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What is Flange Pressure Rating?

flanges pressure rating

“Flange rating” (or class) refers to the maximum pressure a flange can withstand at rising temperatures (in psi or bars). Flanges with a high rating (class) are more robust than flanges with low ratings because, at higher temperatures, they bear more pressure. Flanges made of various materials exhibit different pressure-temperature efficiency at the same rating. In the article, you’ll find the carbon, alloy, and stainless steel flanges pressure rating charts and some information on choosing the correct rating for your piping use.



ANSI / ASME B16.5 Specification

The pressure rating (also known as pressure class) is the highest permissible pressure a flange can handle at elevated temperatures. Also known as Maximum Allowable Working Pressure (MAWP), a pressure rating helps the engineer to decide which material would work best at any point in the pipe system. There are seven ratings of flange pressure according to the ANSI / ASME B16.5 specification: 150, 300, 400, 600, 900, 1500, and 2500.

The words “pressure rating,” “class,” “#,” “Lb”, or “Lbs” are synonymous, i.e. they all adhere to the same precise concept of the pressure/temperature performance of the flange (and other equipment such as valves, fittings, etc.). If we consider a pressure rating of 150, the safe working pressure for this pipe flange or valve at a specific temperature for a given material is 150 pounds per square inch. See below some of the common ways to refer to the pressure rating 150 valves;

  • Class 150# valve
  • 150-pound valve
  • 150-pressure rating valve
  • 150 lb valve
  • 150 class valve

Let’s understand this clearly with an example:

When two flanges have the same bore size (example 3 inches) and the same material (example SS 04). Still, both have different pressure ratings (one flange is class 150, the other 600), the lower-rated flange (class 150) will be smaller, lighter, and less durable than the higher-rated flange (class 600). This is shown in the picture:


So, how to choose the correct flange ratings?

  1. Find below the rating table specific to your flange (this depends on the flange material, as flanges with different material grades have different pressure ratings)
  2. Determine the maximum working temperature in your piping system
  3. Choose a rating based on the maximum pressure you expect at that particular temperature.



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