EPA600/
2-84-187 PB85-122737
RECOMMENDED PRACTICE FOR THE USE OF ELECTROMAGNETIC
FLOWMETERS IN WASTEWATER TREATMENT PLANTS
National Bureau of Standards
Washington, D. C.
NOV 84
U.S. DEPARTMENT OF COMMERCE
National Technical Information Service
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EPA-600/2-84-187
November 1984
RECOMMENDED PRACTICE FOR THE USE OF ELECTROMAGNETIC
FLOWMETERS IN WASTEWATER TREATMENT PLANTS
by
Gershon Kulin
Fluid Ennineerinf Division
National Bureau of Standards
Washington, D. C. 20234
EPA 78-D-X0024-1
Project Officer
Walter W. Schuk
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
REPRODUCED BY
NATIONAL TECHNICAL
INFORMATION SERVICE
U.S. DEPARTMENT OF COMMERCE
SPRINGFIEID, VA. 22161
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(flease read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-S4-137
4. TITLE AND SUBTITLE
RECOMMENDED PRACTICE FOR THE USE OF ELECTROMAGNETIC
FLOWMETERS IN WASTEWATER TREATMENT PLANTS
5. REPORT DATE
November 1984
6. PERFORMING ORGANIZATION CODE
s ACf §s
ON NO.
737
AUTHOR(S)
Gershon Kulin
8. PERFORMING ORGANIZATION REPORT NO.
PERFORMING ORGANIZATION NAME AND ADDRESS
National Bureau of Standards
Fluid Engineering Division
Washington, DC 20234
10. PROGRAM ELEMENT NO.
B113, CAZB1B
11. CONTRACT/GRANT NO.
IAG No. EPA-78-D-X0024-1
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory--Cin., Of
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268__....
13. TYPE OF REPORT AND PERIOD COVERED
Handbook--!0/1 /78-9/30/81
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Walter W. Schuk Telephone - (513) 684-2621
16. ABSTRACT"
Electromagnetic flowmeters that conform to the guidelines described in this
document can be used to measure the volumetric flowrate of all liquids and sludges
normally encountered in wastewater treatment plants, provided that adequate
inspection and maintenance are performed as recommended to contend with potential
deposits and other effects of the harsh fluids.
Electromagnetic flowmeters should be accurate to the lesser of 1 percent of
full-scale or 3 percent of actual flowrate. These flowmeters are not necessarily
immune to the effects of approach velocity distribution. Specific approach
conditions are cited which must be observed during installation in order to
minimize velocity-distribution errors.
Upon installation, electromagnetic flowmeters should be hydraulically tested,
by a method equivalent to that described, to provide an acceptance test and/or
initial calibration. Continuing and frequent performance monitoring subsequent to
initial testing is particularly important for electromagnetic flowmeters in treat-
ment plants. Methods for accomplishing this are suggested.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN LNDED TERMS
COSATI Reid/Group
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
9 'SECURI !"Y CLASS / I Ilix kfp.irn 21. NO OF PAGES
UNCLASSIFIED
39
UNCLASSIFIED
EPA Form 1220-; ;R«
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DISCLAIMER
Although the information described in this document has been funded
wholly or in part by the United States Environmental Protection Agency
through assistance agreement number EPA 78-D-Xti024-l to National Bureau of
Standards, it has not been subjected to the Agency's required peer and
administrative review arid therefore does not necessarily reflect the views
of the Agency and no official endorsement should be inferred.
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FOREWORD
The U. S. Environmental Protection Agency was created because of in-
creasing public and Government concern about the dangers of pollution to the
health and welfare of the American people. Noxious air, foul water, and
spoiled land are tragic testimonies to the deterioration of our natural
environment. The complexity of that environment and the interplay of its
components require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solu-
tion; it involves defining the problem, measuring its impact, and searching
for solutions. The Municipal Environmental Research Laboratory develops new
and improved technology and systems to prevent, treat, and manage wastewater
and solid and hazardous waste pollutant discharges from municipal and communi-
ty sources, to preserve and treat public drinking water supplies, and to mini-
mize the adverse economic, social, health, and aesthetic effects of pollution,
This publication is one of the products of that research and provides a most
vital communications link between the researcher and the user community.
m
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ABSTRACT
Electromagnetic flowmeters that conform to the guidelines described in
this document can be used to measure the volumetric flowrate of all liquids
and sludges normally encountered in wastewater treatment plants, provided that
adequate inspection and maintenance are performed as recommended to contend
with potential deposits and other effects of the harsh fluids.
Electromagnetic flowmeters should be accurate to the lesser of 1 percent
of full scale or 3 percent of actual flowrate. These flowmeters are not nec-
essarily immune to the effects of approach velocity distribution. Specific
approach conditions are cited which must be observed during installation in
order to minimize velocity-distribution errors.
Upon installation, electromagnetic flowmeters should be hydraulically
tested, by a method equivalent to that described, to provide an acceptance
test and/or initial calibration. Continuing and frequent performance moni-
toring subsequent to initial testing is particularly important for electro-
magnetic flowmeters in treatment plants. Methods for accomplishing this are
suggested.
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CONTENTS
Foreword i i i
Abstract iv
1. Scope 1
2. Definitions 2
3. Background 3
3.1 Principle of Operation 3
3.2 Advantages and Disadvantages of Electromagnetic
Flowmeters 4
4. Guidelines for Specifications 6
4.1 Flow Tube 6
4.2 The Secondary 8
4.3 Calibration 9
4.4 Accuracy 9
5. Installation Requirements 10
5.1 Approach Conditions 10
5.2 Bypass 10
5.3 Orientation and Location 10
5.4 Limiting Velocities 11
5.5 Other Considerations 11
6. Error Sources 13
6.1 General 13
6.2 Velocity Profile 13
6.3 Deposits 14
6.4 Air or Other Gases 15
6.5 Zero Drift 15
7. Calibrations and Performance Checks 16
7.1 Hydraulic Calibration of the Measuring System 16
7.2 Calibration Other than Hydraulic 19
7.3 Flow Measurement Methods for Hydraulic Calibrations .... 20
7.4 Flow Measurements for Monitoring 25
7.5 Approximate Flow Measurements 27
8. Operati on and Mai ntenance 30
8.1 Operation/Maintenance Frequencies 30
8.2 Miscellaneous Operation/Maintenance Recommendations .... 31
9. References 32
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1. SCOPE
1.1 This document covers the use of electromagnetic flowmeters for in-plant
flowrate measurement of raw influent, treated effluent, and wastewater in
intermediate stages of treatment as well as various types of liquid sludges,
1.2 For the purposes of this document, "use" of electromagnetic flowmeters is
considered to include: requirements for the design and construction of the
primary and secondary elements; requirements for installation of the equip-
ment; recommendations for performance checks, operation and maintenance.
