Proceedings of the ASME 2014 Pressure Vessels & Piping Conference PVP2014 July 20-24, 2014, Anaheim, California, USA PVP2014-28958 Writing and Reviewing FEA Reports Supporting ASME Section VIII, Division 1 and 2 Designs - Practical Considerations and Recommended Good Practice Trevor Seipp Becht Engineering Canada Ltd. 110-259 Midpark Way, S.E. Calgary, AB CANADA Tel: 403-668-7274 Fax: 403-256-3520 Email: tseipp@becht.com Mark Stonehouse Becht Engineering Canada Ltd. 110-259 Midpark Way, S.E. Calgary, AB CANADA Tel: 403-668-8675 Fax: 403-256-3520 Email: mstonehouse@becht.com ABSTRACT Finite element analysis (FEA) is used, with increasing frequency, to supplement or justify the design of an ASME Section VIII, Division 1 or 2 pressure vessel. When this occurs, good engineering practice indicates that a competent engineer should review the finite element analysis report. In some jurisdictions, it is required that a Professional Engineer review and certify the report. This paper discusses some of the practical aspects of both writing and reviewing a good quality FEA report - both in the context of the technical perspective and in the context of Code compliance. This paper will serve as a practical assistant to an engineer reviewing an FEA report, as well as a guide to an engineer preparing an FEA report. Aspects such as properly following Code requirements, following appropriate Design By Analysis methodologies, and applying good design practices will be discussed. INTRODUCTION FEA can be used to support pressure equipment design where the configuration is not covered by the applicable rules in the ASME code. Whether the pressure vessel design is considered an ASME Section VIII Division 1 [1] or a Division 2 [2] vessel, all finite element analysis for pressure vessels should follow the methodology in ASME Section VIII, Division 2, Part 5 (it is recommended to use the latest Edition, as improvements are continually being made) unless that type of analysis is not covered in Part 5. If specific rules exist in ASME Section VIII, Division 2 for the type of analysis being performed, then those rules must be followed. The FEA is not complete without a detailed report describing the work performed, details of the loads, boundary conditions, mesh discretization, material properties, and compliance with all Part 5 requirements. Since ASME Section VIII, Division 2, Part 5 approaches the design-by-analysis requirements from a protection against failure modes perspective, it is imperative that the analyst determine all reasonably possible failure modes, create a modeling approach that will be appropriate for each failure mode, and ensure that an appropriate margin against failure is maintained. FEA REPORT The goal of the report writer should be to clearly describe the work performed with sufficient detail that another analyst could, with applicable referenced drawings and documentation, independently duplicate the analysis. A quality FEA report consists of the following detailed sections: Executive Summary This should be standard practice and should briefly describe how the FEA was used to support the design, the FEA model, the results and conclusions. This section should be no more than one page, and should be written primarily for the non-technical reader. 1 Copyright 2014 by ASME
Introduction A good introduction should lay out the scope of the FEA, justification for using FEA, the software used, the type of analysis (linear elastic, elastic-plastic, etc.), and the material properties. For example, if the report is in support of a Division 1 vessel, then reference should be made to article U-2(g), with an explanation of how the as safe as provision is accomplished. A complete description of the material properties should be provided. Simply referring to the material specification is not sufficient. The authors recommend that a mixture of tabular data as well as graphical data be used to efficiently provide this data especially if material properties as a function of temperature are used. References All drawings, calculations, and other supplemental information referenced in the report should be detailed in the References section. Since significant changes can occur in drawings after the FEA report is prepared, it is highly recommended that both the Revision Numbers and the dates of all reference material be included in the list. The Edition of the Code of Construction should also be listed, as should the edition of ASME Section VIII, Division 2. Since the Edition of the Code of Construction is dictated by the fabrication contract, it may be not current by several years. Despite that, the authors highly recommend that the current Edition of Division 2 be used at all times. Model Description Dimensional descriptions should be provided from the referenced drawings. The authors experience as reviewers has shown that it is useful to provide some basic dimensional details if for