Flanges - Maximum Allowable Working Pressure

1. Introduction

The maximum allowable working pressure (MAWP) is the maximum pressure at which a flange may be operated at given temperature and is normally expressed in terms of the flange rating.  In the case of ASME B16.5 flanges, as the temperature increases, whilst the class remains the same (i.e. 150#, 300#, 400#, 600#, 900#, 1500# and 2500#), the maximum pressure at which the flange can be used decreases.  The amount by which the pressure decreases with temperature is a function of the material properties.

The pressure-temperature ratings contained in ASME B16.5 have been established based on the procedures described in Nonmandatory Appendix ‘A’ of ASME B16.5.  Whilst the primary consideration when establishing these pressure-temperature ratings was the ability of the flange to sustain pressure and other loads, there are other considerations that may affect or limit the pressure-temperature rating.  One such consideration is the ceiling pressure, pc, which effectively sets a limit on the MAWP regardless of the material properties.

The ceiling pressure values are listed in Table 1.  They are normally only applicable in the case of high-strength materials and are imposed in order to limit deflection in the flange.  ASME B16.5 does not permit pressure-temperature ratings in excess of the values shown in Table 1.

–29 to 38°C20.051.768.9103.4155.1258.6430.9

Table 1 – Rating Ceiling Pressure – pc, bar

2. Material Properties

The procedures for establishing the pressure-temperature ratings of a flange use an allowable stress value which is derived from the yield strength and ultimate tensile strength values listed in Section II, Part D of the ASME Boiler and Pressure Vessel Code.  Materials that are not listed in Section II, Part D of the ASME Boiler and Pressure Vessel Code may be used provided that they conform to a published specification covering chemistry, physical and mechanical properties, method and process of manufacture, heat treatment, and quality control (See Section II, Part D, Appendix 5 of the ASME Boiler and Pressure Vessel Code).

For convenience, ASME B16.5 groups materials as indicated in Table 2 based on identical or closely matching allowable stress and yield strength values.

