ISO/TR 9464:2008

TECHNICAL ISO/TR
REPORT 9464
Second edition
2008-05-15
Guidelines for the use of ISO 5167:2003
Lignes directrices pour l'utilisation de l'ISO 5167:2003

Reference number
©
ISO 2008
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ii © ISO 2008 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 2
4 How the structure of this guide relates to ISO 5167:2003 (all parts). 2
5 Guidance on the use of ISO 5157:2003 (all parts) . 2
5.1 Guidance specific to the use of ISO 5167-1:2003. 2
5.2 Guidance specific to the use of ISO 5167-2:2003. 8
5.3 Guidance specific to the use of ISO 5167-3:2003. 23
5.4 Guidance specific to the use of ISO 5167-4:2003. 24
6 Information of a general nature relevant to the application of ISO 5167:2003 (all parts). 25
6.1 Secondary instrumentation . 25
6.2 Measurement of pressure and differential pressure. 27
6.3 Measurement of temperature . 31
6.4 Determination of density. 35
6.5 Electrical supply and electrical installations . 40
Annex A (informative) Principles of measurement and computation. 41
Annex B (informative) Computation of compressibility factor for natural gases. 57
Annex C (informative) Orifice plate assembly. 59
Bibliography . 68

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.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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/TR 9464 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/TR 9464:1998), which has been technically
revised.
iv © ISO 2008 – All rights reserved

Introduction
The objective of this Technical Report is to assist users of ISO 5167, which was published in 2003 in four
parts. Guidance on particular clauses of ISO 5157:2003 is given.
Some clauses of ISO 5167:2003 (parts 1 to 4) are not commented upon and the corresponding clause
numbers are therefore omitted from this Technical Report, except when it has been thought to be useful to
keep a continuous numbering of paragraphs.

TECHNICAL REPORT ISO/TR 9464:2008(E)

Guidelines for the use of ISO 5167:2003
1 Scope
The objective of this Technical Report is to provide guidance on the use of ISO 5167:2003 (all parts).
ISO 5167:2003 is an International Standard for flow measurement based on the differential pressure
generated by a constriction introduced into a circular conduit (see ISO 5167-1:2003, 5.1). It presents a set of
rules and requirements based on theory and experimental work undertaken in the field of flow measurement.
For a more detailed description of the scope, reference should be made to ISO 5167-1:2003, Clause 1.
Definitions and symbols applicable to this Technical Report are given in ISO 5167-1:2003, Clauses 3 and 4.
Neither ISO 5167-1:2003 nor this Technical Report give detailed theoretical background, for which reference
should be made to any general textbook on fluid flow.
With the application of the rules and requirements set out in ISO 5167-1:2003, it is practicable to achieve flow
measurement within an uncertainty of approximately 1 % of the calculated flowrate. The constraints applicable
to each of the primary devices described in ISO 5167:2003 (parts 2 to 4) need to be given consideration
before determining the most suitable type for a particular application. Parts 2 to 4 can also be used to form the
basis for preliminary design of a metering system.
The information necessary for detailed design, manufacture and final check is specified in the clauses and
paragraphs of ISO 5167:2003 (parts 2 to 4).
Secondary instrumentation is not covered by ISO 5167-1:2003, but Clause 6 of this Technical Report makes
normative reference to ISO 2186.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 2186, Fluid flow in closed conduits — Connections for pressure signal transmissions between primary
and secondary elements
ISO/TR 3313:1998, Measurement of fluid flow in closed conduits — Guidelines on the effects of flow
pulsations on flow-measurement instruments
ISO 4006, Measurement of fluid flow in closed conduits — Vocabulary and symbols
ISO 5167-1:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full — Part 1: General principles and requirements
ISO 5167-2:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full — Part 2: Orifice plates
ISO 5167-3:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full — Part 3: Nozzles and Venturi nozzles
ISO 5167-4:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full — Part 4: Venturi tubes
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4006 apply.
4 How the structure of this guide relates to ISO 5167:2003 (all parts)
Clause 5 of this Technical Report sets out the guidance specific to each of the four parts of ISO 5167:2003:
⎯ 5.1 covers part 1;
⎯ 5.2 covers part 2;
⎯ 5.3 covers part 3;
⎯ 5.4 covers part 4.
Subsequent subclause numbering relates to the clauses in each of the parts. Hence, 5.1.1 covers Clause 1 in
part 1; 5.4.3.1.1 covers Subclause 3.1.1 in part 4.
Guidance applicable to all four parts is given in Clause 6.
5 Guidance on the use of ISO 5157:2003 (all parts)
5.1 Guidance specific to the use of ISO 5167-1:2003
5.1.1 Scope
No comments on this clause.
5.1.2 Normative references
No comments on this clause.
5.1.3 Terms and definitions
No comments on this clause.
5.1.4 Symbols and subscripts
No comments on this clause.
5.1.5 Principle of the method of measurement and computation
5.1.5.1 Principle of the method of measurement
No comments on this subclause.
2 © ISO 2008 – All rights reserved

