This paper presents a comprehensive theoretical and experimental analysis of the friction forces of particulate plugs. Friction forces of particulate plugs play a key role in the estimation of the pressure gradient over a pneumatic conveying pipeline. Therefore, a theoretical model to calculate those forces for variable orientations of the pipe inclination (0 90) was developed. In particular, were analyzed the behavior of various parameters such as plug length, plug diameter, internal friction of the material, wall friction, the pressure gradient due to airflow through the plug, and the stress transmission coefficient. The agreement of the new correlations was found to be in the range of +/- 15% for the stress ratio values. Moreover, by substituting these correlations into the newly developed theoretical model and comparing its values to experimental results, an agreement of +/- 30% for the plug friction force calculation was achieved. The present study enables a better understanding of the characteristics of the two-phase (gas-solids) plug flow regime. (C) 2014 Elsevier BM. All rights reserved.