EN ISO 9300:1995

SLOVENSKI STANDARD
SIST EN ISO 9300:1998
01-avgust-1998
Measurement of gas flow by means of critical flow Venturi nozzles
Measurement of gas flow by means of critical flow Venturi nozzles (ISO 9300:1990)
Durchflußmessung von Gasen mit Venturidüsen bei kritischer Strömung (ISO
9300:1990)
Mesure de débit de gaz au moyen de Venturi-tuyeres en régime critique (ISO
9300:1990)
Ta slovenski standard je istoveten z: EN ISO 9300:1995
ICS:
17.120.10 Pretok v zaprtih vodih Flow in closed conduits
SIST EN ISO 9300:1998 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 9300:1998

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SIST EN ISO 9300:1998

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SIST EN ISO 9300:1998

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SIST EN ISO 9300:1998
INTERNATIONAL
ISO
STANDARD 9300
First edition
1990-08-15
Measurement of gas flow by means of critical
flow Venturi nozzles
Mesure de dkbit de gaz au moyen de Venturi- tuykres en rkgime critigue
Reference number
ISO 9300 : 1990 (El

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SIST EN ISO 9300:1998
ISO 9300 : 1990 (EI
Contents
Page
iii
Foreword. .
1
1 Scope. .
............................................. 1
2 Definitions and Symbols.
.....................................................
2.1 Definitions. 1
2
2.2 Symbols .
..................................................... 2
3 Basicequations
................................................... 2
3.1 State equation
3.2 Flow-rate under ideal conditions . 2
.................................... 4
3.3 Flow-rate under real conditions
4 Applications for which the method is suitable . 4
.................................. 4
5 Standard critical flow Venturi nozzles
5.1 General requirements . 4
4
5.2 Design .
Installation requirements . 6
6
6
6.1 General .
...............................................
6.2 Upstream Pipeline 6
............................................ 6
6.3 Large upstream space
........................................ 6
6.4 Downstream requirements
........................................... 6
6.5 Pressure measurement
..................................................... 7
6.6 Drain holes
.......................................
6.7 Temperature measurement. 7
............................................ 7
6.8 Density measurement
7 Calculation methods. . 8
.................................................. 8
7.1 Massflow-rate
7.2 Discharge coeff icient . 8
............................................. 8
7.3 Critical flow function
8
7.4 Real gas critical flow coefficient .
7.5 Conversion of measured pressure and temperature to Stagnation
conditions . 8
......................... 9
7.6 Maximum permissible downstream pressure
...........................
8 Uncertainties in the measurement of flow-rate 9
9
8.1 General .
8.2 Practical computation of uncertainty . IO
Annexes
................................... 11
A Venturi nozzle discharge coeff icients
12
B Tables of values of the critical flow function C, for various gases .
........... 14
C Computation of real gas critical flow coefficient for natura1 gases
D References from which the Standard critical flow Venturi nozzle discharge
............................................ 15
coefficients were obtained
16
E Bibliography .
0 ISO 1990
All rights reserved. NO part of this publication may be reproduced or utilized in any form or by any
means, electronie or mechanical, including photocopying and microfilm, without permission in
writing from the publisher.
International Organization for Standardization
Case postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
ii

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SIST EN ISO 9300:1998
ISO 9300 : 1990 (E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of
national Standards bodies (ISO member bedies). The work of preparing International
Standards is normally carried out through ISO technical committees. Esch 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, govern-
mental and non-governmental, in Iiaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission (IEC) on all
matters of electrotechnical standardization.
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.
International Standard ISO 9300 was prepared by Technical Committee ISO/TC 30,
Measurement of fluid flow in closed conduits.
Annexes A, B and C form an integral part of this International Standard. Annexes D
and E are for information only.

