ISO/TR 12767:2007

TECHNICAL ISO/TR
REPORT 12767
Second edition
2007-09-01
Measurement of fluid flow by means of
pressure differential devices —
Guidelines on the effect of departure from
the specifications and operating
conditions given in ISO 5167
Mesurage du débit des fluides au moyen d'appareils déprimogènes —
Lignes directrices relatives aux effets des divergences par rapport aux
spécifications et aux conditions de fonctionnement données dans
l'ISO 5167
Reference number
©
ISO 2007
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ii © ISO 2007 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 2
4 Symbols and abbreviated terms . 2
5 Effect of errors on flowrate calculations. 3
5.1 General. 3
5.2 Quantifiable effects. 3
6 Effects of deviations in construction. 4
6.1 Orifice-plate edge sharpness . 4
6.2 Thickness of orifice edge. 5
6.3 Condition of upstream and downstream faces of orifice plate. 6
6.4 Position of pressure tappings for an orifice . 6
6.5 Condition of pressure tappings . 7
7 Effects of pipeline near the meter . 7
7.1 Pipe diameter . 7
7.2 Steps and taper sections . 8
7.3 Diameter of carrier ring . 8
7.4 Undersize joint rings . 11
7.5 Protruding welds. 11
7.6 Eccentricity. 11
8 Effects of pipe layout. 14
8.1 General. 14
8.2 Discharge coefficient compensation. 14
8.3 Pressure tappings. 16
8.4 Devices for improving flow conditions. 17
9 Operational deviations . 17
9.1 General. 17
9.2 Deformation of an orifice plate. 17
9.3 Deposition on the upstream face of an orifice plate. 19
9.4 Deposition in the meter tube . 23
9.5 Orifice-plate edge sharpness . 23
9.6 Deposition and increase of surface roughness in Venturi tubes . 24
10 Pipe roughness . 26
10.1 General. 26
10.2 Upstream pipe . 27
10.3 Downstream pipe. 30
10.4 Reduction of roughness effects. 30
10.5 Maintenance . 30
Bibliography . 32

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
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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 12767 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 12767:1998), which has been technically
revised.
iv © ISO 2007 – All rights reserved

Introduction
ISO 5167 (all parts) specifies methods for flowrate measurement using pressure differential devices.
Adherence to ISO 5167 (all parts) results in flowrate measurements whose uncertainty lies within specified
limits. If, however, a flow-metering installation departs, for whatever reason, from the conditions specified in
ISO 5167 (all parts), the specified limits of uncertainty may not be achieved. Many metering installations exist
where these conditions either have not been or cannot be met. In these circumstances, it is usually not
possible to evaluate the precise effect of any such deviations. However, a considerable amount of data exists
which can be used to give a general indication of the effect of non-conformity to ISO 5167 (all parts), and it is
presented in this Technical Report as a guideline to users of flow-metering equipment.

TECHNICAL REPORT ISO/TR 12767:2007(E)

