The common steps for steel valves and flanges are Classes 150, 300, 600, 900, 1500 and 2500. The number is not a pressure: a carbon steel Class 150 flange is good for about 285 psig at ambient and less as temperature rises, while Class 300 is about 740 psig. Each class has a pressure-temperature table by material group, and all valves and flanges in a line must meet the class the piping spec calls for.
That line is Class 300, don't let anybody bolt a Class 150 valve in there just because the flange fits.
Fitters say 150 pound and 300 pound, the drawings say Class 150 and Class 300; either way the number is a class, not a psi rating.
Also heard as
- class
- pound rating
- ASME class
- lb class
Related terms
PN
Pressure nominal: the metric rating series, such as PN 16 or PN 40, used for European and ISO valves and flanges.
WOG
Water, oil, gas: a cold working pressure rating stamped on small threaded valves, such as 600 WOG.
Raised face
A flange facing where the gasket surface stands slightly proud of the bolt circle area, concentrating gasket load.
Body
The main pressure-containing shell of a valve, carrying the flow passage and the end connections.