How does temperature affect a flange pressure rating?
The allowable pressure of a flange falls as temperature rises, because the allowable stress of the material falls. An ASTM A105 carbon steel Class 150 flange is rated about 285 psig at 100 °F but only about 140 psig at 600 °F. The class number is a designation, not a working pressure.
Key Takeaways
- A pressure class is a rating designation, not a pressure in psi — Class 150 does not mean 150 psi.
- Allowable pressure falls continuously as temperature rises, following the material’s allowable stress curve.
- Material group changes the rating: A105 carbon steel and F316 stainless of the same class and size do not carry the same pressure at temperature.
- Stainless holds its rating better at high temperature but starts lower than carbon steel at ambient.
- The design point is the coincident pressure and temperature, not the maximum of each taken separately.
- Always rate against the current edition of ASME B16.5 for the specific material group.
The chart
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| Temperature | Cl.150 A105 | Cl.300 A105 | Cl.600 A105 | Cl.150 F316 | Cl.300 F316 | Cl.600 F316 |
|---|---|---|---|---|---|---|
| 100 °F / 38 °C | 285 | 740 | 1,480 | 275 | 720 | 1,440 |
| 200 °F / 93 °C | 260 | 675 | 1,350 | 235 | 620 | 1,240 |
| 300 °F / 149 °C | 230 | 655 | 1,315 | 215 | 560 | 1,120 |
| 400 °F / 204 °C | 200 | 635 | 1,270 | 195 | 515 | 1,025 |
| 500 °F / 260 °C | 170 | 605 | 1,215 | 180 | 480 | 955 |
| 600 °F / 316 °C | 140 | 570 | 1,140 | 170 | 450 | 905 |
| 700 °F / 371 °C | 110 | 530 | 1,060 | 160 | 430 | 860 |
| 800 °F / 427 °C | 80 | 410 | 825 | 155 | 415 | 825 |
Reading the chart correctly
Enter the table at the design temperature, not at ambient, and read the allowable pressure for the class and material group in question. The design condition is the coincident pressure and temperature — the pressure that exists when the metal is at that temperature.
Taking the maximum pressure and the maximum temperature independently and checking each against the table is a common and unconservative error, because it can pass a combination the flange would not actually survive.
Note how the two material groups cross over. At ambient, A105 carbon steel is rated slightly higher than F316 stainless. By 600 °F the position has reversed, because carbon steel loses allowable stress faster. Selecting on ambient figures alone therefore gives the wrong answer on any hot line.
What the rating does not cover
The pressure-temperature rating is a flange rating. It does not certify the joint. A joint also depends on the gasket, the bolting and the assembly, and any of the three can fail below the flange rating.
Gasket seating stress has to be achievable with the bolting fitted — see the stud bolt torque chart. Bolting grade has to suit the temperature: A193 B7 is not suitable at cryogenic temperature, where A320 L7 is specified instead. And on sour service the bolting is hardness-limited to B7M, which reduces achievable bolt stress.
The rating also assumes the material is in the condition the specification requires. An A105 flange supplied as-forged where A105N normalised was specified does not carry the tabulated rating in the way the designer assumed.
Frequently Asked Questions
No. The class is a rating designation. A Class 300 flange in ASTM A105 carbon steel is rated about 740 psig at 100 degrees Fahrenheit, falling to about 570 psig at 600 degrees Fahrenheit.
Austenitic stainless has a lower allowable stress than carbon steel at ambient temperature. It retains that stress far better as temperature rises, so above roughly 500 degrees Fahrenheit the stainless rating overtakes the carbon steel rating.
The design temperature of the metal coincident with the design pressure. Using maximum pressure and maximum temperature independently can pass a combination the flange cannot carry.
The flange rating is independent of the gasket, but the joint is not. The gasket must seal at the design condition and the bolting must be able to develop the required seating stress.