EN ISO 20765-5:2022

Natural gas - Calculation of thermodynamic properties - Part 5: Calculation of viscosity, Joule-Thomson coefficient, and isentropic exponent (ISO 20765-5:2022)

This document specifies methods to calculate (dynamic) viscosity, Joule-Thomson coefficient, isentropic exponent, and speed of sound, excluding density, for use in the metering of natural gas flow.

Erdgas - Berechnung der thermodynamischen Eigenschaften - Teil 5: Berechnung der Viskosität, Joule-Thomson-Koeffizient und Isentropenexponent (ISO 20765-5:2022)

Dieses Dokument legt Verfahren zur Berechnung der (dynamischen) Viskosität, des Joule-Thomson-Koeffizienten, des Isentropenexponenten und der Schallgeschwindigkeit, mit Ausnahme der Dichte, zur Anwendung bei der Messung des Durchflusses von Erdgas fest.

Gaz naturel - Calcul des propriétés thermodynamiques - Partie 5: Calcul de la viscosité, du coefficient de Joule-Thomson et de l'exposant isentropique (ISO 20765-5:2022)

Le présent document spécifie une méthode de calcul de la viscosité et d'autres propriétés, à l'exception de la densité, pour le mesurage du débit de gaz naturel.

Zemeljski plin - Izračun termodinamičnih lastnosti - 5. del: Izračun viskoznosti, Joule-Thomsonovega koeficienta in isentropnega eksponenta (ISO 20765-5:2022)

Ta dokument določa metode za izračun (dinamične) viskoznosti, Joule-Thomsonovega koeficienta, isentropnega eksponenta in hitrosti zvoka, razen gostote, za uporabo pri merjenju pretoka zemeljskega plina.

General Information

• Status: Published
• Publication Date: 03-May-2022
• Withdrawal Date: 29-Nov-2022
• ICS: 75.060 - Natural gas
• Technical Committee: CEN/TC 238 - Test gases, test pressures and categories of appliances
• Drafting Committee: CEN/TC 238 - Test gases, test pressures and categories of appliances
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 04-May-2022
• Completion Date: 04-May-2022
• Ref Project: SIST EN ISO 20765-5:2022 - Natural gas - Calculation of thermodynamic properties - Part 5: Calculation of viscosity, Joule-Thomson coefficient, and isentropic exponent (ISO 20765-5:2022)

Overview

EN ISO 20765-5:2022 - Natural gas: Calculation of thermodynamic properties - Part 5 specifies simplified, practical methods to calculate dynamic viscosity, Joule–Thomson coefficient, isentropic exponent, and speed of sound for natural gas metering. The standard targets gas-phase conditions in the temperature range −20 °C to 40 °C and absolute pressures up to 10 MPa. Density is excluded from the scope (density is normally obtained from an equation of state such as GERG‑2008 / ISO 20765-2), but accurate density is required as input for several calculations.

Key topics and technical requirements

• Recommended calculation methods:
  • Lohrenz–Bray–Clark (LBC) method is recommended for gas-phase viscosity because it is simple, widely implemented, and needs minimal component data. LBC requires gas composition and molar density as inputs.
• Use of GERG‑2008 (ISO 20765-2) is recommended to provide molar density when composition is known.
• Simplified formulae for Joule–Thomson coefficient, isentropic exponent, and speed of sound are provided; these are intentionally simple and intended to be fit for metering purposes (though less accurate than full equations of state).
• Uncertainty guidance:
  • The standard gives uncertainty assessments showing density must be calculated with high accuracy for custody metering, while viscosity, isentropic exponent, and Joule–Thomson coefficient may have substantially larger allowable uncertainties. A target uncertainty of about ≤25% for these secondary properties is considered acceptable to achieve a typical 0.1% flowrate uncertainty (k = 2) when density is accurate.
• Informative annexes include example code (Visual Basic) for LBC implementation, component parameter guidance, and methane viscosity examples.

Applications and who uses it

• Primary users:
  • Flow computer developers, custody transfer metering engineers, pipeline operators, gas distribution companies, and laboratory analysts.
• Practical applications:
  • Orifice-plate and differential-pressure metering calculations (ISO 5167), flowrate correction and compensation, metering system verification, and on-line flow measurement where compact, computationally efficient thermophysical property models are required.
• Benefits:
  • Enables robust, standardized calculation of thermodynamic parameters needed in high-pressure natural gas metering and helps ensure consistent measurement uncertainty across systems.

