Abstract
Pipe fittings such as elbows and tees are often used in oil and gas production systems and are vulnerable to erosion damage when sand production is anticipated. Annular flow and low liquid loading flows are mostly observed in multiphase gas production pipelines. Sand erosion of elbows and tees in annular flow is complicated by separation of phases including sand, droplet and sand particle entrainment in the gas core and deposition of entrained droplets and sand to the liquid film. Additionally, experiments have shown that average liquid film thickness along the outer bend of the elbow is lower than that for upward vertical flow in straight sections of pipe. The liquid film thickness in the bend can significantly affect erosion as particles that are entrained in the gas core have to penetrate through the liquid film to impact the pipe wall. In order to accurately predict erosion magnitude within pipe bends, prediction of liquid film thickness in bends is important. In this study, experimental data and CFD calculations are being used to predict annular flow characteristics and liquid film thickness. The simulation results of liquid film thickness trends are in reasonable agreement with the experimental data available in the literature. The use of predicted liquid film thicknesses in high gas superficial velocities and low liquid rates allows more accurate prediction of erosion in elbows. A comparison of predicted erosion magnitude with previously developed models for predicting erosion in annular flows and experimental data indicates that the new modifications improve the predicted erosion data.
Introduction
For many industrial applications dealing with gas-liquid flow, transporting liquid and gas simultaneously causes many challenges. If the flow rates are relatively high then solid particles, which are entrained in pipelines, can cause erosion and lead to severe damage to elbows. Annular flow is characterized by the presence of high gas flow at the center of the pipe and liquid film around the pipe wall. Three major characteristics of annular flow can be stated as follows: droplet entrainment into the gas core, deposition of a fraction of entrained droplets onto the gas-liquid interface and existence of waves at the interface. When the difference between gas and liquid velocities is high enough, large shear velocities are induced due to high gas velocity that result in high interfacial shear stress and consequently a flow rate of liquid droplets is generated at the interface of two phases into the gas core. When sand particles are entrained, the same mechanism that entrains liquid can cause sand entrainment in the gas core. Thus, sand particles that are entrained in the gas core are moving with the high gas velocity in the core and impact pipe fittings such as elbows and tees.
