ratholing

Ratholing is a flow pattern observed in bulk solids handling, typically in silos and hoppers, where material discharges through a narrow central channel (a rathole) while material in the outer portions remains stagnant. This channel forms a funnel-like pathway that allows some material to exit, but at the expense of cross-sectional flow, leaving a substantial amount of the stored material unmoved around the walls.

Causes of ratholing include material properties such as cohesion, moisture, fines, and irregular particle shape, as well as geometric factors like a small outlet relative to the silo diameter, high wall friction, and smooth, rough, or uneven interior surfaces. Materials prone to bridging or arching are especially susceptible. Ratholing is often discussed in contrast to mass flow, where the entire cross-section moves toward the outlet rather than downtown channels forming.

The effects can be problematic: reduced discharge reliability and rate, uneven or inconsistent feeding to downstream processes, and the risk of complete blockage or sudden collapse of the stagnant material when the channel finally fails. It can also lead to contamination or degradation if older stored material remains undisturbed for long periods.

Detection is typically via monitoring of discharge behavior, level changes near the outlet, or direct visual observation. Specialized instrumentation can reveal nonuniform flow patterns or the persistence of unmoved material during discharge trials.

Mitigation focuses on hopper redesign and flow aids. Approaches include adopting mass-flow geometries with steeper walls and larger outlets, ensuring smooth interior surfaces, and using vibration, air jets, or mechanical agitators to disrupt forming channels. Additional measures include material conditioning (moisture control), flow aids, or recirculation strategies to promote uniform cross-sectional flow and prevent ratholing.