## Pressure Vessel Primary Membrane Stress

Primary membrane stresses are tensile or compressive stresses that are essentially uniform through the entire cross-section of a pressure vessel component. Consequently, gross plastic deformation will occur when these stresses exceed the yield strength of the material. Examples of primary membrane stresses in pressure vessel shells are:

• Circumferential and longitudinal stress attributable to internal pressure

• Longitudinal stress in horizontal vessels due to bending between saddle supports

• Axial compression due to the weight of a vertical vessel

• Stresses in a nozzle neck in the area of reinforcement due to internal pressure and to external forces and moments attributable to piping connections. (Exceptions are those related to discontinuity effects.)

• Axial tensile and compressive stresses due to wind and earthquake loads

The design limit for primary membrane stresses is the maximum allowable design stress for the material of construction at the design temperature. Continuous primary membrane stresses cannot exceed two-thirds of the yield strength. However, stresses that act intermittently and for relatively short durations (for example, those attributable to wind and earthquake loads) can be increased to 1.2 times the maximum allowable design stress.

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