Material GroupNominal DesignationPressure Temperature Rating TableApplicable ASTM Specifications
1.1C-Si2-1.1A 105A 216 Gr. WCBA 515 Gr. 70
C-Mn-Si2-1.1A 350 Gr. LF2A 516 Gr. 70
C-Mn-Si2-1.1A 537 Cl. 1
C-Mn-Si-V2-1.1A 350 Gr. LF6 CI. 1
3½Ni2-1.1A 350 Gr. LF3
1.2C-Mn-S2-1.2A 216 Gr. WCC
C-Mn-Si2-1.2A 352 Gr. LCC
C-Mn-Si-V2-1.2A 350 Gr. LF6 Cl. 2
2½Ni2-1.2A 352 Gr. LC2A 203 Gr. B
3½Ni2-1.2A 352 Gr. LC3A 203 Gr. E
1.3C-Si2-1.3A 352 Gr. LCBA 515 Gr. 65
C-Mn-Si2-1.3A 516 Gr. 65
2½Ni2-1.3A 203 Gr. A
3½Ni2-1.3A 203 Gr. D
C-½Mo2-1.3A 217 Gr. WC1
C-½Mo2-1.3A 352 Gr. LC1
1.4C-Si2-1.4A 515 Gr. 60
C-Mn-Si2-1.4A 350 Gr. LF1 Cl. 1A 516 Gr. 60
1.5C-½Mo2-1.5A 182 Gr. F1A 204 Gr. A
C-½Mo2-1.5A 204 Gr. B
1.7½Cr-½Mo2-1.7A 182 Gr. F2
Ni-½Cr-½Mo2-1.7A 217 Gr. WC4
¾Ni-¾Cr-1Mo2-1.7A 217 Gr. WC5
1.91¼Cr-½Mo2-1.9A 217 Gr. WC6
1¼Cr-½Mo-Si2-1.9A 182 Gr. F11 CL.2A 387 Gr. 11 Cl. 2
1.12¼Cr-1Mo2-1.10A 182 Gr. F22 Cl. 3A 217 Gr. WC9A 387 Gr. 22 Cl. 2
1.11C-½Mo2-1.11A 204 Gr. C
1.135Cr-½Mo2-1.13A 182 Gr. F5aA 217 Gr. C5
1.149Cr-1Mo2-1.14A 182 Gr. F9A 217 Gr. C12
1.159Cr-1Mo-V2-1.15A 182 Gr. F91A 217 Gr. C12AA 387 Gr. 91 Cl. 2
1.171Cr-½Mo2-1.17A 182 Gr. F12 Cl. 2
5Cr-½Mo2-1.17A 182 Gr. F5
1.189Cr-2W-V2-1.18A 182 Gr. F92
2.118Cr-8Ni2-2.1A 182 Gr. F304A 351 Gr. CF3A 240 Gr. 304
18Cr-8Ni2-2.1A 182 Gr. F304HA 351 Gr. CF8A 240 Gr. 304H
2.216Cr-12Ni-2Mo2-2.2A 182 Gr. F316A 351 Gr. CF3MA 240 Gr. 316
16Cr-12Ni-2Mo2-2.2A 182 Gr. F316HA 351 Gr. CF8MA 240 Gr. 316H
18Cr-13Ni-3Mo2-2.2A 182 Gr. F317A 240 Gr. 317
19Cr-10Ni-3Mo2-2.2A 351 Gr. CG8M
2.318Cr-8Ni2-2.3A 182 Gr. F304LA 240 Gr. 304L
16Cr-12Ni-2Mo2-2.3A 182 Gr. F316LA 240 Gr. 316L
18Cr-13Ni-3Mo2-2.3A 182 Gr. F317L
2.418Cr-10Ni-Ti2-2.4A 182 Gr. F321A 240 Gr. 321
18Cr-10Ni-Ti2-2.4A 182 Gr. F321HA 240 Gr. 321H
2.518Cr-10Ni-Cb2-2.5A 182 Gr. F347A 240 Gr. 347
18Cr-10Ni-Cb2-2.5A 182 Gr. F347HA 240 Gr. 347H
18Cr-10Ni-Cb2-2.5A 182 Gr. F348A 240 Gr. 348
18Cr-10Ni-Cb2-2.5A 182 Gr. F348HA 240 Gr. 348H
2.623Cr-12Ni2-2.6A 240 Gr. 309H
2.725Cr-20Ni2-2.7A 182 Gr. F310A 240 Gr. 310H
2.820Cr-18Ni-6Mo2-2.8A 182 Gr. F44A 351 Gr. CK3MCuNA 240 Gr. S31254
22Cr-5Ni-3Mo-N2-2.8A 182 Gr. F51A 240 Gr. S31803
25Cr-7Ni-4Mo-N2-2.8A 182 Gr. F53A 240 Gr. S32750
24Cr-10Ni-4Mo-V2-2.8A 351 Gr. CE8MN
25Cr-5Ni-2Mo-3Cu2-2.8A 995 Gr. CD4MCu
25Cr-7Ni-3.5Mo-W-Cb2-2.8A 995 Gr. CD3MWCuN
25Cr-7.5Ni-3.5Mo-N-Cu-W2-2.8A 182 Gr. F55A 240 Gr. S32760
2.923Cr-12Ni2-2.9A 240 Gr. 309S
25Cr-12Ni2-2.9A 240 Gr. 310S
2.125Cr-12Ni2-2.10A 351 Gr. CH8
25Cr-12Ni2-2.10A 351 Gr. CH20
2.1118Cr-10Ni-Cb2-2.11A 351 Gr. CF8C
2.1225Cr-20Ni2-2.12A 351 Gr. CK20
3.135Ni-35Fe-20Cr-Cb2-3.1B 462 Gr. N08020B 463 Gr. N08020
3.299.0Ni2-3.2B 564 Gr. N02200B 162 Gr. N02200
3.399.0Ni-Low C2-3.3B 162 Gr. N02201
3.467Ni-30Cu2-3.4B 564 Gr. N04400127 Gr. N04400
3.572Ni-15Cr-8Fe2-3.5B 564 Gr. N06600168 Gr. N06600
3.633Ni-42Fe-21Cr2-3.6B 564 Gr. N08800409 Gr. N08800
3.765Ni-28Mo-2Fe2-3.7B 462 Gr. N10665333 Gr. N10665
Mn-W2-3.7B 462 Gr. N10675333 Gr. N10675
3.854Ni-16Mo-15Cr2-3.8B 462 Gr. N10276575 Gr. N10276
60Ni-22Cr-9Mo-3.5Cb2-3.8B 564 Gr. N06625443 Gr. N06625
62Ni-28Mo-5Fe2-3.8333 Gr. N10001
70Ni-16Mo-7Cr-5Fe2-3.8434 Gr. N10003
61Ni-16Mo-16Cr2-3.8575 Gr. N06455
42Ni-21.5Cr-3Mo-2.3Cu2-3.8B 564 Gr. N08825424 Gr. N08825
55Ni-21Cr-13.5Mo2-3.8B 462 Gr. N06022575 Gr. N06022
55Ni-23Cr-16Mo-1.6Cu2-3.8B 462 Gr. N06200575 Gr. N06200
3.947Ni-22Cr-9Mo-I8Fe2-3.9435 Gr. N06002
21Ni-30Fe-22Cr-18Co-3Mo-3W2-3.9B 572 Gr. R30556435 Gr. R30556
3.125Ni-47Fe-21Cr-5Mo2-3.10599 Gr. N08700
3.1144Fe-25Ni-21Cr-Mo2-3.11A 479 Gr. N08904A 240 Gr. N08904
3.1226Ni-43Fe-22Cr-5Mo2-3.12B 620 Gr. N08320
47Ni-22Cr-20Fe-7Mo2-3.12B 582 Gr. N06985
46Fe-24Ni-21Cr-6Mo-Cu-N2-3.12B 462 Gr. N08367A 351 Gr. CN3MNB 688 Gr. N08367
3.1349Ni-25Cr-18Fe-6Mo2-3.13B 582 Gr. N06975
Ni-Fe-Cr-Mo-Cu-Low C2-3.13B 564 Gr. N08031B 625 Gr. N08031
3.1447Ni-22Cr-19Fe-6Mo2-3.14B 582 Gr. N06007
40Ni-29Cr-15Fe-5Mo2-3.14B 462 Gr. N06030B 582 Gr. N06030
58Ni-33Cr-8Mo2-3.14B 462 Gr. N06035B 575 Gr. N06035
3.1542Ni-42Fe-21Cr2-3.15B 564 Gr. N08810B 409 Gr. N08810
3.1635Ni-19Cr-1¼Si2-3.16B 511 Gr. N08330B 536 Gr. N08330
3.1729Ni-20.5Cr-3.5Cu-2.5Mo2-3.17A 351 Gr. CN7M
3.1957Ni-22CR-14W-2Mo-La2-3.19B 564 Gr. N06230B 435 Gr. N06230

