Pressure Classes of Flanges

Forged steel , according to ASME B16.5, are made in seven primary ratings as 150Lbs, 300Lbs, 400Lbs, 600Lbs, 900Lbs, 1500Lbs, 2500Lbs.

The concept of flange ratings likes clearly. A Class 300 flange can handle more pressure than a Class 150 flange, because a Class 300 flange are constructed with more metal and can withstand more pressure. However, there are a number of factors that can impact the pressure capability of a flange.
The Pressure Class or Rating for flanges will be given in pounds. Different names are used to indicate a Pressure Class.
For example: 150 Lb or 150 Lbs or 150# or Class 150, all are means the same.

Example of Pressure Rating

Flanges can withstand different pressures at different temperatures. As temperature increases, the pressure rating of the flange decreases. For example, a Class 150 flange is rated to approximately 270 PSIG at ambient conditions, 180 PSIG at approximately 400°F, 150 PSIG at approximately 600°F, and 75 PSIG at approximately 800°F.
In other words, when the pressure goes down, the temperature goes up and vice versa. Additional factors are that flanges can be constructed from different materials, such as stainless steel, cast and ductile iron, carbon steel etc.. Each material have different pressure ratings.
Below an example of a flange NPS 12 with the several pressure classes. As you can see, inner diameter and diameter of the raised face at all the same; but outside diameter, bolt circle and diameter of bolt holes become larger in each higher pressure class.
The number and diameters (mm) of the bolt holes are:

  • Class 150 – 12 x 25.4, Class 300 – 16 x 28.6
  • Class 400 – 16 x 34.9, Class 600 – 20 x 34.9
  • Class 900 – 20 x 38.1, Class 1500 – 16 x 54
  • Class 2500 – 12 x 73
Pressure-Temperature Ratings Example

Pressure-temperature ratings are maximum allowable working gage pressures in bar units at the temperatures in degrees celsius. For intermediate temperatures, linear interpolation is permitted. Interpolation between class designations is not permitted.
Pressure-temperature ratings apply to flanged joints that conform to the limitations on bolting and on gaskets, which are made up in accordance with good practice for alignment and assembly. Use of these ratings for flanged joints not conforming to these limitations is the responsibility of the user.
The temperature shown for a corresponding pressure rating is the temperature of the pressure-containing shell of the component. In general, this temperature is the same as that of the contained fluid. Use of a pressure rating corresponding to a temperature other than that of the contained fluid is the responsibility of the user, subject to the requirements of applicable codes and regulations. For any temperature below -29°C, the rating shall be no greater than the rating shown for -29°C.
As an example, below you will find two tables with material groups acc. to ASTM, and two other tables
with flange pressure-temperature ratings for those ASTM materials acc. to ASME B16.5.

ASTM Group 2-1.1 Materials
Nominal Designation Forgings Castings Plates
C-Si A105(1) A216  r.WCB(1) A515 Gr.70(1)
C-Mn-Si A350 Gr.LF2(1) A516 Gr.70(1),(2)
C-Mn-Si-V A350 Gr.LF6 Cl 1(3) A537 Cl.1(4)
3½Ni A350 Gr.LF3

NOTES:

(1) Upon prolonged exposure to temperatures above 425°C, the carbide phase of steel may be converted to graphite. Permissible but not recommended for prolonged use above 425°C.
(2) Do not use over 455°C.
(3) Do not use over 260°C.
(4) Do not use over 370°C.

ASTM Group 2-2.3 Materials
Nominal Designation Forgings Castings Plates
16Cr-12Ni-2Mo A182 Gr.F316L A240 Gr.316L
18Cr-13Ni-3Mo A182 Gr.F317L
18Cr-8Ni A182 Gr.F304L(1) A240 Gr.304L(1)

(1) Do not use over 425°C.

Pressure-Temperature Ratings for ASTM Group 2-1.1 Materials

Working pressure by Classes, BAR

Temp.°C 150 300 400 600 900 1500 2500
-29 to 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
538 1.4 5.9 7.9 11.8 17.7 29.5 49.2
Pressure-Temperature Ratings for ASTM Group 2-2.3 Materials

Working pressure by Classes, BAR

Temp.°C 150 300 400 600 900 1500 2500
-29 to 38 15.9 41.4 55.2 82.7 124.1 206.8 344.7
50 15.3 40 53.4 80 120.1 200.1 333.5
100 13.3 34.8 46.4 69.6 104.4 173.9 289.9
150 12 31.4 41.9 62.8 94.2 157 261.6
200 11.2 29.2 38.9 58.3 87.5 145.8 243
250 10.5 27.5 36.6 54.9 82.4 137.3 228.9
300 10 26.1 34.8 52.1 78.2 130.3 217.2
325 9.3 25.5 34 51 76.4 127.4 212.3
350 8.4 25.1 33.4 50.1 75.2 125.4 208.9
375 7.4 24.8 33 49.5 74.3 123.8 206.3
400 6.5 24.3 32.4 48.6 72.9 121.5 202.5
425 5.5 23.9 31.8 47.7 71.6 119.3 198.8
450 4.6 23.4 31.2 46.8 70.2 117.1 195.1

--- Hot flanges by Sunny Steel ---

  • Blind Flanges, BL Flange, BLRF
    The blind flange is used to close ends of piping systems.

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