Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full - Part 4: Venturi tubes (ISO 5167-4:2003)

Migrated from Progress Sheet (TC Comment) (2000-07-10): WIs CSF05019 to CSF05022 revises EN ISO 5167-1:1995 (CC/000707)

Durchflussmessung von Fluiden mit Drosselgeräten in voll durchströmten Leitungen mit Kreisquerschnitt - Teil 4: Venturirohre (ISO 5167-4:2003)

Dieser Teil von ISO 5167 legt die geometrische Form und Maße sowie die Anwendung (Einbau- und Betriebsbe-dingungen) von Venturirohren, die in eine voll durchströmte Rohrleitung zur Bestimmung des Durchflusses eingebaut sind, fest.
Dieser Teil von ISO 5167 enthält grundlegende Informationen für die Durchflussberechnung und ist gemeinsam mit den in ISO 5167-1 festgelegten Anforderungen anzuwenden.
Dieser Teil von ISO 5167 gilt nur für Venturirohre, in denen die Strömung im gesamten Messquerschnitt im Unterschallbereich liegt und wo das Fluid als einphasig betrachtet werden kann. Weiterhin kann jedes dieser Geräte nur innerhalb festgelegter Grenzen für Rohrdurchmesser, Rohrrauheit, Durchmesserverhältnis und Reynolds-Zahl verwendet werden. Dieser Teil von ISO 5167 gilt nicht für Messungen bei pulsierenden Strömungen, für Rohrdurchmesser kleiner als 50 mm oder größer als 1 200 mm und für Reynolds-Zahlen unter 2 x 105.
Dieser Teil von ISO 5167 behandelt drei Arten von Venturirohren
a) gussrauhe;
b) bearbeitete;
c) rauhe, aus Stahlblech geschweißte.
Das Venturirohr ist ein Drosselgerät, das aus einem sich verengendem Einlaufteil, an das sich ein zylindrischer Teil anschließt und einem konischen Expansionsteil, Diffusor genannt, besteht. Für die drei Bauarten des klassischen Venturirohres sind die unterschiedlichen Werte für die Unsicherheit der Durchflusskoeffizienten einerseits auf den vorliegenden Umfang der Untersuchungsergebnisse für jede Bauart des klassischen Venturirohres und andererseits auf die mehr oder weniger genaue Definition des geometrischen Profils zurückzuführen. Die Werte beruhen auf Daten, die über viele Jahre gesammelt wurden. Venturidüsen (und andere Düsen) werden in ISO 5167-3 behandelt.

Mesure de débit des fluides au moyen d'appareils déprimogenes insérés dans des conduites en charge de section circulaire - Partie 4: Tubes de Venturi (ISO 5167-4:2003)

L'ISO 5167-4:2003 spécifie la géométrie et le mode d'emploi (conditions d'installation et d'utilisation) de tubes de Venturi insérés dans une conduite en charge dans le but de déterminer le débit du fluide s'écoulant dans cette conduite.
L'ISO 5167-4:2003 fournit également des informations de fond nécessaires au calcul de ce débit, et il convient de l'utiliser conjointement avec les exigences stipulées dans l'ISO 5167-1.
L'ISO 5167-4:2003 est applicable uniquement aux tubes de Venturi utilisés dans les limites spécifiées de diamètre de conduite, de rugosité, de rapport des diamètres et de nombre de Reynolds, dans lesquels l'écoulement reste subsonique dans tout le tronçon de mesurage contenant un fluide pouvant être considéré comme monophasique. Elle n'est pas applicable au mesurage d'un écoulement pulsé. Elle ne couvre pas l'utilisation de tubes de Venturi dans des conduites de diamètre inférieur à 50 mm ou supérieur à 1 200 mm, ni les cas où les nombres de Reynolds rapportés à la tuyauterie sont inférieurs à 2 x 105.
L'ISO 5167-4:2003 traite de trois types de tubes de Venturi classiques: a) à convergent brut de fonderie; b) usiné; c) en tôle soudée brute.
Un tube de Venturi se compose d'un convergent d'entrée, relié à un col cylindrique qui est, à son tour, relié à un évasement de forme tronconique appelé «divergent». Les différences entre les valeurs de l'incertitude du coefficient de décharge pour les trois types de tube de Venturi classique indiquent, d'une part, le nombre de résultats disponibles pour chaque type de tube de Venturi classique et, d'autre part, la définition plus ou moins précise du profil géométrique. Les valeurs sont fondées sur des données recueillies il y a de nombreuses années. Les Venturi-tuyères (et d'autres tuyères) font l'objet de l'ISO 5167-3.

