Which standard defines flange pressure classes?

• A. National Board Inspection Code (NBIC)
• B. B16.5
• C. B31.3
• D. B16.25

Answer: (B)

Flange pressure classes are the rating system that specifies the maximum allowable pressure a flange can handle at a given temperature. The standard that defines these classes is ASME B16.5, Pipe Flanges and Flanged Fittings. It provides the official class designations (such as 150, 300, 600, etc.) and the corresponding pressure–temperature ratings for different sizes and materials, enabling proper matching of flanges and gaskets. Other standards play different roles: NBIC focuses on inspection and repair, B31.3 covers process piping design in general and references flange ratings rather than defining them, and B16.25 deals with flange fabrication requirements and dimensions rather than rating definitions. So the standard that defines flange pressure classes is ASME B16.5.