United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-188 Dec. 1984
4>EPA Project Summary
Recommended Practices for
Flow Measurements in
Wastewater Treatment Plants
Gershon Kulin
A series of three short, concise
manuals were produced to provide
improved guidelines for the selection,
installation, calibration, and mainten-
ance of instruments used for monitor-
ing and control of flow in wastewater
treatment plants. Material and
information are brought together from
several sources and placed in an easily
referenced format. The documents
include the following:
• Recommended Practice for the Use
of Electromagnetic Flowmeters in
Wastewater Treatment Plants.
• Recommended Practice for Flow
Measurement in Wastewater
Treatment Plants with Venturi
Tubes and Venturi Nozzles.
• Recommended Practice for the Use
of Parshall Flumes and Palmer-
Bowlus Flumes in Wastewater
Treatment Plants.
Each document is prefaced by an
executive summary, briefly answers
several of the most common concerns
expressed by treatment plant person-
nel, and references the specific section
of the document that covers the topic in
detail.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, 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
Flow measurement is the most
commonly measured parameter in
wastewater treatment plants and is
required for several diverse purposes,
including permit reports for the National
Pollutant Discharge Elimination System,
billing, the design of new facilities, and
in-plant process control. Moreover, the
devices used to measure flow are even
more diverse and have completely
different geometries, principles of opera-
tion, installation requirements, effective
measuring ranges, and uncertainties of
measurement (accuracy).
Three short, concise manuals were
developed to summarize the key
information required for:
• Selection of the proper device for an
application
• Installation design
• Operation and maintenance
requirements
• Calibration procedures
The information presented in the
manuals also includes the principles of
operation for each device and several
means of verifying the calibration of an
installed flowmeter.
Scope of Recommended
Practices
Meters
The material in the three documents
covers classical venturi tubes, venturi
nozzles; magnetic flowmeters, magnetic
probes in circular pipes flowing full; and
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Parshall and Palmer-Bowlus flumes are
open channel flow measurement. These
devices can be used to measure influent
wastewater, treated effluent, raw
sludges, digested sludge, and activated
sludge. The advantages and disadvan-
tages of each type of meter are discussed
along with the construction, configu-
ration, and installation of the primary
elements; the secondary installation
requirements; and methods of checking
performance and calculating errors.
Installation
The care used in the installation of any
flow-measuring device is critical to both
the accuracy of the flow measurement
and the ability of operating personnel to
maintain the flow rrieter in optimum
condition. This fact is illustrated by the
installation requirements section of the
manual on magnetic flowmeters.
Approach Conditions
At least five diameters of straight pipe,
measured from the plane of the
electrodes, shou Id precede and follow the
tube. Even when this approach length
requirement is met, errors will be
introduced by severe upstream
disturbances. The magnitude of the
errors will depend on the magnetic flux
distribution incorporated in the particular
design.
If a manufacturer claims less
sensitivity to disturbed flows or if
approach lengths shorter than five
diameters are recommended, data and/
or a description of the experiments or
analysis on which this recommendation
based should be furnished to the user.
An exception to these requirements
can be made in the case of a tapered,
axisymmetric reducer, which is installed
immediately upstream of the flow tube to
provide a self-cleaning velocity within the
flow tube. The included angle of such a
taper should not exceed 30 degrees,
either upstream or downstream of the
flowmeter.
Bypass
We strongly recommend that piping
and isolation valves be installed in such a
way that the flow can bypass the meter.
Such an arrangement permits both
removal of the meter for servicing and
convenient stopping of flow through the
meter for zero checks.
We also recommend that a cleanout tee
be installed near the flow tube (see Figure
1 for one suggested layout). This
accessibility is particularly important for
meters that do not have electrode-
cleaning capability or are subject to
deposition or coating.
Orientation and Location
The flowmeter should preferably be
installed in a vertical length of pipe. This
orientation tends to equalize wear on the
lining when an abrasive liquid is flowing,
and it also prevents gases from collecting
at the pipe crown. 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:
• Orient the meter so that the elec-
trodes are on a horizontal line.
• If the pipe is horizontal, install bleed
valves at the crown near the
flowmeter so that the line can be
checked for accumulated gas.
• In any event, locate the flowmeter so
that it will remain full, both during
flow and when the flow is stopped.
