ISO 17089-2:2012

Measurement of fluid flow in closed conduits - Ultrasonic meters for gas - Part 2: Meters for industrial applications

This part of ISO 17089 specifies requirements and recommendations for ultrasonic gas meters (USMs), which utilize acoustic signals to measure the flow in the gaseous phase in closed conduits. This part of ISO 17089 is applicable to transit time USMs and is focused towards industrial flow measurement. Included are meters comprising meter bodies as well as meters with field-mounted transducers. There are no limits on the size of the meter. It can be applied to the measurement of almost any type of gas; such as but not limited to air, hydrocarbon gases, and steam. This part of ISO 17089 specifies performance, calibration (when required), and output characteristics of USMs for gas flow measurement and deals with installation conditions. NOTE It is possible that national or other regulations apply which can be more stringent than those in this part of ISO 17089.

Mesurage de débit des fluides dans les conduites fermées — Compteurs à ultrasons pour gaz — Partie 2: Compteurs pour applications industrielles

General Information

• Status: Published
• Publication Date: 23-Sep-2012
• ICS: 17.120.10 - Flow in closed conduits
• Technical Committee: ISO/TC 30/SC 5 - Velocity and mass methods
• Drafting Committee: ISO/TC 30/SC 5 - Velocity and mass methods
• Current Stage: 9092 - International Standard to be revised
• Start Date: 22-Jan-2024
• Completion Date: 13-Dec-2025

Overview

ISO 17089-2:2012 - "Measurement of fluid flow in closed conduits - Ultrasonic meters for gas - Part 2: Meters for industrial applications" defines requirements and recommendations for transit‑time ultrasonic gas meters (USMs) used in industrial flow measurement. The standard covers meters with integrated meter bodies and field‑mounted transducers, for essentially any gas (air, hydrocarbon gases, steam, etc.), and sets out performance, calibration, output, installation and verification expectations for industrial (Class 3 and 4) applications. It also notes that national regulations may be more stringent.

Key topics and requirements

• Principles of measurement
  • Transit‑time USM operation and acoustic path types (axial, diametrical, chordal).
  • Factors affecting USM performance (gas properties, pressure/temperature, flow profile).
• Meter characteristics
  • Performance indications, operating conditions and classification (typical Class 3: utilities/process; Class 4: flare/vent).
  • Mechanical requirements: meter body, connections, ultrasonic ports, pressure tappings, anti‑roll, flow conditioners and markings.
  • Transducer, electronics, firmware/software requirements and diagnostic capabilities.
• Performance, uncertainty and calibration
  • Requirements for performance specifications, repeatability, resolution and maximum peak‑to‑peak error.
  • Flow test and calibration procedures, static leakage and pressure testing, dimensional and dynamic testing.
  • In‑situ verification and meter diagnostics to support automated condition monitoring.
• Installation and handling
  • Installation conditions, flow profile considerations, handling and transportation guidance to maintain measurement integrity.
• Testing and verification
  • On‑site diagnostics, in‑situ verification methods and guidance for dynamic testing under flowing conditions.

Practical applications and users

ISO 17089-2 is intended for engineers and organizations involved in industrial gas flow measurement, including:

• Plant and process engineers specifying flowmeters for utilities and process control.
• Instrumentation and metering engineers responsible for selection, installation and calibration of ultrasonic gas meters.
• Calibration laboratories and test facilities conducting flow verification and acceptance testing.
• Operators measuring vent, flare or process gases where robust diagnostics and wide rangeability and low pressure loss are required. Benefits of applying ISO 17089-2 include improved measurement repeatability, reduced pressure drop, better handling of pulsating flows, and standardized diagnostics for operational assurance.

Related standards

• ISO 17089-1 (meters for custody transfer and allocation measurement)
• ISO 4006 (vocabulary and symbols for fluid flow in closed conduits)
• Referenced standards on thermodynamic and metering terms cited within ISO 17089-2

Keywords: ISO 17089-2:2012, ultrasonic gas meters, transit-time USM, industrial flow measurement, calibration, diagnostics, installation requirements, meter classification.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 17089-2
First edition
2012-10-01
Measurement of fluid flow in closed
conduits — Ultrasonic meters for gas —
Part 2:
Meters for industrial applications
Mesurage de débit des fluides dans les conduites fermées —
Compteurs à ultrasons pour gaz —
Partie 2: Compteurs pour applications industrielles
Reference number
©
ISO 2012
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO’s
member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO 2012 – All rights reserved

