If you've ever had a fully loaded bin that suddenly stopped discharging, leaving several tons of materials packed in an immovable mass inside, you know material flow problems can be costly. Lost production isn't the only cost:

Breaking up the ratholes or bridges problems can take much labor and cleanup, sending workers inside your large storage vessel to unclog the stoppage can be dangerous and using a sledgehammer or the other device on your vessel's exterior can leave the vessel stressed and permanently deformed.

Is there any good news? Getting even flow-resistant materials to flow from a storage vessel doesn't have to be difficult or costly. And in cases where you don't have enough headroom for a mass-flow storage vessel (whose steep hopper angles promote flow but require a lot of height, as shown in Figure 1), you can use various flow aid devices to get materials flowing smoothly.

With free-flowing materials that occasionally hang up, you may need to apply a localized flow aid to the vessel's wall. Examples of these are a vibrator, which transfers vibration through the vessel wall to the material or an air injector, which injects pulses of air into material near the vessel wall.

Since vibrators are attached directly to the vessel wall, they must be carefully applied to avoid damaging the structure. This limits the force that can be imparted to the material and they are therefore generally used only for smaller bins with flow problems.

Air injection can be used in one area of the bin or over a variety of locations. Localized use targets a particular hang-up while multiple injection points seek to fluidize a large portion of the stored material to achieve flow. When operated as a feeder for downstream equipment, air induced fluidization must be carefully used to avoid upsetting flow through the feeder and loss of accuracy and rate control.

More flow resistant materials often require a bin activator (Figure 2) which replaces a storage vessel's cone section and outlet. The bin activator consists of a cone or dish-shaped housing with angles from 40 to 60 degrees, less steep than required by a mass-flow bin. An adapter ring at the unit's top mounts the activator to the bin bottom. A hanger assembly on the adapter ring suspends the activator from the bin and isolates the bin structure from vibration. A flexible sleeve is used to seal the gap between the bin and the bin activator. An internal baffle (called a pressure cone) is located at the bin activator's center above the outlet, and a vibratory drive is located on the bin activator's exterior.

In operation the baffle relieves the material head pressure on the outlet while the vibratory drive vibrates the entire bin activator, the baffle, and the material in the bin, but not the bin itself. The drive's vibratory thrusts, throw the material horizontally toward the outlet. At the same time, the baffle resolves the horizontal thrusts into strong vertical motion that extend far up into the bin, preventing bridging.

In general, the more flow-resistant the material, the bigger the bin activator must be in relation to the storage vessel's diameter. As shown in Table 1, for a Class I granular, free-flowing material, the bin activator should be one-fourth to one-third the bin diameter, and for a Class IV fibrous or flaky material, the unit should be the same diameter as the bin.  In cases where materials of different classes will be stored in the same vessel, the bin activator should be sized to handle the most flow-resistant material. Also, the faster the desired discharge rate, the larger the bin activator's outlet should be. In general, the bin activator should have the largest possible outlet the downstream equipment will allow.

Table 1:  Material Classifications and Bin Activator Diameter

The choice of which flow aid to use comes down to the difficulty of the flow problem, cost of the solution and logistics of the installation. It makes perfect sense to try a relatively inexpensive bin wall vibrator or air injector first. Bin activators have been successfully employed with the more difficult flow problems, particularly on existing bins. Mass flow bins can be useful in the initial design stages of a new process where their unique space requirements can be accommodated.