Failure Analysis of Valve Cracks in Main Steam Pipelines

Abstract: Cast valves are widely used in the main steam pipelines of power station boilers due to their low cost and flexible design. However, due to their special manufacturing process and operating environment, defects such as cracks, slag inclusions, and gas holes often occur during use. In conjunction with valve inspection cases, the detection process of valve cracks is introduced, the causes of cracks are analyzed, and reasonable suggestions are provided for the safe operation of main steam pipeline valves.
 

The boiler model of the thermal power plant is TG-130/9.82-M2. The boiler was put into operation in December 2016 and has been running for 35,400 hours. The superheated steam outlet temperature is 540℃. After inspection, multiple dendritic cracks were found on the shoulder surface and near the surface of the main steam pipeline electric valve. The specific parameters of the valve are shown in Table 1.
 

 

(1) Before inspecting the valve, the steam and electric systems should be reliably isolated.
(2) The inspection site should be equipped with necessary safety lighting and working power supply.
(3) Before inspection, remove the insulation materials from the inspection site and the parts that hinder the inspection. Set up the scaffolding, inspection platform, and guardrails required for inspection.
(4) After the valve has fully cooled, polish the valve surface until the metallic luster appears.
 

In conjunction with the on-site use of the boiler and valve, focus on reviewing the valve certificate, quality certificate, valve performance test record, boiler main steam pipeline design drawings, and operation records. Through document review, it is determined that the boiler valve selection meets the standards and drawing design requirements, ensures the normal operation of the boiler, and eliminates the influence of improper valve selection on cracks.
 

Macro inspection is primarily based on visual inspection, mainly checking the outer surface of the valve body, valve bonnet, and welded joints to confirm that there are no defects such as cracks, lack of fusion, gas holes, and slag inclusions at the welded joints. For forged valves, there should be no visible cracks, interlayers, folds, or inclusions. For cast valves, the outer surface roughness should meet the requirements of the standard, and there should be no defects such as sand sticking, cracks, and inclusions. The valves used in this unit are cast valves. After the valve surface is polished and cleaned, it is observed through a magnifying glass. The surface condition is good, and no manufacturing or new defects are found. The inner surface of the valve is randomly checked, and there are no defects such as corrosion, erosion, and cracks.
 

According to JB/T 6439-2016 "Standard for Metal Pipe Heating Elements", the magnetic particle inspection of the boiler valve is carried out using the magnetic yoke method. Before the inspection, the outer surface of the valve is polished to reveal the metallic luster, and the contrast enhancer is evenly sprayed to increase the contrast of the inspected surface, ensuring the defect detection rate. During the magnetic particle inspection of the valve's outer surface, the focus should be on all the connections between the gates, risers, and castings, the stress concentration areas such as the valve body shoulder and the cross-section mutation area, and the weak areas such as the weld end processing. After inspection, many small dendritic cracks were found at the shoulder of the main steam pipeline electric valve, as shown in Figure 1, with most cracks originating from defects such as sand particles or shrinkage holes. An angle grinder was used to grind the cracks. After grinding approximately 1.5 mm, the defects were eliminated. The ground parts were re-tested with magnetic particles, and no new defects were detected.
 
 
Figure 1 Schematic diagram of magnetic particle detection of valve cracks
 

For castings, the content of alloy elements and impurity elements directly affects their physical and mechanical properties. For valves, excessive phosphorus (P) content will reduce the toughness and density of the material, increasing the brittleness of the casting. Sulfur (S) elements will increase the possibility of forming defects such as slag inclusions and subcutaneous gas holes. Elements such as chromium (Cr), molybdenum (Mo), and vanadium (V) can improve the wear resistance, compressive strength, and corrosion resistance of castings, as well as the thermal stability of valves. Therefore, detecting chemical composition is also crucial. Table 2 shows the results of spectral analysis of the surface of the valve body of the three valves.It can be seen from Table 2 that the chemical composition of the valve meets the standard requirements. The mass fraction of sulfur (S) and phosphorus (P) impurity elements is less than or equal to 0.03%, and the mass fractions of other elements are within the allowable range: chromium (Cr) is 0.90% to 1.0%, molybdenum (Mo) is 0.5% to 0.7%, and vanadium (V) is 0.2% to 0.3%.
 
