ASME or EN? Comparing standards for pipe, flanges and certificates
Europe builds to EN and DIN; much of the rest of the world builds to ASME and ASTM. On projects with an American specification the two worlds meet – and it gets expensive when someone equates PN 16 with Class 150. This article shows what really corresponds and what only looks alike.
The main pairs of standards
| Component | Europe (EN) | USA (ASME / ASTM) |
|---|---|---|
| Seamless tube, creep-resistant | EN 10216-2 | ASTM A106, A335 |
| Welded pipe | EN 10217 | ASTM A53, A671 |
| Stainless steel pipe | EN 10216-5, EN 10217-7 | ASTM A312 |
| Flanges | EN 1092-1 | ASME B16.5 (to NPS 24), B16.47 |
| Pipe fittings | EN 10253-2 | ASME B16.9 |
| Bolts and nuts | EN ISO 4014, EN 10269 | ASTM A193 Gr. B7, A194 Gr. 2H |
| Gaskets | EN 1514, EN 12560 | ASME B16.20, B16.21 |
| Pipe threads | EN 10226 (R / Rp) | ASME B1.20.1 (NPT) |
| Test certificate | EN 10204 – 2.2, 3.1, 3.2 | Mill Test Report (MTR) |
DN and NPS: almost the same
In the common sizes the outside diameters agree – DN 50 and NPS 2 are both 60.3 mm, DN 100 and NPS 4 both 114.3 mm. That is no coincidence: the European pipe series were aligned with the inch dimensions.
What differs is the wall thickness. In the ASME world it is described by a schedule number, in Europe directly in millimetres. Sch 40 at NPS 4 gives 6.02 mm – a wall that appears in no European preferred series. The basics are in DN, PN and schedule explained.
PN and Class: not the same
The most common and most expensive mistake: PN 16 is not Class 150. A Class 150 carbon steel flange takes around 19.6 bar at room temperature but loses capability with rising temperature considerably faster than a PN 16 flange. Above all: bolt circle and number of bolts do not match. In most nominal sizes the two flanges cannot be bolted together.
The sealing faces differ too. EN 1092-1 uses forms B1 and B2 with a fine serrated finish, ASME B16.5 the raised face with a roughness of roughly 125 to 250 µin AARH. An EN gasket on an ASME face may work – but it has not been tested for it. Which flange form is intended for what is set out under Flange types, the ASME dimensions under ASME B16.5.
Materials: similar is not equal
It is tempting to call P235GH simply “the European A106 Gr. B”. Chemically and mechanically the two are indeed close – but they are not equivalents. Yield strength curves, heat treatment requirements and scopes of testing differ.
| Application | Typical EN | Typical ASTM |
|---|---|---|
| Unalloyed boiler tube | P235GH, P265GH | A106 Gr. B, A192 |
| Creep-resistant, molybdenum-alloyed | 16Mo3 | A335 P1 |
| Creep-resistant, chromium-alloyed | 13CrMo4-5, 10CrMo9-10 | A335 P11, P22 |
| 9 % chromium | P91 | A335 P91 |
| Austenitic stainless | 1.4404, 1.4571 | TP316L, TP316Ti |
| Duplex | 1.4462 | S31803 / S32205 |
The comparison is for orientation, not a release for substitution. Which material is admissible is decided by the project specification – on European plant additionally by the Pressure Equipment Directive.
Certificates: same function, different system
In Europe the certificate to EN 10204 is ordered by type: 2.2, 3.1, 3.2. In the ASTM world people speak of the Mill Test Report. Functionally a good MTR corresponds to a 3.1 – with heat analysis, mechanical properties and heat treatment condition. Formally it is not one.
The practical consequence: anyone building under European law with a CE obligation cannot rely on an MTR alone. Ask for the certificate explicitly to EN 10204 – then the type is on the paper and the argument does not arise.
What a mixed order needs
- Fix the standard per item – not “flange DN 100 Class 150”, but either EN 1092-1 PN 16 or ASME B16.5 NPS 4 Class 150.
- Plan the transitions deliberately – where the two worlds meet you need a defined transition piece, not a compromise flange.
- Think of gasket and bolting – they follow the flange standard, not the pipe standard.
- Name the certificate type – EN 10204 3.1 or MTR, not both left unsaid.
We supply both worlds from one source: pipes to EN and ASTM, flanges and fittings to EN 1092-1 and ASME B16.5 / B16.9, with matching gaskets and bolting. Dimension tables are under Flange dimensions.
Frequently Asked Questions
Does PN 16 correspond to Class 150?
No. A Class 150 carbon steel flange takes around 19.6 bar at room temperature but loses capability with temperature considerably faster than a PN 16 flange. More importantly, bolt circle and number of bolts differ, so in most nominal sizes the flanges do not fit together.
Is DN 100 the same as NPS 4?
In outside diameter yes – both are 114.3 mm. The wall thickness, however, is described differently: by schedule number in the ASME world, directly in millimetres in Europe. The same outside diameter therefore does not mean the same wall or the same bore.
Is P235GH the same as A106 Gr. B?
No, only similar. Chemical composition and mechanical properties are close, but approvals, heat treatment requirements and scopes of testing differ. Substitution is admissible only where the project specification expressly allows it.
Does a Mill Test Report replace a 3.1 certificate to EN 10204?
Functionally an MTR usually contains the same information as a 3.1. Formally it is not a certificate to EN 10204. Where European law applies, the certificate should be ordered explicitly to EN 10204 with the type stated.
Can I mix EN and ASME components in one line?
Only at defined transition points with a suitable transition piece. Within one bolted joint, flange, gasket and bolting must follow the same standard – mixed joints are neither calculated nor tested for tightness.
Do you need exactly these parts?
Grade, size, pressure rating and quantity are enough – you usually receive your quote the same day, with an inspection certificate 3.1 on request.
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