Measurement of wet gas flow by means of pressure differential devices inserted in circular cross-section conduits

ISO/TR 11583:2012 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 consist of gas in the region of 95 % volume fraction or more. ISO/TR 11583:2012 is an extension of ISO 5167. The ranges of gases and liquids from which the equations in ISO/TR 11583:2012 were derived are given. 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 ISO/TR 11583:2012 apply for a wide range of gases and liquids at appropriate gas-liquid density ratios, evaluating gas flow rates depends on information in addition to that required in single-phase flow: a measurement of the pressure loss can be sufficient; measurement of the liquid flow using tracers can be possible; the total mass flow rate 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 ISO/TR 11583:2012.

Mesurage du débit de gaz humide au moyen d'appareils déprimogènes insérés dans des conduites de section circulaire

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Status
Published
Publication Date
22-Mar-2012
Current Stage
9093 - International Standard confirmed
Start Date
13-Aug-2019
Completion Date
13-Aug-2019
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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/TR 11583:2012(E)
ISO 2012
---------------------- Page: 1 ----------------------
ISO/TR 11583:2012(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2012

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,

electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or

ISO's member body in the country of the requester.
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Published in Switzerland
ii © ISO 2012 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 11583:2012(E)
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

© ISO 2012 – All rights reserved iii
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ISO/TR 11583:2012(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies

(ISO member bodies). The work of preparing International Standards is normally carried out through ISO

technical committees. Each member body interested in a subject for which a technical committee has been

established has the right to be represented on that committee. International organizations, governmental and

non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the

International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards

adopted by the technical committees are circulated to the member bodies for voting. Publication as an

International Standard requires approval by at least 75 % of the member bodies casting a vote.

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
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ISO/TR 11583:2012(E)
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.
© ISO 2012 – All rights reserved v
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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

© ISO 2012 – All rights reserved 1
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ISO/TR 11583:2012(E)
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
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ISO/TR 11583:2012(E)

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
Symbol Quantity Dimension SI Unit
C Coefficient of discharge dimensionless 1
Chisholm coefficient dimensionless 1
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
L m
a Venturi tube) at working conditions
Gas densiometric Froude number [see Equation (3)] dimensionless 1
gas
2 2
Acceleration due to gravity LT m/s
2 2
h Specific enthalpy T J/kg
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 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
Mass flowrate MT kg/s
3 1 3
Volume flowrate L T  m /s
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
L ≡ length; M ≡ mass; T ≡ time;  ≡ temperature.
The dimensions and units are those of the corresponding quantity.
© ISO 2012 – All rights reserved 3
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ISO/TR 11583:2012(E)

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
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ISO/TR 11583:2012(E)
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.
© ISO 2012 – All rights reserved 5
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ISO/TR 11583:2012(E)
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

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
CF = 10,046 3 exp 0,05r min 1, (4)
 
gas,th
 
0,016
where
gas
Fr 
gas,th
2,5
6.4.3 Over-reading correction factor
1 CXX (5)
where C is given by the following equation:
n n

 
liquid 1,gas
C

1,gas liquid

where
0,8Fr
gas
n = max0,5830,180,578 exp , 0,392 0,18

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
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ISO/TR 11583:2012(E)
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
down
max 5, 207  9
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
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ISO/TR 11583:2012(E)
If Y/Y < 0,65, X is evaluated from
max
0,28Fr
Y gas
0,75
1 exp35X exp
max
Limits of use in addition to those in 6.4.3:
Fr > 4
gas,th
gas
 5,5
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
max
X obtained from the formulae in 6.4.5
0,6  Y/Y < 0,65
max
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ISO/TR 11583:2012(E)

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 no

...

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