SIST EN ISO 5167-4:2022

SLOVENSKI STANDARD
01-september-2022
Nadomešča:
SIST EN ISO 5167-4:2004
Merjenje pretoka fluida na osnovi tlačne razlike, povzročene z napravo, vstavljeno
v polno zapolnjen vod s krožnim prerezom - 4. del: Venturijeve cevi (ISO 5167-
4:2022)
Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full - Part 4: Venturi tubes (ISO 5167-4:2022)
Durchflussmessung von Fluiden mit Drosselgeräten in voll durchströmten Leitungen mit
Kreisquerschnitt - Teil 4: Venturirohre (ISO 5167-4:2022)
Mesurage de débit des fluides au moyen d'appareils déprimogènes insérés dans des
conduites en charge de section circulaire - Partie 4: Tubes de Venturi (ISO 5167-4:2022)
Ta slovenski standard je istoveten z: EN ISO 5167-4:2022
ICS:
17.120.10 Pretok v zaprtih vodih Flow in closed conduits
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 5167-4
EUROPEAN STANDARD
NORME EUROPÉENNE
June 2022
EUROPÄISCHE NORM
ICS 17.120.10 Supersedes EN ISO 5167-4:2003
English Version
Measurement of fluid flow by means of pressure
differential devices inserted in circular cross-section
conduits running full - Part 4: Venturi tubes (ISO 5167-
4:2022)
Mesurage de débit des fluides au moyen d'appareils Durchflussmessung von Fluiden mit Drosselgeräten in
déprimogènes insérés dans des conduites en charge de voll durchströmten Leitungen mit Kreisquerschnitt -
section circulaire - Partie 4: Tubes de Venturi (ISO Teil 4: Venturirohre (ISO 5167-4:2022)
5167-4:2022)
This European Standard was approved by CEN on 21 May 2022.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 5167-4:2022 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 5167-4:2022) has been prepared by Technical Committee ISO/TC 30
"Measurement of fluid flow in closed conduits" in collaboration with CCMC.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by December 2022, and conflicting national standards
shall be withdrawn at the latest by December 2022.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 5167-4:2003.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 5167-4:2022 has been approved by CEN as EN ISO 5167-4:2022 without any
modification.
INTERNATIONAL ISO
STANDARD 5167-4
Second edition
2022-06
Measurement of fluid flow by means of
pressure differential devices inserted
in circular cross-section conduits
running full —
Part 4:
Venturi tubes
Mesurage de débit des fluides au moyen d'appareils déprimogènes
insérés dans des conduites en charge de section circulaire —
Partie 4: Tubes de Venturi
Reference number
ISO 5167-4:2022(E)
ISO 5167-4:2022(E)
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
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ISO copyright office
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CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 5167-4:2022(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Principles of the method of measurement and computation . 2
5 Classical Venturi tubes . 2
5.1 Field of application . 2
5.1.1 General . 2
5.1.2 Classical Venturi tube with an “as cast” convergent section . 3
5.1.3 Classical Venturi tube with a machined convergent section . 3
5.1.4 Classical Venturi tube with a fabricated convergent section . 3
5.2 General shape . 3
5.2.1 General . 3
5.2.2 Entrance cylinder . 3
5.2.3 Convergent section . 4
5.2.4 Throat. 4
5.2.5 Divergent section . 5
5.2.6 Truncated Venturi tube . 5
5.2.7 Roughness . 5
5.2.8 Classical Venturi tube with an “as cast” convergent section . 5
5.2.9 Classical Venturi tube with a machined convergent section . 6
5.2.10 Classical Venturi tube with a fabricated convergent section . 6
5.3 Material and manufacture . 7
5.4 Pressure tappings . 7
5.5 Discharge coefficient, C . 8
5.5.1 Limits of use . 8
5.5.2 Discharge coefficient of the classical Venturi tube with an “as cast”
convergent section . 8
5.5.3 Discharge coefficient of the classical Venturi tube with a machined
convergent section . 9
5.5.4 Discharge coefficient of the classical Venturi tube with a fabricated
convergent section . 9
5.6 Expansibility [expansion] factor, ε . 9
5.7 Uncertainty of the discharge coefficient, C . 9
5.7.1 Classical Venturi tube with an “as cast” convergent section . 9
5.7.2 Classical Venturi tube with a machined convergent section . 9
5.7.3 Classical Venturi tube with a fabricated convergent section . 10
5.8 Uncertainty of the expansibility [expansion] factor, ε . 10
5.9 Pressure loss . 10
5.9.1 Definition of the pressure loss . 10
