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Flange Pressure Rating Explained (and Charts)

What means ANSI flange pressure rating? How to select the right flange rating?
The term “flange rating” (or class) refers to the maximum pressure (in psi or bars) that a flange withstands at increasing temperatures. Flanges with a higher rating (class) are stronger than flanges with lower ratings, as they bear more pressure at increasing temperatures. 
Flanges made in different materials show different pressure-temperature performance at the same rating. In the article, you find the pressure rating charts for carbon, alloy, and stainless steel flanges and some guidance about selecting the proper rating for your piping application.

ANSI FLANGE PRESSURE RATING EXPLAINED

Pressure rating is defined as the maximum allowed pressure that a flange can withstand at increasing temperatures. According to the ANSI/ASME B16.5 specification, there are seven flange pressure ratings: 150, 300, 400, 600, 900, 1500, and 2500.

The terms “pressure rating”, “class”, “#”, “Lb” or “Lbs” are interchangeable, i.e. they all refer to the same exact concept of pressure/temperature performance of a flange (and other equipment like valves, fittings, etc).

Let’s make this clear with an example:

If two flanges have the same bore size (example 6 inches), the same material (example A105), but two different pressure ratings (one flange is class 150, the other 300), the lower rated flange (class 150) will be smaller, lighter, and less robust than the higher rated flange (class 300). This is represented in the picture:

Flange rating: pressure and temperature ANSI/ASME
Flange Pressure Ratings

Now, let’s look at what this means in terms of pressure rating:

  • the class 150 flange withstands just 140 psi at a temperature of 600 degrees F° (as per the rating chart below)
  • the class 300 flange (bigger and stronger, but with the same bore size) withstands 570 psi at the same temperature of 600 degrees F°
  • finally, a class 2500 flange of the same size bears 34 times more pressure than the class 150 rated flange, reaching a whopping 4730 psi rating at 600 F°!

So, how to select the proper flange rating?

  1. Find the rating table that applies to your flange scrolling below (this depends on the material of the flange, as flanges with different material grades have different pressure ratings)
  2. Determine the max working temperature in your piping system (i.e. select one line in the table)
  3. Select a rating based on the maximum pressure you expect at that temperature level (i.e. select one column in that line)
  4. Voilà, you have the required rating!

To help you do this easily, find below the ASME B16.34 pressure rating charts for the most common flange materials (carbon, alloy, stainless).

CARBON STEEL FLANGE RATING CHART

Carbon steel flange pressure-temperature rating ASME
Carbon steel flange pressure-temperature rating ASME

ANSI FLANGE ASTM A105, A350 Gr. LF2/LF6 Class 1

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures  (Celsius or Fahrenheit) – in PSI

ANSI/ASME B16.34ANSI PRESSURE RATING
Temperature (in F°)150#300#400#600#900#1500#2500#
< 1002857409851480222037056170
2002606809051360203533955655
3002306558701310196532705450
4002006358451265190031705280
5001706058051205181030155025
6001405707551135170528404730
6501255507301100165027454575
7001105307101060159026554425
750955056751015152025354230
80080410550825123520553430
8506532042564095515952655
9005023030546069011501915
950351351852754106851145
10002085115170255430715
Hydrostatic Test Pressure (in Psig)450112515002225335055759275

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures  (Celsius or Fahrenheit) – in bars

ANSI/ASME B16.34ANSI PRESSURE RATING
Temperature in C°150#300#400#600#900#1500#2500#
   -29 / 3819.651.168.1102.1153.2255.3425.5
5019.250.166.8100.2150.4250.6417.7
10017.746.662.193.2139.8233388.3
15015.845.160.190.2135.2225.4375.6
20013.843.858.487.6131.4219365
25012.141.955.983.9125.8209.7349.5
30010.239.853.179.6119.5199.1331.8
3259.338.751.677.4116.1193.6322.6
3508.437.650.175.1112.7187.8313
3757.436.448.572.7109.1181.8303.1
4006.534.746.369.4104.2173.6289.3
4255.528.838.457.586.3143.8239.7
4504.62330.74669115191.7
4753.717.423.234.952.387.2145.3
5002.811.815.723.535.358.897.9

Notes:

  • ASTM A105: An extended exposure to temperatures above 425°C, converts the carbide phase of steel to graphite (therefore this material is not recommended for constant temperatures above this value.
  • ASTM A350 LF6: shall not be used for temperatures above 260 C°

