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According to the provisions of Section II, Part VIII of the US ASME Boiler and Pressure Vessel Code, when using the stress classification method for the strength design of pressure vessels, the stress intensity Sm should be calculated using the third theory of strength: Sm = σ1 – σ3. Here, σ1 and σ3 represent the maximum and minimum principal stresses among the three-dimensional stresses, respectively. The stress intensity limits for various types of stresses must satisfy all of the following 5 strength conditions: ① The stress intensity SⅠ due to the overall membrane stress pm in the primary stresses should be less than the allowable stress KSm; ② The stress intensity SⅡ due to the local membrane stress pl in the primary stresses should be less than 1.5KSm; ③ The stress intensity SⅢ, which is the sum of the primary membrane stress (pm or pl) and the bending stress pb, should be less than 1.5KSm; ④ The stress intensity SⅣ, which is the sum of the overall membrane stress pm or the local membrane stress pl together with the bending stress pb and the secondary stress Q, should be less than 3KSm; ⑤ The stress intensity resulting from the primary stresses (including the overall or local membrane stress along with the bending stress) combined with the secondary stress Q and the peak stress F must not exceed 2 times the allowable stress, 2Sa, as determined by the fatigue curve.