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2. DEFINITIONS
2.1 Accuracy -- The sum of the systematic error (bias or difference from the
"true" value) and the precision at a stated confidence level
(e.g., in terms of a number of standard deviations). In the
case of flowmeters, the true value is usually determined from
a basic calibration method.
2.2 Flow converter — The element of the secondary system that converts the
output electrical signal to flowrate units on a dial, digital,
chart or other readout.
2.3 Flow tube -- The part of the primary that forms a pipe-like conduit.
2.4 Precision — The reproducibility or repeatability of a measurement.
2.5 Primary -- The element of a measuring system that generates a measurable
change in a parameter. In this case, the primary consists of
the flow tube, the coils and yoke that produce a magnetic flux,
and the electrodes.
2.6 Quadrature noise -- Electrical noise that is 90 degrees out of phase with
the desired flow signal.
2.7 Repeatability -- See precision.
2.8 Reynolds number -- A dimensionless number that expresses the ratio between
inertial and viscous effects in a flow. Low Reynolds numbers
(below about 2000 for pipe flow) correspond to laminar flow.
2.9 Secondary -- The portion of the measuring system that amplifies and/or
conditions the signal from the electrodes into an output pro-
portional to the flowrate and further usually converts it to
a readout in flowrate units. Part or all of the secondary
may be mounted directly on the primary.
2.10 Threshold conductivity — A minimum electrical conductivity above which
changes of conductivity of the flowing liquid have a negligi-
ble effect on the measurement.
2.11 Weight function -- A theoretical function that weights the contribution of
each part of the induced voltage in the flow section to the
total sensed by the electrodes.
2.12 Zero drift -- A change with time in the output reading for the no-flow
condition.
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3. BACKGROUND
3.1 Principle of Operation
3.1.1 Flow-tube meters. The operation of electromagnetic flowmeters is
based on the Faraday law of electromagnetic induction. If flow of
a conductive fluid in a pipe is normal to a magnetic field, an
electromotive force is induced across the fluid in a direction
normal to both the magnetic field and the flow. The induced
voltage is measured by placing insulated electrodes across the
pipe so that a line connecting them is perpendicular to the
magnetic field. See figure 1.
Figure 1. Flowmeter schematic.
For an axisymmetric pipe flow and a uniform magnetic field
the output voltage, E, is given by
E = k B L V
[1]
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where L is the distance between electrodes, V is the area
average velocity of the flow, B is the magnetic flux density
and k is a coefficient that is constant for a given meter.
The volumetric flowrate, Q, is then
Q =*D2 V/4 [2]
where D is the diameter of the flow tube and is often, but
not always, the same as L.
The fluid being measured must be conductive. The
threshold conductivity is design dependent but is generally
several micro-mhos per centimeter.
The magnetic field is usually generated by the current alter-
nating in the coils at line frequency. However, in some meters
the line current is rectified and imposed on the coils as a
"pulsed direct current" alternating between a positive value and
zero at a frequency of (typically) several hertz. The claimed
advantage of these meters is that each pulse of current lasts
long enough for an induced voltage to be obtained free of tran-
sients and quadrature signals while the zero-current period is
long enough for the noise signal to be obtained and subtracted
from the measured signal, in effect providing an internal zero
correction with concomitant reduction of drift and improved
accuracy.
3.1.2 Probe-type meters. Although this report deals mainly with the
tube-type flowtneters described above, the availability of other
types of electromagnetic flowmeters is noted here. In these
meters, an electromagnetic sensor measured a "local" velocity to
which the average velocity in the conduit can be related, prefer-
ably by calibration. The following examples do not necessarily
comprise an inclusive list. They include: cylindrical velocity
probes inserted into the flow through the conduit wall or flow
surface; "pitot" probes consisting of a short submersible flow
tube intended for use in large open or closed conduits; stream-
lined sensors mounted at the pipe invert near a manhole, for
example. These kinds of meters are adaptable to conduits other
than circular. In the case of free surface flows the depth must
be monitored simultaneously. The types of fluids in which they
can be used depends in part on the intrusiveness of the probe.
3.2 Advantages and Disadvantages of Electromagnetic Flowmeters
3.2.1 Flow-tube type meters are non-intrusive. This property makes
them particularly attractive for use with sewage and sludge
and also causes the head loss to be no greater than that for
an equivalent length of plain pipe of the same diameter.
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3.2.2 The output of tube-type meters can, under certain conditions, be
made independent of pipe-roughness or Reynolds-number effects
for axisymmetric flow; but the electromagnetic flowmeter does
not necessarily measure a true average velocity for all velocity
distributions. See sections 5.1 and 6.2. However, the output
is independent of fluid density and is essentially linear.
3.2.3 In probe-type meters, the sensing element can be readily withdrawn
for monitoring and cleaning. However, it may be sensitive to
changes in velocity distribution.
3.2.4 Non-conductive liquids and most gases cannot be metered electro-
magneti cally.
3.2.5 The instrument can be subject to extraneous electrical effects
that are not always obvious.
3.2.6 Servicing and trouble-shooting may require substantial equipment
and skilled technicians.
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4. GUIDELINES FOR SPECIFICATIONS
4.1 Flow Tube
4.1.1 Geometry.
4.1.1.1 The flow tube generally should have the same nominal
diameter as the adjoining pipe. An exception can be
made when a smaller diameter is necessary to maintain
a higher velocity through the meter (section 5.4.1).
Specifications for this exception are given in section
5.1.2. There are no restrictions on the length of the
flow tube provided that a proper magnetic field can be
developed; generally the length is greater than the
diameter.
4.1.1.2 The manufacturer's literature should provide, for
design purposes, the actual internal diameter corres-
ponding to each nominal meter size. If this actual
diameter is smaller than the upstream pipe diameter,
and if the flowing material is abrasive, a protective
orifice With diameter slightly smaller than that of the
liner can be installed at the upstream flange to protect
the leading edge of the liner. To insure that perfor
mance is not affected, calibration should be done with
the protective orifice in place.
4.1.1.3 Flow tubes are usually flanged at the ends. Because the
inner lining is usually carried around to the flange face
to provide insulation, gasketing and torquing of the
flange bolts become critically important; the manufacturer
should provide detailed instructions and all necessary parts
to avoid liner damage on installation. For flow tubes that
are not flanged, instructions should similarly be provided
as necessary.
4.1.1.4 Direction of flow must be indicated on the outside of the
flow tube, unless the meter is completely bi-directional
without adjustment.
4.1.2 Materials.
4.1.2.1 The flow tube must be made of non-magnetic material and
the inner liner in contact with the flow must be non-
conductive as well. The materials must be unaffected
by line pressure.
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4.1.2.2 The liner material also must be suitable for the flowing
liquid in terms of resistance to corrosion, abrasion,
etc. In some cases, glass or Teflon lining may be
advantageous for sludge flow. Liners used for digested
sludge should be capable of handling temperatures to
70°C. All liners should be able to resist low tempera-
tures typical of the location.