Merjenje pretoka fluida na osnovi tlačne razlike, povzročene z napravo, vstavljeno v polno zapolnjen vod s krožnim prerezom – 4. del: Venturijeve cevi (ISO 5167-4:2003)

General Information

Status
Published
Publication Date
31-Dec-2003
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
01-Jan-2004
Due Date
01-Jan-2004
Completion Date
01-Jan-2004

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SIST EN ISO 5167-1:1997
SIST EN ISO 5167-1:1997/A1:2001

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Measurement of fluid flow by means of pressure differential devices inserted in circular

cross-section conduits running full - Part 4: Venturi tubes (ISO 5167-4:2003)

Durchflussmessung von Fluiden mit Drosselgeräten in voll durchströmten Leitungen mit

Kreisquerschnitt - Teil 4: Venturirohre (ISO 5167-4:2003)
Mesure de débit des fluides au moyen d'appareils déprimogenes insérés dans des

conduites en charge de section circulaire - Partie 4: Tubes de Venturi (ISO 5167-4:2003)

Ta slovenski standard je istoveten z: EN ISO 5167-4:2003
ICS:
17.120.10 Pretok v zaprtih vodih Flow in closed conduits
SIST EN ISO 5167-4:2004 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST EN ISO 5167-4:2004
---------------------- Page: 2 ----------------------
SIST EN ISO 5167-4:2004
EUROPEAN STANDARD
EN ISO 5167-4
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2003
ICS 17.120.10 Together with EN ISO 5167-1:2003,
EN ISO 5167-2:2003 and EN ISO 5167-3:2003,
supersedes EN ISO 5167-1:1995
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 5167-4:2003)

Mesure de débit des fluides au moyen d'appareils Durchflussmessung von Fluiden mit Drosselgeräten in voll

déprimogènes insérés dans des conduites en charge de durchströmten Leitungen mit Kreisquerschnitt - Teil 4:

section circulaire - Partie 4: Tubes de Venturi (ISO 5167- Venturirohre (ISO 5167-4:2003)

4:2003)
This European Standard was approved by CEN on 20 February 2003.

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 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 Management Centre has the same status as the official

versions.

CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,

Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United

Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels

© 2003 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 5167-4:2003 E

worldwide for CEN national Members.
---------------------- Page: 3 ----------------------
SIST EN ISO 5167-4:2004
EN ISO 5167-4:2003 (E)
CORRECTED 2003-09-03
Foreword

This document (EN ISO 5167-4:2003) has been prepared by Technical Committee ISO/TC 30

"Measurement of fluid flow in closed conduits" in collaboration with CMC.

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 September 2003, and conflicting national

standards shall be withdrawn at the latest by September 2003.

This document, together with EN ISO 5167-1:2003, EN ISO 5167-2:2003 and EN ISO 5167-3:2003,

supersedes EN ISO 5167-1:1995.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of the

following countries are bound to implement this European Standard: Austria, Belgium, Czech

Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,

Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and the

United Kingdom.
Endorsement notice

The text of ISO 5167-4:2003 has been approved by CEN as EN ISO 5167-4:2003 without any

modifications.

NOTE Normative references to International Standards are listed in Annex ZA (normative).

---------------------- Page: 4 ----------------------
SIST EN ISO 5167-4:2004
EN ISO 5167-4:2003 (E)
Annex ZA
(normative)
Normative references to international publications
with their relevant European publications

This European Standard incorporates by dated or undated reference, provisions from other

publications. These normative references are cited at the appropriate places in the text and the

publications are listed hereafter. For dated references, subsequent amendments to or revisions of any

of these publications apply to this European Standard only when incorporated in it by amendment or

revision. For undated references the latest edition of the publication referred to applies (including

amendments).

NOTE Where an International Publication has been modified by common modifications, indicated by

(mod.), the relevant EN/HD applies.
Publication Year Title EN Year
ISO 4006 1991 Measurement of fluid flow in closed EN 24006 1993
conduits - Vocabulary and symbols
ISO 5167-1 2003 Measurement of fluid flow by means of EN ISO 5167-1 2003
pressure differential devices inserted in
circular cross-section conduits running
full - Part 1: General principles and
requirements
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SIST EN ISO 5167-4:2004
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SIST EN ISO 5167-4:2004
INTERNATIONAL ISO
STANDARD 5167-4
First edition
2003-03-01
Measurement of fluid flow by means of
pressure differential devices inserted in
circular cross-section conduits running
full —
Part 4:
Venturi tubes
Mesure 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 5167-4:2003(E)
ISO 2003
---------------------- Page: 7 ----------------------
SIST EN ISO 5167-4:2004
ISO 5167-4:2003(E)
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© ISO 2003

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ii © ISO 2003 — All rights reserved
---------------------- Page: 8 ----------------------
SIST EN ISO 5167-4:2004
ISO 5167-4:2003(E)
Contents Page

