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.

Understanding Flange Pressure Class (ASME B16.5)

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

Pressure and Temperature Rating of Carbon Steel Pipe Flange

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

Class 150 vs 300 vs 600 Flange – Physical Comparison

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.

How to Choose Between Class 150, 300 and 600 Flanges

• Class 150 flange is commonly used for low-pressure and low-temperature services such as water lines, HVAC, and general utility piping.
• Class 300 flange is suitable for moderate pressure and temperature industrial applications.
• Class 600 flange is selected for high-pressure, high-temperature, and critical service piping systems.

Selecting the correct flange pressure class ensures safety, performance, and long-term reliability of the piping system.

Frequently Asked Questions

1. What is a Class 150 flange used for?

A Class 150 flange is typically used in low-pressure and low-temperature piping systems such as water, HVAC, and general utility services.

2. What is the difference between Class 300 and Class 600 flange?

The difference between Class 300 flange and Class 600 flange lies in pressure and temperature capability. Class 600 flanges are thicker and designed for higher-pressure industrial service.

3. Does Class 150 mean 150 psi?

No. Class 150 does not indicate 150 psi. Flange pressure class is defined by ASME B16.5 and varies based on material type and operating temperature.

4. How do I select the correct flange pressure class?

To select the correct flange pressure class, consider operating pressure, temperature, material grade, and applicable ASME standards.