SIST EN ISO 9300:2005

Measurement of gas flow by means of critical flow Venturi nozzles (ISO 9300:2005)

ISO 9300:2005 specifies the geometry and method of use (installation in a system and operating conditions) of critical flow Venturi nozzles (CFVN) used to determine the mass flow-rate of a gas flowing through a system. It also gives the information necessary for calculating the flow-rate and its associated uncertainty. It is applicable to Venturi nozzles in which the gas flow accelerates to the critical velocity at the throat (this being equal to the local sonic velocity), and only where there is steady flow of single-phase gases.

Durchflussmessung von Gasen mit Venturidüsen bei kritischer Strömung (ISO 9300:2005)

Diese Internationale Norm legt die geometrische Gestalt und das Verfahren für die Anwendung (Einbau in ein System und Arbeitsbedingungen) von Venturidüsen fest, die den Massendurchfluss in einem von Gas durchströmten System bei kritischer Strömung bestimmen. Sie enthält Angaben für die Berechnung des Durchflusses und der zugehörigen Unsicherheit.
Sie ist für Venturidüsen anwendbar, in deren Halsteil das Gas auf die kritische Strömungsgeschwindigkeit beschleunigt wird (dies entspricht der örtlichen Schallgeschwindigkeit) und nur für einphasige Gase bei stationärer Strömung. Bei der kritischen Geschwindigkeit hat der Massendurchfluss des durch die Venturidüse strömenden Gases, unter den auf der Einlaufseite vorhandenen Bedingungen, sein Maximum.
Die Venturidüse bei kritischer Strömung (kritische Düse), die in dieser Norm behandelt wird, kann nur innerhalb gegebener Grenzen eingesetzt werden, d. h. Grenzen für das Verhältnis Durchmesser des Halsteils zum Durchmesser des Einlaufrohres und Grenzwerte für die Reynoldszahl im Halsteil. Diese Internationale Norm beschreibt Venturidüsen, die in ausreichend häufigen Versuchen direkt kalibriert wurden, so dass aus diesen Ergebnissen für gleiche Anwendungsbedingungen Koeffizienten mit angebbaren Grenzwerten für die Unsicherheit abgeleitet werden konnten.
Sie enthält Angaben für Fälle, wo
-   die Einlaufstrecke vor der Venturidüse einen kreisförmigen Querschnitt aufweist oder
-   davon ausgegangen werden kann, dass an der Einlaufseite der Venturidüse oder an der Einlaufseite einer als Einheit zusammengefasste Gruppe von Venturidüsen ein großer Raum vorhanden ist. Die zusammengefasste Einheit eröffnet die Möglichkeit, Venturidüsen parallel einzubauen um damit größte Durchflüsse zu erzielen.
Für hochgenaue Messungen bei niedrigen Reynoldszahlen werden hochgenau bearbeitete Venturidüsen beschrieben.

Mesure de débit de gaz au moyen de Venturi-tuyeres en régime critique (ISO 9300:2005)

L'ISO 9300:2005 spécifie la géométrie et le mode d'emploi (installation dans un circuit et conditions opératoires) de Venturi-tuyères en régime critique (CFVN) utilisés pour déterminer le débit-masse de gaz traversant le circuit. Elle donne également les informations nécessaires au calcul du débit et de l'incertitude associée. Elle s'applique aux Venturi-tuyères au sein desquels l'écoulement gazeux est accéléré jusqu'à atteindre la vitesse critique au col (la vitesse d'un écoulement critique est égale à la vitesse locale du son), et uniquement lorsqu'il existe un écoulement stationnaire monophasique de gaz.