Measurement of fluid flow by means of pressure differential
devices — Guidelines on the effect of departure from the
specifications and operating conditions given in ISO 5167
1 Scope
This Technical Report provides guidance on estimating the flowrate when using pressure differential devices
constructed or operated outside the scope of ISO 5167.
Additional tolerances or corrections cannot necessarily compensate for the effects of deviating from ISO 5167
(all parts). The information is given, in the first place, to indicate the degree of care necessary in the
manufacture, installation and maintenance of pressure differential devices by describing some of the effects of
non-conformity to the requirements; and in the second place, to permit those users who cannot comply fully
with the requirements to assess, however roughly, the magnitude and direction of the resulting error in
flowrate.
Each variation dealt with is treated as though it were the only one present. Where more than one is known to
exist, there may be unpredictable interactions and care has to be taken when combining the assessment of
these errors. If there is a significant number of errors, means of eliminating some of them have to be
considered. The variations included in this Technical Report are by no means complete and relate largely to
examples with orifice plates. An example with Venturi tubes has been placed at the end of its section. There
are, no doubt, many similar examples of installations not conforming to ISO 5167 (all parts) for which no
comparable data have been published. Such additional information from users, manufacturers and any others
may be taken into account in future revisions of this Technical Report.
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 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 5167-1 and the following apply.
3.1
square edge
angular relationship between the orifice bore of the flow-measurement device and the upstream face, when
the angle between them is 90° ± 0,3°
3.2
sharpness
radius of the edge between the orifice bore of the flow-measurement device and the upstream face
NOTE The upstream edge of the orifice bore is considered to be sharp when its radius is not greater than 0,000 4d,
where d is the diameter of the orifice bore.
4 Symbols and abbreviated terms
For the purposes of this Technical Report, the symbols given in Table 1 apply.
Table 1 — Symbols and units
Dimensions
M: mass
Symbol Quantity represented SI units
L: length
T: time
c Percentage change in discharge coefficient [≡∆100( CC/ )] dimensionless
C Discharge coefficient dimensionless
C Contraction coefficient dimensionless
c
d Diameter of orifice or throat of primary device at operating conditions L m
D Upstream internal pipe diameter at operating conditions L m
D Carrier ring diameter L m
D Orifice-plate support diameter L m
e Relative uncertainty dimensionless
E Orifice-plate thickness L m
E Orifice thickness L m
e
k Uniform equivalent roughness L m
Distance of upstream pressure tapping from upstream face of plate divided by
L dimensionless
pipe bore, D
Distance of downstream pressure tapping from downstream face of plate
′
L dimensionless
divided by pipe bore, D
−1
q Mass rate of flow MT kg/s
m
r Orifice-plate edge radius L m
Re Reynolds number based on throat bore of device dimensionless
d
Re Reynolds number based on upstream pipe diameter dimensionless
D
−1
u Local axial velocity LT m/s
−1
u Centreline axial velocity LT m/s
CL
−1
U Mean axial velocity LT m/s
−1 −2
Y Modulus of elasticity of orifice-plate material ML T Pa
β Diameter ratio, (= d/D) dimensionless
−1 −2
∆p Differential pressure ML T Pa
−1 −2
∆p Differential pressure required to reach orifice-plate yield stress ML T Pa
y
2 © ISO 2007 – All rights reserved

Table 1 (continued)
Dimensions
M: mass
Symbol Quantity represented SI units
L: length
T: time
ε Expansibility (expansion) factor dimensionless
λ Friction factor dimensionless
−3 3
ρ Fluid density ML kg/m
−3 3
ρ Fluid density at the upstream pressure tapping ML kg/m
−1 −2
σ Yield stress of orifice-plate material ML T Pa
y
5 Effect of errors on flowrate calculations
5.1 General
In this Technical Report, the effects of deviations from the conditions specified in ISO 5167 (all parts) are
described in terms of changes in the discharge coefficient, ∆C, of the meter. The discharge coefficient, C, of a
pressure differential device is given by Equation (1):
4(q 1− β)
m
C = (1)
επ∆ d (2 pρ )
The sharp edge of an orifice plate ensures separation of the flow and consequently contraction of the fluid
stream to the vena contracta. Defining the contraction coefficient, C , as the ratio of the flow area to the
c
geometric area the orifice produces C ≈ 0,6, which mainly accounts for the discharge coefficient, C ≈ 0,6.
c
The effect of change in the discharge coefficient is illustrated by the following example.
Consider an orifice plate with an unduly rounded edge. The result of this will be to reduce the separation and
C
increase , leading in turn to reduced velocities at the vena contracta. The observed differential pressure will
c
therefore decrease. From Equation (1), it can be seen that the discharge coefficient would therefore
increase.
C
Alternatively, as increases, so does C. If no correction is made for this change in C, the meter reading will
c
.
be less than the actual value
It can therefore be concluded that:
a) an effect which causes an increase in discharge coefficient will result in a flowrate reading lower than the
actual value if the coefficient is not corrected;
and conversely,
b) an effect which causes a decrease in discharge coefficient will result in a flowrate reading higher than the
actual value if the coefficient is not corrected.
5.2 Quantifiable effects
When the user is aware of such effects and they can be quantified, the appropriate discharge coefficient can
be used and the correct flowrate calculated. However, the precise quantification of these effects is difficult and
so any flowrate calculated in such a manner should be considered to have an increased uncertainty.
Except where otherwise stated, an additional uncertainty factor, equivalent to 100 % of the discharge
coefficient correction, should be added arithmetically to that of the discharge coefficient when estimating the
overall uncertainty in the flowrate measurement.
6 Effects of deviations in construction
6.1 Orifice-plate edge sharpness
Orifice plates that do not have the specified sharpness of the inlet edge (edge radius r u 0,000 4d in
accordance with 5.1.7.2 of ISO 5167-2:2003), will have progressively increasing discharge coefficients as the
edge radius increases. Tests have shown that the effect on the discharge coefficient, C, is to increase it by
0,5 % for r/d of 0,001, and by about 5 % for r/d of 0,01. This is an approximately linear relationship (see
Figure 1 and Reference [1]). These values apply particularly to Re values above 300 000 and for β values
d
below 0,7, but they can be used as a general guide for other values.
Measurement techniques for edge radius are available, but in general it is better to improve the edge
sharpness to the required value rather than to attempt to measure it and make appropriate corrections.
The effect of nicks in orifice plates has also been measured in Reference [1].

