The influence of the material stress state induced by internal and external forces on the erosion rate of pipelines has rarely been investigated in the literature. In order to fill this gap, using a tensile tester machine, a two–phase gas–solid particles jet erosion test was carried out considering a 316L stainless steel under different tensile stresses and different erosion angles. The results show that: 1) In the elastic range, with the increase of stress, the erosion rate manifests a rising trend; 2) In the metal plastic range, the increase of stress leads to a decrease of the erosion rate; 3) The erosion rate at a small erosion angle is more sensitive to the increment of stress. The present research demonstrates that the combined effect of erosion and material internal stress can contribute to determine the effective resistance of the vulnerable parts of piping systems (such as elbows or similar components).
In oil and gas gathering and transportation processes, sand particles are contained within the extracted oil and gas mixture. When particles move with the fluid to the elbow, tee, or valve, where the flow direction suddenly changes, their inertia will impact the inner pipe wall and possibly damage it. This process is known as erosion and causes serious safety hazards [
By carrying fluids, the erosion wear can be divided into three categories: gas-solid erosion, liquid-solid erosion, and gas-liquid-solid three-phase erosion [
The primary factors affecting the erosion rate are particle speed, impact angle, and target properties [
Scattergood et al. [
In currently used erosion models, the indentation hardness is commonly applied to determine the severity of cutting the material [
The load pipeline bears can be roughly divided into four categories [ Pressure loads caused by the internal temperature and pressure carried by fluid flowing through the pipeline; Continuous external loads, which mainly include the pipe weight, the weight of its support and hanger accessories, outer pipe insulation material weight, and other uniformly distributed external loads; Thermal loads generated by inconsistencies of temperatures during the installation and the operating states. The inconsistencies cause thermal expansion and contraction of the pipeline; Occasional loads, which include occasional temporary loads such as fluid shocks and snow loads, which are generally not considered.
Finally, a comprehensive analysis has shown that stress has a significant impact on the properties of the pipe metal.
Studying the effects of stress on the target material properties started in 1932 by Kokubo et al. [
Based on the above-presented studies, it is evident that the stress state of the target material will significantly affect the hardness, in turn affecting the erosion behavior. Furthermore, in recent years, many studies on pipelines erosion behavior were carried out; however, only a few have considered the influence of pipeline loads on erosion. For this reason, a gas-solid two-phase jet erosion testing equipment was designed as a first step. Next, a tensile test machine was used to carry out an erosion experiment for different tensile stress conditions. Since the erosion mechanism change with the incident angle, erosion experiments under stress were carried out at incident angles of 30°, 45°, 60°, 75°, and 90°. The 316L metal was selected as the target material due to its wide application. Finally, the influence of stress on the erosion rate was investigated and the microscopic morphology of the erosion crater was analyzed to reduce the damage.
Aiming to study the erosion process in target material under stress, a gas-solid two-phase jet erosion experiment was carried out for different tensile stress conditions. The schematic diagram of the experimental device is given in
The tensile tester model used in the experiment was WANCE-ETM502B (technical parameters are provided in
Testing level | Testing range | Measuring range | Relative error | Testing machine resolution | Displacement resolution | Beam speed adjustment range |
---|---|---|---|---|---|---|
1 | 10 N to 2000 N | 0.4% to 100% FS | ± 1% | 1/500000 FS | 0.027 μm | 0.001–500 mm/min |
A nitrogen gas cylinder was used as the gas source in the experiment. The experimental test staple material was 316L stainless steel, chemical composition and mechanical properties of which are given in
Fe | C | Mn | Cr | Ni | Mo | Si | P |
---|---|---|---|---|---|---|---|
69.11 | 0.023 | 1.17 | 17.23 | 10.32 | 2.14 | 0.03 | 0.04 |
Tensile strength (MPa) | Conditional yield strength (MPa) | Yield strength (MPa) | Elastic modulus (GPa) | Poisson’s ratio | Brinell hardness (HB) | Vickers hardness (HV) |
---|---|---|---|---|---|---|
≥480 | ≥177 | 270 | 206 | 0.299 | ≤187 HB | ≤200 |
After assembling the experimental rig, the tensile tester machine is opened and the tensile force was adjusted to 0 N (which resulted in the stress of 0 MPa), 2300 N (100 MPa), 4600 N (200 MPa), 6900 N (300 MPa), 9200 N (400 MPa), and 11500 N (500 MPa), depending on the required testing conditions.
Samples were weighed before and after the erosion, experiment to obtain the masses M1 and M2, respectively. The total weight of erosion particles was recorded as Mp and was used to calculate the erosion rate. The experiments were replicated three times for each experimental condition and the average of the three values was used as the final result to reduce error. The expression for calculating the total erosion rate is given as
The main factors affecting the erosion rate are the particle velocity, the incident angle, particle property, and target martial property, as mentioned earlier. In the experiment, the Al2O3 particles had the same size; therefore, the effects of particle properties were excluded. By keeping the outlet valve opening consistent, it was ensured that the particle velocity will be constant during the experiment. The incident angle was adjusted by fixing the nozzle. Therefore, the only experimental variable in the erosion experiment for a given incident angle was tensile stress.
