What is the difference between Class 150, 300 & 600 flange

We get this question a lot from customers who are new to piping industry. Steel pipe flanges are classified as per ASME B16.5 standard. The pressure rating of flange ranges from 150# to 2500#. The term "lb", "class" and "#" are used interchangeably to designate the pressure rating of the flange. The fact is that 150 lb has no relation to 150 psi and so does the 300 or 600 lb does not correlate to 300 or 600 psi pressure rating. Pressure rating of the flange depends on the material (A105, stainless, nickel alloy etc.), the heat treat condition and pressure "class". Here we will use the term "class" to NOT confuse it with pressure "rating". So what is class 150 flange or ASME B16.5 class 150 flange, as the question raised by many customers.

First lets clarify the pressure rating of steel pipe flange. The table below shows the pressure and temperature rating of A105 carbon steel flange.

As you can see the pressure decreases with increasing temperature and has no real correlation with pressure "class".

Trupply has made a video using slip on flange to clarify the difference between different pressure classes of a flange. The video uses 2 inch slip on flange as an example. The table below shows the difference between 2 inch flange of class 150, class 300 and class 600.

If you have any question about ANSI Flange, ASME Flange, Flange Standards or Pressure Classification, email us at service@trupply.com. We believe in educating our customers so they can make an informed buying decision. You can visit our steel pipe flange collection page for convenient online shopping.