Contents Page
Foreword .iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols and subscripts . 6
3.3 Abbreviations . 7
4 Principles of measurement . 7
4.1 Transit time ultrasonic meters . 7
4.2 Flare or vent gas meters . 8
4.3 Factors affecting performance . 9
4.4 Description of generic types . 9
4.5 Impact of pressure and temperature on the flowmeter geometry .14
4.6 USM measurement uncertainty determination .14
4.7 USM classification .14
5 Meter characteristics .15
5.1 Performance indications .15
5.2 Operating conditions .15
5.3 Meter body, materials, and construction .15
5.4 Connections .16
5.5 Dimensions .16
5.6 Ultrasonic ports .16
5.7 Pressure tapping .16
5.8 Anti-roll provision .16
5.9 Flow conditioner .17
5.10 Markings .17
5.11 Transducers .17
5.12 Electronics .17
5.13 Firmware and software .18
5.14 Inspection and verification functions .19
5.15 Operation and installation requirements .19
5.16 Installation requirements and flow profile considerations .21
5.17 Handling and transportation .22
6 Test and calibration .23
6.1 Flow test and calibration .23
6.2 Static testing for leakage and pressure .23
6.3 Dimensional measurements .23
6.4 Dynamic testing (testing and calibration, adjustment under flowing conditions) .25
6.5 Meter diagnostics .26
6.6 In situ verification .27
Annex A (normative) Special application note on valve characterization and noise .29
Bibliography .36
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International
Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 17089-2 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits,
Subcommittee SC 5, Velocity and mass methods.
ISO 17089 consists of the following parts, under the general title Measurement of fluid flow in closed conduits —
Ultrasonic meters for gas:
— Part 1: Meters for custody transfer and allocation measurement
— Part 2: Meters for industrial applications
iv © ISO 2012 – All rights reserved

Introduction
Ultrasonic meters (USMs) for gas flow measurement have penetrated the market for meters rapidly since 2000
and have become one of the prime flowmeter concepts for operational use as well as custody transfer and
allocation measurement. As well as offering high repeatability and high accuracy, ultrasonic technology has
inherent features like: negligible pressure loss, high rangeability and the capability to handle pulsating flows.
USMs can deliver extended diagnostic information through which it may be possible to verify not only the
functionality of a USM, but also several other components within the system, such as the gas chromatograph,
and the pressure and temperature transmitters. Due to the extended diagnostic capabilities, this part of
ISO 17089 advocates the addition and use of automated diagnostics instead of labour-intensive quality checks.
This part of ISO 17089 focuses on meters for industrial gas applications (class 3 and class 4). Meters for
custody transfer and allocation measurement are the subject of ISO 17089-1.
Typical performance factors of the classification scheme are:
Typical uncertainty 95 %
Class Typical applications confidence level (volume flow Reference
a
rate)
1 Custody transfer ±0,7 % ISO 17089-1
2 Allocation ±1,5 % ISO 17089-1
b
3 Utilities and process ±1,5 % to 5 % for q > q This part of ISO 17089
V V, t
4 Flare gas and vent gas ±5 % to 10 % for q > q This part of ISO 17089
V V, t
a
Meter performance, inclusive of total meter uncertainty, repeatability, resolution and maximum peak-to-peak error, depends upon a
number of factors which include pipe inside diameter, acoustic path length, number of acoustic paths, gas composition and speed of
sound, as well as meter timing repeatability.
b
By specific flow conditioning or when multi-path meters are employed, lower uncertainties may be achieved.
The special application note(s) as presented in Clause 7 as well as information in parentheses are informative.
INTERNATIONAL STANDARD ISO 17089-2:2012(E)
Measurement of fluid flow in closed conduits — Ultrasonic
meters for gas —
Part 2:
Meters for industrial applications
IMPORTANT — The electronic file of this document contains colours which are considered to be
useful for the correct understanding of the document. Users should therefore consider printing this
document using a colour printer.
1 Scope
This part of ISO 17089 specifies requirements and recommendations for ultrasonic gas meters (USMs), which
utilize acoustic signals to measure the flow in the gaseous phase in closed conduits.
This part of ISO 17089 is applicable to transit time USMs and is focused towards industrial flow measurement.
Included are meters comprising meter bodies as well as meters with field-mounted transducers.
There are no limits on the size of the meter. It can be applied to the measurement of almost any type of gas;
such as but not limited to air, hydrocarbon gases, and steam.
This part of ISO 17089 specifies performance, calibration (when required), and output characteristics of USMs
for gas flow measurement and deals with installation conditions.
NOTE It is possible that national or other regulations apply which can be more stringent than those in this part of ISO 17089.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any amendments) applies.
lSO 4006, Measurement of fluid flow in closed conduits — Vocabulary and symbols
3 Terms, definitions, and symbols
3.1 Terms and definitions
3.1.1 General
For the purposes of this document, the terms and definitions given in lSO 4006 and the following apply.
3.1.2 Quantities
3.1.2.1
volume flow rate
q
V
dV
q =
V
dt
where
V is volume;
t is time.
[8]
NOTE Adapted from ISO 80000-4:2006, 4-30.
3.1.2.1.1
actual flow rate
volume of fluid per time at metering conditions
3.1.2.1.2
corrected flow rate
volume of fluid per time measured at metering conditions, but converted to equivalent volume at base conditions
3.1.2.2
indication
flow rate indicated by the meter
3.1.2.3
working range
set of values of quantities of the same kind that can be measured by a given measuring instrument or measuring
system with specified instrumental uncertainty, under defined conditions
[10]
NOTE 1 Adapted from ISO/IEC Guide 99:2007, 4.7, “working interval”.
NOTE 2 For the purposes of this part of ISO 17089, the “set of values of quantities of the same kind” are volume flow
rates whose values are bounded by a maximum flow rate, q , and a minimum flow rate, q ; the “given measuring
V, max V, min
instrument” is a meter.
NOTE 3 The terms “rangeability” and “turndown” can often be found in flowmeter data sheets in connection with the
working range of the meter. These terms are sometimes used interchangeably although their exact meanings are different
and may not mean the same as working range. For example, it is possible to find a stated flowmeter rangeability derived from
the highest measurable flow divided by the minimum measurable flow (typically with flow expressed in terms of flow velocity).
3.1.2.4
metering pressure
p
absolute gas pressure in a meter at flowing conditions to which the indicated volume of gas is related
3.1.2.5
average velocity
v
volume flow rate divided by the cross-sectional area
3.1.3 Meter design
3.1.3.1
meter body
pressure-containing structure of the meter
3.1.3.2
acoustic path
path travelled by an acoustic wave between a pair of ultrasonic transducers
3.1.3.3
axial path
path travelled by an acoustic wave entirely in the direction of the main pipe axis
NOTE An axial path can be both on or parallel to the centre-line or long axis of the pipe.
See Figure 1.
2 © ISO 2012 – All rights reserved

