Cause Analysis of Failures Caused by Plug Valve Erosion

Abstract: Through macroscopic analysis, geometric measurement, chemical composition analysis, mechanical property testing, metallographic analysis, microscopic morphology detection, and force analysis, the process and cause of plug valve erosion failure are demonstrated, and corresponding improvement measures are proposed to improve product quality and eliminate the probability of such accidents.
 

When high-pressure fracturing equipment was in service, and the working pressure increased to about 10,000 psi (1 psi = 6.894757 Pa), the operator found that the pressure loading was abnormal. After stopping the machine for inspection, the 3 in (1 in = 25.4 mm) plug valve male joint of its accessories was found to be eroded, resulting in the leakage of the working fluid. The eroded plug valve is shown in Figure 1. This incident caused the entire high-pressure fracturing unit to shut down, leading to economic losses of more than 1 million RMB. The design material of the failed plug valve is 40CrNi2Mo steel, with a rated working pressure of 15,000 psi. The failed plug valve had been in service for a total of about 120 hours. To find the cause of the failure, a failure analysis was performed.
 

Figure 1 Erosion of plug valves
 

From the eroded plug valve, it can be observed that the working fluid leaked from the male union and pierced the pipe wall along its step, leaving erosion marks between the outer wall of the joint and the joint connector, as shown in Figures 2 and 3.
 

Figure 2 Erosion at the step of the plug valve union

 
Figure 3 Erosion of the three petals of the external connector of the plug valve union
 
The leakage area of the plug valve is fan-shaped, with short cracks on the inner wall and long cracks on the outer wall. The piercing part of the inner wall of the plug valve union is depicted in Figure 4. As depicted in Figure 4, the inner wall crack is about 5 mm wide and 45 mm long, with smooth crack edges, showing obvious piercing erosion characteristics. The crack corresponds to the step of the outer wall, indicating a sudden change in geometric size. The length of the outer wall crack is about 90 mm, and the weak wall thickness of the outer wall is covered with erosion marks.
 
 
Figure 4 Piercing site of the inner wall of the plug valve union end
 
 
Figure 5 Erosion of the outer wall of the plug valve union end
 
After dissecting the erosion area (Figure 5), it is observed that the erosion area is divided into two parts: the piercing area, which is parallel to the inner wall piercing crack, and the erosion area, which is full of holes. The piercing area is parallel to the piercing crack; the piercing surface is smooth with no visible erosion pits, and it accounts for about one-quarter of the wall thickness. From the fracture, it is apparent that its characteristics are similar to those of the inner wall piercing crack. This area is formed by the piercing of the pipe wall due to the overload of high-pressure fluid. The other section is the erosion area, filled with erosion pits of varying sizes. This erosion area is formed by the initial erosion of the crack surface and the outer wall of the pipe by the high-pressure fluid. The initial crack has been destroyed by the erosion of the working fluid, forming an erosion fracture. It is evident that the plug valve male union first produces cracks at the transition of the outer arc between the shafts; the cracks then extend to both sides along the arc angle step and simultaneously expand to the inner wall of the pipe, resulting in erosion failure from the inside to the outside.
 

The arc angle between the shafts was measured by sampling at the plug valve union joint, and the measured arc angle was R 0.55 mm, as shown in Figure 6. The design specification is R 1.2 mm, indicating that the machining does not meet the design requirements.


Figure 6 Arc angle of the leakage site of the plug valve union end
 

The leaking plug valve samples were analyzed using spectroscopy, and the results are shown in Table 1. As shown in Table 1, the chemical composition of the material complies with the provisions of GB/T 3077-2015 "Alloy Structural Steel" for 40CrNi2Mo steel and meets the design requirements.
 
Table 1 Chemical composition of plug valves (mass fraction %)
2.4 Mechanical Properties Test

The mechanical properties of the leaking plug valve were assessed, with the results displayed in Table 2. Table 2 indicates that the mechanical properties of the plug valve meet the design requirements.
 
Table 2 Mechanical properties test of plug valves

 

Samples were taken from the crack of the plug valve for non-metallic inclusion analysis. The analysis results were A0.5, B0, C0.5, and D0.5 for the fine series; A0.5, B0, C0, and D1.0 for the coarse series. No metallurgical quality or processing defects were found. The metallographic structure near the crack consists of tempered troostite and a small amount of free ferrite, forming a normal quenched and tempered structure, as shown in Figure 7.
 

Figure 7 Metallographic structure of plug valves
 

The original defect at the fracture of the plug valve was eroded, leaving only the erosion fracture morphology. The fracture morphology analysis revealed that the main defects were corrosion pits and erosion pits. Under high magnification, they appeared dimple-shaped and oxidized on the surface, characteristics formed by subsequent fluid erosion, as shown in Figure 8.
 

Figure 8 Microscopic morphology of the fracture of the plug valve
 

From the perspective of product structure alone, the male union end has a sudden structural change, resulting in stress concentration. Additionally, the actual arc angle is less than half of the designed arc angle, which exacerbates the stress concentration. Stress cloud analysis shows stress concentration near the root of the male union shoulder. With a yield strength of 930MPa, the maximum internal stress of the male union shoulder is 753.9MPa for an arc of R1.2 mm and 806MPa for an arc of R0.8 mm. The stress increases by about 50MPa for the same yield strength. When the arc between the shafts is reduced to R0.55 mm, the maximum internal stress of the male union shoulder significantly increases, potentially approaching or exceeding the material's yield strength, leading to cracks at the stress concentration site.

Site feedback indicates that during the service of the high-pressure manifold, the plug valve exhibited significant oscillation, especially at the beginning of each operation, as the high-pressure fluid passed through. The oscillation is concentrated on the male union part of the plug valve, which has the smallest wall thickness and is connected to other parts where stress concentration occurs. The stress concentration part of the male union shoulder of the plug valve is subjected to both vibration stress and compressive stress from the working fluid during service. 
 

Based on the above analysis, vibration stress causes the plug valve to initiate cracks on one side of the arc transition angle of the male union shoulder. The small radius of the transition arc angle causes stress concentration, which easily leads to early fatigue cracks. As the number of services and service time increase, the crack extends further along the arc angle into the pipe wall until it reaches 1/4 of the original wall thickness. At this reduced wall thickness, the pipe wall cannot withstand the high-pressure fluid during service. When the pressure reaches 10000 psi, the high-pressure fluid breaks through the weakest part of the pipe wall, forming a puncture crack. It erodes the original crack, its outer diameter, and the three connecting petals, resulting in erosion failure and abnormal pressure.
 

1) Improper arc transition processing at the step of the male union shaft of the plug valve is the primary cause of plug valve leakage.
2) The absence of fixing and vibration reduction measures during the installation of the plug valve subjects it to additional working stress, a secondary cause of its leakage.
3) Control the machining accuracy and increase the arc transition at the step of the plug valve's male union shaft to ≥R2mm, or design it as an inclined surface to reduce stress concentration.
4) Implement vibration reduction and fixing measures to fundamentally reduce the additional vibration stress.