5.1.5.2 Method of determination of the diameter ratio of the standard primary device
See Annex A of this Technical Report.
5.1.5.3 Computation of flowrate
The equations to be used to determine the flowrate of a measuring system are given in ISO 5167-1:2003,
Clause 5. Some results of these calculations will be fixed with installation dimensions and will only need to be
computed once. Other calculations will need to be repeated for every flow measurement point. Annex A gives
worked examples of the iterative computations shown in ISO 5167-1:2003, Annex A.
5.1.5.4 Determination of density, pressure and temperature
5.1.5.4.1 General
No comments on this subclause.
5.1.5.4.2 Density
For details on density measurement, see 6.4.
For details on density computation, see Annex B of this Technical Report.
5.1.5.4.3 Static pressure
No comments on this subclause.
5.1.5.4.4 Temperature
The computation of temperature decrease resulting from expansion of the fluid through the primary device
requires knowledge of the Joule-Thomson coefficient. The coefficient is a function of temperature, pressure
and gas composition. The calculation can be carried out using an equation of state (see, in Annex B, the
“detailed method” using molar composition analysis) or by the use of an approximation valid for natural gas
mixtures that are not too rich, and when p and T are in the range given below. In the last case, the coefficient
is a function of p and T alone.
Provided that, in the molar composition of the natural gas, methane is greater than 80 %, the temperature is in
the range 0 °C to 100 °C and the absolute static pressure is in the range 100 kPa to 20 MPa (1 bar to 200 bar).
µ=−0,35 0,001 42t
JT
(1)
+−0,231 0,002 94tt+ 0,000 0136 0,998+ 0,000 41p− 0,000 111 5p+ 0,000 000 3p
()( )
where
µ is the Joule-Thomson coefficient, in kelvin per bar (K/bar);
JT
t is the temperature of the fluid, in degrees Celsius (°C);
p is the absolute static pressure of the fluid, in bar.
The uncertainty was determined from the differences between this equation and the Joule-Thomson
coefficient of 14 common natural gases and is given by
⎛⎞t
U=−0,066 1 for pu70bar (7 MPa) (2)
⎜⎟
⎝⎠
and
⎡⎤
⎛⎞tt(290− )⎛⎞1 1
U=−0,066 1 1− − for p> 70 bar (7 MPa) (3)
⎢⎥
⎜⎟ ⎜⎟
200 4 70 p
⎝⎠⎢⎥
⎝⎠
⎣⎦
where U is the (expanded) uncertainty in the Joule-Thomson coefficient (K/bar).
NOTE If an orifice plate with β = 0,6 has a differential pressure ∆p = 0,5 bar, the uncertainty in the Joule-Thomson
coefficient corresponds to an uncertainty in flowrate in the range from 0,001 % to 0,009 %, depending on the temperature,
the pressure and the gas composition.
5.1.6 General requirements for the measurements
5.1.6.1 Primary device
5.1.6.1.1 No comments on this subclause.
5.1.6.1.2 No comments on this subclause.
5.1.6.1.3 Table 1, whilst not exhaustive, lists materials most commonly used for the manufacture of primary
devices.
Table 1 — Steels commonly used for the manufacture of primary devices
AISI BS 970 AFNOR DIN
304 304-S15 Z6CN18-09 1.4301
Stainless steels
316 316-S16 Z6CND17-11 1.4401
High elastic limit
420 420-S37 Z30C13
stainless steel
Table 2 gives the mean linear expansion coefficient, elasticity moduli and yield stresses for the materials of
Table 1 according to their AISI designation.
Table 2 — Characteristics of commonly used steels
Mean linear
expansion coefficient
Elasticity modulus Yield stress
AISI designation
between 0 °C and 100 °C
−1
K Pa Pa
−6 9 6
304 17 × 10 193 × 10 215 × 10
−6 9 6
316 16 × 10 193 × 10 230 × 10
−6 9 6
420 10 × 10 200 × 10 494 × 10
The values given in Table 2 vary with both temperature and the treatment process of the steel. For precise
calculations, it is recommended that the data are obtained from the manufacturer.
When the primary device under operating conditions is at a different temperature from the one at which the
diameter “d” was determined (this temperature is referred to as the reference or calibration temperature), the
expansion or contraction of the primary device should be calculated. The corrected diameter “d” to be used in
the computation of diameter ratio and flowrate should be calculated using Equation (4), assuming there is no
restraint due to the mounting:
4 © ISO 2008 – All rights reserved