4.1.3 Electrodes.
4.1.3.1 Electrodes are located diametrically opposite each
other, and are either flush with the inner tube surface
or protrude slightly into the flow. See figure 2.
They must be insulated from the remainder of the flow
tube. The protruding configuration is used to hydro-
dynamically increase the velocity around the electrode
and enhance its self-cleansing capability. A protruding
electrode should be rounded at the edge so that a
curvilinear protuberance is presented to the flow.
Electrode surfaces should never be recessed, since the
resulting cavity may collect debris.
Eledrode
(b)
Figure 2. Electrodes
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4.1.3.2 Electrodes must be made of corrosion-resistant material.
4.1.3.3 It is preferable but not mandatory that the electrodes
be removable without taking the meter out of the line.
4.1.3.4 Small (in area) electrodes are recommended. In
principle, the sensitivity of the flowmeter to velocity
distribution can be eliminated by the use of large
electrodes rather than the conventional or "point"
electrodes. However, the problems introduced in
keeping large electrodes clean are likely to outweigh
the potential advantages and they are not recommended
for wastewater treatment plants.
4.1.3.5 Internal electrode cleaning capability should be built
in, particularly when frequent electrode inspection is
not feasible. This internal cleaning can be accomplished
by mechanical scrapers, ultrasom'cally induced vibrations,
heating, or by other means. It can be either continuous,
intermittent or manually actuated. Continuous operation
is recommended for ultrasonic cleaners, since experience
suggests that grease deposits, once formed, are difficult
to remove by this method.
4.1.4 Grounding. All necessary grounding straps must be furnished with
the meter. Complete instructions for earth grounding and methods
for ascertaining the quality of the ground must be furnished. See
also section 5.5.4.
4.2 The Secondary
4.2.1 The secondary system can be mounted integrally with the flow tube
or separately from it. If it is mounted separately from the primary,
the manufacturer must furnish specifications for all electrical
connectors along with complete instructions, distance restrictions,
etc.
4.2.2 The flow converter must include a display either in flowrate units
or in units that can be readily converted to flowrate. It is prefer-
able that the readout have switch-selectable flow ranges; however,
the zero offset should be checked for each range. A flowrate
display should be mounted near the flowmeter; this is in addition
to any flowrate readout or flow totalizer in the central control
room.
4.2.3 The secondary should be capable of being sealed against excessive
humidity. In some instances, it will be necessary to specify
that all meter elements be able to withstand temporary immersion
for periods up to 48 hours at depths determined by the depth of
pits or vaults that may be accidentally flooded.
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4.2.4 Electrical connections and components must satisfy all code
requirements for the particular area classification.
4.2.5 The secondary system should be able to operate within an ambient
temperature range of -30°C to 60°C.
4.3 Calibration
4.3.1 The flowmeter should be hydraulically calibrated by the manufacturer
prior to delivery to the customer.
4.3.1.1 The conditions under which the factory calibration was
performed, including upstream geometry, should be made
known to the user.
4.3.1.2 If a factory hydraulic calibration was not made, for
whatever reason, the alternate means by which the meter
rating was determined shall be fully explained to the
user.
4.3.2 The manufacturer should furnish, or make available as an option,
equipment for calibrating the flowmeter from the electrodes to
the readout, or shall alternatively furnish instructions for
performing such a calibration of the secondary system.
4.3.3 For probe-type flowmeters, the calibration data should include
information on the effects, if any, of the radial probe location
with regard to changes in pipe roughness or Reynolds number.
4.4 Accuracy
4.4.1 The accuracy of an electromagnetic flowmeter should be within one
percent of full scale, but should not exceed three percent of
flow. This means that if the flowmeter is to be used at flowrates
Tower than about one-third of full scale, a tighter accuracy
specification will have to be put on the meter.
4.4.2 The repeatability shall be within one-half percent of full scale.
For process control purposes, repeatability is often more important
than accuracy. On the other hand, for measurements involving
discharge-permit requirements or allocation of treatment costs or
capacity among jurisdictions or industries, accuracy and repeat-
ability are equally important.
4.4.3 The output should be independent of variations of up to 10 percent
in line voltage and frequency, and of temperature variations within
the ranges cited in sections 4.1.2.3 and 4.2.5.
4.4.4 If the manufacturer claims that electrodes and/or secondary elements
are completely interchangeable, the accuracy limits for the system
with interchanged parts should be furnished to the user.
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5. INSTALLATION REQUIREMENTS
5.1 Approach Conditions
5.1.1 At least five diameters of straight pipe, measured from the plane
of the electrodes, should precede and follow the tube. However,
this approach length does not insure freedom from upstream effects,
since errors described in section 6 can be introduced by severe
upstream disturbances. If approach lengths shorter than 5 dia-
meters are recommended for a specific commercial meter, data
and/or a description of the experiments or analysis on which this
recommendation is based should be furnished to the user.
5.1.2 An exception to the requirement of section 5.1.1 can be made in
the case of a tapered axisymmetric reducer which is installed imme-
diately upstream of a smaller flow tube in order to provide a self-
cleansing velocity within the flow tube. The included angle of
such a taper should not exceed 30 degrees, both upstream and down-
stream of the flowmeter.
5.2 Bypass
5.2.1 It 1s strongly recommended that piping and isolation valves be
installed in such a way that flow can be bypassed around the meter.
This arrangement not only permits removal of the meter when ser-
vicing is necessary, but also allows for convenient stopping of
flow through the meter for zero checks. It is important that
there be no leakage flow through the valves.
5.2.2 It is also recommended that a cleanout tee be installed near the
flow tube. See figure 3 for one suggested layout. This accessi-
bility is particularly important for meters that do not have
electrode-cleaning capability or are subject to deposition or
coating.
5.3 Orientation and Location
5.3.1 It is preferable that the flowmeter be installed in a vertical
length of pipe. This orientation tends to equalize wear on the
lining when an abrasive liquid is flowing and also prevents gases
from collecting at the pipe crown.
5.3.2 If the flowmeter cannot be placed in a vertical pipe and must be
inserted in a horizontal or inclined line, the following steps are
recommended.
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ISOLATING
VALUE.
BY-PASS VALUE
ISOLATING VALUE
FLOW
NOTE: RECOMMENDED 5 D UP AND DOWN STREAM
Figure 3. Typical installation.
5.3.2.1 Orient the meter so that the electrodes are on a
horizontal line.
5.3.2.2 If the pipe is horizontal, install bleed valves at the
crown near the flowmeter so that the line can be
checked for accumulated gas.
5.3.2.3 In any event the flowmeter should always be located so
that it will remain full, both during flow and when the
flow is stopped. See also section 6.5.2.
