ISO/TR 11583:2012

TECHNICAL ISO/TR
REPORT 11583
First edition
2012-04-01
Measurement of wet gas flow by means
of pressure differential devices inserted
in circular cross-section conduits
Mesurage du débit de gaz humide au moyen d'appareils déprimogènes
insérés dans des conduites de section circulaire

Reference number
©
ISO 2012
©  ISO 2012
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ii © ISO 2012 – All rights reserved

Contents Page
Foreword . iv
Introduction . v
1  Scope . 1
2  Normative references . 1
3  Terms and definitions . 2
4  Symbols and subscripts . 2
5  Principle of the method of measurement and computation . 2
5.1  Principle of the method of measurement . 2
5.2  Computation . 4
6  Venturi tubes . 5
6.1  General . 5
6.2  Design requirements . 5
6.3  Pressure tappings . 5
6.4  Computation of gas flowrate . 6
6.5  Uncertainties . 8
7  Orifice plates . 9
7.1  General . 9
7.2  Design requirements . 9
7.3  Use of orifice plates with drain holes . 9
7.4  Pressure tappings . 9
7.5  Computation of gas flowrate . 10
7.6  Uncertainties . 12
8  Tracer techniques . 12
8.1  General . 12
8.2  Technique . 13
8.3  Measuring the gas flowrate using tracer techniques . 13
9  Comparison method . 14
10  Total mass flowrate known . 14
11  Using a throttling calorimeter . 15
12  Installation . 15
12.1  Flow conditioners . 15
12.2  Insulation . 15
12.3  Pressure tappings and impulse lines . 15
12.4  Gas composition . 16
12.5  Densitometers . 16
13  Sampling . 17
13.1  General . 17
13.2  Sampling points at the wet-gas meter . 17
13.3  Sampling points at test separators . 17
Annex A (informative) Calculations . 18
Bibliography . 25

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 11583 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed
conduits, Subcommittee SC 2, Pressure differential devices.
iv © ISO 2012 – All rights reserved

Introduction
ISO 5167-1:2003, ISO 5167-2:2003, and ISO 5167-4:2003 include specifications for Venturi tubes and orifice
plates, but are applicable only where the fluid can be considered as a single phase and the conduit is running
full.
If the fluid being measured is a wet gas there is an overreading which can be corrected using suitable wet-gas
correction equations.
TECHNICAL REPORT ISO/TR 11583:2012(E)