Merjenje pretoka plina na podlagi kritičnega toka v Venturijevi šobi (ISO 9300:2005)

General Information

• Status: Withdrawn
• Publication Date: 31-Oct-2005
• Withdrawal Date: 13-Jul-2022
• ICS: 17.120.10 - Flow in closed conduits
• Technical Committee: IMIN - Measurement instruments
• Current Stage: 9900 - Withdrawal (Adopted Project)
• Start Date: 14-Jul-2022
• Due Date: 06-Aug-2022
• Completion Date: 14-Jul-2022
• Ref Project: EN ISO 9300:2005 - Measurement of gas flow by means of critical flow Venturi nozzles (ISO 9300:2005)

Relations

• Replaced By: SIST EN ISO 9300:2022 - Measurement of gas flow by means of critical flow nozzles (ISO 9300:2022)
• Effective Date: 06-Jul-2022
• Replaces: SIST EN ISO 9300:1998 - Measurement of gas flow by means of critical flow Venturi nozzles (ISO 9300:1990)
• Effective Date: 01-Nov-2005
• Revised: SIST EN ISO 9300:2022 - Measurement of gas flow by means of critical flow nozzles (ISO 9300:2022)
• Effective Date: 01-Sep-2021

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN ISO 9300:2005
01-november-2005
1DGRPHãþD
SIST EN ISO 9300:1998
0HUMHQMHSUHWRNDSOLQDQDSRGODJLNULWLþQHJDWRNDY9HQWXULMHYLãREL,62

Measurement of gas flow by means of critical flow Venturi nozzles (ISO 9300:2005)
Durchflussmessung von Gasen mit Venturidüsen bei kritischer Strömung (ISO
9300:2005)
Mesure de débit de gaz au moyen de Venturi-tuyeres en régime critique (ISO
9300:2005)
Ta slovenski standard je istoveten z: EN ISO 9300:2005
ICS:
17.120.10 Pretok v zaprtih vodih Flow in closed conduits
SIST EN ISO 9300:2005 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 9300:2005

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SIST EN ISO 9300:2005
EUROPEAN STANDARD
EN ISO 9300
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2005
ICS 17.120.10 Supersedes EN ISO 9300:1995
English Version
Measurement of gas flow by means of critical flow Venturi
nozzles (ISO 9300:2005)
Mesure de débit de gaz au moyen de Venturi-tuyères en Durchflussmessung von Gasen mit Venturidüsen bei
régime critique (ISO 9300:2005) kritischer Strömung (ISO 9300:2005)
This European Standard was approved by CEN on 15 July 2005.
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 Central Secretariat 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 Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, 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
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 9300:2005: E
worldwide for CEN national Members.

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SIST EN ISO 9300:2005

EN ISO 9300:2005 (E)





Foreword


This document (EN ISO 9300:2005) 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 February 2006, and conflicting national
standards shall be withdrawn at the latest by February 2006.

This document supersedes EN ISO 9300:1995.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary,
Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.


Endorsement notice

The text of ISO 9300:2005 has been approved by CEN as EN ISO 9300:2005 without any
modifications.

2

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SIST EN ISO 9300:2005


INTERNATIONAL ISO
STANDARD 9300
Second edition
2005-08-15

Measurement of gas flow by means of
critical flow Venturi nozzles
Mesure de débit de gaz au moyen de Venturi-tuyères en régime
critique




Reference number
ISO 9300:2005(E)
©
ISO 2005

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SIST EN ISO 9300:2005
ISO 9300:2005(E)
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©  ISO 2005
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
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Published in Switzerland

ii © ISO 2005 – All rights reserved

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SIST EN ISO 9300:2005
ISO 9300:2005(E)
Contents Page
Foreword. iv
1 Scope . 1
2 Terms and definitions. 1
2.1 Pressure measurement . 1
2.2 Temperature measurement. 2
2.3 Venturi nozzles. 2
2.4 Flow. 2
3 Symbols . 5
4 Basic equations . 6
4.1 State equation . 6
4.2 Flow-rate under ideal conditions . 6
4.3 Flow-rate under real conditions . 6
4.4 Critical mass flux . 7
5 Applications for which the method is suitable . 7
6 Standard critical flow Venturi nozzles (CFVN).7
6.1 General requirements. 7
6.2 Design . 8
7 Installation requirements . 11
7.1 General. 11
7.2 Upstream pipeline. 11
7.3 Large upstream space. 12
7.4 Downstream requirements . 12
7.5 Pressure measurement . 12
7.6 Drain holes . 13
7.7 Temperature measurement. 13
7.8 Density measurement. 13
7.9 Calculated density . 14
8 Calculation methods. 14
8.1 Mass flow-rate. 14
8.2 Discharge coefficient, C . 14
d′
8.3 Critical flow function, C , and real gas critical flow coefficient, C . 15
∗ R
8.4 Conversion of measured pressure and temperature to stagnation conditions. 15
8.5 Maximum permissible downstream pressure. 16
9 Uncertainties in the measurement of flow-rate . 17
9.1 General. 17
9.2 Practical computation of uncertainty . 18
Annex A (normative) Venturi nozzle discharge coefficients . 19
Annex B (normative) Tables of values for critical flow function C — Various gases. 21
∗
Annex C (normative) Computation of critical mass flux for natural gas mixtures. 28
Annex D (normative) Mass flow correction factor for atmospheric air . 32
Annex E (normative) Computation of critical mass flux for critical flow nozzles with high nozzle
throat to upstream pipe diameter ratio, β > 0,25. 33
Bibliography . 36

