Basic Requirements for Performing ASME B&PV Design by Analysis
The ASME Boiler & Pressure Vessel Code permits the use of numerical methods, such as finite element analysis (FEA), in the design of pressure vessels. But who is permitted to perform these analyses and what are the basic requirements imposed by the Code?
Who is Permitted to Perform Numerical Analyses?
The ASME Code (and common sense) recognizes that correctly performing numerical analyses, such as finite element analysis (FEA), requires a certain level of education, training, and experience. These requirements are provided in Section VIII, Division 1, Appendix 47, and in Section VIII, Division 2, Annex 2-J.
In general, engineers and designers performing numerical analyses must have at least four years of pressure vessel design experience (assuming a degreed engineer is doing the work) in addition to having received instruction in the use and understanding of numerical analysis computer programs. Other requirements include fatigue assessments must be performed under the direction of a Certifying Engineer (e.g., a registered Professional Engineer).
For information on the required qualifications of a Certifying Engineer and an overview of Appendix 47 see here.
Requirements for Performing Numerical Analyses
The general steps undertaken in performing a numerical analysis such as FEA are:
Build a model of the component
Define material properties
Apply loading conditions
Perform the analysis
Evaluation of results
Validation of results
The ASME Code (specifically Part 5.1.2 of Section VIII, Division 2) does not provide recommendations on the modeling of pressure vessel components, type of analysis method (e.g., elastic vs. elastic-plastic), and validation of results. It does, however, have requirements pertaining to material properties, loading conditions, and which failure modes are to be investigated.
Material Properties
The ASME Code specifies the material properties used in an analysis shall be determined using the data in Section VIII, Division 2, Part 3. This part of Division 2 provides data for physical properties (e.g. Young’s Modulus), allowable stress limits, stress-strain curves, etc. Section VIII, Division 2, Part 3 typically directs the engineer and designer to Section II, Part D (Material Properties) of the ASME Code for data not explicitly provided in Part 3.
Loading Conditions
The Design by Analysis requirements of Section VIII, Division 2, Part 5 lists the various types of loads to be considered. These include internal/external pressure, hydrostatic pressure, deadweight, live loads, thermal loads, wind, seismic, and snow loads. Much like Section VIII, Division 1, UG-22, which pertains to loading, the list is not meant to be all inclusive. If the vessel is subjected to a load not included in the list it must still be considered.
Load case combinations, including load design factors (scaling factors), are specified. These load case combinations and the corresponding load design factors are based on those provided in ASCE/SEI 7 (American Society of Civil Engineers “Minimum Design Loads and Associated Criteria for Buildings and Other Structures”).
The Design by Analysis evaluation methods of the ASME Code make a distinction between design, operating, and test loading conditions. Load case combinations and load design factors vary depending on the type of load (design, operating, or test).
In addition to static and quasi-static loads, loads that vary with time, and the number of cycles associated with time varying loads, are required to be considered.
Lastly, the type of failure mode being investigated and the acceptance criteria will vary depending on the type of load (design, operating, cyclic, etc.).
Failure Modes to be Evaluated
Part 5 of Section VIII, Division 2 requires evaluation of components for four different failure modes. These are:
Plastic collapse
Local failure
Buckling
Cyclic loading (fatigue)
A component that is being evaluated using finite element analysis must be evaluated for each applicable failure mode.
An overview of the ASME methods used in evaluating pressure vessels for the above failure modes can be found here.
Summary
The ASME Boiler & Pressure Vessel Code provides specific requirements concerning the use of finite element analysis in the design of pressure vessels. These include qualifications of the designer performing the analysis, material properties, loading conditions, and failure modes to be investigated.