Does a Class 150 flange mean 150 psi?
A Class 150 flange does not hold 150 psi. Cold, it holds considerably more than that. Hot, it holds considerably less. The number is a label on a family of ratings, and the only way to use it is to look up the rating at your design temperature.
What the numbers actually are
For Group 1.1 material — ordinary A105 carbon steel, which covers most of a plant — the ratings run like this:
| Temperature | Class 150 | Class 300 | Class 600 |
|---|---|---|---|
| 38 °C | 19.6 bar g | 51.1 bar g | 102.1 bar g |
| 150 °C | 15.8 bar g | 45.1 bar g | 90.2 bar g |
| 250 °C | 12.1 bar g | 41.9 bar g | 83.9 bar g |
| 300 °C | 10.2 bar g | 39.8 bar g | 79.6 bar g |
| 400 °C | 6.5 bar g | 34.7 bar g | 69.4 bar g |
Class 150 starts at 19.6 bar — about 285 psi, nearly double its own name — and falls to 6.5 bar at 400 °C, which is well under half of it. Somewhere around 200 °C the curve passes through 150 psi, and that crossing point is the whole of the coincidence.
The number is historical. The old “pound ratings” were set against saturated steam service, and a Class 150 flange was one suited to roughly 150 psi steam at its corresponding saturation temperature. That convention long outlived the reasoning behind it, which is why the label survives and the arithmetic does not.
Class 150 is the one that falls off a cliff
Look along the rows rather than down them. Between 38 °C and 400 °C, Class 150 loses 67% of its rating. Class 600 over the same span loses 32%.
That difference matters when somebody proposes to “save money on flanges” in a hot line. The higher classes are not merely stronger — they are flatter. A Class 150 joint that looked generous on a cold commissioning water fill can be marginal at operating temperature, and the margin disappears fastest exactly where the process is most demanding.
Three things that change the table
The material group. The table above is Group 1.1. Austenitic stainless sits in a different group with its own curve — slightly lower at ambient, and noticeably better retained when hot. Reading a carbon steel rating for a stainless flange, or the reverse, gets you the wrong number in a direction you cannot predict without looking.
Flange type and facing do not change the pressure–temperature rating, but they do change what the joint will actually survive. A rating assumes the joint is made up correctly with the right gasket and bolting — which is the subject of gaskets and bolting, and where most flanged joints are actually lost.
Bolting material. The ratings assume bolting of adequate strength. Low strength bolting carries a reduced rating of its own, and substituting a bolt grade on site is not a like-for-like swap.
How to use it
Read the design temperature first, then the class, then the table — in that order. The class on its own answers nothing.
If you are specifying rather than checking, the useful habit is to look at the rating at temperature and at the next class up, and see what the step costs. Across a Class 150 to Class 300 jump the rating roughly triples. Flanges are rarely the expensive part of a line, and the hot end of the curve is not where to be economical.
There is more on where the classes come from and how a line class pins them down in flanges and pressure classes and reading a line class.