Key
1 National Engineering Laboratory (NEL, UK) tests —– D = 300 mm
2 ISO limit —– r = 0,000 4d
3 others
4 NEL
5 D = 50 mm (Reference [56])
6 D = 100 mm (Reference [56])
7 D = 150 mm (Reference [34])
8 D = 75 mm (Reference [57])
9 D = 100 mm (Reference [58])
c change in discharge coefficient
r/d radius ratio
Figure 1 — Effect of edge radius on discharge coefficient
4 © ISO 2007 – All rights reserved

6.2 Thickness of orifice edge
For orifice plates, the increase in discharge coefficient due to excessive thickness of the orifice edge (see
5.1.5 of ISO 5167-2:2003) can be appreciable. With a straight-bore orifice plate in a 150 mm pipe, the
changes in discharge coefficient shown in Figure 2 were obtained (see Reference [2]).

Key
1 section of an orifice plate
2 symbol
3 limit of standard
c change in discharge coefficient
E /D orifice thickness to upstream internal pipe diameter ratio
e
Figure 2 — Change in discharge coefficient as a function of orifice thickness
6.3 Condition of upstream and downstream faces of orifice plate
The upstream face should be flat and smooth. Excessive roughness leads to an increase in the discharge
coefficient. Tests have indicated that a surface roughness of 0,000 3d will cause an increase in discharge
coefficient of the order of 0,1 %. Since the requirement for edge sharpness is r u 0,000 4d, an increase in
plate roughness will make it difficult to define the edge sharpness or to confirm that the sharp edge
requirement has been met.
Local damage to the upstream face or edge of an orifice plate does not adversely affect the discharge
coefficient, provided that the damage is kept as far away from the pressure tapping as possible (see
Reference [1]). The discharge coefficient is much less sensitive to the surface condition of the downstream
face of the plate (Reference [1]).
Large-scale lack of flatness, e.g. “dishing”, leads to flow-measurement errors. A “dishing” of 1 % in the
direction of flow causes the reading to be below the actual value, i.e. an increase in C of about 0,2 % for
β = 0,2 and of about 0,1 % for β = 0,7. Distortion against the direction of flow also causes errors which could
be either positive or negative depending on the amount of distortion.
6.4 Position of pressure tappings for an orifice
6.4.1 General
Values of the orifice-plate discharge coefficient for the three standard tapping positions (corner, flange, D
and D/2) can be calculated using Equation (4) of ISO 5167-2:2003 (see Reference [55]). Where the tapping
positions fall outside the tolerances permitted in ISO 5167-2 for the three positions, the discharge coefficient
may be estimated as described in 6.4.2. It should be emphasized that an additional uncertainty factor needs to
be associated with the use of non-standard tapping positions.
6.4.2 Calculation of discharge coefficient
′
Calculate the actual values of L and L . The discharge coefficient can be estimated only if L u 1 and
1 2 1
′
L u 0,47.
′
Using the actual values of L and L , estimate the discharge coefficient using Equation (4) of
1 2
ISO 5167-2:2003.
6.4.3 Estimation of additional uncertainty
If tappings lie between the flange and the corner tappings, the additional uncertainty, e, expressed as a
percentage, can be estimated from:
C
F
e = 25  −1 (2)
C
CT
where
C is the discharge coefficient for flange tappings;
F
C is the discharge coefficient for corner tappings.
CT
If tappings lie between the D and D/2 tappings and the flange tappings, the additional uncertainty, e,
expressed as a percentage, can be estimated from:
C
DD and / 2
e = 25  −1 (3)
C
F
6 © ISO 2007 – All rights reserved

