Difference between class 150, 300 & 600 Flange

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