Figure 1 — Axial path
3.1.3.4
diametrical path
acoustic path whereby the acoustic wave travels through the centre-line or long axis of the pipe
See Figure 2.
Figure 2 — Diametrical paths
3.1.3.5
chordal path
acoustic path whereby the acoustic wave travels parallel to the diametrical path
See Figure 3.
Figure 3 — Chordal paths
3.1.4 Thermodynamic conditions
3.1.4.1
metering conditions
conditions, at the point of measurement, of the fluid whose volume is to be measured
NOTE 1 Metering conditions include gas composition, temperature, and pressure also known as uncorrected conditions.
[5]
NOTE 2 Adapted from ISO 9951:1993, 3.1.6.
3.1.4.2
base conditions
conditions to which the measured volume of the gas is converted
NOTE 1 Base conditions include base temperature and base pressure.
[5]
NOTE 2 Adapted from ISO 9951:1993, 3.1.7.
NOTE 3 Preferred alternatives include reference conditions, standard conditions, normal conditions.
NOTE 4 Metering and base conditions relate only to the volume of the gas to be measured or indicated, and should not
[10]
be confused with rated operating conditions and reference operating conditions (see ISO/IEC Guide 99:2007, 4.9 and
[10]
4.11), which refer to influence quantities (see ISO/IEC Guide 99:2007, 2.52).
3.1.4.3
specified conditions
conditions of the fluid at which performance specifications of the meter are given
[5]
NOTE Adapted from ISO 9951:1993, 3.1.8.
3.1.5 Statistics
3.1.5.1
measurement error
error of measurement
error
measured quantity value minus a reference quantity value
[10]
[ISO/IEC Guide 99:2007, 2.16]
EXAMPLE Difference between the indication of the meter under test and the indication of the reference measurement.
4 © ISO 2012 – All rights reserved