no other reason than to provide the reviewer something to verify from the drawings. Some critical dimensions should also be explicitly described in the report. If there are any geometric simplifications used, those also need to be discussed and justified. The type (2D, 3D, axisymmetric, etc.) and the order of the elements should also be reported. If different types of elements are used, then a description of how the different elements are connected together should be provided. There should be a significant description of the mesh, particularly the mesh size. Mesh features such as the number of elements on fillet radii should also be reported. The accuracy of the model, by way of the discretization, should be indicated. A convergence study is expected although it is not explicitly required for every FEA. Boundary conditions, loads and other similar interactions must also be detailed. It is most helpful when the loads, supports, and restraints are described and shown graphically. The method of restraining the model against rigid-body motion should be described and justified. When symmetry is used, its use must be justified, such that the rationale for the partial model is described with an explanation of the boundary conditions used to compensate for the missing model sections. It is recommended that symmetry models NOT be used when performing buckling calculations. When contact is included in the analysis, such parameters as the normal and tangential conditions should also be presented and justified. Material Properties A complete description of the material properties used in the analysis should be provided. The source for these material properties should also be provided. When available, material properties should be obtained from ASME Section II, Part D [3]. When the material properties vary with temperature, the variation of the material properties should be presented either in tabular form or in a graph that demonstrates that variation. All physical properties, such as: Young s Modulus, the coefficient of thermal expansion, thermal conductivity, thermal diffusivity, specific heat capacity, density and Poisson s ration should all be reported. Strength parameters such as the allowable stress, and minimum specified yield and ultimate strengths should also be reported as appropriate. If the analysis is not linear-elastic, then the appropriate monotonic or cyclic elastic-plastic true stress-true strain (either with hardening or perfectly-plastic) should be detailed as functions of temperature. Presentation of Results Depending on the type of analysis being performed the results that need to be presented may vary. As part of generally accepted good practice, the results should show enough details to demonstrate an appropriate analysis of the failure modes being checked. The results in most analyses will include displacements, deformed shape with un-deformed shape superimposed, stress plot with an appropriate color contour to establish some sense of the magnitude of the stress profile of the model with respect to an allowable stress. When plotting multiple stress contour plots for comparison, it is most useful when the contour scheme/intervals are identical. Analysis of Results and Conclusion This section of the report should follow the approach in ASME Section VIII, Division 2, Part 5. Each Failure Mode must be demonstrated in the results, however, the presentation of the results should be as simple and clear as possible. Hundreds of pages of stress plots need not be presented. On the contrary, industry best practice is to be as concise as possible with 2 Copyright 2014 by ASME
the results. The report should only present those results specifically relevant to the conclusions. In general, all of the above requirements should be presented in paragraph-form. All figures should be referenced in the text of the report if they cannot be referenced, perhaps they are not important/relevant. Based on the authors experience, even a long and complicated FEA report should not be longer than 45 pages. Conversely, even a simple report will likely be 12 pages long, just to satisfy the requirements listed above and below. FAILURE MODES From a technical perspective, the analyst must evaluate the design for all failure mechanisms in 5.2, 5.3, 5.4, and 5.5 of Part 5. The analyst is required to create a modeling approach that will be appropriate for each failure mechanism, and ensure that an appropriate margin against failure is maintained. The appropriate margins for each failure mode depend on the Code of Construction for the component being evaluated. Part 5 of ASME Section VIII, Division 2 approaches the design-byanalysis requirements from this protection against failure modes perspective. The 2013 Edition provides rules for protection against four critical failure modes. For each failure mode, the analyst is required to present a modeling approach and discuss how it is