Paper mill tackles material flow problem at hot lime bin, Solutions!, Online Exclusives, September 2002

To address the problems with maintaining reliable feed from the hot lime bin, the mill planned a joint effort between an outside engineering firm and mill personnel. Jenike & Johanson Inc., Westford, Massachusetts, USA, worked with a Weyerhaeuser team made up of technical, operating and maintenance personnel. The project involved the replacement of critical portions of the bin to improve material flow.

Flow interruptions
The original hot lime bin had a small, square discharge opening that was subject to frequent flow interruptions. Pneumatic vibrators were not successful in maintaining flow. Workers could clear about half of the pluggages by using high pressure air hoses connected to several locations of the lower portion of the bin hopper; however, these air hoses caused fine dust to escape from the feeder area. For the remainder of the pluggages, the standard procedure was for operators to use a sledgehammer on the side of the discharge neck, which was precarious due to equipment access. When the pluggage cleared, additional dust escaped from the feeder area.

These pluggages required frequent clearing, which exposed operators to potential safety incidents. Additionally, the discharge of fine lime dust to the surrounding area was an environmental concern. The pluggages caused swings in slaker operation, which carried through the causticizers, resulting in inconsistent liquor quality. Control room personnel monitored a surveillance camera trained on the feeder discharge to prevent these problems.

Investigating the problem
The hot lime bin consisted of a 6705mm (22 ft.) diameter by 5486mm (18 ft.) tall cylinder section, followed by a 40° from vertical conical hopper that reduced to a 431mm (17 in.) square outlet over a height of 4013mm (13 ft.-2 in.) A 305mm (12 in.) screw feeder used an increase in flight diameter, beginning near the back of the outlet and expanding from 152mm to 305mm (6 in. to 12 in.) at the front of the outlet. The typical hot lime feed rate is 14.5 tons per hour. (See Figure 1.) Note the vibrator in the foreground on the hopper surface, along with one of the high pressure air hoses used to clear pluggages.

Overall View of Hot Lime Bin		Detail View of Hopper and Feeder
Figure 1. Unmodified hot lime bin.

Beneath the feeder outlet was a 356mm (14 in.) diameter primary mixing screw. The hot and fresh lime, after being fed separately into the mixing screw, discharged into a 9° inclined slaker feed screw conveyor.

Mill management asked Jenike & Johanson, Inc., a consulting firm specializing in bulk solids handling, to recommend improvements to the system. First, they considered the problem of no-flow from the hopper. Mill workers knew that the particle size of the hot lime could vary up to 50mm (2 in.), although it was typically 6mm (1/4 in.) or less. No-flow is often caused by the formation of an arch over the hopper outlet. An interlocking arch occurs when large particles (relative to the outlet size of the hopper) mechanically lock to form an obstruction. A cohesive arch occurs when particles adhere to one another to form an obstruction (see Figure 2).

Interlocking Arch		Cohesive Arch
Figure 2. Arching.

The likelihood of arch formation is greatly affected by the flow pattern that develops in the hot lime bin during discharge. In funnel flow, shown in Figure 3, the sloping hopper walls of a bin are not steep enough and smooth enough for material to flow along them. Under these conditions, an internal flow channel develops. As the level of material in the bin decreases, layers of non-flowing material may not slide into the flowing channel, which can result in a stable rathole. In addition, funnel flow can cause caking, results in a first-in last-out flow sequence, and increases the extent to which sifting segregation impacts the discharging material. Funnel flow can also result in flooding of fine materials, due to material falling into the active flow channel from the top surface and becoming fluidized.

Flow Pattern		Stable Rathole
Figure 3. Funnel flow.

Figure 4. Mass flow.

Despite these typical problems, many bins in use are funnel flow, since the discharge pattern is often not considered in the design process.