5.9.2 Relative pressure loss . 10
6 Installation requirements .11
6.1 General . 11
6.2 Minimum upstream and downstream straight lengths for installation between
various fittings and the Venturi tube .12
6.3 Flow conditioners . 16
6.4 Additional specific installation requirements for classical Venturi tubes . 16
6.4.1 Circularity and cylindricality of the pipe and alignment of the classical
Venturi tube . 16
6.4.2 Roughness of the upstream pipe . 17
7 Flow calibration of Venturi tubes .17
iii
ISO 5167-4:2022(E)
7.1 General . 17
7.2 Test facility . 17
7.3 Meter installation . . 18
7.4 Design of the test programme . 18
7.5 Reporting the calibration results . 18
7.6 Uncertainty analysis of the calibration . 18
7.6.1 General . 18
7.6.2 Uncertainty of the test facility . 18
7.6.3 Uncertainty of the Venturi tube . 19
Annex A (informative) Table of expansibility [expansion] factor .20
Annex B (informative) Classical Venturi tubes used outside the scope of ISO 5167-4 .21
Annex C (informative) Pressure loss in a classical Venturi tube .24
Bibliography .26
iv
ISO 5167-4:2022(E)
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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
ISO 5167-4 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits,
Subcommittee SC 2, Pressure differential devices, in collaboration with the European Committee for
Standardization (CEN) Technical Committee CEN/SS F05, Measuring instruments, in accordance with
the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This second edition of ISO 5167-4 cancels and replaces the first edition of ISO 5167-4:2003, which has
been technically revised.
The main changes are as follows:
— The use of single pressure tappings on Venturi tubes is permitted.
— The discharge coefficient and uncertainty are given in Clause 5 for a Venturi tube with a machined
convergent section for Re > 10 .
D
— Flow calibration of Venturi tubes is included.
— There is improved wording of the rules for spacing of multiple fittings but no change in actual
requirements.
A list of all parts in the ISO 5167 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
ISO 5167-4:2022(E)
Introduction
ISO 5167, consisting of six parts, covers the geometry and method of use (installation and operating
conditions) of orifice plates, nozzles, Venturi tubes, cone meters and wedge meters when they are
inserted in a conduit running full to determine the flow rate of the fluid flowing in the conduit. It also
gives necessary information for calculating the flow rate and its associated uncertainty.
ISO 5167 (all parts) is applicable only to pressure differential devices in which the flow remains
subsonic throughout the measuring section and where the fluid can be considered as single-phase, but
is not applicable to the measurement of pulsating flow. Furthermore, each of these devices can only be
used uncalibrated within specified limits of pipe size and Reynolds number, or alternatively they can be
used across their calibrated range.
ISO 5167 (all parts) deals with devices for which direct calibration experiments have been made,
sufficient in number, spread and quality to enable coherent systems of application to be based on their
results and coefficients to be given with certain predictable limits of uncertainty. ISO 5167 (all parts)
also provides methodology for bespoke calibration of differential pressure meters.
The devices introduced into the pipe are called primary devices. The term primary device also includes
the pressure tappings. All other instruments or devices required to facilitate the instrument readings
are known as secondary devices, and the flow computer that receives these readings and performs
the algorithms is known as a tertiary device. ISO 5167 (all parts) covers primary devices; secondary
devices (see ISO 2186) and tertiary devices will be mentioned only occasionally.
Aspects of safety are not dealt with in ISO 5167-1 to ISO 5167-6. It is the responsibility of the user to
ensure that the system meets applicable safety regulations.
vi
INTERNATIONAL STANDARD ISO 5167-4:2022(E)
Measurement of fluid flow by means of pressure
differential devices inserted in circular cross-section
conduits running full —
Part 4:
Venturi tubes
1 Scope
This document specifies the geometry and method of use (installation and operating conditions) of
1)
Venturi tubes when they are inserted in a conduit running full to determine the flow rate of the fluid
flowing in the conduit.
