COOLING TECHNOLOGY INSTITUTE
CTI CODE TOWER
Standard Specifications
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Standard for
Liquid Flow Measurement
September 2008
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CTI Bulletin STD-146 (08)
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FOREWORD
This Cooling Technology Institute (CTI) publication is published as an aid to cooling tower purchasers and designers.
It may be used by anyone desiring to do so, and efforts have been made by CTI to assure the accuracy and reliability of
the data contained herein. However, CTI makes no warranty of fitness for particular purpose or merchantability nor
any other warranty expressed, implied or statutory. In no event shall CTI be liable or responsible for Incidental,
Consequential or Commercial losses or damage of any kind resulting from this publication's use; or violation of any
federal, state, or municipal regulation with which this publication may conflict or for the infringement of any patent
resulting from the use of this publication.
Nothing contained herein is to be construed as granting any right for the manufacture, sale or use in connection with
any method, apparatus, or product covered by letters patent, nor as insuring anyone against liability for infringement of
letters patent.
This guideline document summarizes the best current state of knowledge regarding the specific subject. This document
represents a consensus of those individual members who have reviewed this document, its scope and provisions. It is
intended to aid all users or potential users of cooling towers.
Approved by the CTI Executive Board.
This document has been reviewed and approved
as part of CTI’s Five Year Review Cycle. This
document is again subject to review in 2011.
Approved by the
CTI Executive Board
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Copyright 2008
by Cooling Tower Institute
Printed in U.S.A.
CTI - Bulletin
STD-146 (08)
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All CTI codes and standards are copyrighted with all rights reserved to CTI. The reproduction of any part of this or
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STANDARD FOR LIQUID FLOW MEASUREMENT
TABLE OF CONTENTS
1.0
Scope and Purpose
1
2.0
Nomenclature & Symbols
1
3.0
Piping Requirements
2
4.0
Contaminants or Disturbances
3
5.0
Primary Flow Measurement Device and Methods
3
6.0
Primary Device Calibration
4
7.0
Flow Signal Conditioning and Readout
4
A
References (Normative)
6
B
References (Informative)
6
C
Pitot Tube
7
Appendix
D
Figure C-1 Pitot Tap Installation
12
Figure C-2 Air-Over Water Inverted “U” Tube Manaometer
13
Table C-3 Density and Specific Gravity of Water
14
Table C-3 (IP)
14
Figure C-4 Equal Annular Area Mid-point Factors for Pitot Traverse Points
15
Figure C-5 Water Flow Measurement Pitot Tube Data Sheet
16
Figure C-5 (IP) Water Flow Measurement Pitot Tube Data Sheet
17
Alternative Flow Meters
18
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STANDARD FOR LIQUID FLOW MEASUREMENT
1.1
1.2
the Cooling Technology Institute test codes.
The accuracy of the flow measurements depends
on the location and type of instrument used and
the stability of the test.
1.0 SCOPE AND PURPOSE
Scope - This standard covers methods for liquid
flow measurement. Its application is limited to
the measurement of water, water/glycol
mixtures, and other homogencous single-phase
liquids for which acceptable physical property
data are available.
1.3
Purpose - The basic intent of this standard is to
provide liquide flow measurement procedures
that are accurate and meet the requirements of
Units of Measure – This standard is written in
Primary Rational SI (International System of
Units) with secondary I-P (inch-Pound) units.
For full details on appropriate conversions
between SI and I-P units refer to CTI Standard
STD-145.
______________________________________________________________________________________________
2.0 NOMENCLATURE & SYMBOLS
The symbols used in this code are identified in the following table.
Symbol
Description &
Units of Measure
A
Cross sectional area of the pipe at the measurement location,
m2 (ft2)
β
Beta Ratio – ratio of small to large diameter in orifices,
venturis or nozzles, and contractions or enlargements in pipes
(d/D), dimensionless
Co
Calibration coefficient for measurement device,
d
Orifice or nozzle throat diameter,
mm (in)
D
Pipe inside diameter,
mm (in)
Dc
Characteristic dimension. Used in Reynolds Number computation. It may
refer to pipe diameter or pitot tip diameter depending on application,
DP
Abbreviation for “Differential Pressure” when referring to a class of
f low measurement devices.
dimensionless
m (ft)
δ
Manometer reading ,
mm (in)
∆P
Differential pressure,
Pa (psi)
f
Multiplier on diameter to determine measurement position for equal
annular area sectors within a closed conduit,
dimensionless
. (See Figure C-4)
g
Acceleration due to gravity,
γ
Specific gravity, dimensionless
H
Velocity head of flowing fluid,
n
Number of points in a pitot traverse,
dimensionless
Nx
Specific station number of a pitot measurement location,
dimensionless
ν
Kinematic viscosity,
PS
Static pressure,
Pa (psi or lbf/in2)
PT
Total pressure,
Pa (psi or lbf/in2)
nominally 9.80665m/s2
(32.1740 ft/s2)
m (ft) of flowing fluid
m2/s (ft2/s)
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Pa (psi or lbf/in2)
Pv
Velocity pressure,
Q
Volumetric liquid flow rate,
Re
Reynolds Number – Defined by the formula vDc/ν,
SL
Straight, unobstructed length of pipe in measurement location
expressed in multiples of pipe diameter,
L/s (gal/min or gpm)
dimensionless
D
ρ
Fluid density,
kg/m3 (lb/ft3)
ρff
Density of flowing fluid,
kg/m3 (lb/ft3)
ρmf
Density of manometer fluid,
kg/m3 (lb/ft3)
v
Fluid velocity,
m/s (ft/s)
V
Velocity, weighted average,
m/s (ft/s)
X
Distance from a reference point to a pitot location,
mm (in)
_____________________________________________________________________________________________
Measurement locations with disturbance free
straight pipe less than five pipe diameters (5D)
upstream or one pipe diameter (1D) downstream
lead to unacceptable flow measurement errors for
nearly all flow measurement methods. In these
installations, location insensitive measurement
methods such as dye dilution must be used.
3.0 PIPING REQUIREMENTS
The flow measuring instrument position and
orientation in the pipe is critical to the accuracy of the
flow measurement. The contracting parties to a test
shall evaluate the piping configuration to determine the
optimum flow measurement location and instrument to
be used.
Available Pipe
Straight Length,
SL
3.1 Measurement Location - Most of the commonly
used flow measurement instruments require a
fully developed velocity profile at the point of
measurement, which is achieved with long,
obstruction and disturbance free straight runs of
pipe. To ensure the stated accuracy of the flow
measurement devices, the flow measurement
location should be situated in a section of straight
pipe, free of any valves or fittings, extending
ideally twenty pipe diameters (20D), but a
minimum of fifteen pipe diameters (15D) in
length. The measurement location should have a
minimum of ten pipe diameters (10D), or 2/3 of
the length, of the straight pipe upstream and a
minimum of five pipe diameters (5D), or 1/3 of
the length, of the straight pipe downstream.
20D
Device
Location –
Upstream
Device
Location –
Downstream
15D
5D
10D
5D
2/3 · SL,
but > 5D
1/3 · SL,
but > 1D
Ideal
15D
Minimum SL for
desired levels of
uncertainty.
<15D
Compromised
levels of
uncertainty.
Practical limitations on field piping often require
taking flow measurements where the 15D
minimum criteria for straight, obstruction-free
pipe length is unattainable. Recognizing that
compromising the 15D minimum straight run of
pipe decreases the measurement accuracy,
measurements with reduced, but generally
acceptable, levels of accuracy shall be made with
the measurement location set according to the 2/3
upstream - 1/3 downstream ratio of the available
straight length of pipe. In very short straight
lengths, deference should always be given to
more upstream length than downstream length.
<6D
Unacceptable
levels of
uncertainty.
Special flow
measurement
techniques
required.
When the flow measurement location is in a
vertical riser, the location must also be in a
region of positive static head.
3.2 Safety - In addition to the considerations for
accuracy, it is also imperative that the flow
measurement location be safely accessible to test
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3.3 Flow Area - The average measured pipe internal
diameter should be used in the flow area
calculations. The pipe internal diameter should
be precisely measured on at least two (2)
diameters 90° opposed at the plane of the flow
measurement. It is the intent to establish the true
cross sectional area of the pipe by the best means
available. For flow measurements made with
pitot tubes, the area used to calculate the water
flow rate will depend on the method of pitot tube
calibration. If the pitot tube is calibrated based
on a “blockage correction” basis, then the area of
the flow measurement, AC, is determined by the
total cross sectional area minus the average area
of the cross section of the pitot tube at each of the
measurement locations. If the pitot tube
calibration is based on a “gross” area, then the
area is equal to the uncorrected cross sectional
area of the pipe at the pitot measurement plane.