5.4 Limiting Velocities
5.4.1 To assure scouring action, design for minimum velocities of at
least 1.5 m/s (5 ft per second) for primary sludge and at least
0.9 m/s (3 ft per second) for other sludges and raw influent
sewage.
5.4.2 To avoid excessive abrasion, velocities should never exceed 8
m/s (25 ft per second).
5.5 Other Considerations
5.5.1 Place the flowmeters on the high pressure side of pumps and
control valves; this will minimize the effect of,gas bubbles.
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5.5.2 Avoid proximity to heavy induction machinery.
5.5.3 Refer to section 4.1.1.3 for mechanical installation procedures.
5.5.4 Proper grounding is essential (see section 4.1.4); improper ground-
ing is one of the most frequent causes of failure of electro-
magnetic flowmeters in wastewater treatment plants. Grounding
requirements may vary, depending for example on whether the pipe
is metallic or nonmetallic, or on the meter material. An internal
grounding ring (which also provides abrasion protection) may be
required. In all cases the manufacturer's instructions must be
followed closely.
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6. ERROR SOURCES
6.1 General. The errors considered in this section are over and above the
basic accuracy inherent in the instrument under optimum conditions of
flow and use; this basic accuracy (as defined in 2.1) shall not exceed
the limits given in section 4.4. It can be less than these limits if
so indicated by in-place calibration or by complete data furnished by
the manufacturer.
6.2 Velocity Profile
6.2.1 Background.
6.2.1.1 It has been shown theoretically that it is impossible
to design an electromagnetic flowmeter with "point"
electrodes that will be completely insensitive to all
types of velocity profiles (1). Compromises can be
attained as given in the following.
6.2.1.2 It has been shown theoretically that meters designed
with a uniform magnetic flux are insensitive to the
velocity profile provided it remains axisymmetric (2).
This means that changes in viscosity or in (uniformly
distributed) pipe roughness will not affect the
flowmeter response. But in order to attain
axisymmetry, long straight upstream approach piping is
needed.
6.2.1.3 Commercial electromagnetic flowmeters are designed with
non-uniform magnetic flux density in an attempt to even
out the weight function and render the meters less
sensitive to axisymmetric velocity distributions. This
may result in losing the inherent insensitivity to
axisymmetric changes described in section 6.2.1.2;
however, this effect will generally be small -- well
under 1 percent (1) of full scale.
6.2.1.4 Only limited data based on systematic experimental
investigation of upstream effects are available in the
literature, and even these show some disagreement.
Sensitivity to upstream conditions depends in part upon
meter design, and experimental results are not
necessarily transferable to other types of meters.
Nevertheless, the recommendations in section 6.2.2 are
proposed as practical guidelines based on published
information.
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6.2.2 Effects of upstream pipe fittings.
6.2.2.1 For the purpose of this section, a straight approach
length of 10 diameters will be considered to restrict
errors due to velocity distribution to less than 1
percent and will be treated as an essentially
error-free condition (3).
6.2.2.2 Axisymraetric fittings, e.g., expanders, reducers, etc.,
located at least 5 diameters upstream of the electrode
plane probably will not measurably affect the perfor-
mance, provided that any asymmetric fittings in the
line are at least 5 diameters farther upstream.
Fittings that introduce asymmetric disturbances, e.g.,
elbows, gate valves, etc., and are located 5 diameters
(and up to 10 diameters) upstream can be estimated to
add an additional 1 to 3 percent uncertainty to the
flow (4). Fitting, even closer to the electrode plane
can cause substantial errors. For example, a half-
closed gate valve located 1 diameter upstream can cause
errors of 8 percent and higher (4). It is not possible
to further quantify errors from these sources using
published information. The size of the error may also
depend upon the orientation of the eccentricity
relative to the line connecting the electrodes.
6.2.2.3 The effect of swirl has not been established; an
uncertainty of 1 percent should be allowed (5).
6.2.2.4 There is little information on effects of downstream
fittings; a straight length of 3 diameters is suggested
for asymmetric fittings.
6.3 Deposits
6.3.1 Deposits of non-conducting material on the electrodes will tend
to Insulate the electrodes and cause erroneous readings. Such
deposits can be minimized by electrode shape (section 4.1.3.1),
automatic cleaning capability (section 4.1.3.5), or maintenance
of high flow velocities.
6.3.2 Deposits of non-conducting material on the tube liner (but not
on the electrodes) will not affect the result unless the deposit
becomes thick enough to measurably change the tube diameter and
affect the output obtained from equation [2]. Such accumula-
tions are discouraged by maintenance of high flow velocities.
6.3.3 Deposits of non-conducting material on the (originally
conducting) pipe immediately upstream or downstream of the
flowmeter will cause the liquid to lose its grounding. Where
this problem is anticipated, the grounding should be done with
grounding spikes or rings (or their equivalent) protruding into
the flow.
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6.3.4 The effect of conductive deposits within the flow tube depends
in part on the ratio of the conductivity of the deposit to that
of the liquid. Deposits of the same conductivity as the liquid
may introduce no error except for that associated with change of
diameter. However, conductive deposits can cause shorts to
develop between the electrodes, or between the electrodes and
ground, and they should be discouraged by the use of high
velocities, elevated temperatures, or frequent cleaning.
6.4 Air or Other Gases
6.4.1 Air or gas bubbles diffused through the flow will not introduce
an error in the volumetric flowrate measurement. However,
mass flowrates deduced from these measurements will be in error.
See section 5.5.1. If gas collects so that the pipe is not
full (as distinguished from bubble dispersion), an error will be
introduced even if the electrodes remain submerged. See section
5.3.
6.4.2 Small gas bubbles effervescing from the liquid and adhering to
the electrodes will introduce errors by partially insulating
the electrodes. Maintenance of recommended velocities (section
5.4.1) should preclude this occurrence.
6.5 Zero Drift
6.5.1 Zero drift is a commonly occurring error source in electro-
magnetic flowmeters, and it can be monitored only by frequent
checking with the flow stopped. Flowmeters that have built-in
zero correction as described in section 3.1.1 may require only
infrequent monitoring. See section 7.1.2.7.
6.5.2 There is limited evidence that alternate immersion and
drying of the electrodes encourages the formation of thin
coatings that contribute to zero drift (5). However, keeping
the flowmeter tube full at all times, even during extended
periods of no flow, is suggested only for clean flows in
non-corroding lines. In view of the proclivity of sewage and
sludge to form grease deposits, these flows should be stopped
under meter-full conditions only long enough for zero checks.
A clean water flush is required following longer term shut down.
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7. CALIBRATIONS AND PERFORMANCE CHECKS
71.1 Hydraulic Calibration of the Measuring System
7.1.1 General.
7.1.1.1 Even though a flowmeter is delivered pre-calibrated
hydraulically by the manufacturer in accordance with
section 4.3.1, an in-place hydraulic calibration
provides the most assured method of evaluating its
performance.
