United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-83-051 Nov. 1983
Project Summary
Technical Assistance
Document: Quality Assurance
Guideline for Process Feed
Rate Monitors in the Portland
Cement and Lime Industries
Robin R. Segall and John R. Richards
This study, sponsored by the Quality
Assurance Division of the U.S. Environ-
mental Protection Agency's Environ-
mental Monitoring Systems Laboratory,
evaluated the performance of process
feed rate monitors used in the Portland
cement and lime industries. Particulate
emission regulations applicable to both
industries are specified as kilograms of
particulate per megagram of feed to the
process. Regulatory agencies have
assumed that feed rate monitors are
highly accurate due to process control
considerations; this study evaluated the
validity of this assumption.
This study has found that six major
types of feed rate monitors are currently
used in the Portland cement and lime
industries. Each of these monitor types
is reliable and accurate; seldom do the
errors exceed ±3 percent, and in most
cases the monitor performance allows
control of the process flows to within +1
percent. This performance level is
necessary (1) to ensure that the resulting
products meet specifications, and (2) to
minimize kiln and calciner operating
problems. Operating personnel routinely
check the operation of the feed rate
monitors and calibration is carried out
frequently.
In the majority of cases, regulatory
agencies could assume that the process
feed rate monitors are sufficiently
accurate. However, if some question
should arise concerning the monitors,
the evaluation procedures listed in this
report may be used to verify accuracy.
The following material also includes a
summary of typical problems and
calibration techniques associated with
the monitors.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The New Source Performance Stand-
ards (NSPS) and many State Implementa-
tion Plan (SIP) regulations contain several
emission rate limits specified per process
operating rate, as shown below:
Pollutant emission rate
Process operating rate
In the NSPS, this type of emission
standard is found in Subparts F and HH,
which contain the Standards of Perform-
ance for Portland cement plants and
lime manufacturing plants, respec-
tively.1'2 In these two subparts, the
allowable emission limits for particulate
matter are expressed per unit of process
weight, as indicated by the following:
Portland Cement Plants (Subpart F)
0.15 Kg per metric ton of feed (dry basis)
to the kiln
Lime Manufacturing (Subpart HH)
0.15 Kg per megagram of limestone feed
[to the rotary kiln]
0.075 Kg per megagram of lime feed [to
the lime hydrator]
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In the past, a substantial effort has
been directed towards the quality assu-
rance (QA) of the numerator in these
expressions, while the accuracy of the
denominator remained unquestioned.
For process control reasons, agencies
have routinely assumed the denominator
is accurate.
This report examines the information
available on the accuracy and reliability of
data from these monitors. In addition to
the current literature, equipment vendors
and plant operators were consulted to
obtain information concerning types and
applications of these monitors, reported
accuracy, sources of error, operation and
maintenance procedures, calibration
procedures, and general evaluation
procedures. This report includes an
introductory section that discusses the
types and typical locations of process feed
rate monitors for the Portland cement a nd
lime industries. The operation and
routine calibration of each of the major
types of monitors typical of these
industries are described in section 2.0.
This section also addresses the quality
assurance aspects of each type of
monitor including accuracies, typical
errors and problems, preventive mainte-
nance, and visual evaluation techniques.
Operation, Calibration and
Evaluation of Process Feed
Rate Monitors
This examination into the use of
process feed rate monitors in the Portland
cement and lime industries revealed that
six main types of monitors are commonly
used. These are: (1) the gravimetric
weigh belt scale, (2) the gravimetric
weigh belt feeder, (3) the nuclear weigh
belt scale, (4) the impact flowmeter, (5)
the pneumatic weigh feeding system, (6)
the plunger-type feeder, and (7) the
magnetic flowmeter. This section will
examine each of these major monitor
types.
Gravimetric Weigh Belt Scales
Gravimetric weigh belt scales are the
most basic form of gravimetric contin-
uous-weighing devices. The normal
weigh belt scale is part of a material-
handling conveyor belt system and
supports a portion of the moving conveyor
belt. It uses a mechanical lever or load
cells to instantaneously weigh the amount
of material carried on that portion of the
belt. Weigh belt scales are used in both
the lime industry and the Portland cement
industry. When used for kiln monitoring,
weigh belt scales usually act as a
materials balance check, in conjunction
with another weigh feeding device which
actually provides the consistent kiln feed
rate.