Related standards

• ISO 20765-2: GERG‑2008 equation of state for density calculations
• ISO 20765 series (other parts addressing thermodynamic properties)
• ISO 5167 (measurement of fluid flow by pressure-differential devices / orifice metering)

Keywords: natural gas, viscosity calculation, Lohrenz‑Bray‑Clark, Joule‑Thomson coefficient, isentropic exponent, GERG‑2008, metering, flow measurement, ISO 20765-5.

Standards Content (Sample)

SLOVENSKI STANDARD
01-september-2022
Zemeljski plin - Izračun termodinamičnih lastnosti - 5. del: Izračun viskoznosti,
Joule-Thomsonovega koeficienta in isentropnega eksponenta (ISO 20765-5:2022)
Natural gas - Calculation of thermodynamic properties - Part 5: Calculation of viscosity,
Joule-Thomson coefficient, and isentropic exponent (ISO 20765-5:2022)
Erdgas - Berechnung der thermodynamischen Eigenschaften - Teil 5: Berechnung der
Viskosität, Joule-Thomson-Koeffizient und Isentropenexponent (ISO 20765-5:2022)
Gaz naturel - Calcul des propriétés thermodynamiques - Partie 5: Calcul de la viscosité,
du coefficient de Joule-Thomson et de l'exposant isentropique (ISO 20765-5:2022)
Ta slovenski standard je istoveten z: EN ISO 20765-5:2022
ICS:
75.060 Zemeljski plin Natural gas
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 20765-5
EUROPEAN STANDARD
NORME EUROPÉENNE
May 2022
EUROPÄISCHE NORM
ICS 75.060
English Version
Natural gas - Calculation of thermodynamic properties -
Part 5: Calculation of viscosity, Joule-Thomson coefficient,
and isentropic exponent (ISO 20765-5:2022)
Gaz naturel - Calcul des propriétés thermodynamiques Erdgas - Berechnung der thermodynamischen
- Partie 5: Calcul de la viscosité, du coefficient de Joule- Eigenschaften - Teil 5: Berechnung der Viskosität,
Thomson et de l'exposant isentropique (ISO 20765- Joule-Thomson-Koeffizient und Isentropenexponent
5:2022) (ISO 20765-5:2022)
This European Standard was approved by CEN on 3 April 2022.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 20765-5:2022 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 20765-5:2022) has been prepared by Technical Committee ISO/TC 193
"Natural gas" in collaboration with Technical Committee CEN/TC 238 “Test gases, test pressures,
appliance categories and gas appliance types” the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by November 2022, and conflicting national standards
shall be withdrawn at the latest by November 2022.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 20765-5:2022 has been approved by CEN as EN ISO 20765-5:2022 without any
modification.
INTERNATIONAL ISO
STANDARD 20765-5
First edition
2022-04
Natural gas — Calculation of
thermodynamic properties —
Part 5:
Calculation of viscosity, Joule-
Thomson coefficient, and isentropic
exponent
Gaz naturel — Calcul des propriétés thermodynamiques —
Partie 5: Calcul de la viscosité, du coefficient de Joule-Thomson et de
l'exposant isentropique
Reference number
ISO 20765-5:2022(E)
ISO 20765-5:2022(E)
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
ISO 20765-5:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Background . 1
5 Viscosity, η . . 3
5.1 Viscosity as a function of temperature, pressure, and composition . 3
5.2 Viscosity as a function of temperature and mass density . 6
6 Other properties . 6
6.1 Preamble . 6
6.2 Joule-Thomson coefficient, μ . 8
6.3 Isentropic exponent, κ . 9
6.4 Speed of sound, W . 10