- Hydraulic in-place calibration is recommended when
the system is first installed, at which time it
provides not only an initial calibration but also an
acceptance test.
- Hydraulic calibrations are recommended also at times
subsequent to the initial calibration, if electrodes
or secondary elements are changed, or if monitoring
indicates an unexplained change in the measured flow-
rate.
- General procedures for hydraulic calibration are
described in section 7.1.2. Methods of obtaining
reference flow measurements for hydraulic calibrations
are given in section 7.3.
- Exceptions to the requirement for hydraulic 1n-place
calibration are given in section 7.2.
7.1.1.2 It is emphasized that even an in-place hydraulic
calibration reflects the performance of an electro-
magnetic flowmeter only so long as the electrodes
and the tube interior are not adversely affected and
remain effectively in the same condition as during
calibration. Therefore, continued monitoring and
follow-up calibrations are important, especially in
sewage and sludge flows.
7.1.2 General procedure for hydraulic calibration.
7.1.2.1 Be sure that the electromagnetic flowmeter has been
set up and that preliminary checks have been made
completely in accordance with manufacturer's instruc-
tions.
7.1.2.2 With the line full but with the discharge valves closed
(pump, if not positive displacement, should be running)
check the zero-flow reading of the instrument. If there
16
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is a zero offset, correct it according to manufacturer's
instructions.
7.1.2.3 Set up a flowrate through the meter that is within the
anticipated working range of flows. Allow enough time
for it to become steady, and measure the reference or
"true" flowrate, QR, by one of the methods in section 7.Z.3.
7.1.2.4 During the time that the measurement of QR is being made,
the flowrate as measured by the electromagnetic flowmeter,
Q, should be recorded. If the measurement of QR is made
by a method that requires a substantial length of time,
e.g., the volumetric method, readings of Q should be
made at intervals during the test period and an average
value determined.
7.1.2.5 Compute the percentage difference in the two flowrates
from 100 (Q-QR)/Qn and enter this value on an error vs.
flowrate curve as shown in figure 4.
7.1.2.6 The procedure of sections 7.1.2.2 through 7.1.2.5
should be done for a minimum of three flowrates (low,
medium and high) within the anticipated working range.
It is preferable that numerous runs be made (above the
minimum three) to provide an indication of repeatability
as shown in figure4 . (As a practical matter, however,
it is recognized that some of the calibration flow
measurements of section 7.2 are very time and resource
consuming and that repeated runs may not be feasible.)
Draw curve A averaged through the points.
7.1.2.7 A zero check (section 7.1.2.2) is recommended between
each run in order to check the short-term zero drift.
- If there is substantial zero drift, it should be
ascertained whether it is due to local conditions,
such as changes in nearby inductive machinery, which
should not necessarily be charged against the flowmeter.
- If the zero drift appears to be characteristic of
the instrument itself, it should be monitored before
each test point but no adjustment should be made.
- Even if a flowmeter has automatic internal zero
correction, a no-flow reading is desirable as an
initial check on the correction.
7.1.2.8 Estimate the percentage error in measuring QR and enter
it as curve B in figure 4.
- This error estimate for QR is especially important
in acceptance tests, since the flowmeter under test
cannot be held to an accountability stricter than the
user's ability to check it.
- The estimated error will clearly depend upon which
method of section 7.3 is used and the manner in which
17
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QJ
O)
_O
U.
5
4
3
2
0
-3
t Q
8
Figure 4. Flowmeter error determination.
18
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it is executed. It may or may not vary with flowrate.
Some of the methods, e.g., several of the transfer-
standard devices, have documentation that provides
guidelines for estimating errors (7,8). In any event,
estimated errors that are agreeable to the involved
parties must be assigned to QR.
7.1.2.9 If curve A is outside of curve B by more than the
amounts allowed in section 4.4.1 (or alternate specifi-
cations), the performance may be unsatisfactory and
these options should be considered.
- If the test results in figure 4 suggest a constant
percentage error, it may be that a span adjustment of
the secondary is required. The manufacturer's manual
should provide instructions by which the user can
make this adjustment. The calibration process of
section 7.1.2 should be repeated after such an
adjustment.
- If the flowmeter cannot be brought within specifica-
tions with adjustments made by the user, it must be
repaired or rejected. However, if the data
differences can be ascribed to faulty approach
conditions and/or are otherwise consistent and
repeatable, the calibration results can be used to
develop a new, in-place rating for the flowmeter.
7.1.2.10 The error comparison described in sections 7.1.2.8 and
7.1.2.9 should be regarded as a suggested procedure
only. Other comparison procedures that are agreeable
to the involved parties are acceptable; the important
point is that the reliability of the reference measure-
ment must be taken into account in some way.
7.2. Calibration Other Than Hydraulic
7.2.1 It is recognized that there are situations where in-place
hydraulic calibrations are not feasible and the user must
resort to other methods.
7.2.1.1 One alternative is to calibrate the secondary only,
using an electronic "flow simulator" offered as an
option by the manufacturer or following instructions
provided by the manufacturer to accomplish the same
result. (See section 4.3.2). This alternative should
be used in a place of a complete calibration only when
there is reasonable certainty as to the condition and
performance of the primary. This requires (assuming
the flux strength can be verified) that the electrode
and flow tube surfaces be closely monitored and that
the adjacent piping is suitable.
19
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7.2.1.2 If there are upstream pipe fittings, valves, etc.,
closer than 5 diameters to the electrodes, and particu-
larly if these fittings produce asymmetric flows, the
calibration should include the entire measuring system,
i.e., primary as well as secondary.
7.2.2 Situations such as those described in section 7.2.1.2 can be
approached also by "dry calibration" techniques, which involve
direct measurement of the magnetic field strength in combination
with weight functions and velocity distributions to arrive at a
flowrate. Details of this method are beyond the scope of this
report. See, e.g., (6). The velocity-distribution data needed
for this method normally would be difficult to obtain, particu-
larly in a sludge. However, it may be feasible to estimate a
"worst case" velocity distribution based on published information
so that error limits can be estimated.
7.3 Flow Measurement Methods for Hydraulic Calibrations
7.3.1 General.
7.3.1.1 The purpose of section 7.3 is to provide a general
overview of methods available for obtaining the
reference flowrate, QR» needed in section 7.1. The
methods most likely to be applicable here are:
- Volumetric
- Comparison with a reference flowmeter
- Dilution
- Salt velocity
- Velocity traverse
7.3.1.2 The methods described vary considerably in accuracy and
difficulty. Advantages, disadvantages, and areas of
application of each method are cited. The method
selected by the user will depend upon the purpose of
the calibration, the type of liquid flowing, and the
resources available to conduct the tests.
7.3.1.3 For highest accuracy, the in-place hydraulic calibration
should be conducted with the actual process liquid.
