SIST EN ISO 13686:2005

SLOVENSKI STANDARD
SIST EN ISO 13686:2005
01-julij-2005
=HPHOMVNLSOLQ±2]QDþHYDQMHNDNRYRVWL,62
Natural gas - Quality designation (ISO 13686:1998)
Erdgas - Bestimmung der Beschaffenheit (ISO 13686:1998)
Gaz naturel - Désignation de la qualité (ISO 13686:1998)
Ta slovenski standard je istoveten z: EN ISO 13686:2005
ICS:
75.060 Zemeljski plin Natural gas
SIST EN ISO 13686:2005 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 13686:2005

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SIST EN ISO 13686:2005
EUROPEAN STANDARD
EN ISO 13686
NORME EUROPÉENNE
EUROPÄISCHE NORM
May 2005
ICS 75.060
English version
Natural gas - Quality designation (ISO 13686:1998)
Gaz naturel - Désignation de la qualité (ISO 13686:1998)
This European Standard was approved by CEN on 17 April 2005.
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 Central Secretariat 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 Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 13686:2005: E
worldwide for CEN national Members.

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SIST EN ISO 13686:2005
EN ISO 13686:2005 (E)






Foreword



The text of ISO 13686:1998 has been prepared by Technical Committee ISO/TC 193 "Natural
gas” of the International Organization for Standardization (ISO) and has been taken over as EN
ISO 13686:2005 by CMC.

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 2005, and conflicting national
standards shall be withdrawn at the latest by November 2005.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary,
Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.


Endorsement notice

The text of ISO 13686:1998 has been approved by CEN as EN ISO 13686:2005 without any
modifications.

2

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SIST EN ISO 13686:2005
INTERNATIONAL IS0
STANDARD 13686
First edition
1998-05-01
Natural gas -
Quality designation
Gaz nature/ - Dhignation de la qua/it6
Reference number
IS0 13686: 1998(E)

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SIST EN ISO 13686:2005
IS0 13686:1998(E)
Contents
V
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1 Scope .*.,.,.,.,.,.,.,.,,.,,,,.~.~,,~,,~~,~,~,~,~~~~,,~
1
2 Normative references . . . . . . . . . . . . . . . . . . . . . . . . . . . .*.*.
3
3 Definitions . . . . . . . . . . . . . . . . . . . .~.~.~.
7
4 Symbols, abbreviations and units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*.
8
5 Quality designation parameters .
8
..............................................................................................
5.1 Gas composition
9
5.2 Gas properties .
9
6 Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~~.~~~.~~~~.~~~.~~~.~.~~~.~.~.~~.~~~.~~~,,~~.~,~,~,~~~~,,,
10
..........................................
Annex A (informative) Introduction to informative annexes
10
A 1 Quality specification .
11
A 2 Interchangeability .
15
...........................................................
A 3 Natural gas, Local distribution system
16
A 4 Condensation curves .
17
.....................................................................................................
A.5 Odorization
17
...................................................
A 6 Nominal range of natural gas components
0 IS0 1998
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 the publisher.
International Organization for Standardization
Case postale 56 l CH-1211 Geneve 20 l Switzerland
Internet central @I iso.ch
x.400 c=ch; a=400net; p=iso; o=isocs; s=central
Printed in Switzerland
ii

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SIST EN ISO 13686:2005
IS0 13686: 1998(E)
@ IS0
Annex B (informative) German Regulations Code of Practice DVGW G 260 I:
.................. .22
1983, G 260/l I: 1990 Extract of the relevant parts for natural gases
22
B 1 Basic gases, substitute gases, additive gases .
............................................................................................... 22
B 2 Standard state
22
B 3 Standard values .
24
B 4 Gas families, groups .
24
............................................................................................
B 5 Gas composition
.............................................................. 24
B 6 Notes on the technical burning data
.25
.................
B 7 Notes on the gas constituents and gas secondary substances.
..................................................... 27
B 8 Data and guide values for the gas quality
Annex C (informative) European standard EN 437 “Test gases, test pressures
29
. . . . .*.*.
and categories of appliances”
................................... 31
Annex D (informative) Interchangeability A.G.A. Index Method
31
..............................................................................
D 1 Example for a calculation
...................... .37
Annex E (informative) British Gas Hydrocarbon Equivalence Method.
....................................................................... 37
E 1 Composition-based prediction
..................................................................... 38
E 2 Prediction of Interchangeability
39
.........................................................
E 3 Three Dimensional Prediction Diagram
. . . . . . . . . . . . . . . . . .*.*. 41
Annex F (informative) Weaver Index Method
Annex G (informative) French Method for Determining Gas Interchangeability
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
(Delbourg Method)
G 1. Calculation of Interchangeability Indices on the Basis of Chemical
43
Composition of the Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G 2. Interchangeability Limits for Second-family Gases for Domestic
44
Appliances at 20 mbar .,,.,.
48
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Annex H (informative) Bibliography

