Forged steel flanges, according to ASME B16.5, are made in seven primary ratings:

Class 150 300 400 600 900 1500 2500

The concept of flange ratings is relatively simple. A Class 300 flange can handle more pressure than a Class 150 flange, because a Class 300 flange are constructed with more metal and can withstand more pressure. However, there are a number of factors that can impact the pressure capability of a flange.

The Pressure Class or Rating for flanges will be given in pounds. Different names are used to indicate a Pressure Class.

For example: 150 Lb or 150 Lbs or 150# or Class 150