Pressure Vessel Stresses

A pressure vessel is subjected to various loads which develop stresses that are categorized by the ASME Code as:
• Primary
• Secondary
• Peak

The maximum allowable design stress for a pressure vessel is based on the strength of the material. Stress limits depend on the category of the stress and its relationship to the various potential failure modes. Figure 100-1 classifies typical stresses in a vessel.

Under certain conditions, each category or combination of stresses can cause the vessel to fail in a different way. For this reason, the limits for each category of stress are related to the potential failure modes. Primary stress limits are set to prevent deformation and ductile burst. Primary plus secondary stress limits are set to prevent plastic deformation leading to collapse. Peak stress limits are set to prevent fatigue failure due to cyclic loading.

Because of differences between the simplified equations used in the codes and the complexity of the theoretical equations, a factor of safety is applied to various material properties used to determine allowable stress values. These safety factors differ according to the specific criteria of each section or division of the Code and according to the required levels for the actual stresses

Since each stress category requires a different safety factor to protect against failure, the designer has to evaluate each type of stress to achieve the most economical and safe design.

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