COMPARISON OF DIFFERENT ANALYSIS DESIGN METHODS IN THE CALCULATION OF HYDROGENATION REACTOR SKIRT STRUCTURE. Jianhua PAN*, Xuedong CHEN, Cui JUN

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1 Proceedings of the ASME 2014 Pressure Vessels & Piping Conference PVP2014 July 20-24, 2014, Anaheim, California, USA PVP COMPARISON OF DIFFERENT ANALYSIS DESIGN METHODS IN THE CALCULATION OF HYDROGENATION REACTOR SKIRT STRUCTURE Jianhua PAN*, Xuedong CHEN, Cui JUN National Engineering Technical Research Center on PVP Safety, Hefei General Machinery Research Institute, Hefei, , PR China * address: Tel: ; FAX: ABSTRACT The skirt support structure of hydrogenation reactor works in the conditions of high temperature and high pressure. There is not only high mechanical stress but also high temperature difference stress in this zone. The service environment of the structure is in poor conditions. In this paper, temperature field analysis on the local structure of skirt hot box is carried out. The stress classification method and direct route method are used to calculate the skirt support structure of hydrogenation reactor. The calculation results are compared. Results of these two analysis and design methods are both meet with the requirements of relevant standards. The results obtained by two methods are discussed. Some meaningful conclusions are obtained. 1. INTRODUCTION Hydrogenation Reactor is the core equipment in processing of petroleum products, working in high temperature and high pressure environment. In such severe operation condition, there are not only mechanical stress but also a certain high thermal stress existing. Skirt support zone of hydrogenation reactor is one of key parts of the equipment. H- type forging is usually used at the lower head. Upper part of h- type forging connected with the cylinder and the lower part of h-type forging connected with lower head and skirt. It usually exists high temperature gradient in h-type forging, so the stress situation is complex and severe. It needs higher reliability requirements. So it is necessary to conduct a detailed analysis of temperature field and thermal stress about the structure of skirt support part. Currently, in China pressure vessel analysis and design methods are mostly based on stress classification method, which is an engineering approximation. It uses elastic analysis instead of plastic analysis. This method is simple, mature and low computational cost. But It exists a certain difficulties in correct classification. The direct route method is based on elastic-plastic analysis of pressure vessel structure. Different failure modes appear under different loads for pressure vessels, different failure criteria are used. Unlike stress classification method, this method effectively avoids difficulties of stress classification. global plastic deformation and progressive plastic deformation are used as failure determination conditions. It is more truly reflect structural damage limit state. Two analysis design methods are used to calculate the skirt support structure of hydrogenation reactor. First, linear elastic method is used to calculate the hydrogenation reactor support skirt structure. Stress classification and assessment is carried out with stress classification in Chinese standard JB/T The direct route method in standard EN Appendix B is used according to global plastic deformation failure mode of skirt support structure. The two calculation results are compared and discussed. Some meaningful conclusions are obtained. 2. DESIGN PARAMETERS The relative design parameters of hydrogenation reactor in this paper are shown in table 1. The material parameters used for two methods are shown in table2 and table3. Table 1 Design parameters material 2.25Cr-1Mo- 0.25V design pressure operation pressure Design Temperature operation Temperature 21.7 MPa 19 MPa 454/ Copyright 2014 by ASME

2 Temp Young s modulus (10 3 MPa ) Table 2 Meterial parameters Yield Design strengt stress h intensity Poisson (MPa) (MPa) s ratio Partial safety factor s Tresca Yield strengt h design value (MPa) Table 3 Linear expansion coefficient(10-6 mm/mm ) material between lower head and shirt reducing temperature gradient compared with traditional design, it is still existing relatively high temperature gradient in adjacent area between h- type forging and skirt. Fig.1 Temperature analysis model In this paper, indirect method is used to calculate structure stress. Firstly, structure thermal analysis is used to obtain the temperature field distribution. Then, the thermal element is converted to structure element, stress analysis is carried out by loading temperature filed obtained above as body load in structure model. Due to long time between start and stop of hydrogenation reactor, the problem can be treated as a steady-state temperature field analysis. The analysis area involves following heat transfer problems: convective heat transfer between internal medium and wall, convective heat transfer between insulation layer and external air, heat conduction between vessel wall metal and insulation layer. The film coefficient between medium and wall is 580 W/m 2 K, the air temperature inside skirt is 50, the film coefficient between air and insulation layer is 5 W/m 2 K, the film coefficient between air inside skirt and skirt wall is 5 W/m 2 K, the film coefficient between outside air and out wall of skirt is 15 W/m 2 K, the film coefficient between outside air and out wall of insulation is 15 W/m 2 K, the conduction coefficient of aluminum silicate insulation layer is W/m K. 3. TEMPERATURE FIELD CALCULATION Finite element model used in temperature analysis is shown in fig.1. There are three kinds of heat transfer boundaries, viz insulation and air, skirt and air, inside medium and vessel wall. PLANE 77 is adopted as heat analysis element. There are 941 nodes and 3128 elements in all. The result of temperature field calculation is shown in Fig.2. Although there is a hot box where is no insulation Fig.2 Results of steady-state temperature field 4. CLCULATION RESULTS OF TWO DIFFERENT DESIGN METHODS 4.1 STRESS CLASSIFICAITON METHOD Heat elements PLANE 77 are converted to structure elements PLANE183 once temperature field distribution is obtained. After defining parameters used in structure analysis and heat transfer analysis and boundary conditions required in structure analysis, thermal stress calculation can be carried out. Finite element thermal stress calculation result based on elastic linear material constitutive is shown in Fig.3. Result of finite element analysis without temperature difference load is shown in Fig.4. 2 Copyright 2014 by ASME