Other types of insertion meters may require
similar flow area blockage correction.
3.4 Flow Conditioners – Flow conditioners in the
flow measurement section may be used to reduce
swirl and improve the velocity profile.
4.0 CONTAMINANTS OR DISTURBANCES
Contaminants or disturbances in the flow stream can
have a significant and adverse effect on flow
measurement.
4.1 Particulates and/or Solids - Particulates and
solids may cause partial or total plugging of
pressure taps and reduction in cross-section of
the flow area. Appropriate flow measurement
instruments tolerant to the liquid conditions must
be selected prior to a test.
4.2 Air (Gas) Bubbles - Any source of free air (or
gas) bubbles should be eliminated during the test.
Bleed valves should be installed at all high points
in the instruments and on the instrument lines
where air (or gas) may collect. These high points
should be purged to eliminate all air (or gas)
from the system at a frequency appropriate for
the application.
4.3 Pulsating or Surging Flow - Pulsating or surging
adversely affects measurement accuracy. Flow
measuring instruments shall be located as far as
practical from the source of the pulsation. Any
noticeable pulsation effects should be
documented in the test report.
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4.4 Swirl Flow - Swirl is spiraling flow, which
strikes the flow meter at an oblique angle. The
severity of the swirl component is directly related
to the pipe or valve configuration and
disturbances which generate swirl flow. Flow
swirl is inversely related to the distance between
the disturbance and the flow measuring location.
Installation of flow conditioners may reduce
swirl flow.
5.0 PRIMARY FLOW MEASUREMENT DEVICE
AND METHODS
5.1 Pitot Tube - The CTI's recommended flow
measurement instrument is the pitot tube (see
Appendix C). Properly applied, the pitot tube
provides acceptable levels of accuracy over a
broad range of pipe sizes and flow rates
encountered in field testing. Assumptions about
flow profile and flow area, required of some
meters for stated accuracy, are eliminated
because a pitot traverse measures actual local
velocity profile in the pipe as well as pipe
diameter. The pitot tube is simple in operating
principle and construction, durable, portable, and
may be inspected with each use for damage,
clogging, or wear.
5.2 Other Flow Measurement Instruments - The
following listed flow measurement instruments,
when properly applied, are acceptable to CTI for
testing when the selected instrument is mutually
agreed upon by the contracting parties.
A brief synopsis of these flow instruments is
presented in the Appendix D of this Standard.
Specific and full details for the flow instrument
selected should be obtained from the instrument
manufacturer.
5.2.1
Orifice Meter (Appendix D.2.).
5.2.2
Flow Nozzle (Appendix D.3.).
5.2.3
Venturi (Appendix D.4.).
5.2.4
Multiport Averaging Pitot Flow Meter
(Appendix D.5.).
5.2.5
Vortex Shedding Meter
(Appendix D.6.).
5.2.6
Turbine Meter (Appendix D.7.).
5.2.7
Magnetic Flow Meter (Appendix D.8.).
5.2.8
Transit time Ultrasonic Flow Meter
(Appendix D.9.).
5.2.9
Tracer Injection Measurement Method
(Appendix D.10.).
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personnel during the test. The tower owner
and/or site contractor shall provide the necessary
equipment for safe access.
6.0 PRIMARY DEVICE CALIBRATION
6.1 Calibration Laboratory - Flow measurement
instruments must be calibrated at flow
laboratories having both experience and
qualification for a minimum of engineering grade
primary device calibration. There are a number
of flow laboratories available for calibration of
flow meters. The typical considerations are flow
capacity, maximum line size and cost.
Traceability of the calibration to primary
measurement standards is required and should be
considered in selection of the laboratory. The
individual flow laboratory certification accuracy
is generally from ¼% to 1% of true flow rate.
The uncertainty of the calibration reference flow
meter or laboratory flow standard shall not
exceed ±1%.
Some laboratories state their uncertainty in terms
of the % of reading value. The reporting of
uncertainty in the calibration should be specified
prior to calibration. Calibration reporting shall
include sufficient data, description, and
discussion of the calibration to satisfy the parties
to the test where the calibrated device is to be
used.
6.2 Calibration Requirements - Calibration shall be
completed for a minimum of two (2) average
flow velocities on the order of 1.5 m/s (5 ft/sec)
and 3 m/s (10 ft/sec) and on the order of the test
flow velocity if it is significantly out of this
range.
Calibration intervals shall be at least as frequent
as specified in the governing CTI test code or
immediately upon known or suspected instrument
damage. For example, ATC-105 “Acceptance
Test Code for Water Cooling Towers” and Multiagency licensing agreements specify a maximum
three (3) year calibration interval, if undamaged.
7.0 FLOW SIGNAL CONDITIONING AND
READOUT
Regardless of the type of flow measurement
instrument, all signal conditioning and readout
components need to be consistent with the specified
accuracy and operating principles of the flow
measurement instrument. The operator must be aware
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of the condition of the flow measurement readout
device, its calibration and its use.
7.1 Manometer - Manometers of various types are
simple instruments for differential pressure (DP)
measurements. The type of manometer selected
must be suitable for both the differential and
system pressures expected in a particular
application.
7.1.1 Air over Liquid Manometer - The inverted
U-tube manometer with air over the measured
liquid is the CTI's standard instrument for DP
measurements. The height differential between
the two liquid menisci in the manometer
indicates the velocity head caused by fluid flow.
(See example of air-over-liquid manometer in
Figure C-2).
7.1.2 Liquid-Liquid "U"-Tube Manometer - The
upright U-tube manometer uses an indicating
fluid other than air over the measured liquid. The
indicating fluid must be totally immiscible and
non-reactive with the measured liquid, must have
a readily discernable interface to the measured
liquid and its density must be accurately known.
Refer to the manometer manufacturer for specific
information regarding different indicating fluids
and proper manometer operating procedures.
7.1.3 Manometer Readings
7.1.3.1 Manometer Fluid Fluctuations
Because most flows have fluctuations in
velocity pressure (Pv) and static pressure
(Ps), the time average of the deflection is
used to indicate the ∆P. (Note: Averaging
of the maximum and minimum deflection is
not equivalent to the time average and
should not be used.)
7.1.3.2 Manometer Indicating Fluid Choice
- If an indicating fluid other than air is used
in the manometer, the density of the fluid
must be low enough to produce an average
deflection of 250 mm (10 inch) or greater
DP indication.
7.1.3.3 System Pressure Considerations Total system pressure values should be
taken into account in choosing hardware
and tubing.
7.1.3.4 Gas in the System - All gas bubbles
must be removed from the tubing and
fittings.
Valve
combinations
and
sequencing should facilitate filling of the
lines and expulsion of gas bubbles.
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Flow measuring instruments and methods other
than those listed may be required in certain
applications if properly applied and agreeable to
the parties of the test.
7.1.3.5 Parallax - Caution should also be
exercised in reading the manometer to
prevent reading errors due to parallax.
7.1.3.6 Liquid Meniscus - The meniscus, or
interface between the manometer indicating
fluid and the measured liquid, can have a
convex, concave or flat surface, depending
on the particular manometer fluids present
and fluid movement.
Therefore, care
should be exercised to read the manometer
meniscus the same way (on the edge or at
the top or bottom of the curvature) for each
reading.
7.2 Other "DP" Readout Devices - There are a
number of other types of readout devices for DP
type flow meters which can be classified as
mechanical, pneumatic or electronic. The most
commonly used would be the electronic DP
transmitter.
The DP transmitters, must be
accurate, stable, repeatable and linear. These
units must also be properly calibrated for zero
and span.
7.4 Calibration and Use - It is recommended that all
flow
signal
conditioning
and
readout
instrumentation be calibrated and inspected in
accordance with accepted engineering practice
and in accordance with the manufacturer's
recommendations.
Any required instrument
calibration correction curves should be prepared
in advance of the test and such curves, operating
characteristics or other related factors are to be
recorded in the test report. Flow measurement
instruments as well as signal conditioning and
readout instrumentation must be used in a
manner consistent with their calibration method.
Regardless of the type of flow measurement
device, all signal conditioning and readout
components need to be consistent with the
specified accuracy and operating principles of the
flow measurement device. The operator must be
aware of the condition of the flow measurement
readout device, its calibration, and its use.
