ASME’s Definition of a Pressure Vessel

The American Society of Mechanical Engineers (ASME) defines and provides standards for the design, fabrication, and inspection of pressure vessels through its Boiler and Pressure Vessel Code (BPVC), specifically Section VIII. According to ASME, a pressure vessel is generally described as a container designed to hold or store gases or liquids under pressure.

Here’s a more detailed definition based on the ASME BPVC:

A pressure vessel is a container with a pressure differential between the inside and outside, designed for pressures above 15 psi (103.4 kPa) and, irrespective of size, is not intended to be fired. This 15 psi threshold distinguishes pressure vessels from other containers that operate at atmospheric pressure or those very close to it.

It’s important to note a few caveats and specifics:

• Exclusions: 

There are certain exclusions to what ASME considers a pressure vessel under Section VIII. For instance, piping systems, turbines, transport tanks, and some others are not covered under this section, even though they might contain pressurized fluids.

• Divisions:

 ASME Section VIII, which covers unfired pressure vessels, is divided into several divisions:

• Division 1: Provides requirements applicable to the design, fabrication, inspection, testing, and certification of pressure vessels operating at either internal or external pressures exceeding 15 psi.
• Division 2: Contains alternative rules, which are more rigorous than Division 1. It offers a mix of design-by-rule and design-by-analysis considerations.
• Division 3: Addresses design and fabrication of high-pressure vessels, typically those with internal or external pressures more than 10,000 psi.

• Size Considerations:

 ASME also has size considerations. Containers that are very small, even if they exceed 15 psi, might be excluded from being classified as pressure vessels under specific conditions.

• Boilers vs. Pressure Vessels: 

It’s important to distinguish between boilers and pressure vessels, even though both are covered under the BPVC. Boilers are designed to generate steam and fall under a different section (Section I) of the BPVC.

• Specific Configurations:

 Some configurations, like thin-walled pressure vessels (e.g., soda cans, aerosol cans) that are mass-produced, might fall under different standards or exceptions.

• Joint Categories:

 ASME BPVC Section VIII provides specific details about different joint categories and types, which relate to the quality and inspection requirements of the welds. Depending on the location and function of the weld in the vessel, different levels of inspection rigor might be mandated.

• Materials: 

The materials used for constructing pressure vessels are extensively detailed in the ASME BPVC. These materials are carefully selected based on their mechanical properties, and they have to meet specific criteria for ductility, toughness, and strength. Section II of the BPVC provides material specifications, ensuring that vessels are constructed from materials that have well-understood and documented properties.

• Nondestructive Examination (NDE):

 Depending on the design and service conditions of the vessel, various nondestructive examination methods might be required, including radiography (RT), ultrasonic testing (UT), magnetic particle testing (MT), and dye penetrant testing (PT), among others. The BPVC provides criteria for when and how these tests are to be conducted.

• Overpressure Protection: 

To avoid dangerous overpressure situations, pressure vessels must have protection mechanisms. This often takes the form of safety or relief valves. Section VIII provides requirements for the installation of such devices to ensure they operate correctly under potential overpressure conditions.

• Documentation and Certification: 

Once a pressure vessel is designed, fabricated, and tested, it often needs to be stamped with the ASME “U” symbol to indicate its compliance with the BPVC’s standards. This stamp is a sign of quality assurance, signifying that the vessel has been fabricated according to ASME’s rigorous standards. Additionally, a data report is created and provided, serving as a record of the vessel’s compliance.

• Repairs and Alterations:

 Over the life of a pressure vessel, it might require repairs or alterations. The ASME BPVC, through the National Board Inspection Code (NBIC) and the “R” stamp, provides guidelines for performing these tasks while ensuring the vessel’s continued safety and compliance.

• Regular Inspection:

 Depending on the service conditions and local regulations, pressure vessels often require regular inspections during their operational life. These inspections ensure the vessel’s integrity over time and might include both internal and external evaluations, thickness measurements, and other NDE methods.

• Importance of Training:

 Given the complexity and importance of the BPVC, engineers, inspectors, and fabricators need proper training. ASME and other organizations offer courses and certifications to ensure professionals are well-versed in the standards and their application.

While the ASME BPVC provides comprehensive standards and guidelines for the design and fabrication of pressure vessels, one must always refer to the latest version of the code and ensure adherence to any local or national regulations that might apply.

In conclusion, the ASME BPVC is a comprehensive document that covers almost every facet of pressure vessel design, fabrication, testing, and maintenance. Its primary goal is to ensure the safety of these containers, which, if failed, could result in catastrophic events. Adherence to the BPVC isn’t just about regulatory compliance; it’s about ensuring the protection of both life and property.