Flange Ratings and Flange Classes for ASME Flanges

In this article, I will explain to you about flange ratings. In definition, you can say that it is a combination of pressure class and temperature class. Or simply it is an ability of flange to withstand pressure at a given temperature. I have covered the following topics in this article to explain this.

  1. Definition of Pressure-Temperature Rating
  2. What is Temperature Rating?
  3. What is Pressure Rating?
  4. Flange Rating Equation
  5. Flange Class

While working with ASME/ANSI Flange, the terms such as Flange Ratings, Flange Pressure Ratings, Flange Class, pressure-temperature rating of flanges are quite confusing. In this article, I have tried to simplify these terms for a better understanding of young engineers.

What is Pressure-Temperature rating?

Pressure- temperature rating is the maximum allowable non-shock gauge pressure at the specific temperature for a given material. I know it is still confusing. Let me further break this term.

ASME B16.5 is a standard for Pipe Flanges and Flanged Fittings that covers flanges sizes from NPS ½” to 24”. In this standard, flanges are classified based on their pressure-temperature rating which is also known as a flange class.

What is Rating?

As per ASME B31.3, ratings are maximum allowable working gauge pressure at the given temperature and pressure class for applicable material.

Temperature Rating

Piping materials such as carbon steel, stainless steel, alloy steel have different mechanical and chemical properties. The same material can handle different amounts of stress at a different temperature. Based on the ability of a material to handle the stresses at a given temperature, ASME B31.3 has devolved the maximum allowable stress value of the material at a specific temperature and listed in Table A1.

In short, the reason behind establishing the temperature rating is to calculate the adequate wall thickness of the pipe, flange, and flanged fittings so that they can withstand the stresses due to pressure and other loads.

Piping Courses

Pressure Rating

The pressure rating is safe working or maximum operating pressure with respect to the working temperature. It depends on the materials’ Stress-Strain characteristics. It is available in different Codes and Standards.

Flange Ratings

ASME B16.5 has listed the Pressure-Temperature ratings for flanges. These ratings are established based on the prime factor of hydro testing of the flanged fittings to the bursting and by adding a factor of safety of 3.0 at the rated working pressure and ambient temperature.

ASME/ANSI B16.5 has established these temperature-pressure ratings by using Formula as given below and listed in the tabular form, for all materials at different flange ratings.

PT = (Pr x SI) / 8750

  • PT = rated working pressure in psig for specified material at temperature T.
  • Pr = Pressure rating as per Class in Psig.
  • SI = Selected stress in Psig for specified material at Temperature T.

Refer the sample table below;

Flanges pressure rating in psig for common carbon steel ASME flanges
( A105 / A350 LF2 / A350 LF3 / A350 LF6)
Temperature (F°) 150# 300# 400# 600# 900# 1500# 2500#
-20 to 100 285 740 985 1480 2220 3705 6170
200 260 680 905 1360 2035 3395 5655
300 230 655 870 1310 1965 3270 5450
400 200 635 845 1265 1900 3170 5280
500 170 605 805 1205 1810 3015 5025
600 140 570 755 1135 1705 2840 4730
650 125 550 730 1100 1650 2745 4575
700 110 530 710 1060 1590 2655 4425
750 95 505 675 1015 1520 2535 4230
800 80 410 550 825 1235 2055 3430
850 65 320 425 640 955 1595 2655
900 50 230 305 460 690 1150 1915
950 35 135 185 275 410 685 1145
1000 20 85 115 170 255 430 715

From the above table, you can see that a 150 lb carbon steel flange, the allowable pressure is 285 psi at 100°F, 170 psi at 500°F, and 20 psi at 1000°F. The only temperature at which the flange is rated for 150 psi is near 500°F. Higher class flanges are rated at their nominal pressure rating only at near 850°F. See the cells in yellows.

Flange Class

ASME has developed a flange Class considering temperature and pressure rating. There are seven Class 150#, 300#, 400#, 600#, 900#, 1500#, and 2500#.

Higher the flange ratings, heavier the flange, and can withstand higher pressure and temperature. So, when the temperature goes up for a given material, the maximum allowable pressure goes down, and vice versa. See the below image comparison of 6” Class 150 and Class 2500 weldneck flange.

Comparison of Class 150 and class 2500 ansi flange ratings

Now, these pressure classes of flanges are commonly known as “pounds” rather than “pounds per square inch.” Also note that while describing the pressure class of flanges, terms such as “pound” and “class” may be considered interchangeable.

What is ASME / ANSI 150 pressure rating?

People refer to the same flange in different ways. For example, when we say class 150 flange, it means that safe working pressure for this flange at rated temperature for a given material is 150 pounds per square inch.  See below the some of the common way to refer the 150 Class flange

  • Class #150 flange
  • 150-pound flange
  • class 150 flange pressure rating
  • 150 pressure rating flange
  • 150 lb flange
  • Class flange ratings

How to determine flange ratings?

Well, it is very simple.  Let’s assume that we want to use the ASTM A105 flange for our service that has a pressure rating of 1,200 PSIG and at 500 °F.

Flanges pressure rating in psig for common carbon steel ASME flanges
( A105 / A350 LF2 / A350 LF3 / A350 LF6)
Temperature (F°) 150# 300# 400# 600# 900# 1500# 2500#
-20 to 100 285 740 985 1480 2220 3705 6170
200 260 680 905 1360 2035 3395 5655
300 230 655 870 1310 1965 3270 5450
400 200 635 845 1265 1900 3170 5280
500 170 605 805 1205 1810 3015 5025
600 140 570 755 1135 1705 2840 4730
650 125 550 730 1100 1650 2745 4575
700 110 530 710 1060 1590 2655 4425
750 95 505 675 1015 1520 2535 4230
800 80 410 550 825 1235 2055 3430
850 65 320 425 640 955 1595 2655
900 50 230 305 460 690 1150 1915
950 35 135 185 275 410 685 1145
1000 20 85 115 170 255 430 715

Now refer to the pressure-temperature table for carbon steel flanges given above. Flange with 600# rating meets the requirement. You can see that Class 600# can withstand 1205 psig pressure at 500 °F. See the cell highlighted in Green.

The information listed in this article is based on my research on the topics and may not be 100% correct. Any correction is welcome.

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