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7.3 Other Readout Devices - Several other flow
meters such as magnetic meters, ultrasonic
meters, turbine meters and vortex meters use
flow signal conversion and readout other than
DP. The signal conditioning may be quite
complex and non-linear and not easily verifiable.
Caution should be exercised in choosing and
operating these meters and related readout.
These devices must be proven for their
application, accurate, stable, and repeatable. The
unit calibration must be known and verified on a
regular basis.
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APPENDIX A. REFERENCES
(Normative)
A.1 REFERENCES, STANDARDS AND CODESNORMATIVE
Listed here are standards, handbooks, and other
publications essential to the implementation of
this standard. All references in this section are
considered a part of this standard.
1. ASME, Application, Part II of Fluid Meters, Sixth
Edition 1971, Interim Supplement 19.5 on
Instruments and Apparatus, ASME PTC19.51972.
2. ASME Standard, Measurement of Fluid Flow in
Pipes using Orifice, Nozzle and Venturi, ASME
MFC-3M-1989.
APPENDIX B. REFERENCES
(Informative)
B.1 REFERENCES, STANDARDS AND CODESINFORMATIVE
Listed here are standards, handbooks and other
publications which may provide useful
information and background, but are not
considered essential to the implementation of this
standard. References in this section are not
considered a part of this standard.
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1. American National Standard, Orifice Metering of
Natural Gas, ANSI/API 2530, First Edition
2. ANSI/ASME, Measurement of Liquid Flow in
Closed Conduits Using Transit-Time Ultrasonic
Flow Meters, ANSI/ASME MFC-5M-1985.
3. ASME, Fluid Meters, their Theory and
Application, Sixth Edition 1971.
4. ASME, Fluid Flow in Closed Conduits –
Connections for Pressure Signal Transmissions
Between Primary and Secondary Devices, MFC8M-1988.
5. ASME, Glossary of Terms Used in the
Measurement of Fluid Flow in Pipes, MFC-1M1991.
6. ASME, Measurement of Fluid Flow in Closed
Conduits by Means of Electromagnetic
Flowmeters, MFC-16M-1995.
7. ASME, Measurement of Fluid Flow in Pipes
Using Vortex Flow Meters, MFC-6M-1987.
8. ASME, Measurement Uncertainty for Fluid Flow
in Closed Conduits, MFC-2M-1983(R1988).
9. ASME, Method for Establishing Installation
Effects on Flowmeters, MFC-10M-1994.
10. ASME, Power Test Codes, Hydraulic Prime
Movers, PTC-18, 1949.
11. ASME, Power Test Codes, Instruments and
Apparatus,
Chapter
3,
Fluid
Velocity
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12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
Measurement, Part 5 - Measurement of Quantity
of Materials, PTC-19.5.3, 1965.
ASME, Power Test Codes, Instruments and
Apparatus, Chapter 4 - Flow Measurement of
Quantity of Materials, PTC-19.5-4, 1959.
ASME, Power Test Codes, Atmospheric Water
Cooling Equipment, PTC-23-1986.
ASME, Flow Meter Computational Handbook,
1961.
ASME Transactions, Volume 61, Pages 465 to
476, 1939. Pitot Tubes in Large Pipes, Edward
S. Cole and E. Shaw Cole.
ASME Transactions, August, 1935, Pitot Tube
Practice, Edward S. Cole.
ASME Transactions, August, 1939, Investigation
of Errors of Pitot Tubes, C.W. Hubbard.
CTI Bulletin TP-3A, Kent J. Bordelon, 1963.
CTI Bulletin FSP-156(00), 2000.
CTI Paper No. TP-183A, John P. Fath, 1978.
CTI Paper No. TP03-02, Mark S. Huber &
Robert P. Miller,2003.
Flow Measurement Engineering Handbook, R.W.
Miller, McGraw-Hill Book Company, 1984.
ISA, Flow, Its Measurement and Control in
Science and Industry, Volume 1, Parts 1, 2, and
3, 1974.
ISA, Industrial Flow Measurement, David W.
Spitzer, First Printing, May, 1984.
ISO 2186:1973, Flow in Closed Conduits –
Connections for Pressure Signal Transmissions
Between Primary and Secondary Elements,
International Organization for Standardization,
Switzerland.
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34. ISO 11631:1998, Measurement of Fluid Flow -Methods of Specifying Flowmeter Performance,,
International Organization for Standardization,
Switzerland.
35. ISO/TR 12767:1998, Measurement of Fluid Flow
by Means of Pressure Differential Devices –
Guidelines to the Effect of Departure from the
Specifications and Operating Conditions Given
in ISO 5167-1, International Organization for
Standardization, Switzerland.
36. ISO/TR 15377:1998, Measurement of Fluid Flow
by Means of Pressure Differential Devices –
Guidelines for the Specification of Nozzle and
Orifice Plates Beyond the Scope of ISO 5167-1,
International Organization for Standardization,
Switzerland.
37. Shell Flow Meter Engineering Handbook, 1968.
38. U.S. Department of Commerce, National Bureau
of Standards, Survey of Micromanometers, NBS
Manograph 114, 1970.
39. U.S. Department of Commerce, National Bureau
of Standards, Calibration and Related
Measurement Services of the National Bureau of
Standards, NBS Special Publication 250, 1982
Edition.
40. U.S. Department of Commerce, National Bureau
of Standard, A Guide to Methods and Standards
for the Measurement of Water Flow, NBS
Special Publication 421, 1975.
41. United States Department of the Interior, Bureau
of Reclamation, Water Measurement Manual,
Second Edition, 1974.
42. United States Federal Register, Volume 36,
Number 247, Part II, December, 1978,
Environmental Protection Agency, Standards of
Performance for New Stationary Sources,
Method 1, Sample and Velocity Traverses for
Stationary Sources.
APPENDIX C. PITOT TUBE
C.1.0 PITOT TUBE
C.1.1 Operating Principle - The pitot tube is an
instrument designed and constructed to
incorporate one opening facing upstream (facing
the liquid flow) which senses the total pressure
(PT) in the pipe. The total pressure (PT) consists
of the static pressure (PS), plus the velocity
impact pressure (PV) due to liquid flow. A
second opening is incorporated in the pitot tube
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to sense the static pressure (PS). Depending on
the pitot tube design, there could be more than
one static pressure opening. The differential
pressure (∆P), or the difference between the
total pressure (PT) and the static pressure (PS), is
a direct measurement of the velocity pressure
(PV). For incompressible, single-phase liquids,
the ∆P or velocity pressure (PV) is proportional
to the square of the fluid velocity according to
Bernoulli’s Law. A differential pressure gauge
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26. ISO 4006:1991, Measurement of Fluid Flow in
Closed Conduits -Vocabulary and Symbols,
International Organization for Standardization,
Switzerland.
27. ISO 5167-1:1991, Measurement of Fluid Flow by
Means of Pressure Differential Devices – Part 1:
Orifice Plates, Nozzles and Venturi Tubes
Inserted in Circular Cross-Section Conduits
Running Full, International Organization for
Standardization, Switzerland.
28. ISO/TR 5168:1998, Measurement of Fluid Flow
– Estimation of Uncertainties, International
Organization for Standardization, Switzerland.
29. ISO/TR 6817:1992, Measurement of Conductive
Liquid FlowRate in Closed Conduits – Method
Using Electromagnetic Flowmeters, International
Organization for Standardization, Switzerland.
30. ISO 7066-1:1997, Assessment of Uncertainty in
the Calibration and Use of Flow Measurement
Devices – Part 1: Linear Calibration
Relationships, International Organization for
Standardization, Switzerland.
31. ISO 7066-1:1998, Assessment of Uncertainty in
the Calibration and Use of Flow Measurement
Devices – Part 2: Non-Linear Calibration
Relationships, International Organization for
Standardization, Switzerland.
32. ISO 7194:1983, Measurement of Fluid Flow in
Closed Conduits -- Velocity-area Methods of
Flow Measurement in Swirling or Asymmetric
Flow Conditions in Circular Ducts by Means of
Current-meters
or
Pitot
Static
Tubes,
International Organization for Standardization,
Switzerland.
33. ISO/TR 9464:1998, Guidelines for the Use of
ISO 5167-1:1991, International Organization for
Standardization, Switzerland.
or manometer connected between the total
pressure and static pressure taps of the pitot tube
conveniently provides the measurable ∆P, or
velocity pressure (PV), of the flowing fluid.
The pitot shall not be used for flow
measurements outside the calibration span
unless additional calibration points are taken at
the velocities being measured.