Foreword............................................................................................................................................................ iv

Introduction ........................................................................................................................................................ v

1 Scope...................................................................................................................................................... 1

2 Normative references ........................................................................................................................... 2

3 Terms and definitions........................................................................................................................... 2

4 Principles of the method of measurement and computation........................................................... 2

5 Classical Venturi tubes......................................................................................................................... 3

5.1 Field of application ............................................................................................................................... 3

5.2 General shape ....................................................................................................................................... 3

5.3 Material and manufacture..................................................................................................................... 7

5.4 Pressure tappings................................................................................................................................. 7

5.5 Discharge coefficient, C ....................................................................................................................... 8

5.6 Expansibility [expansion] factor, ε ...................................................................................................... 9

5.7 Uncertainty of the discharge coefficient C....................................................................................... 10

5.8 Uncertainty of the expansibility [expansion] factor ε...................................................................... 10

5.9 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 .............................................................................................................. 11

6.3 Flow conditioners ............................................................................................................................... 15

6.4 Additional specific installation requirements for classical Venturi tubes .................................... 15

Annex A (informative) Table of expansibility [expansion] factor ................................................................ 17

Annex B (informative) Classical Venturi tubes used outside the scope of ISO 5167-4............................. 18

Annex C (informative) Pressure loss in a classical Venturi tube ................................................................ 22

Bibliography ..................................................................................................................................................... 24

© ISO 2003 — All rights reserved iii
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SIST EN ISO 5167-4:2004
ISO 5167-4:2003(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.

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 5167-4 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits,

Subcommittee SC 2, Pressure differential devices.

This first edition of ISO 5167-4, together with the second edition of ISO 5167-1 and the first editions of

ISO 5167-2 and ISO 5167-3, cancels and replaces the first edition of ISO 5167-1:1991, which has been

technically revised, and ISO 5167-1:1991/Amd.1:1998.

ISO 5167 consists of the following parts, under the general title Measurement of fluid flow by means of

pressure differential devices inserted in circular cross-section conduits running full:

 Part 1: General principles and requirements
 Part 2: Orifice plates
 Part 3: Nozzles and Venturi nozzles
 Part 4: Venturi tubes
iv © ISO 2003 — All rights reserved
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SIST EN ISO 5167-4:2004
ISO 5167-4:2003(E)
Introduction

ISO 5167, divided into four parts, covers the geometry and method of use (installation and operating

conditions) of orifice plates, nozzles and Venturi tubes when they are inserted in a conduit running full to

determine the flowrate of the fluid flowing in the conduit. It also gives necessary information for calculating the

flowrate and its associated uncertainty.

ISO 5167 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 within specified limits of

pipe size and Reynolds number.

ISO 5167 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.

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 for the measurement are known as “secondary

devices”. ISO 5167 covers primary devices; secondary devices will be mentioned only occasionally.

ISO 5167 is divided into the following four parts.

a) Part 1 of ISO 5167 gives general terms and definitions, symbols, principles and requirements as well as

methods of measurement and uncertainty that are to be used in conjunction with Parts 2 to 4 of ISO 5167.

b) Part 2 of ISO 5167 specifies orifice plates, which can be used with corner pressure tappings, D and D/2

pressure tappings , and flange pressure tappings.

c) Part 3 of ISO 5167 specifies ISA 1932 nozzles , long radius nozzles and Venturi nozzles, which differ in

shape and in the position of the pressure tappings.
d) This part of ISO 5167 specifies classical Venturi tubes .

Aspects of safety are not dealt with in Parts 1 to 4 of ISO 5167. It is the responsibility of the user to ensure

that the system meets applicable safety regulations.

1) See ISO 2186:1973, Fluid flow in closed conduits — Connections for pressure signal transmissions between primary

and secondary elements.

2) Orifice plates with “vena contracta” pressure tappings are not considered in ISO 5167.

3) ISA is the abbreviation for the International Federation of the National Standardizing Associations, which was

succeeded by ISO in 1946.

4) In the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.

© ISO 2003 — All rights reserved v
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SIST EN ISO 5167-4:2004
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SIST EN ISO 5167-4:2004
INTERNATIONAL STANDARD ISO 5167-4:2003(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 part of ISO 5167 specifies the geometry and method of use (installation and operating conditions) of

Venturi tubes when they are inserted in a conduit running full to determine the flowrate of the fluid flowing in

the conduit.

This part of ISO 5167 also provides background information for calculating the flowrate and is applicable in

conjunction with the requirements given in ISO 5167-1.

