Why we call “Pound” for the Pipe Flange Classes?
Piping flanges are identified by classes. Each class represents with pressure-temperature rating related to allowable pressure with operating temperature. At a given operating temperature, the selected flange class offers an allowable pressure that is either equal or greater than the design pressure. This table (from ASME B16.5) shows pressure-temperature rating for A-105 forged carbon steel flanges.
Flange class are commonly being used as “Pound” by many engineers. For example, Class 300 flanges are called 300-Pound Flanges, and so forth. This was because Class 300 flanges were rated 300 pounds per square inch (psi) pressure at benchmark temperature. Pound (lb) meant psi (pressure) and there is no relation with the weight of the flange itself.
Benchmark temperatures used were different, for different materials. However, currently for A-105 flanges, Class 150 has a rating pressure of 170 psi at 500 F benchmark temperature and Class 300 has
a rating pressure of 270 psi at 850 F benchmark temperature. It is clear that current pressure-temperature rating is no longer correspond to the original benchmark idea, see Table 2-1.1.
If you know the reason, you can still continue to say “Pound”.
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