Choosing a flange class and gasket from ASME B16.5
A flange class is not a pressure limit on its own — it has to be read at the design temperature. A Class 150 carbon steel flange holds 285 psi at 100°F but only 200 psi at 400°F and 80 psi at 800°F. This page uses the ASME B16.5 pressure-temperature ratings for material group 1.1 (A105 carbon steel) and returns the lowest class that still covers your design point. At 300 psi and 500°F, for example, Class 150 allows only 170 psi while Class 300 allows 605 psi, so Class 300 is the answer.
| Class | 100°F | 400°F | 800°F |
|---|---|---|---|
| Class 150 | 285 psi | 200 psi | 80 psi |
| Class 300 | 740 psi | 635 psi | 410 psi |
| Class 600 | 1,480 psi | 1,265 psi | 825 psi |
| Class 900 | 2,220 psi | 1,900 psi | 1,235 psi |
The gasket follows from the class and the service. Low pressure ambient water lines are fine on compressed non-asbestos sheet, steam and hot process piping should move to spiral wound, and Class 900 and above normally uses a ring type joint. Ratings change with the material group, so stainless and alloy flanges must be read from their own table rather than this one, and NPS 26 and larger fall under B16.47 instead of B16.5. Check the values against the published standard and have a mechanical engineer or the inspection authority confirm the selection before purchase.
Frequently Asked Questions
No. The class is only a label. A carbon steel Class 150 flange is rated 285 psi at 100°F and the allowable falls steadily as temperature rises, reaching 20 psi at 1,000°F.
ASME B16.5 permits straight-line interpolation between listed temperatures, which is what this calculator does. Above the 1,000°F limit of the carbon steel table you need a different material.