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SIST EN ISO 9300:1998
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SIST EN ISO 9300:1998
INTERNATIONAL STANDARD ISO 9300 : 1990 IE)
Measurementof gas fl’ow by means of critical flow
Venturi nozzles
1 Scope internal surface of the conduit. The tapping is achieved such
that the pressure within the hole is the static pressure at that
This International Standard specifies the geometry and method
Point in the conduit.
of use (installation in a System and operating conditions) of
critical flow Venturi nozzles used to determine the mass flow-
2.1.1.2 ’ static pressure of a gas: Actual pressure of the
rate of a gas flowing through a System. lt also gives the infor-
flowing gas which tan be measured by connecting a pressure
mation necessary for calculating the flow-rate and its
gauge to a wall pressure tapping.
associated uncertainty.
lt applies to Venturi nozzles in which the gas flow accelerates to
NOTE - Only the value of the absolute static pressure is used in this
the critical velocity at the throat (this being equal to the local International Standard.
sonic velocity). At the critical velocity, the mass flow-rate of the
gas flowing through the Venturi nozzle is the maximum poss-
2.1.1.3 Stagnation pressure of a gas: Pressure which
ible for the existing upstream conditions.
would exist in the gas in a flowing gas stream if the stream were
brought to rest by an isentropic process.
This International Standard is applicable only where there is
steady flow of Single-Phase gases. The critical flow Venturi
NOTE - Only the value of the absolute Stagnation pressure is used in
nozzles dealt with tan only be used within specified limits, e.g.
this International Standard.
limits for the nozzle throat to inlet diameter ratio and throat
Reynolds number. lt deals with Venturi nozzles for which direct
calibration experiments have been made in sufficient number
2.1.2 Temperature measurement
and quantity to enable inherent Systems of application to be
based on their results and to enable coefficients to be given
2.1.2.1 static temperature of a gas : Actual temperature of
with cet-tain predictable limits of uncertainty.
the flowing gas.
The Venturi nozzles specified in this International Standard are
NOTE - Only the value of the absolute static temperature is used in
called “Primar-y devices”. The other instruments necessary for
this International Standard.
the measurement of the flow-rate are known as “secondary
devices”. This International Standard principally covers primary
devices; secondary devices are discussed only occasionally.
2.1.2.2 Stagnation temperature of a gas : Temperature
which would exist in the gas in a flowing gas stream if the
Information is given in this International Standard for cases where
stream were brought to rest by an isentropic process.
a) the Pipeline upstream of the Venturi nozzle is of circular
NOTE - Only the value of the absolute Stagnation temperature is used
Cross-section, or
in this International Standard.
b) it tan be assumed that there is a large space upstream
of the Venturi nozzle.
2.1.3 Critical flow nozzles
2.1.3.1 Venturi nozzle : Convergentldivergent restriction
2 Dbfinitions and Symbols
inserted in a System, intended for the measurement of flow-
rate.
2.1 Definitions
For the purposes of this International Standard, the following
2.1.3.2 throat : Section of minimum diameter of a Venturi
definitions apply.
nozzle.
2.1 .l Pressure measurement
2.1.3.3 critical Venturi nozzle : Venturi nozzle for which the
2.1.1.1 wall pressure tapping : Hole drilled in the wall of a nozzle geometrical configuration and conditions of use are
conduit in such a way that the edge of the hole is flush with the such that the flow-rate is critical.
1