7.3.1.4 If the calibration flowrate measurements are made at a
location in the plant that is not directly in the line
containing the electromagnetic flowmeter, equivalence
of the flowrate at the measurement location to that
through the flowmeter must be assured.
7.3.2 Volumetric calibration.
7.3.2.1 The feasibility of volumetric calibration of course depends
upon the availability of suitable tank space and connecting
conduits. Of the methods described here, it is probably
20
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the one most suitable for difficult sludges. The
potential accuracy is high, provided that:
- The tank is regular in configuration so that its
lateral dimensions can be measured within acceptable
limits of accuracy.
- The tank is large enough to permit a test run of
sufficient length for the effect of timing errors at
the start and finish to be kept within acceptable limits.
- The change in liquid level during the run is large enough
so that the starting and finishing depths (probably
obtained by the "on-the-run" method) can be measured
within acceptable relative error limits.
- The flowrate remains relatively constant during the
run. (See section 7.1.2.4.)
- It is noted that gravimetric tests can be substituted
for volumetric where suitable tanks and scales are
available.
7.3.2.2 Estimate the uncertainty of the resulting QR from a
combination of the estimated errors of measurement of
the lateral area, the depth change, the elapsed time.
This uncertainty should be used in figure 4 as
indicated in section 7.1.2.
7.3.3 Comparison with a reference meter (transfer standard).
7.3.3.1 In this context a reference meter is a flowrate measuring
device whose performance can be referenced to published
standards or to recommended practices that are acceptable
to the involved parties. Examples include:
- Standard venturi tubes and venturi nozzles (7,8)
- Orifice plates (7,8)
- Parshall flumes (9)
- Thin plate weirs (10)
7.3.3.2 Flowmeters used as reference devices must meet all
requirements of the accepted standard practices in
fabrication, installation and use, so that their flow
coefficients and uncertainties can be used for determining
Qn. In practical treatment-plant situations, it may be
difficult to find or install a reference flowmeter meeting
all of these requirements. For example, the stringent
upstream approach conditions may not be met. In that case,
the flowmeter cannot be used as a transfer standard
unless rationally based and defensible modification to
the flow coefficients and uncertainties can be agreed
to by the involved parties.
7.3.3.3 When a differential-pressure type of flowmeter is used
as the reference device, measure the pressure difference
with a U-tube manometer. If a commercial secondary
device is used in place of a manometer, it must have had
21
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a recent calibration and complete information on its
performance must be available; its error should be
included in the QR uncertainty.
7.3.3.4 When a standard weir or Parshall flume is used as the
transfer flowmeter, measure its head with a point gage
(carefully zeroed to the crest) or equivalent device
and determine QR from standard equations. If a
commercial secondary device is used, see section
7.3.3.3.
7.3.3.5 It may be acceptable to use as a reference device a
flowmeter for which there are no published standards,
e.g., acoustic flowmeters, segmental orifices,
provided: the device has been recently calibrated and
its current accuracy and repeatability can be
quantified to the satisfaction of involved parties; and
the device is used under effectively the same
conditions for which it was calibrated.
7.3.4 Dilution method.
7.3.4.1 In the dilution method the flowrate is deduced from the
dilution of measurable properties (e.g., color, conduc-
tivity, or fluorescence) or tracer chemicals added to a
turbulent flow in known amounts. The calibration can
be done by either the constant-rate injection method,
or the slug injection method. The constant-rate method
is recommended here because it appears more practical
for In-plant use. See reference (11) for details.
7.3.4.2 In the constant rate Injection method, a tracer solu-
tion of accurately known concentration 1s Injected
upstream at a rate which 1s constant and accurately
measurable. At a downstream distance long enough for
complete mixing, the flow is sampled and the concentra-
tion "plateau" is attained. The flowrate, Q, Is then
determined from
Q = q(c] - c2)/(c2 - c0) [3]
where: q is the rate at which the sample of concentra-
tion C] is Injected; c2 is the measured "plateau"
concentration downstream; and c0 (which may be close
to zero) is the background concentration of the tracer
chemical existing in the flow.
7.3.4.3 This method requires accurate measurement of q and of
all concentrations; skilled personnel and specialized
equipment are needed. However, under optimum condi-
tions the potential accuracy is high. See reference
(11) for methods of estimating errors.
22
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7.3.4.4 The tracer property must be conservative, since losses
by absorption to solids in the flow will result in an
apparent reduction in Cp. The fluorescent dye
Rhodamine WT has been used successfully in sewage
without losses; its behavior in sludge is not known.
7.3.5 Salt-velocity method.
7.3.5.1 In the salt-velocity method, brine is injected suddenly
at an upstream station in such a way that it becomes
well-distributed across the section very rapidly. The
time of passage of the salt pulse between two downstream
stations is measured by means of electrodes which detect
the increased conductivity associated with the passage
of the brine. The flowrate then can be determined
provided the volume of the conduit between the electrodes
is accurately known. This method has a potential for
1 percent accuracy under optimum conditions. The
accuracy actually obtained depends upon the tranverse
mixing and coherence of the injected brine slug, upon
the accuracy of determination of the centers of gravity
of the tracer conductivity and the time separating them,
as well as upon the accuracy of the aforementioned
volume determination.
7.3.5.2 Published standards for the salt-velocity method are
written for circular pipes flowing full (8,12), and these
or similar references must be consulted for details of
the method. A sufficient length of (preferably straight)
pipe upstream of the first electrode is necessary to insure
complete lateral mixing of the salt slug when it reaches
the electrode. This length can be short as four diameters
when the injection is done internally in the standard
manner (8,12). However, a substantially longer approach
distance is needed if injections are made from the periphery
of the pipe. The distance between the two sets of electrodes
must be at least four diameters.
7.3.5.3 The liquid being measured must have a significantly
smaller electrical conductivity than the brine.
7.3.5.4 The brine injection must be sudden, with an injection
interval of the order of 1 second and no leakage
thereafter.
7.3.5.5 The electrodes must provide equal increments of
conductivity for equal segments of cross-sectional
area. This requires the electrodes to be intrusive,
so that the method cannot be used in raw sewage or
sludge. However, it is suitable for treated effluent.
23
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7.3.5.6 In principle, this method can be adapted to other
shapes of conduits or channels provided that the
approach length and the electrode spacing and confi-
guration are modified to compensate for the shape
change in a manner that is hydrodynamically sound and
agreeable to the involved parties.
7.3.6 Velocity-area method.
7.3.6.1 The velocity-area method is applied to a flow cross
section by measuring a number of velocities over the
section, each representative of the average velocity
within an incremental area, and summing the resulting
velocity-area products. The method can be applied to
both open and closed conduit flows, but it is much more
convenient in accessible open channels.
7.3.6.2 This method 1s likely tp be suitable mainly for elec-
tromagnetic flowmeters of relatively large capacity
discharging a fairly clean liquid.
