Rule of thumb · applies

Class 150 is not 150 psi – what the ASME pressure class really means

Class 150 does not mean 150 psi.

This rule of thumb does not replace the design calculation or the applicable standard.

Category: robust rule
Standards referenced: ASME B16.5 Table 2-1.1 (Group 1.1), ASME B16.34

Rule of thumb

The number after "Class" is a pressure-temperature class, not a pressure limit.

Where does the rule come from?

The number looks like a pressure, and in the early days of American flange standardisation (ASA B16, from the 1920s) it was indeed tied to a working pressure in psi at a defined reference temperature. Materials, strength values and temperature steps have been revised many times since; the class designation stayed – as a name, not as a number. Anyone who works out "Class 150 = 150 psi = about 10 bar" today is almost a factor of two off at room temperature – and right at 300 °C.

What is technically correct?

The class assigns each flange, per material group, a table of allowable pressure against temperature. The allowable pressure falls with temperature because the strength of the material falls. For material group 1.1 (carbon steel, e.g. A105) Class 150 allows 19.6 bar up to 38 °C – not the 10.3 bar that 150 psi would be –, 13.8 bar at 200 °C, 10.2 bar at 300 °C and only 6.5 bar at 400 °C. A flange of the same class in stainless steel (group 2.2, e.g. A182 F316) has a different curve: same connection, different allowable pressures.

Two things follow. First, "Class 150" without material and temperature is not a pressure statement. Second, the class does say something about the connection dimensions: two flanges of the same NPS and class fit together regardless of material – just like DN and PN in the EN world.

Where does the rule end?

The rule says what the number is not. What it is can only be found in the pressure-temperature table of the material group – which has to be opened, every time.
The table applies to the flange. The flanged joint (gasket, bolts, mating flange) may limit earlier.
Linear interpolation between two temperature steps is permitted; above the last step there is no value.
Class designations cannot be converted into PN. Class 150 and PN 16 are matched to each other in purchasing because they occur in similar plants – not because they mean the same pressure or fit together (they do not).

What does the standard say?

ASME B16.5 sets the pressure-temperature ratings in Section 2, one table per material group (Table 2-1.1 for group 1.1 with A105, A350 LF2, A216 WCB …). The classes are 150, 300, 400, 600, 900, 1500 and 2500. The standard explicitly calls the class number dimensionless. Valves follow the same system under ASME B16.34.

DIN/EN versus ASME/ASTM

Both worlds separate connection dimension from allowable pressure: EN 1092-1 via PN plus Annex G (one p/T table per material group), ASME B16.5 via class plus Section 2. The difference is in the numbers: PN 16 allows 16 bar at room temperature and is therefore "more honest" than Class 150, which allows 19.6 bar; from about 200 °C both are close (13.3 versus 13.8 bar for the carbon-steel groups). What they share: no statement without material and temperature. See the sister rule "PN 16 is not always 16 bar".

Worked example

Welding neck flange 2" Class 150 RF, A105 (core range): allowable 19.6 bar up to 38 °C, 17.7 bar at 100 °C, 13.8 bar at 200 °C, 10.2 bar at 300 °C (ASME B16.5, Table 2-1.1). Rejecting Class 150 for a hot-water line at 130 °C and 16 bar "because 150 psi is 10 bar plus margin" is wrong – the flange is allowed about 16.6 bar at 130 °C. Choosing Class 150 for steam at 250 °C and 13 bar "because 150 psi is enough" is wrong the other way – 12.1 bar allowable is below the requirement.

Zickwolff reference

No tradition attached to this rule. (Block stays empty on the page; O-Z hides it.)

Checked today

Traffic light: applies (class A). As a warning the rule is correct without restriction; the actual statement is in the p/T table of the material group.

Frequently asked questions

How many bar is Class 150?

It depends on material and temperature. For carbon steel (A105) it is 19.6 bar at room temperature, 13.8 bar at 200 °C, 6.5 bar at 400 °C – figures from ASME B16.5, Table 2-1.1.

Is Class 150 the same as PN 16?

No. The connection dimensions differ; the flanges do not mate. Both are matched to each other in purchasing because they occur in similar plants.

Then why is it called 150?

Historically the number was tied to a steam working pressure of about 150 psi; for decades it has been a class name only.

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