Measurement of wet gas flow by means of pressure differential
devices inserted in circular cross-section conduits
1 Scope
This Technical Report describes the measurement of wet gas with differential pressure meters. It applies to
two-phase flows of gas and liquid in which the flowing fluid mixture consists of gas in the region of 95 %
volume fraction or more (the exact limits on the mixture are defined in 6.4.3, 6.4.5, 7.5.3 and 7.5.5). This
Technical Report is an extension of ISO 5167. The ranges of gases and liquids from which the equations in
this Technical Report were derived are given in 6.4.1 and 7.5.1. It is possible that the equations do not apply
to liquids significantly different from those tested, particularly to highly viscous liquids.
Although the over-reading equations presented in this Technical Report apply for a wide range of gases and
liquids at appropriate gas-liquid density ratios, evaluating gas flowrates depends on information in addition to
that required in single-phase flow: under certain conditions, a measurement of the pressure loss is sufficient;
tracers can be used to measure the liquid flow; the total mass flowrate may be known (this is more likely in a
wet-steam flow than in a natural gas/liquid flow); in a wet-steam flow a throttling calorimeter can be used.
Wet-gas measurement using Venturi tubes or orifice plates is covered in this Technical Report.
This Technical Report is only applicable to wet gas flows with a single liquid and is not intended for the oil and
gas industry.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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 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 5167-4:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full — Part 4: Venturi tubes
ISO/TR 15377, Measurement of fluid flow by means of pressure-differential devices — Guidelines for the
specification of orifice plates, nozzles and Venturi tubes beyond the scope of ISO 5167
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4006, ISO 5167-1 and the following
apply.
3.1
stratified flow
common regime in horizontal pipes at low gas velocities (typically 5 m/s or less) in which the free liquid runs
along the bottom of the pipe with the gas flowing at the top of the pipe
3.2
annular flow
flow regime that in horizontal pipes occurs at medium gas velocities (typically 5 m/s to 15 m/s) in which the
liquid flows around the pipe wall with the gas flowing through the centre of the pipe
NOTE In horizontal pipes, annular flow is not uniform; owing to gravitational effects, the liquid is present in higher
quantities around the wall at the bottom of the pipe than higher up the pipe wall.
3.3
mist flow
flow regime that in horizontal pipes requires high gas velocities (typically 15 m/s or higher) to keep the liquid
suspended in the gas and describes liquid in the flow being carried along in small-droplet form within the body
of gas
3.4
slug flow
flow regime in which liquid travels along the pipe intermittently but in significant quantity, often due to the liquid
becoming trapped in the flow line, for example at the bottom of a vertical pipe or when the flow is started after
shutdown
3.5
liquid volume fraction
LVF
ratio of the liquid volume flowrate to the total volume flowrate, where the total volume flowrate is the sum of
the liquid volume flowrate and the gas volume flowrate, all volume flowrates being at actual (not standard)
conditions
3.6
gas volume fraction
GVF
ratio of the gas volume flowrate to the total volume flowrate, where the total volume flowrate is the sum of the
liquid volume flowrate and the gas volume flowrate, all volume flowrates being at actual (not standard)
conditions
4 Symbols and subscripts
See Table 1.
5 Principle of the method of measurement and computation
5.1 Principle of the method of measurement
The principle of the method of measurement using differential-pressure meters is based on the installation of a
primary device (such as an orifice plate or a Venturi tube) into a pipeline. The installation of the primary device
causes a pressure difference between the upstream side and the throat or downstream side of the device.
The flowrate can be determined from the measured value of this pressure difference and from the knowledge
2 © ISO 2012 – All rights reserved

of the characteristics of the flowing fluid as well as the circumstances under which the device is being used. It
is assumed that the device is geometrically similar to one on which calibration has been carried out and that
the conditions of use are the same, i.e. that it is in accordance with ISO 5167-2 or ISO 5167-4.
Table 1 — Symbols
a
Symbol Quantity Dimension SI Unit
C Coefficient of discharge dimensionless 1
C
Chisholm coefficient dimensionless 1
Ch
C
Concentration of tracer in fluid dimensionless 1
fluid
Diameter of orifice or throat of Venturi tube at working
d L m
conditions
Upstream internal pipe diameter (or upstream diameter of
D
L m
a Venturi tube) at working conditions
Fr
Gas densiometric Froude number [see Equation (3)] dimensionless 1
gas
2 2
g
Acceleration due to gravity LT m/s
2 2
h Specific enthalpy T J/kg
L
H
Function of the surface tension of the liquid (see 6.4.3) dimensionless 1
Distance between the downstream end of the Venturi tube
divergent section (measured from the end of the cone not
L
L m
down
the flange) and the downstream pressure tapping used to
measure the pressure loss
1 2
p Absolute static pressure of the fluid ML T Pa
1
q
Mass flowrate MT kg/s
m
3 1 3
q
Volume flowrate L T  m /s
V
t Temperature of the fluid  °C
X Lockhart-Martinelli parameter [see Equation (2)] dimensionless 1
 Diameter ratio:  = d/D dimensionless 1
1 2
p Differential pressure ML T Pa
Pressure loss (without correction for the pressure loss that
1 2
 would have taken place if the Venturi tube or orifice plate ML T Pa
had not been present)
b b
 Absolute uncertainty — —
 Expansibility [expansion] factor dimensionless 1
 Isentropic exponent dimensionless 1
Density of the fluid (subscript 1 denotes the value at the
3 3
 ML kg/m
upstream tapping plane)
 Over-reading correction factor [see Equation (1)] dimensionless 1

a
L ≡ length; M ≡ mass; T ≡ time;  ≡ temperature.

b
The dimensions and units are those of the corresponding quantity.

