Determination of the thickness of the connection side between H-shaped forging and skirt of hydrogenation reactor

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author: Cui Jing, Gao Bingjun, Zhang Jirui, Wang Junbao, summary: The finite element method was used to analyze the mechanical stress and temperature stress of the connection area between the H-shaped forging and the cylinder, head and skirt of the hydrogenation reactor, and the stress assessment was carried out on key positions. It was found that the thickness of the connection side between the H-shaped forging and the skirt cannot be determined solely according to JB4710-199 2. The thickness of the skirt determined in "Steel Tower Vessel" should be determined from the perspective of analytical design to fully consider the edge stress and temperature stress caused by the deformation coordination between the h-shaped forging and the skirt, and then determine the reasonable thickness of the connection side between the h-shaped forging and the skirt. keywords: Hydrogenation reactor ； Skirt ； analysis design ； Deformation coordination ； Temperature stress 1 Introduction Hydrogenation reactor is a key equipment in petroleum product processing, and H-shaped forgings are usually used at the lower head (Fig. 1). The upper part of the H-shaped forging is connected to the cylinder, and the lower part is connected to the head and skirt. The three basic thicknesses of H-shaped forgings are generally designed to be equal to the thickness of the barrel, head and skirt respectively. Although the hydrogenation reactor is designed according to analytical design standards, since the skirt is a non-pressure component, its thickness is generally still designed and calculated based on JB4710-1992 "Steel Tower Vessel" [1]. However, it may not be appropriate to use this thickness to determine the thickness of the h-shaped forging skirt connection side. The main reason is that the pressure endured by the main body of the hydrogenation reactor is relatively high. There is bound to be edge stress in the connection between the skirt and the main body of the hydrogenation reactor. Moreover, within a certain pressure range, this edge stress will be very large. At the same time, the existence of temperature stress makes the stress situation in the connection area more complicated. Therefore, the thickness of the skirt seat determined according to JB4710-1992 should be used as a reference to conduct a detailed stress analysis on this part, and conduct stress evaluation according to the analysis and design method, so as to reasonably determine the thickness of the h-shaped forging skirt seat connection side. Figure 1 Schematic diagram of h-shaped forging structure. Taking an in-use gasoline and diesel hydrotreating reactor with an annual output of 600,000 tons as an example, the author used the ANSYS finite element program to conduct stress analysis and evaluation, and determined the reasonable thickness of the h-shaped forging skirt connection side. 2 h type forging stress analysis 2.1 The original design conditions and size design conditions of the hydrogenation reactor are design pressure P = 8.83MPa and design temperature T = 347℃ ； The material is forged steel 2.25Cr-1Mo ； The allowable stress intensity Sm = 115.5MPa at the design temperature. The original dimensions are the inner radius of the cylinder R1 = 1406.5 mm, and the wall thickness t1 = 87 mm ； The inner radius of the ball head R2 = 1424 mm, the wall thickness t2 =52 mm ； Skirt wall thickness t3= 22 mm ； Transition fillet radius r = 20mm, forging height H = 568 mm. The thickness of the skirt seat 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, using an axial symmetry model, in which the length of the barrel and skirt connected to the H-shaped forging is long enough, much greater than 2.5 times the edge stress attenuation length [2, 3]. Figure 2: The mechanical stress calculation model mainly discusses the stress distribution law of the connection area of ​​the H-shaped forging, and ignores the opening pipe of the lower head. The loads and constraints are shown in Figure 2, in which the end of the cylinder simulates the stress of the closed cylinder with surface force P1. (2) Mechanical stress heating stress calculation model The mechanical stress heating stress calculation model is shown in Figure 3. The filled part in the figure is the insulation layer, and the thickness of the insulation layer is 180mm. An 8-node quadrilateral thermal unit (plane55) is used for thermal stress analysis [4]. Figure 3 Mechanical stress heating stress calculation model. Boundary 1 is the convection boundary between the insulation layer and the air, and the skirt and the air. Boundary 2 is the convection boundary between the fluid medium inside the hydrogenation reactor and the container wall. Boundary 3 is the adiabatic boundary. The medium temperature is Tf = 347°C, the air temperature is T3 = 20°C, the measured outer wall temperature of the container is T1 = 325°C, the air convection heat transfer coefficient is α1 = 12W/m2·°C, the thermal conductivity coefficient of the insulation layer (microporous calcium silicate) is λ1 = 0.134W/m·°C, and the thermal conductivity coefficient of the h-shaped forging is λ2 = 35W/m·°C. The convective heat transfer coefficient between the internal fluid medium and the container wall can be obtained by the inverse method based on the actual measured container wall temperature and medium temperature. For cylindrical stationary heat conduction, the heat transfer rate through each layer is the same. The heat conduction rate equation [5] is as follows: In the formula, Q1——heat conduction rate, W λ——heat transfer coefficient, W/m·℃ r——cylinder radius, m S——cylinder inner and outer wall surface area, m2 For convective heat transfer, the convective heat transfer rate equation [5] is as follows: Q2 = αS△t (3) In the formula, Q2——convective heat transfer rate, W α——convective heat transfer coefficient, W/m2·℃ △t——temperature difference between the fluid and the wall surface, °C. For cylindrical steady-state heat conduction, the heat transfer rate through each layer is the same, so Q1=Q2, from which it can be deduced that the convective heat transfer coefficient of the medium α=14W/m2·℃. 