This document also provides background information for calculating the flow rate and is applicable in
conjunction with the requirements given in ISO 5167-1.
This document is applicable only to Venturi tubes in which the flow remains subsonic throughout
the measuring section and where the fluid can be considered as single-phase. In addition, Venturi
tubes can only be used uncalibrated in accordance with this standard within specified limits of pipe
size, roughness, diameter ratio and Reynolds number, or alternatively they can be used across their
calibrated range. This document is not applicable to the measurement of pulsating flow. It does not
cover the use of uncalibrated Venturi tubes in pipes sized less than 50 mm or more than 1 200 mm, or
where the pipe Reynolds numbers are below 2 × 10 .
This document deals with the three types of classical Venturi tubes:
a) “as cast”;
b) machined;
c) fabricated (also known as “rough-welded sheet-iron”).
A Venturi tube consists of a convergent inlet connected to a cylindrical throat which is in turn connected
to a conical expanding section called the divergent section (or alternatively the diffuser). Venturi
nozzles (and other nozzles) are dealt with in ISO 5167-3.
NOTE In the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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.
ISO 4006, Measurement of fluid flow in closed conduits — Vocabulary and symbols
ISO 5167-1, Measurement of fluid flow by means of pressure differential devices inserted in circular cross-
section conduits running full — Part 1: General principles and requirements
ISO 5168, Measurement of fluid flow — Procedures for the evaluation of uncertainties
1)  In the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.
ISO 5167-4:2022(E)
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM: 1995)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4006 and ISO 5167-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Principles of the method of measurement and computation
The principle of the method of measurement is based on the installation of a Venturi tube into a pipeline
in which a fluid is running full. A static pressure difference exists between the upstream section and the
throat section of the device. Venturi tube geometries and designs have been extensively tested across
a wide range of flow conditions and shown to have a reproducible value of the discharge coefficient, C,
within a given uncertainty. Uncalibrated Venturi tubes of one of these geometries and designs, within
that same range of flow conditions, can be used to determine the flow rate from the measured value of
this pressure difference and from a knowledge of the fluid conditions.
The mass flow rate can be determined by Formula (1):
C π
q = ερdp2Δ (1)
m 1
4 4
1−β
The uncertainty limits can be calculated using the procedure given in ISO 5167-1:2022, Clause 8.
Similarly, the value of the volume flow rate can be calculated since
q
m
q =
V
ρ
where ρ is the fluid density at the temperature and pressure for which the volume is stated.
Computation of the flow rate, which is an arithmetic process, is performed by replacing the different
items on the right-hand side of Formula (1) by their numerical values. Table A.1 gives Venturi tube
expansibility factors (ε). They are not intended for precise interpolation. Extrapolation is not permitted.
The diameters d and D mentioned in Formula (1) (since D is required to calculate β) are the values of
the diameters at working conditions. Measurements taken at any other conditions should be corrected
for any possible expansion or contraction of the primary device and the pipe due to the values of the
temperature and pressure of the fluid during the measurement.
It is necessary to know the density and the viscosity of the fluid at working conditions. In the case
of a compressible fluid, it is also necessary to know the isentropic exponent of the fluid at working
conditions.
5 Classical Venturi tubes
5.1 Field of application
5.1.1 General
The field of application of the classical Venturi tubes dealt with in this document depends on the way in
which they are manufactured.
ISO 5167-4:2022(E)
Three types of standard classical Venturi tube are specified according to the method of manufacture of
the internal surface of the entrance cone and the profile at the intersection of the entrance cone and the
throat. These three methods of manufacture (and hence roughness) are described in 5.1.2 to 5.1.4, and
the resulting Venturi tubes have somewhat different characteristics.
There are limits given for the roughness of the internal surfaces and the Reynolds number for each
type.
5.1.2 Classical Venturi tube with an “as cast” convergent section
This is a classical Venturi tube made by casting in a sand mould, or by other methods which leave a
finish on the surface of the convergent section similar to that produced by sand casting. The throat
is machined and the junctions between the cylinders and the convergent and divergent sections are
rounded.
These classical Venturi tubes can be used in pipes of diameter between 100 mm and 800 mm and with
diameter ratios β between 0,3 and 0,75 inclusive.
