On this page 10 sections
What do flange pressure classes mean?
Short answer
Flange pressure classes, 150, 300, 600, 900, 1500 and 2500 under ASME B16.5, set the pressure a flange can carry. The allowable pressure falls as temperature rises and depends on the material group, so a Class 150 carbon steel flange carries less pressure at 300 °C than at ambient.
Types
Pressure classes
ASME B16.5 classes (150, 300, 600, 900, 1500, 2500) are rating designations. The actual allowable pressure depends on material group and temperature. Class 150 carbon steel isn't limited to 150 psi at ambient temperature.
Facings
Raised face (RF) with spiral-wound gaskets is standard. Flat face (FF) mates with cast-iron or non-metallic flanges. Ring-type joint (RTJ) is used for high pressure and temperature.
Specifying
Example: 6" Class 300 WNRF, bore Sch 40, ASTM A182 F316L, ASME B16.5.
Choosing the flange type
| Type | Best for |
|---|---|
| Weld neck (WN) | High pressure, temperature and cyclic loads |
| Slip-on (SO) | Low-pressure, easy alignment |
| Socket weld (SW) | Small bore, high pressure |
| Threaded | Small bore where welding is not allowed |
| Lap joint | Expensive alloy lines with a carbon steel backing flange |
| Blind | Closing line ends and nozzles |
Large diameters
Above NPS 24, flanges follow ASME B16.47 Series A or B, which are not interchangeable. State the series on the order. See ASME B16.5.
Related guides, comparisons and articles
Frequently asked questions
What is the difference between RF and RTJ facing?
Raised face (RF) seals with flat gaskets; ring-type joint (RTJ) uses a metal ring in a groove for high pressure and temperature.
Do stainless flanges have the same ratings as carbon steel?
No. Ratings depend on the material group in B16.5, so stainless and carbon steel flanges of the same class have different pressure-temperature ratings.
Can I bolt a Class 150 flange to a Class 300 flange?
No. Bolt circles and dimensions differ between classes.
Which standards define pipe, fitting and flange dimensions?
ASME B36.10M and B36.19M for pipe, ASME B16.9 and B16.11 for fittings, and ASME B16.5 and B16.47 for flanges. EN and IS equivalents are used on some projects.
Does Bright Tubes supply the materials in this guide?
Yes. The grades discussed and related products are supplied from stock or mill-sourced, with EN 10204 certificates.