C.1.2 Types of Pitot Tubes - A large variety of pitot
tubes have been developed over the years to
accommodate the many different flow
measurement applications. The types of pitot
tubes that may be encountered include the basic
impact pitot tube, pitot-static tube, Simplex pitot
tube, Kiel pitot tube, Wedge pitot tube,
combined-reverse or "S" pitot tube, and heavy
duty pitot tube. The particular specifications or
operating characteristics of any given pitot type
that may be considered should be obtained from
the individual manufacturer or from the many
technical articles and publication on pitot tubes.
The calibration uncertainty shall not exceed
2.5%. Pitot tube calibration uncertainty shall
incorporate
the
following
uncertainty
components:
a. Primary flow measurement uncertainty;
b. Variation of pitot tube coefficient between
each calibration velocity setting (e.g. 1.2, 2.1
and 3.0 m/s);
c. Uncertainty in the pipe diameter at the
measurement plane;
d. Spatial variation within the measurement
plane during the calibration at each velocity
setting.
The pitot tube coefficient for a given flow
measurement depends upon the particular
design or type pitot tube being used and the
Reynolds Number of the pitot tip. Some types
of pitot tubes do not measure the true static
pressure in the pipe but static pressure with
some component of velocity pressure. The
magnitude of the difference between true pipe
static pressure and observed static pressure,
gives rise to the many calibrations on the
Simplex tube showing values of calibration
coefficient (Co) between 0.79 and 0.83. An
ideal pitot tube would have a calibration
coefficient of unity (1.0), independent of tip
Reynolds Number.
C.1.4 Pitot Tap Installation - Two (2) pitot taps, 90°
apart but in the same cross-sectional plane of the
pipe, are required in each liquid flow line in
which the liquid flow rate is to be determined.
The mounting connections shall be installed in a
straight run of pipe free from obstructions in
accordance with Section 3.0, "Measurement
Location Requirements", of this Standard.
Refer to Figure C-1 for pitot tap installation.
C.1.3 Calibration - Each pitot tube must be accurately
calibrated prior to its initial usage. Any signs of
physical damage require the pitot tube
calibration be verified in accordance with
Section 6.0 of this Standard.
The Simplex pitot tube requires multiple
calibration points over its expected range of
operation. A minimum of 3 approximately
equally spaced calibration points are required at
flow velocities spanning 1.2 to 3.0m/s (4 to
10ft/s). Typical calibration points would be
1.2, 2.1 and 3.0 m/s (4, 7 and 10 ft/s). Within
the calibrated span, the pitot coefficient shall be
established by either of the following methods:
a. Using a single coefficient at all flow
velocities, the value of which is the average of
the calibration coefficients;
b. Using a velocity dependent coefficient, the
value of which is determined by the best
linear fit through the calibration coefficients.
The edge of the pitot tap holes, at the inside
surface of the pipe wall, must be clean and
square (or rounded slightly), free from burrs,
wire edges or other irregularities. The pitot tap
hole size should be the smallest hole practicable,
32mm (1-1/4 in) minimum pipe size for
standard un-reinforced tubes and 50mm (2 in)
minimum pipe size for reinforced tubes. This
minimizes erroneous flow readings due to
possible flow stream distortions, turbulent
eddies, and vortices near the pipe wall at the
insertion point.
If there is a choice of flow measurement in a
horizontal or vertical flow line, the traverse
should be made in a vertical flow line because
this eliminates the possibility of air being
entrapped at the top of the pipe. If the line is
horizontal, horizontal and vertical traverses
should be made.
Vertical traverses in a
horizontal pipe serve to indicate if there is
sediment in the bottom of the pipe or air (gas)
pockets in the top of the pipe, restricting flow
area. If horizontal and vertical traverses are not
feasible because of the physical surroundings
(i.e., underground, adjacent pipe, etc.), any
feasible orientation where two traverses at 90°
to each other and the pipe axis should be
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acceptable. However, in these installations,
measures must be taken to verify that no air
(gas) is trapped in the top of the pipe and no
sediment accumulated in the bottom of the pipe
that would reduce flow area. A small valve on
top of the pipe should be provided to assure any
air (gas) present in the line is released prior to
the flow measurement. Likewise, a small valve
on the bottom of the pipe may be used to
indicate the presence of heavy sediment.
Extreme care should be taken at the time of
installation to be certain that the taps are
perpendicular in the pipe wall to ensure that the
traverse is made on a diametric chord rather
than an oblique chord. Caution should be taken
in locating the taps so that there is room for the
full length of the retracted pitot tube when
mounted on the back of the tap valves.
C.1.5 Traverse and Flow Rate Calculation The basic
pitot tube device only senses the fluid velocity
pressure at one point (local velocity) in the flow
stream. In order to establish the average
velocity of the pipe, the pipe diameter must be
traversed taking ∆P measurements at the
centroid of each of several equal area annuluses.
A flow velocity at each of these points must
then be calculated and the average velocity used
to compute the total flow rate. Note that
½
velocity, which is proportional to (∆P) , is
averaged not the ∆P itself.
Following are the basic flow equations for the
pitot tube, along with supporting data tables for
completing pitot flow calculations. The tables
are not exhaustive but representative of the most
common situations. It is noteworthy that the
basic equations and data for pitot tubes apply to
almost
all
differential
pressure
flow
measurement devices.
Basic Equations:
V = Co ⋅
2 ⋅ g ⋅ ∆P
ρ ff
[1]
Recognizing PV = ∆P, Equation [1] is the Bernoulli
formulation for velocity, which may be also be written
in the following form:
V = Co ⋅ 2 ⋅ g ⋅ H
[2]
where H is the head of the flowing fluid. Volume flow
is the product of velocity and cross-sectional area.
Q =V ⋅ A
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C.1.5.1 Normal U-Tube Manometer:
In this type of manometer, the manometer fluid is
denser than the flowing fluid. The manometer
differential (δ) will represent velocity pressure in terms
of a column of fluid or head (H) of net differential
density.
ρ mf − ρ ff
1000
ρ ff
δ ρ mf − ρ ff
H= ⋅
12
ρ ff
δ
H=
⋅
[4]
[4 I-P]
C.1.5.2 Inverted U-Tube Manometer:
In this type of manometer, the manometer fluid is less
dense than the flowing fluid. An air over liquid
manometer, the most commonly used type of
manometer in pitot measurements is in this
classification. The manometer differential (δ) will
represent velocity pressure in terms of a column of
fluid or head (H) of net differential density.
ρ ff − ρ mf
1000
ρ ff
δ ρ ff − ρ mf
H= ⋅
12
ρ ff
δ
H=
⋅
[5]
[5 I-P]
For air over fluid manometers, the density of the air is
usually very small in comparison to the fluid density
and is taken to be negligible.
Equation [5] then simplifies to
H=
H=
δ
1000
δ
12
[6]
[6 I-P]
In most cases this assumption does not introduce more
than 0.1-0.3% error in the final flow analysis. For
example, assume the case of an air over water
manometer with the air columns saturated and
pressurized to 202 kPa absolute at 40°C.
Water density, ρff = 992.21 kg/m3
Air density, ρmf = 2.22 kg/m3
ρ ff − ρ mf 992.21 − 2.22
=
= 0.998
992.21
ρ ff
[3]
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Taking the square root of the head makes the
manometer fluid (air) density correction value even
smaller in the final flow calculation.
C.1.5.3 Working Equations – General Case:
Equations [2] and [3] are normally combined to the
fundamental form:
Q = Co ⋅ A ⋅ 2 ⋅ g ⋅ H
[7]
and then simplified further for practical units of
measure and manometer type.
In SI units:
Q = 4430 ⋅ C o ⋅ A ⋅ H
where
[8]
Co is the pitot calibration coefficient,
A is flow area given in m2,
H is fluid head given in m, and
Q is flow rate computed as L/s.
The value of the constant 4430 is derived from
1000 ⋅ 2 ⋅ 9.80665 = 4428.7 ≈ 4430
3
1000 is the conversion from m to L,
9.80665 is the nominal value of g, m/s2,
4430 is a conveniently rounded value with less than
0.1% introduced error.
where
In I-P units:
Q = 3600 ⋅ C o ⋅ A ⋅ H
where
[8 I-P]
Co is the pitot calibration coefficient,
A is flow area given in ft2,
H is fluid head given in ft, and
Q is flow rate computed as US gpm.
The value of the constant 3600 is derived from
60 ⋅ 7.48052 ⋅ 2 ⋅ 32.1740 = 3600.4
where
60 converts seconds to minutes
7.48052 is the conversion from ft3 to gal,
32.1740 is the nominal value of g, ft/s2,
3600 is a conveniently rounded value.
