For the reliable design of fluidized dense-phase pneumatic conveying systems, it is of paramount importance to accurately estimate blockage conditions or the minimum transport boundary. Existing empirical models for the fluidized dense-phase conveying of fine powders are either based on a limited number of products and pipelines or have not been tested for their accuracy and stability over a wide range of scale-up conditions. In this paper, based on the test results of 22 different powders conveyed through 38 pipelines, a unified model for the minimum transport boundary has been developed that represents gas Froude number as a function of solid loading ratio and particle Froude number. The model has been validated by predicting the minimum transport boundary for 3 different products, conveyed through 5 different pipelines. Various other existing models have also been validated for the same products and pipelines. Comparisons between experimental blockage boundary and predicted results have shown that the new particle Froude number and solid loading ratio based model provides more accurate and stable predictions compared to the other existing models, which can unexpectedly provide significant inaccuracies. The model incorporates both pipe diameter effect and some important physical properties of the particles. The model is believed to be useful in predicting minimum conveying velocities to avoid pipe blockage and to ensure optimum operating point for industrial pneumatic conveying systems. (C) 2015 Elsevier B.V. All rights reserved.