Article
One of the most common misunderstandings in piping is reading a flange's pressure class as its pressure rating. A Class 150 flange is not rated for 150 psi, and a Class 300 flange is not rated for 300 psi. The class is a designation; the working pressure depends on the flange material and the temperature.
What the class actually is
ASME B16.5 covers flanges up to 24 inches in seven classes: 150, 300, 400, 600, 900, 1500 and 2500. Each class has its own dimensions, bolting and, for every material group, a table of the maximum working pressure at each temperature. The class number is a label for that set of tables. It is related to pressure historically, but it is not the pressure itself.
An example with carbon steel
For the material group that includes ASTM A105 forged carbon steel, the ratings at ambient temperature (from minus 29 °C up to 38 °C) are approximately:
| Class | Approximate rating at ambient |
|---|---|
| 150 | 19.6 bar (285 psi) |
| 300 | 51.1 bar (740 psi) |
| 600 | 102.1 bar (1,480 psi) |
So a Class 150 carbon steel flange holds almost twice its "number" at room temperature. As temperature rises, the allowable pressure falls. At several hundred degrees Celsius the same Class 150 flange is rated far below its ambient value. Always read the rating at the actual design temperature from the current edition of the standard; the figures above are for orientation only.
Why material changes the rating
B16.5 sorts materials into groups by their strength behaviour with temperature. A stainless steel flange of the same class does not have the same ratings as a carbon steel one, and alloy steels such as F11 or F22 hold their strength better at high temperature. Two flanges that look identical and carry the same class can therefore be rated quite differently. This is why the material grade must be on the drawing, not only the class.
The weakest part sets the limit
A flanged joint is only as good as its gasket and bolting. The flange rating assumes suitable bolts and a suitable gasket. A spiral wound gasket and ASTM A193 B7 stud bolts are the usual pairing for carbon steel flanges in process service; a soft gasket or under-strength bolts can make the joint weaker than the flange.
How to specify a flange correctly
- Type: weld neck, slip-on, blind, socket weld, threaded or lap joint.
- Size (NPS) and pressure class.
- Facing: raised face, flat face or ring type joint.
- Material standard and grade, for example ASTM A105 or A182 F316L.
- For weld neck flanges, the bore or the matching pipe schedule.
- The dimensional standard: ASME B16.5 up to 24 inches, ASME B16.47 above.
See our weld neck flanges, slip-on flanges, and the full flange range.
Frequently asked questions
Does Class 150 mean 150 psi?
No. The allowable pressure depends on material group and temperature; Class 150 carbon steel is rated around 285 psi at ambient temperature.
Where are flange pressure-temperature ratings found?
In the pressure-temperature rating tables of ASME B16.5 (and B16.47 for larger sizes).
Does flange material affect the rating?
Yes. Each material group has its own pressure-temperature table.
Key takeaways
- Class is a rating designation, not a pressure.
- Allowable pressure falls as temperature rises.
- Always check the B16.5 table for your material group.
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