2.3 Calculation results and analysis The original size of the model is used for calculation. The equivalent stress contour cloud diagram of the third strength theory is shown in Figures 4 and 5. Figure 4 Original design mechanical stress cloud diagram Figure 5 Original design mechanical stress heating stress cloud diagram It can be seen from the cloud diagrams in Figure 4 and Figure 5 that for the special structural form of h-shaped forgings, under the original design, the outside of the skirt connection area of ​​h-shaped forgings has a higher level of deformation coordination stress. It can be seen from the cloud images in Figures 4 and 5 that for the special structural form of h-shaped forgings, under the original design, the outside of the skirt connection area of ​​h-shaped forgings has a higher level of deformation coordination stress. Select sections 1-1 and 2-2 (Figure 6) for stress evaluation. Section 1-1 corresponds to the maximum stress point when pure mechanical stress occurs, and section 2-2 corresponds to the maximum stress point when mechanical stress and heating stress occur. The assessment results (see Table 1) indicate that the stress level of the original design structure exceeds the standard and cannot meet the requirements of the analytical design stress assessment standards. Figure 6 Schematic diagram of stress assessment section 3. Reasonable determination of the thickness of the skirt connection side of the type forging. From the above stress analysis and evaluation, it can be seen that the thickness of the skirt connection side of the h-type forging should not be determined directly based on the skirt thickness calculated by JB4710-1992 "Steel Tower Vessel". Instead, it should be used as a reference to conduct detailed stress analysis and evaluation, and then determine the reasonable thickness of the skirt connection side of the forging. The author uses the APDL language provided by ANSYS to perform parametric modeling, and uses its OPT module to search and optimize to determine the reasonable thickness of the connection side of the h-shaped forging skirt. 3.1 Parametric modeling The standard process for finite element analysis includes: Define the model and its loads, solve and interpret the results. If the solution results indicate 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 more complex or has many modifications, this process can be complicated and time-consuming. The ANSYS parametric design language provides the function of automatically completing the above cycle by establishing intelligent analysis. As long as the parameter value to be modified is changed, the other parameters can be changed accordingly, thereby achieving the purpose of changing the model geometry. In the problem discussed in this article, the stress level of the h-shaped forging is mainly related to the thickness t3 of the skirt connection side. Taking t3 as the design variable and using the OPT module provided by ANSYS for search and optimization, a reasonable thickness of the connection side of the h-shaped forging skirt can be obtained. 3.2 Calculation results When mechanical stress is simply considered, the thickness of the h-shaped forging skirt connection side t3 = 22mm can meet the analysis and design assessment requirements. However, when temperature load is applied, the stress increases and the thickness cannot meet the assessment requirements. To determine the thickness, not only the mechanical and mechanical stress must be considered, but the stress calculation must be performed to obtain the skirt stress, and the temperature stress is also required. 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 The relationship between the thickness of the skirt connection side and the maximum equivalent stress. It can be seen from Figure 7 that the maximum equivalent stress decreases as the support thickness increases. When the skirt support thickness is 33mm, the stress requirements can be met, so it can be determined that t3=33mm. Compare the stress distribution curves on the outside of the original designed h-shaped forging and the thickness of the h-shaped forging skirt connection side after optimization, 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. As can be seen from Figure 8, the stress level of the optimized model is significantly reduced. The stress distribution curve in Figure 8 also carries out stress assessment on sections 1-1 and 2-2 in Figure 6. The optimized structure can meet the assessment requirements. The assessment results are shown in Table 1. 4 Conclusion For the special h-shaped forging of the hydrogenation reactor, because it works in an environment with high pressure and temperature, the cylinder and the head are free to deform greatly. The deformation coordination process between the h-shaped forging and the skirt will produce large edge stress at the connection. At the same time, the existence of temperature stress increases the stress at the connection. The thickness of the h-shaped forging skirt connection side should not be determined based on the thickness calculated in JB4710-1992 "Steel Tower Vessel", but should be used as a reference to conduct stress analysis, and the thickness of the h-shaped forging skirt connection side 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 Standards (First Edition) [S]. [3] He Kuangguo. Fundamentals of Pressure Vessel Analysis and Design [M] Beijing: Mechanical Industry Press, 1995 [4] Wang Guoqiang. Practical engineering numerical simulation technology and its practice on ANSYS [M] Xi'an: Northwestern Polytechnical University Press 1999 [5] Yao Yuying. Principles of Chemical Engineering [M] Tianjin: Tianjin University Press 1999 About the author: Cui Jing (1978- ), graduated from Hebei University of Technology in 2000 with a major in chemical process machinery. She is a master's student at Hebei University of Technology. Her research direction is pressure vessel structural optimization and CAD. Correspondence address: Box 300, Teaching and Research Office of Chemical Process Machinery, Hebei University of Technology. (end)