Limiting Velocities
To assure scouring action, design from
minimum velocities of at least 1.5 m/s (5
ft/sec) for primary sludge and at least 0.9
m/s (3 ft/sec) for other sludges and raw
influent sewage. To avoid excessive
By-Pass
Value
Isolating
Value
Flow
Note: Recommended 5 D Up and Down Stream
Figure 1. Typical installation.
abrasion, velocities should never exceed
8 m/s (25 ft/sec).
Other Considerations
Place the flowmeters on the high
pressure side of pumps and control valves
to minimize the effect of gas bubbles.
Avoid proximity to heavy induction
machinery. Proper grounding is
essential.
Even when the flow device is correctly
installed, the measuring system should
be checked for uncertainty of
measurement and estimated errors
should be recorded. Error estimates
provide an assessment of the uncertainty
of a measurement. One method of
estimating the uncertainty of a flowrate
determination based on the principles of
a venturi tube is to combine individual
uncertainties as follows.
8Q/Q = [(8C/C)2 + M2 (8D/D)2 + N2(Sd/d)2 +
(1/4)(8Ah/Ah)2] 1/2
where M = 2fiV(1 -J34)
N = 2/(1 -O4)
S - uncertainty
d = throat diameter
Ah = differential head on meter in
terms of height of the flowing
fluid
C = flow coefficient
D = inlet diameter
Q = volumetric flowrate
]8 = diameter ratio, d/D
The second and third terms inside the
brackets are usually small, since the
diameter uncertainties are limited.
Volumetric Method
Verification of flowmeter accuracy
becomes more difficult with increasing
size. For small meters of 7.5 cm (3") or
less, it is usually possible to set up a
means of diverting the flow to a tank of
known geometry so that the accuracy of
the meter can be checked volumetrically.
Even for large meters this is still the pre-
ferred method; but if this technique is not
possible, alternate methods can be used.
Dilution Method
In the dilution method, the flowrate is
deduced from the dilution of measurable
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properties (e.g., color, conductivity, or
fluorescence) of tracer chemicals added
to a turbulent flow in known amounts.
The calibration can be done either by the
constant-rate injection method or the
slug injection method. The constant-rate
method is recommended here because it
appears to be more practical for in-plant
use. In the constant-rate injection
method, a tracer solution of accurately
known concentration is injected
upstream at a rate that is constant and
accurately measurable. At a downstream
distance long enough for complete
mixing, the flow is sampled and the con-
centration determined after a steady
state of concentration "plateau" is
attained. The flowrate, Q, is then
determined from
Q = q(c, - c2)/(c2 - CQ)
where q = the rate at which the sample of
concentration c, is injected,
c2 = the measured "plateau" con-
centration downstream, and
CQ = (which may be close to zero) =
the background concentration of
the tracer chemical existing in
the flow.
This method requires accurate
measurement of q and of all
concentration. Skilled personnel and
specialized equipment are needed, but
under optimum conditions, the potential
accuracy is high.
The tracer property must be conserva-
tive, since losses by absorption to solids
in the flow will result in an apparent
reduction in c2:. The fluorescent dye
Rhodamine WT has been used
successfully in sewage without losses,
but its behavior in sludge is not known.
These recommended practices were
prepared in fulfillment of Interagency
Agreement No. 78-D-X0024-1 by the
National Bureau of Standards under the
sponsorship of the U.S. Environmental
Protection Agency.
Gershon Kulin is with the National Bureau of Standards. Washington, DC 20234.
Walter Schuk is the EPA Project Officer (see below).
The complete reports, entitled:
"Recommended Practice for Flow Measurement in Wastewater Treatment
Plants with Venturi Tubes and Venturi Nozzles," (Order No. PB 85-121 663;
Cost: $10.00)
"Recomended Practice for the Use of Parshall Flumes and Palmer-Bowlus
Flumes in Wastewater Treatment Plants,"(Order No. PB 85-122 745; Cost:
$10.00)
"Recommended Practice for the Use of Electromagnetic Flowmeters in
Wastewater Treatment Plants, "(Order No. PB 85-122 737; Cost: $8.50)
The above reports will be available only from: (cost subject to change)
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
US GOVERNMENT PRINTING OFFICE; 559-016/7873
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