dd=+[1 λ(]T−T ) (4)
0d 0
where
d is the primary device diameter in flowing conditions;
d is the primary device diameter at reference temperature;
λ is the mean linear expansion coefficient of the primary device material;
d
T is the primary device temperature in flowing conditions;
T is the reference or calibration temperature.
Where automatic temperature correction is not required in the flow computer, the uncertainty for “d ” included
in the overall uncertainty calculations should be increased for the change in “d ” due to temperature variation
(see ISO 5167-1:2003, 8.2.2.4). An initial calculation may show that this additional uncertainty is small enough
to be considered negligible.
5.1.6.2 Nature of the fluid
No comments on this subclause.
5.1.6.3 Flow conditions
5.1.6.3.1 No comments on this subclause.
5.1.6.3.2 If there is a likelihood of such a change of phase, a way of overcoming the problem is to increase
the diameter ratio, so that the differential pressure is reduced.
5.1.6.3.3 No comments on this subclause.
5.1.7 Installation requirements
5.1.7.1 General
The following list of inspection equipment is not exhaustive, but provides a basis for inspection control:
⎯ calipers (thickness, diameters);
⎯ internal micrometer (diameters);
⎯ micrometer (thickness);
⎯ gauge block, feeler gauge (relative position, absolute standard for checking micrometers);
⎯ protractor (angles);
⎯ profile measuring apparatus (edge);
⎯ straight edge rule (flatness);
⎯ three point bore gauge (internal diameter).
Only instruments which may be calibrated to primary standards should be used if optimum accuracy is
required.
5.1.7.1.1 No comments on this subclause.
5.1.7.1.2 No comments on this subclause.
5.1.7.1.3 No comments on this subclause.
5.1.7.1.4 No comments on this subclause.
5.1.7.1.5 No comments on this subclause.
5.1.7.1.6 The requirements in this subclause of ISO 5167-1, where drain or vent holes are located near to
the primary device, are illustrated in Figure 1. This figure illustrates the importance of placing the drain or vent
hole in the annular chamber where one is used. It should be noted that the location of a drain or vent hole
relative to a pressure tapping is of greater importance where there is no annular chamber and the drain or
vent hole enters the pipe itself.
It should be realized that the flowing fluid may cause deposition, corrosion or erosion of the inner wall of the
pipe. The installation may therefore not conform to the requirements of ISO 5167-1. Internal inspection of the
pipe should be carried out at intervals appropriate to the conditions of application.

Key
1 pressure tapping
2 orifice plate
3 drain holes and/or vent holes
a
Flow direction.
Figure 1 — Location of drain holes and/or vent holes
6 © ISO 2008 – All rights reserved