Accuracy
For factory approved installations,
manufacturers guarantee load cell-type
weigh belt scales to weigh and totalize
with an error not to exceed 0.5 percent of
full scale, 3'4 while mechanical type
scales may have claimed accuracies of
±0.25 percent or better.5
Calibration
There are three common methods for
calibration of gravimetric weigh belt
scales. These are: using a material test,
using test chains, or using test weights. A
fourth method, electronic calibration, is
not as commonly seen.6'7
A typical calibration scheme involves
use of a material test for initial belt scale
calibration upon installation8'9 and then
use of a test weight or test chain
calibration monthly thereafter.9 Evidence
that an instrument is performing incor-
rectly, of course, indicates that a calibra-
tion should be performed immediately.
Evaluation Checklist
In summary, the following checklist
may be useful as a guide for routi ne visual
examinations of gravimetric weigh belt
scales.
• Belt scale is being operated at
between 50 and 90 percent of its
rated capacity.
• Belt loading is uniform in flow and
symmetry.
• No material lumps are larger than
15 percent of belt width.
• There is no material or water
movement in the belt scale area.
• There is a gravity or tension take-up
on a belt longer than 40 ft, and it is
located near the head pulley.
• Unloaded belt runs completely in the
idler trough (at correct belt tension).
• Belt scale is located near the bottom
or tail end of the conveyor.
• Belt scale is acceptable distance
from convex or concave curves,
load-in and load-out points, skirt-
boards, and trippers.
• Belt conveyor does not have "V"-
type idlers, five roll deep trough
idlers, rope or cable support rolls
type idlers, offset carry rolls, or
center rolls.
• Scale mounted idlers and three
stationary idlers to either side are
dimensionally aligned (within 1/32
in.).
• Belt tracks centrally at all load
conditions.
• There are no training idlers within
six idlers of the scale.
• Troughing angle of idlers is < 35°.
• A windbreak is provided if the scale
area is subjected to >5 mph winds.
• The ambient temperature in the scale
area is within the specified operating
range for that model.
• There is no dust build-up on the
weighbridge, belt, or speed sensor.
Gravimetric Weigh Belt
Feeders
Gravimetric weigh belt feeders are
somewhat similar to gravimetric weigh
belt scales in that they weigh flowing
material with a scale applied to a
conveyor belt. However, the scales weigh
material (usually on its way from one place
to another) without concern for the
amount delivered per unit of time; the
weigh belt feeder, on trie other hand,
weighs material (on a short belt) in order
to deliver a specified amount in a
specified time and usually at a constant
rate. A weigh belt feeder usually includes
feedback controls to achieve this goal.
Accuracy
Because of their shorter belt lengths,
weigh feeders tend to be more accurate
than comparatively sophisticated weigh
belt scales. The literature and vendors
cite accuracies of up to ±0.25 percent10'11
of set rate, and at least ±0.5 percent11 or
±0.5 percent to ±1 percent of set rate.5
Calibration
Gravimetric weigh belt feeders are
calibrated in the same manner as
gravimetric weigh belt scales; that is, by
using material tests, test chains, and test
weights (see previous section).
Evaluation Checklist
The following items may be used tc
perform a visual evaluation of a weigh
feeder.
• Load is at least 50 percent ol
capacity.
• Belt is loose enough to permi'
deflection at weigh area.
• Belt has tension roll or some othei
assurance that it does not slip or
drive and measurement pulleys.
• Weigh idler(s) or weighdeck i)
aligned.*
• There is little or no dust build-up 01
weigh deck, platform, etc.
• There is no dust build-up on pulleys
• Belt is tracking correctly.
*To check al ignment place a string between the fixe
or stationary idlers; it should barely touch the weic
idlers.
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Nuclear Weigh Belt Scales
Like the gravimetric weigh belt scale
systems, nuclear weigh belt scale systems
are not generally used as direct kiln feed
rate monitors. Instead, they are usually
part of a larger materials balance check
system. However, the nuclear monitor
can be used to measure mass flow in
material transfer systems other than a
conveyor belt.12 It has been applied
directly to proportioning screw feeders,
and to the combination system used for
kiln feed rate monitoring and control.13
Accuracy
Stated accuracy varies between ±0.75
percent and ±1 percent depending upon
application and manufacturer. 3l 4'15
Discussions with plant operators who
use these instruments have indicated
typical accuracies of 1 percent with
source drift occasionally causing errors as
great as 3 percent.9'16
Calibration
The two principal methods of nuclear
weigh belt scale calibration are the dyna-
mic material test procedure and the static
calibration plate (or material) procedure.