C.1.5.4 Working Equations – Specific Case - Air
Over Fluid Manometer:
If the pitot DP measurement is being taken with an
inverted air over fluid manometer, and the fluid has a
density similar to or greater than water, Equation [8]
can be reduced further, with negligible error, to the
following forms:
In SI units:
Q = 140 ⋅ Co ⋅ A ⋅ δ
Co is the pitot calibration coefficient,
A is flow area given in m2,
δ is manometer deflection given in mm, and Q is flow
rate computed in L/s.
where
The value of the constant 140 is derived from
4428.7 ⋅ 1 / 1000 = 140.05 ≈ 140
where
1000 is the conversion of manometer
deflection from mm to m, and
140 is a conveniently rounded value with less than
0.1% introduced error.
In I-P units:
Q = 1040 ⋅ Co ⋅ A ⋅ δ
[9 I-P]
Co is the pitot calibration coefficient,
A is flow area given in ft2,
δ is manometer deflection as inches, and Q is flow rate
computed in US gpm.
where
The value of the constant 1040 is derived from
3600.4 ⋅ 1 / 12 = 1039.3 ≈ 1040
where
12 is the conversion of manometer deflection
from in to ft, and
1040 is a conveniently rounded value with less than
0.1% introduced error.
Equation [9] will be sufficiently accurate for most pitot
measurements with inverted U-tube air over fluid
manometers. If actual values of g, ρff or ρmf differ
significantly from those used in the development of
these equations, or the manometer is a different type,
the fundamental working equation, Equation [8] along
with Equation [4] or [5] should be used.
C.1.6 Vibration of the Pitot Tube.
With large
diameter pipes and moderate (2 m/sec [7
ft/sec]) velocities, or with moderate diameter
pipes and high velocities, attention should be
given to possible vibrations of the pitot tube.
Slight vibrations do not seem to affect the
readings of velocity head but large vibrations
may influence them. A reinforced pitot tube
should be used in pipes 1200 mm (48 in) in
diameter or larger to minimize the vibration
effects and prevent excessive tube deflection.
C.1.7 Correction for Projected Area of Pitot Tube.
The area occupied by the pitot tube may or may
not be taken into consideration in the calibration
of the tube. If the calibration was calculated
recognizing the tube area, then this effect must
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possible in applications where the measurement
location, equipment calibration, tap installation,
readout device, and pitot tube are properly
applied.
C.1.8 Accuracy – Using a Simplex pitot tube,
measurement accuracy of plus or minus three
percent (± 3%) of the true value of liquid flow is
Accuracy values for other pitot tube types have
not been established.
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be similarly recognized in calculating flow from
a field traverse. The method used will be
determined by the pitot tube calibration
coefficient (i.e., blockage or non-blockage).
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Figure C-1 PITOT TAP INSTALLATION
Gate Valve or Equal For Pitot Taps
Pipe Size
Water Velocity
Pitot Tap
Close Nipple
Standard
150 mm – 1500 mm
(6 in – 60 in)
Reinforced
900 mm – 3660 mm
(36 in – 144 in)
Consult CTI if larger
90°
Half Coupling
< 3 m/s
(<10 ft/s)
recommended
Higher velocities
may be tolerated
Contact CTI
Valve and
Nipple Opening
≥ 32 mm*
(≥ 1 1/4 in)
≥ 50 mm*
(≥ 2 in)
*Caution:
Many commercial valves and/or heavy wall pipe nipples have openings less
than their nominal size, therefore, the nominal size used may have to be
increased one or more sizes larger than the minimum shown.
Remove all Burrs
Extending Into Pipe
Rule to Measure
Manometer
Deflection
Gate Valve
or Equal
Measuring Rule to
Locate Pitot Tube
Tip on Line
Water Flow
Pipe Bushing
if Required
Pitot Pipe Packing Gland
Pitot
Tube
Larger Nipple
Coupling
Water Manometer
Circulating
Water Line
Consult CTI Test Representative for
Minimum Clearance Requirements
Pitot Entry
Hole Burr
Free
Typically 0.6 m to 1.2 m + Pipe Diameter
(2 ft to 4 ft + Pipe Diameter)
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Figure C-2
AIR-OVER WATER INVERTED “U” TUBE
MANOMETER
Air
V4
DP = H
V1
Water
V2
V3
Device
LEGEND
V1, V2 : Simple Stop Cock
V3, V4 : 3-way Stop Cock
Cooling Tower
Water Flow
OPERATING SEQUENCE
Step 1:
Connect lines to DP connections on measuring device.
Step 2:
Open valves 1, 2, and 4; Close valve 3; fill all lines, expel all bubbles.
Step 3:
Close valves 1 and 2; open 4; open valve 3 slowly to drain some water out and let air in
valve 4 about half way down upper loop.
Step 4:
Close valves 3 and 4; open 1 and 2; elevate 4 until menisci are at comfortable position for
reading. Keep all air in one “bubble”.
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Primary
Table C-3
Temperature
o
C
Density
kg/m3
Specific
Gravity*
0
4
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
999.84
999.97
999.70
999.10
998.21
997.05
995.65
994.03
992.22
990.21
988.04
985.69
983.20
980.55
977.76
974.84
971.79
968.61
965.31
961.89
0.99987
1.00000
0.99973
0.99913
9.99824
0.99708
0.99568
0.99406
0.99225
0.99024
0.98807
0.98572
0.98323
0.98058
0.97779
0.97487
0.97182
0.96864
0.96534
0.96192
_______________________________________________
* Specific Gravity is typically referenced against water at 4oC.
Thermo-physical properties are per IAWPS-95 for water at 1 atm.
Reference: http://webbook.nist.gov/cgi/
Table C-3 (IP)
Density and Specific Gravity of Water
Temperature
o
F
Density
lbm/ft3
Density
lbm/gal
Specific
Gravity**
32
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
62.418
62.426
62.409
62.367
62.301
62.216
62.113
61.994
61.860
61.712
61.552
61.379
61.195
61.000
60.795
60.680
60.355
60.121
8.3441
8.3451
8.3429
8.3373
8.3284
8.3171
8.3033
8.2874
8.2695
8.2497
8.2283
8.2052
8.1806
8.1545
8.1271
8.0984
8.0683
8.0370
1.00082
1.00095
1.00067
1.00000
0.99894
0.99758
0.99593
0.99402
0.99187
0.98950
0.98693
0.98416
0.98121
0.97808
0.97479
0.97135
0.96774
0.96399
o
** Specific gravity is customarily referenced against water at 60 F.
Thermo-physical properties are per IAWPS-95 for water at 14.696 psia.
Reference: http://webbook.nist.gov/cgi/
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Density and Specific Gravity of Water
Fiigure C-4 Equal
E
Annu
ular Area Mid-point
M
Faactors for Pitot
P
Traverrse Points
Value
es of Factors,, f
--`,`,,,``,``,`,`,`,`````,,,`,,-`-`,,`,,`,`,,`---
20
18
1
16
14
12
10
8
6
4
N
1
.0127
.0141
.0159
9
.0182
.0213
.0257
.0323
.043
36
.0670
1
2
.0390
.0436
.0493
3
.0568
.0670
.0817
.1047
.146
64
.2500
2
3
.0670
.0751
.0854
4
.0991
.1181
.1464
.1938
.295
59
.7500
3
4
.0969
.1091
.1250
0
.1464
.1773
.2261
.3232
.704
41
.9330
4
5
.1292
.1464
.1693
3
.2012
.2500
.3419
.6768
.853
36
5
6
.1646
.1882
.2205
5
.2685
.3557
.6581
.8062
.956
64
6
7
.2042
.2365
.2835
5
.3664
.6443
.7739
.8953
7
8
.2500
.2959
.3750
0
.6336
.7500
.8536
.9677
8
9
.3064
.3821
.6250
0
.7315
.8227
.9183
9
10
.3882
.6
6179
.7165
5
.7988
.8819
.9743
10
11
.6118
.7041
.7795
5
.8536
.9330
11
12
.6936
.7635
.8307
7
.9009
.9787
12
13
.7500
.8118
.8750
0
.9432
13
14
.7958
.8536
.9146
6
.9818
14
15
.8354
.8909
.9507
7
15
16
.8708
.9249
.9841
16
17
.9031
.9564
17
18
.9330
.9859
18
19
.9610
19
20
.9873
20
N = 1 through N =
For stations
s
For stations
s
N=
n
n - 2N + 1
: f = 0.5 2
4n
n
+ 1 through N = n : f = 0.5 +
2
2N - 1 - n
4n
Exam
mple:
L
XN for
f 10 point
Station Locations
traverse of 600 mm ID
pipe:
Stattion #1
10 - 2 + 1
= 0.0257
40
00.0) = 15.42 mm
m
X1 = (0.0257) (60
f1 = 0.5 -
Stattion #8
16 - 1 - 10
= 0.8536
6
40
0
00.0) = 512.2 mm
X 8 = (0.8536) (60
f 8 = 0.5 +
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Appendix D – Alternative Flow Meters
ADVANTAGES AND DISADVANTAGES
Advantages:
• Low Cost
• Easily installed
• Good accuracy when properly installed
Disadvantages:
• Large, unrecovered pressure loss
• Requires significant lengths of straight pipe
and/or flow conditioners up stream
• Silt or particulate can build up on upstream face
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Governing Standards – Orifice plates shall be selected
and applied in accordance with ASME MFC-3M1989(R95), Measurement of Fluid Flow in Pipes using
Orifice, Nozzle, and Venturi and ASME PTC19.5-1972,
Application, Part II of Fluid Meters: Interim Supplement
on Instruments and Apparatus.
