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 that piping be classified as Class 150, 300, 400, 600, 900, 1500, and 2500. The piping 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.
Piping Class Ratings Based on ASME B16.5 and Corresponding PN Designators
Above table lists the standard pipe class ratings based on ASME B16.5 along with corresponding pression nominal (PN) rating designators. Pression nominal is the French equivalent of pressure nominal.
In addition, the piping may be classified by class ratings covered by other ASME standards, such as ASME B16.1, B16.3, B16.24, and 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). Above table 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. Refer to Chap. B2 for a more detailed discussion of class rating of piping systems.
Other Industrial Piping Class Ratings Classifications
Based upon a unique or proprietary design of a pipe, fitting, or joint, the manufacturer may assign a pressure-temperature rating that may form the design basis for the piping system. Examples include Victaulic couplings and the Pressfit system.
In no case shall the manufacturer’s rating be exceeded. In addition, the manufacturer may impose limitations which must be adhered to.
The piping systems within the jurisdiction of the National Fire Protection Association (NFPA) requirements are required to be designed and tested to certain required pressures. These systems are usually rated for 175 psi (1207.5 kPa), 200 psi (1380 kPa), or as specified.
The American Water Works Association (AWWA) publishes standards and specifications, which are used to design and install water pipelines and distribution system piping. The ratings used may be in accordance with the flange ratings of AWWA C207, Steel Pipe Flanges; or the rating could be based upon the rating of the joints used in the piping.
Specific or Unique Rating
When the design pressure and temperature conditions of a piping system do not fall within the pressure-temperature ratings of above-described rating systems, the designer may assign a specific rating to the piping system. Examples of such applications include main steam or hot reheat piping of a power plant, whose design pressure and design temperature may exceed the pressure-temperature rating of ASME B16.5 Class 2500 flanges. It is normal to assign a specific rating to the piping.
This rating must be equal to or higher than the design conditions. The rating of all pressure-containing components in the piping system must meet or exceed the specific rating assigned by the designer.
Sometimes a piping system may be subjected to full-vacuum conditions or submerged in water and thus experience external pressure, in addition to withstanding the internal pressure of the flow medium. Such piping systems must be rated for both internal and external pressures at the given temperatures. In addition, a piping system may handle more than one flow medium during its different modes of operation. Therefore, such a piping system may be assigned a dual rating for two different flow media. For example, a piping system may have condensate flowing through it at some lower temperature during one mode of operation while steam may flow through it at some higher temperature during another mode of operation. It may be assigned two pressure ratings at two different temperatures.