This part of ISO 5167 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, each of these devices

can only be used within specified limits of pipe size, roughness, diameter ratio and Reynolds number. This

part of ISO 5167 is not applicable to the measurement of pulsating flow. It does not cover the use of 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 part of ISO 5167 deals with the three types of classical Venturi tubes:
a) cast;
b) machined;
c) rough welded sheet-iron.

A Venturi tube is a device which consists of a convergent inlet connected to a cylindrical throat which is in turn

connected to a conical expanding section called the “divergent”. The differences between the values of the

uncertainty of the discharge coefficient for the three types of classical Venturi tube show, on the one hand, the

number of results available for each type of classical Venturi tube and, on the other hand, the more or less

precise definition of the geometric profile. The values are based on data collected many years ago. Venturi

nozzles (and other nozzles) are dealt with in ISO 5167-3.

NOTE 1 Research into the use of Venturi tubes in high-pressure gas [ W 1 MPa ( W 10 bar)] is being carried out at

present (see References [1], [2], [3] in the Bibliography). In many cases for Venturi tubes with machined convergent

sections discharge coefficients which lie outside the range predicted by this part of ISO 5167 by 2 % or more have been

found. For optimum accuracy Venturi tubes for use in gas should be calibrated over the required flowrate range. In high-

pressure gas the use of single tappings (or at most two tappings in each plane) is not uncommon.

NOTE 2 In the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.

© ISO 2003 — All rights reserved 1
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SIST EN ISO 5167-4:2004
ISO 5167-4:2003(E)
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.

ISO 4006:1991, Measurement of fluid flow in closed conduits — Vocabulary and symbols

ISO 5167-1:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular

cross-section conduits running full — Part 1: General principles and requirements

3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 4006 and ISO 5167-1 apply.

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. In a Venturi tube a static pressure difference exists between the upstream section

and the throat section of the device. Whenever the device is geometrically similar to one on which direct

calibration has been made, the conditions of use being the same, the flowrate can be determined from the

measured value of this pressure difference and from a knowledge of the fluid conditions.

The mass flowrate can be determined by the following formula:
C π
qd=∆ερ2p (1)
m 1
1− β

The uncertainty limits can be calculated using the procedure given in Clause 8 of ISO 5167-1:2003.

Similarly, the value of the volume flowrate can be calculated since
q =

where ρ is the fluid density at the temperature and pressure for which the volume is stated.

Computation of the flowrate, which is a purely arithmetic process, is performed by replacing the different items

on the right-hand side of Equation (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 Equation (1) 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.

2 © ISO 2003 — All rights reserved
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SIST EN ISO 5167-4:2004
ISO 5167-4:2003(E)
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 part of ISO 5167 depends on the way in

which they are manufactured.

Three types of standard classical Venturi tube are defined 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 are described in 5.1.2 to 5.1.4 and have somewhat different

characteristics.

There are limits to the roughness and Reynolds number for each type which shall be addressed.

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 cones 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 cones may or may not be

rounded.

These classical Venturi tubes can be used in pipes of diameter between 50 mm and 250 mm and with

diameter ratios β between 0,4 and 0,75 inclusive.
5.1.4 Classical Venturi tube with a rough-welded sheet-iron convergent section

This is a classical Venturi tube normally fabricated by welding. 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 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 The minimum cylinder length, measured from the plane containing the intersection of the cone

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.
© ISO 2003 — All rights reserved 3
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SIST EN ISO 5167-4:2004
ISO 5167-4:2003(E)

The entrance cylinder diameter D shall be measured in the plane of the upstream pressure tappings. The

number of measurements shall be at least equal to the number of pressure tappings (with a minimum of four).

The diameters shall be measured near each pair of pressure tappings, and also between these pairs. The

arithmetic mean value of 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 tappings.

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
1 conical convergent E
2 cylindrical throat, C
3 conical convergent B
4 entrance cylinder A
5 connecting planes
7° u ϕ u 15°
Flow direction
See 5.4.7
Figure 1 — Geometric profile of the classical Venturi tube
4 © ISO 2003 — All rights reserved
---------------------- Page: 16 ----------------------
SIST EN ISO 5167-4:2004
ISO 5167-4:2003(E)

5.2.3 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 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 by means of a template. The deviation between the

template and the conical section 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 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 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 tube.

3 2 3

The diameter d shall be measured very carefully in the plane of the throat pressure tappings. The number of

measurements shall be at least equal to the number of pressure tappings (with a minimum of four). The

diameters shall be measured near each pair of pressure tappings and also between these pairs. 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 tappings.

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 revolution as

2 3

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 by means of a template.

2 3

The deviation between the template and the classical Venturi tube shall evolve in a regular way for each

curvature so that the single maximum deviation that is measured occurs at approximately midway along the

template profile. The value of this maxi
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