A common misconception among beginner piping designers is that a "Class 150#" or "Class 300#" flange has an operating limit of exactly 150 psi or 300 psi. Under ASME B16.5, a flange's pressure class is a dimensionless designation representing a standardized envelope of pressure-temperature (P-T) ratings. The actual maximum allowable working pressure (MAWP) depends on operating temperature and alloy metallurgy.
ASME B16.5 standardizes flanges from NPS 1/2 through NPS 24 in seven primary pressure classes:
Up to 285 psi ambient
Up to 740 psi ambient
Rarely specified
Up to 1480 psi ambient
Up to 2220 psi ambient
Up to 3705 psi ambient
Up to 6170 psi ambient
As operating temperature increases, material yield strength drops. Consequently, the allowable pressure falls significantly below the ambient rating:
| Operating Temp | Class 150# (psi) | Class 300# (psi) | Class 600# (psi) | Class 900# (psi) |
|---|---|---|---|---|
| -29°C to 38°C (-20°F to 100°F) | 285 psi | 740 psi | 1480 psi | 2220 psi |
| 200°C (392°F) | 200 psi | 635 psi | 1270 psi | 1910 psi |
| 300°C (572°F) | 140 psi | 570 psi | 1145 psi | 1715 psi |
| 400°C (752°F) | 80 psi | 490 psi | 975 psi | 1465 psi |
Under ASME B16.5 paragraph 7, finished flanges must be capable of withstanding a factory hydrostatic proof test conducted at 1.5 times the 38°C (100°F) pressure rating rounded to the next higher 25 psi increment:
No. The bolt circle diameter (BCD), bolt hole size, and number of bolts differ completely between pressure classes to intentionally prevent mismatched connections in the field.
Historically, Class 400 was introduced as an intermediate rating, but piping engineers found it more practical and cost-effective to standardize on Class 600 to minimize warehouse spares and simplify inventory management.
Remax Forge manufactures Class 150# through 2500# forged flanges in size ranges from 1/2" to 48", fully hydro-tested and certified to EN 10204 3.1.