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Principles for selecting non-destructive testing methods: 1) Each non-destructive testing method has its own range of capabilities and limitations, and sufficient operational space must be ensured； 2) Non-destructive testing methods that can only detect surface opening defects include penetrant testing and visual inspection. Penetrant testing is mainly used for non-porous materials, while visual inspection is primarily used to detect defects visible to the naked eye ； 3) Non-destructive testing methods capable of detecting surface opening defects and near-surface defects include magnetic particle testing and eddy current testing. Magnetic particle testing is mainly used for ferromagnetic materials, while eddy current testing is mainly used for conductive metal materials ； 4) Non-destructive testing methods that can detect defects at any location in materials include radiographic testing, ultrasonic testing, diffraction time-of-flight ultrasonic testing, and X-ray digital imaging testing. Generally speaking, ultrasonic testing and diffraction time-of-flight ultrasonic testing have lower capabilities for detecting surface openings or near-surface defects compared to magnetic particle testing, penetrant testing, or eddy current testing ； 5) Acoustic emission testing can be used to determine the strength and approximate location of active defects present inside or on the surface of pressure equipment. Acoustic emission testing requires pressurization tests on pressure-bearing equipment; when active defects are detected, other non-destructive testing methods should be used for reinspection ； 6) The non-destructive testing method that can only detect through-hole defects or overall integrity of pressure-bearing equipment is leak detection ； 7) For ferromagnetic materials, magnetic particle testing should be preferred to detect surface or near-surface defects; other non-destructive testing methods can only be used when magnetic particle testing is not feasible due to reasons such as the shape of the structure ； 8) When using a non-destructive testing method and conducting tests under different procedures, if the test results differ, the assessment level with the higher degree of risk shall prevail ； 9) When two or more testing methods are used to inspect the same part of a pressure-bearing equipment, the grade shall be determined according to each respective method.