The Effect of Aeration-Assisted Discharge in a Cylindrical Silo with an Inverted Conical Hopper

Aeration-assisted discharge is used on many large cylindrical silos storing very fine dry powders, such as cement and raw cement meal. Most of the more recent large cylindrical silos also have an inverted conical hopper with multiple discharge openings around the bottom perimeter of the inverted conical hopper. An annular ring of aeration pads with a minimal inward radial slope is present between the bottom of the inverted hopper and the silo wall. These silos have generally had both bonded post-tensioned tendons and conventional circumferential wall reinforcement.

Many of these silos have suffered significant structural wall damage. The present Eurocode for silo design specifically excludes silos which have inverted conical hoppers. The ACI 313-16 Standard does not exclude them, but does require designing for asymmetric funnel flow channels at the wall.

Structural inspection of these damaged silos with an inverted hopper and aeration-assisted discharge reveals the primary cause of the silo wall damage is overstress from circumferential flexure combined with circumferential tension. The circumferential flexure is the primary source of overstress. The circumferential flexure is created by the difference in lateral silo wall pressures between the low pressures in the funnel flow channels and the high wall pressures in the static material adjacent to the flow channels.

The funnel flow channels form at the wall due to the discharge openings being close to the silo wall, the slope of the inverted cone, and the aeration pressure in the pads which are immediately adjacent to the discharge openings. The aeration pressure fluidizes a thin layer of material immediately adjacent to the opening. As the material starts to flow into the discharge opening, a flow channel starts to form above the discharge opening. The flow channel increases in size until it reaches the silo wall. The final size of the flow channel is controlled by the magnitude of the aeration pressure. The vertical pressure at the bottom of the channel must be equal to the aeration pressure for the flow channel to be in equilibrium.

Field observations of the top surface of asymmetric flow channels at the wall show them to be semi-circular in plan with the chord being adjacent to the silo wall. The pressures in the flowing material are similar to those in a small silo of similar plan shape with material having a high friction coefficient.  With finely ground material like cement or raw cement meal, the friction coefficient is equal to the tangent of the internal friction angle. Due to the high friction and a low hydraulic radius, the vertical and lateral pressures in the flow channel are very low compared to initial silo wall pressures.

In addition, observation of flexible, transparent silo models demonstrates that the stress field created in the flow channel remains after discharge ceases for that channel. The flow pressures in a channel do not change due to cessation of flow. Therefore, multiple areas of low silo wall pressures can exist simultaneously if multiple discharge openings have been activated.

The high friction of multiple flow channels means that most of the weight of the flow channels is transferred to the static material, increasing the vertical and lateral pressure in the static material. In addition, the multiple flow channels reduce the perimeter of static material in contact with the silo wall, increasing the hydraulic radius of the static material compared to the initial value. Both of these effects cause a significant increase in the vertical and lateral pressures in the static material compared to the initial pressure before flow. Pressure measurements in a large silo in Colombia, S.A. demonstrated that the static pressure was approximately five times as high as the flow channel pressures. The silo had an aeration pressure of 8.6 psi.

Photograph of wall damage of a 72’ diameter by 292’ high blending silo in the Philippines between 145’ to 165’ below the roof. The silo had an inverted conical hopper with seven discharge openings around the perimeter of the hopper. The silo had aeration-assisted discharge in an inward-sloping annular area between the hopper and silo wall. The silo had both bonded post-tensioning tendons and conventional circumferential reinforcement.

Observation of flexible, transparent silo models also demonstrates that multiple asymmetric flow channels can sequentially form immediately adjacent to one another, creating even larger areas of low pressure on the silo wall. The number of immediately adjacent flow channels is controlled by the capacity of the static material to arch horizontally around the area of low pressure. The capacity of the static material to arch horizontally around the low-pressure area is directly proportional to the compressive strength of the static material with the existing confining pressure from the flow channels.