In a wet gas flow the gas flowrate is determined by evaluating an over-reading. The over-reading is due to the
mass of liquid passing through the primary device. The over-reading is affected by the flow regime, which in a
wet gas flow is generally stratified, annular or mist, although, in practice, wet gas flows may be a combination
of these flow regimes. Other flow regimes can occur intermittently, particularly the slug flow regime if liquid
has become trapped in the flow line, for example at the bottom of a vertical pipe.
Combinations of line conditions, pipe orientations, and gas-liquid ratios influence the type of flow regime
present. An appreciation of which, if any, flow regime is likely to prevail can be extremely useful. The
application of the same wet-gas measurement technique can produce widely different results depending on
which flow regime predominates, and knowledge of the likely flow regime can therefore influence the correct
choice of measurement principle to be applied.
NOTE Even in a horizontal pipe, liquid can be held-up by gas flows of 1 m/s or less and can remain almost stationary
rather than flow with the gas.
5.2 Computation
The gas mass flowrate, q , is given by
m,gas
2Δp
C π 1,gas
qd  (1)
m,gas
4 4 
1
where
C is given in 6.4.2 or 7.5.2 as appropriate;
 is determined from the appropriate part of ISO 5167;
 is the upstream gas density;
1,gas
 is the over-reading correction factor.
NOTE In evaluating , the actual values of p and p measured in wet gas are used.
1 2
Factor  depends on the primary device, on the gas-liquid density ratio,  / , where  is the
1,gas liquid liquid
density of the liquid, on the Lockhart-Martinelli parameter, X, as defined in Equation (2):
q 
m,liquid 1,gas
X (2)

q 
m,gas liquid

and on the gas densiometric Froude number, Fr , as defined in Equation (3):
gas
4q 
m,gas 1,gas
Fr  (3)
gas

 πDgD
liquid 1,gas
1,gas
where g is the acceleration due to gravity and q is the liquid mass flowrate.
m,liquid
4 © ISO 2012 – All rights reserved

6 Venturi tubes
6.1 General
Venturi tubes are widely used for wet-gas applications. Among their advantages are:
a) they do not ‘dam’ the flow (unlike orifice plates);
b) they can be operated at higher differential pressures than orifice plates without incurring permanent meter
damage [differential pressures up to and above 2 bar (200 kPa) can be contemplated; for a fixed gas
mass flowrate the presence of liquid may greatly increase the differential pressure];
c) therefore, they have a relatively high turndown (typically 10:1) when used with suitably ranged differential
pressure transmitters.
6.2 Design requirements
The design requirements for Venturi tubes are specified in ISO 5167-4. However, special attention should be
paid to the following: the finish of the Venturi tube internal surface, which should be smooth and free from
machining defects including burrs and ridges; the pressure tappings, which at the point of entry into the meter
internal bore should have sharp edges and be free from burrs and wire edges; and the edge of the conical
inlet, which should be sharp and free from manufacturing defects.
The equations in this Technical Report should only be applied to meters that have been installed horizontally.
Installation of the Venturi tube at a low point of the piping configuration where liquid could collect should be
avoided.
In respect of the number and location of the pressure tappings, the meter should not conform to ISO 5167-4;
see 6.3.
In many situations, it is desirable that the Venturi tube be installed with suitable “double block and bleed”
isolation valves so that the meter can be removed and inspected as required.
The presence of liquid in the flow line affects the flow profile as it enters the Venturi tube. This is a source of
measurement uncertainty over and above that normally expected for dry-gas measurement. In order to
minimize this additional uncertainty, upstream pipe work should be designed so that bends immediately
upstream of the meter encourage any stratified liquid to flow at the bottom of the pipe. Moreover, it is not
recommended that the reduced straight lengths outlined in ISO 5167-4 be used. Where possible, the longer
lengths should be used in order to minimize measurement uncertainty. The use of flow conditioners in wet-gas
applications is not recommended (see 12.1).
6.3 Pressure tappings
The meter should be installed horizontally with a single pair of pressure tappings. The recommended location
for the tappings circumferentially is given in 12.3.
Any double block and bleed valve fitted to the tappings should be a full-bore valve.The use of compact or
wafer double block and bleed valves introduces liquid traps into the impulse line.
In addition, a third pressure tapping may be located downstream of the Venturi conical expander outlet (the
diffuser section) to facilitate the measurement of the fully recovered pressure. The optimum position for this
third pressure tapping has not been definitively established, but is approximately 6D from the downstream end
of the divergent section.
The ratio of the pressure loss to the differential pressure can be much higher than in a single phase flow. This
ratio can be used under certain circumstances to determine the Lockhart-Martinelli parameter. Where the
liquid mass flowrate is only measured discontinuously, significant variations in this ratio can help indicate
when a new measurement of the liquid mass flowrate is required.
6.4 Computation of gas flowrate
6.4.1 General
The general model for the over-reading correction factor is reported in References [6] and [1]. Reference [3]
includes an improved correlation. Extensive research (References [22] to [28]) includes the collection of data
on which the equations in 6.4.2, 6.4.3 and 6.4.5 are based. Gas flowrate equations in this subclause appear in
Reference [19].
Further research into the use of Venturi tubes in wet gas is still required.
The range of gases and liquids in the database from which the gas flowrate equations in this subclause have
been derived is: nitrogen, argon, natural gas and steam; water (at ambient temperature and in a wet-steam
1)
flow), Exxsol D80 , Stoddard solvent (white spirit), and decane. It is possible that the equations do not apply
to liquids significantly different from those tested, particularly to highly viscous liquids.
The wet gas flowrate is calculated from Equation (1) where C and  are obtained from Equations (4) and (5),
respectively.
Examples of how to perform the computations are given in Annex A.
6.4.2 Discharge coefficient