3.1.5.2
error curve
interconnection of the curve (e.g. polynomial) fitted to a set of error data as a function of the flow rate of the
reference meter
3.1.5.3
maximum peak-to-peak error
maximum difference between any two error values
3.1.5.4
repeatability
measurement precision under a set of repeatability conditions of measurement
[10]
[ISO/IEC Guide 99:2007, 2.21]
EXAMPLE The closeness of agreement among a number of consecutive measurements of the output of the test
meter for the same reference flow rate under the same metering conditions.
NOTE The repeatability corresponds to the 95 % confidence interval of the error.
3.1.5.5
resolution
smallest difference between indications of a meter that can be meaningfully distinguished
[6]
NOTE Adapted from ISO 11631:1998, 3.28.
3.1.5.6
velocity sampling interval
time interval between two consecutive gas velocity measurements
3.1.5.7
zero flow reading
flowmeter indication when the gas is at rest, when both axial and non-axial velocity components are essentially zero
Figure 4 shows the flow rates in relation to the uncertainty budget.
q
V,0
Error limit (q < q )
V,0 V,t
Error limit (q ≤ q ≤ q )
V,t V V,max
q q q q /(m /h)
V,min V,t V,max V
Figure 4 — Typical error curve as function of the flow rate
(Δq / q ) / %
V V
3.2 Symbols and subscripts
The symbols and subscripts used in this part of ISO 17089 are given in Tables 1 and 2. Examples of uses of
the volume flow rate symbol are given in Table 3.
Table 1 — Symbols
a
Quantity Symbol Dimensions SI unit
2 2
A
Cross-sectional area L m
−1
c
Speed of sound in fluid LT m/s
Inside diameter of the meter body D L m
b
Weighting factors (live inputs) f 1 —
i
b
Integers (1,2,3, …) i,n 1 —
b
Calibration factor K 1 —
b
Flow profile correction factor k 1 —
n
b
Valve noise L 1 dB
p,N,v
Path length l L m
p
b
Attenuation factor N 1 —
d
b
Valve weighting factor N 1 —
v
b
Number of samples used in the signal processing. n 1 —
s
−1 −2
Absolute pressure p ML T Pa
−1 −2
Emitted acoustic pressure p ML T Pa
n
−1 −2
Pressure difference Δp ML T Pa
−1
Mass flow rate q MT kg/s
m
3 −1 3
Volume flow rate q L T m /s
V
t
Transit time T s
−1
v
Average velocity LT m/s
−1
v
Velocity of the acoustical path i LT m/s
i
b
Weighting factors (fixed value) w 1 —
i
Path angle f — rad
−3 3
Density of the fluid ρ ML kg/m
a
M ≡ mass; L ≡ length; T ≡ time ; Θ ≡ temperature.
b
”Dimensionless” quantity.
Table 2 — Subscripts
Subscript Meaning
min minimum
max maximum
t transition
Table 3 — Examples of flow rate symbols
Symbol Meaning
q Designed maximum flow rate
V, max
q Designed minimum flow rate
V, min
q Transition flow rate for defining accuracy
V, t
requirements
6 © ISO 2012 – All rights reserved

3.3 Abbreviations
ES electronic system
FAT factory acceptance test
FC flow conditioner
MSOS measured speed of sound
SNR signal-to-noise ratio
SOS speed of sound
TSOS theoretical speed of sound
USM ultrasonic flowmeter
USMP USM package, including upstream pipe, flow conditioner and thermo-well when bi-directional
4 Principles of measurement
4.1 Transit time ultrasonic meters
Figure 5 outlines the basic system setup to demonstrate the transit time principle. A pair of transducers capable
of transmitting and receiving ultrasonic pulses is located on both sides of the pipe at positions A and B.
The transducers transmit and receive pulses sequentially. Under zero flow conditions, the time taken for an
ultrasonic pulse to travel from A to B, t , is equal to that from B to A, t , and there is no difference in time.
AB BA
When a flow is introduced, the ultrasonic pulse from A to B is assisted by the flow, and as a result the time taken
decreases. In addition, the pulse from B to A is opposed by the flow and subsequently the time taken increases.
The resulting measured difference in transit time is directly proportional to the axial velocity of the flowing
gas. Providing that the distance between the transducers is known, the axial gas velocity passing between
transducer A and B can be measured. Ignoring second order effects such as path curvature, the travel times
of the acoustic pulse, t and t , can be shown to be given by:
AB BA
lp
t = (1)
AB
cv+ cosφ
()
and
lp
t = (2)
BA
cv− cosφ
()
where
l is the path length;
p
c is the speed of sound (SOS) in the gas;
v is the average velocity of the gas;
f is the path angle.
Formula (3) for the measured gas velocity can be derived by subtracting Formula (2) from Formula (1):
l
 
p
v =− (3)
 
