The Difference Between Class 150 and Class 300 Flanges

This is the comparison made most often when choosing a flange class. Both exist in the same nominal sizes, yet they are not interchangeable.

Dimensional differences

At the same nominal size, a Class 300 flange has a thicker body, a larger outside diameter, a wider bolt circle and usually larger bolts than Class 150. Bolt count may increase as well, which is why the two classes do not bolt together.

Pressure temperature difference

For A105 carbon steel, roughly: Class 150 carries about 19 bar at ambient, Class 300 about 50 bar. Capacity falls for both as temperature rises, but proportionally faster for Class 150. Around 200 °C the Class 150 rating drops markedly, which is why steam lines often use Class 300.

Exact values are read from the standard table by material group and temperature.

Which line gets which

• Class 150: Water, air, low pressure general service, utility lines
• Class 300: Steam, hydrocarbons, medium pressure process lines, equipment connections
• Class 600 and above: High pressure, high temperature and critical service

Cost

Class 300 means more material, so the flange price rises roughly 1.5 to 2 times, with larger gaskets, larger studs and longer installation on top. Raise the class where it is needed rather than lifting a whole line "to be safe".

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FAQ

Can Class 150 and Class 300 be bolted together?
No. Their bolt circles and often bolt counts differ.

How many bar does Class 150 carry?
About 19 bar at ambient for A105; the value falls with temperature and varies by material group.

When should Class 300 be used?
When pressure or temperature pushes the Class 150 rating, and on steam and hydrocarbon lines.