February 25, 2025
In industrial valve technology, choosing the type of flange is much more than just a matter of dimensions. It's a key element that determines the safety, compatibility, and compliance of a project.
In North America, as in Europe, flanges adhere to different standards: PN (Nominal Pressure) in Europe, ASA/Class (ASME) in North America, and ISO PN in certain petrochemical markets. Misinterpreting these standards can cause delays, additional costs, and disputes upon delivery.
This guide clarifies the major differences and offers practical markers for your specifications.
1. PN, ASA, ISO: three different logics
PN (Nominal Pressure) — European approach
The designation PN (e.g., PN 10, PN 25, PN 40) corresponds to a maximum pressure in bar at room temperature.
Example: PN 10 = 10 bar max at 20 °C.
However, caution: as the temperature increases, the mechanical strength of the flange decreases.
ASA / ASME Class — North American approach
In North America, ASME pressure classes are used (e.g., Class 150, 300, 600...).
These classes are not expressed in bar but define specific pressure/temperature ranges according to the material (carbon steel, stainless steel, alloy...).
Main reference: ASME B16.5 (flanges up to NPS 24) and ASME B16.47 (beyond).
ISO PN — a hybrid standard
Some petrochemical industries use ISO PN (PN 20, 50, 100...), similar to American classes but without perfect correspondence.
Indicative example:
ISO PN 20 ≈ Class 150
ISO PN 50 ≈ Class 300
ISO PN 100 ≈ Class 600
ISO PN 150 ≈ Class 900
ISO PN 250 ≈ Class 1500
ISO PN 420 ≈ Class 2500
👉 Remember: a PN 100 is not equivalent to an ISO PN 100. This confusion is common and must be clarified contractually.
2. Drillings and dimensional compatibility
Another classic trap lies in the drilling of flanges:
DN 15 to DN 150: identical drilling for PN 10 and PN 16.
DN 15 to DN 50: identical drilling for PN 10/16/25/40.
DN 65 to DN 150: identical drilling for PN 25 and PN 40.
This sometimes allows for mechanical interchangeability, but not pressure/temperature compatibility.
3. Types of flange faces
The type of face plays a crucial role in sealing:
RF (Raised Face): most common standard.
FF (Flat Face): used on cast iron or low-pressure applications.
RTJ (Ring Type Joint): special machining for high pressures/temperatures and hazardous fluids.
Stamped collar: to be avoided for sleeved valves and high pressures.
👉 For toxic fluids, high pressures, or high temperatures, RTJ or special machining faces are preferred.
4. Practical comparison PN ↔ ASA ↔ ISO
Standard | Example | Typical application |
|---|---|---|
PN (Europe) | PN 10, PN 40 | Water, heating, utilities |
ASA / ASME Class | Class 150, Class 300... | North America, heavy industries |
ISO PN (petrochemistry) | PN 20, 50, 100... | Refining, international projects |
👉 In North America, ASME B16.5 is the essential reference. The tables of correspondence are only used to translate a European specification, never to dimension.
5. Common risks and how to avoid them
PN / Class confusion
➡️ Risk: specifying PN 40 in a Canadian project → misunderstanding from the supplier.
✅ Solution: always translate to ASME Class for the NA market.
Mixing PN and ISO PN
➡️ Risk: believing that PN 100 = ISO PN 100 → non-compliant upon delivery.
✅ Solution: require written validation from the client and specify the exact standard.
Poor flange face choice
➡️ Risk: leakage or incompatibility with the gasket.
✅ Solution: specify the face (RF, RTJ, FF) and the associated gasket class in the specifications.
6. Best practices for purchasing and specification
Always start with ASME B16.5/B16.47 if the project is North American.
Translate European PNs to Class via an indicative table, but systematically check pressure/temperature curves.
Specify the flange face (RF, FF, RTJ) according to the fluid and pressure.
Document material certificates (NA compliant MTRs, EN 10204 accepted if equivalent).
Check local availability of gaskets and flanges to avoid downtime.
7. Practical cases
Municipal potable water project: Class 150 RF, epoxy-coated ductile iron body, FF gaskets.
High pressure steam project: Class 600 RF, steel body, spiral metal/graphite gaskets.
Refinery project: Class 900 RTJ, alloy steel body, API 6FA fire-safe requirements.
Conclusion
The choice and specification of flanges determine the safety, compliance, and durability of an installation.
👉 The right reflex in North America:
talk ASME Class rather than PN,
check the flange face and gasket compatibility,
clarify any mention of ISO PN before ordering.
