ASME Section II: A Deep Dive into the Foundation of Pressure Vessel Materials ASME Section II of the Boiler and Pressure Vessel Code (BPVC) serves as the comprehensive materials book for the construction of pressure vessels under ASME Section VIII, Divisions 1 and 2. It is a foundational and supplementary section, providing the necessary material specifications and properties required for design and fabrication. This in-depth rundown explores the structure of Section II, its application for both Division 1 and Division 2 pressure vessels, and clarifies why the same material can have different allowable stress values depending on its intended use. First: A General Rundown of ASME Section II At its core, ASME Section II is a four-part compilation of material specifications and their associated properties. It ensures that materials used in pressure vessel construction have the required chemical composition, mechanical strength, and physical characteristics for safe operation. How It Is Composed: The Four Parts ASME Section II is organized into four distinct parts, each addressing a different category of materials or properties: • • • • Part A: Ferrous Material Specifications: This part contains specifications for carbon steel, low alloy steel, stainless steel, and other ferrous materials. The specifications, designated with an "SA" prefix (e.g., SA-516), are often identical or similar to those from ASTM International. They detail requirements for manufacturing processes, heat treatment, chemical composition, and mechanical properties like tensile and yield strength. Part B: Nonferrous Material Specifications: Following a similar structure to Part A, this section covers nonferrous materials such as aluminum, copper, nickel, and titanium alloys. These specifications are identified by an "SB" prefix (e.g., SB-209). Part C: Specifications for Welding Rods, Electrodes, and Filler Metals: This part is crucial for welded vessel construction. It provides specifications for welding consumables, which are largely derived from American Welding Society (AWS) standards and are designated with an "SFA" prefix (e.g., SFA-5.1). Part D: Properties (Customary and Metric): This is the most frequently used part for design calculations. It does not contain material specifications but rather compiles the properties of the materials specified in Parts A and B into a usable format for engineers. Part D is intricately structured into subparts, tables, charts, and curves. Breaking Down Part D: Subparts, Tables, and Charts Part D is where the raw material information is translated into design values. Its key components are: • • • Subpart 1 - Stress Tables: This is the most critical section for designers. It contains tables of maximum allowable stress values for a vast range of materials at various temperatures. These tables are the cornerstone of pressure vessel thickness calculations.[1] Subpart 2 - Physical Properties Tables: This subpart provides essential physical data needed for design, such as thermal expansion coefficients, thermal conductivity, and moduli of elasticity, all presented as a function of temperature.[2] Subpart 3 - Charts and Tables for Determining Shell Thickness of Components Under External Pressure:This section contains the well-known external pressure charts (e.g., CS-2 for carbon steel) used to design vessels subjected to vacuum or external pressure.[2] • Mandatory and Non-Mandatory Appendices: These provide the basis for how the stress values are established, guidelines on rounding data, and other supplementary information.[1] Second: Using ASME Section II for Pressure Vessel Design The specific tables and values used from ASME Section II, Part D depend entirely on the division of Section VIII under which the vessel is being constructed. Application for Section VIII Division 1 Division 1 employs a "design-by-rule" philosophy, which is a more conservative approach.[3] When designing a Division 1 vessel, engineers refer to the following in Section II, Part D: • Tables 1A and 1B: These tables provide the maximum allowable stress values (S) for Division 1 construction. o Table 1A is for Ferrous Materials (e.g., carbon and stainless steels).[4] o Table 1B is for Nonferrous Materials (e.g., aluminum and nickel alloys).[5] These allowable stress values are established with a design factor of 3.5 on the material's specified minimum tensile strength (SMTS).[5][6] Application for Section VIII Division 2 (Up to the 2023 Edition) Division 2 utilizes a more rigorous "design-by-analysis" approach, which allows for less conservative designs and often results in thinner, more efficient vessels.[6] Until the 2023 edition, Division 2 was split into two classes with different requirements: • • For Division 2, Class 2: This class was introduced in 2017 and is still more conservative than the original Division 2 rules.[7] For these vessels, you would use: o Table 2A and 2B (Note: Prior to the introduction of classes, these tables were used for Division 2. With the class system, the design margin aligns with Class 1). The allowable stresses are based on a design factor of 3.0 on the SMTS.[6][7] For Division 2, Class 1: This represents the more advanced and less conservative option within Division 2.[7] The allowable stress values are found in: o Table 5A for Ferrous Materials.[8] o Table 5B for Nonferrous Materials.[8] These tables are based on a design factor of 2.4 on the SMTS, which is why they permit higher allowable stresses.[6][7] It is important to note that the 2023 Edition of ASME Section VIII Division 2 has largely eliminated the "Class 1" and "Class 2" designations, streamlining the rules. Revisions to Part 2 of Division 2 have removed the Certifying Engineer requirements for many design-by-rule applications that were previously tied to the class designation.[9][10] Users should always refer to the specific edition of the code being used for a project. Why Different Tables and Values for the Same Material? The existence of different allowable stress values for the same material in different tables is a direct reflection of the varying safety margins and design philosophies of Division 1 and Division 2. The core reason is the Design Factor. A higher design factor means a greater margin of safety is built into the calculation, resulting in a lower allowable stress and, consequently, a thicker vessel component for the same pressure. • • Division 1 (Design Factor of 3.5): This is a conservative, rules-based approach intended for a wide range of standard applications. The higher safety factor accounts for a less detailed stress analysis.[3][6] Division 2 (Design Factors of 3.0 and 2.4): This division's "design-by-analysis" approach requires a more comprehensive and detailed stress analysis of the vessel.[3][11] This rigorous analysis provides a deeper understanding of how the vessel will behave under load. To compensate for this increased engineering effort and more stringent fabrication and examination requirements (such as more extensive non-destructive testing), the code permits the use of a lower design factor, leading to higher allowable stresses and more optimized, efficient designs.[6][12] In essence, the trade-off is between the level of engineering analysis and the efficiency of the material use. By mandating a more in-depth understanding of the stresses within a vessel, Division 2 justifies the use of higher allowable stresses for the very same material that would be assigned a lower value under the more generalized rules of Division 1.
