Pressure class, PN and schedule are three different numbers that often appear in the same RFQ – and they describe different things. Treating a flange class as a pipe wall thickness, or a PN rating as an ASME class, leads to wrong quotations and mismatched components.
What Each Number Means
| Term | Used for | What it describes |
|---|---|---|
| Pressure class (Class 150, 300, 600 …) | ASME flanges, valves, forged fittings | A pressure–temperature rating group defined in the standard (for example ASME B16.5); the allowable pressure falls as temperature rises |
| PN (PN 10, 16, 40 …) | EN / ISO flanges and valves | A nominal pressure designation in the EN system (for example EN 1092-1); also temperature- and material-dependent |
| Schedule (Sch 10S, 40, 80 …) | Pipe and buttweld fittings | A wall-thickness designation for a given pipe size (ASME B36.10 / B36.19) |
Why They Are Separate RFQ Fields
- A Class 300 flange can be welded to Sch 40 or Sch 80 pipe – the class does not set the pipe wall.
- The weld neck flange bore must match the pipe schedule, which is a separate field from the flange class.
- PN and ASME classes are not direct equivalents; flanges from the two systems generally do not mate.
- Forged fitting classes (Class 3000, 6000) are a different scale from flange classes (Class 150–2500).
A Clean Line Item
Write each value in its own field: Weld neck flange, 4", Class 300, RF, bore to Sch 80, ASTM A105, ASME B16.5; Pipe, 4", Sch 80, ASTM A106 Gr B. If the project uses EN components, state the PN and the EN standard instead of an ASME class.
Related: weld neck flange bore vs pipe schedule and ASME B16.5 vs B16.47 ordering.
Frequently Asked Questions
Is Class 300 the same as PN 40?
No. They are designations in two different systems (ASME and EN) with their own pressure–temperature tables and dimensions. They are not direct equivalents and the flanges generally do not mate.
Does flange class determine pipe schedule?
No. The flange class is a pressure–temperature rating; the pipe schedule is a wall thickness. A weld neck flange bore is then matched to the chosen schedule.
Is a Class 3000 fitting the same as a Class 300 flange?
No. Forged fitting classes under ASME B16.11 use a different scale from flange classes under ASME B16.5.
