Manufacturer’s Rating
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 no case shall the manufacturer’s rating be exceeded. In addition, the manufacturer may impose limitations which must be adhered to.
NFPA Ratings
The piping systems within the jurisdiction of the National Fire Protection Associa-tion (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 (1380kPa), or as specified.
AWWA Ratings
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.
Dual Ratings
Sometimes a piping system may be subjected to full-vacuum conditions or submerged in water and thus experience external pressure, in addition to with standing 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. #Little_PEng