Piping includes pipe, flanges, fittings, bolting, gasket, valves, and the pressure containing portions of other piping components.It also includes pipe hangers and supports and other items necessary to prevent over pressurization and overstressing of the pressure -containg components.It is evident that pipe is one element or a part of piping.Therefore, pipe sections when joined with fittings, valves, and other mechanical equipment and properly supported by hangers and supports, are called piping.
Pipe is classified in usual idustry paractice that is in accordance with the pressure temperature rating system used for classifying flanges.
The piping rating must be govered by the pressure-temprature 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.
The following table shows Piping Class Ratings Based on ASME b16.5 and Corresponding PN Designators.
*PN- Flange Preesure Number
Pressure-temperature ratings of different classes vary with the temperature and the material of construction.
For pressure-temperature ratings, refer to tables in ASME B165.5, or ASME B16.34.
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 preesures and temperatures not covered by any standard.
OTHER PIPE RATINGS
Based upon a unique or proprietary design of a pipe, fitting or joint, the manufactyurer may asign a pressure-temperature ratings that may form the design basis for the piping system.In no case manfacturer rating may be exceeded.In addition the manufacturer may impose limitations which may 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 sytems are usually rated for 175psi (1207.5 kPa), 200psi(1380 kPa), or as specified.
The American Water Works Association(AWWA) publishes standards and specification, which are used to design and install water pipelines and distribution system piping. The ratings used may be in accordance withthe flange ratings of AWWA C207, Steel Pipe Flanges; or the rating could be based upon the ratings of the joints used in the piping.
Unique or Specific 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. It is normal to assign a specific rating to the piping. this ratings mustbe equal ot or higher thyan the design conditions. The ratings of allpressure-containing components in the piping systemmust meet or exceed the specific rating assignesd by the designer.
Sometimes the 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 temperaturs. In addition piping system may handle more than one flow of medium duringits different modes of operation.Therefor such a piping system may be assigned to dual ratings for two differenty flow media.