss [ANSI A58.1] .Wind speeds in the zone where the cumene column is to be erected are in the range of 90 mph. Fig 2 shows the wind loads will exert a pressure on the face of the vessel facing the wind and will cause bending of te supports .Wind design is influence on structural stability of pressure vessel. It is important to find out the wind force and moments at each elevation to check if the calculated shell thicknesses are adequate. The overturning moment at the base is used to determine all of the anchorage and support details. These details include the number and size of anchor bolts, thickness of skirt, size of legs, and thickness of base plates. As a loading, wind differs from seismic in that it is more or less constant; whereas, seismic is of relatively short duration. In addition, the wind pressure varies with the height of the vessel. A vessel must be designed for the worst case of wind or seismic, but need not be designed for

      both simultaneously. While typically the worst case for seismic design is with the vessel full (maximum weight), the worst design case for wind is with the vessel empty. This will produce the maximum uplift due to the minimum restraining weight. The wind forces are obtained by multiplying the projected area of each element, within each height zone by the basic wind pressure for that height zone and by the shape factor for that element. The total force on the vessel is the sum of the forces on all of the elements. The forces are applied at the centroid of the projected area.

      Fig. 2 Wind Load Fig. 3 General Vessel Dimensions

      The Vessel for the Cumene column will have the following general dimensions. Fig 3 shows the general vessel dimensions.

      Table 1 General Vessel Dimensions

      Description	Value(mm)	Value (ft)
      Vessel Diameter	1524	5
      Vessel Thickness	12.7	1/2
      Length of Cylindrical Portion	254	10
      Total Height of Vessel from Ground	4127.5	13.6
      Height of Legs for Support	1066.8	3.6

      The Legs of the Vessel are I section beams with dimensions as shown below.

      Description	Symbol	Value(mm)
      Depth(mm)	G	152.4
      Width(mm)	H	152.4
      Web thickness mm	I	15
      Thickness(mean) mm	J	15

      Description	Symbol	Value(mm)
      Depth(mm)	G	152.4
      Width(mm)	H	152.4
      Web thickness mm	I	15
      Thickness(mean) mm	J	15

      Table 2 Leg Support Cross Section Dimensions

      Fig. 4 Leg Support cross section dimensions

   4. Calculation.

      Structure category= III (Buildings and other structures containing sufficient quantities of toxic or explosive substances to be dangerous to the public if released. Buildings or structures where the primary occupancy is one in which more than 300 people congregate in one area)

      Exposure category= D (Flat, unobstructed coastal areas directly exposed to wind blowing over open water; applicable for structures within distance from shoreline of 1500ft or 10 times the structure height)

      Table 3 Wind Load Calculations Factors [Pressure Vessel Design Manual by D. Moss Table 3.1& 3.2]

      Description	Symbol	Value	Unit
      Structure category	–	III
      Exposure category	–	D
      Basic Wind velocity	V	90	mph
      Effective diameter	D	5	feet
      Importance factor	I	1.15
      Force coefficient	Cf	0.7-0.9
      Velocity Pressure exposure	Kz	1.03
      Topographic Factor	KZT	1
      Height Of Vessel	H	13.6	feet

      First determine vessel is rigid or flexible

      1. If h/D < 4, then vessel is rigid.

      2. If h/D > 4, then vessel is flexible.

Calculate h/ D ratio = 13.6/5 = 2.72 < 4 then vessel is rigid

Wind Load/ Force (F) = qz*G*Cf*Af

Calculate,

Velocity Pressure at height z above the ground (qz) = 0.00256*Kz *KZT * V2* I (1)

(qz) = 0.00256* Kz * KZT * V2*

(qz) = 0.00256* 1.03 * 1* 902 * 1.15

(qz) = 245.6 .PSF (pounds per square feet) Hence,

Wind Load/ Force (F) = qz *G

*Cf*Af (2)

Wind Load/ Force (F) = 245.6 * 0.85 * 0.81 *13.6 * 5

Wind Load/ Force (F) = 11500 N

Based on the above calculations wind load used for simulation is 11.5kN in addition to this, the vessel is subjected to internal pressure of 1.03Mpa, self- weight 6820Kg.

1. FEA Approach

   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.

2. 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.

   Table 4 Material Properties.

   Sr. No	Particular	Value
   1	Material	Structural Steel
   2	Mass	8620 Kg
   3	Density	31500 Kg/3
   4	Volume	2.7338e+008 mm³
   5	Poisson ratio	0.3
   6	Youngs modulus E	2e+11 Pa
   7	Compressive yield strength	2.5e+8Pa
   8	Tensile ultimate strength	4.6e+8 Pa

3. 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 ANSYS workbench software Later on to model was imported to ANSYS 12 where symmetric model was prepared, and then accordingly vessel supports was tilted to required inclinations.

Fig. 5 Modeling of Pressure Vessel with 6 Leg Support.

1. 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 tetrahedron element was used for meshing. The element is defined by 10nodes.

   Fig. 6 10 Node Tetrahedron Element

2. Meshing of the model

   Meshing is the method of dividing the model into the number of element to obtain the good accuracy in the analysis. As the number of element increases the accuracy of analysis increases. In this paper meshing size is taken as 100mm for pressure vessel body and 30 mm for pressure vessel support. Figure 7 shows the meshing of pressure vessel.

   Fig. 7 Meshing Of Pressure Vessel

3. Boundary conditions.

Applying the boundary conditions applied to the pressure vessel model. All the supports are fixed. Pressure applied inside the pressure vessel is 150psi.Wind load of 11500 N is applied on one of its face.

