EN ISO 9300:2005

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

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.
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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.

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.
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
ISO 9300:2005(E)
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ii © ISO 2005 – All rights reserved

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

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

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 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
2 © ISO 2005 – All rights reserved

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:
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 ∗
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

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
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 −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
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
T Absolute stagnation temperature of the gas at nozzle inlet Θ K
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
−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
−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.
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
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:
6 © ISO 2005 – All rights reserved

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
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
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 to the extension of the toroidal contour.
6.2.2.4 The toroidal surface of the CFVN located between the inlet plane and the divergent section (see
Figure 1) shall not deviate from the shape of a torus by more than ± 0,001d. The radius of curvature r of this
c
toroidal surface in a plane in which the axis of symmetry lies shall be 1,8d to 2,2d.
6.2.2.5 The divergent section of the CFVN downstream of the point of tangency with the torus shall form
a frustum of a cone with a half-angle between 2,5° and 6°. The length of the divergent section shall be not
less than the throat diameter.
6.2.2.6 The uncertainty in the measurement of flow-rate using CFVN built in accordance with this
International Standard depends in particular on the uncertainly in the throat cross-sectional area. It is difficult
to measure precisely the throat diameter of a toroidal throat CFVN, particularly in the case of small nozzles,
and great care should be taken.

8 © ISO 2005 – All rights reserved

ISO 9300:2005(E)
Key
1 inlet plane
2 intersection of toroidal surface and divergent section
3 location of pressure indicating device
a −6
In this region the arithmetic average roughness Ra shall not exceed 15 × 10 d and 0,04 µm for normally and
accurately machined Venturi nozzles, respectively, and the contour shall not deviate from toroidal form by more
than ± 0,001d.
b −4
In this region the arithmetic roughness value shall not exceed 10 d.
c
Inlet surface shall lie outside this contour.
Figure 1 — Toroidal-throat Venturi nozzle
6.2.3 Cylindrical-throat Venturi nozzles
6.2.3.1 The CFVN shall conform with the specifications shown in Figure 2.
6.2.3.2 The inlet plane is defined as that plane which is tangential to the inlet contour of the CFVN and
perpendicular to the nozzle centre-line.
6.2.3.3 The convergent section of the CFVN (inlet) shall be a quarter of a torus tangential on one hand to
the inlet plane (see 6.2.3.2) and on the other hand to the cylindrical throat. The length of the cylindrical throat
and the radius of curvature r of the quarter of torus shall be equal to the throat diameter.
c
6.2.3.4 The inlet toroidal surface of the CFVN shall not deviate from the shape of a torus by more than
± 0,001d.
6.2.3.5 The flow-rate shall be calculated from the mean diameter at the cylindrical throat outlet section.
The mean diameter shall be determined by measuring at least four angularly equally distributed diameters on
the cylindrical throat outlet. No diameter along the throat length shall deviate by more than ± 0,001d from the
mean diameter.
The length of the throat shall not deviate from the throat diameter by more than 0,05d. The connection
between the quarter of torus and the cylindrical throat shall be inspected visually and no defect should be
observed. When a defect of connection is observed, it shall be checked that the local radius of curvature in a
ISO 9300:2005(E)
plane in which the axis of symmetry lies is never less than 0,5d throughout the inlet surface (quarter of torus
and cylindrical throat). Figure 3 illustrates this requirement.
The total area of the inlet and throat surfaces shall be properly polished so that the arithmetic average
−6
roughness Ra does not exceed 15 × 10 d.
The connection between the cylindrical throat and the divergent section shall also be visually inspected and
no defect shall be observed.
6.2.3.6 The divergent section of the CFVN comprises a frustum of a cone with a half-angle between 3°
and 4°. The length of the divergent section shall be not less than the throat diameter.