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SIST EN ISO 9300:1998
ISO 9300 : 1990 (El
.
2.1.4 Flow 2.1.4.7 real gas critical flow coefficient, CR : Alternative
form of the critical flow function, more convenient for gas mix-
tures. lt is related to the critical flow function as follows:
2.1.4.1 mass flow-rate, qm: Mass of gas per unit time pass-
ing through the Venturi nozzle.
c 21’2
CR= +
NOTE - In this International Standard, the term flow-rate always
2.1.4.8 critical pressure rat-io, r,: Ratio of the absolute
refers to mass flow-rate.
static pressure of the gas at the nozzle throat to the absolute
Stagnation pressure for which the gas mass flow-rate through
2.1.4.2 throat Reynolds number, Red: Dimensionless
the nozzle is a maximum.
Parameter calculated from the gas velocity, the gas density at
the nozzle throat and the gas dynamic viscosity at nozzle inlet
2.1.4.9 back-pressure ratio : Ratio of the absolute nozzle
Stagnation conditions. The characteristic dimension is taken as
exit static pressure to the absolute nozzle upstream Stagnation
the throat diameter at working conditions. The throat Reynolds
pressure at which the flow becomes critical.
number is given by the formula
2.1.4.10 Mach number, Ma, (at nozzle upstream static con-
ditions) : Ratio of the mean axial fluid velocity to the velocity of
Sound at the inlet of the Venturi nozzle.
2.1.4.3 isentropic exponent, K : Ratio of the relative vari-
2.1.4.11 compressibility factor, 2 : correction factor
ation in pressure to the corresponding relative Variation in
expressing numerically the deviation from the ideal gas law of
density under elementary reversible adiabatic (isentropic) trans-
the behaviour of a real gas at given pressure and temperature
formation conditions :
conditions. lt is defined by the formula
Q c2 PM
Q
=-
z=-------
K
P P @RT
where R, the molar gas constant, equals 8,314 3 J/(moLK).
where
p is the absolute static pressure of the gas;
2.1.5 uncertainty : Estimate characterizing the range of values
within which the true value of a measurand lies, at 95 % probability.
Q is the density of the gas;
In some cases, the confidence Ievel whichcan be attached to
c is the local Speed of Sound;
this range of values will be greater than 95 %, but this will be so
only where the value of a qua.ntity used in the calculation of
the subscript S means “at constant entropy”.
flow-rate is known with a confidence level in excess of 95 %; in
such a case, reference should be made to ISO 5168.
For an ideal gasl), K is equal to the ratio of specific heat
capacities y and is equal to 5/3 for monatomic gases, 7/5 for
diatomic gases,‘9/7 for triatomic gases, etc.
2.2 Symbols
The Symbols used in this International Standard are specified in
2.1.4.4 discharge coefficient, C: Dimensionless ratio of the
table 1.
actual flow-rate to the ideal flow-rate that would be obtained
with one-dimensional isentropic flow for the same upstream
Stagnation conditions. This coefficient corrects for viscous and
3 Basic equations
flow field cun/ature effects. For the nozzle design and instal-
lation conditions specified in this International Standard, it is a
3.1 State equation
function of the throat Reynolds number only.
The behaviour of a real gas tan be described by the formula
2.1.4.5 critical flow : Maximum flow-rate for a particular
(RlM) TZ
Pl@ =
Venturi nozzle which tan exist for the given upstream con-
ditions. When critical flow exists the throat velocity is equal to
the Iocal value of the Speed of Sound (acoustic velocity), the 3.2 Flow-rate under ideal conditions
velocity at which small pressure disturbances propagate.
For ideal critical flow-rates to exist, three main conditions are
necessa ry :
2.1.4.6 critical flow function, C, : Dimensionless function
which characterizes the thermodynamic flow properties of an a) the flow is one-dimensional;
isentropic and one-dimensional flow between the inlet and the
b) the flow is isentropic;
throat of a Venturi nozzle. lt is a function of the nature of the
gas and of Stagnation conditions (see 3.2). c) the gas is perfett (i.e. 2 = 1 and K = y).
1) 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 volu me occupied by the molecules tan be neglected.
2

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SIST EN ISO 9300:1998
ISO 9300 : ,199O (EI
Table 1 - Symbols
Symbol - Quqntity
Dimensionsl) SI unit
Cross-sectional area of Venturi nozzle exit
L* m*
A2
Cross-sectional area of Venturi nozzle throat
A* L* d
C Discharge coeff icient dimensionless
Real gas critical flow coefficient (for one-dimensional flow of a real gas)
dimensionless
CR
Critical flow function (for one-dimensional flow of a real gas)
dimensionless
C*
Critical flow function (for one-dimensional isentropic flow of a pet-fect gas)
c*i dimensionless
D Diameter of upstream conduit
L
m
d Diameter of Venturi nozzle throat
L m
E Relative uncertainty
dimensionless
2) ~
e Absolute uncertainty
M Molar mass
M kg kmol-’
Mach number at nozzle inlet static conditions
dimensionless
Ma1
Absolute static pressure of the gas at nozzle inlet ML-1 T-2
Pa
Pl
Absolute static pressure of the gas at nozzle exit ML-’ T-2 Pa
P2
Absolute Stagnation pressure of the gas at nozzle inlet ML-’ T-2 Pa
po
Absolute static pressure of the gas at nozzle throat
ML-1 T-2 Pa
P*
Absolute static pressure of the gas at nozzle throat for one-dimensional isentropic flow ML-’ T-2
Psi Pa
of a per-fett gas
Ratio of nozzle exit static pressure to nozzle inlet Stagnation pressure for one-
(P2IP()) i dimensionless
dimensional isentropic flow of a perfett gas
Mass flow-rate
MT-’ kg-s-’
4 m
Mass flow-rate for one-dimensional isentropic flow of an inviscid gas MT-’ kg-s-’
qrni
ML2 T-2 a-1
Universal gas constant
R J l kmol-’ K-1
Nozzle throat Reynolds number dimensionless
Red
j Radius of curvature of nozzle inlet L m
rc
i
j Ctit~ca& pressure ratio QI&
dimensionless
r*
Absolute stagna&on temperature of the gas at nozzle inlet 0
K
%
Absolute static temperature of the gas at nozzle inlet 0 K
Tl
Absolute static temperature of the gas at nozzle throat 0 K
Ti
Throat sonic flow velocity; critical flow velocity at the throat LT-’
m-s-’
“*
z Compressibility factor dimensionless
Diameter ratio dlD dimensionless
ß
Ratio of the specific heat capacity at constant pressure cp to the specific heat capacity at dimensionless
Y
constant volume cy
dimensionless
K Isentropic exponent
Dynamit viscosity of the gas at Stagnation conditions at nozzle inlet ML-’ T-1 Pas
PO
Dynamit viscosity of the gas at nozzle throat ML-’ T-1 Pas
Pu,
ML-3 kg-m-3
Gas density at Stagnation conditions at nozzle inlet
@O
Gas density at nozzle throat ML-3 kgmm-3
@*
= mass; L = length; T = time; 0 = temperature.
1) M
2) The dimension of this Parameter is the dimension of the quantity to which it relates.
3