7.3.6.3 The velocity can be measured by point-velocity measur-
ing instruments such as current meters (rotating or
electromagnetic), pltot tubes, etc., or by acoustic
velocity meters that measure an average velocity
component along a line path.
- The point velocity instruments are intrusive and
generally could not be used effectively 1n raw sewage
or sludges. However, there may be an opportunity to
use a rotating-element or electromagnetic current
meter 1n an open channel discharging treated
effluent. In that event, see section 7.3.6.5.
- Pltot tubes are usually restricted to full conduit
flows, where velocities are more likely to be high
enough to generate a readily measurable dynamic
pressure.
7.3.6.4 The accuracy of this method depends upon whether the
sampling points are distributed so as to yield an
average velocity and whether each velocity is accurate-
ly measured. The latter depends in part on the
accuracy of the velocity-measuring instrument itself
and upon the duration of the sampling time at each
point. The sampling requirements tend to make it a
lengthy measurement, so 1t can be used only where
sufficiently long periods of essentially steady flow
are available.
7.3.6.5 In cases where a point-velocity instrument is used in
an open channel, the following conditions must be
observed.
- The average velocity in the section preferably should
exceed 0.30 tn/s (1 ft per second).
24
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- Only velocity-measuring instruments that have been
recently calibrated and whose present accuracy and
uncertainty can be estimated to the satisfaction of
involved parties should be used.
- Consult reference (12) for distribution of velocity
sampling points in the cross section, and reference (13)
for error estimates.
7.4 Flow Measurements for Monitoring
7.4.1 General.
7.4.1.1 Measurements for monitoring are defined in this context
as those that are made subsequent to an initial calibration
for the purpose of detecting anomolous behavior of the
electromagnetic flowmeter. These measurements in
themselves cannot furnish a calibration coefficient as
can those listed in section 7.3. Nevertheless, the
measurements must be carefully made so that their
precision can be depended upon.
7.4.1.2 Frequent monitoring is especially important for users
of electromagnetic flowmeters, since the system is
electrical as well as generally obscured from view and
consequently the quality of its performance is not as
visible and readily checked as, say, an open channel
flume with a float gage.
7.4.1.3 Measurements for monitoring frequently involve
measuring a pressure difference at a location where
the flowrate has a unique and repeatable relationship
to that difference so long as the geometry remains
unchanged. This requirement for constant geometry means
that, in the case of valves, the settings for disc or
gate position must be reproducible. It also means that
there cannot be internal deposits or coatings that
significantly affect the measured pipe diameter.
Locations for monitoring can include, among others:
- Pipe elbows, usually 90 degrees (see also section 7.5.2)
- Butterfly or gate valves (when partly closed)
- Pumps
- Pipe reducers
7.4.1.4 The capability for monitoring should be set up prior to
calibration, so that a relation between the pressure
difference at the monitor and the reading of the newly
installed electromagnetic flowmeter can be established
from the results of the hydraulic calibration (section
7.1.2) or by other means (section 7.2).
7.4.1.5 It should be noted that this type of monitoring
procedure is valid only so long as the viscosity of the
25
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liquid does not change radically; caution should be
exercised with sludges.
7.4.2 Measuring the pressure difference.
7.4.2.1 Pressure taps.
- Pressure taps should be flush with the inner surface
of the pipe and free of burrs. Hole diameter is not
critical because we are looking for repeatability
rather than accuracy; diameters of from 0.5 cm to
1.0 cm for small to large pipes are usually adequate.
The condition of the inner surface at the tap must
remain essentially the same and should be monitored
to that end. For example, rust or grease ridges or
tubercles at or near the hole will affect the reading.
- Pressure taps for elbows are installed as shown in
figure 5. For the other locations cited in
section 7.4.1.3, the taps are placed upstream and
downstream of the device producing the pressure
difference.
- For sewage and sludges it is recommended that only
one pressure tap be used at each upstream and
downstream point. (Elbows use only one tap at each
position in any event.) The use of several taps
evenly spaced around the circumference and connected
by a piezometer ring is preferably restricted to
relatively clean liquids.
- Single pressure taps in horizontal or near-horizontal
lines should be located in a horizontal diametric
plane, or slightly above that position, in order to
minimize entry of gases and solids into the manometer
lines.
7.4.2.2 Differential-pressure sensor.
- The pressure difference can be measured using
conventional U-tube manometry, for example, a water-air
manometer or, for large differences, a water-mercury
manometer. Manometer fluids should be selected so
that the manometer deflection is never less than
approximately 3 inches (7.6 cms). The glass tubes
should be large enough to avoid meniscus shape errors.
- The pressure difference can be measured also with one
of various types of commercial differential pressure
cells. Such sensors should be calibrated frequently
against a liquid-column manometer. This is best
accomplished by providing fittings in the connecting
tubing so that a manometer can be placed conveniently
in parallel with the sensor.
- Always bleed manometer lines of gas before each use,
and check for zero deflection before and after each use.
26
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- Make several manometer readings (more, if the columns are
oscillating) for each measurement and use the average.
7.4.2.3 Connecting tubing.
- The recommendations for the connecting tubing between
the pressure taps and the manometer or sensor are
intended to discourage accumulations of gas or
sediment in the lines.
- To that end, lengths of horizontal tubing should be
avoided in favor of moderate slopes. Further,
elevation peaks (inverted U's) in liquid lines
should have gas bleed valves, and troughs should
have sediment drains or traps.
- The connecting tubing should be corrosion resistant
and its bore should be at least 3/8 inch (1 cm). It
should be valved in a way that permits all portions
of the lines to be flushed when necessary.
- Valves and by-pass tubing should be provided so that
the manometer or sensor can be isolated from the pipe
flow for zero checks.
- Clean water should be kept in the lines by the purge
flow (see section 7.4.2.4).
7.4.2.4 Purge flows.
*
- Continuous purging of pressure taps is required in
sewage and sludge flows. This purging should be
done with tap water or clean non-potable water.
- The pressure loss in the tubing between the purge-
water connection and the tap should be the same in
both of the lines so that the pressure differential
is essentially unaffected by the purge flow. This
can be accomplished by making the two paths geometrically
similar and by keeping the purge flowrate the same in both
legs. Install a variable area flowmeter (or equivalent)
and a control valve in each of the purge water lines for
flow adjustment.
- Purge flows should be high enough to keep dirty
liquid out of the connecting lines under all
conditions; however they should remain negligible
relative to the primary flow.
7.5 Approximate Flow Measurements
7.5.1 It is useful on occasion to have a relatively quick and
inexpensive way of knowing whether a flowmeter is even
approximately correct, particularly when an in-place calibration
cannot be made. Measurements in the range of +_ 10 percent
uncertainty may be adequate for this purpose. Some of the
monitoring locations listed in section 7.4 are adequate for
this purpose, as shown in the following examples.