Accuracy - Calibrated with its transmitter and the
required lengths of straight pipe and flow conditioners in
place, a properly installed orifice plate will achieve
measurement accuracies of ± 0.75-2%.
D.2. FLOW NOZZLE
Operating Principle - The flow nozzle operates on the
same principle as the orifice plate. Unlike the orifice
plate, however, the flow nozzle has an elliptical or
rounded inlet leading to a short, constant diameter throat
section. The contoured inlet produces somewhat less
overall pressure drop than that generated by an orifice
plate of the same Beta Ratio but the impact on the
required system head (and operating cost) is still
significant.
Installation - A flow nozzle is installed in a flanged
spool section in the pipeline. It requires the same
minimum lengths of straight pipe and/or flow
conditioners as an orifice plate to achieve the specified
accuracies.
ADVANTAGES AND DISADVANTAGES
Advantages:
• Less costly than Venturi tubes
• Good accuracy when properly installed
Disadvantages:
• Large, unrecovered pressure loss, essentially the
same as an orifice plate of same capacity
• Requires significant lengths of straight pipe and/or
flow conditioners up stream
Governing Standards – Flow nozzles shall be selected
and applied in accordance with ASME MFC-3M1989(R95), Measurement of Fluid Flow in Pipes using
Orifice, Nozzle, and Venturi and ASME PTC19.5-1972,
Application, Part II of Fluid Meters: Interim Supplement
on Instruments and Apparatus.
Accuracy - Calibrated with its transmitter and the
required lengths of straight pipe and/or flow conditioners
in place, a properly installed flow nozzle will achieve
measurement accuracies of ± 0.75-2%.
D.3. Venturi
Operating Principle – The Venturi tube operates on the
same principle as the orifice plate. Its configuration is
similar to the flow nozzle but, with a diverging exit cone
to recover most of the velocity pressure created at the
throat, thereby substantially reducing the permanent
pressure loss.
Since dirt and sediment will not
accumulate in the contoured sections, this meter is more
suited to dirty water applications.
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D.1. ORIFICE PLATE
The orifice plate is one of several classical differential
pressure type flow meters, a category that also includes
the Venturi tube and flow nozzle. These meters all have
long application histories and a large amount of
published data exists on their design and application.
Operating Principle -- The operating principle
underlying classical differential pressure type flow
meters, such as the orifice plate, utilizes Bernoulli’s
energy equation for streamline flow. Basically this states
that when the flow in a conduit is contracted, there is an
increase in the velocity and velocity pressure (kinetic
energy) at the point of contraction and a corresponding
reduction in the static pressure (potential energy). The
pressure differential determined by measuring the static
pressure upstream of and in the vicinity the contraction
(orifice plate) is related to the difference between the
squares of the fluid velocities at the two points. Since
the velocity times the pipe area is the volumetric flow
rate, a basic flow equation can be developed relating
flow rate to the square root of the pressure differential.
In this relationship, the ratio of the orifice diameter (d) to
the pipe diameter (D) is typically termed the Beta Ratio
(β).
In its final form, the equation includes an
empirically derived constant to account for the discharge
coefficient, contraction characteristics, velocity profile,
etc.
Installation - A thin plate, usually of stainless steel, with
a precisely drilled, sharp-edged orifice hole is mounted
between two flanges in the pipe. The orifice is sized to
produce a pressure drop no greater than necessary to
produce desired accuracy of flow measurement. For
most applications, the Beta ratio (ratio of orifice diameter
to pipe diameter) typically falls between 0.35 and 0.65.
Various other types of mountings are available to contain
the basic orifice plate depending on service and operating
capability required.
Orifice meters, particularly in the larger Beta ratios, are
adversely affected by non-symmetric flow profiles and
swirl. The use of upstream flow conditioners and/or
significant lengths of straight pipe on the order of 15 to
20 pipe diameters are necessary for accurate
measurement.
ADVANTAGES AND DISADVANTAGES
Advantages:
• Low system head loss
• Requires shortest length of straight pipe upstream
• Excellent accuracy when properly calibrated and
installed
Disadvantages:
• Costlier than orifice plates and flow nozzles
• Greatest weight and larger size for a given line
size
Governing Standards – Venturi tubes shall be selected
and applied in accordance with ASME MFC-3M1989(R95), Measurement of Fluid Flow in Pipes using
Orifice, Nozzle, and Venturi and ASME PTC19.5-1972,
Application, Part II of Fluid Meters: Interim Supplement
on Instruments and Apparatus.
Accuracy - Calibrated with its transmitter and the
required lengths of straight pipe or flow conditioners in
place, a properly installed Venturi tube will achieve
measurement accuracies of ± 0.5-2%.
D.4.
Multiport Averaging Flow Meter
Operating Principle - The multiport averaging flow
meter is a differential pressure device which operates on
physical principles similar to the pitot tube. A multiport
averaging flow meter consists of a tube spanning the full
diameter of the pipe, the tube having upstream and
downstream pressure measurement ports spaced at
specific intervals across the pipe’s diameter to measure
the “average” velocity pressure of the flowing fluid. The
multiple upstream and downstream measuring ports are
interconnected such that the single output pressure
differential of the device is presumed to represent the
“average” velocity pressure of the flowing fluid.
Whereas a pitot tube is progressively inserted in a pipe to
measure discrete flow velocities at predetermined
stations, thus developing the complete velocity profile,
the multiport averaging flow meter assumes a uniformly
fully developed flow profile. The flow rate is calculated
from the measured differential pressure using the same
equipment and equations as for the pitot tube in
Appendix C.
Installation - The multiport averaging flow meter sensor
is inserted through a connection in the pipe wall and
spans the full diameter of the pipe. A far wall support to
minimize tube vibration may be required in large
diameter pipes and high velocity flows. It is not
recommended that the device be left in-situ in dirty water
and open systems because of the potential for fouling the
pressure ports.
Multiport averaging flow meters are adversely affected
by non-symmetric flow profiles and swirl. The use of
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upstream flow conditioners and/or significant lengths of
straight pipe on the order of 15 to 20 pipe diameters are
necessary for accurate measurement. The device shall be
calibrated for the size of pipe and piping geometry in
which it is to be used and shall be inspected for port
cleanliness before use.
ADVANTAGES AND DISADVANTAGES
Advantages:
• Low system head loss
• Simplifies flow measurement to a single reading
and calculation, reducing test and monitoring time
• Fairly inexpensive device relative to other meters
Disadvantages:
• Does not resolve non-uniform flow profiles well
• Requires significant straight runs of pipe for flow
conditioning
• Subject to tube vibration in large pipes and high
velocity flows
• Unlike a pitot tube, the instrument is limited to a
specific pipe diameter
Governing Standards – There are no internationally
recognized governing standards on this method of flow
measurement. Multiport averaging flow meters shall be
selected and applied in accordance with manufacturer’s
recommendations and guidance from Appendix C of this
standard.
Accuracy – Calibrated with its transmitter and the
required lengths of straight pipe and flow conditioners in
place, a properly installed multiport averaging flow
meter will achieve measurement accuracies of ± 2-5%.
D.5. Vortex Shedding Meter
Operating Principle - The vortex shedding meter is an
electronic sensor consisting of a bluff body of a width
approximately 1/4 to 1/3 of the pipe diameter with pulse
pickup sensors extending across the full diameter of the
pipe. The bluff body generates an alternating fluid flow
pattern on each side in accordance with a flow
phenomenon referred to as Von Karmen vortex shedding.