X
CF = 10,046 3 exp 0,05r min 1, (4)
 

gas,th
 
0,016

where
Fr
gas
Fr 
gas,th
2,5

6.4.3 Over-reading correction factor
1 CXX (5)
Ch
where C is given by the following equation:
Ch
n n

 
liquid 1,gas
C
Ch


1,gas liquid

where
0,8Fr

gas
n = max0,5830,180,578 exp , 0,392 0,18


H



H depends on the liquid and is equal to 1 for hydrocarbon liquid, 1,35 for water at ambient temperature, and
0,79 for liquid water in a wet-steam flow. It is a function of the surface tension of the liquid.

1) Product available commercially. This information is given for the convenience of users of this document and does not
constitute an endorsement by ISO of this product.
6 © ISO 2012 – All rights reserved

Limits of use:
0,4    0,75
0 < X  0,3
Fr > 3
gas,th

1,gas
 0,02

liquid
D ≥ 50 mm
6.4.4 Determination of X
To perform the flowrate computation, X is required. This can be obtained by one of the following methods:
a) by measuring the liquid flowrate using tracer techniques (see Clause 8);
b) by comparing the results from the wet-gas meter with those from gas and liquid meters downstream of a
separator in series with the wet-gas meter;
c) by comparing the results with those from another wet-gas meter (see Clause 9);
d) by calculating from the known total mass flowrate (see Clause 10);
e) by using a throttling calorimeter in a steam/water flow (see Clause 11);
f) by using the third pressure tapping and applying an additional correlation (see 6.4.5).
6.4.5 Use of the pressure loss ratio to determine X
For a limited range of X, it is possible to use the pressure loss to determine the Lockhart-Martinelli parameter.
The formulae given here are valid for a Venturi tube with divergent total angle in the range 7° to 8°.
The pressure loss, , from the upstream pressure tapping to a tapping a distance L downstream of the
down
downstream end of the Venturi tube divergent section is measured. L should be such that
down
L
down
max 5, 207  9

D
Then evaluate (this is an iterative procedure)

Y 0,089 6 0,48
 p
and


 Fr
1,gas gas
Y0,61exp11 0,045

max


 H

liquid



If Y/Y  0,65, it is not possible to use the pressure loss ratio to determine X.
max
If Y/Y < 0,65, X is evaluated from
max
0,28Fr

