Erosion in a Pipe Bend

Erosion often occurs during the development of oil and gas fields and is a major issue affecting their long-term safe production. Erosion can lead to thinning and perforation of equipment and pipeline wall materials, eventually causing equipment failure. By the end of 2017, the total mileage of China's long-distance oil and gas pipelines reached 131,400 kilometers, ranking third in the world. It is expected to reach 169,000 kilometers by 2020 and 240,000 kilometers by 2025. With the expansion of the pipeline, the problem of erosion has become increasingly prominent, and its impact on oil field production efficiency has become more evident.
 
Oil pipes are primarily made from steel, and the internal medium consists of multiphase flow. Erosion is likely to occur when the medium contains sand, oxygen, CO2, and minerals. As oil fields develop, the proportion of water and sand in crude oil increases, leading to an accelerated erosion rate in pipelines. Long-term collisions between sand particles and walls of pipe bends make erosion more likely, creating a safety hazard. Erosion varies at different locations. To provide an effective anti-erosion solution, it is necessary to thoroughly analyze the erosion mechanism of liquid-solid two-phase flow in pipelines.

 
Figure 1 Erosion in a pipe bend
 
Erosion reduces the integrity of the material structure and leads to mass loss, mainly caused by the direct impact of solid particles and bubble collapse. Its damage mechanism includes mass loss due to particle collisions and the destruction of the surface passivation film, which accelerates corrosion. Erosion by solid particles not only directly damages the wall surface but also destroys the passivation film, maintaining a high corrosion rate over time. In severe cases, erosion can cause oil and gas leaks, seriously affecting production and safety. Although erosion is not the primary cause of pipeline damage, it is a key factor in accelerating corrosion damage. When corrosion and erosion occur simultaneously in a pipeline, erosion damages the passivation film, exposes the underlying material, and causes continued damage, thereby gradually increasing the impact on the pipeline.
 
When the liquid-solid two-phase flow passes through a bend, the sudden change in flow direction intensifies the collision of solid particles with the wall surface, leading to particularly severe erosion loss at the bend. As the liquid-solid two-phase flow moves through the bend, the fluid impacts the outer arch side due to inertia, causing particles to follow the fluid and collide with the outer arch side. Some particles rebound, collide with the wall surface, and then flow out with the fluid. Erosion in the bend section can be dozens of times greater than that in the straight pipe. Therefore, studying the erosion mechanism of bends caused by liquid-solid two-phase flow is crucial for the safety assessment of oil and gas pipelines.
 

The bend is a common pipe fitting in the oil and gas industry. Due to its curved structure, it is susceptible to erosion, and the degree of damage can be more severe than that of a straight pipe. The partial change in direction at the bend causes turbulence in the medium flow, which leads to sand particles in the liquid impacting the pipe wall strongly, resulting in erosion. In recent years, the number of pipeline and equipment damage incidents caused by erosion has gradually increased, seriously affecting the safe production of oil and gas fields.
 
The erosion at the bend is complex and related to factors such as fluid properties, solid-phase particle characteristics, and pipeline geometry. Studies have shown that the maximum erosion loss at the bend is closely related to the number of particles and kinetic energy, with kinetic energy being the more significant factor. Damage is usually concentrated in the outer area where the bend connects to the downstream straight pipe and on the side wall of the straight pipe section near the bend outlet. Xuewen Cao and others used Fluent to simulate and analyze the erosion at the bend and found that the maximum degree of damage increases with speed and concentration, while the effect of sand particle size initially decreases and then increases. Additionally, the placement direction of the bend affects the degree of damage. The placement method with the outlet aligned with the direction of gravity experiences the least damage, while the method with the outlet opposite to gravity experiences the most damage. Damage and destruction at the bend become more complex due to factors such as inertial force, pressure differences, and varying flow directions. Understanding particle movement in liquid-solid two-phase flow is crucial for studying erosion. Additionally, the varying speeds at which particles bypass wall obstructions also affect the degree of damage to the pipeline wall.
 
The change in fluid direction at the bend leads to sudden changes in velocity and pressure, increasing the complexity of pipeline damage. To predict and reduce erosion, numerical simulation has become an important tool. Using the Liaohe Oilfield gas pipeline as an example, LiangLi and others simulated erosion in different bends using various computational models and obtained parameters such as the bend flow field, pressure field, particle movement, and their interrelationships. Their research shows that the particle distribution in the 45° bend is more concentrated, the flow velocity and pressure change significantly, and the degree of damage is also higher. For the right-angle bend, the quartz sand particle distribution is most concentrated at twice the diameter of the bend outlet, resulting in the highest erosion rate. The U-shaped bend shows a greater degree of damage at the 60°–90° outlet. Additionally, the bend's upward inclination angle significantly affects the degree of erosion. When the upward inclination angle is less than 45°, the pressure gradient increases with the angle; when it exceeds 45°, the pressure gradient changes less. Fluent numerical simulations also show that erosion wear on the outside of the bend is the most severe. Installing a guide vane can improve this situation. The optimal position is 0.36D from the inner wall of the bend. Peyman conducted a comprehensive inspection of the 90° bend damage using non-invasive ultrasonic technology and flow visualization. The results show that pressure on the outer arch side is the highest, quartz sand particles accumulate near the outer arch side wall, and the degree of damage is the greatest. Flow separation and secondary circulation in the 90° right-angle bend cause the most severe erosion of the outer arch side wall and the downstream straight pipe section. The size of quartz sand particles, fluid velocity in the pipe, and specific gravity in the two-phase flow significantly affect the erosion rate.
 
Besides the effects of fluid and solid particles, bend erosion is also related to the material and surface roughness of the bend. Pascal Fede and others studied the erosion of the 90° bend using a multi-fluid method and analyzed the effect of wall roughness on erosion. They established a prediction model, evaluated its accuracy using experimental data, and analyzed the effects of particle number, wall recovery coefficient, friction coefficient, and surface roughness on the erosion rate. Thiana A and others studied the erosion rate of bends under different flow conditions using CFD numerical simulations and found that the maximum erosion rate increased with the velocity of multiphase flow, identifying the location of maximum erosion. Additionally, they combined the Reynolds-Stokes equation with the random Lagrangian particle tracking scheme to study bend erosion caused by particle impact. Bends made from different materials exhibited varying erosion patterns. The failed wall of the copper bend was examined using scanning electron microscopy to determine the location and cause of the maximum erosion rate. Establishing a mathematical model helps predict erosion in oil pipeline bends. Studies have shown that erosion mainly occurs near the bend's outlet. CFD simulations and array electrodes were used to study the erosion behavior of X65 steel bends, revealing the main factors and differences in the erosion-corrosion rate and its variations at different positions of the bend.