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." |