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SIST EN ISO 9300:1998
ISO 9300 : 1990 (EI
Under these conditions, the critical flow-rate is given by nozzle, the flow is directly proportional to the nozzle upstream
Stagnation pressure and not, as in the case of the subsonic
meter; tobthe Square root of a measured differential pressure.
The maximum flow range which tan be obtained for a given
critical Venturi nozzle is generatly limited to the range of inlet
pressures which are available above the inlet pressure at which
= A*C,i(fJO@O)“2
the flow becomes critical.
qmi
where
The most common applications to date of critical flow Venturi
nozzles have been for tests, calibration and flow control.
Standard critical f.low Venturi nozzles
33 . Flow-rate under real conditions
5.1 General requirements
For fJow-rates under real conditions, the formulae for critical
flow-rates become
5.1 .l The Venturi nozzle shall be inspected to determine that
it conforms with the requirements of this International Stan-
A*CC*Po
4m = dard.
w?lM) TOI u2
or 5.1.2 The Venturi nozzle shall be manufactured from material
suitable for the intended application. Some considerations are
qm= A*CC~(PO@O11’2
that
since
a) it should be possible to finish the material to the re-
quired condition; some materials are unsuitable owing to
c*zp
CR =
the inclusion of pits, voids and other inhomogeneities,
where Z. is the value of the compressibility factor at Stagnation
b) the material, together with any su tface treatment u sed,
conditions at nozzle inlet:
shall not be subject to corrosion in the intended Service, and
= P(+k@ To
20
c) the material should be dimensionally stable and should
have known and repeatable thermal expansion charac-
It should be noted that C, and CR are not equal to C,i because
teristics (if it is to be used ata temperature other than that at
the gas is not perfett, C is less than unity since the flow is not
which the throat diameter has been measured) so that the
one-dimensional and a boundary layer exists owing to viscous
appropriate throat diameter correction tan be made.
effects.
5.1.3 The throat and toroidal inlet up the conical divergent
section of the Venturi nozzle shall be smoothly finished so that
4 Applications for which the method is
the arithmetic average roughness R, does not exceed
suitable
15 x IO-6d.
Esch application should be evaluated to determine whether a
critical flow Venturi nozzle or some other device is the most
5.1.4 The throat a nd to roidal inlet up the conical divergent
suitable. An important consideration is that the flow through
section sha II be free from dirt, films or other contamination.
the Venturi nozzle be independent of the downstream pressure
(see 7.6) within the pressure range for which the Venturi nozzle
5.1.5 The form of the conical divergent section of the Venturi
tan be used for critical flow measurement.
nozzle shall be checked to ensure that any Steps, disconti-
nuities, irregularities and lack of concentricity do not exceed
Some other considerations are as follows.
1 % of the local diameter. The arithmetic average roughness R,
of the conical divergent section shall not exceed IO-4d.
For critical flow Venturi nozzles the only measurements re-
quired are the gas pressure and the gas temperature or density
upstream of the critical Venturi nozzle since the throat con-
5.2 Design
ditions tan be calculated from thermodynamic considerations.
There are two designs of Standard Venturi nozzles, i.e. the
The velocity in the critical Venturi nozzle throat is the maximum
toroidal throat Venturi nozzle and the cylindrical throat Venturi
possible for the given upstream Stagnation conditions, and
nozzle.
therefore the sensitivity to installation effects is minimized
except for those of swirl which shall not exist in the inlet part of
5.2.1 Toroidal throat Venturi nozzle
the Venturi nozzle.
sonic Venturi nozzles with subsonic pressure-
When comparing 5.2.1.1 The Venturi nozzle shall conform with the specifi-
it tan be noted that in the case of the critical cations shown in figure 1.
differente meters
4

