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Key takeaways
- ASME B16.5 covers flanges NPS 1/2 to 24 in seven classes: 150, 300, 400, 600, 900, 1500 and 2500.
- The allowable pressure depends on the class, the material group and the temperature. It drops as temperature rises.
- For carbon steel (Group 1.1), Class 150 is rated 19.6 barg up to 38 °C and 13.8 barg at 200 °C.
Classes are not pressures
The class number is a name for a pressure–temperature rating table, not a pressure. A Class 300 flange in carbon steel is rated at 51.1 barg at ambient temperature, but only 43.8 barg at 200 °C and 34.7 barg at 400 °C.
Material groups
B16.5 groups flange materials with similar strength. The most common are:
- Group 1.1: common carbon steels such as A105 forgings, A216 WCB castings and A350 LF2 low-temperature forgings.
- Groups 1.2 to 1.17: other carbon and low-alloy steels, each with its own table.
- Groups 2.1 to 2.3: austenitic stainless steels (304, 316, 316L and their variants).
- Group 2.8: duplex and super duplex.
Each group has its own rating table.
Group 1.1 ratings (barg)
| Temperature | Class 150 | Class 300 | Class 600 | Class 900 | Class 1500 | Class 2500 |
|---|---|---|---|---|---|---|
| −29 to 38 °C | 19.6 | 51.1 | 102.1 | 153.2 | 255.3 | 425.5 |
| 100 °C | 17.7 | 46.6 | 93.2 | 139.8 | 233.0 | 388.3 |
| 200 °C | 13.8 | 43.8 | 87.6 | 131.4 | 219.0 | 365.0 |
| 300 °C | 10.2 | 39.8 | 79.6 | 119.5 | 199.1 | 331.8 |
| 400 °C | 6.5 | 34.7 | 69.4 | 104.2 | 173.6 | 289.3 |
Values between temperatures can be interpolated linearly. Use the flange rating tool on this site for any temperature and to find the lowest class for a given pressure.
Facings
- Raised face (RF): the most common, used with spiral wound and sheet gaskets.
- Ring type joint (RTJ): a groove for a metal ring gasket, used at high pressure and in some offshore and wellhead services.
- Flat face (FF): used when mating with cast iron or non-metallic flanges.
Beyond NPS 24 and beyond B16.5
- ASME B16.47 covers NPS 26 to 60 flanges, in Series A and Series B, which are not interchangeable.
- API 6A covers wellhead and Christmas tree flanges rated in psi (2000 to 20 000 psi).
- Compact flanges (NORSOK L-005) are used offshore to save weight.
Code reference
ASME B16.5 Pipe flanges and flanged fittings, NPS 1/2 through NPS 24.
ASME B16.47 Large diameter steel flanges, NPS 26 through NPS 60.
API 6A Wellhead and tree equipment.
What to take back to your desk
When you set a piping class rating, check the flange rating at the highest design temperature, not at ambient. That’s where most class selection mistakes happen.
Educational content. Always design to your project specification.