5.1.7.1.7 This subclause is intended to ensure a reliable measurement of temperature. Although the
flowing temperature is not a quantity directly involved in the equation for calculating flowrate, it is an important
parameter since it may be used to calculate “d ” and “D” plus critical process parameters under flowing
conditions.
5.1.7.2 Minimum upstream and downstream straight lengths
5.1.7.2.1 No comments on this subclause.
5.1.7.2.2 When designing a metering pipe installation, it is recommended that the required minimum
straight lengths are determined by the maximum diameter ratio that is expected in the life of the installation.
For diameter ratios not actually shown in ISO 5167-2:2003, Table 3, ISO 5167-3:2003, Table 3 or
ISO 5167-4:2003, Table 1 but which are inside the limits of the standard, it is reasonable practice to
interpolate linearly between the values obtained at the nearest two diameter ratios.
If an orifice meter is designed to measure the flowrate in either direction, the minimum straight lengths of pipe
on both sides of the orifice plate should conform to the minimum requirements for upstream and downstream
straight lengths as specified in ISO 5167-2:2003, 6.2 and Table 3.
5.1.7.3 General requirement for flow conditions at the primary device
No comments on this subclause.
5.1.7.4 Flow conditioners
It should be noted that although swirl is generally not detectable in visual inspection of the pipe, swirl and
asymmetry are sometimes visible in the coating, if present, on an orifice plate. A typical herring bone or
chevron pattern that may be seen on a plate that has been in service for some time may indicate that the flow
at the orifice plate may be swirling or asymmetrical. Swirl has a greater effect on measurement than any other
fluid dynamic mechanism and, although straight lengths of pipe will eliminate swirl, decay may occur very
slowly and the swirl may persist over considerable distances. The use of straight lengths of pipe to eliminate
swirl is questionable, especially in large pipe sizes, as the decay of induced swirl from common pipe
components may not be sufficient to ensure fully developed profiles within the minimum lengths required in
the tables.
Flow conditioners are strongly recommended for use downstream of a metering system header and in the
following circumstances:
a) where the upstream fittings or arrangement of fittings are not defined in the tables;
b) where a primary device of high β ratio is to be used for a given fitting, a flow conditioner which has passed
the compliance test may reduce the upstream length necessary to achieve a good velocity profile, or may
improve the velocity profile for a given straight length.
Many new flow conditioners have been developed since the previous edition of ISO 5167 published in 1991,
and ISO 5167-1:2003 describes compliance testing for flow conditioners.
Various flow conditioners and straighteners are described in ISO 5167-1:2003, Annex C and ISO 5167-2:2003,
Annex B, respectively. Not all of the conditioners described have been subjected to or have necessarily
passed the compliance testing procedure.
5.1.8 Uncertainty on the measurement of flowrate
In 1995, ISO in cooperation with BIPM, IEC, IFCC, IUPAC, IUAP and OIML published the Guide to the
expression of uncertainty in measurement (GUM). The content of this document and ISO 5168 should be
taken into account when performing uncertainty analyses.
Any manufacturer’s specification of error should be studied carefully to ensure that the limits of error are
known at the measured value concerned. Some points to note include the following:
a) uncertainties are often expressed as a percentage of full scale or range;
b) uncertainties are often defined at specified reference conditions. Additional uncertainties may arise when
operating conditions differ from reference conditions.
5.2 Guidance specific to the use of ISO 5167-2:2003
5.2.1 Scope
This part of ISO 5157:2003 is concerned solely with orifice plates and their geometry and installation. It is
necessary to read ISO 5167-2 in conjunction with ISO 5167-1.
Orifice plate meters with three arrangements of tappings are described and specified: flange tappings; corner
tappings; and D and D/2 tappings.
5.2.2 Normative references
No comments on this clause.
5.2.3 Terms, definitions and symbols
No comments on this clause.
5.2.4 Principles of the method of measurement and computation
The density and viscosity of the fluid can be measured (see 6.4) or calculated (see Annex B) from the gas
composition. A number of computer programs are available for carrying out the calculation of density and
viscosity. In the case of a compressible fluid, the isentropic exponent at working conditions is necessary for
the flow calculation and this can be calculated from gas composition.
5.2.5 Orifice plates
5.2.5.1 Description
5.2.5.1.1 General
No comments on this subclause.
5.2.5.1.2 General shape
5.2.5.1.2.1 No comments on this subclause.
5.2.5.1.2.2 No comments on this subclause.
5.2.5.1.2.3 Referring to Annex C, three factors need to be taken into consideration in designing an orifice
plate to avoid excessive deformation.
⎯ First, the mounting arrangements should not impose any forces on the orifice plate which would cause
the limit of 0,5 % slope given in ISO 5167-2:2003, 5.1.3.1 to be exceeded under the condition of no
differential pressure.
⎯ Secondly, the thickness of the plate, E, should be such that, taking account of the modulus of elasticity of
the plate material, the differential pressure for the maximum design flowrate should not cause a 1 % slope
to be exceeded. When the flowrate is reduced to zero, the plate will return to the original maximum 0,5 %
slope.
8 © ISO 2008 – All rights reserved

