Chemical Engineering July 2012 - 24

Automated
Department Editor: Scott Jenkins
utomated systems for batch
weighing can improve
overall product quality by
raising the accuracy and consistency
of the mixture compared to
what can be achieved with the
manual addition of pre-weighed
bags. Automated batch-weighing
systems integrate load cells with
connections, valves, relay hardware
and process control software.
These systems control one
or more pieces of feed equipment
that deliver different ingredients
to a common receiving vessel at
user-defi ned quantities. To take
advantage of the benefi ts offered
by automatd batch-weighing
systems, consider the following
items, some of which can also be
applied to the selection of other
weighing instruments.
A
the material, load cells transmit
loss-of-weight information to the
controller. In general, loss-ofweight
systems are more suitable
for weighing a smaller number of
larger-volume ingredients.
In many cases, the most suitable
system depends on how
and where the bulk material is
received and stored. If the material
is delivered by railcar or bulk
truck, for example, loss-of-weight
systems would be impractical
because the material containers
could not be mounted on the load
cells required for that type of
system. Conversely, if the material
arrives in groups of bulk bags,
loss-of-weight may be the most
appropriate approach.
Two approaches
When using a manual batchweighing
approach, a common
practice may be to work with
pre-weighed bulk bags, which
can introduce error through incomplete
manual emptying of the
bag, or by having bag contents
that are not exactly the weight
stated. In addition to increasing
accuracy, automated weighbatching
systems can simplify the
addition of bulk product ingredients
to blenders or other process
vessels, as well as increase plant
productivity.
In general, two automated
weigh-batch methods exist: gainin-weight
systems and loss-ofweight
systems.
Gain-in-weight. In this arrangement,
batch ingredients are
typically conveyed in sequence
into a hopper above the process
vessel or blender. The hopper
is set on load cells that transmit
weight-gain data to a programmable
logic controller (PLC) that
starts the conveyor for each
ingredient and then stops it when
the preset weight for that ingredient
is reached. Gain-in-weight
systems are generally preferable
for weighing a larger number
of smaller-volume ingredients.
Costs can be minimized because
only a single set of load cells is
required for the entire gain-inweight
system.
Loss-of-weight. In this type
of system, the source of each
ingredient, such as a bulk-bag
unloader, is mounted on load
cells that transmit weight loss
data to a controller that starts and
stops each conveyor or rotary
airlock valve to weigh each ingredient.
As a conveyor unloads
Loss-of-weight batch-weighing
systems have the advantage of
increased batch speed, if two or
more ingredients are being added
to a process vessel. For loss-ofweight
systems, all ingredients
can be weighed and discharged
simultaneously, as opposed to
sequentially, as is required in a
gain-in-weight system.
Key parameters
In gain-in-weight systems, an
important parameter that must be
addressed is how to account for
material that is still on its way to
the scale after the batch controller
has deactivated the material-feed
device. The material-in-fl ight variable
can be minimized by proper
control sequencing and equipment
positioning. If the discharge of the
material conveyor is immediately
above the gain-in-weight hopper,
little in-fl ight material will result. In
such an arrangement, the amount
of in-fl ight material will tend to be
relatively constant from batch to
batch, so it can be compensated
for by having the weigh-batch
controller stop the feed device
at a point prior to the hopper
achieving the desired weight. In
this way, the in-fl ight material fi lls
in the difference and the desired
weight is reached.
When load cells are located at
fl oor level, they are more susceptible
to damage and more likely
to require frequent calibration
after impacts with mobile plant
equipment, such as pallet jacks
or forklifts. In situations where
the weigh hopper is suspended
above the fl oor, the possibility
can be virtually eliminated.
Load cells
As a weighing device for
automated batch-weighing, load
cells should be used for weighing
ingredients over 10 kg up to several
metric tons. When selecting
a weighing system, always make
an effort to stay in the mid-range
of the unit's specifi ed capacity
with the loads that will be
weighed typically. Heavy-range
load cells cannot weigh small
volumes of batch ingredients with
high accuracy.
The moving parts of a load cell
are constantly under stress, which
can cause the accuracy of the
cell to drift. Regularly scheduled
calibration is needed to ensure
consistently accurate results.
Calibration gauges the
response of a load cell in an
attempt to prevent out-of-specifi -
cation results in a manufacturing
process. Typically, calibrating a
load cell involves using certifi ed
test weights to generate readings
on the cell. If the cell readings
do not match the test weights,
manual or automatic adjustments
can be made to correct the drift.
Operators can perform the task
for routine accuracy checks, but
certifi cation can be awarded only
if a trained and certifi ed technician
performs the task with certifi
ed and traceable test weights.
Accuracy
No defi ned specifi cation exists for
accuracy in the weighing industry.
Accuracy can be considered a
combination of several different
factors, including four quantifi able
specifi cations: resolution, reproducibility,
linearity and uncertainty
of measurement.
Resolution. Resolution refers to
the smallest change in mass that
can be read on a particular scale
(regardless of capacity).
Reproducibility. Reproducibility
is the weighing device's ability to
perform consistently over time and
with multiple different operators.
Reproducibility is often expressed
as a standard deviation.
Linearity. Linearity represents
the measurement of the weighing
system's variance in accuracy
over the weight values within its
capacity range.
Uncertainty of measurement.
The uncertainty of measurement
refers to the difference between
the measured weight of a given
sample when compared to the
true weight, with variances attributed
to weighing environment
and other factors. A common
value for uncertainty of measurement
is not to exceed 0.1% of the
sample quantity.
Editor's note: This edition of " Facts at
Your Fingertips " was adapted from
the following articles: 1) Boger, D.,
Automating Your Weigh Batching
System, Chem. Eng., June 2008, pp.
75-79; and 2) Titmas, R. and Carey,
S., Weighing Your Options: The
10 Most Important Scale Considerations,
Chem. Eng. December 2007,
pp. 61-65.
FIGURE 1.
Mechanical (above)
and pneumatic
(below) gain-inweight
batching
systems can
transport material
from silos,
manual dumping
stations, process
equipment, bulk
bags or any other
source to a hopper,
a blender, or other
downstream equipment
mounted on
load cells
FIGURE 2.
Mechanical (above)
and pneumatic
(below) loss-ofweight
batching
systems transport
material from
one or more bulk
bags to a common
hopper, blender,
conveyor, shipping
container, or any
process vessel
Weigh-Batching
Systems
Chemical Engineering July 2012

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