4.0 Results

Area of interest of this paper is the leg so result taken into consideration is total deformation in the leg and Maximum stress developed in the leg support. Fig 8 shows the minimum deformation in 18 degree angle in finite element method.

Fig. 8 Total Deformation in mm

Fig. 9 Equivalent Stress in Mpa

By using numerically means finite element method 18 degree gives minimum deformation and minimum stress.

Fig. 10 Equivalent Stress in Mpa

In this above fig.10 shows the equivalent stress of pressure vessel by simulating the structure for the 15 to 180 degree angle shows the minimum allowable stress of structural stress and hence the structure is safe. Allowable stress for the structural steel is near about 167 Mpa.

1. Experimental

   The typical shape of pressure tank is cylindrical to effectively maintain the internal hydrostatic gas pressure. The pressure is near about 1.03 Mpa. The test for pressure vessel leakage finding is hydro test to check the leakage in the pressure vessel.

   1. Ultrasonic Testing

      Weld spot at nozzle vessel intersection 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. 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 corresponding 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 Pressure Test

      Hydrostatic test is an indispensable method to inspect the strength of the pressure vessel and to verify the fabrication quality. We performed the hydrostatic test to study the stress distribution of the integrated pressure vessel. Hydrostatic testing is the most common procedure used to qualify newly manufactured pressure vessel, cylinders, spheres and tubes used for the transportation of dangerous goods and other chemical process. Hydrostatic testing is also required periodically to re-qualify these pressure vessels for continued service. Vessel was also tested for hydro-test pressure of 1.03Mpa 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 vertical leg supports for measuring the deformations.

   3. Procedure

During a hydrostatic test, a pressure vessel is placed inside a closed system, usually a test jacket filled with water, and a specified internal water pressure is applied to the container inside this closed system. The applied internal pressure causes an expansion of the container being tested, and the total and permanent expansion that the container undergoes is measured. These volumetric expansion measurements, in conjunction with an internal and external visual inspection of the container, are used to determine if a pressure vessel is safe for continued use, or have suffered from degradation in its structural integrity and must be condemned. This method, also known as a modified hydrostatic test, consists of subjecting a pressure vessel to a specified internal pressure and inspecting the pressurized container for leaks, bulges or other defects. This

method is permitted only when the applicable regulations do not require the determination of volumetric expansion measurements. The equipment shall be maintained at test pressure for 30 min.

Fig. 11 Test Pressure Vessel

5.3 Design Inputs.

Table 5 Design Input Data

6.0 Results and Plots

Variation of stresses and deformation with respect to inclination of pressure vessel support with 800 opening on one side.

Angle Vs Equivalent Von Mises Stress

120

100

80

60

40

20

0

Angle Vs Equivalent Von Mises Stress

120

100

80

60

40

20

0

0 20 40

Leg Inclination , Degree

0 20 40

Leg Inclination , Degree

Equivalent

Von-

Equivalent

Von-

Equivalent Von Mises Stress, Mpa

Equivalent Von Mises Stress, Mpa

Fig. 12 Graph of Leg Inclination Vs stress

Total Deformation,MM

Total Deformation,MM

From graph shown in fig.12 shows that from 0 to 30 degree stress increases gradually and in between 0 to 30 degree the stress value is minimum at 18 degree and again it increases. From this graph conclude that the structure of the 18 degree gives the minimum stress from that considering this as an optimizeddesign. The Value stress is at 18 degree 49.087 Mpa.

Angle Vs Total Deformation

Angle Vs Total Deformation

0.350

0.300

0.250

0.200

0.150

0.100

0.050

0.000

Angle Max.

0.350

0.300

0.250

0.200

0.150

0.100

0.050

0.000

Angle Max.

0 20 40

Leg Inclination, Degree

0 20 40

Leg Inclination, Degree

Fig. 13 Graph of Leg Inclination Vs stress

From graph shown in fig.13 shows that from 0 to 30 degree deformation increases but 18 degree give minimum deformation and minimum stress value. From this graph conclude that the structure of the 18 degree gives the minimum deformation from that considering this as an optimized design. The Value of total deformation in vertical leg support is 0.1857mm.

0 100 200

Angle, Degree

0 100 200

Angle, Degree

Equivalent Von Mises Stress , Mpa

Equivalent Von Mises Stress , Mpa

Fig.13 Graph of Leg Inclination Vs Stress

In fig. 13 graph shows the by simulating angle from 15 to 180 degree structure gives the minimum stress which is less than allowable stress of structural steel(less than 167 Mpa) so, the structure is safe.

7.0 Conclusion

The pressure vessel is being designed to implement the Cumene Process. The process is extremely sensitive to pressure and temperature conditions and requires a lot of control systems to monitor it. In this paper exertion observed that maximum equivalent Von-mises stress observed was about 49.087 Mpa around 6 leg supports with vertical inclination on one side 80 degree opening.. By using finite element analysis conclude that 18 degree gives the almost minimum equivalent von mass stress and minimum total deformation in pressure vessel. And also experimentally the deformation is 0.1319mm. Which is close to the analysis results. And for simulating the structure for the 15 to 180 degree gives minimum stress and it less than allowable stress of structural steel. So structure is safe.

Acknowledgement

We sincerely thank Mr. I. M. Jamadar for his continuous support in providing advances in Pressure Vessel analysis technology and for guidance to prepare this paper.

References

Angle Vs Equivalent Von Mises Stress

60

50

40

30

20

10

0

Angle Vs Equivalent Von Mises Stress

60

50

40

30

20

10

0

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