Most nuclear scale manufacturers rec-
ommend conducting a material test for
the initial calibration. Recalibration is
then performed on a monthly basis using
the calibration plate that is usually
supplied with the weigh scale.17
Evaluation Checklist
The previous discussion suggests the
following checklist for nuclear weigh belt
scale evaluation.
• Belt loading is between 70 and 100
percent of rated capacity (or at least
greater than 2.7 Ib/ft2).
• Belt loading does not vary by more
than 30 percent on a regular basis.
• Product profile does not appear to
change significantly.
• Product does not include lumps
larger than 15 percent of belt width.
• Scale is not used to weigh more than
one material or material bulk density
without recalibration.
• There is no material slippage in the
vicinity of the scale.
• There is no material build-up on the
detector.
• There is no material build-up on the
belt.
• There is no water running on the
belt in the weighing area.
Impact Flowmeters
This type of process monitor is designed
for use as an in-line flowmeter for
measuring solids, liquids, and slurries. In
the Portland cement and lime industries,
it finds typical application in the weight
measurement of solids less than 3 in. in
diameter. Impact flowmeters make
material flow measurements by sensing
the force produced by the material as it
free falls (by gravity) onto a sensing
device. Two types of sensing devices are
currently used in impact flowmeters: the
impaction plate and the impaction cone.
The former device is used in the majority
of installations in the lime and cement
industries, therefore the following dis-
cussion will focus almost exclusively on
the impaction plate type of flowmeter.
Accuracy
Impact flowmeters are quite accurate;
manufacturers generally claim 0.5 percent
of full scale, and comments from plant
instrument operators confirm this.9'18'19
Calibration
As with the weigh belt scales and the
nuclear scales discussed previously,
initial calibration of an impaction plate
flowmeter is usually accomplished by
using a material test and subsequent
recalibration uses a static test "weight."
The initial material test calibration is
recommended upon installation, with
recalibrations performed at monthly
intervals (more often if required by
special circumstances or if warranted by
inconsistent readings).9
Some of the newer flowmeter system
installations may have a third calibration
check that utilizes a bin-load cell weighing
system.8
Evaluation Checklist
The previous section suggests the
following visual inspection points to be
used in routine impact flowmeter evalua-
tion.
• The material flow impacts on the
center of the sensing plate or cone.
• Flowmeter is used for only one
material (unless material change
calibration compensation is built
into the electronics).
• There is no appreciable buildup of
dust'in the impingement area of the
impaction plate (or cone).
• There is no airflow against the plate
(or cone). Note: Most of these
devices are enclosed so this may not
be a problem.
• Material measured by the flowmeter
does not change in particle shape or
hardness.
• Plant maintenance procedures for
flowmeter include ensuring that it is
kept properly leveled.
• The flowguide assembly is the
proper one for current material and
flowrate.
Pneumatic Weigh Feeding
Systems
Polysius Corporation, a major design/
construction firm for cement plants,
recently installed pneumatic weigh
feeding systems in five U.S plants. The
system is marketed under the name
POLDOS®.20'21'22'23
Raw meal is fed from the homogenizing
silos through a set of control valves to a
bucket elevator. The meal is dumped into
a calibration tank mounted on load cells,
then metered out of this tank by a control
valve. The meal passes down an airslide
to the pneumatic conveyor, which trans-
ports the material to the top of the
preheater. The mass flow from the
pneumatic conveyor is linearly related to
the floor pressure below the pneumatic
conveyor (AEROPOL®) aeration surface.
The pressure is converted by a transducer
into an electrical control signal for the
main solids flow valve located under the
calibration bin. By using the pressure
level as an indicator of flow, it is possible
to eliminate the weigh belt feeder which
used to be installed below the calibration
bin.
Accuracy
Polysius claims an accuracy of better
than 1 percent which is equivalent to the
accuracy claimed for conventional weigh
feeding systems.21
Calibration
The POLDOS® system is calibrated by
interrupting the material flow to the
calibration bin and allowing the raw meal
to empty out for a period of time.22'23 The
difference in the calibration bin weight
before and after the test is compared with
the pressure value. A number of tests at
different feeding rates are used to
generate the complete calibration curve.