2cosφ tt
 AB BA 
It is important to note that, in Formula (3), the term for the SOS in the gas has been eliminated. This means that
the measurement of the gas velocity is independent of the properties of the gas, such as pressure, temperature,
and gas composition. Nonetheless, if the transducers are recessed, it is possible that there is an additional
influence which is SOS dependent.
In a similar way, the SOS is derived by adding Formula (1) and Formula (2):
l
 
p 11
c =+ (4)
 
2 tt
 AB BA 
In multipath meters, the individual path velocity measurements are combined by a mathematical function to
yield an estimate of the mean pipe velocity:
vf= vv. (5)
()
1 n
where n is the total number of paths. Due to variations in path configuration and different proprietary approaches
of solving Formula (5), even for a given number of paths, the exact form of f(v . v ) can differ.
1 n
To obtain the volume flow rate, q , the estimate of the mean pipe velocity, v, is multiplied by the cross-sectional
V
area of the measurement section, A, as follows:
qA= v (6)
V
Figure 5 — Basic system setup
4.2 Flare or vent gas meters
In addition to class 1, 2, and 3 meters, ultrasonic transit time meters are also utilized extensively in class 4
measurement of flare or vent gas. Although the application is very different from that associated with class 1,
2 or 3 m, the transit time principle outlined above is still applicable.
8 © ISO 2012 – All rights reserved

The pipe sizes used for vent or flare systems within refineries, chemical plants or on production platforms
can be very large in diameter, and the composition and process conditions of flare or vent gas often vary
substantially between steady-state conditions and upset conditions. Rapid changes in pressure, temperature,
gas composition, and flowing velocity frequently occur as a result of a plant or process upset. The user should
ensure that a USM purchased for flare gas duty has been designed to accommodate such conditions. The
addition of temperature and pressure inputs is also required to enable standard volume flow to be derived, and
this may be a requisite for emissions reporting or even operational consent. A further requirement for plant
mass balance and steam injection control for the flare stack tip is mass flow calculation. Some USMs may
employ proprietary algorithms that utilize the MSOS, and absolute pressure and temperature inputs of the gas
to infer the average molecular weight, and hence mass flow.
The user is advised to check with the USM manufacturer that the gas composition, process temperature
and pressure remain compatible with any molecular weight or mass flow algorithms involved throughout the
expected range of these process variables.
4.3 Factors affecting performance
The performance of a USM is dependent on a number of intrinsic and extrinsic factors.
Intrinsic factors (i.e. those related to the meter and its calibration prior to delivery) include:
a) the geometry of the meter body and ultrasonic transducer locations and the uncertainty with which these
are known (including the temperature and pressure coefficient);
b) the accuracy and quality of the transducers and electronic components used in the transit time measurement
circuitry (e.g. the electronic clock stability);
c) the techniques utilized for transit time detection and computation of mean velocity (the latter of which
determines the sensitivity of the meter to variations in the flow velocity distribution).
Extrinsic factors, i.e. those related to the process and ambient conditions of the application, include:
1) the flow velocity profile;
2) the temperature distribution;
3) flow pulsations;
4) the noise, both acoustic and electromagnetic;
5) solid and liquid contamination;
6) temperature and pressure;
7) acoustic attenuation by specific gases (such as carbon dioxide);
8) acoustic effects by specific gases (such as hydrogen).
4.4 Description of generic types
4.4.1 General
This generic description of USMs for gases recognizes the scope for variation within commercial designs and
the potential for new developments. For the purpose of description, USMs are considered to be comprised of
several components, namely:
a) transducers;
b) meter body with acoustic path configuration;
c) electronics;
d) a data-processing and presentation unit.
4.4.2 Transducers
Transducers for a USM function as pairs of known acoustic characteristics. Each individual transducer
comprises an acoustic element with electrical connections and a supporting mechanical structure with which
the process connection is made.
The transducers may be in contact with the process fluid (also termed invasive) as part of a meter body in a
factory manufactured USM, but may also be field mounted as part of a retrofit installation to an existing pipe.
Alternatively, transducers may be clamp-on (also termed non-invasive) with reference to a closed conduit
(commonly termed: meter body; spool piece; process pipe; or parent pipe).
a) Wetted transducers are in direct contact with the fluid and may be supplied as an integral part of a meter
body, or separately as part of a cold-tap or hot-tap field-mounting kit, to be fabricated on an existing
process pipe.
b) A cold-tap installation requires that the process pipe is out of service, isolated, empty, and regarded as
safe for cutting and welding.
c) A hot-tap installation is by contrast performed on a process pipe which is in active service and thus
full of process fluid and at a pressure and/or temperature that are different to ambient and considered
hazardous. Such an installation requires a special transducer insertion mechanism which achieves a leak-
and pressure-tight seal of the process during the requisite hole-tapping operation.
d) Isolation valves are also employed on spool, hot-tap, and cold-tap installations to allow the transducer to
be inserted or withdrawn with the valve opened.
It may be necessary for wetted transducers to be inserted into the bore of the meter body or parent pipe,
possibly in conditions of atmospheric or very low pressure. In this situation, the transducer is termed intrusive
in addition to being invasive.
Wetted transducers may also include a buffer which serves to isolate the transducer from aspects of the
process fluid which may be harmful to it, such as cryogenic or very high temperature and/or high pressure.
Such a buffer design typically also serves to maintain the pipe integrity in the event of transducer removal being
necessary, and may even serve to enhance acoustic transmission by acting as a waveguide.
Clamp-on transducers may be made of metal or composite material and attached to the pipe by an appropriate
clamping fixture. The pipe wall is an integral part of the flowmeter and the acoustic characteristics of the
material, the thickness, the inside and the outside conditions as well as the position of the flanges need to be
considered. The maximum angle of the acoustic path is limited and mostly determined by the ratio of the SOS
between the pipe wall material and the fluid.
Typical diagrams of wetted, clamp-on and buffered transducers are given in Figure 6.
Due to the wide range of operating conditions within industry, there is no single design of transducer that is
technically and commercially viable for all situations, and thus designs are many and varied. Accordingly, the
user is advised to consult the USM manufacturer for correct transducer selection and application advice.
4.4.3 Meter body and acoustic path configurations
4.4.3.1 General
Clamp-on and wetted USMs for measurement of gases and vapours are available in a variety of single and
multi-path configurations. The number of measurement paths required, and the configuration of those paths,
may be influenced by the accuracy requirement or any potential variations in velocity distribution.
As well as variations in the radial position of the measurement paths in the cross-section, the path configuration
can be varied in orientation to the pipe axis. By utilizing reflection of the ultrasonic wave from the interior of the
meter body or from a fabricated reflector, the path can traverse the cross-section several times.
10 © ISO 2012 – All rights reserved