Key
1 inlet plane
−4
2 conical divergent section with an arithmetic average relative roughness not exceeding 10 d
3 transition region
a −6
In this region the arithmetic average roughness Ra shall not exceed 15 × 10 d and the contour shall not deviate from
toroidal and cylindrical forms by more than ± 0,001d.
Figure 2 — Cylindrical-throat Venturi nozzle

Figure 3 — Detail of connection between quarter of torus and cylindrical throat (transition region)
10 © ISO 2005 – All rights reserved

ISO 9300:2005(E)
7 Installation requirements
7.1 General
This International Standard applies to the installation of CFVN when either
a) the pipeline upstream of the CFVN is of circular cross-section, or
b) it can be assumed that there is a large space upstream of the CFVN or a set of CFVN mounted in a
cluster.
In the case of a), the CFVN shall be installed in a system meeting the requirements of 7.2.
In the case of b), the CFVN shall be installed in a system meeting the requirements of 7.3.
In either case, swirl shall not exist upstream of the CFVN. Where a pipeline exists upstream of the nozzle,
swirl-free conditions can be ensured by installing a flow straightener of the design shown in Figure 4 at a
distance l > 5D upstream of the nozzle inlet plane or any type of other flow conditioners of recognised type
having equivalent or better performance — see [1] and [2] in the Bibliography.

Key
1 inlet plane
2 etoile straightener with vane thickness adequate to prevent buckling
3 location of temperature sensor
4 location of pressure tapping
a −4
In this region the surface roughness shall not exceed 10 D.
Figure 4 — Installation requirements for upstream pipework configuration
7.2 Upstream pipeline
The primary device may be installed in a straight circular conduit which shall be concentric within ± 0,02D with
the centre line of the CFVN. The inlet conduit up to 3D upstream of the CFVN shall not deviate from circularity
−4
by more than 0,01D and shall have an arithmetic average roughness Ra which shall not exceed 10 D. The
diameter of the inlet conduit shall be a minimum of 4d (β u 0,25).
In cases where upstream installation constraints are such that the above requirement cannot be met, specific
tests are recommended to investigate the influence of the installation conditions on the uncertainty of the flow-
rate measurement and/or the determination of C , when running a primary calibration. A correction method is
d′
given in this International Standard for the calculation of the mass flow-rate when β > 0,25.
ISO 9300:2005(E)
7.3 Large upstream space
It can be assumed that there is a large space upstream of the primary device if there is no wall closer than 5d
to the axis of the primary device or to the inlet plane of the primary device, as defined in 6.2.2.2 or 6.2.3.2.
In cases of a large upstream space, or for high flow-rates, multiple CFVNs may be used.
7.4 Downstream requirements
No requirements are imposed on the outlet conduit except that it shall not restrict the flow in a manner such as
to prevent critical flow in the CFVN.
7.5 Pressure measurement
7.5.1 When a circular conduit is used upstream of the primary device, the upstream static pressure shall
preferably be measured at a wall pressure tapping at a distance 0,9D to 1,1D from the inlet plane of the
Venturi nozzle (see Figures 1 and 4). The wall pressure tapping may be located upstream or downstream of
this position, provided that it has been demonstrated that the measured pressure can be used reliably to give
the nozzle inlet stagnation pressure.
7.5.2 When it can be assumed that there is a large space upstream of the primary device, the upstream
wall pressure tapping shall preferably be located in a wall perpendicular to the inlet face of the primary device
and within a distance of 10d ± 1d from that plane. The wall pressure tapping may be located upstream or
downstream of this position, provided that it has been demonstrated that the measured pressure can be used
reliably to give the nozzle inlet stagnation pressure.
7.5.3 For the wall pressure tapping referred to in 7.5.1, and preferably also for that in 7.5.2, the centreline
of the wall pressure tapping shall meet the centreline of the primary device and be at right angle to it. At the
point of the breakthrough, the hole shall be circular. The edges shall be free from burrs, and shall be square or
lightly rounded to a radius not exceeding 0,1 times the diameter of the wall pressure tapping. It shall be
confirmed by visual inspection that the wall pressure tappings comply with these requirements. When an
upstream pipeline is used, the diameter of the wall pressure tapping shall be less than 0,08D and less than
12 mm. The wall pressure tapping shall be cylindrical for a minimum length of 2,5 times the diameter of the
tapping (see Figure 5).
Dimensions in millimetres
a
Edge of hole flush with internal surface of conduit, burr-free and square to a radius not exceeding 0,1d .
t
Figure 5 — Detail of wall pressure tapping when upstream pipeline is used
7.5.4 The downstream pressure shall be measured to ensure that critical flow is maintained. This pressure
shall be measured by using a conduit wall pressure tapping located within 0,5 times the conduit diameter of
the exit plane of the divergent section.
12 © ISO 2005 – All rights reserved

