Strength verification of the rectangular valve body cavity in orifice plate valves?

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I would like to ask whether, to determine the strength of the cavity in a rectangular valve body, it is necessary to refer to Table 5-71 on page P1016 of Lu Peiwen’s \"Practical Valve Design Manual\", which deals with rectangular shapes of type T1. Using the formulas provided there, the stresses calculated for a and b are more than twice the tensile and compressive stresses associated with ZG230~450 material. The actual wall thickness values have been in use for several years now, and no problems have occurred even under conditions of overpressure. Could experts please help me figure out what went wrong? Half of the long side: a = 102 mm; half of the short side: b = 40 mm. Design pressure: P = 6.3 MPa. Actual thickness: SB = 22 mm. Corrosion allowance: C = 3 mm. At room temperature, ZG230-450 has a strength of 102 MPa; it is 82 MPa under other conditions