Product bridging describes the obstruction condition where a stable powder arch forms above the outlet of a silo or forms across opposite walls of a silo. In both of these circumstances normal product flow is interrupted. In the worst case scenario when a large product bridge collapses, serious, often catastrophic damage can result to the silo.
Rat holing is a condition where a narrow channel of product preferentially flows to the outlet of the silo, leaving a stable internal mass of product that does not flow at all.
Proper design and sufficient bulk product experience are required to prevent these flow obstructions from occurring. There are minimum discharge or outlet diameters, different for each bulk solid, that must be taken into consideration during the design phase of a properly engineered system.