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SIST EN ISO 9300:2005
ISO 9300:2005(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 9300 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits,
Subcommittee SC 2, Pressure differential devices.
This second edition cancels and replaces the first edition (ISO 9300:1990), which has been technically revised.

iv © ISO 2005 – All rights reserved

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SIST EN ISO 9300:2005
INTERNATIONAL STANDARD ISO 9300:2005(E)

Measurement of gas flow by means of critical flow Venturi
nozzles
1 Scope
This International Standard specifies the geometry and method of use (installation in a system and operating
conditions) of critical flow Venturi nozzles (CFVN) used to determine the mass flow-rate of a gas flowing
through a system. It also gives the information necessary for calculating the flow-rate and its associated
uncertainty.
It is applicable to Venturi nozzles in which the gas flow accelerates to the critical velocity at the throat (this
being equal to the local sonic velocity), and only where there is steady flow of single-phase gases. At the
critical velocity, the mass flow-rate of the gas flowing through the Venturi nozzle is the maximum possible for
the existing upstream conditions while CFVN can only be used within specified limits, e.g. Iimits for the nozzle
throat to inlet diameter ratio and throat Reynolds number. This International Standard deals with CFVN for
which direct calibration experiments have been made in sufficient number to enable the resulting coefficients
to be used with certain predictable limits of uncertainty.
Information is given for cases where the pipeline upstream of the CFVN is of circular cross-section, or it can
be assumed that there is a large space upstream of the CFVN or upstream of a set of CFVN mounted in a
cluster. The cluster configuration offers the possibility of installing CFVN in parallel, thereby achieving high
flow-rates.
For high-accuracy measurement, accurately machined Venturi nozzles are described for low Reynolds
number applications.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1 Pressure measurement
2.1.1
wall pressure tapping
hole drilled in the wall of a conduit in such a way that the edge of the hole is flush with the internal surface of
the conduit
NOTE The tapping is achieved such that the pressure within the hole is the static pressure at that point in the conduit.
2.1.2
static pressure of a gas
actual pressure of the flowing gas which can be measured by connecting a pressure gauge to a wall pressure
tapping
NOTE Only the value of the absolute static pressure is used in this International Standard.
2.1.3
stagnation pressure
pressure which would exist in a gas in a flowing gas stream if the stream were brought to rest by an isentropic
process
NOTE Only the value of the absolute stagnation pressure is used in this International Standard.
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SIST EN ISO 9300:2005
ISO 9300:2005(E)
2.2 Temperature measurement
2.2.1
static temperature
actual temperature of a flowing gas
NOTE Only the value of the absolute static temperature is used in this International Standard.
2.2.2
stagnation temperature
temperature which would exist in a gas in a flowing gas stream if the stream were brought to rest by an
isentropic process
NOTE Only the value of the absolute stagnation temperature is used in this International Standard.
2.3 Venturi nozzles
2.3.1
Venturi nozzle
convergent/divergent restriction inserted in a system intended for the measurement of flow-rate
2.3.2
normally machined Venturi nozzle
Venturi nozzle machined by a lathe and surface polished to achieve the desired smoothness
2.3.3
accurately machined Venturi nozzle
Venturi nozzle machined by a super-accurate lathe to achieve a mirror finish without polishing
2.3.4
throat
section of minimum diameter of a Venturi nozzle
2.3.5
critical flow Venturi nozzle
CFVN
Venturi nozzle for which the nozzle geometrical configuration and conditions of use are such that the flow-rate
is critical at the nozzle throat
2.4 Flow
2.4.1
mass flow-rate
q
m
mass of gas per unit time passing through the CFVN
NOTE In this International Standard, the term flow-rate always refers to mass flow-rate.
2.4.2
throat Reynolds number
Re
nt
dimensionless parameter calculated from the gas flow-rate and the gas dynamic viscosity at nozzle inlet
stagnation conditions
NOTE The characteristic dimension is taken as the throat diameter at stagnation conditions. The throat Reynolds
number is given by the formula:
4q
m
Re =
nt
πd µ
0
2 © ISO 2005 – All rights reserved