7 Example calculations .10
8 Conclusions .11
9 Reporting of results .11
Annex A (informative) Symbols and units .12
Annex B (informative) Example LBC viscosity function .13
Annex C (informative) Example routine to convert CV, RD, and CO mole fraction to an
equivalent C -C -N -CO mixture .15
1 3 2 2
Annex D (informative) Viscosity of methane .16
Bibliography .17
iii
ISO 20765-5:2022(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 1,
Analysis of natural gas, in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 238, Test gases, test pressures and categories of appliances, in accordance
with the agreement on technical cooperation between ISO and CEN (Vienna Agreement).
A list of all parts in the ISO 20765 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
ISO 20765-5:2022(E)
Introduction
This document gives simplified methods for the calculation of (dynamic) viscosity, Joule-Thomson
coefficient, and isentropic exponent for use in natural gas calculations in the temperature range −20 °C
to 40 °C, with absolute pressures up to 10 MPa, and only within the gas phase. For the Joule-Thomson
coefficient and isentropic exponent, the uncertainty of the formulae provided is greater than that
[1]
obtained from a complete equation of state such as GERG-2008 (see ISO 20765-2) but is considered to
be fit for purpose. The formulae given here are very simple.
v
INTERNATIONAL STANDARD ISO 20765-5:2022(E)
Natural gas — Calculation of thermodynamic properties —
Part 5:
Calculation of viscosity, Joule-Thomson coefficient, and
isentropic exponent
1 Scope
This document specifies methods to calculate (dynamic) viscosity, Joule-Thomson coefficient, isentropic
exponent, and speed of sound, excluding density, for use in the metering of natural gas flow.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
4 Background
The main motivation for this document is to provide simplified methods for the calculations required,
according to ISO 5167, to measure flow of high-pressure natural gas with an orifice plate meter.
Useful references for the work herein are given below:
a) ISO 5167-1:1991, Measurement of fluid flow in closed conduits — Part 1: Pressure differential devices
b) EN 5167-1:1997, Measurement of fluid flow by means of pressure differential devices — Part 1: Orifice
plates, nozzles and Venturi tubes inserted in circular cross-section conduits running full
c) 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
d) 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
The basic mass flowrate, q, formula is:
C π
q= ε dP2⋅⋅Δ D (1)
4 4
1−β
where
C is a function of β, Re, and the type of orifice pressure tappings;
ε is a function of β, P, ΔP, and κ.
The symbols are defined in Annex A. The standards above differ in the functions for C and ε. Although
q is given by Formula (1), iteration is required since C is a function of Re, and Re is a function of q.
Similarly, given q in Formula (1) does not directly give ΔP since ε is a function of ΔP.
ISO 20765-5:2022(E)
The use of the formulae in ISO 5167 for calculating flowrate (q) for an orifice plate meter, over a typical
input range of temperature, pressure, differential pressure, and gas composition, gives the following
formula for the standard uncertainty, u, (when the only source of uncertainties is considered to be in
the calculation of the required gas thermophysical properties):
2 =±05,, 0 000 2 ⋅u ρρ/ 
()
   
