"Pression Nominal" is the rating designator followed by a designation number indicating the approximate pressure rating in bars.
Piping Class Ratings based on the ASME B16.5 - Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard - class and the corresponding ISO 7005 PN (Pression Nominal*) ratings:
Flange Class 150 300 400 600 900 1500 2500
Flange Pressure Nominal (PN) 20 50 68 110 150 260 420
1 bar = 1x105 Pa (N/m2) = 0.1 N/mm2 = 10,197 kp/m2 = 10.20 m H2O = 0.98692 atm = 14.5038 psi (lbf/in2)
PN ratings do not provide a proportional relationship between different PN numbers, whereas class numbers do. Class numbers are therefore recommended before PN ratings.
Note! The piping rating must follow the pressure-temperature rating of the weakest pressure containing item in the system.
These pressure classes also have differing pressure and temperature ratings for different materials. Unique pressure classes for piping can also be developed for a process plant or power generating station; these may be specific to the corporation, engineering procurement and construction (EPC) contractor, or the process plant owner. The ANSI/ASME pressure classes for Flat-Face flanges are 125# and 250#. The classes for Ring-Joint, Tongue & Groove, and Raised-Face flanges are 150#, 300#, (400# - unusual), 600#, 900#, 1500#, and 2500#.
The flange faces are also made to standardized dimensions and are typically "flat face", "raised face", "tongue and groove", or "ring joint" styles, although other obscure styles are possible.
Source: China Flange Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)