Evaluating Vessel Stresses - CAESAR II - Help

CAESAR II Users Guide

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ASME Section VIII Division 2 — CAESAR II applies rules prior to the 2007 Edition — provides a procedure to analyze the local stresses in vessels and nozzles. For this example, we will only discuss the nozzle analysis approach.

Always refer to the applicable design code if any of the limits described in this section are approached, or if any unusual material, weld, or stress situation exists, or there are non-linear concerns such as the operation of material within creep range.

The first step is to determine if the elastic approach is satisfactory. To summarize, Section AD-160 states that if the model meets all of the following conditions, then a fatigue analysis is not required:

  1. The expected design number of full-range pressure cycles does not exceed the number of allowed cycles corresponding to a Sa value of 3Sm (4Sm for non-integral attachments) on the material fatigue curve. Sm is the allowable stress intensity for the material at the operating temperature.

  2. The expected design range of pressure cycles other than startup or shutdown must be less than ⅓ (¼ for non-integral attachments) the design pressure times (Sa/Sm), where Sa is the value from the material fatigue curve for the specified number of significant pressure fluctuations.

  3. The vessel does not experience localized high stress due to heating.

  4. The full range of stress intensities due to mechanical loads including piping reactions does not exceed Sa, from the fatigue curve, for the expected number of load fluctuations.

After deciding if an elastic analysis is satisfactory, you must determine whether to take either a simplified or a comprehensive approach to do the vessel stress analysis. For more information on the simplified or the comprehensive approach, see ASME Section VIII Division 2-Elastic Nozzle Simplified Analysis or ASME Section VIII Division 2-Elastic Nozzle Comprehensive Analysis. For more information on Section VIII Division 2 requirements, refer to the latest version of the ASME code.