molar or mass density
()
uq /q
()
 
+00,,000 60±⋅ 000 2 u ηη/   viscosity
() ()
   
(2)
isentropic exponent
()
+±0,,002 0 001 2 ⋅u κκ/ 
()
   
JJoule-Thomson coefficient
()
2 22
+− 0,,000 40±⋅ 000 2 u μμ/ 
()
   
Formula (2) may be used to estimate the required uncertainty for the calculation of the properties that
are part of this document.
For the mass flowrate expanded uncertainty (U) (coverage factor k=2, with a 95 % confidence interval)
to be less than 0,1 % it is required that
U ρρ/,< 01 %
()
U ηη/%< 85
()
(3)
U κκ/%< 25
()
U μμ/%< 125
()
For the uncertainty contribution of these properties to the complete flowrate uncertainty to be less
than 0,02 % requires that
U ρρ/,< 002 %
()
U ηη/%< 17
()
(4)
U κκ/%< 5
()
U μμ/ < 255 %
()
Thus, density needs to be calculated as accurately as possible, while the uncertainty in the calculation
of the other properties can be much higher, with a target uncertainty of less than about 25 % for a 0,1 %
[1]
uncertainty in the flowrate (k = 2). The use of the GERG-2008 equation of state provides calculations
of density that are generally within the required 0,1 % uncertainty.
ISO 20765-5:2022(E)
5 Viscosity, η
5.1 Viscosity as a function of temperature, pressure, and composition
There are many methods for the calculation of gas phase (dynamic) viscosity, some of which that
are based on theory are quite complicated (see Reference [10] for details). The Lohrenz-Bray-Clark
method (LBC) is relatively simple, requires minimal component data, and is a method that is widely
implemented, and is the method recommended here. One disadvantage is the sensitivity to the input
density; but for the application considered here, accurate densities will be available.
This method requires that the gas composition is available. With inputs of temperature, pressure, and
composition, the GERG-2008 equation of state (ISO 20765-2) may be used to obtain the molar density
required in the formulae below. When the composition is not known, the method in 5.2 may be used.
The formulae needed to implement this method are outlined below (Annex B contains an example
Visual Basic program), where the required parameters consist of the following component values for
the N components:
— molar mass M [g/mol]
i
— critical temperature T [K]
c,i
— critical pressure P [MPa]
c,i
— critical density ρ [mol/dm ]
c,i
— mole fraction x [mol/mol]
i
These mixture parameters may be estimated with the following formulae:
N
Mx= M (5)
mix ∑ ii
i=1
N
Tx= T (6)
c,mixc∑ ii,
i=1
N
Px= P (7)
c,mixc∑ ii,
i=1
N
x
i
V = (8)
c,mix ∑
ρ
c,i
i=1
The component values are obtained from any suitable source, e.g. ISO 20765-2:2015, Annex B.
ISO 20765-5:2022(E)
The viscosity of a natural gas mixture is calculated as:
η =+ηξ⋅−δ 1 (9)
()
mix
The generalized mixture viscosity, which is based on the pure fluid viscosities, is:
N
xMη
∑ ii i
i=1
η = (10)
mix
N
xM
ii
∑
i=1
The parameter ξ is dependent only on the molar mass and the critical temperature and pressure, and is
given as:
1 1 2
−
6 3
2 T P
M
     
c,mixc,mix
mix
ξ =u (11)
     
η
u u u
 MT    P 
This formula is made dimensionless with the use of the following constants:
u = 0,·000 1 mPas
η
u = 1 g/ mol
M
(12)
u = 1 K
T
u = 0,101 325 MPa
P
The parameter δ in Formula (9) is density dependent, and given as:
23 4
δρ=+1,,023 0 23364 ++0,,58533ρρ−0 40758 0,093324ρ (13)
rr rr
ρρ=⋅V (14)
r c,mix
where ρ is the molar density at T and P, calculated from ISO 20765-2.
The pure fluid component viscosity is:
1 2
−
6 3
2 T P
M    
 
ci,,ci
i
η =u α (15)
   
i η  
u u u
 M   T   P 
where α is given as:
09, 4
TT≤=15,: α 34, ⋅ (16)
rr
0,625
TT>=15,: α 1,,778⋅⋅()4581− ,67 (17)
rr
The reduced temperature in these formulae is:
TT= /T (18)
ric,
From the experimental data given in References [3] to [9] the estimated uncertainty of this method is
about 4 % (95 % confidence interval). (Bias=-0,31 %, RMS=1,59 %). Note that using Formula (9) these
are predicted calculations. The experimental data was not used in the development of the method.
The number of points and ranges are:
ISO 20765-5:2022(E)
Total number of points 721
Temperature range 260 K to 344 K (−13 °C to 71 °C)
Pressure range 0,1 MPa to 12,7 MPa
Figure 1 shows the distribution of the errors compared with the following experimental data:
[3]
1) Carr (1953) 3 mixtures 55 points
[4]
2) Golubev (1959) 1 mixture 17 points
[5]
3) Gonzalez et al. (1970) 8 mixtures 35 points
[6]
4) Nabizadeh & Mayinger (1999) 1 mixture 32 points
[7]
5) Assael et al. (2001) 1 mixture 22 points
[8]
6) Schley et al. (2004) 3 mixtures 521 points
[9]
7) Langelandsvik et al. (2007) 2 mixtures 39 points

a)  Y as a function of T b)  Y as a function of P
Key
T temperature (K) Y viscosity error (%)
P pressure (MPa)
Figure 1 — Comparisons of viscosities calculated from the Lohrenz-Bray-Clark method
(Formula 9) with experimental data
If only bulk properties are available rather than a detailed composition, e.g. calorific value (CV), relative
density (RD), and CO mo
...