Engineers at Martin Marietta Magnesia Specialties, Inc. were in the process of designing two steel silos to handle dolomite lime, at the Manistee, MI plant. Each silo was to be 21 ft. in diameter by approximately 56 ft. tall with a 30° (from vertical) cone converging to a 2 ft. diameter outlet. Below the outlet, a round-to-square transition with a slide gate opened onto a vibratory pan feeder/conveyor.

Martin Marietta's engineers asked us to evaluate the proposed silo design. In particular, they required that the silos provide mass flow in order to minimize segregation [220K QuickTime video] and ensure reliable flow.

As with a large percentage of our work, we began be requesting representative samples of the dolomite lime for testing. In this case, Martin Marietta's engineers selected two samples, described as "fines" and "nuggets." Tests were performed by our highly experienced technicians in our specialized testing facilities. These tests, which provided the necessary information to reliably predict the handling behavior of the material, are based on the widely accepted storage and flow of solids theories developed by our founder, Dr. Andrew Jenike.

The test results showed that this dolomite lime is easy flowing, and thus able to flow through small outlets without forming a cohesive arch. However, it was also very frictional, requiring wall angles much steeper than 30° (from vertical) for flow to occur along steel walls. We concluded that the proposed design would not provide mass flow when handling this material.

With a potential problem discovered, our next step was to determine the best method to correct the situation. Using the extensive solids handling experience of our engineers as well as proven techniques for solving difficult material handling problems, we investigated several alternatives, to find the most practical in terms of cost and performance.

The solution recommended by J&J, and selected by Martin Marietta, incorporated a unique BINSERT® design. We took the normal cone-in-cone concept one step further with the addition of an internal third cone as shown in the figure. In doing so, we were able to utilize the original silo envelope with a 30° cone, thus maintaining the required silo capacity in the same height.

The BINSERT® system is unique in that it allows less steep wall angles to be used, but still achieves mass flow. It works by altering the pressures that are exerted on the walls of a conventional single cone hopper to force the material to slide along the walls, thus creating mass flow.

With a BINSERT® system, segregation can be controlled better than with a mass flow silo with a single cone. A single cone design minimizes segregation as long as an adequate material level remains in the cylinder section of a silo. As the level drops into the hopper section, material at the center flows faster than at the perimeter of the hopper, which can cause some segregation. The BINSERT® system eliminates this velocity difference, thereby virtually eliminating segregation regardless of the material head.

In order to prevent interlocking of large two inch particles of dolomite lime, the minimum outlet diameter of the innermost cone had to be 1 ft. Therefore, we had to terminate the outer cone at about a 3 ft. diameter. This change in outlet diameter required a new transition piece to mate with the feeder.

The original transition piece design called for a 2 ft. square outlet, fed by a vibrating pan feeder. If the original design were used, the material would have fed from only one end of the outlet, thereby destroying the desired mass flow pattern. Therefore, we designed a new transition hopper with a slotted outlet. This design eliminated the vibrating pan feeder, and utilized a belt feeder with an interface designed to withdraw material uniformly from the entire area of the outlet.

The overall design was accepted by Martin Marietta, and the two silos were fabricated.

After the silos were erected, Martin Marietta's engineers found that certain construction tolerances had been exceeded. They asked us to review the measured deviations and predict their effect on flow in the silos. We found that the internal cones were out of round and not concentric. After analyzing these measurements, we recommended corrective action—taking into consideration cost as well as material flow through the silos. Our continued involvement with the project helped to ensure that potential problems were eliminated and that the final product would work as intended.

A contractor made the recommended modifications, and Martin Marietta put the silos into service. R. M. Racey Jr., the senior staff engineer at Martin Marietta with whom we worked on this job, wrote in an unsolicited letter, "I want to thank you for the design work your firm provided us on our lime storage silo project. We are pleased that the silos function as intended and all of the equipment is working well."

We at Jenike & Johanson are pleased that we were given the opportunity to be of service at an early stage of this project. It is very satisfying to help a client by diagnosing and preventing a potential problem, instead of correcting an existing problem that could have been avoided.