Silo Arching & Ratholing Minimum Outlet Calculator
Determine critical cohesive arching outlet dimension ($B_{min}$) and stable rathole piping diameter ($D_{rh}$) based on powder shear cell yield strength.
Powder Cohesion & Bulk Properties
Minimum Outlet Sizing to Prevent Stoppages
Silo Obstructions: Arching vs Ratholing
Bulk solids flow blockages occur primarily through two physical mechanisms:
• Cohesive Arching (Bridging): A mechanical dome or bridge of powder forms across the discharge opening, holding up the entire contents above. To prevent arching, the discharge outlet dimension $B$ must exceed the critical cohesive arch span $B_{min} = H(\theta) f_c / \gamma$.
• Ratholing (Piping): In funnel flow silos, material empties only from a core cylinder above the outlet. The surrounding stagnant annular bulk consolidates into vertical freestanding walls. Flow stops completely once the central core empties unless the outlet exceeds $D_{rh}$.
Circular vs Slotted Outlets
A circular opening requires a diameter twice as large as a slotted rectangular opening ($B_{slot} \approx 0.5 \times B_{round}$) because a rectangular slot allows the cohesive bridge to collapse in pure plane strain.