Table 2 Valve elements and mass fraction %

 

To eliminate the residual stress generated in the casting process of the valve, eliminate the segregation of the structure, refine the grain, and improve the mechanical properties of the valve, heat treatment is generally necessary after casting. If the heat treatment process is not properly controlled, the surface hardness of the valve shell can exceed the standard. Therefore, hardness is an important parameter for determining the heat treatment process and a crucial factor affecting the generation of cracks. Table 3 shows the hardness test values ​​of the outer surface of the valves of the three main steam pipelines of the boiler. During the actual testing process, the hardness of each valve was measured at three locations, with the average value obtained by measuring five points at each location. The test revealed that the hardness values of most valves were within the range of 160-200 HB. However, only the main steam electric valve had a hardness slightly exceeding the standard, and this location was precisely at the shoulder of the valve. The valve hardness is too high, causing the plasticity at this location to decrease, which reduces the bearing capacity of the valve and increases the likelihood of cracks.
 
Table 3 Actual hardness values ​of valves

 

In addition to macroscopic inspection, the microstructure state is also a significant factor affecting the generation of cracks. In conjunction with the actual test conditions, the main steam electric valve with slightly exceeded hardness was subjected to metallographic testing according to DL/T 884-2019. The metallographic structure of the test part is shown in Figure 2. As seen in Figure 2, there is no graphitization or decarburization in the metallographic structure of the valve, and the grain structure is in good condition. This indicates that long-term high-temperature operation has not affected its microstructure, reduced the bonding force between grains, or weakened its mechanical properties.
 
 
Figure 2 Valve metallographic structure diagram
 

Combined with the actual use of the valve and after a comprehensive analysis of the test results, it was found that the design selection, chemical composition, and metallographic structure of the valve were not the direct causes of the cracks in the boiler valve. The main reasons for the cracks in the valve are as follows:
(1) The valve is a cast valve. During the casting process, "sequential solidification" is used, meaning a "riser" is set at the part of the casting that may have shrinkage holes or solidify last, so that the casting solidifies first in the part away from the riser, then in the part close to the riser, and finally in the riser. Sequential solidification causes a large temperature difference, significant thermal stress, and considerable deformation of the casting. The shoulder position of the valve itself is a stress concentration area, which increases the possibility of cracks.
(2) During the solidification process of the valve casting, after the liquid metal is poured into the mold, heat is mainly dissipated through the mold wall, so solidification always starts from the surface of the casting. When a large number of dendrites appear in the late stage of solidification and overlap into a complete skeleton, solid-state shrinkage begins to occur. However, there is still a layer of unsolidified liquid metal film between the dendrites. When the solidified casting shell shrinks, it is hindered by external factors such as the sand core, molding sand, and friction between the casting surface and the sand mold surface. The shell cannot shrink freely, and tensile stress is generated. When the tensile stress exceeds the material strength limit, cracks occur between the dendrites. If the dendrite skeleton is pulled apart quickly, and there is not enough liquid metal around the separated part to flow into the cracks in time and replenish them, cracks occur on the casting surface.
(3) Cast valves generally need to undergo heat treatment after casting. An improper heat treatment process can cause the valve hardness to exceed the standard, reduce plasticity, and decrease valve load capacity. Due to the internal stress of the cast valve itself, it will be in an unstable state. Additionally, the main steam valve system is subjected to complex forces, and the size and shape of the structure itself vary. The valve neck stress concentration effect is significant. Under the combined effect of working stress and temperature difference stress during the start-stop process, the original casting crack source expands to form macro cracks. For cracks found during the inspection process, the user unit should use grinding to eliminate them. After grinding, the wall thickness of the ground part is measured. If the wall thickness is lower than the thickness required for the safe operation of the valve, the ground part should be repaired by welding. Additionally, the user unit should strengthen monitoring during operation to prevent new defects.
 

The cracks in the valve of the main steam pipeline of the power station boiler are closely related to the casting process, valve structure, and actual operating conditions. Therefore, to ensure the safe operation of the equipment, the equipment acceptance work should be done before the valve is installed to ensure that the model and specifications of the valve meet the design requirements, the chemical composition is within the deviation range, and the valve has no defects such as inclusions, shrinkage holes, or cold shuts left over from manufacturing. Additionally, during the inspection process, the inspector should combine the actual operating conditions on site to formulate a reasonable inspection plan, focusing on weak parts such as the valve body shoulder that are prone to cracks, to ensure that new defects are discovered in a timely and accurate manner.