ANSI FLANGE ASTM A350 Gr. LF3, A350 LF6, Class 2

The flange rating chart shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures  (Celsius or Fahrenheit) – in PSI

ANSI/ASME B16.34ANSI PRESSURE RATING
Temperature in °F150#300#400#600#900#1500#2500#
-20 to 10029075010001500225037506250
20026075010001500225037506250
3002307309701455218536406070
4002007059401410211535305880
5001706658851330199533255540
6001406058051210181530255040
6501255907851175176529404905
7001105707551135170528404730
750955056701010151025204200
80080410550825123520603430
8506527035553580513402230
900501702303455158601430
95035105140205310515860
1000205070105155260430

ANSI FLANGE ASTM A350 Gr. LF1 

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures  (Celsius or Fahrenheit) — in PSI

ANSI/ASME B16.34ANSI PRESSURE RATING
Temperature °F150#300#400#600#900#1500#2500#
-20 to 1002356208251235185030851545
2002155607501125168528104680
3002105507301095164027354560
4002005307051060158526454405
500170500665995149524904150
600140455610915137022853805
650125450600895134522453740
700110450600895134522453740
75095445590885132522103685
80080370495740111018503085
8506527035553580513402230
900501702303455158601430
95035105140205310515860
1000205070105155260430

ALLOY STEEL FLANGE RATING CHART

Alloy steel flange pressure-temperature rating ASME
Alloy steel flange pressure-temperature rating ASME

ANSI FLANGE ASTM A182 Gr. F1 (Chrome Moly)

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures  (Celsius or Fahrenheit) – in PSI

ANSI/ASME B16.34ANSI PRESSURE RATING
Temperature °F150#300#400#600#900#1500#2500#
-20 to 1002656959251390208534705785
2002606809051360203533955660
3002306558701305195532605435
4002006408551280192032005330
5001706208301245186531055180
6001406058051210181530255040
6501255907851175176529404905
7001105707551135170528404730
750955307101065159526604430
800805106751015152525404230
85065485650975146024354060
90050450600900135022453745
9503528037556084514052345
1000201652203304958251370

STAINLESS STEEL FLANGE RATING CHART

Stainless steel flange pressure-temperature rating ASME
Stainless steel flange pressure-temperature rating ASME

ANSI FLANGE ASTM A182 Gr. F304, 304L

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures  (Celsius or Fahrenheit) – in PSI

ANSI/ASME B16.34ANSI PRESSURE RATING
Temperature °F150#300#400#600#900#1500#2500#
-20 to 1002757209601440216036006000
2002306008001200180030005000
3002055407201080162027004500
400190495660995149024854140
500170465620930139523303880
600140435580875131021853640
650125430575860129021503580
700110425565850127521253540
75095415555830124520753460
80080405540805121020153360
85065395530790119019803300
90050390520780116519453240
95035380510765114519103180
10002032043064096516052675
10502031041061592515452570
11002025534551577012852145
1150202002654005959951655
1200201552053104657701285
125020115150225340565945
13002085115170255430715
1350206080125185310515
140020506595145240400
145015354570105170285
15001025355580135230

ANSI FLANGE ASTM A182 Gr. F316, 316L

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures  (Celsius or Fahrenheit) – in PSI

ANSI/ASME B16.34ANSI PRESSURE RATING
Temperature °F150#300#400#600#900#1500#2500#
-20 to 1002757209601440216036006000
2002356208251240186030955160
3002155607451120168027954660
4001955156851025154025704280
500170480635955143523903980
600140450600900135522553760
650125445590890133022203700
700110430580870130521703620
75095425570855128021353560
80080420565845126521103520
85065420555835125520903480
90050415555830124520753460
95035385515775116019303220
100020350465700105017502915
105020345460685103017202865
11002030540561091515252545
11502023531547571011851970
1200201852453705559251545
1250201451952954407351230
130020115155235350585970
13502095130190290480800
14002075100150225380630
1450206080115175290485
150020405585125205345

HOW TO SELECT FLANGE RATING

If you are still in doubt about how to select the flange rating, take a look at this useful video:

 

Comments

This post currently has one response

  • {08/28/19 @ 15:12 EDT}
    I found this site to contain much useful information but not exactly what I need. I hope you can help regarding the correct
    ASME flange to use for methane service @ 1000°C and 0.2 MPa. I think 900# RF or RTJ? Please, where could I find that information?
    Thanks for any help.

    Ps. Information not found in “Flange Pressure Rating Explained (and Charts)” article. Charts end at 1500 F (815°C)

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