5.1.3 Classical Venturi tube with a machined convergent section
This is a classical Venturi tube cast or fabricated as in 5.1.2 but in which the convergent section is
machined as are the throat and the entrance cylinder. The junctions between the cylinders and the
convergent and divergent sections may or may not be rounded.
These classical Venturi tubes can be used in pipes of diameter between 50 mm and 350 mm and with
diameter ratios β between 0,4 and 0,75 inclusive.
5.1.4 Classical Venturi tube with a fabricated convergent section
This is a classical Venturi tube normally fabricated by rolling sheet iron (or an alternative sheet
material) to form the sections of the Venturi tube, welding to complete the cylindrical, convergent
and divergent sections, and then welding these together. For larger sizes it may not be machined if the
tolerance required in 5.2.4 can be achieved, but in the smaller sizes the throat is machined.
These classical Venturi tubes can be used in pipes of diameter between 200 mm and 1 200 mm and
with diameter ratios β between 0,4 and 0,7 inclusive.
5.2 General shape
5.2.1 General
Figure 1 shows a section through the centreline of the throat of a classical Venturi tube. The letters
used in the text refer to those shown on Figure 1.
The classical Venturi tube is made up of an entrance cylinder A connected to a conical convergent
section B, a cylindrical throat C and a conical divergent section E. The internal surface of the device
is cylindrical and concentric with the pipe centreline. The coaxiality of the convergent section and the
cylindrical throat is assessed by visual inspection.
5.2.2 Entrance cylinder
The minimum cylinder length, measured from the plane containing the intersection of the convergent
section frustum B with the cylinder A, may vary as a result of the manufacturing process (see 5.2.8 to
5.2.10). It is, however, recommended that it be chosen to be equal to D.
The entrance cylinder diameter D shall be measured in the plane of the upstream pressure tapping(s).
The number of measurements shall be at least four, of which one shall be measured near each pressure
tapping. The arithmetic mean value of all these measurements shall be taken as the value of D in the
calculations.
ISO 5167-4:2022(E)
Diameters shall also be measured in planes other than the plane of the pressure tapping(s).
No diameter along the entrance cylinder shall differ by more than 0,4 % from the value of the mean
diameter. This requirement is satisfied when the difference in the length of any of the measured
diameters complies with the said requirement with respect to the mean of the measured diameters.
Key
a
1 entrance cylinder A Direction of flow.
b
2 conical convergent section B 7° ≤ φ ≤ 15°.
c
3 cylindrical throat C See 5.4.7.
4 conical divergent section E
5 connecting planes
6 upstream pressure tapping(s)
7 throat pressure tapping(s)
Figure 1 — Geometric profile of the classical Venturi tube
5.2.3 Convergent section
The convergent section B shall be conical and shall have an included angle of 21°± 1° for all types of
classical Venturi tube. It is limited upstream by the plane containing the intersection of the cone frustum
B with the entrance to cylinder A (or their prolongations) and downstream by the plane containing the
intersection of the cone frustum B with the throat C (or their prolongations).
The overall length of the convergent section B measured parallel to the centreline of the Venturi tube is
therefore approximately equal to 2,7(D − d).
The convergent section B is blended to the entrance cylinder A by a curvature of radius R , the value of
which depends on the type of classical Venturi tube.
The profile of the convergent section shall be checked. The maximum deviation of the convergent
section shall not exceed, in any place, 0,004D.
The internal surface of the conical section of the convergent section is taken as being a surface of
revolution if two diameters situated in the same plane perpendicular to the axis of revolution do not
differ from the value of the mean diameter by more than 0,4 %.
It shall be checked in the same way that the joining curvature with a radius, R , is a surface of revolution.
5.2.4 Throat
The throat C shall be cylindrical with a diameter, d. It is limited upstream by the plane containing the
intersection of the cone frustum B with the throat C (or their prolongations) and downstream by the
plane containing the intersection of the throat C with the cone frustum E (or their prolongations). The
ISO 5167-4:2022(E)
length of the throat C, i.e. the distance between those two planes, shall be equal to d ± 0,03d whatever
the type of classical Venturi tube.