Examples of these calibration curves are
available in references 22 and 23. As part
of the overall calibration procedure,
the load cells on the calibration bin are
checked by hanging various weights on the
side of the bin.21
Evaluation
Polysius reports no serious problems
with the operation of the POLDOS®
system,21 which is entirely enclosed and
has no external mechanical parts which
could be checked. Because of this, agency
observers must rely strictly on plant
calibration data.
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Plunger Type Feeders
Lime plants with rotary kilns and
preheater towers generally use a plunger
type feeding mechanism to del iver the hot
limestone feed to the kiln.24'25'26'27'28 By
controlling the length of the stroke and
the frequency of operation, it is possible
to achieve the desired volumetric feed
rate.24'28 Depending on the design of the
preheater, the system may be equipped
with 4 to 18 of these feeders.24'2*27''*
Calibration
Calibration of the feeders can be
accomplished in a manner analogous to
that used to calibrate the impaction flow
monitors and the POLDOS® system. The
stone feed bin can be mounted on load
cells and emptied over a period of time in
order to relate the plunger operation to
the mass of material fed. High- and low-
level indicators in the stone bin could be
used in lieu of the load cells. They can
also be calibrated using material balance
checks.
Evaluation
Since the feeding mechanisms are
mounted internally on most types of
preheaters, it is impossible for any
agency observer to evaluate their per-
formance. The kiln rotational rate may
provide one index of the feed rate since
the feeding system is synchronized with
the kiln speed.
Magnetic Flowmeters
Because of their ability to measure
volumetric flows of abrasive slurries,
magnetic flowmeters find prevalent use
for kiln feed rate monitoring in the wet
process cement industry. These flow-
meters measure, over a period of time,
the flowrate and total flow of the blended
raw material slurry sent intothe kiln.This
flowrate can be converted to dry raw
material mass flowrate by taking into
account slurry density (which remains
reasonably constant). One plant uses a
magnetic flowmeter in conjunction with a
density meter.29'30 The density meter is
generally a nuclear density gauge that
measures slurry weight per gallon.
Accuracy
Magnetic flowmeter accuracy differs
slightly from manufacturer to manufact-
urer but many claim ±1 percent of full
scale or better.31 Some manufacturers
claim accuracies of ±1 percent of actual
flow,32 and others go as low as ±0.5
percent of full scale.33
Calibration
Magnetic flowmeter calibration is
almost always performed using an
electrical simulator;32 however, in cases
where maximum accuracy is required, a
comprehensive water calibration that
uses a calibration rig is essential.34 The
simulator is connected to the electrode
wires in the transmitter, and simulates
normal conductor fluid voltages through
the flowtube for different flow velocities.32
The transmitter is checked to ensure that
these readings agree with the simulator
readings. Because this type of calibration
requires that the process be shut down,
and because magnetic flowmeters sel-
dom, if ever, drift from calibration, these
flowmeters are usually calibrated only
once, at the time of installation.32
Evaluation Checklist
A magnetic flow monitor is usually not
subject to serious operational problems.
However, if data confirmation is desirable,
the following checklist may be of assist-
ance. Since the instrument is totally
enclosed, the extent to which proper
operation can be checked is quite limited.
• Check the monitor installation to
confirm that the site is free of
serious flow disturbances.
• Confirm that instrument is properly
grounded, and grounding is free of
corrosion, by visually checking
ground strap or area.
• Compare instrument reading to kiln
rotational speed.
References
1. U.S. Environmental Protection Agency,
Standards, of Performance for Port-
land Cement Plants, CFR Part 60,
page 248767, December 23, 1971.
2. U.S. Environmental Protection Agency,
Standards of Performance for Lime
Plants, CFR Part 60, page 9472,
March 7, 1978.
3. Hyer Industries, Inc. Thayer R.F. —A
Belt Scale of Proven Stability in
Severe Environments, Bulletin 5D66
Rev-1, 1977.
4. Ramsey Engineering Co., Ramsey
Precision Belt Scale System Model
10-14, Bulletin TDS 10-14/40-17REV,
1979.
5. Kurylchek, A.L. "Belt Conveyor
Scales and Weigh Feeders," Process
Instruments and Controls Handbook,
Douglas M. Considine (Editor), McGraw
Hill Book Company, Second Edition,
pages 732-738, 1974
6. National Bureau of Standards. Speci-
fications, Tolerances, and Other
Technical Requirements for Weighing
and Measuring Devices, NBS Hand-
book 44, September 1980.
