Pressure classes: ASME, PN and forged fittings
"Class 150", "PN 16" and "class 3000" look like the same thing, but they measure different things. See what each one means and how much it withstands.
ASME class (flanges and valves)
- Used in ASME B16.5 and B16.47 flanges and in ASME B16.34 valves.
- B16.5 has seven classes: 150, 300, 400, 600, 900, 1,500 and 2,500. B16.47 goes from class 75 (Series B only) to 900, and B16.34 (valves) reaches 4,500.
- The class is not the pressure: the allowable pressure depends on the material group and falls with temperature.
| Class | −29 to 38 °C | 100 °C | 200 °C | 300 °C | 400 °C | Approximate PN |
|---|---|---|---|---|---|---|
| 150 | 19.6 | 17.7 | 13.8 | 10.2 | 6.5 | PN 20 |
| 300 | 51.1 | 46.6 | 43.8 | 39.8 | 34.7 | PN 50 |
| 400 | 68.1 | 62.1 | 58.4 | 53.1 | 46.3 | PN 68 |
| 600 | 102.1 | 93.2 | 87.6 | 79.6 | 69.4 | PN 100 |
| 900 | 153.2 | 139.8 | 131.4 | 119.5 | 104.2 | PN 150 |
| 1,500 | 255.3 | 233.0 | 219.0 | 199.1 | 173.6 | PN 250 |
| 2,500 | 425.5 | 388.3 | 365.0 | 331.8 | 289.3 | PN 420 |
For other materials (stainless, duplex, Cr-Mo), see the pressure table on each B16.5 class page. Groups available: Carbon steel A105 / A350 LF2, Stainless F304 / F304H, Stainless F316 / F316H, Duplex F51, super duplex F53 / F55 (up to 315 °C), 6Mo F44, Inconel 625, Incoloy 825, Hastelloy C-276 / C-22.
PN (European standard)
- Used in EN 1092-1 and DIN flanges: PN 6, 10, 16, 25, 40, 63, 100, 160.
- The number is the nominal pressure in bar at 20 °C for the reference material. It also falls with temperature.
- The equivalence to ASME classes (table above) is approximate and does not guarantee that the flanges mate: bolt patterns and bolts are different.
Classes 2000, 3000, 6000 and 9000 (forged fittings)
In ASME B16.11 forged fittings, the class indicates the matching pipe:
| Class | Threaded (NPT) | Socket weld (SW) |
|---|---|---|
| 2000 | SCH 80 | – |
| 3000 | SCH 160 | SCH 80 |
| 6000 | XXS | SCH 160 |
| 9000 | – | XXS |
Class 2000 exists only threaded; class 9000 only socket weld. A threaded fitting loses wall thickness at the thread, which is why it needs the next class up. See the schedule converter and the pipe threads guide.
Class 800 in valves
Small forged valves (API 602, generally up to NPS 2) also use class 800, between 600 and 900. It exists only for valves; there is no class 800 flange.
Frequently asked questions
Is class 150 the same as 150 psi?
No. The class is a designation, not the pressure. In A105 carbon steel, class 150 allows 19.6 bar (285 psi) at 38 °C and drops to 14 bar at 200 °C. The actual pressure depends on material and temperature (ASME B16.5 tables).
Are 150 lb and class 150 the same thing?
Yes. "150 lb" or "150#" is the old name for class 150. The current standard uses only "Class 150".
Does a class 150 flange mate with a PN 16 flange?
Generally not. The systems have different bolt circles, bolt counts and bolt diameters. Some dimensions coincide, but check the drawing before assembling.
What are classes 3000 and 6000 in forged fittings?
They are the classes of ASME B16.11 forged fittings (socket weld and threaded couplings, elbows, tees). The class indicates which pipe wall thickness the fitting matches, not the pressure: a socket weld class 3000 fitting works with SCH 80 pipe; a threaded one, with SCH 160.