In mass flow, as shown in Figure 4, all of the material is in motion during withdrawal from the bin. Mass flow bins provide first-in first-out flow sequence, eliminate stagnant material, reduce sifting segregation, and provide a steady discharge with a consistent bulk density. Mass flow also significantly reduces fine material flooding during discharge. Requirements for achieving mass flow include sizing the outlet large enough to prevent arching and ensuring that the hopper is sufficiently smooth and steep enough to promote flow at the walls.

Figure 5. Expanded flow.

A third type of flow pattern, called expanded flow and shown in Figure 5, develops when a mass flow hopper is placed beneath a funnel flow hopper. The mass flow hopper activates a flow channel in the funnel flow hopper, the outlet of which is large enough to prevent the formation of a stable rathole. The wall angles of the funnel flow hopper can remain shallower than the angles necessary for mass flow, significantly reducing the required headroom. The mass flow hopper retains the benefits of discharging material with a consistent bulk density while preventing flooding.

Analyzing the problem
To analyze the situation and redesign the bin hopper to reliably handle the hot lime, the project team had to answer some basic questions via flow property tests.

Hopper angle and smoothness: The required steepness for mass flow depends on the friction that develops between the particles and the hopper surface. This friction is measured using a Jenike direct shear tester in accordance with ASTM D 6128 [1], with small samples of material placed in a test cell and slid along wall surfaces of interest. From the wall friction tests, engineers can determine limiting hopper angles for mass flow by using a method developed by Dr. Andrew Jenike [2]. Factors that can affect the results, including time at rest, corrosion, abrasive wear and temperature, must be reproduced during the tests. The mill team ran tests using a specially modified furnace that allowed them to apply loads and make measurements externally (see Figure 6).

Figure 6. Furnace testing apparatus and test cell.

Hopper outlet size:
To achieve mass flow, the outlet must be large enough to prevent arching. Mills can avoid interlocking arches by making sure that the outlet diameter is six to eight times the largest particle size in a circular opening. Cohesive arching can be analyzed by determining the cohesive strength of the material. These tests are also conducted using a Jenike direct shear tester. Consolidating forces are applied to material in a test cell and the force required to shear the material is measured. One can then calculate the minimum outlet sizes to prevent arching or ratholing (in funnel flow) using the theory developed by Jenike [2]. As before, conditions that can affect the results must be reproduced during the tests.

At the New Bern mill, flow property tests confirmed that funnel flow was occurring with the hot lime bin’s 40° (from vertical) conical hopper; tests showed that ratholing was likely for continuous flow conditions. The discharge pattern resulted in a very narrow flow channel forming above the outlet. This narrow channel made arching more likely due to its small size and the higher material pressures associated with funnel flow.

The flow channel was further narrowed due to the configuration of the feed screw auger. The smaller diameter flights did not allow for the full outlet width to be active. Stagnant material remained on either side the flights, which propagated up into the bin above and further increased the potential for flooding.

A new approach
To prevent the pluggage problems, the mill needed to convert at least a portion of the hopper to mass flow, with a larger outlet size and shape. Jenike & Johanson recommended that the existing cone be cut off at the 2438mm (8 ft.) diameter and replaced with a mass flow transition hopper, resulting in an expanded flow pattern. For this effort to be successful, it was also necessary to replace the screw feeder with one properly designed for mass flow so that the entire outlet would become active.

Jenike & Johanson selected a transition hopper design with an outlet width of 737mm (29 in.), which is large enough to overcome arching (determined by the tests). The transition hopper featured vertical end walls, which are most conservative in promoting flow. The side wall angle was selected based on the aforementioned wall friction test results for 304 stainless steel in plate thickness with a cold rolled, 2B finish. Due to this critical requirement, the quality of finish plate used to form the hopper had to be verified during the fabrication phase.

