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-tuyères 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
14-Aug-2005
Current Stage
9960 - Withdrawal effective - Withdrawal
Completion Date
29-Jun-2022

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SIST EN ISO 9300:2005
01-november-2005
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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.
---------------------- Page: 3 ----------------------
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.
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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
---------------------- Page: 5 ----------------------
SIST EN ISO 9300:2005
ISO 9300:2005(E)
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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

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

© ISO 2005 – All rights reserved iii
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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.

© ISO 2005 – All rights reserved 1
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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
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

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:
Re =
πd µ
2 © ISO 2005 – All rights reserved
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SIST EN ISO 9300:2005
ISO 9300:2005(E)
2.4.3
isentropic exponent

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:
ρρdpc
κ==
ppdρ
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

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

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
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 ∗
© ISO 2005 – All rights reserved 3
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SIST EN ISO 9300:2005
ISO 9300:2005(E)
2.4.8
critical pressure ratio

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

〈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

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:
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
A Cross-sectional area of Venturi nozzle throat L m
C Coefficient of discharge Dimensionless
C Critical flow coefficient for one-dimensional flow of a real gas Dimensionless
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

D Diameter of the upstream conduit L m
d Diameter of Venturi nozzle throat L m
M Molar mass M kg mol
Ma Mach number at the location of the upstream pressure tapping Dimensionless
−1 −2
p Absolute static pressure of the gas at nozzle inlet ML T Pa
−1 −2
p Absolute static pressure of the gas at nozzle exit ML T Pa
−1 −2
p Absolute stagnation pressure of the gas at nozzle inlet ML T Pa
−1 −2
p Absolute static pressure of the gas at nozzle throat ML T Pa
Absolute static pressure of the gas at nozzle throat for one-dimensional
−1 −2
p ML T Pa
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
−1 −1
q Mass flow-rate for one-dimensional isentropic flow of an inviscid gas MT kg·s
2 −2 −1 −1 −1
R Universal gas constant M L T Θ J·mol K
Re Nozzle throat Reynolds number Dimensionless
r Radius of curvature of nozzle inlet L m
r Critical pressure ratio p /p Dimensionless
∗ nt 0
U′ Relative uncertainty Dimensionless
T Absolute temperature of the gas at nozzle inlet Θ K
T Absolute stagnation temperature of the gas at nozzle inlet Θ K
T Absolute static temperature at nozzle throat Θ K
−1 −1
v Throat sonic flow velocity; critical flow velocity at nozzle throat LT m·s
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
−1 −1
µ Dynamic viscosity of the gas at nozzle throat ML T Pa·s
−3 −3
ρ Gas density at stagnation conditions at nozzle inlet ML kg·m
−3 −3
ρ Gas density at nozzle throat ML kg·m
M = mass
L = length
T = time
Θ = temperature
Same as the corresponding quantity.
© ISO 2005 – All rights reserved 5
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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
q = (2)
R
 0
qA= C p ρ (3)
mi nt ∗ 0 0
where
γ +1
 γ −1
C = γ (4)
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)
R
 0
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:

6 © ISO 2005 – All rights reserved
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SIST EN ISO 9300:2005
ISO 9300:2005(E)
p M
Z = (8)
ρ RT

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
For the flow-rate under ideal conditions, critical mass flux =
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
© ISO 2005 – All rights reserved 7
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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

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

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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