Repair of surface defects on pressure vessels

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Surface cracks: All surface cracks shall be ground away. If, after removal of the cracks, the actual wall thickness of the pressure vessel is not less than the design thickness, and the grinding depth is not more than 7% of the nominal thickness or 3 mm, then no welding repair is required. However, to reduce stress concentration, the ground area is required to be smooth with a gentle transition, and the side slope should not exceed 1:4. If the grinding depth is significant and the remaining thickness is less than the minimum wall thickness permitted by design, strict welding repair measures should be taken for restoration. When the weld repair depth on the surface of the spherical tank shell plate exceeds 3 mm, ultrasonic testing should also be conducted. Corrosion: For scattered pitting corrosion, if the depth of corrosion does not exceed 1/5 of the wall thickness (excluding the corrosion allowance). Within a diameter of 200 mm, if the area of pitting corrosion does not exceed 45 cm2, or the total length of pitting corrosion along any diameter does not exceed 50 mm, no treatment is generally required ； For uniform surface corrosion, if the strength is found to be satisfactory based on the remaining average wall thickness (after deducting the amount of corrosion that has occurred up to the next inspection period), no action is required ； For non-uniform surface corrosion, if the strength is found to be satisfactory when checked based on the minimum remaining wall thickness (subtracting the amount of corrosion that has occurred up to the next inspection period), no action is required. Otherwise, pressure reduction, welding repair, replacement, or scrapping should be carried out depending on the specific circumstances. In cases of severe corrosion, failure in strength verification, difficulty in repair, or reaching the end of its service life, it should be considered for scrapping. For other defects such as mechanical damage, arc scarring, cratering, and weld marks from tools and fixtures, if no abnormalities are found around them, the pressure vessel generally does not require any treatment. For pressure vessels with special requirements where the defects are severe, they should be ground out to remove them. Layers within the base material that are parallel to the free surface of the base material or have an angle of less than 10° require no treatment; when the angle is 10° or more, it is necessary to calculate the projected length in the direction of the plate thickness in order to determine the direction and location of the defect. In severe cases, the material must be discarded. Bulges that occur during use should have their causes identified, and it should be determined whether such defects are stable. If the cause can be identified and it is confirmed that the bulges will not continue to grow, the item can be repaired and then used ； If the cause cannot be identified, or if it is identified but the problem continues to spread, it should be scrapped.