⎯ Thirdly, it is necessary to ensure that, if it is possible for differential pressures in excess of those for
maximum design flowrate to be applied, plastic buckling (i.e. permanent deformation) will not occur.
For the first point, great care is needed in both the design and manufacture of the mounting arrangements.
Single or double chamber mounting devices are satisfactory. When mounting orifice plates between standard
flanges, the flanges shall be at 90° ± 1° to the pipe axis. The pipe sections on both sides of the orifice plate
should be adequately supported to ensure that no undue strain is placed on the orifice plate.
For the second point, it should be understood that elastic deformation of an orifice plate introduces an error in
the flow measurement results. As long as the deformation does not exceed the 1 % slope required by
ISO 5167-2:2003, 5.1.2.3, no additional uncertainty will result. Theoretical and experimental research (see
Reference [13]) indicates that the maximum change in discharge coefficient for a 1 % slope is 0,2 %.
Therefore, orifice plates that conform to the 0,5 % slope specified in ISO 5167-2:2003, 5.1.3.1 can deform an
additional 0,5 % slope (i.e. 0,1 % change in discharge coefficient) whilst still conforming to the requirements of
this subclause. Table 3 tabulates the plate thickness to plate support diameter ratios (E/D') for various values
of β and differential pressures, valid for an orifice plate manufactured from AISI stainless steel 304 or 316, and
simply supported at its rim.
Table 3 — Minimum E/D' ratios for orifice plates manufactured in AISI 304 or AISI 316 stainless steel
∆p for maximum flowrate
β
kPa
10 30 50 75 100 200 400
0,2 0,009 0,011 0,013 0,014 0,014 0,016 0,018
0,3 0,010 0,013 0,015 0,016 0,017 0,020 0,022
0,4 0,010 0,014 0,016 0,018 0,019 0,022 0,025
0,5 0,010 0,014 0,016 0,018 0,020 0,023 0,027
0,6 0,010 0,014 0,016 0,018 0,019 0,023 0,026
0,7 0,009 0,012 0,014 0,016 0,017 0,020 0,024
0,75 0,008 0,011 0,013 0,014 0,016 0,018 0,021

Table 3 is based on the use of Equation (5) when 100 ∆q /q is not to exceed 0,1 in magnitude and
m m
E* = 193 × 10 Pa:
∆q ∆pD′′D
⎛⎞⎛ ⎞
m
100 =− ab− (5)
⎜⎟⎜ ⎟
*
qE E
E⎝⎠⎝ ⎠
m
where
a = β (13,5 − 15,5β);
1,3
b = 117 − 106 β ;
E* is the modulus of elasticity of plate material;
D' is the plate support diameter (this may differ from pipe bore D);
E is the plate thickness.
For the third point, the maximum differential pressure (which can be greater than ∆p in Table 3) that could be
applied has to be determined by the designer. This could occur when the metering section is isolated and then
vented to reduce it to atmospheric pressure to enable the orifice plate to be removed for inspection, or when
pressurizing the metering section before putting into service.
To avoid plastic deformation (buckling), the orifice plate thickness should be such that:
Ep∆
>−0,681 0,651β (6)
()
D′ σ
y
where
∆p is the maximum differential pressure determined by the designer, in pascals (Pa);
σ is the yield stress of the orifice plate material, in pascals (Pa).
y
NOTE 1 For stainless steel, σ = 300 MPa, but it is advisable to use a value of 100 MPa for design purposes.
y
The thickness of the orifice plate chosen should be whichever is the greater when determined by
Equations (5) and (6), but should not exceed the 0,05D required in ISO 5167-2:2003, 5.1.5.3. Should the
calculations indicate that the E required is greater than 0,05D, the designer should either reduce ∆p or else
introduce a stronger material.
EXAMPLE
⎯ Equation (5):
β = 0,2
E* = 193 GPa
∆p = 50 kPa (0,5 bar)
gives E/D′ > 0,013 from Equation (5) or Table 3.
⎯ Equation (6):
β = 0,2
σ = 300 MPa for stainless steel, but for design purposes it is advisable to use
y
σ = 100 MPa
y
∆p = 100 kPa (1 bar) (see NOTE 2)
gives E/D′ > 0,023.
Consequently, E/D′ should be at least 0,023.
NOTE 2 100 kPa (1 bar) is the maximum anticipated differential pressure.
5.2.5.1.3 Upstream face A
5.2.5.1.3.1 Table 4 gives values of deflection of the inner edge of the orifice corresponding to the 0,5 %
slope for various pipe diameters and diameter ratios, β, assuming the deformation is rectilinear.
10 © ISO 2008 – All rights reserved

