Análisis del agrietamiento de la válvula de cierre de la tubería de vapor principal

The main steam pipe globe valve is made from cast carbon steel, and approximately 900kg. Through macroscopic inspection and surface flaw detection, it was found that the valve body had defects such as cracks, shrinkage holes, and porosity. After metallographic inspection, hardness testing and other technical inspections, no defects such as structural deterioration and excessive hardness were found. The causes of cracks, shrinkage cavities, and porosity were analyzed and the following conclusions came.
 
2.1 Casting processes
The casting quality of the valve body is poor and there are many casting defects. In the casting process of valve bodies, process control is poor. The globe valve body has poor flowability and high shrinkage rates in the cooling and solidification process due to the high melting point of cast steel, causing the volume of the molten metal to shrink. It is not effectively replenished, causing serious shrinkage holes and porosity, and reducing the strength of the shell flange surface. After the defective valve is put into operation, the crack expansion will be accelerated under the action of stress.
 
2.2 Casting stress
The different cooling rates of each part of the metal in the condensation process led to inconsistent shrinkage of each part due to the large size, complex shape and structure of the valve. In addition, the obstruction of the mold and the mold core restricts the solid shrinkage of the valve, resulting in casting stress. Greater stress will accelerate the cracking of the valve body. Heat treatment is required after valve casting is completed. The heat treatment process has a decisive impact on the quality of casting. Residual internal stress is inevitable due to the characteristics of the valve. As the operation time accumulates, cracks will occur in the globe valve when partial stress is excessive.
 
2.3 Combined stress
When the valve is installed, the flange surface bears the pressure of the nut. Especially when the nut is unevenly tightened, the force on the flange surface is also uneven. When the valve is operating, under the pressure of the internal medium, the flange surface is subjected to tensile stress. Under the action of internal pressure, stress concentration will occur at partial defect locations, and the valve is subjected to thermal stress when it is operated under high temperatures for a long time. When the tensile strength of the material is exceeded, cracks appear in partial locations, and gradually expand under the action of alternating stresses such as thermal stress and fatigue load. As the cracks expand, the cracks extend to both sides, causing the valve body to crack.
 
3. Precautions
3.1 Casting processes
Quality control and inspection systems are strictly implemented in the valve casting process. By lowering the pouring temperature, reducing the shrinkage of the casting, increasing the boost pressure, improving the density of the product and the pouring system, fully transmitting the pressure, solidification and shrinkage; and strengthening cooling for the special parts of the product can prevent shrinkage cavities and porosity. Proper heat treatment after valve casting can effectively reduce residual stress after casting.
 
3.2 Installation processes
When connecting two flange surfaces, press the flange sealing surface and the gasket evenly to ensure the same bolt stress; when tightening the bolts, avoid exceeding the specified torque; avoid uneven force in the flange tightening process. Bolts should be tightened in sequence in a symmetrical and intersecting manner to ensure uniform stress on the flange surface.
 
3.3 Usage processes
In the complex steam pipeline system of a thermal power plant, there are often many valves. To ensure the safety and reliability of the pipe valves during the production and operation of the power station boiler, maintenance personnel must strengthen online inspection and shutdown maintenance. High-temperature insulation sleeves can be customized for the valve insulation layer for easy disassembly. Increase the frequency of valve inspection.
 
3.4 Statutory inspection
During the statutory internal inspection of power station boilers, the inspection agency should formulate a special inspection and testing plan and conduct random inspections for the main steam valves, select reasonable inspection and testing methods, inspect and detect defects on surfaces, near-surfaces and internal surfaces of the valve, promptly discover and eliminate defects. Conduct metallographic analysis of the material of the valve body used for high temperature and high pressure to prevent the valve material from deteriorating.
 
4. Conclusion
The valve bodies used in the main steam pipelines of thermal power plants are difficult to control and are prone to casting defects due to the casting process. The valve bodies are also very brittle. They bear internal medium pressure during operation and are often insulated during high-temperature operation. It is necessary to inspect and detect surface cracks. Therefore, strengthening valve inspection and testing, and adopting reasonable inspection and testing plans to promptly discover and eliminate defects is of great significance to ensure the safe operation of power station boilers.