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Determination of the thickness of the connecting side between the H-shaped forgings and the skirt of the hydrogenation reactor

Abstract: the mechanical stress and temperature stress of the connecting area between the H-shaped forgings and the cylinder, the head and the skirt of the hydrogenation reactor are analyzed by using the finite element method, and the stress evaluation is carried out on the key positions. It is found that the thickness of the connecting side between the H-shaped forgings and the skirt cannot be determined only according to the skirt thickness determined in jb4710-1992 steel tower vessels, The stress analysis should be carried out according to the point of view of analysis and design, so as to fully consider the edge stress and temperature stress caused by the deformation coordination between H-shaped forgings and skirt, and then determine the reasonable thickness of the connecting side between H-shaped forgings and skirt

key words: hydrogenation reactor; Skirt seat; Analysis and design; Deformation coordination; Temperature stress

1 preface

hydrogenation reactor is a key equipment in the processing of petroleum products. H-type forgings are usually used at the lower head (Fig. 1). The upper part of H-type forgings is connected to the cylinder, and the lower part is connected to the head and skirt. The three basic thicknesses of H-type forgings are generally designed to be equal to the thicknesses of cylinder, head and skirt respectively. Although the hydrogenation reactor is designed according to the analysis and design standard, because the skirt is a non pressure component, its thickness is generally designed and calculated according to jb4710-1992 steel tower vessel [1]. However, using this thickness to determine the thickness of the connecting side of the skirt of H-shaped forgings may not be appropriate. The main reason is that the main body of the hydrogenation reactor is under high pressure, and the connection between the skirt and the main body of the hydrogenation reactor is bound to have edge stress. Moreover, within a certain pressure range of 69 metal faced polystyrene sandwich plate, this edge stress will be very large, and the existence of temperature stress makes the stress situation in the connection area more complex. Therefore, we should take the determination of skirt thickness according to jb4710-1992 as a reference, carry out a detailed stress analysis on this part, and evaluate the stress according to the analysis and design method, so as to reasonably determine the thickness of the connecting side of the skirt of H-type forgings

Figure 1 structural diagram of H-shaped forgings

the author takes an in-service gasoline and diesel Hydrofining Reactor with an annual output of 600000 tons as an example, carries out stress analysis and evaluation by using ANSYS finite element program, and determines the reasonable thickness of the skirt connecting side of H-shaped forgings

2 stress analysis of H-shaped forgings

2.1 original design conditions and dimensions of hydrogenation reactor

design conditions are design pressure p= 8.83mpa, design temperature T = 347 ℃; The material is forged steel 2.25Cr-1Mo; Allowable stress intensity at design temperature SM = 115.5mpa

the original size is the radius of the cylinder R1 = 1406.5 mm, and the wall thickness t1 = 87 mm; Inner radius of ball head R2 = 1424 mm, wall thickness T2 =52 mm; Skirt wall thickness t3= 22 mm; Transition fillet radius r= 20mm, forging height h = 568 mm. The skirt thickness meets the requirements of jb4710-1992

2.2 finite element calculation model

(1) mechanical stress calculation model

the mechanical stress calculation model is shown in Figure 2. The axisymmetric model is adopted, in which the length of the cylinder and skirt connected with the H-shaped forging is long enough, which is far greater than 2.5 times [2, 3] of the edge stress attenuation length

Figure 2 mechanical stress calculation model

because it mainly discusses the stress distribution gauge in the connection area of H-shaped forgings. 7. In case of power failure or common failure, the shutdown time law ignores the perforated nozzle of the lower head. See Fig. 2 for the load and constraint. The surface force P1 is used at the end of the cylinder to simulate the stress of the closed cylinder

(2) calculation model of mechanical stress heating stress

the calculation model of mechanical stress heating stress is shown in Figure 3. The filled part in the figure is the insulation layer, and the thickness of the insulation layer is 180mm. 8-node quadrilateral thermal element (plane55) is used for thermal stress analysis [4]

Figure 3 mechanical stress heating stress calculation model

on boundary 1 is the convection boundary between insulation layer and air, as well as skirt and air, boundary 2 is the convection boundary between fluid medium in hydrogenation reactor and vessel wall, and boundary 3 is the adiabatic boundary

the medium temperature is TF = 347 ℃, the air temperature is T3 = 20 ℃, the measured outer wall temperature of the vessel is T1 = 325 ℃, and the air convection heat transfer coefficient is α 1 = 12w/m2 · ℃, the thermal conductivity of insulation layer (microporous calcium silicate) is λ 1 = 0.134w/m · ℃, the heat conductivity of H-type forgings is λ 2= 35W/m·℃。

the convective heat transfer coefficient between the internal fluid medium and the vessel wall can be obtained by the inverse method according to the measured vessel wall temperature and medium temperature

for the steady-state heat conduction of the cylinder, the heat conduction rate through each layer is the same. The heat conduction rate equation [5] is as follows:

where Q1 - heat conduction rate, w

λ——— Heat conduction coefficient, w/m ·℃

r -- cylinder radius, m

s -- surface area of inner and outer walls of the cylinder, m2

for convective heat transfer, the convective heat transfer rate equation [5] is as follows:

q2= α S △ t (3)

where Q2 -- convective heat transfer rate, w

α——— Convective heat transfer coefficient, w/m2 ·℃

△ T -- the temperature difference between the fluid and the wall, ℃

for the steady-state heat conduction of the cylinder, the heat transfer rate through each layer is the same, so q1=q2, so the convective heat transfer coefficient of the medium is derived α= 14W/m2·℃。