appropriate. Protection Against Plastic Collapse Paragraph 5.2 discusses the evaluation methods available for protection against plastic collapse. Three alternative analysis methods are presented: Elastic stress analysis method Limit-load method Elastic-plastic stress analysis method the rationale for the selection. Special attention shall be paid to the cautions in Paragraphs 5.2.1.2, 5.2.1.3, and 5.2.1.4. All loading scenarios referenced by Paragraph 5.2 shall be evaluated. Each step in 5.2.2.4, 5.2.3.5, or 5.2.4.2 (Assessment Procedure) shall be detailed. Post-processing of results for the elastic stress analysis method shall be performed in strict accordance to the rules of Paragraph 5.2.2.3 and Annex 5.A. Stress classification lines shall be described in the body of the report and shown graphically. If the built-in stress linearization methods of the finite element analysis software are used, compliance with the linearization methods in Annex 5.A must be demonstrated. (Note that the default built-in linearization in some software is NOT compliant with 5.A.4.1.2, and modifications to the default setting must be made.) Reporting of the results of the limit-load analysis and the elastic-plastic analysis need only consist of a confirmation that a converged solution has been obtained with the required design margins. Where appropriate, a plot of the extent of the plastic region should be provided. Protection Against Local Failure Paragraph 5.3 discusses the evaluation methods available for protection against local failure. Two alternative analysis methods are presented: Elastic analysis method Elastic-plastic analysis method the rationale for the selection. If the elastic-plastic analysis method is used, each step of 5.3.3.1 shall be detailed. As shown by Seipp [5], extra care must be taken in this failure mode with respect to mesh discretization; the same discretization used for Protection Against Plastic Collapse may be inadequate for Protection Against Local Failure. Protection Against Buckling Failure This check only needs to be performed when there exists compressive stresses due to the design loads. If this check does not need to be performed, a single statement to that effect needs to be included in the report. Paragraph 5.4 discusses the evaluations methods available for protection against buckling failure. Note that each evaluation method has a particular design factor. Three alternative methods are presented: Bifurcation buckling analysis performed using an elastic stress analysis without geometric nonlinearities to determine the pre-stress in the component. Bifurcation buckling analysis performed using an elastic-plastic analysis with the effects of nonlinear geometry to determine the pre-stress in the component. Non-linear buckling analysis performed in accordance with the elastic-plastic stress analysis method, and explicitly considering imperfections. the rationale for the selection. The analyst shall demonstrate that simplification of the model does not result in exclusion of critical buckling mode shapes. The analyst shall also describe how the modeling approach will account for axisymmetric as well as non-axisymmetric buckling modes. 3 Copyright 2014 by ASME
Protection Against Failure From Cyclic Loading Paragraph 5.5 discusses the evaluation methods that must be performed, and provides alternatives for fatigue analysis and protection against ratcheting. If the component under evaluation has been screened using the screening criteria in paragraph 5.5.2, then the analyst need only report that the screening criteria has been satisfied. Otherwise, a fatigue assessment in accordance with 5.5.3 must be performed. Three alternative methods are provided for fatigue assessments: Elastic stress analysis and equivalent stresses Elastic-plastic stress analysis and equivalent strains Analysis of welds elastic stress analysis and structural stress the rationale for the selection. Each step in 5.5.3.2, 5.5.4.2, or 5.5.5.2 shall be detailed. Where fatigue strength reduction factors (FSRFs) are required in the elastic stress analysis method, the rationale for the choice of FSRF shall be detailed, including appropriate references to published recommendations. Regardless of whether or not a fatigue assessment is required, all components are required to complete the evaluation for protection against ratcheting. Two alternative methods are provided for protection against ratcheting: Elastic stress analysis Elastic-plastic stress analysis the rationale for the selection, and the results of the assessment. Protection Against Additional Failure Modes The analyst must also consider all other additional failures modes (such as those that may be described in a UDS). Although no rules for these types of analyses exist in Part 5, the analyst must ensure that an appropriate margin against failure is maintained. All methods must be fully described, and the rationale for each method