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SIST EN ISO 13686:2005
@ IS0
IS0 13686:1998(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
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. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
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.
International Standard IS0 13686 was prepared by Technical Committee
ISO/TC 193, NaturaI gas.
Annexes A to H of this International Standard are for information only.
iv

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SIST EN ISO 13686:2005
@ IS0
IS0 13686: 1998(E)
Introduction
The need for an International Standard concerning the designation of natural gas
quality was a basic reason for the establishment of lSO/TC 193 in 1989.
Standardisation of the designation of quality is specifically stated in the scope of the
TC. Natural gas, supplying 20 % of the world ’s primary energy, is likely to increase
its market share greatly. Yet there is currently no generally accepted definition of
natural gas quality.
To meet this need, it was decided that a general statement of the parameters (i.e.
components and properties) required should be established and that the resulting
International Standard would not specify values of, or limits for, these parameters.
Furthermore, it was decided that general-purpose natural gas transmitted to local
distribution systems (LDS), referred to as “natural gas ”, should be the first
consideration. Thus, this International Standard was developed. Informative
annexes are attached as examples of actual natural gas quality specifications that
already exist.
This International Standard does not impose any quality restrictions on raw gas
transported via pipelines or gathering systems to processing or treating facilities.
It should be understood that this International Standard covers natural gas at the
pipeline level prior to any treatment by LDS for peakshaving purposes. This covers
the vast percentage of the natural gas that is sold in international trade and
transmitted for custody transfer to local distribution systems.
V

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SIST EN ISO 13686:2005
IS0 13686: 1998(E)
INTERNATIONAL STANDARD o IS0
Quality designation
Natural gas -
1 Scope
This International Standard specifies the parameters required to describe
finally processed and, where required, blended natural gas. Such gas is
referred to subsequently in this text simply as “natural gas ”.
The main text of this standard contains a list of these parameters, their units
and references to measurement standards. Informative annexes give examples
of typical values for these parameters, with the main emphasis on health and
safety.
In defining the parameters governing composition, physical properties and
trace constituents, consideration has also been given to existing natural gases
to ensure their continuing viability.
The question of interchangeability is dealt with in annex A clause A.2.
2 Normative references
The following standards contain provisions which, through reference in this
text, constitute provisions of this International Standard. At the time of publica-
tion, the editions indicated were valid. All standards are subject to revision,
and parties to agreements based on this International Standard are
encouraged to investigate the possibility of applying the most recent editions of
the standards indicated below. Members of IEC and IS0 maintain registers of
currently valid International Standards.
IS0 6326-l : 1989, Natural gas - Determination of sulfur compounds -
Part I: General introduction.
IS0 6326-2: 1981,
Gas analysis - Determination of sulphur compounds
in natural gas - Part 2: Gas chromatographic method
using an electrochemical detector for the
determination of odoriferous sulphur compounds.
IS0 6326-3: 1989, Natural gas - Determination of sulfur compounds -
Part 3: Determination of hydrogen sulfide, mercaptan
sulfur and carbonyl sulfide sulfur by potentiometry.
IS0 6326-4: 1994, Natural gas - Determination of sulfur compounds -
Pat? 4: Gas chromatographic method using a flame
photometric detector for the determination of
hydrogen sulfide, carbonyl sulfide and other sulfur-
containing odorants.