where
C is the discharge coefficient for D and D/2 tappings.
D and D/2
6.4.4 Example
Consider an orifice meter with β = 0,6, Re = 10 , D = 250 mm and tappings at 0,15D upstream and
D
downstream of the plate.
′
To estimate the discharge coefficient, use Equation (4) of ISO 5167-2:2003 with L = L = 0,15.
1 2
The tappings in this example lie between the flange tapping and D and D/2 tapping positions. From
Tables A.8 and A.2, respectively, of ISO 5167-2:2003: C = 0,605 1; C = 0,607 0. Therefore
F D and D/2
0,605 1
e =−25 1= 0,078
0,607 0
The uncertainty in the discharge coefficient is 0,5 % (see 5.3.3.1 of ISO 5167-2:2003).
Therefore, overall uncertainty is 0,5 + 0,078 ≈ 0,6 % (i.e. the uncertainties have simply been added together).
6.5 Condition of pressure tappings
Experience has shown that large errors can be created by pressure tappings which have burrs or deposits on,
or close to, the edge where the tapping penetrates the pipe wall. This is particularly the case where the
tappings are in the main flow stream, such as throat tappings in nozzles or Venturi tubes, where small burrs
can give rise to significant percentage errors. Upstream corner tappings and downstream tappings in relatively
dead zones are much less liable to cause this problem.
The installation shall be inspected before use and at regular intervals to ensure that these anomalies are not
present.
7 Effects of pipeline near the meter
7.1 Pipe diameter
The internal diameter of the pipe upstream and downstream of the primary device should always be measured
to ensure that it is in accordance with 6.4 of ISO 5167-2:2003, 6.4 of ISO 5167-3:2003 or 6.4.1 of
ISO 5167-4:2003. Errors in the upstream internal diameter measurement cause errors in the calculated
flowrate, which are given by:
δq
−2 δD
β
m
=  (4)
q D
(1− )
β
m
These errors become significant for large β, e.g. with β = 0,75, a positive 1 % error in D will cause a negative
1 % error in q .
m
The downstream pipe is far less critical, as for an orifice plate, an ISA 1932 nozzle or a long radius nozzle its
diameter need only be within 3 % of that of the upstream pipe (see 6.4.6 of ISO 5167-2:2003 or 6.4.6 of
ISO 5167-3:2003) and for a Venturi nozzle or a Venturi tube its diameter need only be W 90 % of the diameter
at the end of the divergent section (see 6.4.6 of ISO 5167-3:2003 or 6.4.1.3 of ISO 5167-4:2003).
7.2 Steps and taper sections
Sudden enlargements of the pipe in the vicinity of the primary device should always be avoided as large errors
in flow measurement result from their use. Similarly, tapering sections of pipe can lead to significant errors, as
can be seen from Table 2 which gives the order of errors to be expected when an orifice plate with corner
tappings is immediately preceded or followed by a taper piece.
The information in Table 2 indicates that a taper piece divergent in the direction of flow, and placed
immediately upstream, is not recommended, since discharge-coefficient increases of up to 50 % result. On the
other hand, a convergent taper piece, whether installed before or after the orifice plate, and provided it is not
of a steeper angle than those shown, results in coefficient changes of generally less than 2 %.
Table 2 — Effect of taper pieces
Order of the discharge coefficient
Position of orifice plate β
change to be expected
%
a) Immediately downstream from a divergent taper piece
0,4 + 10
0,7 + 50
b) Immediately downstream from a convergent taper piece
0,4 − 0,5
0,7 − 2
c) Immediately upstream from a convergent taper piece
0,4 0 to − 1
0,7 + 1
7.3 Diameter of carrier ring
The requirements for the sizing and concentric mounting of carrier rings for orifice plates and nozzles are
specified in 6.4 and 6.5 of ISO 5167-2:2003, 6.4 and 6.5 of ISO 5167-3:2003 and Figure 4 of ISO 5167-2:2003.
If the requirement of 6.5.4 of ISO 5167-2:2003 and 6.5.4 of ISO 5167-3:2003 (i.e. that the centred carrier ring
should not protrude into the pipe) is not met, relatively large flow-measurement errors will be introduced.
Figure 3 shows such an installation and Figure 4, using the same notation, shows the approximate errors
introduced for the given conditions, where a is the width of the portion of the carrier ring upstream of the
upstream face of the orifice plate or nozzle. It is emphasized that in arriving at these errors, the internal carrier
ring diameter, D , and not the diameter of the main line, has been used in determining the calculated flowrate
and is to be used for D in determining the correction factor when making use of the values shown.
Where the carrier is oversize, experimental results indicate that for β = 0,74 a carrier 11 % oversize and
extending 0,05D upstream from the plate increased the discharge coefficient by approximately 0,5 %.
However, for a similar geometry but with β = 0,63, no effect was found.
8 © ISO 2007 – All rights reserved

