Comparison of API Design Requirements
6/18/2009 Page 1
API 6A DIS API 16A 4.3.3.2 ASME method
The design methodology as described in ASME BPV:2004 with 2005 and 2006 addenda, Section VIII, Division 2, Appendix 4, may be used for design calculations for pressure-containing equipment. Design-allowable stresses shall be limited by the following criteria:
65 Y
TSS = and Y
m 23
SS =
where
Sm is the design stress intensity at rated working pressure;
ST is the maximum allowable general primary membrane stress intensity at hydrostatic test pressure;
SY is the material-specified minimum yield strength.
1.1.1.1 ASME method
The design methodology is described in ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, Appendix 4. Design-allowable stresses shall be limited by the following criteria:
ySS 9,0T ≤
and
ym SS )32(≤
where
Sm is the design stress intensity at rated working pressure;
ST is the maximum allowable general primary membrane stress intensity at hydrostatic test pressure;
SY is the material’s specified minimum yield strength.
API 16C Recommendation 3.1.1 ASME method
The design methodology as described in ASME Boiler Code & Pressure Vessel Code, Section VIII, Division 2, Appendix 4, shall be used for design calculations. Design allowable stresses shall be limited by the following criteria:
Ym 2
3S
S = and ySS 9.0T =
where Sm is the design stress intensity at rated
working pressure;
Sy is the material minimum specified yield strength.
ST is the maximum allowable primary membrane stress intensity at hydrostatic test pressure;
Classic ASME method
The design methodology as described in the 2004 ASME Boiler & Pressure Vessel Code, Section VIII, Division 2, Appendix 4, shall be used for design calculations. Design allowable stresses shall be limited by the following criteria:
Ym 2
3S
S = and ySS 9.0T =
where Sm is the design stress intensity at rated
working pressure;
Sy is the material minimum specified yield strength.
ST is the maximum allowable general primary membrane stress intensity at hydrostatic test pressure;
Comparison of API Design Requirements
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API 6A DIS API 16A 4.3.3.3 Theory of constant energy of distortion
The theory of constant energy of distortion, also known as the Von Mises Law, may be used for design calculations for pressure-containing equipment. Rules for the consideration of discontinuities and stress concentrations are beyond the scope of this International Standard. However, the basic pressure-vessel wall thickness may be sized by combining triaxial stresses based on hydrostatic test pressure and limited by the following criterion:
SE = SY where
SE is the maximum allowable equivalent stress at the most highly stressed distance into the pressure vessel wall, computed by the distortion energy theory method;
SY is the material-specified minimum yield strength.
5.4.2.3 Distortion Energy Theory
This design methodology for the basic pressure vessel wall thickness uses a combination of the triaxial stresses based on the hydrostatic test pressure and is limited by the following criterion:
ye SS = Where
eS is the maximum allowable equivalent stress computed by the distortion energy theory method;
yS is the material’s specified minimum yield strength.
API 16C Recommendation 3.1.2 Distortion Energy Theory Method
The Distortion Energy Method shall be used for design calculations for unions. The minimum pressure design thickness shall be calculated by combining triaxial stresses based on the distortion energy method, using either one of the limits below. Secondary stresses and stress concentrations are beyond the scope of this method.
SE = SY where
SE is the maximum allowable equivalent stress at the most highly stressed distance into the pressure vessel wall, computed by the distortion energy theory method;
SY is the material-specified minimum yield strength.
4.3.3.3 Distortion energy theory method
The distortion energy theory, also known as the Von Mises Law, may be used for design calculations for pressure-containing equipment. Rules for the consideration of discontinuities and stress concentrations are beyond the scope of this method. However, the basic pressure-vessel wall thickness may be sized by combining triaxial stresses based on hydrostatic test pressure and limited by the following criterion:
SE = SY where
SE is the maximum allowable equivalent stress at the most highly stressed distance into the pressure vessel wall, computed by the distortion energy theory method;
SY is the material’s specified minimum yield strength.
Comparison of API Design Requirements
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Non-standard materials (only 6A addresses this issue). The problem pointed out by Jean Brunjes is that if we merely change the limit for test conditions to 0.9 Sy, then the stress would almost always be limited by the tensile strength and there would be no benefit from the change. By limiting Sm only, the limit for secondary stress range then automatically defaults to 3 Sm, and the limit for ST remains 0.9 Sy as for standard materials.
API 6A Recommended for 6A, 16A, and 16C 4.3.3.6 Non-standard materials design requirements
The design methodology as described in ASME Section VIII Division 2 Appendix 4 shall be used for design and calculations for pressure-containing equipment utilizing non-standard materials. Design allowable stresses shall be limited by the following criteria: ST = the smaller of 5/6 SY or 2/3 Rm,min
Sm = the smaller of 2/3 SY or 1/2 Rm,min
Ss = the smaller of 2 SY or Rm,min
Where Sm is the design stress intensity at rated working pressure; Ss is the maximum combined primary and secondary stress intensity; ST is the maximum allowable general primary membrane stress intensity at hydrostatic test pressure; Rm,min is the material-specified minimum ultimate tensile strength; and Sy is the material-specified minimum yield strength; and
4.3.3.6 Non-standard materials design requirements
The ASME method shall be used for design calculations for pressure-containing equipment utilizing non-standard materials, with the following exception: Sm = the smaller of 2/3 SY or 1/2 Rm,min, where Sm is the design stress intensity at rated working pressure; Rm,min is the material specified minimum ultimate tensile strength; and Sy is the material specified minimum yield strength.
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Design of Bolting
API 6A API 16A 4.3.4 Closure Bolting The maximum allowable tensile stress for closure bolting shall be determined considering initial bolt-up, rated working pressure and hydrostatic test pressure conditions. Bolt stress, based on the root area of the thread, shall not exceed the following limit: SA = 0.83 SY where SA is the maximum allowable tensile stress; SY is the bolting material minimum specified yield strength. Bolting stresses shall be determined considering all loading on the closure including pressure acting over the seal area, gasket loads, and any additive mechanical and thermal loads.
5.4.2 Closure Bolting Stresses shall be determined considering all loading on the closure including pressure acting over the seal area, gasket loads, and any additive mechanical loads. The maximum tensile stress shall be determined considering initial make-up loads, working conditions and hydrostatic test conditions. The stresses, based on the minimum cross-sectional area, shall not exceed the following limits: Sa ≤ 0.83 Sy where Sa is the maximum allowable tensile stress; Sy is the bolting material minimum specified yield strength.
API 16C Recommendation 3.6 CLOSURE BOLTING Identical to API 6A 4.3.4
Closure Bolting The maximum tensile stress for closure bolting shall be determined considering:
a) initial bolt-up; b) operating conditions including
pressure loads, external mechanical loads, and thermal stress;
and c) hydrostatic test pressure conditions.
Bolt tensile stress, based on the minimum cross-sectional area of the bolt or stud, shall not exceed the following limits: Sa = 0.83 Sy and Sb = 1.0 Sy where Sa is the maximum allowable tensile membrane stress; Sb is the maximum allowable tensile membrane plus bending stress; Sy is the bolting material minimum specified yield strength.