Multi-path configurations may also offer redundancy of measurement paths, if so configured, to provide a
safeguard if one path becomes inoperative. Alternatively, they may be utilized to offer reduced measurement
uncertainty over the entire flow range, i.e. low-flow measurement paths and high-flow measurement paths (e.g.
by pre-setting the angle of the sensor head).
a)  wetted, intrusive b)  wetted, non-intrusive
c)  clamp-on d)  buffered
Figure 6 — Typical transducer arrangements
4.4.3.2 Basic acoustic path configurations
Common acoustic path configurations are illustrated in Figure 7.
Path configurations may be described as shown in Figure 7 as:
a) diametric paths — paths that are across the diameter of the pipe;
b) chordal paths — paths that follow a chord of the pipe diameter;
c) bounced paths — paths with one reflection;
d) bounced paths — paths with more than one reflection;
e) partial path — path that partially follows a chord of the pipe diameter having intrusive transducers;
f) partial path — path that partially follows a chord of the pipe diameter, single probe mounted.
4.4.3.3 Flow profile correction factors, k
n
USMs are sensitive to velocity profile effects in both fully developed and disturbed flow conditions, since the
devices estimate the average velocity across the whole of the pipe cross-section by measuring the average
velocity along the path. Even in fully developed flow, the value of the profile correction factor, k , is not unity
n
and depends on the Reynolds number and pipe wall roughness.
Depending on the path configuration, a correction based on the Reynolds number might or might not be
incorporated by the manufacturer. Where used, effective correction normally depends upon the input during
commissioning of relevant viscosity data for the process fluid, from which a Reynolds number can be calculated,
and a correction factor established and applied.
The manufacturer should be asked whether there is a recommended meter orientation for upstream piping
configurations and operational conditions which are known to produce flow profile distortions.
If profiles are disturbed, the profile correction factor for fully developed flow is no longer applicable. The flow
profile can be disturbed by upstream pipe work configurations such as bends, expansions and contractions and
the presence of valves and pumps.
4.4.3.4 Meters with paths at multiple radial displacements
In these meters, the velocity is measured at different radial positions. Several methods can be used when
combining the velocities to obtain the mean pipe velocity. These can be classified as follows.
a) Summation with constant weighting:
n
vw= v (7)
∑ ii
i= 1
where the radial displacements of the paths and the weightings w to w are determined on the basis of
1 n
documented numerical integration methods.
b) Summation with variable weighting:
n
vf= v (8)
∑ ii
i= 1
where the radial displacements of the paths are fixed at design and the weightings f to f may be determined
1 n
from input parameters and/or measured variables (e.g. velocities).
In any of the given configurations, a multiplying or meter factor, K (either constant or variable), may be applied
after summation to correct for deviations due to manufacturing tolerances:
qK= Av (9)
V
12 © ISO 2012 – All rights reserved