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SIST EN ISO 13686:2005
0 IS0
IS0 13686: 1998(E)
IS0 632695:1989, Natural gas - Determination of sulfur compounds -
Par? 5: Lingener combustion method.
Gas analysis - Determination of the water dew point
IS0 6327:1981,
of natural gas - Cooled surface condensation
hygrometers.
Natural gas - Simple analysis by gas
IS0 6568:1981,
chromatography.
IS0 65709t1983, Natural gas - Determination of potential hydrocarbon
liquid content - Part 1: Principles and general
requirements.
IS0 657002:1984, Natural gas - Determination of potential hydrocarbon
liquid content - Part 2: Weighing method.
IS0 6570-3:1984, Natural gas - Determination of potential hydrocarbon
liquid content - Part 3: Volumetric method.
Natural gas - Determination of hydrogen, inert gases
ISO6974:1984,
and hydrocarbons up to Cs - Gas chromatographic
method
Natural gas - Extended analysis - Gas
IS0 6975: 1997,
chroma tographic method.
IS0 6976:1995, Natural gas - Calculation of calorific values, density,
relative density and Wobbe index from composition.
Natural gas - Determination of water by the
IS0 lOlOl-1:1993,
Karl Fischer method - Part I: Introduction.
IS0 lOlOl-2:1993, Natural gas - Determination of water by the
Karl Fischer method - Part 2: Titration procedure.
IS0 10101-3:1993, Natural gas - Determination of water by the
Karl Fischer method - Part 3: Coulometric procedure.
IS0 10715:1997, Natural gas - Sampling.
IS0 11541:1997, Natural gas - Determination of water content at
high pressure.
IS0 12213-1:1997, Natural gas - Calculation of compression factor - Part
1: Introduction and guidelines.
IS0 13443:1996, Natural gas - Standard reference conditions.
2

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SIST EN ISO 13686:2005
IS0 13686:1998(E)
0 IS0
Definitions
3
For the purposes of this International Standard, the following definitions and
explanations apply.
3.1 natural gas
A gaseous fuel obtained from underground sources and consisting of a
complex mixture of hydrocarbons, primarily methane, but generally also
including ethane, propane and higher hydrocarbons in much smaller amounts.
It generally also includes some inert gases, such as nitrogen and carbon
dioxide, plus minor amounts of trace constituents.
Natural gas remains in the gaseous state under the temperature and pressure
conditions normally found in service.
It is produced by processing raw gas or from liquefied natural gas and, if
required, blended to give a gas suitable for direct use.
As pipeline quality natural gas it may then be transmitted within a local dis-
tribution system, within a country, or across national borders. It is subject to
contractual requirements between buyer and seller, and in some cases to
national or state requirements as to quality (see annex A, clause A. 1).
3.2 liquefied natural gas
Natural gas which, after processing, has been liquefied for storage or trans-
portation purposes. Liquefied natural gas is revapourized and introduced into
pipelines for transmission and distribution as natural gas.
3.3 substitute natural gas
Manufactured or blended gas with properties which make it interchangeable
with natural gas. Substitute natural gas is sometimes called synthetic natural
gas.
3.4 raw gas
II heads through gathering I ines to processing
Unprocessed gas taken from we
facilities.
3.5 local distribution system
The gas mains and services which supply natural gas directly to consumers.
3.6 gas quality
The quality of a natural gas is defined by its composition and t he following
physical properties:
Major components: calorific value, Wobbe index
Minor components: density, compression factor
Trace constituents: relative density, dew points

