Answer: (1) Most defects generated during the use of pressure vessels are surface defects, and buried defects are relatively rare. The defects generated during use are mainly cracks, deformation and corrosion, and most of them are generated on the surface. Most surface defects can be detected by macroscopic inspection.
(2) The effective size of surface cracks is twice that of buried cracks, and the danger of surface cracks is greater than that of buried cracks. The focus should be on checking surface cracks.
(3) From the perspective of the stress of the pressure vessel, the surface stress is relatively large. The additional stress generated by the angular deformation and misalignment at the weld is superimposed on the surface of the pressure vessel, and the temperature difference stress caused by the temperature difference of the vessel wall is also superimposed on the surface, which makes it easier to produce surface defects and is easy to expand after they are produced.
(4) The surface is in contact with the medium. When the inner and outer surfaces are in contact with the corrosive medium, surface defects will occur under certain working conditions.
(5) The probability of accidents caused by surface defects is greater than that of buried defects, and most of these defects can be detected by macroscopic inspection methods.
(6) When performing macroscopic inspection of surface defects, the method is simple, the workload is small, the cost is low, and it is easy to use.
(7) The method of using macroscopic inspection as the main method is conducive to the implementation of regular inspection of pressure vessels and to grasping the safety status of pressure vessels in use as soon as possible.