a) b)
c) d)
e) f)
Figure 7 — Basic acoustic path types for USMs (front and top view)
NOTE The three-dimensional geometry cannot be depicted faithfully.
4.5 Impact of pressure and temperature on the flowmeter geometry
When a wide range of pressures and or temperatures are envisaged, the user should consult the manufacturer
(If required the equations for this are given in ISO 17089-1).
4.6 USM measurement uncertainty determination
The in situ measurement uncertainty of systems based on USMs comprises:
a) verification uncertainties associated with the meter testing such as:
1) timing verification,
2) geometric verification;
b) uncertainties arising from differences between the process conditions and the conditions under which the
meter was tested and or calibrated, including those which are a function of the pressure and temperature
correction or compensation, flow conditions, fluid characteristics or contamination;
c) uncertainties associated with secondary instrumentation, such as pressure and temperature transmitters,
gas composition measurement, and flow computers.
Additionally for clamp-on meters:
d) the pipe material and dimensions;
e) the mounting of the transducers;
f) the internal and external pipe wall surface conditions.
[3]
When a system uncertainty analysis is required, guidance can be found by reference to ISO 5168 and or
[9]
ISO/IEC Guide 98-3.
4.7 USM classification
To aid the user in making a meter selection based on the overall uncertainty required for the measurement,
a USM can be classified. This process involves dividing the available meters into classes of performance as
outlined in Table 4. Aside from this, there are also other classes dealing with other measurement applications
(custody transfer or fiscal measurement and allocation) as detailed within ISO 17089-1.
Table 4 — USM classification
Typical uncertainty 95 % confidence level
Class Typical applications
a
(volume flow rate)
Within ±1,5 % to 5 % for q > q
V V, t
By specific flow conditioning or when multi-path
3 Process, utilities or fuel gas
meters are employed, lower uncertainties can be
achieved
4 Emission monitoring as flare or vent gas Within ±5 % to 10 % for q > q
V V, t
a
Meter performance, inclusive of total meter uncertainty, repeatability, resolution, and maximum peak-to-peak error, depends upon
a number of factors which include pipe inside diameter, acoustic path length, number of acoustic paths, gas composition and SOS, and
meter timing repeatability.
The classes detailed within this part of ISO 17089 represent two different measurement specifications commonly
applied in industry. Depending on the importance of measurement with respect to operational demands, the
total uncertainty budget for the complete measurement system is different (greater) than for the meter.
14 © ISO 2012 – All rights reserved

