Class 150 Flange Temperature Limits
Class 150 flange temperature limits define how quickly the allowable working pressure of a Class 150 flange decreases as the service temperature rises. At ambient temperature, a Class 150 carbon steel flange is rated at 285 psig (19.6 barg), but at 500°F (260°C) this drops to just 170 psig (11.7 barg)—a 40% reduction. Understanding these limits is critical for utility steam, cooling water, and low-pressure process piping where Class 150 is the standard specification.
Class 150 PT Ratings by Material
The following table shows the maximum allowable pressure at each temperature step for Class 150 flanges in three common material groups.
| Temperature | A105 / A350 LF2 (Group 1.1) | A182 F304 (Group 2.1) | A182 F316 (Group 2.3) |
|---|---|---|---|
| -20 to 100°F (-29 to 38°C) | 285 psig (19.6 bar) | 275 psig (19.0 bar) | 275 psig (19.0 bar) |
| 200°F (93°C) | 260 psig (17.9 bar) | 235 psig (16.2 bar) | 240 psig (16.5 bar) |
| 300°F (149°C) | 230 psig (15.9 bar) | 215 psig (14.8 bar) | 215 psig (14.8 bar) |
| 400°F (204°C) | 200 psig (13.8 bar) | 195 psig (13.4 bar) | 195 psig (13.4 bar) |
| 500°F (260°C) | 170 psig (11.7 bar) | 170 psig (11.7 bar) | 170 psig (11.7 bar) |
| 600°F (316°C) | 140 psig (9.7 bar) | 140 psig (9.7 bar) | 145 psig (10.0 bar) |
| 700°F (371°C) | 110 psig (7.6 bar) | 125 psig (8.6 bar) | 130 psig (9.0 bar) |
| 800°F (427°C) | 80 psig (5.5 bar) | 110 psig (7.6 bar) | 115 psig (7.9 bar) |
| 1,000°F (538°C) | 20 psig (1.4 bar) | 55 psig (3.8 bar) | 60 psig (4.1 bar) |
Values from ASME B16.5-2020. Linear interpolation is permitted for intermediate temperatures.
When Class 150 Becomes Insufficient
| Service | Typical Design Conditions | Class 150 Sufficient? |
|---|---|---|
| Cooling water | 75 psig / 150°F | Yes |
| Utility air | 100 psig / ambient | Yes |
| Low-pressure steam | 50 psig / 300°F | Yes |
| Medium-pressure steam | 200 psig / 400°F | No (only rated 200 psig) |
| Hot oil | 150 psig / 600°F | No (only rated 140 psig) |
| High-temp process gas | 100 psig / 800°F | No (only rated 80 psig) |
When Class 150 is insufficient, the next standard class is Class 300, which provides approximately 2.5 to 4 times the pressure capacity depending on temperature.
For the complete PT rating tables across all pressure classes, refer to the ASME flange rating guide.
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