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7.5.2 Elbow meters.
7.5.2.1 The accelerations associated with flow around a
curve of radius r cause a pressure difference in the
radial direction which can be used to deduce a
flowrate (as well as monitor) in a full pipe. This
method, which has been investigated mainly for
90-degree elbows, requires an arrangement as shown
in figure 5.
A-A
Figure 5. Elbow meter.
7.5.2.2 The flowrate can be estimated from the following
analytically determined expression (14).
Q= (r/2D)1/2UD2/4)(29Ah)1/2 [4]
Here, r is the radius of curvature of the elbow
centerline, D is the elbow (and pipe) diameter and
Ah is the measured head difference in terms of height
of flowing liquid. ''
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7.5.2.3 The elbow performance is more sensitive to the radius
of curvature of the inside bend than to that of the
outside bend. Therefore it is desirable, when
practicable, to determine r by measuring the inner
bend curvature and adding half the diameter rather
than to use a nominal value of r.
7.5.2.4 The elbow should be preceded by about 10 diameters of
straight upstream pipe. There is insufficient
information with which to evaluate such effects as
pipe roughness and Reynolds number, except to note
that with decreasing Reynolds number the flow is less
than that predicted by equation [4]. Therefore added
caution must be exercised in its application to sludge
f1ows.
7.5.2.5 Examination of published experimental results suggests
that equation [4] cannot be depended upon for accuracies
better than roughly +_ 10 percent. However, it should
be noted that an elbow meter that is carefully fabricated
and installed and properly calibrated can be as effective
a flowmeter as other types of pressure-differential
devices. The foregoing paragraphs of section 7.5.2
pertain to uncalibrated elbows only.
7.5.3 Valves. Butterfly valves are sometimes furnished with flowrate
vs. angle-of-opening data which can be used for approximate
checks on flowmeters. Such measurements would of course be
affected to an unknown extent by upstream pipe conditions
and viscosity.
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8. OPERATION AND MAINTENANCE
8.1 Operation/Maintenance Frequencies
8.1.1 Flowmeters without monitoring stations Csection 7.4).
8.1.1.1 The frequency of required routine operation checks and
maintenance can best be determined from experience.
However, until experience provides guidelines,
relatively frequent checks must be made, particularly
where there is no monitoring station.
- Stop flow through the meter briefly at least once a
day to check for zero drift. This should be done
initially even for meters that have internal zero
correction in order to check for possible offsets;
the frequency of subsequent checks can be reduced
as appropriate.
- Check the secondary span weekly. See section 4.3.2.
- Visually inspect the liner and electrodes monthly
and clean as necessary.
8.1.1.2 The frequency of the above checks can be adjusted as a
data base is developed.
8.1.2 Flowmeters with monitoring stations.
8.1.2.1 If there is a monitoring station as described in
section 7.4, the initial operation checks can consist
of a daily comparison of the electromagnetic flowmeter
reading with the monitor, provided that the satisfactory
condition of the internal surfaces and the pressure
taps of the monitor can be assured. If this condition
is satisfied, the operations in section 8.1.1.1 need
not be done at specified intervals until the previously
determined relation (section 7.4.1.4) between the flow-
meter and monitor readings changes, at which time zero
and secondary span checks should be made, followed by
inspection and cleaning if necessary.
8.1.2.2 If the internal condition of the monitoring station cannot
be verified satisfactorily, treat the flowmeter according
to section 8.1.1 and use the monitor only to detect gross
malfunctions.
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8.2 Miscellaneous Operation/Maintenance Recommendations
8.2.1 Follow all maintenance procedures recommended by the manufacturer
in the user manual, in addition to those in section 8.1.
8.2.2 If the flowmeter has been off line, always turn on the electrical
supply several hours before the readings are started.
8.2.3 Measure the actual internal diameter of the flow tube before
it is installed.
8.2.4 If the flowmeter is in a horizontal line (not recommended)
vent the gas bleeds (section 5.3.2.2) at least once per shift
until a more realistic frequency is learned from experience.
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9. REFERENCES
(1) Theenhaus, R., "Modern Developments and New Applications of Magnetic
Flowmeters," International Conference on Modern Developments in Flow
Measurement, Atomic Energy Research Establishment, Harwell, G. B. 1971,
Paper No. 29.
(2) Shercliff, J. S., "The Theory of Electromagnetic Flow Measurement,"
Cambridge University Press, 1962.
(3) British Standards Institution, "Specification for Electromagnetic
Flowmeters," BS5792:1980.
(4) de Jong* J., "Comparison of Some 500-mm Diameter Electromagnetic
Flowmeters," in Flow Measurement of Fluids, H. H. Dijstelbergen
and E. A. Spencer, eds., North-Holland Publishing Co., 1978.
(5) Scott, R. W. W., "A Practical Assessment of the Performance of Electro-
magnetic Flowmeters," in Fluid Flow Measurement in the Mid 1970's,
Vol. 1, edited by E. A. Spencer and W. J. Ramsay, Nat. Eng. Lab.,
G. B., 1977.
(6) Haacke, A. C., "Calibration of Electromagnetic Flowmeters Using a
Flow Simulator," in Flow Measurement of Fluids, H. H. Dijstelbergen
and E. A. Spencer, eds.* North-Holland Publishing Co., 1978.
(7) International Standards Organization, "Measurement of Fluid Flow by
Means of Orifice Plates, Nozzles and Venturi tubes Inserted in
Circular Cross-Section Conduits Running Full," ISO/DIS 5167, 1976,
draft revision of R781.
(8) American Society of Mechanical Engineers, "Fluid Meters -- Their Theory
and Application," 6th ed., 1971, 345 E. 47 St., New York, N.Y. 10017.
(9) American Society for Testing and MaterialSi "Standard Method for Open
Channel Flow Measurement of Industrial Water and Industrial Waste Water
by the PaYshall Flume," ASTM D1941-67.
(10) British Standards Institution, Standard No. 2680-4A, "Methods of
Measurement of Liquid Flow in Open Channels: Part 4A, Thin Plate
Weirs and Venturi Flumes," 1965.
(11) International Standards Organization, "Measurement of Water Flow in
Closed Conduits—Tracer Methods, Part I: General," ISO No. 2975/1,
1974; "Part II: Constant Rate Injection Method Using Non-radioactive
Tracers," ISO No. 2975/2^ 1974.
(12) Hydraulic Institute, "Standards for Centrifugal, Rotary and Reciprocating
Pumps," 12th edition.
(13) International Standards Organization, "Liquid Flow Measurement in
Open Channels — Velocity Area Methods," ISO 748, 1976.
(14) Replogle, J. A., et al., "Evaluation of Pipe Elbows as Flow Meters,"
Proc. Am. Soc. Civil Eng., Vol. 92, IR3, Sept. 1966, 17-34.
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