PN vs ASME Class: Conversion Chart and Why PN16 Is Not Class 150

The Short Answer

There is no exact conversion between PN and ASME Class. Any table you find, including the one below, is an approximation for orientation only. The two systems were built on different logic:

• PN (Pression Nominale) is a nominal pressure in bar, referenced to a base temperature of 20 degrees C. It comes from the European EN, DIN and ISO family.
• ASME Class is a rating category, not a pressure. The allowable pressure at any given temperature is looked up in ASME B16.34 tables and depends on the body material group.

Treating them as interchangeable is one of the most common and most expensive specification errors in cross-border projects.

The Two Competing PN Systems

This is the source of most confusion, and it is rarely explained clearly.

System one: true EN 1092-1 PN series. These are the pressure steps actually defined in European flange and valve standards:

PN 2.5, PN 6, PN 10, PN 16, PN 25, PN 40, PN 63, PN 100, PN 160, PN 250, PN 320, PN 400

System two: ISO 7005 metric designations for ASME classes. When ASME classes are expressed in metric form, they are given PN numbers that approximate their bar value:

PN 20, PN 50, PN 68, PN 110, PN 150, PN 260, PN 420

Notice these two lists barely overlap. PN20, PN50, PN110, PN150, PN260 and PN420 are not EN pressure steps at all. They are metric labels for Class 150, 300, 400, 600, 900, 1500 and 2500.

So when a supplier quotes "PN20", they mean an ASME Class 150 component described in metric. When they quote "PN16", they mean a genuinely European component. These are different parts.

Approximate Conversion Table

Values shown are for ASME B16.34 material group 1.1 (A105 forged carbon steel, WCB castings) at 38 degrees C. Read the "nearest EN step" column with care: in most rows the European step is lower than the ASME class, meaning a straight substitution downgrades your pressure envelope.

Why PN16 Is Not Class 150: The Dimensional Proof

Even setting pressure aside, the flanges do not fit. Take DN100 / NPS 4 as a worked example:

The bolt circles differ by 10.5 mm between ASME Class 150 and PN16. The bolts will not line up. No amount of gasket compression fixes a mismatched bolt circle.

Note also that GB/T 9113 PN16 matches EN 1092-1 PN16 exactly at this size. Chinese metric flanges follow the European geometry, not the American one. If your project is ASME-based, a Chinese PN16 valve will not mate to your piping regardless of pressure adequacy.

Material Group Changes the Rating

A second trap: the same ASME Class delivers different pressures depending on body material.

Stainless steel is not stronger in this context. Its allowable stress at elevated temperature falls off differently, and B16.34 places it in a lower-rated group at ambient. Engineers who assume "stainless is always better" occasionally under-specify without realising it.

Temperature Is the Missing Variable

Both systems derate with temperature, but from different baselines:

• PN ratings are referenced to 20 degrees C.
• ASME Class ratings are referenced to 38 degrees C (100 F).

A Class 150 carbon steel valve rated 285 psi at ambient drops to roughly 200 psi at 204 degrees C, and continues falling. Any comparison made at ambient becomes invalid the moment your service temperature rises. Our pressure-temperature rating chart covers the derating curves in detail.

A Practical Decision Rule

1. Never substitute across systems on the basis of a conversion table alone. Use the table to shortlist, then verify against the actual standard.
2. Match the flange standard to the piping standard, not to the pressure. If the line is ASME B16.5, the valve must be ASME B16.5.
3. When a rating falls between two steps, go up, not down. Class 600 sits between PN100 and PN110; specify PN160 if the European alternative must carry the same duty.
4. Confirm the body material group before accepting any pressure figure.
5. Ask the supplier which PN system they mean when you see PN20, PN50 or PN110. Those numbers signal an ASME part described in metric.

Cross-Checking Real Products

Pressure class, body material and end connection standard for products across ninety-plus manufacturers are catalogued in the ValveSpecs Pro database, which makes it straightforward to confirm whether a quoted valve genuinely meets an ASME or EN requirement.

If you are reading a Chinese supplier quotation, the pressure unit inside the model number follows a separate convention. See our Chinese valve model number decoding guide.