a)  Orifice plate
b)  Nozzle
Key
1 flow
Figure 3 — Carrier having internal diameter, D , less than pipe diameter, D
a)  Orifice plate
b)  Nozzle
Key
1 a = 0,2D to 0,3D
c change in discharge coefficient
β diameter ratio
Figure 4 — Effect of incorrect carrier diameter
10 © ISO 2007 – All rights reserved

7.4 Undersize joint rings
When the inside diameter of a joint ring or gasket is smaller than the pipe diameter, especially on the
upstream side of an orifice plate or nozzle, very large flow-measurement errors may occur. The magnitude
and sign of the effect in relation to the measurement of flowrate is dependent on the combination of a number
of variables, e.g. the thickness of the joint ring upstream of the orifice plate, the extent of its protrusion into the
flow, its position relative to the orifice plate and pressure tappings, and the degree of roughness of the
upstream pipe.
7.5 Protruding welds
The effect of an undressed circumferential weld protruding into the pipe bore adjacent to the primary device
will be similar to that of an undersize joint ring. Such an effect may arise from the fitting of a weld-neck flange,
and the magnitude of the effect will depend on the height uniformity, or otherwise, of the protruding weld, and
its position in relation to the single or multiple pressure tapping arrangement employed to measure the
differential pressure across the primary device. To quantify the resulting error in a specific situation is difficult
without a direct calibration.
It should be noted that seamed pipe may be used, provided that the internal weld bead is parallel to the pipe
axis throughout the entire length of the pipe required, to satisfy the installation requirements for the primary
device being used. Any weld bead shall not have a height greater than the permitted step in diameter. Unless
an annular slot is used, the seam shall not be situated within any sector of ± 30° centred on any individual
pressure tapping to be used in conjunction with the primary device. If an annular slot is used, the location of
the seam is not significant. If spirally wound pipe is used, then it shall be machined to a smooth bore.
(See 7.1.4 of ISO 5167-1:2003.)
7.6 Eccentricity
The requirements for concentric mounting of the device are given in 6.5.3 and 6.5.4 of ISO 5167-2:2003, 6.5.3
and 6.5.4 of ISO 5167-3:2003 and 6.4.3 of ISO 5167-4:2003. The geometric measure of eccentricity is the
distance between the pipe and orifice-plate centrelines and is often expressed as a percentage of the pipe
diameter, D. Deviations from the permitted eccentricity values for the mounting of an orifice plate relative to
the upstream and downstream pipe sections will result in errors in the measurement of flowrate.
Figure 5 shows the eccentric mounting of an orifice plate in a sideways direction relative to the upstream
pipeline. The displacement is to the right and the eccentricity is a combination of the dimensional tolerances
arising from the bolt-hole pitch-circle diameter, the bolt diameter, the bolt-hole diameter and the outer
diameter of the orifice plate.
Experimental evidence on the effects of eccentricity is limited, but it has been shown that for orifice plates, the
effect on discharge coefficient is a function of β, pipe size and roughness, pressure-tapping type, location and
magnitude, as well as the position of the orifice centre relative to the pressure tapping.
Experimental work indicates that the errors due to eccentricity increase in general with β. For β = 0,2 and
eccentricity up to 5 % of D, discharge coefficient increases are unlikely to exceed 0,1 %. For larger β, the
changes are best shown graphically as in Figure 6.
Below 3 % eccentricity, the error varies with type of tappings and direction of eccentricity. The meter is least
sensitive to eccentricity perpendicular to the tappings. Above 3 % eccentricity, errors for all tappings and
directions increase rapidly.
NOTE No data are available for corner tappings, but the errors are probably similar to those for flange tappings since
the above data were obtained from a test line with D = 150 mm.
A further effect of eccentric positioning of an orifice plate is an increased unsteadiness of the differential
pressure signal obtained. Observations have shown, for example, a marked increase in differential pressure
reading fluctuations with increasing eccentricity for all values of β between 0,4 and 0,7.
Because of the number of variants contributing to the effect of eccentricity on the measurement of flow, the
effect is difficult to quantify. Every effort should be made to restrict eccentricity to less than 3 % of D,
particularly in the direction of the tappings.
The effect may be minimized by employing four equally-spaced upstream and downstream tappings on the
flowmeter, as illustrated in Figure 1 of ISO 5167-1:2003. The pressure lines from these are then coupled in the
widely used triple-T tapping arrangement in order to obtain an average differential pressure reading.
As a general guide, it may be assumed that the effects of eccentric mounting for multi-tapped nozzles will be
less than those for orifice plates of equivalent β. Venturi tubes are less likely to be installed off-centre.
NOTE Combined installation faults: it is recommended that errors arising from the combined effects of eccentricity,
carrier ring steps, etc., are not taken into account additively. The total possible error will be governed by the strongest of
the effects present.
Key
1 bolt-hole pitch circle
2 flange centreline
3 orifice bore
4 orifice-plate outside diameter
5 flange bore
6 pipe inside diameter
7 pipe centreline
8 orifice centreline
9 eccentricity
Figure 5 — Possible orifice-plate eccentricity resulting from specified tolerances on bolt hole, bolt
hole pitch circle, pipe outside diameter and flange bore
12 © ISO 2007 – All rights reserved

