It is usual industry practice to classify the pipe in accordance with the pressure-temperature rating system used for classifying flanges. However, it is not essential rating must be governed by the pressure-temperature rating of the weakest pressure-containing item in the piping. The weakest item in a piping system may be a fitting made of weaker material or rated lower due to design and other considerations.
In addition, the piping may be classified by class ratings covered by other ASME standards, such as ASME B16.1, ASME B16.3, ASME B16.24, and ASME B16.42. A piping system may be rated for a unique set of pressures and temperatures not covered by any standard.
Pression nominal (PN) is the rating designator followed by a designation number, which indicates the approximate pressure rating in bars. The bar is the unit of pressure, and 1 bar is equal to 14.5 psi or 100 kilopascals (kPa). Table A1.2 provides a cross-reference of the ASME class ratings to PN rating designators. It is evident that the PN ratings do not provide a proportional relationship between different PN numbers, whereas the class numbers do. Therefore, it is recommended that class numbers be used to designate the ratings. #Little_PEng