2.3 calculation results and analysis

the original size of the model is used for calculation. The equivalent stress contour cloud map of the third strength theory is shown in Figures 4 and 5

Figure 4 original design mechanical stress nephogram

Figure 5 original design mechanical stress heating stress nephogram

it can be seen from the nephogram of Figure 4 and figure 5 that for the special structural form of H-shaped forgings, under the original design, the stress level outside the skirt connection area of H-shaped forgings is relatively high due to deformation coordination. It can be seen from the cloud pictures in Figure 4 and figure 5 that for the special structural form of H-shaped forgings, under the original design, the stress level outside the skirt connection area of H-shaped forgings is high due to deformation coordination

select the section and 2 - 2 (Figure 6) for stress evaluation, where section 1 - 1 corresponds to the maximum stress point under pure mechanical stress, and section 2 - 2 corresponds to the maximum stress point under mechanical stress and heating stress. The evaluation results (see Table 1) show that the stress level of the original design structure exceeds the standard and cannot meet the requirements of the analysis and design stress evaluation standard

Figure 6 schematic diagram of stress evaluation section

reasonable determination of the thickness of the connecting side of the skirt of type 3 forgings

from the above stress analysis and evaluation, it can be seen that the thickness of the connecting side of the skirt of type H forgings should not be determined directly according to the skirt thickness calculated by jb4710-1992 steel tower vessels, but should be used as a reference for detailed stress analysis and evaluation, so as to determine the reasonable thickness of the connecting side of the skirt of forgings. The author uses APDL language provided by ANSYS to carry out parametric modeling, and uses its opt module to search and optimize to determine the reasonable thickness of the connecting side of the skirt of H-type forgings

3.1 parametric modeling

the standard process of finite element analysis includes: defining the model and its load, solving and interpreting the results. If the solution results show that it is necessary to modify the design, then the geometry of the model must be changed and the above steps must be repeated, especially when the model is complex or modified more, this process may be complex and time-consuming. ANSYS parameter design language provides the function of automatically completing the above cycle by means of establishing intelligent analysis. As long as the parameter value to be modified is changed, the other parameters can be changed accordingly, so as to achieve the purpose of changing the geometry of the model

in the problems discussed in this paper, the stress level of H-shaped forgings is mainly related to the thickness T3 of the connecting side of the skirt. Taking T3 as the design variable, the reasonable thickness of the connecting side of the skirt of H-type forging can be obtained by using the opt module provided by ANSYS

3.2 calculation results

when the mechanical stress is considered only, the thickness of the connecting side of the skirt of H-type forging t3=22mm can meet the analysis and design evaluation requirements. However, when the temperature load is applied, the stress increases, and the thickness cannot meet the evaluation requirements. Therefore, it is determined that the thickness does not meet the demand explosion again. Only the mechanical and mechanical stress should be considered to calculate the stress, and the skirt stress and temperature stress should be obtained. Therefore, the relationship curve between the temperature stress thickness T3 and the maximum equivalent stress of the model is shown in Figure 7

Figure 1 Relationship between the thickness of the connecting side of the skirt and the maximum equivalent stress

it can be seen from Figure 7 that the maximum equivalent stress decreases with the increase of the support thickness. When the support thickness of the skirt is 33mm, the stress requirements can be met, so t3=33mm can be determined

compare the stress distribution curve of the outer side of the original design H-shaped forging with that of the outer side after the thickness optimization of the connecting side of the skirt of the H-shaped forging, as shown in Figure 8. The left side of the figure is the stress distribution curve under the combined action of mechanical stress and temperature stress, and the right side is the stress distribution curve under the action of mechanical stress. It can be seen from figure 8 that the stress level of the optimized model is significantly reduced

figure 8 stress distribution curve

at the same time, the stress evaluation of section 1 - 1 in Figure 6 is carried out. The optimized structure can meet the evaluation requirements. The evaluation results are shown in Table 1

4 conclusion

for hydrogenation reactor, a special H-shaped forging, because it works in the environment of high pressure and temperature, the free deformation of the cylinder and head is large. During the deformation coordination process of H-shaped forging and skirt, a large edge stress will be generated at the joint, and the existence of temperature stress will increase the stress at the joint. The thickness of the connecting side of the skirt of H-type forgings should not be determined according to the thickness calculated in jb4710-1992 steel tower vessels, but should be used as a reference for stress analysis, and the thickness of the connecting side of the skirt of H-type forgings should be reasonably determined according to the analysis and design requirements

References:

[1] jb4710-1992, steel tower vessels [S]

[2] JB4732-1995, steel pressure vessels - Analysis and design standard (First Edition) [S]

[3] he kuangguo Basis of pressure vessel analysis and design [M] Beijing: China Machine Press, 1995

[4] Wang Guoqiang Practical engineering numerical simulation technology and its practice on ANSYS [M] Xi'an: Northwestern Polytechnic University Press 1999

[5] yaoyuying Principles of chemical engineering [M] Tianjin: Tianjin University Press, 1999

about the author: Cui Jing (1978-), graduated from Hebei University of technology in 2000, majoring in chemical process machinery. He is a postgraduate student at Hebei University of technology. His research direction is pressure vessel structure optimization and CAD. His communication address is mailbox 300, teaching and Research Office of chemical process machinery, Hebei University of technology. (end)

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