ISO 9300:2005(E)
The critical flow may also be checked by measuring the wall pressure at the step located immediately
downstream of the nozzle throat. That method requires special machining of the CFVN (see 6.1.3).
7.5.5 In some applications, the outlet pressure can be determined without the use of a wall pressure
tapping. For example, the CFVN may discharge directly into the atmosphere or other region of known
pressure. In these applications the outlet pressure need not be measured.
7.6 Drain holes
The conduit may be provided with the necessary drain holes for the removal of condensate or other foreign
substances that may collect in some applications. There should be no flow through these drain holes while the
flow measurement is in progress. If drain holes are required, they shall be located upstream of the nozzle
upstream wall pressure tapping. The diameter of the drain holes should be smaller than 0,06D. The axial
distance from the drain hole to the plane of the upstream wall pressure tapping shall be greater than D and
the hole shall be located in an axial plane different from that of the wall pressure tapping.
During measurement, flow must be single-phase upstream and in the throat with no condensation and all
surfaces must retain their cleanliness and hence surface finish. If this cannot be guaranteed, the
measurement shall not be claimed to conform to this International Standard.
7.7 Temperature measurement
The inlet temperature shall be measured using one or more sensors located upstream of the CFVN. When an
upstream pipeline is used, the recommended location of these sensors is 1,8D to 2,2D upstream of the inlet
plane of the CFVN. The diameter of the sensing element shall be not larger than 0,04D and the element shall
not be aligned with a wall pressure tapping in the flow direction. If it is impracticable to use a sensing element
of diameter less than 0,04D, the sensing element shall be so located that it can be demonstrated that it does
not affect the pressure measurement. The sensor may be located further still upstream, provided that it has
been demonstrated that the measured temperature can be used reliably to give the nozzle inlet stagnation
temperature.
Particular care has to be exercised in the selection of the temperature sensor and the insulation of pipework if
the stagnation temperature of the flowing gas differs from that of the medium surrounding the pipeline by more
than 5 K. In such cases, the sensor selected shall be insensitive to radiation error and the pipework shall be
well lagged to minimize heat transfer between the flowing gas and the surrounding medium. If the
temperatures of the flowing gas and the pipe wall differ significantly, it is extremely difficult to measure the gas
temperature accurately.
7.8 Density measurement
For some applications, it may be desirable to measure directly the gas density at the nozzle inlet — for
instance when the molar mass of the gas is not known with a sufficient accuracy.
When a densitometer is used, it shall be installed upstream of the nozzle and of the upstream pressure and
temperature tappings. To achieve correct measurement of the inlet gas density, particular attention shall be
given to the following points.
a) The installation of the densitometer shall not disturb the pressure and temperature measurements.
b) When the densitometer is located outside the main upstream pipe, checks shall be carried out to ensure
that the gas in the device is the same as the gas flowing in the main conduit.
c) The pressure and temperature conditions at the densitometer should be as close as possible to the
nozzle inlet conditions in order to avoid corrections. If necessary, the inlet density shall be computed from
the measured density using the equation of state:
pT Z
0 den den
ρρ= (9)
0den
pTZ
den 0 0
ISO 9300:2005(E)
where
den as a subscript signifies “relative to the densitometer”;
T is the temperature that should be measured;
den
p is the pressure that should be determined by measurement of the difference from p ;
den 0
Z /Z is calculated using the specifications of 7.9.
den 0
7.9 Calculated density
Instead of the measurement of the density, a calculation may be performed using the gas composition
determined by gas chromatography, combined with a recognised equation such as the one proposed by
[3]
ISO 6976:1995 in particular. The uncertainty of the method is as good as the uncertainty obtained with a
densitometer.
8 Calculation methods
8.1 Mass flow-rate
The actual mass flow-rate shall be computed from one of the following equations:
AC C p
′
nt d ∗ 0
q =
m
R
T
 0
M

or
qA= CC p ρ
m nt d′ R 0 0
where A is calculated from the value of d.
nt
8.2 Discharge coefficient, C
d′
8.2.1 The discharge coefficient depends largely on the shape of the CFVN and it shall be noted that at
small values of the throat diameter the nozzle geometry is very difficult to control and measure (see 6.2.2.6).
8.2.2 The discharge coefficient for the CFVN may be obtained from the following equation:
−n
Ca=−bRe (10)
dn′ t
The coefficients a, b and n are given in Table 1 for each type of CFVN for the range of throat Reynolds
number over which they may be used.
14 © ISO 2005 – All rights reserved

ISO 9300:2005(E)
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