When you're shopping for flanges, valves and fittings, you're likely to see products identified by their ANSI rating. If you're not already familiar with the term, you might wonder what it means and how it affects the flanges you plan to purchase. As it turns out, the ANSI rating is very important when it comes to picking the right flange and is something you should always research before purchasing.


The ANSI rating of a flange, valve or fitting can help ensure you purchase the right flange given the material and the maximum pressure and temperature to which it will be exposed.

What Is ANSI?

The American National Standards Institute, or ANSI, is an independent nonprofit organization that helps oversee the standards of many products, services, processes, systems and personnel in America. The main goal of the organization is to promote and facilitate "voluntary consensus standards and conformity assessment systems" and to safeguard their integrity in order to enhance global competitiveness of American businesses and the American quality of life.

How Pressure Rating Works

A stainless steel pipe that is 1/4-inch thick will not be able to sustain as much pressure as a 1/2-inch thick piece of stainless steel pipe of the same length. A pressure rating states the maximum allowable stress value of a material at a specific temperature to help ensure the flange will not fail when used under its maximum temperature and pressure combination.

There are many standards when it comes to flanges, valves and fittings, but ANSI pressure ratings are perhaps the most popular set of standards. When discussing this standard, people may use the terms "pressure rating," "class," "#," "lb" or "lbs," as these are all interchangeable and simply refer to the concept of the pressure/temperature performance of a flange. This pressure/temperature performance is important because it helps you choose the right flange for your given conditions. There are seven ANSI flange pressure ratings: 150, 300, 400, 600, 900, 1500 and 2500.

To understand what this means, imagine you have multiple flanges made from the same material and of the same bore size. The lower the flange rating, the thinner, lighter and less sturdy the flange will be. While a class-150 carbon steel flange heated to a temperature of 600 degrees Fahrenheit can withstand only 140 pounds per square inch of pressure, a class-300 flange with all the same specs otherwise will be able to withstand 570 psi. A class-2500 flange with the same material and bore size can bear 34 times the pressure of the 150-class flange, withstanding up to 4730 psi.

Selecting the Right ANSI Rating

As temperatures go up, the maximum allowable pressure will go down, so it is important to know the maximum temperature and pressure to which your flange will need to hold up before selecting the right flange. Once you have that information, you can look up a flange-rating chart online for the material with which you want to work and then search on the left side of the table for the maximum temperature to which your flange will be subjected.

From there, follow the flange-rating table to the right until you get to the lowest number that is still higher than the maximum pressure you'll need. Once you have that number, follow the table up until you get to the row stating the correct ANSI pressure class.

Alternatively, you can look for a flange pressure-rating calculator that will allow you to enter your desired material and your maximum temperature and pressure before automatically pulling up the correct ANSI rating for your flange.