Abstract: A silo for gravity flow storage of bulk particulate solids comprises downwardly converging and downwardly diverging walls. The downwardly converging walls form angles to the vertical that satisfy the conditions for mass flow. The walls impose particle velocity gradients in the horizontal cross sections of the silo, reducing interparticle cohesion, preventing the formation of arches and promoting uniformity of residence time within the silo. The modules may be vertically stacked, and may be clustered to increase capacity and to reduce pressure stress levels where required.