Selective Removal of Particles and Shifting of Particle-Size Distribution during Pipeline Transport of Sediment Deposits under Turbulent Flow

An experimental study was conducted to investigate selective removal of particles and shifting in particle-size distribution during a pipeline transportation of sediment deposits under turbulent flow of water. Presence of sediment deposits is encountered in various pipeline transportation operations (e.g., transport of oil, gas, oil sands, and slurry). Effectively removing solids from pipelines is an important part of the pipeline transportation-operation design process. Mechanisms of particle removal from sediment deposits in pipelines could vary depending on the particle size. Understanding how the transport mechanisms are related to particles sizes would help in designing better operational practices. Four different particle-size ranges were selected for experiments: (1) small particles ($d_p<60\mathrm{\mu m}$), (2) intermediate-size particles ($50\mathrm{\mu m}<d_p<150\mathrm{\mu m}$), (3) coarse particles ($d_p>150\mathrm{\mu m}$), and (4) poly-dispersed particles (mixed range). Experiments were performed at three different flow rates, all ensuring turbulent flow conditions, but velocities well below the critical velocity for particle removal (i.e., suspension from the bed). Samples from sediment deposits were taken during the transport experiments using custom-designed bottom extractors at different time intervals. The particle-size distributions (PSDs) of the samples were analyzed using an automated image analyzer equipped with specialized software. Results showed that interparticle attractive forces were dominant among small-size particles, which makes removal of smaller particles within the group more difficult; as a result, concentration of smaller particles within the group increased over the time. For intermediate-size particles, interparticle attractive forces became insignificant. With intermediate-size particles, the turbulent burst forces were able to lift the smaller particles within the group from the interface of the sediment deposits into the main flow. Consequently, the concentration of larger particles increased at the bed, whereas the concentration of the smaller particles, which can be lifted by the bursting force (velocity dependent), was reduced. For large-size particles, the hydrodynamic drag was the major force mobilizing the particles. The particles were moved by rolling, sliding, and saltation mode; therefore, the PSD for the large-sized particles stayed almost the same.