ASME B16.5 Flange Pressure Classes Explained

ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 to NPS 24. Larger sizes, NPS 26 to NPS 60, are covered by ASME B16.47 (Series A and Series B). Both standards group flanges into pressure classes. A common mistake is to read the class number as a pressure in psi. It is not: it is a rating designation, and the pressure a flange can actually hold depends on its material and the operating temperature.

The seven B16.5 classes

B16.5 defines Class 150, 300, 400, 600, 900, 1500 and 2500. Higher classes have thicker flanges, larger or more bolts and bigger bolt circles, so a Class 600 flange is not interchangeable with a Class 300 flange of the same pipe size.

Typical ratings for carbon steel

The table shows maximum working pressures at ambient temperature (−29 °C to 38 °C) for Material Group 1.1, which includes common forged carbon steel such as ASTM A105.

Always confirm against the pressure–temperature tables in the current edition of ASME B16.5 for your exact material group.

Why temperature matters

As temperature rises, the allowable stress of the material falls, so the pressure rating falls too. A Class 150 carbon steel flange that holds about 19.6 bar at ambient temperature is rated for much less at 400 °C. Stainless steel, alloy steel and duplex grades sit in different material groups with their own tables, so two Class 300 flanges in different materials can have different ratings at the same temperature.

Choosing the right class

1. Find the design pressure and design temperature of the line.
2. Identify the material group of the flange material.
3. Pick the lowest class whose rating at the design temperature is equal to or above the design pressure.
4. Check that the class matches connected valves and equipment, since bolting dimensions differ by class.

Need help matching a class to your conditions? Share your design pressure, temperature and material with our team.