Numerical Study of Gas-Sand Two-Phase Flow Erosion in a Standard 90° Elbow ^†

Abstract

Erosive wear is a common problem in hydrocarbon industry pipelines transporting a multi-phase flow. Erosion is more critical in flow-changing devices, like tees, elbows, U- bends, valves, and chokes. By performing numerical simulations, this research investigation focuses on pre-eminent parameters that influence elbow wear, including erodent size and flow velocity. In this analysis, particle diameters of 150 and 300 μm and velocities of 11, 15, and 23 m/s were used. At a velocity of 11 m/sec, the erosion rate induced by a particle size of 300 μm was 1.62 times higher than with a particular size of 150 μm. Similarly, when the velocity was increased from 15 m/s to 23 m/s for the same particle size of 300 μm, the erosion rate was increased by 3.23 times.

1. Introduction

In the hydrocarbon industries, erosion is a common problem that leads to pipeline leakage, component damage, and even overall pipeline system failure. Erosive wear takes place when a fluid-carrying erodent consistently impacts the target surface [1]. This consistent impact force applied by the erodent causes gradual material loss from the target material [2]. The angle of impaction between carrier fluid and target surface plays a crucial role in defining the erosion rate [3]. Therefore, flow-changing devices, like elbows, valves, chokes, tees, U-bends, and offset elbows are comparatively more prone to erosive wear and tear [4].

The standard 90° elbow is widely used in the hydrocarbon industry as a flow-changing device [5]. Erosive wear in the 90° elbows has been the central focus of various experimental and numerical investigations [6]. Erosion in elbows is quite large compared to straight pipes [7]. Though erosion is inevitable and cannot be avoided fully, engineers and scientists have utilized a diverse range of erosion mitigation techniques to minimize devastating damages to elbows due to erosive wear and tear [8].

Design modification is a popular technique for mitigating erosive wear and tear in elbows. A vertex chamber was examined in comparison to a standard 90° elbow [9]. The basic geometric parameters, such as pipe diameter, domain size, and curvature radius, were not modified. A semi-sphere was constructed at the upstream section. The research work concluded that the erosion rate in the vertex chamber was considerably reduced, and in the worst-case scenario it was half of the 90° elbow.

This research study presents a numerical approach to predict the erosion rate and mechanism in a standard 90° elbow. The key focus of this research work aims at analyzing the influence of two-phase, air-solid flow on the erosion of the elbow at different locations. To decouple the effect of two-phase, air-sand flow on erosion in a standard 90° elbow, it is subjected to a range of flow velocities and mass flow rates.

This research work numerically validates the experimental and computational claims made by Viera et al. [10].

2. Methods

The 3D model displayed in Figure 1 was created in SOLIDWORKS. To compare and analyze the results, the geometry of Viera et al. [10] was utilized for the simulations. Flow enters the pipe through a straight section that is 1000 mm in length. This section contains a 90° elbow with r/D equal to 1.5 followed by another straight segment that is a 600 mm flow outlet. The geometry and other details are shown in Figure 1. The inside diameter (D) of the pipe is equal to 76.2 mm. In this study, a grid with 2.7 million cells was used for all the simulation cases due to its optimal computational cost and accuracy.

The experimental results of Viera et al. [10] were validated using CFD simulations. In Viera et al.’s experiment, various tests were performed at different air velocities and different sand flow rates. The experimental results were then validated by CFD simulations. For the vertical horizontal (V-H) configuration tests, sand sizes of 150 and 300 μm were used.

3. Results and Discussion

This research work presents a numerical approach to verify the experimental findings of Viera et al. [10]. Six cases with different velocities and particle sizes were carried out. The operating conditions were consistent with those used in the experiments. The numerical results, which are displayed in

Table 1. Comparison of numerical and experimental results.

Tests	Air Velocity (m/s)	Particle Size (μm)	Sand Rate (kg/day)	Erosion Rate (mm/year)		%Error
				Experimental Results	Numerical Results
1	11	150	254	6.5	7.31	12.46
2	11	300	288	16.9	19.2	16.36
3	15	150	237	13.2	15.03	13.86
4	15	300	192	19.3	22.83	18.29
5	23	150	257	36.2	42.33	16.93
6	23	300	227	80.3	96.70	20.42

, were in close agreement with the experimental results.

A literature study reveals that elliptical and V-shape erosion scars appear on the largest inner surface bend (extrados) of the elbow due to erosive wear. In this research work, both elliptical and V-shape scars were developed, as shown in Figure 2a. The erosion rate is negligible from 0° to 20° and increases gradually from 20° to 55° of the 90° elbow, with severe erosion observed at 45° to 55°. Figure 2b shows the erosion rate at different curvature angles of the standard 90° elbow.

It was observed that the erosive wear rate increased with erodent size diameter, as shown in Figure 3. Erodents of 150 and 300 μm size were used in this case study, and carrier fluid velocity was kept at 23 m/s for both particle sizes.

The influence of fluid velocity on erosion rate was analyzed. Two flow velocities, 11 and 23 m/s were considered in this case. The particle size of 300 μm was used for both velocities. It was concluded that the erosion rate increases with the fluid velocity.

It is evident from the numerical results that the highest pressure is observed at the outer surface, and the lowest pressure is achieved at the inner surface of the elbow. Gradual pressure drop is observed from outer surface towards inner surface. The pressure distribution across the entire elbow is presented in Figure 4.

Particle trajectory shows that particles enter through the inlet and follow a straight path and then move in an irregular pattern after hitting the elbow. The majority of the erodent impacts the outer wall, which produces maximum wear. The inner wall of the pipe is a no-erodent zone with minimum erosion, as shown in Figure 5.

4. Conclusions

A CFD-based numerical approach was utilized to validate previous experimental and numerical results of erosion in a 90° elbow bend. The influence of different parameters on erosive wear rate and future recommendations are extensively discussed below.

• A detailed study of the impact of particle size on erosive wear was conducted. It was found that erosive wear increased with the particle diameter size when the flow velocity and mass flow rate were kept constant.
• An increase in erosive wear rate with flow velocity was observed. Maximum erosion was detected at 45° to 55° of the largest inner surface bend (extrados) of the elbow. V-shape and elliptical erosion scars were produced at the extrados of the elbow.
• Maximum pressure was detected at the extrados of the elbow due to maximum particle impact. A gradual decrease in pressure was observed from extrados towards intrados.
• Maximum flow velocity was observed at the intrados of the elbow due to low pressure. A gradual decrease in velocity was observed from intrados towards extrados.

The influence of non-spherical sand particles could be studied in future for a better understanding of sand erosion, as sand particles are non-spherical in nature.