Table 2 – List of Material Specifications

3. Pressure Temperature Rating Method

3.1. Rating Equation Class 300# and Higher

For flanges in for Class 300# and above, the pressure-temperature ratings for the materials listed in Table 2 were established using the equation

Formula 1


C1 = 10 when S1 is expressed in MPa and the resultant value of pt will be expressed in bar;

pc = ceiling pressure in bar at the material temperature, T, see Table 1;

Pr = pressure rating class index, e.g. for Class 300#, Pr = 300;

pt = rated working pressure in bar for the specified material at the material temperature, T;

S1 = selected stress in MPa for the specified material at the material temperature, T.  The method for establishing S1 is described in Paragraphs 3.2, 3.3 and 3.4, as applicable.

3.2. Ratings for Group № 1 Materials

The selected stress for Group № 1 materials in Table 2 is determined as follows:

a) At temperatures below the creep range, S1 shall be equal to or less than

  1. 60% of the specified minimum yield strength at 38°C.
  2. 60% of the yield strength at temperature, T.
  3. 1.25 times 25% of the ultimate tensile strength value at temperature, T, as listed in Section II, Part D of the ASME Boiler and Pressure Vessel Code for either Section I or Section VIII, Division 1.

b) At temperatures in the creep range, the value of S1 shall be the allowable stress at temperature, T, as listed in Section II, Part D of the ASME Boiler and Pressure Vessel Code, for either Section I or Section VIII, Division 1, but not exceeding 60% of the listed yield strength at temperature.

