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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 B16.34 ANSI PRESSURE RATING
Temperature (in F°) 150# 300# 400# 600# 900# 1500# 2500#
< 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
Hydrostatic Test Pressure (in Psig) 450 1125 1500 2225 3350 5575 9275

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 B16.34 ANSI PRESSURE RATING
Temperature in C° 150# 300# 400# 600# 900# 1500# 2500#
   -29 / 38 19.6 51.1 68.1 102.1 153.2 255.3 425.5
50 19.2 50.1 66.8 100.2 150.4 250.6 417.7
100 17.7 46.6 62.1 93.2 139.8 233 388.3
150 15.8 45.1 60.1 90.2 135.2 225.4 375.6
200 13.8 43.8 58.4 87.6 131.4 219 365
250 12.1 41.9 55.9 83.9 125.8 209.7 349.5
300 10.2 39.8 53.1 79.6 119.5 199.1 331.8
325 9.3 38.7 51.6 77.4 116.1 193.6 322.6
350 8.4 37.6 50.1 75.1 112.7 187.8 313
375 7.4 36.4 48.5 72.7 109.1 181.8 303.1
400 6.5 34.7 46.3 69.4 104.2 173.6 289.3
425 5.5 28.8 38.4 57.5 86.3 143.8 239.7
450 4.6 23 30.7 46 69 115 191.7
475 3.7 17.4 23.2 34.9 52.3 87.2 145.3
500 2.8 11.8 15.7 23.5 35.3 58.8 97.9

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 B16.34 ANSI PRESSURE RATING
Temperature in °F 150# 300# 400# 600# 900# 1500# 2500#
-20 to 100 290 750 1000 1500 2250 3750 6250
200 260 750 1000 1500 2250 3750 6250
300 230 730 970 1455 2185 3640 6070
400 200 705 940 1410 2115 3530 5880
500 170 665 885 1330 1995 3325 5540
600 140 605 805 1210 1815 3025 5040
650 125 590 785 1175 1765 2940 4905
700 110 570 755 1135 1705 2840 4730
750 95 505 670 1010 1510 2520 4200
800 80 410 550 825 1235 2060 3430
850 65 270 355 535 805 1340 2230
900 50 170 230 345 515 860 1430
950 35 105 140 205 310 515 860
1000 20 50 70 105 155 260 430

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 B16.34 ANSI PRESSURE RATING
Temperature °F 150# 300# 400# 600# 900# 1500# 2500#
-20 to 100 235 620 825 1235 1850 3085 1545
200 215 560 750 1125 1685 2810 4680
300 210 550 730 1095 1640 2735 4560
400 200 530 705 1060 1585 2645 4405
500 170 500 665 995 1495 2490 4150
600 140 455 610 915 1370 2285 3805
650 125 450 600 895 1345 2245 3740
700 110 450 600 895 1345 2245 3740
750 95 445 590 885 1325 2210 3685
800 80 370 495 740 1110 1850 3085
850 65 270 355 535 805 1340 2230
900 50 170 230 345 515 860 1430
950 35 105 140 205 310 515 860
1000 20 50 70 105 155 260 430

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 B16.34 ANSI PRESSURE RATING
Temperature °F 150# 300# 400# 600# 900# 1500# 2500#
-20 to 100 265 695 925 1390 2085 3470 5785
200 260 680 905 1360 2035 3395 5660
300 230 655 870 1305 1955 3260 5435
400 200 640 855 1280 1920 3200 5330
500 170 620 830 1245 1865 3105 5180
600 140 605 805 1210 1815 3025 5040
650 125 590 785 1175 1765 2940 4905
700 110 570 755 1135 1705 2840 4730
750 95 530 710 1065 1595 2660 4430
800 80 510 675 1015 1525 2540 4230
850 65 485 650 975 1460 2435 4060
900 50 450 600 900 1350 2245 3745
950 35 280 375 560 845 1405 2345
1000 20 165 220 330 495 825 1370

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 B16.34 ANSI PRESSURE RATING
Temperature °F 150# 300# 400# 600# 900# 1500# 2500#
-20 to 100 275 720 960 1440 2160 3600 6000
200 230 600 800 1200 1800 3000 5000
300 205 540 720 1080 1620 2700 4500
400 190 495 660 995 1490 2485 4140
500 170 465 620 930 1395 2330 3880
600 140 435 580 875 1310 2185 3640
650 125 430 575 860 1290 2150 3580
700 110 425 565 850 1275 2125 3540
750 95 415 555 830 1245 2075 3460
800 80 405 540 805 1210 2015 3360
850 65 395 530 790 1190 1980 3300
900 50 390 520 780 1165 1945 3240
950 35 380 510 765 1145 1910 3180
1000 20 320 430 640 965 1605 2675
1050 20 310 410 615 925 1545 2570
1100 20 255 345 515 770 1285 2145
1150 20 200 265 400 595 995 1655
1200 20 155 205 310 465 770 1285
1250 20 115 150 225 340 565 945
1300 20 85 115 170 255 430 715
1350 20 60 80 125 185 310 515
1400 20 50 65 95 145 240 400
1450 15 35 45 70 105 170 285
1500 10 25 35 55 80 135 230

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 B16.34 ANSI PRESSURE RATING
Temperature °F 150# 300# 400# 600# 900# 1500# 2500#
-20 to 100 275 720 960 1440 2160 3600 6000
200 235 620 825 1240 1860 3095 5160
300 215 560 745 1120 1680 2795 4660
400 195 515 685 1025 1540 2570 4280
500 170 480 635 955 1435 2390 3980
600 140 450 600 900 1355 2255 3760
650 125 445 590 890 1330 2220 3700
700 110 430 580 870 1305 2170 3620
750 95 425 570 855 1280 2135 3560
800 80 420 565 845 1265 2110 3520
850 65 420 555 835 1255 2090 3480
900 50 415 555 830 1245 2075 3460
950 35 385 515 775 1160 1930 3220
1000 20 350 465 700 1050 1750 2915
1050 20 345 460 685 1030 1720 2865
1100 20 305 405 610 915 1525 2545
1150 20 235 315 475 710 1185 1970
1200 20 185 245 370 555 925 1545
1250 20 145 195 295 440 735 1230
1300 20 115 155 235 350 585 970
1350 20 95 130 190 290 480 800
1400 20 75 100 150 225 380 630
1450 20 60 80 115 175 290 485
1500 20 40 55 85 125 205 345

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:

 

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