Shaul & Kalman (2014-2015) defined three different types of plugs (Plug-I, II and III) of different particulate materials for dense phase pneumatic conveying. The Present experimental study is undertaken to determine the borderline between Plug-I and the other type of plugs. The borderline is established by using the concept of detachment velocity of the particles from the front of the plug. For the current study, thirty three particulate materials have been tested having the Archimedes number (Ar) in the range of 10(-3) to 10(6). The experiments conducted in three transparent Plexiglas pipes of diameters 1 in., 2 in. and 3 in. For each experiment, a single artificial plug of type I of varying length in the range of 10-100 cm was inserted into the test section of 1.5 m length. Further, as the plug flow is a low velocity phenomenon, the Reynolds number (R-e) during experimentation, is varied in the range of 10(-3) to 10(2). The results of the experiments show that for the particulate materials with Ar > 10(2), first the phenomenon of detachment occurs at a corresponding velocity and on further increasing the flow rates the plug dissembled. The variation of the detachment velocity is found to be a power function of the Ar number (for Ar > 10(2)), whereas, the detachment velocity is found to be independent of the pipe diameter and plug length. For materials with Ar < 10(2) no detachment of the particles from the plug front is observed and the plug starts to move as Plug-I at a critical air flow-rate and a critical plug length corresponding to respective material. (C) 2017 Elsevier B.V. All rights reserved.