Joe Marinelli, president, Solids Handling Technologies Inc.

This is a very broad question. Obviously proper storage silo design is critical to the successful everyday operation of a production facility. Storage silos must operate properly to ensure reliable product flow to your process, mixer, truck, etc. Improperly designed silos can cause production interruptions, increased workload, safety hazards, and more importantly, can affect the bottom line.

First, be aware that the science of bulk solids handling was developed mainly due to the work of Dr. Andrew W. Jenike, who pioneered the theory of bulk solids flow in the 1950s that is still relevant today. Flow interruptions such as material arching, ratholing, flushing, and segregating are troublesome and, in many cases, not easily prevented. These flow problems result in limited live storage, caking and spoilage, shaking, and potentially structural failure.

Bulk solids typically flow in one of the following two patterns: funnel flow and mass flow. In funnel flow, some material moves, while most remains stationary or stagnant. Flow problems such as ratholing, flushing, segregation, and erratic flow are common problems in funnel flow. Mass flow occurs when all the material moves whenever any is withdrawn. Arching can be avoided, fine powders tend to deaerate, and segregation is minimized, so the problems of funnel flow can be avoided.

You might ask if there is a way to determine how my material flows? The work of Dr. Jenike involved development of a laboratory test procedure to identify material flow properties. This test procedure is the standard in Europe and in the US through ASTM. It involves determining a material’s cohesive properties (ability to arch and rathole), wall friction properties, and compressibility.

Most materials have cohesive strength, which requires determining minimum opening sizes to prevent the problems of arching and ratholing. This requires measurement of the material’s cohesive strength as a function of pressure (everything in bulk solids flow is affected by pressure). Mass flow--the preferred flow patten--requires all the material in the storage silo to move upon discharge and this requires material flowing at hopper wall surface. To determine the hopper angle for mass flow, the material’s wall friction properties are determined on a representative surface such as stainless steel, carbon steel, UHMW polyethylene, aluminum, etc. Lastly, your material bulk density is determined at various solids pressures (compressibility).

Factors that affect flowability are moisture content, particle size, temperature, and time of storage at rest. Moisture content increases affect flowability. Therefore, the higher the moisture, the more cohesive a solid is. Particle size affects flow as typically: the finer the particles, the more cohesive they are. Temperature affects flowability with higher temperatures causing material particles to “stick” together. Lastly, time of storage at rest is a big factor. Typically, solids gain strength after some period of storage at rest under load.

With the above in mind, let’s talk about silo geometry. Most importantly, the hopper section in the lower portion of the silo is required to ensure reliable solids flow. Hoppers can take many shapes, the most common of which are cones and wedges. Conical hoppers are inherently strong and easily fabricated, but typically require steep walls to ensure mass flow and use a circular opening. Wedge-type hoppers use flat surfaces to form the wedge hopper. Wedge hoppers ensure mass flow at shallower hopper angles than cones, and still maintain mass flow. Wedge hoppers converge to a slotted opening and are slightly more costly. Wedge hoppers typically require feeders such as screws and belts to control discharge.

This brings up another concept in that the feeder design is just as important as your silo design. An improperly designed feeder will destroy a mass flow pattern. Typical feeders used are screws, belts, rotary valves, etc. With screws and belts (used on slotted openings) the feeders must increase in capacity along the length of the slot to maintain mass flow. Rotary valves are typically used on circular or square openings. Interestingly, they tend to create preferential flow channels that can initiate a funnel flow pattern destroying mass flow.

In summary, proper silo design is not easy. Knowledge of a material’s flow properties and proper design techniques are critical to avoiding silo flow problems and provide uninterrupted, uniform flow to your process.

Joe Marinelli is president, Solids Handling Technologies Inc.