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SIST EN ISO 9300:2005
ISO 9300:2005(E)
2.4.3
isentropic exponent
κ
ratio of the relative variation in pressure to the corresponding relative variation in density under elementary
reversible adiabatic (isentropic) transformation conditions
NOTE 1 The isentropic exponent is given by the formula:
2
ρρdpc
κ==

ppdρ

s
where
p is the absolute static pressure of the gas;
ρ is the density of the gas;
c is the local speed of sound;
s signifies “at constant entropy”.
NOTE 2 For an ideal gas, κ is equal to the ratio of specific heat capacities γ and is equal to 5/3 for monatomic gases,
7/5 for diatomic gases, 9/7 for triatomic gases, etc.
NOTE 3 In real gases, the forces exerted between molecules as well as the volume occupied by the molecules have a
significant effect on the gas behaviour. In an ideal gas, intermolecular forces and the volume occupied by the molecules
can be neglected.
2.4.4
discharge coefficient
C
d′
dimensionless ratio of the actual flow-rate to the ideal flow-rate of non-viscous gas that would be obtained with
one-dimensional isentropic flow for the same upstream stagnation conditions
NOTE This coefficient corrects for viscous and flow field curvature effects. For each type of nozzle design and
installation conditions specified in this International Standard, it is only a function of the throat Reynolds number.
2.4.5
critical flow
maximum flow-rate for a particular Venturi nozzle, which can exist for the given upstream conditions
NOTE When critical flow exists, the throat velocity is equal to the local value of the speed of sound (acoustic velocity),
the velocity at which small pressure disturbances propagate.
2.4.6
critical flow function
C

∗
dimensionless function which characterises the thermodynamic flow properties of an isentropic and one-
dimensional flow between the inlet and the throat of a Venturi nozzle
NOTE It is a function of the nature of the gas and of stagnation conditions (see 4.2).
2.4.7
real gas critical flow coefficient
C
R
alternative form of the critical flow function, more convenient for gas mixtures
NOTE It is related to the critical flow function as follows:
CC= Z
R ∗
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SIST EN ISO 9300:2005
ISO 9300:2005(E)
2.4.8
critical pressure ratio
r
∗
ratio of the static pressure at the nozzle throat to the stagnation pressure for which the gas mass flow-rate
through the nozzle is a maximum
NOTE This ratio is calculated in accordance with the equation given in 8.5.
2.4.9
back-pressure ratio
ratio of the nozzle exit static pressure to the nozzle upstream stagnation pressure
2.4.10
Mach number
Ma
〈at nozzle upstream static conditions〉 ratio of the mean axial fluid velocity to the velocity of sound at the
location of the upstream pressure tapping
2.4.11
compressibility factor
Z
correction factor expressing numerically the deviation from the ideal gas law of the behaviour of a real gas at
given pressure and temperature conditions
NOTE It is defined by the formula:
pM
Z =
ρ RT
where R, the universal gas constant, equals 8,314 51 J/(mol·K).
2.5
uncertainty
parameter, associated with the results of a measurement, that characterizes the dispersion of the values that
could reasonably be attributed to the measurand
4 © ISO 2005 – All rights reserved