Table 4 — Plate flatness tolerances
Nominal diameter of the measuring pipe in millimetres
β 50 100 200 300 400 500 600 700 800 900 1 000
Maximum deflection h in millimetres for 0,5 % slope
0,20 0,10 0,20 0,40 0,50 0,80 1,00 1,20 1,40 1,60 1,80 2,00
0,25 0,09 0,19 0,38 0,56 0,75 0,94 1,13 1,31 1,50 1,69 1,88
0,30 0,09 0,18 0,35 0,52 0,70 0,88 1,05 1,22 1,40 1,57 1,75
0,35 0,08 0,16 0,32 0,49 0,65 0,81 0,97 1,14 1,30 1,46 1,63
0,40 0,07 0,15 0,30 0,45 0,60 0,75 0,90 1,05 1,20 1,35 1,50
0,45 0,07 0,14 0,27 0,41 0,55 0,69 0,82 0,96 1,10 1,24 1,38
0,50 0,06 0,13 0,25 0,38 0,50 0,63 0,75 0,88 1,00 1,13 1,25
0,55 0,06 0,11 0,22 0,34 0,45 0,56 0,67 0,79 0,90 1,01 1,13
0,60 0,05 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00
0,65 0,04 0,09 0,18 0,26 0,35 0,44 0,52 0,61 0,70 0,79 0,88
0,70 0,04 0,07 0,15 0,22 0,30 0,38 0,45 0,52 0,60 0,67 0,75
0,75 0,03 0,06 0,13 0,19 0,25 0,31 0,38 0,44 0,50 0,56 0,63
Reference [13].
5.2.5.1.3.2 The roughness criterion in this subclause may not be adequate to ensure that the edge
−5
sharpness requirements of ISO 5167-2:2003, 5.1.7.2 can be achieved. It is recommended that Ra u 10 d
should be used. The roughness of the orifice bore should conform to the same criterion.
5.2.5.1.3.3 It is very important that the bevelled side of the plate (if applicable) is located downstream. If
the plate is inserted with the bevel upstream, the flowrate can be as much as 20 % underestimated. It should
be normal practice to mark the plate, if practical, to indicate the upstream face in such a way that the marking
can be seen when the plate is installed.
One common method of identifying the upstream face where the orifice plate is installed between flanges is to
install a “paddle plate” where the critical details are engraved on the handle which extends from the flange
joint.
In no circumstances should the upstream face of the orifice plate within diameter “D” be indented by any
marking.
5.2.5.1.4 Downstream face B
No comments on this subclause.
5.2.5.1.5 Thicknesses E and e
No comments on this subclause.
5.2.5.1.6 Angle of bevel
No comments on this subclause.
5.2.5.1.7 Edges G, H and I
5.2.5.1.7.1 No comments on this subclause.
5.2.5.1.7.2 The last paragraph of this subclause requires the edge radius to be measured should there
be any doubt that it conforms to the requirements of ISO 5167-2:2003, 5.1.7.1 and 5.1.7.2. In those
exceptional cases, some suitable techniques are given below.
a) Casting method (see Reference [8])
A replica of the edge is produced using a casting technique. The casting is made in two stages, firstly with
a coloured cold-forming plastic which takes up a negative form of the orifice plate edge, and then backed
with a semi-transparent epoxy resin taking the place of the orifice plate. The completed casting is cut into
two halves exposing the replica of the orifice plate edge, polished and photographed with magnification.
The edge condition can then be measured.
b) Lead foil impression method (see Reference [8])
An impression of the edge is made by pressing lead foil, 0,1 mm thick, onto the orifice plate edge. The
lead foil is held in a micrometer controlled inspection gauge and pressed onto the edge to give an
indentation 0,12 mm deep. The indentation is examined using a projection microscope or similar
equipment where the image is magnified, and a tracing of the outline is drawn. The edge condition can
then be measured.
c) Paper-recording roughness method (see Figure 2)
This instrument records on a magnified scale the movements of a tracing stylus. To obtain an enlarged
reproduction of the orifice edge, the paper speed should be chosen equal to the driving velocity times the
magnification of the transverse movements. To establish the correct edge radius of the orifice, the tip
radius of the stylus has to be subtracted from the edge radius measured from reproduction and divided by
the degree of magnification. It should be noted that the finite dimensions of the stylus, such as tip angle,
tip radius and stylus length, can invalidate the measurement or conceal irregularities on the edge.
When edge sharpness is to be measured, it should be in at least 4 positions, equally spaced around the bore.
When a defect is visible to the naked eye, the edge sharpness should also be measured at this point.
Interpretation of the edge profile, whatever the reproduction technique, is a matter of expert judgement.
Standard machining practice can cause the profile to be very irregular, even though the orifice plate conforms
to all the requirements for dimensions and surface roughness.
All edges lying within the shaded region of Figure 3, with an additional margin for surface roughness, can be
considered as acceptable. Some surface roughness is tolerable in accordance with ISO 5167-2:2003, 5.1.3.2,
but very irregular edges should be rejected.
A simple way of estimating the actual edge radius is by comparing the profile with curves (see examples in
Figure 4) reproduced on a transparent foil.
Edge sharpness measurement is a specialist activity. There are laboratories in many countries that are
capable of measuring edge sharpness to the standard required in ISO 5167-2:2003, 5.1.7.2. See
Reference [8].
12 © ISO 2008 – All rights reserved

