Get expert answers to pressing questions in PVP Design
What are the primary design codes and standards for pressure vessels, boilers, and heat exchangers?
The most common design codes used for pressure vessels, boilers and shell-and-tube heat exchangers are those contained in the ASME Boiler and Pressure Vessel Code. ASME BPV VIII, Division 1 is the most commonly used pressure vessel code in the world, and many heat exchangers are designed to meet it. ASME BPV I is used for Power Boilers and ASME BPV IV is used for Heating Boilers.
How often does ASME Section VIII go through a code update?
How is the minimum thickness of a pressure vessel determined?
There are a variety of methods to determine the minimum thickness of a pressure vessel. These different methods are dependent on the stress theory, whether the geometry is considered thick or thin, the types of loading and failure mechanisms considered and if design by rule or design by analysis is employed.
What are the key differences between Design By Analysis and Design By Rule?
Design By Rule (DBR), a conservative and standardized approach, relies on predefined rules, formulas, and tables provided in the code of construction, such as ASME Section VIII, Division 1, which are based on empirical data, historical experience, and simplified assumptions. The approach does not require stress analysis or sophisticated engineering judgment beyond applying the code’s specified methods. Design By Analysis (DBA) requires a more detailed and rigorous engineering analysis, e.g. finite element analysis, such as that outlined in ASME Section VIII, Division 2 Part 5. The approach focuses on evaluating actual stresses, strains, and failure modes under specific design and operating conditions instead of relying solely on prescriptive rules.
What methods are used to analyze pressure vessel stress and integrity?
From an inspection/quality standpoint: Visual Inspection, Liquid Penetrant Inspection for any material surfaces in question and/or any critical welds is often employed. Hydrostatic or Pnuematic Pressure testing is typically for new vessels. Also used is Ultrasonic Testing on larger and more critical welds, Radiographic Testing on smaller but more critical welds like pipe-to-pipe joints that would experience internal pressure.
What welding techniques are commonly used in pressure vessel fabrication?
Any large critical weld will typically be completed using Submerged Arc Welding (SAW) when welding seams; long shell seams, shell to shell circumference seams, shell to head circumference seams. Most internal welds are completed using Gas Metal Arc Welding in the spray arc transfer mode (GMAW-Spray) and that's typically completing Backing welds for the seams and welding any internal parts along with any nozzles that can be welded in the shop (internal & external joint welds.) In some cases for the installation process of the PV, some flanges, fixtures, and even nozzles have to be field welded and those are accomplished using Shielded Metal Arc Welding (SMAW) and typically using 6010 rods of the initial welding (root weld) and then completing the weld using a low hydrogen electrode such as a 7018 rod.
What are the different design methodologies used for pressure vessels?
These different methods are dependent on the stress theory, whether the geometry is considered thick or thin, the types of loading and failure mechanisms considered and if DBR or DBA is employed. Ultimately, the choice of method is limited to the code of construction used. A variety of codes and standards exist for the design of pressure vessels with the most common being ASME BPV VIII, Division 1. The most used stress theories for ductile materials are Maximum Shear Stress Theory (Tresca) and the Maximum Distortion Energy Theory (Von Mises).
How do ASME codes address different materials used in pressure vessel construction?
The ASME BPV and Piping Codes such as ASME BPV VIII, Division 1 (S8D1) and B31.1, have a variety of requirements for materials. First, a material must satisfy specification requirements to show it can be classified as that specification (spec); this is done through the material test report (MTR). Materials satisfying spec requirements may or may not be permitted for use by the code of construction; for example, S8D1 lists acceptable carbon and low alloy stills in Table UCS-23. A material must be listed in UCS-23, or a similar table for other material types, if it is to be used for a S8D1 vessel. Assuming the material meets the spec and is listed in the code of construction, it then must meet all the requirements for materials of its type as listed. Expanding on the UCS-23 example, the material listed here must meet all the requirements in Part UCS. This includes requirements on post weld heat treatment, low temperature limits, limits on certain specs, sizes, products forms, etc. The final major consideration is the material properties used in the design. These are primarily found in ASME Section II, Part D for use with S8D1. Tables here specify the values that can be used for allowable stress, yield and ultimate tensile strength, in design formulas. In addition, several other properties can be found including Young’s Modulus, Poisson’s ratio, etc.
