Pressure Vessel Secondary Stresses

Secondary stresses differ from primary stresses because they are self-limiting. Secondary stresses develop at structural discontinuities.

Examples of secondary stresses are:
1. Bending stresses at head-to-shell junctions.
2. Bending stress at conical-transition-to-cylindrical-shell junction.
3. Bending stress in the shell at nozzles.
4. Bending stress at vessel supports and external attachments.
5. Thermal stresses produced by temperature gradients in the shell, or by differences
in temperature between the nozzle and shell.

Unlike primary stresses, secondary stresses are reduced in magnitude by the local yielding, before gross plastic deformation or bursting can occur. The first application of load during hydrotest will generally suffice to significantly reduce the secondary stresses in a pressure vessel, but subsequent load applications could further reduce the secondary stresses.

The stress limit for secondary stresses is 3.0 times the maximum allowable design stress for the material of construction at the design temperature. Therefore, the secondary stress is permitted to be as high as twice the yield strength, but it is reduced in magnitude by local yielding. Unless a detailed stress analysis is made, structural discontinuities that develop secondary stresses should be separated by a distance of at least 2.5 x R(t)1/2 to avoid additive effects that could increase the total secondary stress above 3.0 times the maximum allowable design stress.

A distinction must be made between local primary membrane stresses and secondary stresses. Like secondary stresses, local primary membrane stresses also develop at structural discontinuities, and are essentially self-limiting. However, they are categorized as primary stresses because the plastic deformation associated with the yielding (required to redistribute the local membrane stress) may be excessive.

Therefore, in effect, the membrane component of the stress developed by the selfconstraint at structural discontinuities is categorized as a primary stress, whereas the bending component of the stress is categorized as a secondary stress.

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