c) In no case shall the selected stress value increase with increasing temperature.

d) The creep range is considered to be at temperatures in excess of 370°C for Group № 1 materials.

e) When the allowable stresses listed for the reference ASME Boiler and Pressure Vessel Code Section show a higher and lower value for allowable stress and the higher value is noted to the effect that these stress values exceed two-thirds of the yield strength at temperature then the lower value shall be used.  If lower allowable stress values do not appear and it is noted in the allowable stress table that the allowable stress values exceed two-thirds of the yield strength at temperature, then the allowable stress values used shall be determined as two thirds of the tabulated yield strength at temperature.

f) Yield strength shall be as listed in Section II, Part D of the ASME Boiler and Pressure Vessel Code, for either Section III or Section VIII, Division 2.

g) Allowable stress values listed in Section II, Part D of the ASME Boiler and Pressure Vessel Code, for Section III, Class 2 or Class 3 values may only be used for a material not listed for either Section I or Section VIII, Division 1.

3.3. Method for Group № 2 and № 3 Materials

Pressure-temperature ratings for Class 300# and higher, of materials corresponding to those in Group № 2 and № 3 of Table 2, are established using the method described in Paragraphs 3.1 and 3.2, except that in Paragraphs 3.2(a)(1) and 3.2(a)(2), the 60% factor shall be changed to 70%.  For Group № 2 materials, the creep range is considered to be at temperatures in excess of 510°C unless the material properties indicate a lower temperature should be used.  For Group № 3 materials, the creep range onset temperature shall be determined on an individual basis.

3.4. Method for Class 150# – All Materials

Pressure-temperature ratings for Class 150# rating designation are established by the method given for the related materials in Paragraphs 3.1, 3.2, and 3.3 subject to the following exceptions:

a) The value used for the pressure class rating index, Pr, in above equation shall be 115.

b) The value used for the selected stress, S1, at the material temperature, T, shall be in accordance with the requirements of either Paragraph 3.1 or 3.2, as applicable.

c) The value used for the rated working pressure, pt, shall not exceed value at temperature T as given by the following equation.

Formula 2


C2 = 21.41;

C3 = 0.03724 with T expressed in °C, the resultant pt will be in bar;

T = material temperature, °C.

The value of T in the above equation shall not exceed 538°C.  For values of T less than 38°C, use T equal to 38°C.

4. Discussion

When establishing the pressure-temperature ratings for a particular piping class, the engineer must first determine the appropriate pressure class rating, i.e. 150#, 300#, 400#, 600#, 900#, 1500# and 2500#.  In this respect, it is not enough for the engineer to simply consider the design pressure.  As noted in the introduction, there are other loads that need to be considered, namely applied bending moments, ME, and axial forces, FA, which can be significant  The engineer will normally factor in the effects of these loads by converting them to an equivalent pressure, Peq.  This is done using the following equation; commonly referred to as the Kellogg formula.

Formula 3

Where, G is the diameter of the gasket load reaction, which is normally assumed to be the mean diameter of the gasket contact face/ring groove.  Having calculated Peq, the engineer will add this to the design pressure and, as a rule of thumb, provided the sum is not more than one and a half times the MAWP, the engineer will assume that the selected pressure class rating is suitable.  If the sum is more than one and a half times the MAWP, the engineer needs to select the next pressure class rating up and check that.

For more critical applications, the above rule of thumb may provide sufficient assurance as to the adequacy of the flange.  Where this is the case, the engineer needs to carry out design calculations for each flange size under consideration.

Rules exist that allow the engineer to determine the stresses not only in the flange, but also in the bolting.  In some cases, it is the bolt stresses that determine the pressure class rating rather than the flange stresses, i.e. if the required bolt stresses are excessive or close to the limit, the engineer may decide to select the next pressure class rating up.  The loads on the flange/bolting due to internal pressure may increase slightly (due to the larger value of G), but the reduction Peq (for the same reason) combined with the increase in the total bolt cross-sectional area (which increases with increasing pressure class rating), will more than compensate for this.

How the rules for calculating the stresses in a bolted flange connection are applied will be the subject of a separate blog.