Key
1 recorded movement
2 driving movement
3 stylus
4 traced path
5 square edge
6 R = edge radius
7 radius
Figure 2 — Paper-recording roughness method
Figure 3 — Maximum edge radius

Figure 4 — Edge radius curves
5.2.5.1.7.3 No comments on this subclause.
5.2.5.1.7.4 No comments on this subclause.
5.2.5.1.8 Diameter of orifice, d
5.2.5.1.8.1 Because of the uncertainty of the discharge coefficient, and strict requirements on eccentricity,
pipe roughness and upstream straight lengths, the user is advised to remain below a diameter ratio, β, of 0,6
for the most accurate measurements.
5.2.5.1.8.2 No comments on this subclause.
14 © ISO 2008 – All rights reserved

5.2.5.1.8.3 To enable the requirements of this subclause (i.e. 0,05 % difference) to be shown to have
been met, it is necessary to measure or compare with an uncertainty of at most 0,02 %.
5.2.5.1.9 Bidirectional plates
5.2.5.1.9.1 A symmetrical plate is intended to be used for the measurement of a fluid that may flow in
either direction. Such a plate should not be bevelled.
The thickness, E, of the plate should then not be greater than 0,02D. As a consequence, symmetrical plates
should only be used with low values of differential pressure to prevent deformation (see ISO 5167-2:2003,
5.1.2.3).
5.2.5.1.9.2 The appropriate tappings for the direction of flow should be used.
5.2.5.1.10 Material and manufacture
Subclause 5.1.6.1.3 gives some information on the most commonly used materials and their characteristics.
5.2.5.2 Pressure tappings
This subclause means that pressure tappings have to be installed as follows: at least one upstream tapping
and one downstream tapping of the same type, i.e. D and D/2, flange or corner (see ISO 5167-2:2003, 5.2.1).
Tappings of several types may be installed at the same location. In such cases, each type of tapping (each
“set”) will be totally independent from the others: the various sets should not interfere in any way and failure to
comply with this will result in an inaccurate measurement.
This implies that, on the same side of the orifice plate, several tappings should not lie on the same axial plane
(see Figure 5). Moreover, if they are of different types (e.g. flange and D and D/2), they should be offset by at
least 30°. If they are of the same type (e.g. all flange), then no guidance on the acceptable offset in terms of
angle is given. No tappings should affect the readings of any other tappings.
a)  Example of incorrect positioning

b)  Example of correct positioning
Key
1 pressure tappings
Figure 5 — Relative position of pressure tappings of different types
5.2.5.3 Coefficients and corresponding uncertainties of orifice plates
No comments on this subclause.
5.2.5.4 Pressure loss, ∆ϖ
Figure 5 in ISO 5167-2:2003 does not take account of frictional pressure losses in the pipe.
∆T, as shown, is appropriate for a gas metering system.
16 © ISO 2008 – All rights reserved