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SIST EN ISO 9300:2005
ISO 9300:2005(E)
3 Symbols
Symbol Description Dimension SI unit
2 2
A Cross-sectional area of Venturi nozzle exit L m
2
2 2
A Cross-sectional area of Venturi nozzle throat L m
nt
C Coefficient of discharge Dimensionless
d′
C Critical flow coefficient for one-dimensional flow of a real gas Dimensionless
R
C Critical flow function for one-dimensional flow of a real gas Dimensionless
∗
C Critical flow function for one-dimensional isentropic flow of a perfect gas Dimensionless
∗i
D Diameter of the upstream conduit L m
d Diameter of Venturi nozzle throat L m
−1
M Molar mass M kg mol
Ma Mach number at the location of the upstream pressure tapping Dimensionless
1
−1 −2
p Absolute static pressure of the gas at nozzle inlet ML T Pa
1
−1 −2
p Absolute static pressure of the gas at nozzle exit ML T Pa
2
−1 −2
p Absolute stagnation pressure of the gas at nozzle inlet ML T Pa
0
−1 −2
p Absolute static pressure of the gas at nozzle throat ML T Pa
nt
Absolute static pressure of the gas at nozzle throat for one-dimensional
−1 −2
p ML T Pa
i
∗
isentropic flow of a perfect gas
Ratio of nozzle exit static pressure to inlet stagnation pressure for one-
(p /p ) Dimensionless
2 0 i
dimensional isentropic flow of a perfect gas
−1 −1
q Mass flow-rate MT kg·s
m
−1 −1
q Mass flow-rate for one-dimensional isentropic flow of an inviscid gas MT kg·s
mi
2 −2 −1 −1 −1
R Universal gas constant M L T Θ J·mol K
Re Nozzle throat Reynolds number Dimensionless
nt
r Radius of curvature of nozzle inlet L m
c
r Critical pressure ratio p /p Dimensionless
∗ nt 0
U′ Relative uncertainty Dimensionless
T Absolute temperature of the gas at nozzle inlet Θ K
1
T Absolute stagnation temperature of the gas at nozzle inlet Θ K
0
T Absolute static temperature at nozzle throat Θ K
nt
−1 −1
v Throat sonic flow velocity; critical flow velocity at nozzle throat LT m·s
nt
Z Compressibility factor Dimensionless
β Diameter ratio d/D Dimensionless
γ Ratio of specific heat capacities Dimensionless
a a
δ Absolute uncertainty
κ Isentropic exponent Dimensionless
−1 −1
µ Dynamic viscosity of the gas at stagnation conditions ML T Pa·s
0
−1 −1
µ Dynamic viscosity of the gas at nozzle throat ML T Pa·s
nt
−3 −3
ρ Gas density at stagnation conditions at nozzle inlet ML kg·m
0
−3 −3
ρ Gas density at nozzle throat ML kg·m
nt
M = mass
L = length
T = time
Θ = temperature
a
Same as the corresponding quantity.
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SIST EN ISO 9300:2005
ISO 9300:2005(E)
4 Basic equations
4.1 State equation
The behaviour of a real gas can be described by the formula:
pR
= TZ (1)

ρ M

4.2 Flow-rate under ideal conditions
For ideal critical flow to exist, three main conditions are necessary:
a) the flow must be one-dimensional;
b) the flow must be isentropic;
c) the gas must be perfect (i.e. Z = 1 and κ = γ).
Under these conditions, the critical flow-rate is given by:
A Cp
nt ∗ 0
i
q = (2)
mi
R
T
 0
M

or
qA= C p ρ (3)
mi nt ∗ 0 0
i
where
γ +1
 γ −1
2
C = γ (4)

∗
i
γ +1

4.3 Flow-rate under real conditions
For flow-rates under real conditions, the formula for critical flow-rate becomes:
A CC p
nt d′ ∗ 0
q = (5)
m
R
T
 0
M

or
qA= CC p ρ (6)
m nt d′ R 0 0
since
CC= Z (7)
R0∗
where Z is the value of the compressibility factor at upstream stagnation conditions:
0
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SIST EN ISO 9300:2005
ISO 9300:2005(E)
p M
0
Z = (8)
0
ρ RT
00
It should be noted that C and C are not equal to C because the gas is not perfect. C is less than unity

