Solid particles are encountered in oil and gas pipelines when producing from unconsolidated reservoirs. Solid accumulation can lead to increase in pressure loss, erosion, corrosion, production decline etc. The stratified flow regime is most vulnerable to particle deposition, owing to low phase velocities that occur in this regime. Various sand flow regimes may occur, whereby particles can stop moving and accumulate along the pipe either as stationary dunes or a stationary bed. The objective of this paper is to investigate experimentally and theoretically the transition boundary between two of the regimes, namely, concentrate solid flow (moving bed) and moving dunes. Experimental data were acquired in a 10 cm (4 in.) ID pipe, using air and water. Spherical glass beads with specific gravity of 2.65, 45-90 mu m and 125-250 mu m in diameter, at concentration up to 10,000 PPM, were utilized. Acquired data include the effect of superficial velocities, particle size and particle concentration on the different sand flow regimes, including the moving bed and the moving dunes. The acquired data showed that the transition between the moving dunes and the moving bed is strongly dependent on the above mentioned parameters. A mechanistic model was developed for the prediction of the transition between the moving dunes and the moving bed, based on a rolling mechanism for particles located between two sand piles. When the particles rolled up on a pile, transition to the separated moving dunes occurred, otherwise, the moving bed was formed along the pipe. Comparison between the acquired data and the model predictions showed a discrepancy of 15%. (C) 2019 Elsevier B.V. All rights reserved.