must be justified. SPECIAL CONSIDERATIONS FOR DIVISION 1 An FEA cannot be used to supersede existing rules in ASME Section VIII, Division 1. However, where no Code rules exist for a given situation, an FEA may be required. When performing an FEA on a Division 1 pressure vessel, or a component of a Division 1 vessel, the analyst is applying Article U-2(g) of Division 1 which states: This Division of Section VIII does not contain the rules to cover all details of design and construction. Where complete details are not given, is intended that the Manufacturer, subject to the acceptance of the Inspector, shall provide details of design and construction which will be as safe as those provided by the rules of this Division There exist no other rules currently in ASME Section VIII Division 1 to provide guidance for the performance of FEA. However, there are several considerations that are specific to the application of Division 2, Part 5 for a Division 1 vessel that constitute good industry practice. When an elastic analysis is performed, the allowable stress for all product forms except bolting need to be from Section II, Part D, Table 1 and Table 1A (i.e. the allowable stress for Section VIII Division 1 construction). Bolting material allowable stresses should be determined from Section II, Part D, Table 3. Limiting values that are not calculated using the allowable stress, S, such as fatigue and compressive allowable stresses, can be determined from the current rules in Section VIII Division 2. The Design By Analysis Rules in ASME Section VIII, Division 2, Part 5, as described above, should be followed. This is only possible for temperatures not in the creep regime. All of the load case combinations of the applicable Division 2 assessment procedure need to be considered in addition to any other combinations defined by the User. In evaluating load cases involving the pressure term, P, the effects of the pressure being equal to zero need to be considered. All of failure mechanisms in 5.2, 5.3, 5.4, and 5.5 of Part 5 need to be evaluated. In 5.3, a component is exempt from the Local Failure Criteria evaluation if the component design is in accordance with the standard details of Part 4. That said, there are many details permitted in Division 1 that are not permitted in Division 2. If the component being evaluated is not covered by a standard detail from Part 4, then an evaluation per 5.3 is required. When elastic-plastic analysis is performed, the required load case combinations from ASME FFS-1/API- 579 [4] Table B1.4 Note 6 need to be used, applying an RSFa=1. Evaluation of the test condition per paragraph 4.1.6.2 of Section VIII, Division 2 is not mandatory, but consideration of the test condition per UG-22(j) of Section VIII, Division 1 is mandatory. 4 Copyright 2014 by ASME
REVIEWING AN FEA REPORT The recommendations provided above in this paper represent the authors opinions on what constitutes good practice when it comes to preparing an FEA report. Based on these guidelines, the foregoing also forms a type of checklist for reviewing and/or certifying any reports. The absence of any of these essential items represents a deficiency that should be discussed with the author of the FEA report. CONCLUSION The use of FEA for a design implies that the design is different from that typically undertaken in a Design-By- Rules context. Therefore, engineers should hold themselves to a high standard for such Design-By- Analysis approaches. Performing an FEA, writing a report, and reviewing that report should adhere to the highest reasonable standard. The guidelines presented in this paper should be considered as the minimum of such high standards. An FEA, whether performed on an ASME Section VIII Division 1 or 2 vessel, should follow the rules of ASME Section VIII Division 2 Part 5. A quality, complete FEA report will include sufficiently detailed sections including an Executive Summary, Introduction, References, Model Geometry, Results, Analysis of Results and Conclusion. The report must demonstrate evaluation against all applicable failure modes as defined in Part 5. These guidelines are a good guide for both engineers preparing an FEA report, as well as engineers who are reviewing and/or certifying an FEA report. REFERENCES 1. ASME, 2013, ASME Boiler and Pressure Vessel Code, Section VIII Division 1, American Society of 2. ASME, 2013, ASME Boiler and Pressure Vessel Code, Section VIII Division 2, American Society of 3. ASME, 2013, ASME Boiler and Pressure Vessel Code, Section II Part D, American Society of 4. API 579-1/ASME FFS-1, Fitness For Service, American Petroleum Institute/American Society of Mechanical Engineers, 2007 5. Seipp, Trevor G., An Evaluation of The Protection Against Local Failure in ASME Section VIII, Division 2: Finite Element Model Considerations, ASME PVP2013-98028, ASME Pressure Vessels and Piping Conference, July 2013. 5 Copyright 2014 by ASME