Lime Company Installs Mass Flow Feeders
Mass-Flow Feeders Replace Lime Company's Previous Vibrating Feeders
Dravo Lime, Maysville, Ky., stores kiln-run pebble lime in crete silos. The lime ranges from 0 to 1¾ inches and
from 65 to 68 lb/ft³. The silos discharge through feeders to a weighbelt, then to a barge loadout system with a
1,140-t/h capacity.
Silo Maintenance Prompts Consultant's Study
The lime company previously
discharged each silo through seven vibratory feeders. One feeder was located in each silo's center and six were
spaced equidistant around the perimeter. As the lime entered, large pieces (pebble lime) moved toward the silo walls
as the smaller particles moved toward the center. By operating several feeders, the lime company blended these variously
sized particles.
However, a silo consultant found that discharging lime from multiple points with vibratory feeders caused channel
flow and vibration that was undesirable from a silo maintenance standpoint. The original feeders also raised dust
and required extensive maintenance. In addition, having seven feeders per silo meant seven feeders to maintain.
Responding to Study Results, Lime Company Opts For New Feeders
Based on the silo consultant's recommendations, the lime company considered switching to one mass-flow feeder in
each silo's center to eliminate channel flow and vibration. The silo consultant suggested a mass-flow feeder manufacturer
that then supplied the lime company with literature, drawings, and a user's list.
Explaining his company's eventual equipment selection, Dravo Lime's John Lohr said, "Two people from our plant
visited several mass-flow feeder installations. Based on what we learned about the feeders - and recognizing that
they had never been used for lime - we nonetheless decided to install the feeders."
The silo consultant
tested the feeders to determine the lime's minimum arching dimension and to ensure the lime wouldn't block the feeders.
The consultant determined that a 24-inch-square feeder would permit mass flow and could be retrofit into each silo's
center discharge. The lime company required feed-rates from 200 to 1,140 t/h per silo, which the 24-inch feeders
could provide. To maintain an appropriate blend of material, the lime company would need to simultaneously feed
material from two of the three silos.
Gates and Feeders Control Lime Flow While Hydraulic Unit Powers and PLC Oversees
Once the feeder configuration was finalized and the equipment was delivered, installing the feeders required no
silo modifications. The only change was to add a rolling slide gate above each feeder. The slide gate has externally
lubricated cam-follower bearings, a hydraulic cylinder, and a ½-inch-thick blade. The blade thickness, hydraulic
cylinder size, and bearing size and spacing were based on the material headload and column height within the silos,
so the blade could cut through a standing material column. In slide gate operation, the hydraulic cylinder opens
and closes the blade, which rolls on the cam-follower bearings.
Each mass-flow feeder (Figure 1)
has two flat rectangular
feeder
blades mad of reinforced carbon steel overlaid with Type 304 stainless steel with a 2B finish. The blades are mounted
on opposite sides within a housing and connect to external swinging radius arms that pivot on flange-block bearings.
Hydraulic cylinders and a synchronizing mechanism attach to the radius arms, which are connected by cross beams.
In feeder operation, the hydraulic cylinders swing the radius arms, varying the width of the opening between the
blades. The opening is aligned lengthwise with the existing weighbelt's center, and material discharges volumetrically
between the feeder blades. The synchronizing mechanism ensures that both blades open at the same speed and to the
same angle.
A hydraulic power unit runs the rolling slide gate and feeders. A 5-horsepower motor runs a pump that pushes
hydraulic fluid to directional valves and actuators. An additional motor and pump serve as on-line back-ups.
A programmable logic controller (PLC) oversees the equipment. To program the PLC for varying feed rates, workers
first take measurements from the weighbelt with the feeder blades open a fixed percentage and then extrapolate feed
rate control information. For example, lime feeds at 1,100 t/h with the blades 50 percent open.
New Feeders Are Installed
For equipment installation, workers remove the existing feeders and closed up the perimeter discharge openings.
Workers disassembled the slide gates to fit them through the silo archways, then hoisted the gates into place and
reassembled them. Factory installation support included computer-aided design (CAD) drawings, electrical information
about linear transducers, and startup assistance.
"Installations went
smoothly," Lohr said. "At startup time, we made some adjustments to the feeder arm connections because we inadvertently
lost some shims. Once the manufacturer's startup worker handled that, it was all square and running smoothly."
The hydraulically powered, PLC-controlled feeders cam deliver lime at 150 to 1,140 t/h. Workers have found that
the lime's particle size affects the federate. "We've noticed a difference in the required feeder blade opening
for large-sized and small-sized material," Lohr said. "And we've found that when we're handling smaller sized material,
we sometimes have pulsing. We believe this problem is largely the result of the lime's material-flow characteristics;
ratholing or material shearing off the column sometimes causes a surge into the feeder."
"Finding the appropriate discharge opening for the feeders require far more time and effort than we anticipated,"
Lohr said, adding, "Each of our silos has three center discharge openings, and we had to move each of the feeders
before we arrived at the best overall system configuration for product blending."
Eliminating the vibratory feeders has reduced airborne dust at Dravo Lime. "We have about a 75 percent dust reduction
compared with the vibratory feeders," Lohr said. "That's a definite reward, and we also expect to see a maintenance
cost reduction."