5.2.6 Installation requirements
5.2.6.1 General
No comments on this subclause.
5.2.6.2 Minimum upstream and downstream straight lengths for installation between various
fittings and the orifice plate
No comments on this subclause.
5.2.6.3 Flow conditioners
No comments on this subclause.
5.2.6.4 Circularity and cylindricality of the pipe
NOTE To conform to the given specifications, the pipe lengths adjacent to the primary device may have to be
specially machined. As no significant diameter difference may exist between the various lengths of the measuring pipe
(ISO 5167-2), the ones adjacent to the primary device may have to be made of a thicker pipe so that the correct internal
diameter can be obtained after machining a length of 2 pipe diameters upstream of the primary device. This method will
result in a measuring pipe having homogeneous dimensions.
5.2.6.4.1 A check should be made so that, over a length of 2D upstream of the primary device, any
diameter measured in any plane does not vary by more than 0,3 % from the mean diameter previously
obtained by ISO 5167-2:2003, 6.4.2.
In addition to the diameters measured in three cross-sections to establish “D”, additional diameters should be
measured in at least each of two different cross sections at locations dependent on the device to be installed:
⎯ 0,5D and 2D for orifice plates with D and D/2 pressure tappings;
⎯ D and 2D for orifice plates with corner and flange tappings.
In those cases where few cross-sections are used, a check should be made that no systematic variation of the
measured diameters can be found.
5.2.6.4.2 The value of “D”, corrected for thermal expansion (see below), is that used for the computation of
the diameter ratio. This value of “D” is also used as the basis for establishing the circularity of the pipe over a
length of at least 2D upstream and downstream of the primary device (see ISO 5167-2:2003, 6.4).
The distance to each of the measurement locations is expressed in terms of “D”, which is not known before
taking measurements at the prescribed locations. For the purpose of establishing the position of these
locations, it is permissible to take “D” as equal to the nominal bore of the pipe.
Figure 6 gives an example for orifice meters where diameters are measured in only three different cross-
sections:
⎯ A , B , C for orifice plates with corner tappings;
1 1 1
⎯ A , B , C for orifice plates with flange tappings;
2 2 2
⎯ A , B , C for orifice plates with D and D/2 tappings.
3 3 3
In any case, individual diameters should be measured with an accuracy of at least 0,1 %, as the overall
tolerance is 0,3 % (see ISO 5167-2:2003, 6.4.1).
When the measuring pipe under flowing conditions is at a significantly different temperature from the one at
which diameter D was determined (this temperature is referred to as the reference or calibration temperature),
the expansion or contraction of the pipe should be taken into account in the computation of diameter ratio and
flowrate, using Equation (7):
D=+DT[]1(λ −T) (7)
00D
where
D is the diameter of the pipe in flowing conditions;
D is the diameter of the pipe at reference temperature;
λ is the mean linear expansion coefficient of the pipe material;
D
T is the pipe temperature in flowing conditions;
T is the reference or calibration temperature.
The value for λ should be obtained from the manufacturer of the measuring pipe.
D
Where automatic temperature correction is not required in the flow computer, the uncertainty for “D” included
in the overall uncertainty should be increased for the change in “D” due to temperature variation (see
ISO 5167-1:2003, 8.2.2.4). An initial calculation may show that this additional uncertainty is small enough to
be considered negligible.
18 © ISO 2008 – All rights reserved

Dimensions in millimetres
Key
1 plate upstream face
Internal diameter D to be used in flowrate computation:
44 4
1⎡⎤
DD=+D+D
⎢⎥∑∑iA iB∑ iC
nn n
⎣⎦ii==11 i=1
n = 1 for corner tappings
n = 2 for flange tappings
n = 3 for D and D/2 tappings
Figure 6 — Measurement of internal diameter, D
5.2.6.4.3 It shall be noted that measuring the internal diameter at the ends of each section of pipe is not
sufficient to ensure conformity with ISO 5167-2:2003, 6.4.3. In addition, a check should be made to determine
that the different sections of pipe are properly mounted and do not have a step in excess of the limits given in
ISO 5167-2 when connected together. See Figure 7.
The use of self-centring pipe joints is recommended. Consideration should be given to the use of tongue and
groove flanges, male and female flanges, dowel pins or spigot and recess.
Check that the maximum internal step “e” between any two adjacent sections of pipe (A and B) more than two
pipe diameters upstream of the primary device does not exceed the required value in ISO 5167-2:2003, 6.4.3,
where D is the mean pipe diameter computed over 0,5D (see Figure 6).

Figure 7 — Inspection of measuring pipe sections
It is possible to determine the step between coupled pipe lengths with sufficient accuracy by fixing external
reference points whilst the pipe is uncoupled. Reference points can be on the extension of a matching piece
or plane and should be constructed in pairs, just over the joint, one on each side of it. Four or six pairs of
reference points equally spaced around the circumference of the pipe joint will usually be adequate.
The distance from the pipe wall to the reference point should be measured while uncoupled. To determine the
position of a reference point in space on the extension of a plane [Figure 8 a), left hand side], the plane should
be extended by a sliding reference piece.
Once coupled, the distance between two reference po
...