7. Pike, C.E. Requirements for the
Approval of Belt Conveyor Scales,
Southern Weighing and Inspection
Bureau, Circular 9585-5, June 1971.
8. Ted Houseconnect, Fuller Inc., per-
sonal communication with J.
Richards, Engineering-Science,
December 14, 1982.
9. Karl Middour, Gifford-Hill, personal
communication with R. Segall, En-
gineering-Science, December 6,
1982.
10. Wallace & Tiernan Division, Pennwalt
Corporation, Digital Weighbelt Feeder
Series 31 -160, Catalog File 310.160,
No Date.
11. Hyer Industries, Inc. Thayer — A
Weigh Feeder Designed for Comput-
erized Plants, Bulletin 5D65-10/76-
5M-SP, 1976.
12. Kay-Ray Inc., Digital Weigh Scale
Model 6000X, Bulletin 110-880, No
Date.
13. Kay-Ray Inc., Weigh Scale Systems,
Bulletin KR123-281, 1981.
14. Ohmart, Weighart Series 4000,
Bulletin SDBW-8177, No Date.
15. Kay-Ray Inc., On-Line Weigh Scale
Model 6000, Bulletin KR266-1081,
No Date.
16. Ken Levin and Morris Yarrow, Ontario
Cement, personal communication
with J. Richards, Engineering-
Science, December 13, 1982.
17. Charles, J., System Service Corpora-
tion, personal communication to R.
Segall, Engineering-Science, De-
cember 8, 1982.
18. Milltronics Inc., Sankyo Flowmeter
System Specification Sheet 1051,
No Date.
19. Yarway, Solids Flow Transmitter,
Bulletin 5M875C, No Date.
20. Wittmann, J.P. Raw Meal Kiln
Weigh Feeding Systems, Polysius
Corporation, October 1978.
21. Mueller, K., Polysius Corporation,
personal communication with J.
Richards, Engineering-Science, De-
cember 21, 1982.
22. Polysius Corporation, Dosier-System
POLDOS, Bulletin 1240 DIE (6000
BU), July 1979.
23. Polysius Corporation, Fordertechnik,
Dosier-System POLDOS, Bulletin
1247 <3,5679FM), No Date.
24. Levine, S. High-Value End Uses
Underlie Plant Design, Pit & Quarry,
pages 53-56 and 123, May 1980.
25. Trauffer, Walter E. Recent Plam
Improvement Program Gives Austir
White Lime Well Balanced Operation
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Pit & Quarry, pages 106-110, May
1965.
26. Gori, T., and Schwarzkopf, F. Metal-
lurgical and Chemical Lime Production
via a Coal-Fired Preheater System,
Pit & Quarry, pages 100-104, June
1979.
27. Sklavwer, M.M., and Yearham, B.
Preheater Cuts Heat Input Demand,
Pit & Quarry, pages 52-55, May
1979.
28. No Author. Parallel Flow Lime Kiln
Lower As Heat Consumption, Pit &
Quarry, pages 85-87, May 1981.
29. Trauffer, Walter E., Peerless' New
Detroit Plant, Pit & Quarry, Volume
84, pages 84-93, July 1972.
30. Liptak, B. G. (Editor). Magnetic Flow-
meters, Instrument Engineers' Hand-
book, Volume I — Process Measure-
ment, pages 478-487, Chilton Book
Company, 1969.
31. Webb, A. S. Electromagnetic Flow-
metering ... Basics, Products, and
Applications; Instrumentation Tech-
nology, Volume 21, Number 3, pages
29-36, March 1974.
32. Grzenda, J.H., Foxboro Corporation,
personal communication with R.
Segall, Engineering-Science, De-
cember 9, 1982.
33. Liptak, B.C. (Editor). Instrumentation
in the Processing Industries, Chilton
Book Company, 1973.
34. Harrison, R. Flow Measurement — A
State of the Art Review, Chemical
Engineering, Volume 87, Number 1,
pages 97-104, January 14, 1980.
R. R. Segall and J. R. Richards are with Engineering-Science, Durham, NC
27701.
Thomas J. Logan is the EPA Project Officer (see below).
The complete report, entitled "Technical Assistance Document: Quality Assur-
ance Guideline for Process Feed Rate Monitors in the Portland Cement and
Lime Industries,"(Order No. PB84-101 468; Cost: $1O.OO, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
•&U. S. GOVERNMENT PRINTING OFFICE: 1983/759-102/0800
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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