∗ R ∗i d′
since the flow is not one-dimensional and a boundary layer exists owing to viscous effects.
4.4 Critical mass flux
q
mi
For the flow-rate under ideal conditions, critical mass flux =
A
nt
q
m
For the flow-rate under real conditions, critical mass flux =
A C
nt d′
5 Applications for which the method is suitable
Each application should be evaluated to determine whether a CFVN or some other device is the most suitable.
An important consideration is that the flow through the Venturi nozzle is independent of the downstream
pressure (see 9.5) within the pressure range for which the Venturi nozzle can be used for critical flow
measurement.
Some other considerations are as follows.
For CFVN the only measurements required are the gas pressure and the gas temperature or density
upstream of the critical Venturi nozzle, since the throat conditions can be calculated from thermodynamic
considerations.
The velocity in the CFVN throat is the maximum possible for the given upstream stagnation conditions, and
therefore the sensitivity to installation effects is minimized, except for those of swirl which shall not exist in the
inlet part of the CFVN.
When comparing CFVN with subsonic pressure-difference meters, it can be noted that in the case of the
CFVN, the flow is directly proportional to the nozzle upstream stagnation pressure and not, as in the case of
the subsonic meter, to the square root of a measured differential pressure.
The maximum flow range which can be obtained for a given CFVN is generally limited to the range of inlet
pressures which are available above the inlet pressure at which the flow becomes critical.
The most common applications to date for CFVN have been for tests, calibration and flow control.
6 Standard critical flow Venturi nozzles (CFVN)
6.1 General requirements
6.1.1 Materials
The CFVN shall be manufactured from material suitable for the intended application. Some considerations are
that
a) it should be possible to finish the material to the required condition (as given in 6.1.2 and 6.1.3), taking
into account that some materials are unsuitable owing to the inclusion of pits, voids and other
inhomogeneities,
b) the material, together with any surface treatment used, shall not be subject to corrosion in the intended
service, and
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SIST EN ISO 9300:2005
ISO 9300:2005(E)
c) the material should be dimensionally stable and should have known and repeatable thermal expansion
characteristics (if it is to be used at a temperature other than that at which the throat diameter has been
measured), so that the appropriate throat diameter correction can be made.
6.1.2 Surface finish of the throat and the inlet
The throat and toroidal inlet up to the conical divergent section of the CFVN shall be smoothly finished so that
−6
the arithmetic average roughness Ra does not exceed 15 × 10 d and 0,04 µm for normally and accurately
machined Venturi nozzles, respectively.
The throat and toroidal inlet up the conical divergent section shall be free from dirt or any other contaminants.
For a normally machined CFVN, it is allowable to use a toroidal throat CFVN with a diameter step at the throat
not larger than 10 % of the throat diameter.
6.1.3 Conical divergent
The form of the conical divergent section of the CFVN shall be checked to ensure that any steps,
discontinuities, irregularities and lack of concentricity do not exceed 1 % of the local diameter. The arithmetic
−4
average roughness Ra of the conical divergent section shall not exceed 10 d.
6.2 Design
6.2.1 General
There are two designs of standard CFVN: the toroidal-throat Venturi nozzle and the cylindrical throat Venturi
nozzle. Accurately machined Venturi nozzles shall be built according to the toroidal design.
6.2.2 Toroidal-throat Venturi nozzle
6.2.2.1 The CFVN shall conform with the specifications shown in Figure 1.
6.2.2.2 For purposes of locating other elements of the CFVN metering system, the inlet plane of the
CFVN is defined as that plane perpendicular to the axis of symmetry which intersects the inlet at a diameter
equal to 2,5d ± 0,1d.
6.2.2.3 The convergent section of the CFVN nozzle (inlet) shall be a portion of a torus which shall extend
from the inlet plane through the minimum area section (throat) and be tangential to the divergent section. The
contour of the inlet upstream of the inlet plane (see 6.2.2.2) is not specified, except that the surface at each
axial location shall have a diameter greater than or equal
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