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Understanding the Distinction: Class 1 vs Class 2 Pressure Vessels


In the dynamic world of pressure vessel standards, the ASME Boiler & Pressure Vessel Code stands as a guiding beacon. A few years ago, Section VIII, Division 2 introduced a transformative classification system, delineating pressure vessels into Class 1 and Class 2. For engineers and manufacturers, grasping the intricacies of these classifications is paramount. This article dives into the nuances, providing a consolidated overview based on the 2019 Edition of the ASME Code.


The distinctions between Class 1 and Class 2 pressure vessels neatly fall into four categories: allowable stresses, design requirements, Manufacturer’s Design Report (MDR), and User’s Design Specification (UDS).


Allowable Stresses:

Class 1: Allowable stress values are derived from Section II, Part D, Subpart 1, Table 2A or Table 2B, with design margins against ultimate tensile and yield strengths set at 3.0 and 1.5, respectively.

Class 2: Allowable stress values are extracted from Section II, Part D, Subpart 1, Table 5A or Table 5B, with design margins against ultimate tensile and yield strengths at 2.4 and 1.5, respectively.


Design Requirements:

Class 1: Governed by Design by Rule (Part 4 of Section VIII, Division 2), with Design by Analysis (Part 5) as a fallback. Design by Analysis cannot substitute Design by Rule.

Class 2: Design by Analysis (Part 5) is obligatory in the absence of design rules in Part 4. Components can be designed using a blend of Part 4 and Part 5, providing flexibility in establishing thickness and configuration.


Manufacturer’s Design Report (MDR):

Class 1: MDR certification is required for specific scenarios, including fatigue analysis, Part 5 usage for thickness determination, design of quick-actuating closures, and dynamic seismic analysis.

Class 2: MDR certification is a standard requirement.


User’s Design Specification (UDS):

Class 1: UDS certification is unnecessary unless fatigue analysis is mandated.

Class 2: UDS must be certified.


Advantages of Class 2 Pressure Vessels:

When contemplating Class 2 pressure vessels, the primary advantage lies in potential material cost savings. The lower design margin allows for thinner components, translating to reduced material costs. Thickness determined using Part 5 (Design by Analysis) may replace that determined using Part 4 (Design by Rule), contributing to additional savings. However, it's vital to note that Class 2 vessels come with higher engineering costs due to mandatory MDR and UDS certification, along with potential expenses tied to analyses for reducing design thickness. This trade-off proves most advantageous for large and/or thick-walled pressure vessels.

Exploring Class 1 and Class 2 Pressure Vessels in Industry Applications: In industries where liquids, gases, and substances are handled under pressure, the design and classification of pressure vessels play a pivotal role in ensuring safety and efficiency. The primary classifications, Class 1 and Class 2 pressure vessels, categorize these containers based on design, construction, and application. In this blog, we'll explore the distinctive characteristics and applications of these vessels, shedding light on their unique roles in diverse industries.



Class 1 Pressure Vessels:

Class 1 pressure vessels are engineered for low-pressure applications, operating at pressures not exceeding 15 psi (1.03 bar). The design specifications for Class 1 vessels are less stringent, reflecting the lower risk associated with their use.

Characteristics of Class 1 Pressure Vessels:

  1. Lower Pressure Ratings: Designed for applications where lower pressure levels suffice, Class 1 vessels are suitable for processes that do not demand high pressure, offering versatility and adaptability.

  2. Less Stringent Design Standards: The design and construction standards for Class 1 pressure vessels are less demanding, contributing to cost savings in manufacturing without compromising safety.

  3. Common Applications: Class 1 pressure vessels find applications in various industries, including heating and ventilation systems, water storage, and processes where moderate pressure containment is adequate.

Class 2 Pressure Vessels:

In contrast, Class 2 pressure vessels are engineered to handle higher pressure levels and more demanding applications, operating at pressures exceeding 15 psi (1.03 bar) up to a defined limit, depending on their specific design and purpose.

Characteristics of Class 2 Pressure Vessels:

  1. Higher Pressure Ratings: Designed to withstand higher pressures, Class 2 vessels are essential in industries such as chemical processing, petrochemicals, and power generation, where elevated pressure conditions are necessary.

  2. Rigorous Design Standards: The design and manufacturing standards for Class 2 pressure vessels are more stringent to ensure structural integrity and safety under high-pressure conditions. This involves detailed engineering analysis and quality control measures.

  3. Diverse Applications: Class 2 pressure vessels are utilized in a broad spectrum of applications, including gas storage, chemical reactors, steam boilers, and other processes where high-pressure containment is crucial for operational success.

Conclusion: Understanding the distinctions between Class 1 and Class 2 pressure vessels is fundamental in choosing the right container for specific industrial applications. Whether seeking versatility in moderate pressure conditions or robustness in high-pressure environments, the appropriate classification ensures optimal safety and efficiency in diverse operational settings.



Universal Engineering services(UES) can do Engineering of modular substations. Our Structural Engineers can design offshore container modules, skids packages, cargo baskets, steel structural frames, lifting analysis of offshore structures,etc to clients all over the middle east countries like UAE, Oman, Qatar, Saudi and Kuwait for petrochemical, power and oil and gas industry. We also offers reliable offshore and onshore design of structures as per D.N.V. 2.7.1 and we are diversified into Structural Design, Engineering & Analysis for the Industrial, Offshore and Oil & Gas industry.



**The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.

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