Although most pressure vessels in the USA have been built to Section VIII of the ASME Boiler and Pressure Vessel Code (BPVC), the actual requirements for the design, manufacture and operation of pressure vessels are defined at state level. This means that not all states have the same requirements.

At another level, there’s also the fact that operating conditions vary considerably. Some areas experience exceptionally low temperatures while others are exposed to high temperatures, high humidity and marine corrosion. This means the pressure vessel requirements for Alaska are significantly different to those in Florida.

Different Ambient Conditions Affect Pressure Vessel Design

Pressure vessels are designed for a specific purpose that is primarily determined by their intended use. However, climatic conditions have to be taken into account because of their potential impact on the integrity of these vessels. Some environmental factors that impact on pressure vessel requirements include:

• Low temperature: The resistance of steel to brittle fracture is reduced at low temperatures. According to ASME VIII, the minimum design metal temperature (MDMT) and the lowest anticipated temperature the vessel will be exposed to must be taken into account and certain procedures followed during manufacture of low temperature pressure vessels. Section UCS-65 of ASME VIII lists the design and manufacturing requirements of pressure vessels for low temperature operation.
• Moisture: Pressure vessels exposed to the weather and high humidity can suffer rapid external corrosion if moisture is trapped, especially under lagging, and steps must be taken to prevent such corrosion.
• Marine environment: Pressure vessels in coastal environments are vulnerable to corrosion from salt spray and must have corrosion-resistant coatings.

State Pressure Vessel Requirements

Pressure vessel requirements vary quite considerably over the 50 states although virtually all have legislated that pressure vessels should be manufactured in accordance with ASME VIII, and none appear to have any legislation that supersedes the BPVC. Typical code requirements for most states include the following:

• Design code: Virtually all states require that pressure vessels are designed and manufactured to ASME VIII. Some states accept vessels with Canadian registration numbers constructed in conformance with ASME, and a few will accept pressure vessels built to foreign codes after individual review. Several states require that ASME PVHO-1 is used for pressure vessels designed for human occupancy.
• Inspection codes: Most states require that pressure vessels are inspected to either the API-150 Pressure Vessel inspection code or to the National Board Inspection Code.
• Repairs: A number of states require the National Board of Boiler and Pressure Vessel Inspectors (NBBI) R certification for pressure vessel repair and the NBBI VR certification for the repair of pressure relief valves.
• Registration: Nearly all states require that pressure vessels are registered with the NBBI and that the Manufacturers Data report is filed.
• Inspection frequency: Several states specify the required frequency for pressure vessel and boiler inspections
• Special considerations: All states have detailed schedules listing which pressure vessels must be inspected and which are exempt.
• Boilers: As a rule, states require that boiler power piping is designed to ASME B31.1 and that boiler safety devices conform to ASME CSD-1

Customer and Fabricator Responsibilities

Individual pressure vessel requirements vary considerably and it’s crucial that end customers understand the legal requirements for the design, manufacture, registration and operation of a pressure vessel in their state to ensure compliance with local laws. These can be obtained from the NBBI, although it’s best to contact local authorities to be certain. Additionally, it’s the designer’s responsibility to take into account specific climatic conditions that trigger the need for the design to incorporate specific requirements, such as those for low temperature operation.