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Methods for Internal Pressure Design

Methods for Internal Pressure Design

The ASME B31.3 Code provides four basic methods for the design of components for internal pressure, as described in para. 302.2:

  • (a) Components in accordance with listed standards for which pressure ratings are provided in the standard, such as ASME BI6.5 for flanges, are considered suitable by ASME B31.3 for the pressure rating specified in the standard. Note that the other methods of pressure design provided in ASME B31.3 can be used to rerate such listed components and/or extend their temperature range.

  • (b) Some listed standards, such as ASME B16.9 for pipe fittings, state that the fitting has the same pressure rating as matching seamless pipe. ASME B31.3 modifies this slightly by stating that the fittings are accepted to have the same rating as the matching seamless pipe, considering only 87.5% of the wall thickness (removing the 87.5% allowance is presently under consideration). This takes into consideration the typical mill tolerance for pipe. Note that design calculations are not usually performed for these components; design calculations are performed for the straight pipe, and matching fittings are simply selected.

  • (c) Design equations for some components, such as straight pipe and branch connections, are provided in para. 304 of ASME B31.3. These can be used to determine the required wall thickness with respect to internal pressure of components. Furthermore, some specific branch connection designs are assumed to be acceptable.

  • (d) Components that are not in accordance with a listed standard and for which design equations are not provided in the Code are treated in para. 304.7.2. This paragraph provides accepted methods, such as burst testing, for determining the pressure capacity of unlisted components.

The equations in the Code provide the minimum thickness required to limit the membrane, and in some cases bending stresses in the piping component, to the appropriate allowable stress. Mechanical and corrosion/ erosion allowances must be added to this thickness. Finally, the nominal thickness selected must be such that the minimum thickness that may be provided, per specifications and considering mill tolerance, is at least equal to the required minimum thickness.

The pressure design rules in the Code are based on maximum normal stress, or maximum principal stress, versus maximum shear stress, or von Mises stress intensity. When the rules were developed in the 1940’s, it was understood that stress intensity provided a better assessment of yielding, but it was felt that the maximum principal stress theory could generally provide a better measure of pressure capacity in situations where local yielding could simply lead to stress redistribution [Rossheim and Markl (I960)).

Mechanical allowances include physical reductions in wall thickness, such as due to threading and grooving the pipe. Corrosion and erosion allowances are based on the anticipated corrosion and/or erosion over the life of the pipe. This is based on estimates, experience, or literature, such as National Association of Corrosion Engineers (NACE) publications. These allowances are added to the pressure design thickness to determine the minimum required thickness of the pipe or component when it is new.

For threaded components, the nominal thread depth (dimension h of ASME B 1.20.1; see Appendix I or equivalent) is used for the mechanical allowance. For machined surfaces or grooves where the tolerance is not specified, the Code requires that a tolerance of 0.5 mm (0.02 in.) on the depth of the cut be assumed.

Mill tolerances are provided in specifications. The most common tolerance on the wall thickness of straight pipe is 12.5%. This means that the wall thickness at any given location around the circumference of the pipe must not be less than 87.5% of the nominal wall thickness. Note that the tolerance on pipe weight is typically tighter, so that the volume of metal and its weight may be present although a thin region would control design for hoop stress due to internal pressure.

The appropriate specification for the pipe must be referred to in order to determine the specified mill tolerance. For example, plate typically has an under tolerance of 0.25 mm (0.01 in.). However, pipe formed from plate does not have this under tolerance; it can be much greater. The pipe specification, which can permit a greater under tolerance, governs for the pipe. The manufacturer of pipe can order plate that is thinner than the nominal wall thickness for manufacturing the pipe, as long as the pipe specification mill tolerances are satisfied. However, the weight tolerance could then govern. For example, the thickness tolerance for A53 pipe is 12.5%, but the weight tolerance is 10%. As a result, the minimum thickness for A53 welded pipe made from plate material would be 10% under thickness because of the weight tolerance. #Little_PEng


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