a)  β = 0,75 b)  β = 0,66
c)  β = 0,57
Key
1 D and D/2 tappings
2 flange tappings
3 ± 0,3 %
4 (away from tapping 1) ← → (towards tapping 1)
5 ± 0,5 %
6 ± 0,7 %
c change in discharge coefficient
x eccentricity
Figure 6 — Discharge coefficient error vs. eccentricity for an orifice plate with D and D/2
and flange tappings
8 Effects of pipe layout
8.1 General
Minimum values of the straight lengths required between the primary device and various upstream fittings are
given in 6.2 of ISO 5167-2:2003, 6.2 of ISO 5167-3:2003, and 6.2 of ISO 5167-4:2003. Minimum straight
lengths are given both for zero additional uncertainty and for 0,5 % additional uncertainty in the discharge
coefficient.
When the minimum requirements for even 0,5 % additional uncertainty cannot be satisfied, the user should
make a correction to compensate for the change in the discharge coefficient and should also increase the
value of the percentage uncertainty.
Corrections and additional uncertainties for square-edged orifice plates with corner, flange and D and D/2
tappings are given in Tables 3 and 4 for a variety of upstream pipe bends and fittings. Shifts in columns 4
and 5 are particularly variable, depending on the exact details of the double bend.
Additional data on shifts in orifice-plate discharge coefficients for a large number of upstream fittings are given
in References [3-6].
8.2 Discharge coefficient compensation
8.2.1 Corrections
The discharge coefficient can be corrected using the data in Table 3 as illustrated in the following examples:
a) percentage change in coefficient is + 1,1 %, therefore the coefficient should be multiplied by 1,011;
b) percentage change in coefficient is − 2,3 %, therefore the coefficient should be multiplied by 0,977.
14 © ISO 2007 – All rights reserved

