For straight pipe under external pressure, there is a membrane stress check in accordance with Eq. (3a) [or (3b)] of ASME B31.3 [the equation for internal pressure; Eq. (4.2) or Eq. (4.9) here), as well as a buckling check in accordance with the external pressure design rules of ASME BPVC, Section VIII, Division 1.
Flanges, heads, and stiffeners that comply with ASME BPVC, Section VIII, Division 1, para. UG-29 are considered stiffeners. The length between stiffeners is the length between such components. The buckling pressure is a function of geometrical parameters and material properties.
Buckling pressure calculations in ASME BPVC, Section VIII, Division 1 require first the calculation of a parameter A, which is a function of geometry, and then a parameter B, which depends on A and a material property curve. The charts that provide the parameter B account for plasticity between the proportional limit of the stress-strain curve and the 0.2% offset yield stress. The chart for determination of A is provided in Fig. 4.5. A typical chart for B is provided in Fig. 4.6.
Two equations are provided for calculating the maximum permissible external pressure. The first uses the parameter B,
The second equation is based on elastic buckling, so the elastic modulus is used. A chart of B could be used, with the linear elastic portion of the curve extended to lower values of B, but this would unnecessarily enlarge the charts. The charts provided in ASME B31.5 have this form, with the elastic lines extended.
The procedures of ASME BPVC Section VIII include consideration of the allowable out-of-roundness in pressure vessels, and use a design margin of three. Although pipe is not generally required to comply with the same out-of-roundness tolerance as is required for pressure vessels, this has historically been ignored, and has not led to any apparent problems.
The basis for the approach of ASME BPVC Section VIII is provided in Bergman (1960), Holt (1960), Saunders and Windenburg (1960), Windenburg and Trilling (1960), and Windenburg (1960).A new buckling evaluation procedure, provided in Code Case 2286, is more relevant to piping, because it permits consideration of combined loads, including external pressure, axial load, and gross bending
moment. Code Case 2286 also provides a more uniform margin of safety and a higher allowable external pressure. It is not at present explicitly recognized in ASME B31.3, but could be considered, given para. 300(c)3. #Little_PEng.