General Wastage Internal Corrosion

General wastage of a pressure vessel shell over a relatively large area, that reduces the thickness of the shell below the minimum thickness required by the ASME Code, can result in failure by rupture. The primary membrane stress in the corroded area will be increased above the maximum allowable design stress permitted by the ASME Code, and failure will occur when the increased stress exceeds the tensile strength of the material.

The weld-joint efficiency factor used for the design of the pressure vessel should be used when calculating the minimum thickness required for the vessel shell when weld joints are within the corroded area. In addition, the minimum required thickness calculations should include wind and earthquake loadings according to the same criteria used for the design of the vessel (see Section 400). It may be possible to exclude wind and earthquake loads from the minimum thickness requirement, depending on the location and geometry of the corrosion, but this should be discussed with a pressure vessel or structural engineer.

API 510 permits “averaging” the remaining wall thickness in corroded areas. If the average thickness is less than the minimum thickness required by the ASME Code or can be expected to be reduced below this thickness by the prevailing corrosion rate before the next scheduled inspection, either the shell must be restored to the minimum required thickness plus a corrosion allowance by weld build-up (see Section 800), or the vessel must be replaced. An acceptable alternative is to rerate the vessel (Section 800). Rerating reduces the maximum allowable working pressure so the maximum allowable design stress will not be exceeded in the corroded areas with reduced wall thickness.

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