Table 3 — Percentage change in discharge coefficient, c, when the straight pipe lengths before the
orifice are less than those specified in ISO 5167-2
Upstream
straight β Type of fitting (for details of nomenclature, see key)
length
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
4D 0,5 −1,4 −1,4 −0,5 +2,9 +2,9 −0,4 +8,2 — +0,2 +0,2 −1,0 −0,8 +0,3 +0,5 +0,2 — —
0,6 −2,3 −2,2 −1,1 +1,7 +1,3 −1,2 +8,5 — −0,2 −0,3 −2,4 −1,7 +0,3 0 −0,2 — —
0,7 −3,8 −3,2 −1,8 +0,1 +0,4 −2,1 +8,2 — −0,9 −0,7 −4,4 −2,3 +0,3 −0,6 0 — —
b b b b b
0,8 −5,6 — −2,6 −2,4 — −3,1 +3,4 — −2,2 — −7,5 — +0,3 −1,3 — — —
a
8D 0,5 −0,7 −0,7 −0,3 +2,4 +2,4 0 +6,3 +6,4 −0,2 −0,2 −0,6 −0,4 — −0,2 −0,2 −0,8 −0,7
a
0,6 −1,4 −1,2 −0,7 +1,4 +1,2 −0,7 +5,6 +6,1 −0,6 −0,4 −1,3 −1,2 — −0,7 −0,8 −1,3 −1,2
0,7 −2,2 −1,9 −1,2 +0,3 +0,4 −1,3 +4,4 +6,1 −1,1 −0,8 −2,1 −1,9 +0,1 −1,2 −1,2 −1,7 −1,7
b b b b b b
0,8 −3,2 — −1,8 −1,7 — −2,0 +2,3 — −1,9 — −3,1 — +0,1 −1,8 — −2,0 −2,1
a a a a c
12D 0,5 — — — +2,0 +2,0 0 +5,5 +5,5 -0,2 −0,1 −0,4 −0,3 — −0,3 −0,2 — —
a c
0,6 −0,8 −0,8 −0,4 +1,2 +1,0 −0,4 +3,9 +4,3 −0,4 −0,3 −0,9 −0,9 — −0,7 −0,6 −0,8 −0,8
a
0,7 −1,4 −1,4 −0,8 +0,3 +0,3 −0,8 +2,6 +3,2 −0,8 −0,7 −1,3 −1,3 — −1,1 −1,0 −1,2 −1,1
b b b b b b
0,8 −2,0 — −1,3 −1,3 — −1,3 +1,5 — −1,3 — −1,7 — — −1,5 — −1,5 −1,4
a a a a
16D 0,5 — — — +1,7 +1,7 0 +5,1 +5,0 −0,1 0 −0,2 −0,2 — −0,2 −0,2 — —
a a a
0,6 — — −0,3 +1,1 +0,9 −0,3 +3,5 +3,6 −0,3 −0,2 −0,6 −0,6 — −0,4 −0,4 — —
a
0,7 −0,8 −0,8 −0,5 +0,3 +0,3 −0,5 +2,1 +2,4 −0,5 −0,5 −0,9 −1,0 — −0,7 −0,6 −0,9 —
b b b b b b
0,8 −1,3 — −0,7 −1,1 — −0,8 +0,8 — −0,8 — −1,0 — — −1,0 — −1,2 —
a
Refer to Table 3 of ISO 5167-2:2003.
b
For D and D/2 tappings, discharge coefficient changes measured for β > 0,75 should not be used for interpolation to give discharge
′
coefficient changes for β u 0,75, as the downstream tapping is in the pressure recovery region if L > 2(1 − β).
c
For a concentric expander 0,5D to D over a length of D to 2D refer to Table 3 of ISO 5167-2:2003.
Key
No. Type of upstream fitting Type of No. Type of upstream fitting Type of
tappings tappings
1 Single short radius 90° bend Corner, flange 10 Butterfly valve, fully open D and D/2
2 Single short radius 90° bend D and D/2 11 Butterfly valve, 52° open Corner, flange
3 Two 90° bends in the same plane, U- or S- All 12 Butterfly valve, 52° open D and D/2
configuration, 0D to 10D spacer
4 Two 90° bends at right angles, no spacer Corner, flange 13 Gate valve, fully open All
5 Two 90° bends at right angles, no spacer D and D/2 14 Gate valve, ⅔ open Corner, flange
6 Two 90° bends at right angles, 5D to 11D All 15 Gate valve, ⅔ open D and D/2
spacer
7 Two 90° mitre bends at right angles, no spacer Corner, flange 16 Gate valve, ¼ open and globe valve All
8 Two 90° mitre bends at right angles, no spacer D and D/2 17 Symmetrical enlargement, tapered or abrupt All
9 Butterfly valve, fully open Corner, flange
Table 4 — Formulae for additional uncertainty in the orifice discharge coefficient, to be used with the
percentage changes given in Table 3, for all tapping arrangements
Type of upstream fitting Additional uncertainty formulae
a
Piezometer ring Single tapping
e.g. Triple-T
Single short radius 90° bend. Bend radii 1D to 1,5D 0,5(1 + 0,6⏐c⏐) 0,5 + 0,6⏐c⏐
Two 90° bends, U- or S-configuration, in same plane 0,5(1 + ⏐c⏐) 0,5 + ⏐c⏐
Two 90° bends at right angles, no spacer (where X is the 0,5(1 + ⏐c⏐) + (10D/X) 0,5 + ⏐c⏐ + (10D/X)