5 Meter characteristics
5.1 Performance indications
The manufacturer shall specify the repeatability and the maximum peak-to-peak error over the measurement
range above q .
V, t
5.2 Operating conditions
5.2.1 Volume flow rates and fluid velocities
The maximum volume flow rate and the minimum volume flow rate shall be specified.
5.2.2 Pressure classes
Ultrasonic transducers used in USMs require a minimum density to ensure acoustic coupling of the sound
pulses to and from the fluid. Therefore, the expected minimum operating pressure as well as the maximum
operating pressure shall be specified.
5.2.3 Temperature
The manufacturer or supplier shall specify the operating and ambient temperature ranges that can be met by
the equipment being offered.
5.2.4 Gas quality
The meter shall operate within the relevant accuracy limits for all gases for which the meter is intended to be used.
The presence of some components in the gas can impact on the performance of a meter. In particular, high
levels of carbon dioxide and hydrogen in a gas mixture can influence and even inhibit the operation of a USM
owing to their acoustic absorption properties.
The manufacturer should be consulted if any of the following are expected:
a) when highly attenuating gases, such as carbon dioxide and hydrogen, can be present;
b) when the operational conditions are near the critical point of the gas mixture;
c) when non-hydrocarbon gases can be present and the sound velocity is being used to determine the
molecular weight;
d) when the total sulfur level, from materials such as mercaptans (thiols), hydrogen sulfide, and elemental
sulfur, exceeds 320 µmol/mol;
e) when there is the possibility of a liquid carry-over from separators or scrubbers;
f) salt deposits.
Deposits which may be present in a process pipeline (e.g. condensates, glycol, amines, inhibitors, water or
traces of oil mixed with mill-scale, dirt or sand) affect the accuracy of the meter by reducing its cross-sectional
area and by reducing the effective acoustic path length.
5.3 Meter body, materials, and construction
5.3.1 General
This subclause applies to meters delivered and installed with a meter body only. For field-mounted transducers,
the manufacturer should be contacted to clarify the installation requirements.
5.3.2 Materials
The meter body shall be manufactured of materials suited to the service conditions including temperature,
pressure, and gas composition of the fluid which the meter is to handle. Special care should be taken on
corrosion resistance of the meter body. Exterior surfaces of the meter shall be protected as necessary against
environmental corrosion. Insulation of the meter body is outside of the scope of this part of ISO 17089.
5.3.3 Meter body
The meter body and all other invasive or pressure- and/or force-loaded parts should be made in accordance with
accepted International Standards or based on sound engineering practice agreed between the manufacturer
and the user of the meter. Special care should be taken when meter bodies are to be welded into the pipeline.
5.4 Connections
The inlet and outlet connections of the meter shall conform to recognized standards e.g. ANSI/ASME (class 300,
600, 900, etc.), DIN, EN, and JIS. For a meter body that is to be welded into the pipeline, the welding ends
shall conform to recognized standards (e.g. ANSI) or shall be agreed between the manufacturer and the user
of the meter.
5.5 Dimensions
The inlet flange of the meter should have the same inside diameter to within 3 % compared with the adjacent
pipeline. Any step change at the meter inlet beyond this should not prevent the meter from meeting the accuracy
requirements of its performance class. In the case of bidirectional use of the meter, both flanges shall be
considered as inlet flanges.
For welded-in meter bodies, the requirements of the welding procedure shall be considered.
The dimension of the measuring section (section of the meter body where the transducers are installed) shall
be stated in the manufacturer’s documentation.
5.6 Ultrasonic ports
Since the measured gas may contain impurities, transducer ports in connection with the transducer shall be
designed so as to reduce the possibility of liquids or solids accumulating in the transducer ports or to ensure
that the performance of the meter is not influenced by such conditions.
The USM may be equipped with valves, or necessary additional devices, mounted on the transducer ports,
in order to make it possible to replace the ultrasonic transducers without depressurizing the meter run. The
manufacturer shall ensure that the operation of the replacement mechanism is safe under the design conditions
of the meter.
5.7 Pressure tapping
Where a pressure measurement is required (i.e. for volume flow at standard conditions), at least one metering-
pressure tapping shall be provided on the meter or on the piping adjacent to the meter. For a horizontal pipe,
this shall be drilled perpendicular to the top ±85° of the meter body or pipe.
5.8 Anti-roll provision
For a USM constructed using a meter body, the meter shall be designed so that the meter body does not roll
when resting on a smooth surface with a slope of up to 10 %. This is to prevent damage to the protruding
transducers and electronic system (ES) when the USM is temporarily set on the ground during installation or
maintenance work.
The meter shall be designed to permit easy and safe handling of the meter during transportation and installation.
Hoisting eyes or clearance for lifting straps shall be provided.
16 © ISO 2012 – All rights reserved

5.9 Flow conditioner
A flow-conditioning device may be delivered with the USM. In this case, the flow conditioner (FC) with its
associated upstream pipe shall be considered as a part of the meter. The installation conditions (up and
downstream runs) of the FC shall be agreed with the meter manufacturer. The additional pressure drop of the
FC should be considered.
5.10 Markings
As a minimum, the following information shall be stated on the meter:
a) manufacturer, model number, serial number;
b) direction of forward flow;
c) minimum and maximum operating pressure and temperature;
Additionally, for wetted meters:
d) meter size, flange class, and mass;
e) meter body design code and material.
This information should be placed on a nameplate or stamped on the meter body. The legal requirements
and/or requirements from codes and standards on marking of pressure bearing parts shall be considered.
5.11 Transducers
5.11.1 Rate of pressure change
Rapid pressurization and depressurization of a USM may cause damage to the transducer or change the
characteristics of the meter. Meter users should therefore ensure that the transducers are pressurized or
depressurized as slowly as possible and, in the absence of information from the manufacturer, a rate of no
greater than 0,5 MPa/min is recommended. This
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