In the design and manufacture of pressure vessels, safety and regulatory compliance are paramount. One of the most widely adopted design methodologies is the Design-by-Formula (DBF) approach, codified in standards such as ASME Section VIII, Division 1 or EN13445. With the increasing reliance on pressure vessel software to streamline this process, a crucial step is the appraisal of design submissions, ensuring the vessel meets all applicable requirements. This article explores how such appraisals are typically conducted when DBF is employed, focusing on the role of software and inspector responsibilities.
The Appraisal Process
Design appraisal typically begins with a review of the submitted drawing and the corresponding calculation package. The first step is a cross-check between the drawing and the design calculations: Are the geometries (e.g., shell diameter, head type, nozzle positions) consistent between both? Are the materials specified in the drawings also those used in the calculations? This ensures that the design intent has been properly captured and analyzed.
Next, inspectors turn to the calculation report generated by the software. These reports should clearly document input – such as internal pressure, material properties, joint efficiencies, and corrosion allowances – and output, such as required thicknesses, stress values, and safety factors. It’s important that the output follow the formulas laid out in the code, and the input assumptions must be technically sound and traceable.
Depth of Evaluation: Output Only or Full Cross-Check?
A common question in modern design appraisals is whether inspectors need to recalculate values independently or use another software package for validation. While using a second software tool for cross-verification is sometimes practiced, it is not always necessary – especially when the primary software’s output is thorough, code-referenced, and transparent.
A well-structured output report should provide:
v Clear references to the clauses and equations from the applicable code
v Intermediate calculations and not just final values
v Graphical representations where appropriate
v Documentation of all input values with appropriate sketches, to match the drawing dimensions
If the above elements are present and the software is validated (incorporating a QA/QC validation manual), then the inspector’s primary role becomes evaluating the accuracy and validity of the input and the interpretation of the results rather than repeating all the calculations. Such software incorporating all of the above features is VCLAVIS.
However, if the software output is opaque or incomplete, or if errors are suspected, then a more rigorous check is warranted, possibly involving manual calculation or cross-verification using another code-compliant program.
Best Practices for Inspectors
Inspectors should be familiar with both the governing code and the specific software being used, and should have a background experience on pressure vessel analysis. Training in the software allows inspectors to understand default settings, built-in assumptions, and how load cases are applied.
Conclusion
Design appraisal using software under the DBF method requires a balanced approach—verifying the consistency of design documentation, the validity of input, and the clarity and completeness of output. While recalculating with another tool is sometimes useful, especially for critical components, a comprehensive and transparent output often suffices for effective evaluation. Ultimately, the goal is to ensure that the pressure vessel will perform safely under its intended service conditions, in full compliance with applicable standards.