During in-service inspection of ASME pressure vessels, crack treatment must adhere to strict specifications to ensure safe operation. The core objective of ASME pressure vessel in-service inspection is to identify defects such as cracks, assess their impact on structural integrity, and develop scientifically sound treatment measures. In this process, crack location, nature assessment, and subsequent treatment must all be carried out according to the specific provisions of the ASME code.
Crack location and identification are the primary steps in in-service inspection. The ASME code requires a comprehensive inspection of the pressure vessel's surface and near-surface cracks using non-destructive testing techniques such as ultrasonic testing, magnetic particle testing, or penetrant testing. The inspection scope must cover stress concentration areas such as welds, heat-affected zones, connections between nozzles and the shell, and locations of abrupt structural changes. For internal cracks, in-depth inspection using radiographic testing or acoustic emission techniques is necessary. Crack location must be precise down to the specific component and area to provide a basis for subsequent treatment.
Determining the nature of the crack is crucial to the treatment principle. The ASME code classifies cracks into three categories: surface cracks, buried cracks, and penetrating cracks, and assesses their hazard level based on their morphology, size, and propagation trend. For example, surface cracks located in high-stress areas, even if small, can propagate rapidly due to stress concentration; buried cracks require analysis of their depth and distance from the vessel wall using tomography or phased array ultrasound. Furthermore, whether the crack is accompanied by structural deterioration, such as overheating, burning, or intergranular corrosion, is also an important basis for assessing its hazard.
The core principle of treatment lies in risk control and ensuring structural integrity. ASME standards clearly state that for surface cracks, if the remaining wall thickness meets strength verification requirements, they can be treated by grinding and smoothing the transition, but complete crack removal must be confirmed by magnetic particle or penetrant testing. For buried or penetrating cracks, if the crack depth exceeds a specific proportion of the vessel wall thickness, or is located in critical components, patching or replacement of pressure-bearing components is necessary. During patching, the beveling design must avoid sharp corners, the welding material must be compatible with the base material, and preheating, interpass temperature, and post-weld heat treatment processes must be strictly controlled to prevent the formation of new cracks.
Crack treatment under special operating conditions requires additional consideration. For high-strength steel ASME pressure vessels, crack treatment must be combined with fracture mechanics analysis to assess whether the stress intensity factor at the crack tip is lower than the material's fracture toughness. If the crack depth is close to the critical value, even if it does not penetrate the vessel wall, leakage may occur due to the "leak before bursting" failure criterion. In this case, patching or replacement should be prioritized. For cryogenic, highly toxic, or high-pressure vessels, crack treatment requires even greater caution. Fatigue analysis or failure safety design may be necessary to ensure the treated vessel operates safely for its remaining service life.
Post-treatment inspection and verification are crucial for closed-loop management. ASME standards require that after crack treatment, non-destructive testing confirms complete defect elimination, and hydrostatic testing verifies the sealing and strength of the treated area. For patched or replaced components, surface flaw detection and hardness testing are required to ensure welding quality meets requirements. Furthermore, the inspection cycle for treated vessels should be shortened, and the treated area should be designated as a key monitoring area, regularly tracking for crack recurrence or new crack formation.
The crack treatment principles during in-service inspection of ASME pressure vessels reflect the dual objectives of risk control and structural integrity assurance. By scientifically locating cracks, determining their properties, and implementing appropriate treatment measures, combined with rigorous post-treatment inspections, the impact of cracks on vessel safety can be effectively controlled, ensuring the safe and stable operation of the asme pressure vessel during its service life.
