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Key takeaways
- ASTM A106 Gr. B is seamless carbon steel pipe for high-temperature service and the workhorse of process plants.
- ASTM A333 Gr. 6 is impact-tested carbon steel pipe for low temperatures, commonly down to −45 °C.
- Pipe, fittings and flanges come in matching families: A106 with A234 WPB and A105; A333 Gr. 6 with A420 WPL6 and A350 LF2.
The common pipe grades
ASTM A106 Gr. B: seamless carbon steel pipe for high-temperature service. It is the default for most plant process lines.
ASTM A53 Gr. B: seamless or welded pipe for general use. It is common in utility services, but many oil and gas specifications prefer A106 for process lines.
API 5L Gr. B and X grades: line pipe, widely used for pipelines and also for plant piping. Dual-certified pipe (A106 B / A53 B / API 5L B) is common and simplifies purchasing.
ASTM A333 Gr. 6: seamless or welded pipe with Charpy impact testing at low temperature, commonly −45 °C. It is used for cold services, depressurisation lines and places where low ambient temperatures or auto-refrigeration can occur.
ASTM A671 / A672: electric-fusion-welded pipe made from plate, used for large diameters where seamless pipe isn’t available.
Matching families
Each pipe grade has matching fittings and flanges with the same properties.
| Service | Pipe | Butt-weld fittings | Flanges and forgings |
|---|---|---|---|
| General / high temperature | A106 Gr. B | A234 WPB | A105 |
| Low temperature | A333 Gr. 6 | A420 WPL6 | A350 LF2 Class 1 |
Why low temperature matters
Carbon steel becomes brittle when it is cold. B31.3 has a minimum temperature curve for each material, and below it the steel must be impact tested. Low-temperature cases are easy to miss. Examples are gas blowdown (depressurisation cools the gas sharply), relief valve outlets, LPG flashing across a valve, and winter ambient conditions in cold regions. Process should give a minimum design metal temperature for every line.
Onshore
Onshore plants in cold climates often use A333 Gr. 6 for outdoor lines because winter ambient temperatures fall below the A106 curve.
Offshore
Offshore, low-temperature carbon steel is common for flare, blowdown and gas lines downstream of pressure letdown, where depressurisation can cause very low temperatures.
Code reference
ASTM A106 Seamless carbon steel pipe for high-temperature service.
ASTM A333 Seamless and welded steel pipe for low-temperature service.
ASME B31.3, para. 323.2.2 Lower temperature limits and impact testing requirements.
What to take back to your desk
For every carbon steel class, check that the minimum design metal temperature is stated and that the pipe, fittings, flanges and bolting are all qualified to that temperature.
Educational content. Always design to your project specification.