The throat C is connected to the convergent section B by a curvature of radius, R , and to the divergent
section E by a curvature of radius, R . The values of R and R depend on the type of classical Venturi
3 2 3
tube.
The diameter, d, shall be measured very carefully in the plane of the throat pressure tapping(s). The
number of measurements shall be at least four, of which one shall be measured near each pressure
tapping. The arithmetic mean value of all these measurements shall be taken as the value of d in the
calculations.
Diameters shall also be measured in planes other than the plane of the pressure tapping(s).
No diameter along the throat shall differ by more than 0,1 % of the value of the mean diameter. This
requirement is satisfied when the difference in the length of any of the measured diameters complies
with the said requirement in respect of the mean of the measured diameters.
The throat of the classical Venturi tube shall be machined or be of equivalent smoothness over the
whole of its length to the surface roughness specified in 5.2.7.
It shall be checked that the joining curvatures into the throat with radii R and R are surfaces of
2 3
revolution as described in 5.2.3. This requirement is satisfied when two diameters, situated in the same
plane perpendicular to the axis of revolution, do not differ from the value of the mean diameter by more
than 0,1 %.
The values of the radii of curvature, R and R , shall be checked. The deviation shall evolve in a regular
2 3
way for each curvature so that the maximum deviation that is measured occurs approximately midway
along the profile. The value of this maximum deviation shall not exceed 0,02d.
5.2.5 Divergent section
The divergent section E shall be conical and may have an included angle, φ, of between 7° and 15°.
For low pressure-loss applications, it is recommended that an angle between 7° and 8° be chosen. Its
smallest diameter shall not be less than the throat diameter.
5.2.6 Truncated Venturi tube
A classical Venturi tube is called “truncated” when the outlet diameter of the divergent section is less
than the diameter, D and “not truncated” when the outlet diameter is equal to diameter, D. The divergent
portion may be truncated by about 35 % of its length without significantly modifying the pressure loss
of the device or its discharge coefficient.
5.2.7 Roughness
The roughness criterion, Ra, of the throat and that of the adjacent curvature shall be as small as possible
−4
and shall always be less than 10 d. The internal surface of the divergent section shall be clean and
smooth. Other parts of the classical Venturi tube have specified roughness limits depending on the type
considered.
5.2.8 Classical Venturi tube with an “as cast” convergent section
The profile of the classical Venturi tube with an “as cast” convergent section has the following
characteristics.
The internal surface of the convergent section B is sand cast. It shall be free from cracks, fissures,
depressions, irregularities and impurities. The roughness criterion, Ra, for the surface shall be less
−4
than 10 D.
ISO 5167-4:2022(E)
The minimum length of the entrance cylinder A shall be equal to the smaller of the following two values:
— D; or
— 0,25D + 250 mm (see 5.2.2).
The internal surface of the entrance cylinder A may be left “as cast” provided that it has the same
surface finish as the convergent section B.
The radius of curvature, R , shall be equal to 1,375D ± 0,275D.
The radius of curvature, R , shall be equal to 3,625d ± 0,125d.
The length of the cylindrical part of the throat shall be no less than d/3. In addition, the length of the
cylindrical part between the end of the joining curvature, R , and the plane of the pressure tappings,
as well as the length of the cylindrical part between the plane of the throat pressure tappings and the
beginning of the joining curvature, R , shall be no less than d/6 (see also 5.2.4 for the throat length).
The radius of curvature, R , shall lie between 5d and 15d. Its value shall increase as the divergent angle
decreases. A value close to 10d is recommended.
5.2.9 Classical Venturi tube with a machined convergent section
The profile of the classical Venturi tube with a machined convergent section has the following
characteristics.
The minimum length of the entrance cylinder A shall be equal to D.
The radius of curvature, R , shall be less than 0,25D and preferably equal to zero.
The radius of curvature, R , shall be less than 0,25d and preferably equal to zero.
The length of the throat cylindrical part between the end of the curvature, R , and the plane of the
throat pressure tappings shall be no less than 0,25d.
The length of the throat cylindrical part between the plane of the throat pressure tappings and the
beginning of the joining curvature, R , shall be no less than 0,3d.
The radius of curvature, R , shall be less than 0,25d and preferably equal to zero.
The entrance cylinder and the convergent section shall have a surface finish equal to that of the throat
(see 5.2.7).