The variation of total erosion rate with the stress and impact angle was obtained using the gas-solid two-phase experimental system (see
Additionally, the erosion rate with a small incident angle was more sensitive to the stress increment. For the test sample with a 30° impact angle, the erosion rate at 0 MPa was 2.8591 × 10–5 kg/kg⋅h, while its value was 7.2666 × 10–5 kg/kg⋅h at 200 MPa. Hence, the erosion rate increased 2.5 times. In contrast, under the same stress condition with the impact angle of 90°, the erosion rate only increased 1.5 times.
Following the experiment, a wire was used to cut the test sample erosion position and the surface morphology of the erosion parts was scan using an electron microscope. The impact angle of the observed specimen was 90° and test piece stresses were 0 MPa, 100 MPa, 200 MPa, 300 MPa, 400 MPa, and 500 MPa, respectively. The surface image of the center pit part eroded under those conditions is shown in
After observing the scanning electron microscope (SEM) pictures shown above, it can be seen that the erosion surface at 90° incident angle was dominated by pits caused by repeated particle impacts. Among them, the surface craters at the 0 MPa test sample were not noticeable and the overall surface morphology remained relatively flat. The density of surface craters started to increase with the 100 MPa test sample, which had a rugged surface with evident traces of repeated particle impacts. Further, the diameter of surface craters became larger at 200 MPa, with the pit depth being much deeper compared to the 100 MPa test sample. Moreover, the surface pit diameters of the 300 MPa test sample were smaller than that of the 200 MPa sample; similarly, the pit depth was also lower. The surface of 400 MPa had no easily noticeable pits and was relatively flat. Similar was observed for the surface of the 500 MPa sample; there were practically no pits and the surface was flatter than that of the 400 MPa.
Based on the analysis above, it can be concluded that as the stress increases from 0 to 200 MPa, the surface impact marks gradually become noticeable, while the pit density and diameters increase. This implies that, within the elastic range, it is easier for particles to embed within the material and remove the surface pieces as the stress increases, causing increasing both the impact crater diameter and density. After particles impact the crater, a lip-like protrusion is formed around it. In the subsequent impact, the elevated material will be taken away, forming a crater through subsequent particles. This is consistent with the fact that those pits were found using the scanning electron microscope; however, there were no large lip-like protrusions around them. Additionally, in the plastic range, the material impact resistance increased with the increase in stress.
In addition to the pits, the forging signs such as small flakes and cracks can be seen on the target surface using the scanning electron microscope. According to the erosion theory proposed by Levy [
It should be noted that there were more flakes found on the 300 MPa and 400 MPa test samples, which also exhibited a greater flake size compared to the remaining samples. However, there are no visible flakes and cracks produced by forging on the surface of 100 MPa and 200 MPa samples. That is, the forging behavior had more influence on the surface morphology in the plastic stress zone. Thus, when the target is not stressed, inconspicuous pits and forging marks are found on the surface. When the target is in the elastic region, the density and diameter of surface pits increase with stress. On the other hand, when the target is in the plastic region, the surface morphology pits decrease as stress increases. The size of small flakes produced by the forging marks firstly increases with stress, then decreases after reaching the maximum value at 400 MPa. In other words, when the impact angle is 90°, the mechanism of removing the target surface material is closely related to its stress.
In this paper, a gas-solid two-phase erosion experiment was carried out at different stress; the relationship between the erosion rate, stress, and incident angle was obtained. Aiming to further study the erosion pattern, the influence of stress on the microscopic erosion morphology was analyzed using SEM. Based on the results, the following conclusions were made: For the 316L metal in the elastic range 0–200 MPa, the erosion rate increases rapidly with the stress, weakening the material erosion resistance. In the plastic range (300 MPa to 500 MPa), the erosion rate decreased as the stress increased; however, pipelines used in engineering rarely reach the plastic stage. Thus, within the allowable engineering stress range, the stress will significantly increase the metal erosion rate. Although within the elastic range, the erosion rate increased with the increase in stress for all incident angles. Moreover, the erosion rate at each angle increased at a different range. The erosion rate at large impact angles was not sensitive to stress as the erosion rate at 90° incident angle only increased 1.5 times, while at a small impact angle was it was more sensitive to the stress increment. The erosion rate at 30° incident angle increased 2.5 times. The target material properties will change with the stress increment, leading to changes in both the diameter and depth of the pits, affecting the erosion rate. Lastly, by observing the microscopic morphology, it was also noticed that the stress will change the way the particles cut the material. Erosion at elbows and similar places should consider the influence of stress. Thus, measures such as increasing the wall thickness, increasing the elbow curvature, and installing the additional features to reduce stress should be implemented to reduce the erosion risk and damage to the pipeline.