Flange pressure ratings explained: ASME B16.5 classes, material groups and temperature
A flange class (150, 300, 600, 900, 1500, 2500) is not a pressure. ASME B16.5 places each forging material in a material group and tabulates the allowable working pressure for every class at each temperature. The rating falls as temperature rises, and two Class 300 flanges in different materials can have different ratings at the same temperature.
Class, group and temperature
Three things set the allowable pressure of a B16.5 flange: its pressure class, the material group of the forging, and the design temperature. The class fixes the dimensions — bolt circle, thickness, number of bolts. The group reflects how strong the material is at temperature. B16.5 then gives one table per group, with a column for each class and a row for each temperature.
So the question "what pressure can a Class 300 flange take?" has no single answer. You read the Class 300 column of the right group's table at your design temperature — and your piping class sheet, which already does this, governs.
Material groups for common flange materials
| Forging material | B16.5 group |
|---|---|
| A105 · A350 LF2 | 1.1 |
| A182 F11 Class 2 | 1.9 |
| A182 F22 Class 3 | 1.10 |
| A182 F304 | 2.1 |
| A182 F304L · F316L | 2.3 |
| A182 F321 | 2.4 |
| A182 F347 | 2.5 |
| F51 · F53 · F55 (duplex) | 2.8 |
| B564 N04400 (Monel 400) | 3.4 |
| B564 N06600 (Alloy 600) | 3.5 |
| B564 N08825 · N06625 · B462 / B564 N10276 | 3.8 |
Group numbers from ASME B16.5 Table 1A. Take pressures from the edition your project uses.
Temperature limits and notes
Some materials carry limits in the table notes. B16.5 does not permit N08825 (Alloy 825) above 538 °C. Low-temperature service needs impact-tested materials such as A350 LF2, and chrome-moly groups (1.9, 1.10) are used for hot hydrocarbon and steam service, where welds usually need post-weld heat treatment.
Duplex grades (group 2.8) are strong, so they rate well at ambient temperature, but the group 2.8 table stops at about 316 °C (600 °F) because duplex steels embrittle at moderately high temperatures — check the table and your code.
Above 24 inches: B16.47
B16.5 stops at NPS 24. Larger flanges follow ASME B16.47, in Series A (MSS SP-44 pattern, heavier) and Series B (API 605 pattern, lighter). The series do not bolt together, so name the series on the order. European projects use EN 1092-1 with PN ratings, which are not interchangeable with ASME classes.
What to put on a flange order
- Type — weld neck, slip-on, blind, socket weld, lap joint
- Size, class and facing (RF, RTJ, FF)
- Bore / schedule for weld neck and socket weld flanges
- Material standard and grade, e.g. A182 F316L or B564 N08825
- Dimensional standard and edition (B16.5, B16.47 Series A/B, EN 1092-1)
- Certificate (EN 10204 3.1 or 3.2) and any NACE MR0175 requirement