5.2.10 Classical Venturi tube with a fabricated convergent section
The profile of the classical Venturi tube with a fabricated convergent section has the following
characteristics.
The minimum length of the entrance cylinder A shall be equal to D.
There shall be no joining curvature between the entrance cylinder A and the convergent section B other
than that resulting from welding.
There shall be no joining curvature between the convergent section B and the throat C other than that
resulting from welding.
There shall be no joining curvature between the throat C and the divergent section E other than that
resulting from welding.
The internal surface of the entrance cylinder A and the convergent section B shall be clean and free
from encrustation and welding deposits. It may be galvanized. Its roughness criterion, Ra, shall be
−4
about 5 × 10 D.
ISO 5167-4:2022(E)
The internal welded seams shall be flush with the surrounding surfaces. They shall not be located in
the vicinity of the pressure tappings.
5.3 Material and manufacture
5.3.1 The classical Venturi tube may be manufactured from any material, provided that it is in
accordance with the foregoing description and will remain so during use.
5.3.2 It is also recommended that the convergent section B and the throat C be joined as one part. It
is recommended that in the case of a classical Venturi tube with a machined convergent, the throat and
the convergent section be manufactured from one piece of material. If, however, they are made in two
separate parts they shall be assembled before the internal surface is finally machined.
5.3.3 Particular care shall be given to the centring of the divergent section E on the throat. There
shall be no step in diameters between the two parts.
This can be established by physical inspection before the classical Venturi tube is installed, but after
the divergent section has been assembled with the throat section.
5.4 Pressure tappings
5.4.1 The upstream and throat pressure tappings shall be made in the form of separate pipe wall
pressure tappings.
One or more pressure tappings in each plane are permissible. Multiple tappings at the same plane may
be interconnected by annular chambers, piezometer rings or, if there are four tappings, a “triple-T”
arrangement (see ISO 5167-1:2022, 5.4.3).
5.4.2 If d is greater than or equal to 33,3 mm, the diameter of these tappings shall be between 4 mm
and 10 mm and moreover shall never be greater than 0,1D for the upstream pressure tapping(s) and
0,13d for the throat pressure tapping(s).
If d is less than 33,3 mm, the diameter of the throat pressure tapping(s) shall be between 0,1d and 0,13d
and the diameter of the upstream pressure tapping(s) shall be between 0,1d and 0,1D.
It is recommended that pressure tappings as small as compatible with the fluid be used (for example
with its viscosity and cleanness).
5.4.3 The centrelines of the pressure tappings shall meet the centreline of the classical Venturi tube,
and shall be contained in planes perpendicular to the centreline of the classical Venturi tube.
Where interconnected multiple tappings are used, they shall be equally distributed around the cross
section of the pipe.
5.4.4 At the point of break-through, the hole of the pressure tapping shall be circular. As small burrs
or a wire edge can cause significant shifts in differential pressure, it is critical that the pressure tapping
edges be flush with the pipe wall and free from any burrs. If joining curvatures are required, the radius
shall not exceed one-tenth of the diameter of the pressure tapping.
5.4.5 The pressure tappings shall be cylindrical over a length at least 2,5 times the internal diameter
of the tapping, measured from the inner wall of the pipeline.
5.4.6 Conformity of the pressure tappings with the two foregoing requirements is assessed by
physical inspection.
ISO 5167-4:2022(E)
5.4.7 The spacing of a pressure tapping is the distance, measured on a straight line parallel to the
centreline of the classical Venturi tube, between the centreline of the pressure tapping and the reference
planes specified below.
For the classical Venturi tube with an “as cast” convergent section, the spacing between the upstream
pressure tappings situated on the entrance cylinder and the plane of intersection between the
prolongations of the entrance cylinder A and the convergent section B shall be:
— 0,5D ± 0,25D for 100 mm < D < 150 mm;
— 05, D for 150 mm < D < 800 mm.
−02, 5D
For classical Venturi tubes with a machined or fabricated convergent section, the spacing between the
upstream pressure tappings and the plane of intersection between the entrance cylinder A and the
convergent section B (or their prolongations) shall be:
0,5D ± 0,05D
For all types of classical Venturi tube, the spacing between the plane
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