The frequency of the vorticies, alternating from one side
to the bluff body to the other, is proportional to the fluid
flow rate in the pipe.
Vortices will not form until a sufficient Reynolds
Number is reached for stable operation, so there are some
limits on flow velocity. Vortex formation is also affected
by velocity profile, swirl, pulsating flow, gas entrapment,
pipeline vibration and sensor deflection/vibration.
Installation - The vortex shedding meter is normally
installed between flanges in the pipeline. Because of its
sensitivity to flow profile and swirl, upstream and
downstream straight pipe length and flow conditioning
requirements are at least as severe as orifice meters (15
to 20 pipe diameters) and more length may be required in
certain piping configurations according to the
manufacturer’s recommendations.
ADVANTAGES AND DISADVANTAGES
Advantages:
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Installation - Venturi tubes are available commercially
in pipe diameters to 1000 mm and larger with ends
flanged or beveled for welding. The minimum lengths of
straight upstream pipe required for a venturi to achieve
specified accuracies are significantly less than those
required for an orifice plate or flow nozzle, often on the
order of only 5 to 7 pipe diameters.
• Good accuracy when properly calibrated and
installed in a clean fluid with a uniform velocity
profile
Disadvantages
• Costly and sensitive electronic sensor not practical
for very large pipe diameters
• Offers significant blockage to flow area,
increasing pumping head
• Requires significant straight lengths of pipe for
flow conditioning
• Sensitive to flow profile and entrained gas
Governing Standards – Vortex shedding meters shall
be selected and applied in accordance with ASME MFC6M-1987(R95), Measurement of Fluid Flow in Pipes
using Vortex Flow Meters.
Accuracy – Calibrated with its transmitter and the
required lengths of straight pipe and flow conditioners in
place, a properly installed vortex shedding flow meter
will achieve measurement accuracies of ± 1-3% on a
clean, homogenous fluid with uniform velocity profile.
D.6. Turbine Meter
Operating Principle – Turbine meters come in both
fixed pipe size devices where all the flow passes the
turbine element (fully ported) and insertion devices
where a turbine assembly is mounted on the end of a
support rod for insertion into the flow stream. Insertion
devices are only exposed to part of the total flow in the
immediate vicinity of the probe. Fixed pipe size (fully
ported) meters mount between flanges or threaded pipe
connections and are typically limited in size to what may
be used for make-up and bleed off water line sizes and
small primary loops, typically up to 2”. Insertion devices
are more practical for profiling and monitoring flow in
larger pipe diameters and open channels.
Both types of turbine meters work on the same principle
and consist of a meter body and a turbine rotor assembly
with fixed blades set at an angle to the flow stream. The
fluid passing the meter strikes the rotor blades causing
the rotor to spin at a rotating speed proportional to the
fluid flow rate. The moving rotor blade speed is detected
by a magnetic or capacitive pickup coil, providing a
digital pulse whose frequency is related to the fluid flow.
Turbine meters require a minimum flow velocity to
initiate rotation of the turbine and maintain steady
rotation to overcome friction forces in the rotor bearings.
Turbine meters should not be operated above
recommended flow velocities because bearings may
overheat causing binding, reduced accuracy, and
permanent bearing damage. The rotor bearings are
generally lubricated by the process fluid being measured
so it is important that the fluid remain clean to prevent
particulates from entering the bearing cavity and binding
the bearings.
Insertion meters may be used for single point flow
indication in a large pipe or may be used to profile
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velocity across the pipe diameter, much like a pitot tube.
Because of their size, insertion turbine meters will
measure velocity over a broader cross-section of pipe
than a spot measuring device like a pitot tube, and
special consideration must be given to the number and
location of the velocity profile points to ensure an
accurate profile has been measured.
Further, the
insertion turbine meter will have some difficulty
measuring velocity profiles close to the pipe wall.
Therefore, insertion types of meters will not be able to
develop the same accuracy as fully ported meters.
Insertion meters may be used in a single point
(centerline) mode for flow monitoring and checking, but
this does not provide acceptable accuracy for CTI
Acceptance Testing. Acceptance testing would require
velocity profiling. Both fully ported and insertion types
of turbine meters are subject to higher error in pulsing
and swirling flows and are not recommended for these
situations.
Installation - A minimum length of straight pipe 10 to
20 diameters upstream and 5 diameters downstream of
the turbine meter, plus upstream flow conditioning is
required to achieve the specified flow measurement
accuracy.
ADVANTAGES AND DISADVANTAGES
Advantages:
• Relatively inexpensive class of flow meters,
compact in design and easy to install
• Full ported models offer excellent accuracy when
properly calibrated and installed in clean fluids
Disadvantages:
• Subject to damage from particulates and debris in
the flow stream
• Reduced accuracy when insertion devices are used
for velocity profiling near pipe walls
• Insertion devices require large pipe opening and
offer significant flow blockage
• Must operate within prescribed flow velocities
Governing Standards – There are no internationally
recognized governing standards on this method of flow
measurement. Turbine flow meters shall be selected and
applied
in
accordance
with
manufacturer’s
recommendations.
Accuracy – Properly calibrated and installed with
sufficient straight lengths of pipe in a clean fluid, fully
ported precision turbine meters may achieve accuracy of
±.75-2%. An insertion device requires calibration in a
pipe of similar diameter and flow rate and may achieve
accuracy of ±3-5%.
D.7. Magnetic Flow Meter
Operating Principle – Magnetic flow meters come in
both fixed pipe size devices where all the flow passes the
sensing elements (fully ported) and insertion devices
where a magnetic sensing head is mounted on the end of
an insertion tube. Fixed pipe size (fully ported) meters
mount between flanges or threaded pipe connections
while insertion devices enter a pipe through a pipe wall
tap.
Magnetic flow meters measure the induced
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electromotive force (emf) created by a flowing fluid
passing through a magnetic field. The basic elements of
the magnetic flow meter are (a) the permanent magnet or
electromagnet, (b) the pipe or tube through which the
fluid flows, and (c) the insulated electrodes which are
attached to the inside of the pipe or tube. The secondary
meter element is the electronic signal conditioning
equipment used to measure the induced emf.
Besides calibration, these flow meters require several
precautions be taken to achieve accurate results. First,
because the induced emf is a function of the fluid
conductivity, the meter requires fluid conductivity well
above its sensing threshold or large flow measurement
errors will result. Therefore, if the fluid conductivity is
less than the threshold sensing value, salts should be
added to the fluid to raise this value. This may not be
practical in large systems or ultra-pure water closed
systems. The meter manufacturer should report the
required minimum fluid conductivity for meter
operation. Second, because the electrodes are inside the
pipe or tube, they are subject to corrosion and collection
of deposits. Therefore, before any flow measurement is
taken, the electrodes should be inspected and, if
necessary, cleaned. Third, entrained gasses break the
electrical continuity of the magnetic field, introducing
measurement errors. The meter should not be used on
flows with high levels of entrained gasses.
Fully ported magnetic flow meters can be highly accurate
and offer no obstruction to fluid flow. Commercial fully
ported meters are available in sizes up to about 3m (10
ft), but are generally limited by practical concerns such
as size, weight, power, and cost, to much smaller pipe
sizes. Insertion devices can be used for velocity profiling
on large diameter pipe but offer some obstruction to
flow, are subject to deflection/vibration, and have
sensitivity limitations near the pipe wall. Because of their
sensor size and pipe wall proximity limitations, insertion
electromagnetic meters may not be capable of measuring
flow velocity in the same equiannular locations as a pitot
tube in the same pipe. Therefore special consideration
must be given to the number and location of the velocity
profile points to ensure an accurate profile has been
measured. The insertion type of meter must be used in a
velocity traversing mode to have satisfactory accuracy
for CTI Acceptance Testing.
Installation – The meter must be mounted in a location
where gasses will not be trapped, and the pipe remains
full. The sensing electrodes must be mounted in a
position not subject to gas accumulation. In a horizontal
pipe, the electrodes should ideally be located at least 45°
off the vertical axis. Whether fully ported or insertion
type, the magnetic flow meter normally requires at least
five pipe diameters of straight pipe before and after the
meter to properly condition the flow to achieve the
specified flow measurement accuracy. Additional length
up to 15 diameters may be required when valves and
multiple elbows are located upstream. An AC electric
power source is normally required for the unit to be
operational.
