Class 1 vs Class 2 Pressure Vessels
Several years ago, Section VIII, Division 2 of the ASME Boiler & Pressure Vessel Code started classifying Division 2 pressure vessels as either Class 1 or Class 2. The differences in the requirements and design basis between these two classes are not provided in a single location in Section VIII, Division 2 but rather spread out across the Code. This article presents a compilation of the differences between these two classifications. The information in this article is based on the 2019 Edition of the ASME Boiler & Pressure Vessel Code. Since the ASME Code is revised on a regular basis, the applicable edition of the Code for your pressure vessel should be consulted.
The differences between Class 1 and Class 2 pressure vessels can be placed into four groups: allowable stresses, design requirements, Manufacturer’s Design Report (MDR), and User’s Design Specification (UDS).
Allowable Stresses
Class 1
Allowable stress values are taken from Section II, Part D, Subpart 1, Table 2A or Table 2B
Design margin against ultimate tensile strength is 3.0
Design margin against yield strength is 1.5
Class 2
Allowable stress values are taken from Section II, Part D, Subpart 1, Table 5A or Table 5B
Design margin against ultimate tensile strength is 2.4
Design margin against yield strength is 1.5
Design Requirements
Class 1
Part 4 of Section VIII, Division 2 (Design by Rule) shall be used in the design of Class 1 pressure vessels. If design rules are not provided in Part 4, Design by Analysis (Part 5 of Section VIII, Division 2) shall be used. If Design by Analysis (Part 5) is used, all loadings specified in the User’s Design Specification (UDS) must be considered. If the component cannot be designed using Part 4 or Part 5, a design method consistent with the overall design philosophy of Class 1 and acceptable to the Authorized Inspector (AI) shall be used.
The Design by Analysis rules in Part 5 cannot be used in lieu of the rules in Part 4.
Class 2
Design by Analysis (Part 5) is required when design rules are not provided in Part 4. All loadings specified in the UDS must be considered.
Components for Class 2 pressure vessels may be designed using a combination of Part 4 or Part 5. Part 5 (Design by Analysis) may be used to establish design thickness and/or configuration in lieu of Part 4 (Design by Rule) for any geometry or loading conditions. Thickness can be less than that determined using Part 4.
Manufacturer’s Design Report (MDR)
Class 1
The MDR must be certified when any of the following are performed:
Fatigue analysis
Part 5 (Design by Analysis) is used to determine thickness of pressure parts when design rules are not provided in Part 4
A quick-actuating closure is designed using Part 4.8
A dynamic seismic analysis is performed
Class 2
The MDR must be certified
User’s Design Specification (UDS)
Class 1
The UDS does not have to be certified unless fatigue analysis is required.
Class 2
The UDS must be certified
So when is specifying a Class 2 pressure vessel advantageous?
Assuming allowable stresses are based on the ultimate tensile strength of the material, the lower design margin for Class 2 pressure vessels allows for potentially thinner components than what would be required for a Class 1 pressure vessel. This may result in lower material costs. Design thickness of components determined using Part 5 (Design by Analysis) of Section VIII, Division 2 may be used in lieu of those determined using Part 4 (Design by Rule) for Class 2 pressure vessels. This can also lead to lower material costs. So the advantage of specifying a pressure vessel as Class 2 is the potential for lower material costs.
Class 2 pressure vessels, however, require the Manufacturer’s Design Report (MDR) and User’s Design Specification (UDS) to be certified resulting in higher engineering costs. There may also be higher engineering costs associated with performing analyses to reduce the design thickness of components. This trade-off of higher engineering costs to lower material costs is often only advantageous with large and/or thick-walled pressure vessels.