Temperature vs Flange Pressure Rating
Temperature is the single most important factor that reduces the pressure rating of a flange. how temperature affects flange pressure rating matters for safe piping design, particularly in steam, hot oil, and high-temperature process systems. As the service temperature increases, the yield and tensile strength of the flange material decrease, which directly reduces the maximum working pressure defined in the ASME B16.5 PT rating tables.
The Derating Mechanism
The PT rating tables in ASME B16.5 are derived from the allowable stress values of the flange material at each temperature. As temperature rises:
- The yield strength of the steel decreases
- The tensile strength decreases
- For temperatures above approximately 700°F, time-dependent creep strength becomes the governing factor
- The maximum allowable pressure is proportionally reduced
Temperature Effects on Class 300 Flanges
| Temperature | A105 (psig) | A105 % of Ambient | F316 (psig) | F316 % of Ambient |
|---|---|---|---|---|
| Ambient (100°F) | 740 | 100% | 720 | 100% |
| 200°F | 675 | 91% | 620 | 86% |
| 400°F | 635 | 86% | 530 | 74% |
| 600°F | 550 | 74% | 480 | 67% |
| 800°F | 410 | 55% | 430 | 60% |
| 1,000°F | 170 | 23% | 345 | 48% |
At 1,000°F, an A105 flange retains only 23% of its ambient rating, while F316 retains 48%.
When Materials Behave Differently
The rate at which temperature reduces the pressure rating depends on the metallurgy of the flange material. The table below shows the crossover points where stainless steel begins to outperform carbon steel.
| Comparison | Crossover Temperature | Explanation |
|---|---|---|
| A105 vs F304 (Class 300) | ~750°F | Above 750°F, F304 retains more pressure capacity |
| A105 vs F316 (Class 300) | ~700°F | F316 outperforms A105 above 700°F |
| A105 vs F11 (Class 300) | All temperatures | F11 alloy steel matches or exceeds A105 at every point |
Below the crossover temperature, carbon steel flanges actually have higher pressure ratings than austenitic stainless steel flanges of the same class, because A105 has higher ambient-temperature yield strength than F304 or F316.
Practical Examples
Steam system at 500°F and 400 psig: A Class 300 A105 flange is rated at 600 psig at 500°F, providing a 50% margin. Class 150 (rated at 170 psig at 500°F) is clearly insufficient.
Hot oil loop at 750°F and 300 psig: A Class 300 A105 flange is rated at 505 psig, adequate with a 68% margin. However, if the design temperature were raised to 850°F, the A105 rating drops to 270 psig—below the design pressure—and either a higher class or alloy material would be required.
Cryogenic service at -50°F: Temperature below -20°F does not increase the pressure rating. The ambient-temperature rating applies for all temperatures at or below 100°F. However, the flange material must be suitable for low temperatures (e.g., A350 LF2 instead of A105).
For complete PT rating tables, see the ASME flange rating guide.
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