distance from the orifice plate to the nearest bend)
Two 90° bends at right angles, 5D to 11D spacer 0,5 + ⏐c⏐ 0,5(1 + 3⏐c⏐)
Two 90° mitre bends at right angles, no spacer 0,5 + ⏐c⏐ 0,5(1 + 3⏐c⏐)
Butterfly valve, fully open 0,5 + ⏐c⏐ 0,5(1 + 3⏐c⏐)
Butterfly valve, 52° open 0,5 + ⏐c⏐ 0,5(1 + 3⏐c⏐)
Gate valve, fully open 0,5(1 + ⏐c⏐) 0,5 + ⏐c⏐
Gate valve, ⅔ open 0,5(1 + ⏐c⏐) 0,5 + ⏐c⏐
Gate valve, ¼ open and globe valve 0,5 + ⏐c⏐ 0,5 + ⏐c⏐
Symmetrical restriction or enlargement, tapered or abrupt 0,5 + ⏐c⏐ 0,5 + ⏐c⏐
a
The tapping axis should be at right angles to the plane of the nearest upstream bend.
8.2.2 Additional uncertainty
The formulae for calculating the additional percentage uncertainty in discharge coefficient are given in Table 4
for each type of fitting. This is in addition to the basic uncertainty in the discharge coefficient of: 0,5 % for
0,2 u β u 0,6, (1,667β − 0,5) % for 0,6 < β u 0,75. In deriving the formulae, the quantity of data, its
consistency and corroboration from different sources have been taken into account. Their use is illustrated in
the following examples.
a) If the equation to be applied is:
ec=+0,5 1 (5)
()
where ⏐c⏐ is the modulus of percentage change (i.e. the magnitude irrespective of sign) and if the change in the
coefficient is + 1,4 %, then e = 1,2 %.
b) If the equation to be applied is:
ec=+0,5 (6)
and if c = - 2,8 %, then e = 3,3 %.
8.3 Pressure tappings
It is emphasized that the change in the coefficient when D and D/2 tappings are used is often different from
those obtained with corner or flange tappings.
When the upstream straight pipe length is less than that required for zero additional uncertainty, it is
recommended that multiple tappings with triple-T connections, as shown in Figure 1 of ISO 5167-1:2003, be
used. If single tappings are used, their axes should be at right angles to the plane of the nearest upstream
bend.
16 © ISO 2007 – All rights reserved

8.4 Devices for improving flow conditions
Flow conditioners should be used where asymmetric or swirling flow has to be measured. Descriptions of
various flow conditioners are provided in Annex B of ISO 5167-2:2003. Even where the installation
requirements of ISO 5167-2:2003 [6.3 or Annex B (informative)] cannot be met, the use of a flow conditioner
may reduce errors especially in swirling flow.
9 Operational deviations
9.1 General
Metering systems that conform to ISO 5167 when new or recently maintained may be subject to a significant
degradation in accuracy over the passage of time.
This degradation may result from several causes:
a) deformation of the orifice plate;
b) deposition on the upstream face of an orifice plate;
c) deposition in the meter tube;
d) rounding of the orifice-plate edge;
e) deposition in the pressure tappings;
f) deposition and increase of surface roughness in a Venturi tube.
An indication of the effect of sources of error a) to d) and f) is given in 9.2 to 9.6.
It cannot be emphasized too strongly that the continued achievement of high accuracy requires the
expenditure of considerable effort. In particular, regular inspection and maintenance are essential. Inspection
periods depend on the nature of the fluid being metered and on the manner of operation of the system in
which the meter is installed, and can only be determined from experience.
9.2 Deformation of an orifice plate
9.2.1 General
An or
...