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SIST EN ISO 9300:1998
ISO 9300: 1990 (El
0,9Dtol,l D
0
Met plane
f-Y
t
12,5Oto 6’
I ntersection of toroidal
surface and divergent section
In this region the arithmetic
average roughness R, of the
1) P, pressure measurement. surface shall not exceed 10S4d
3) In this region the arithmetic average roughness R, shall not
Inlet surface shall lie
exceed 15 x IO-W and the contour shall not deviate from
within the hatched zone
toroidal form by more than It: 0,001 d.
Figure 1 - Toroidal throat Venturi nozzle
5.2.1.2 For purposes of locating other elements of the Venturi 5.2.2.3 The convergent section of the Venturi nozzle (inlet)
nozzle critical flow metering System, the inlet plane of the
shall be a quarter of a torus tangential on one hand to the inlet
Venturi nozzle is defined as that plane perpendicular to the axis plane (see 5.2.2.2) and on the other hand to the cylindrical
of symmetry which intersects the inlet at a diameter equal
throat. The length of the cylindrical throat and the radius of
to 2,5d + 0,l d. curvature rC of the quarter of torus shall be equal to the throat
diameter.
5.2.1.3 The convergent section of the Venturi nozzle (inlet)
5.2.2.4 The inlet toroidal surface of the Venturi nozzle shall
shall be a Portion of a torus which shall extend through the
not deviate from the shape of a torus by more than * 0,001 d.
minimum area section (throat) and shall be tangential to the
divergent section. The contour of the inlet upstream of the inlet
plane (see 5.2.1.2) is not specified, except that the surface at
5.2.2.5 The flow-rate shall be calculated from the mean
each axial location shall have a diameter equal to or greater
diameter at the cylindrical throat outlet section. The mean
than the extension of the toroidal contour.
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
5.2.1.4 The toroidal sur-face of the Venturi nozzle located
more than + 0,001 d from the mean diameter.
between the inlet plane and the divergent section (see figure 1)
shall not deviate from the shape of a torus by more+ than
The length of the throat shall not deviate from the throat
+ 0,001 d. The radius of curvature rC of this toroidal surface in a
diameter by more than 0,05d.
plane in which the axis of symmetry lies shall be 1,8d to 2,Zd.
The connection between the quarter of torus and the cylindrical
throat shall be inspected visually and no defect should be
5.2.1.5 The divergent section of the Venturi nozzle
observed. When a defect of connection is observed, it shall be
downstream of the Point of tangency with the torus shall form
checked that the local radius of curvature in a plane in which
a frustum of a cone with a half-angle between 2,5O and 6O. The
the axis of symmetry lies is never less than 0,5d throughout the
length of the divergent section shall be not less than the throat
inlet surface (quarter of torus and cylindrical throat). The total
diameter.
area of the inlet surface shall be properly polished so that the
arithmetic average roughness R, does not exceed 15 x IO-Gd.
5.2.2 Cylindrical throat Venturi nozzle
The connection between the cylindrical throat and the
divergent section shall also be visually inspected and no defect
5.2.2.1 The Venturi nozzle shall conform with the specifi-
shall be observed.
cations shown in figure 2.
l
5.2.2.6 The divergent section of the Venturi nozzle comprises
5.2.2.2 The inlet plane is defined as that plane which is
a frustum of a cone with a half-angle between 3O and 4O. The
tangential to the inlet contour of the Venturi nozzle and perpen- length of the divergent section shall be not less than the throat
dicular to the nozzle centre-line.
diameter.