What factors should be considered when selecting materials for pressure vessels?
The process drives the material selection. Certain materials may not be suitable for hygienic service or to contain certain chemicals for example. Some materials will degrade quickly when exposed to certain fluids, etc. In addition, low temperature service, high temperature service, etc. all further narrow the acceptable material options.
What are the key differences between ASME Section VIII Division 1 and Division 2 codes?
a. General Scope: Both Division 1 (D1) and Division 2 (D2) cover fabrication, inspection, testing, materials, and design. They also define responsibilities, though D2 formalizes them more deeply.
b. Design By Analysis (DBA): D2 includes Part 5, which is the section dedicated to DBA in Division 2. Division 1 does not include a section on DBA; though it does not prohibit its use. Division 1 allows DBA to supplement the rules included in it, but DBA may not be used to replace rules in Division 1. The user of DBA in D2 may be used to get a more favorable answer for the designer. For example, if a designer is working on a D1 stamped vessel and an aspect of the design is clearly in scope of the D1 method, DBA cannot be used to get around that. A D2 vessel design can get around a DBR method using a DBA method.
c. Design By Rule (DBR): The DBR methods in D2 are often identical or near identical to methods in D1. However, in many cases, the methods have additional scope of coverage or are a different method altogether. For example, the ellipsoidal head method in D2 is the same as Appendix 1-4(f) in D1. The custom flange method in D2 4.16 is the same as Appendix 2 in D1, though the hub factors are determined in an updated way, and 4.16 has built-in support for external loads. The external pressure method in D2 4.4 is a completely different method from the external pressure methods in D1 UG-28, UG-29 and UG-33. It is often more favorable to the designer as well.
d. Style: D1 is in two-column style with figures, nomenclature, tables, etc, placed throughout the text. D2 is in a one-column style with text later followed by nomenclature, tables, and figures in a consistent and ordered fashion in line with ISO standards.
e. Harmonization: As an additional note, the common rules effort is eliminating design rules in D1 that are common with design rules in D2. Instead, the D1 location will point to the D2 location for these common rules. There is also language guiding the D1 user to D2 to supplement D1 methods with the D2 methods, including DBR and DBA.
a. General Scope: Both Division 1 (D1) and Division 2 (D2) cover fabrication, inspection, testing, materials, and design. They also define responsibilities, though D2 formalizes them more deeply.
b. Design By Analysis (DBA): D2 includes Part 5, which is the section dedicated to DBA in Division 2. Division 1 does not include a section on DBA; though it does not prohibit its use. Division 1 allows DBA to supplement the rules included in it, but DBA may not be used to replace rules in Division 1. The user of DBA in D2 may be used to get a more favorable answer for the designer. For example, if a designer is working on a D1 stamped vessel and an aspect of the design is clearly in scope of the D1 method, DBA cannot be used to get around that. A D2 vessel design can get around a DBR method using a DBA method.
c. Design By Rule (DBR): The DBR methods in D2 are often identical or near identical to methods in D1. However, in many cases, the methods have additional scope of coverage or are a different method altogether. For example, the ellipsoidal head method in D2 is the same as Appendix 1-4(f) in D1. The custom flange method in D2 4.16 is the same as Appendix 2 in D1, though the hub factors are determined in an updated way, and 4.16 has built-in support for external loads. The external pressure method in D2 4.4 is a completely different method from the external pressure methods in D1 UG-28, UG-29 and UG-33. It is often more favorable to the designer as well.
d. Style: D1 is in two-column style with figures, nomenclature, tables, etc, placed throughout the text. D2 is in a one-column style with text later followed by nomenclature, tables, and figures in a consistent and ordered fashion in line with ISO standards.
e. Harmonization: As an additional note, the common rules effort is eliminating design rules in D1 that are common with design rules in D2. Instead, the D1 location will point to the D2 location for these common rules. There is also language guiding the D1 user to D2 to supplement D1 methods with the D2 methods, including DBR and DBA.