ADVANTAGES AND DISADVANTAGES
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Advantages:
• Low system head loss
• Requires shortest length of straight pipe upstream
• Excellent accuracy when properly calibrated and
installed
• Simple set-up and operation
Disadvantages:
• Fully ported models are among costliest types of
flow meters
• High weight and size for larger pipe sizes make
portability impractical
• Insertion type meter is less sensitive for velocity
profiling near pipe walls and subject to vibration
and flow area blockage
Governing Standards – Electromagnetic flow meters
shall be selected and applied in accordance with ASME
MFC-16M-1995, Measurement of Fluid Flow in Closed
Conduits by Means of Electromagnetic Flowmeters.
Accuracy - Properly “wet” calibrated, and installed with
sufficient straight length of pipe flowing a sufficiently
conductive fluid, fully ported electromagnetic flow
meters may achieve accuracy of ±.5-2%. An insertion
device requires calibration in a pipe of similar diameter
and flow rate and may achieve accuracy of ±1.5-5%.
D.8. Transit Time Ultrasonic Flow Meter
Operating Principle - With these meters, an high
frequency ultra-sonic signal is beamed at an acute angle
across the conduit. The time required for the signal to
travel from the sending to the receiving transducer
depends on whether it is moving with or against the flow
of fluid and the speed of sound through the fluid. By
measuring and comparing the “transit time” of the signal
moving with the flow to that moving against the flow,
the average velocity of the fluid in the path of the beam
can be determined, assuming a fully developed and
uniform velocity profile. Given average fluid velocity
and pipe diameter, the flow rate is automatically
computed.
Locating the meter with sufficient upstream straight
length of pipe and proper calibration has shown these
instruments can produce repeatable and accurate flow
measurements, but the operating technician must be very
aware of instrument location and upstream &
downstream flow conditions. Because the transit time is
directly related to the average fluid velocity across the
diametric path, various non-uniform velocity profiles
may produce a common “average” velocity across this
linear path, which may not represent the cross-sectional
area weighted average velocity and either overstate or
understate the true flow rate. Taking multiple diametric
measurements and averaging the results does not correct
this error but may be used to indicate the presence of a
uniform velocity profile.
Transit time meters are available in a number of
configurations, the two basic considerations being the
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number of beam paths (which can range from one to
eight or more) and whether the transducers are in direct
contact with the fluid stream (wetted), or the beam must
pass through some intervening material (non-wetted).
Single beam instruments and some multiple beam
instruments perform transit time measurements across
one or more diametrical chord paths. Measurements of
this type are most sensitive to the velocity profile in the
conduit. Some multiple beam instruments measure flow
velocity over several non-diametrical chord paths and
may be less sensitive to non-uniform velocity profiles.
Both single and multiple beam meters are sensitive to
swirl. Transit time meters are generally used in clean
fluid applications because excessive suspended solids,
entrained air or other particulates may adversely affect
the measurement.
Installation – Transit time meters shall be installed in a
straight run of pipe, free of obstacles extending a
minimum distance of fifteen (15) pipe diameters
upstream and five (5) diameters downstream of the
instrument.
The transducers used to send and receive the signals may
be factory mounted on a spool piece for installation on
site or field installed by welding or bonding special
mounts to the conduit. Portable meters utilize simple
clamping devices to affix the transducers to the external
wall of the conduit. When mounting the transducers to
the external surface, it should first be thoroughly cleaned
to remove all dirt; scale, paint or galvanizing that could
interfere with the signal. Internal scale and certain kinds
of pipe material may also significantly attenuate the
meter’s signal. In horizontal runs, the transducers should
be mounted between two and four o’clock. Avoid
mounting them directly at twelve and/or six o’clock.
Installation in a vertical riser is preferred to eliminate gas
(air) pockets. Readings taken at multiple diametrical
chords is recommended to confirm no gas pockets,
velocity profile effects and/or pipe wall introduced error.
Instrument parameter set-up, which may include inputs
such as sensor spacing and angle, pipe wall data, and the
fluid’s density, sound speed, viscosity, and temperature,
is as equally critical to achieving good flow measurement
as the physical installation of the device. A skilled
technician must be able to properly install, set-up, and
then interpret the signal data and instrument feedback.
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ADVANTAGES/DISADVANTAGES
OF TRANSIT TIME METERS
Advantages:
• Imposes no system head or flow losses
• Clamp-on type is non-intrusive, easily and quickly
installed
• Wetted type of devices may be used to accurately
measure very large diameter conduits where other
devices are impractical
Disadvantages:
• Relatively costly compared to some other types of
meters
• Requires properly trained and skilled technician
for set-up and operation
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• Does not operate well on bubbly or particulate
laden water
• Concrete or FRP pipes can attenuate signal to
point of making meter unusable
Governing Standards – Transit-Time Ultrasonic flow
meters shall be selected and applied in accordance with
ASME MFC-5M-1989(R94), Measurement of Liquid
Flow in Closed Conduits using Transit-Time Ultrasonic
Flowmeters, and the manufacturer’s instructions.
Accuracy – Multi-path, Transit-Time Ultrasonic flow
meters, properly installed with the required lengths of
straight pipe in place, may achieve measurement
accuracies of ±0.5-1%.
Two-path, clamp-on meters
properly installed with the required lengths of straight
pipe in place may achieve measurement accuracies of
±2-4%. Accuracy will be highly affected by the quality
of the pipe, homogeneity and cleanliness of the fluid, and
skill of the operating technician. A study documenting
the magnitude of the effect of non-uniform velocity
profile on flow accuracy can be found in Reference No.
21, CTI Paper No. TP03-02.
D.9. Tracer Injection Measurement Method
Operating Principle – The tracer injection method, also
known as dye dilution method, requires the injection of a
traceable substance at a consistent and known rate into a
flowing fluid stream that can be detected or measured at
two separate locations in the system. Normally tracers
will be capable of detection by either chemical or
photocell methods.
Concentration Method – The ‘Concentration Method’
involves introducing the tracer material into the system at
a constant and precisely known volumetric or mass flow
rate while continuously monitoring the tracer
concentration at a downstream location. For simplistic
illustration, if the tracer injection rate is evaluated on a
volumetric basis, then the flow is calculated as follows:
Q=K⋅
FRt ⋅ γ t
ct
Where: Q is flow rate, L/s (gpm)
FRt is feed rate of the tracer, ml/min
γt is the tracer specific gravity
ct is tracer concentration, parts per million
(ppm)
K is a constant for unit conversion: 360x10-3
(264.15 IP Units)
Highly accurate tracer measurement will compensate for
temperature, density and background concentration of the
tracer, and the measurements should only be performed
by skilled personnel familiar with these techniques.
Tracer – The choice of tracer will have a very significant
impact on the accuracy and ease of testing. Types of
tracer that are most commonly used are chemical,
colorimetric and fluorescent. For best results the tracer
needs to have the following characteristics:
• Low level of detect ability
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• No detection interferences
• Not degraded by normal cooling water treatment
chemicals
• Availability of reliable in-line continuous
monitoring equipment
• Safe to handle
• Environmentally acceptable
Prior to any tracer flow study being undertaken it is
necessary to carry out a background tracer test on the
water system to be tested to establish a datum zero
calibration for the in-line analyzer.
Tracer Feed Equipment – The tracer feed equipment
must be sized for the system to be tested to ensure that
the tracer is fed at a sufficient rate to facilitate the
measurement of concentration. The feed pump should be
operated at maximum frequency and flow rate adjusted
by stroke. The tracer feed point should be located such
that good mixing of the tracer in the bulk water will
occur upstream of the detection point(s).
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ADVANTAGES AND DISADVANTAGES
Advantages:
• Imposes no system head or flow losses
• Unlike other flow measurement devices long
lengths of straight pipe upstream of the
measurement plane are not required
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• May be the only means to measure flow in
complex piping systems where other flow
measurement devices cannot be used
Disadvantages:
• Relatively costly compared to some other types of
meters
• Requires highly trained and skilled technician for
set-up and operation
• May be difficult to implement in a closed loop
system
Governing Standards – The techniques shall follow the
guidelines of ASME PTC-19.5 and ASME PTC-18.
Further the manufacturer’s application data for process
equipment and tracers must be followed as their methods
and formulas may be proprietary.
Accuracy – Accuracy is dependent on many factors
including choice of tracer, tracer feed equipment, tracer
injection point, tracer mixing, tracer on-line analyzer and
operator expertise. Companies specialized in these
methods and equipment can achieve high levels of
accuracy.
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PO Box 73383 Houston, Texas 77273
281.583.4087 – Fax: 281.537.1721 – www.cti.org
September 2008 – Printed in USA
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Cooling Technology Institute