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SIST EN ISO 9300:1998
ISO 9300 : 1990 (EI
1) In this region the arithmetic average roughness R, of the sutface shall not
exceed 15 x IO-Gd and the contour shall not
deviate from toroidal and cylindrical form by more than I!I 0,001 d.
In the conical divergent section the arithmetic average roughness R, shall not exceed IO-4d.
2)
Figure 2 - Cylindrical throat Venturi nozzle
6 Installation requirements the primary device or to the inlet plane of the primary device, as
defined in 5.2.1.2 or 5.2.2.2.
6.1 General
6.4 Downstream requirements
This International Standard applies
to the installation of
Venturi nozzles when either
No requirements are imposed on the outlet conduit except that
it shall not restritt the flow so as to prevent critical flow in the
a) the Pipeline U m of the Venturi nozzle is of circular
Venturi nozzle.
Cross-section, 0
b) it tan be assumed that there is a large
space U m
6.5 Pressure measurement
of the Venturi nozzle.
6.5.1 When a circular conduit is used upstream of the primary
For case a), the primary device shall be installed in a System
device the upstream static pressure shall preferably be
meeting the requirements of 6.2. For case b), the Primar-y
measured at a wall pressure tapping at a distance 0,9D to 1 ,l D
device shall be installed in a System meeting the requirements
from the inlet plane of the Venturi nozzle (see figure 1). The
of 6.3. In both cases, swirl shall not exist upstream of the Venturi
wall pressure tapping may be located upstream or downstream
nozzle. Where a Pipeline exists upstream of the nozzle, swirl-free
of this Position provided that it has been demonstrated that the
conditions tan be ensured by installing a flow straightener of the
measured pressure tan be used reliably to give the nozzle inlet
design shown in figure 3 at a distance I, > 50 upstream of the
Stagnation pressure.
nozzle inlet plane.
6.5.2 When it tan be assumed that there is a large space
6.2 Upstream Pipeline
upstream of the primary device the upstream wall pressure
The primary device may be installed in a straight circular con- tapping shall preferably be located in a wall perpendicular to the
inlet face of the primary device and within a distance of
duit which shall be concentric within + 0,020 with the centre-
IOd + 1 d from that plane. The wall pressure tapping may be
line of the Venturi nozzle. The inlet conduit up to 30 upstream
of the Venturi nozzle shall not deviate from circularity by more located upstream or downstream of this Position provided that
than 0,Ol D and shall have an arithmetic average roughness R, it has been demonstrated that the measured pressure tan be
used reliably to give the nozzle inlet Stagnation pressure.
which shall not exceed 10-40. The diameter of the inlet con-
duit shall be a minimum of 4d.
6.5.3 For the wall pressure tapping mentioned in 6.5.1, and
63 . Large upstream space preferably also for that mentioned in 6.5.2, the centre-line of
the wall pressure tapping shall meet the centre-line of the
lt tan be assumed that there is a large space upstream of the Primar-y device and be at right angles to it. At the Point of the
primary device if there is no wall closer than 5d to the axis of
breakthrough, the hole shall be circular. The edges shall be free

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SIST EN ISO 9300:1998
-
0,9D to IJ D
12> D
, Nozzle inlet
{plane
Vane thickness to be adequate
to prevent buckling
1) P, pressure measurement.
2) T, temperature measurement.
3) In this region the surface roughness shall not exceed IO-W.
Figure 3 - Installation requirements for an upstream pipework configuration
from burrs, and shall be Square or lightly rounded to a radius through these drain holes while the flow measurement is in pro-
not exceeding 0,l times the diameter of the wall pressure tap- gress. If drain holes are required they shall be located upstream
ping. lt shall be confirmed by visual inspection that the wall of the nozzle upstream wall pressure tapping. The diameter of
pressure tappings comply with these requirements. When an the drain holes should be smaller than 0,06D. The axial distance
upstream Pipeline is used the diameter of the wall pressure tap-
from the drain hole to the plane of the upstream wall pressure
ping shall be less than 0,080 and preferably less than 12 mm. tapping shall be greater than D and the hole shall be located in an
The wall pressure tapping shall be cylindrical for a minimum axial plane different from that of the Wall pressure tapping.
length of 2,5 times the diameter of the tapping (see figure 4).
6.7 Temperature measurement
D ( < 12 mm preferably)
The inlet temperature shall be measured using one or more sen-
sors located upstream of the Venturi nozzle. When an
upstream Pipeline is used the recommended location of these
Sensors is 1,8D to 2,ZD upstream of the inlet plane of the Ven-
...