Pressure Vessel Local Primary Membrane Stress
Local primary membrane stress is a subcategory of primary membrane stress that is developed by sustained internal and external loads similar to primary membrane stresses. A local primary membrane stress exceeds the stress limit for a primary membrane stress, but as the higher stress is localized it can be redistributed to the surrounding portions of the pressure vessel if yielding occurs. Although the redistribution of stress upon localized yielding normally prevents failure of the pressure vessel, the plastic deformation associated with such yielding is unacceptable. Therefore, the stress limit for a localized stress is set at 1.5 times the maximum allowable stress of the material of construction at the design temperature, which can be as high as the minimum yield strength.
In order to prevent excessive elastic distortion, a local primary membrane stress is not permitted to extend in a longitudinal direction more than R(t)1/2, where R is the radius of curvature of the vessel component and t is its thickness. Furthermore, individual regions of localized stress must be separated by at least 2.5 x R(t)1/2.
Examples of local primary membrane stresses in pressure vessels are:
• Membrane stress at head-to-shell junctions.
• Membrane stress at conical-transition-to-cylindrical-shell junctions.
• Membrane stress in the shell at nozzles.
• Membrane stress at vessel supports or external attachments.
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