3 Fig.3 Calculation result based on elastic linear material Fig.4 Result without temperature difference Fig.5 Stress assessment paths In principle, the mechanical loads and thermal loads should be calculated separately. Principle stresses are extracted according to mechanical load calculation results. Secondary stresses are extracted according to thermal load calculation results. The thermal load calculation results are overlaid with the former, then they are classified. The disadvantage is that the thermal stress results extracting from FEA software can not be used directly. Only stress components along paths could be used to superimposed. A more easy approach to deal with is extracting stresses from only mechanical load case and mechanical load thermal load case respectively. Principle stresses are extracted from former one, such as P L, P b, PL Pb. Secondary stresses are extracted from the latter one, such as PL Pb Q. In this paper, two paths are selected for stress classification and assessment. Paths are shown in Fig.5. Stress classification results are in table4. The assessment results meet with Chinese standard JB/T Steel Pressure Vessel- Analysis and Design Standard (2005 confirmation). Table 4 Stress classification and assessment P L,MPa PL Pb Q,MPa Path (mechanical load (mechanical and case) thermal load case) PATH <1.5S m = <3S m =678.0 PATH <1.5S m = <3S m =678.0 Note: S m =226MPa 4.2 THE DIRECT ROUTE METHOD According to the load case of hydrogenation reactor, the failure mode of hydrogenation reactor skirt support zone structure is global plastic deformation. Direct route method load case is established based on this failure mode. Elastic perfectly plastic material constitutive relation are used in direct method, material parameters are shown in table2. Calculation result of temperature field is applied as temperature load to the finite element modal. When the internal pressure load, the initial state is stress-free. Then the pressure is proportional loaded by the same percentage from zero to maximum. In EN standard, the maximum absolute strain value of pressure vessel and its main structure should not be more than 5% in normal operation conditions for global plastic deformation design verification. Stress calculation results of FEA model contours is shown in Fig.6. The absolute value of total strain contours of FEA model is shown in Fig.7. The change process of maximum equivalent strain absolute value and the increasing load is shown in Fig.8. In the direct route method analysis of skirt support structure, when the normal load design values is applied to FEA model, the absolute maximum strain is 2.56% in the end of load path, which is less than the allowable value. The calculation results meet with the requirements in EN standard. The results can be consistent with global plastic 3 Copyright 2014 by ASME

4 deformation design checking requirements in normal operating conditions. Fig.6 Stress assessment paths Fig.7 Strain distribution contours Fig.8 Strain and load rate relationship 5. ANALYSIS AND DISCUSSION Both of the results got from two methods above could meet with relevant standards requirements. It is shown that both of these two methods can give correct evaluation results. The calculation results of stress classification method is nominal elastic stress. In fact, when the stress exceeds actual yield stress, local structure will change and stress redistribution will occurred. Therefore, the result of direct route method is more close to the actual situation. Although there are some typical cases of stress classification list in JB/T standard to avoid designers asunder classification, the situation encountered in actual structure design is changing. It is difficult to classify each stress uniquely, for example, primary bending stress is hard to distinguish from secondary stress. In these cases, the stress classification method may produce ambiguous results. In direct route method, limit condition of global plastic deformation under normal operation cases is that the absolute maximum strain should not more than 5%. Elastic perfect plastic material constitutive relations without strain hardening effect are used. This treatment methods make the process of analysis and calculation more easy. Direct route method resolve the problems that it is difficult to classify stress happened in stress classification method. In addition, direct route method can prevent structure failure from large deformation with the strain limit condition. Direct route method has advantages form these aspects. Although the result of direct route method is closer to actual situation than the results obtained from stress classification method, the solution process is very complex. It is afraid not suitable for wide range engineering design. It is used in the complex structure and load conditions or the analysis design which is in apparent dispution. Direct route method only specifies the material constitutive relation, yield condition and eligibility requirements. Therefore, direct route method is more safety and economy, but cost is the analysis more timeconsuming. And requirements for analysts will also be greatly improved. It needs the perpetrators have enough experience and supporting manufacture, testing and other requirements. Nevertheless, the direct route method is advanced unquestioned in technical. The benefits from safety and reliability of some important pressure vessels is higher than the consideration to be paid for the complexity of direct route method. 6. CONCLUSION The hydrogenation reactor skirt support structure calculation results from the two design methods show that they can reach right conclusions. Stress classification method is an engineering approximation. The analysis is simple, mature, low computation cost. But there are some difficulties in correct stress classification. The direct route method avoids the problems caused by stress classification, the results are more reasonable. But the analysis method is more complex and high 4 Copyright 2014 by ASME

5 requirements for designers. It is suitable for complex and important pressure equipments. The direct route method is more advanced after a reasonable comparison. It represents the future trends of pressure vessel analysis and design methods. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support by Projects of National Science and Technology Plan (No. 2012CB626813). REFERENCES [1] JB (2005 confirmed edition), Steel Pressure Vessels-Analysis of design standard[s]. [2] Ding Bomin. Review and analysis of development of design by analysis method[j]. Journal of Pressure Vessel, 2008, (9). [3] Ding Bomin. An introduction and analysis of new ASME Ⅷ-2(2007) alternative rules for construction of pressure vessels[j]. Journal of Pressure Vessel,2008,(25). [4] Thomas P. Pastor, Overview of ASME 2007 Section Ⅷ, Division 2, ASME Technical Presentation, Beijing, May 28-29,2007,Shanghai, May 30-31,2007. [5] Companion Guide to the ASME Boiler & Pressure Vessel Code [6] 2007 ASME Boiler& Pressure Vessel code Ⅷ Division 2, Alternative Rules-Rules For construction of Pressure Vessel[s]. [7] Ding BM. Analysis and Application of ASME Pressure Vessel Code[M]. 2009: Copyright 2014 by ASME