Jenike & Johanson chose a dual 356mm (14 in.) mass flow screw feeder to provide the required activation of the entire outlet of the new transition hopper. The augers were designed to allow for an increase in capacity in the direction of feed; features include a conical shaft section near the back of the screw, as well as an expansion of pitch closer to the front of the screw. The screw flights remain constant at 356mm (14 in.) in diameter, so as not to result in a narrowing of the flow channel as was likely occurring with the existing feeder. The hopper and feeder solution, in comparison to the previous arrangement, is shown in Figure 7.

Figure 7. Before and after hopper and feeder arrangements.

Some height compensation was required for the modified arrangement. The mill installed a new inclined screw conveyor, approximately 7010mm (23 ft.) in length, in place of the existing mixing and inclined slaker conveyors. The fresh lime screw conveyor was able to remain in its current position, with its discharge moved to mate with the new screw conveyor.

Getting up to speed
The mill implemented the changes to its hot lime feed system during a 14 day mill outage in 1999. The mill chose Jenike & Johanson to fabricate the dual 356mm (14 in.) screw feeder, transition hopper, and a shutoff gate. Jenike & Johanson also provided drawings for the plant modifications, which included the cutting and attachment of a new flange on the exiting cone and shortening of the fresh lime screw conveyor. After confirmation with a structural review, cross bracing between the bin legs on one side was removed (temporarily) to gain crane access to the area of construction.

First, workers dismantled and removed the old equipment. They then cut and removed the lower cone section, and prepared the remaining cone for installation of the new hopper (see Figure 8).

Cross bracing removal		Lower cone removal
Figure 8. Bin modifications in progress.

The hopper mounting flange had to be level, which was made difficult since the existing cone was not completely round. Once the hopper was properly in place, the new screw feeder was lifted up and drifted into position with a crane, chain falls and come-alongs. Workers added horizontal support steel beneath the new screw feeder as well. The interface between the new hopper and screw feeder is a slip fit with a heat resistant, fiberglass gasket, which provides a reasonable seal against dust; however, use has shown that some improvements in this arrangement can be made in future installations.

The mixing and slaker feed conveyors were replaced, and the fresh lime screw conveyor shortened as part of the installation. Installers fabricated a chute to discharge fresh lime into the lead end of the new slaker feed screw conveyor, just before the entry point of the hot lime from the new screw feeder. Weyerhaeuser personnel executed the installation plan efficiently, resulting in a flawless startup that was on time. Figure 9 shows views of the final arrangement after the installation.

Side View - Showing Conveyors		End View
Figure 9. Modified hot lime bin.

The team has gained considerable experience with the new equipment since its installation. The plant needed a way to isolate the metering screw for maintenance. A solid gate mechanism was precluded early in the design process because of clearance issues, expense, and fear that if not used over time it might not be functional when needed. Jenike & Johanson proposed a pin gate design that would induce bridging across the hopper outlet and stop flow, which was included with the feeder as part of its supply. The gate proved to be successful when it was used just a year after installation to access the screw feeder for maintenance.

The initial design of the screw feeder housing was not robust enough for the severe duty and heat of hot lime service at the mill. Within months of start-up, the bottom of the feeder trough began to distort and crack along its seam. Maintenance executed some temporary repairs, and the feeder continued operation until the next shutdown. Meanwhile, Jenike & Johanson redesigned and provided a replacement trough. During a shutdown and the first opportunity to service the feeder, the trough was replaced, and there have been no further issues with it since. Some minor damage to the screws has been discovered over time. The first occurrence was caused by chunks of kiln brick that made it past the crusher, while the second occurrence was from a solid steel sounding weight, used to determine bin level, that fell and jammed the screws. Beyond the initial items noted, the new feed arrangement is still in operation, without any pluggages occurring in the bin. The video camera, monitor and sledge hammer that the old system required were gladly retired.

REFERENCES

1. "Standard Shear Testing Method for Bulk Solids Using the Jenike Shear Cell," American Society of Testing Methods (ASTM), D 6128, 2000.
   
2. Jenike, A.W.: Storage and Flow of Solids, University of Utah Engineering Experiment Station, Bulletin No. 123, 1964.