The Short Answer
PN and Class are two different ways of stating a valve’s pressure rating, and they are not directly interchangeable. Class ratings (150, 300, 600 and so on) come from the ASME/ANSI pressure-temperature system and define allowable pressure as a function of temperature. PN (nominal pressure) values such as PN10, PN16 and PN25 come from the European ISO/EN system and are based on a nominal reference pressure, with derating for temperature handled separately. Rough engineering approximations are often used — Class 150 is commonly matched with PN20, Class 300 with PN50 and Class 600 with PN100 — but there is no official precise conversion, and the correct answer for any project comes from comparing the pressure-temperature tables of the two standards for the actual service temperature.
What Class Ratings Mean
In the ASME/ANSI system, valve pressure ratings are expressed as Class numbers: 150, 300, 600, 900, 1500 and 2500. A Class is not a single pressure number — it is a designation tied to a pressure-temperature table. The allowable working pressure decreases as temperature rises, because material strength falls off at elevated temperature. The governing design standard is ASME B16.34 for pressure-rated valves, with the flange rating standard ASME B16.5 (Class 150 through 2500, up to a given size) providing the flange pressure-temperature data. When an engineer says a “Class 300 gate valve,” the rating is only fully defined when the temperature is also known.
What PN Ratings Mean
In the European system, valve ratings are expressed as PN values — PN6, PN10, PN16, PN25, PN40 and higher. PN originally stood for nominal pressure in bar, and the designation reflects a reference pressure at a defined reference temperature, typically with the rating based on standard materials and flange groupings under EN 1092 for flanges and EN 12516 / ISO 7005 series for pressure ratings. Like Class, the PN value is a designation that works with a pressure-temperature table rather than a single fixed number, so the allowable pressure at the actual service temperature must be read from the relevant standard’s table.
PN vs Class: Rough Correspondence and Real Differences
| ASME/ANSI Class | Common Approximate PN Match | Note |
|---|---|---|
| 150 | PN20 | Common approximate pairing; not an official conversion |
| 300 | PN50 | Approximate; verify with pressure-temperature tables |
| 600 | PN100 / PN110 | Approximate; depends on the flange standard edition |
| 900 | PN150 | Approximate; rarely a straight swap |
| 1500 | PN250 / PN260 | Approximate; verify for the exact size and material |
| 2500 | PN420 | Approximate; verify for the exact size and material |
These pairings are practical shortcuts used in the industry, not standard-defined equivalences (data needs further verification for your exact size, material and temperature). The real differences go deeper than the number: flange dimensions and bolt circles differ between the two systems, face-to-face lengths follow different series, and the temperature derating curves are not aligned. A valve built to PN25 does not automatically meet a Class 300 requirement even though the numbers look similar — the same flange compatibility logic applies as in ASME B16.5 vs B16.47 flanges.
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Application Example: A Project That Mixed the Systems
A plant in Southeast Asia specifies “gate valves, class 300” for a steam line at 260 °C, and a supplier proposes a “PN40” valve as an equivalent, arguing that both are rated for the service. The engineering team checks the pressure-temperature tables: at 260 °C, the Class 300 rating for the specified material is lower than the room-temperature figure, and the PN40 rating under EN 12516 at the same temperature is different again — in this case the PN40 valve’s flange bolt circle also did not match the ASME B16.5 flanges already in the line. The valve is rejected and the specification is corrected to require ASME B16.34 Class 300 with B16.5 flanges. The same cross-system confusion appears when a project moves between Chinese and US standards — see China valve standards vs ASME/API, where the marking language decides whether a valve fits the piping.
Common Misconceptions
The first misconception is that PN and Class can be converted with a fixed factor — they cannot, because both systems depend on temperature, material and the flange standard edition. The second is assuming a higher PN number always beats a lower Class number at the same nominal size; the comparison is only meaningful within one system at one temperature. The third is mixing flange systems in one line: an EN 1092 flange and an ASME B16.5 flange of the same nominal size are not interchangeable without adapters, so the rating question is inseparable from the flange question. The fourth is reading a rating off a nameplate without the temperature: a nameplate pressure is typically the reference value, and the allowable pressure falls with temperature.
Practical Recommendation
State the pressure rating in the specification together with the design standard, the flange standard and the maximum service temperature — a rating is not a standalone number. When a supplier offers a valve from the other system, ask for the pressure-temperature table for the actual material and temperature, and confirm the flange and face-to-face dimensions against the piping. For cross-system projects, decide one governing standard for the whole line and require all components to follow it. Where a conversion is unavoidable, treat the approximate pairings above as a starting point only, and have the final pressure-temperature comparison reviewed before purchase.
Conclusion
PN and Class describe pressure ratings in two different systems, and the shortcut numbers are approximations, not conversions. The correct comparison is always the pressure-temperature table for the actual material, temperature and flange standard. Specify the governing standard, verify the table, and check the flanges — and the two systems stop being a source of field failures.
This article provides general educational knowledge only and is not professional engineering advice.
Sources & Further Reading
ASME, EN, ISO
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Disclaimer: This article is provided for general educational and informational purposes only and does not constitute professional engineering advice. Valve applications vary with pressure, temperature, media, and applicable codes. Always verify specifications against the manufacturer’s data sheet and consult a qualified engineer before selecting, installing, or repairing any valve. Valve Knowledge Hub makes no warranties about the accuracy or completeness of the information on this site and assumes no liability for any use of it.