Low concentration particle transport in multiphase horizontal pipes in the presence of a viscous liquid is experimentally investigated. The experiments were conducted for a wide range of liquid and gas flow rates in both intermittent and stratified flows. Critical flow rates (velocity) is defined as the minimum required liquid and gas flow rates (velocities) to keep particles constantly moving in the pipe. The effects of physical parameters such as sand concentration, sand size, pipe size, and liquid viscosity are also experimentally investigated. It is observed that that critical velocity is a function of sand concentration and sand size and increases by increasing either within the ranges of particle concentrations and sizes examined. Regarding the effect of carrier liquid viscosity, the experimental data reveal that by increasing viscosity the minimum required flow rates to constantly move sand along the pipe increases within the range examined. (C) 2016 American Institute of Chemical Engineers