Y gas
0,75
1 exp35X exp


YH

max


Limits of use in addition to those in 6.4.3:
Fr > 4
gas,th
Fr
gas
 5,5
H

1,gas
 0,09

liquid
These limits reflect the available data: see Reference [19].
Then  is obtained from 6.4.3 .
6.5 Uncertainties
The uncertainty, q , of the gas mass flowrate is given by
m,gas

qq(/C)
(/C )
mm,gas ,gas


qC//q C
mm,gas ,gas

where
(/C )
C /
the relative uncertainty of C/, is as given in Table 2 and
(/qC)
m,gas
qC/
m,gas
is obtained by considering Equation (1). The denominator, q /C, consists of the terms of Equation (1)
m,gas
excluding the factor C/, and thus the uncertainty of each term can be estimated either from ISO 5167 or from
calibration (see ISO 5167-1:2003, 8.2.2.1).
Table 2 — The relative uncertainty of C/ in Equation (1) for a Venturi tube using the equations in 6.4
Relative uncertainty of C/ in
Range of X or of Y/Y
max
Equation (1)
X  0,15 3%
X known without error
X > 0,15 2,5 %
Y/Y < 0,6
4%
max
X obtained from the formulae in 6.4.5
0,6  Y/Y < 0,65
6%
max
8 © ISO 2012 – All rights reserved

There are very limited data for wet-steam flow. Because of the uncertainty in the value of H, both  , the
0,94
value of  given H = 0,94, and  , the value of  given H = 0,79, should be calculated and
0,79


0,79 0,94
100 %


0,79

added to the relative uncertainty of C/.
7 Orifice plates
7.1 General
Orifice plates have been historically used for a wide range of applications including wet gas. Provided that the
orifice plate remains undamaged, orifice plates perform well in wet gas. There is a risk that a slug of liquid
could bend an orifice plate.
7.2 Design requirements
The design requirements for orifice plate assemblies are contained within ISO 5167-2.
The equations in this Technical Report should only be applied to meters that have been installed horizontally.
Installation of the orifice plate assembly at a low point of the piping configuration where liquid could collect
should be avoided.
In many situations it is desirable that the orifice plate be installed with suitable double block and bleed
isolation valves, so that the orifice plate can be removed and inspected as required.
The presence of liquid in the flow line affects the flow profile as it enters the orifice plate. This is a source of
measurement uncertainty over and above that normally expected for dry gas measurement. In order to
minimize this additional uncertainty, upstream pipe work should be designed so that bends immediately
upstream of the meter encourage any stratified liquid to flow at the bottom of the pipe. Moreover, it is not
recommended that the reduced straight lengths outlined in ISO 5167-2 be used. Where possible, the longer
lengths should be used in order to minimize measurement uncertainty. The use of flow conditioners in wet-gas
applications is not recommended (see 12.1).
7.3 Use of orifice plates with drain holes
The use of orifice plates fitted with drain holes is covered by ISO/TR 15377. Where the liquid flow only occurs
infrequently for brief periods, orifice plates with drain holes may be sufficient without any correction for over-
reading. There are insufficient data to provide general formulae for  and C for the orifice plate with a drain
hole.
The drain hole should be located at the bottom of the orifice plate (corresponding to the 6 o'clock position).
7.4 Pressure tappings
The meter should be installed horizontally with a single pair of pressure tappings. The recommended location
for the tappings circumferentially is given in 12.3.
In addition, a third pressure tapping may be located downstream of the orifice plate to facilitate the
measurement of the fully recovered pressure. A distance of 6D from the plate is regarded as reasonable.
The ratio of the pressure loss to the differential pressure can be significantly higher than in a single-phase flow.
This ratio can be used under certain circumstances to determine the Lockhart-Martinelli parameter. Where the
liquid mass flowrate is only measured discontinuously, significant variations in this ratio can help indicate
when a new measurement of the liquid mass flowrate is required.
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