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SIST EN ISO 13686:2005
0 IS0
IS0 13686: 1998(E)
3.7 reference conditions
The preferred reference conditions are referred to as standard reference
conditions and denoted by the subscript “s” (see IS0 13443):
=
101,325 kPa
S
P
W
T 288,1!5 K
s -
3.8 calorific values
Divided into two types: superior calorific value and inferior calorific value,
defined as follows (see IS0 6976).
3.8.1 superior calorific value
The amount of heat which would be released by the complete combustion in
air of a specified quantity of gas, in such a way that the pressure at which the
reaction takes place remains constant, and all the products of combustion are
returned to the same specified temperature as that of the reactants, all of these
products being in the gaseous state except for water formed by combustion,
which is condensed to the liquid state at the above mentioned temperature.
The above mentioned pressure and temperature must be specified.
3.8.2 Inferior calorific value
The amount of heat which would be released by the complete combustion in
air of a specified quantity of gas, in such a way that the pressure at which the
reaction takes place remains constant, and all the products of combustion are
returned to the same specified temperature as that of the reactants, all of these
products being in the gaseous state. The above mentioned pressure and
temperature must be specified.
Both superior and inferior calorific values, which differ by the heat of
condensation of water formed by combustion, can be specified on a molar,
mass or volumetric basis. For the volumetric basis the pressure and tempera-
ture shall be stated at standard reference conditions.
Calorific values can also be stated as dry or wet, depending on the water
vapour content of the gas prior to combustion.
The effect of water vapour on the calorific values, either directly measured or
calculated, is described in annex F of IS0 6976.
Normally, the calorific value is expressed as the superior, dry value specified
on a volumetric basis under standard reference conditions.
3.9 density
The mass of a gas divided by its volume at specified pressure and
temperature.

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SIST EN ISO 13686:2005
IS0 13686:1998(E)
3.10 relative density
Often called specific gravity, it is the mass of natural gas, dry or wet, per unit
volume divided by the mass of an equal volume of dry air, both at the same
specified pressure and temperature (see IS0 6976).
3.11 Wobbe index
The Wobbe index is a measure of the heat input to gas appliances, derived
from the orifice flow equation. It is defined as the specified calorific value,
always on a volume basis, divided by the square root of the corresponding
relative density. The heat input for different natural gas compositions is the
same if they have the same Wobbe index and are used under the same gas
pressure (see IS0 6976).
3.12 compression factor
The compression factor Z is the quotient of the volume of an arbitrary mass of
gas, at a specified pressure and temperature, and that of the same gas under
the same conditions as calculated from the ideal gas law.
The terms compressibility factor and Z-factor are synonymous with compres-
sion factor (see IS0 12213-l ).
3.13 water dew point
The dew point defines the temperature above which no condensation of water
occurs at a specified pressure. For any pressure lower than the specified pres-
sure there is no condensation at this temperature
(see A.4.1 and IS0 6327).
3.14 hydrocarbon dew point
The dew point defines the temperature above which no condensation of hy-
drocarbons occurs at a specified pressure.
At a given dew point, there is a pressure range within which condensation
occurs except at one point, the cricondentherm (see A.4.2).
3.15 molar composition
The molar composition of a gas is the term used when the proportion of each
component is expressed as a molar (or mole) fraction, or molar (mole) percen-
tage, of the whole.
Thus the mole fraction, xi, of component i is the quotient of the number of
moles of component i and the number of moles of the whole mixture present in
the same arbitrary volume. One mole of any chemical species is the amount of
substance which has the relative molecular mass in grams. A table of
recommended values of relative molecular masses is given in IS0 6976.
For an ideal gas, the mole fraction (or percentage) is identical to the volume
fraction (percentage), but this relationship cannot in general be assumed to
apply to real gas behaviour.
5

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SIST EN ISO 13686:2005
IS0 13686:1998(E)
@ IS0
3.16 gas composition
The concentrations of the major and minor components and trace constituents
in natural gas as analysed.
3.17
gas analysis
The use of test methods and other techniques for determining the gas
composition, as stated in this International Standard.
3.18 interchangeability
A measure of the degree to which the combustion characteristics of one gas
resemble those of another gas. Two gases are said to be interchangeable
when one gas may be substituted for the other without affecting the operation
of gas burning appliances or equipment.
3.19
odorization
Natural gas is normally odourless. It is necessary to add an odorant to the gas
fed into the distribution system for safety reasons. It permits the detection of
the gas by smell at very low concentrations.
3.20 methane number
The methane number is a rating indicating the knocking characteristics of a
fuel gas. It is comparable to the octane number for petrol.
The methane number expresses the volume percentage of methane in a met-
hane/hydrogen mixture which, in a test engine under standard conditions, has
the same tendency to knock as the fuel gas to be examined.

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SIST EN ISO 13686:2005
ISO13686:1998( E)
0 IS0
4 Symbols, abbreviations and units
Symbol/Abbreviation Meaning and units
d Relative density
Molar basis calorific value (kJ/mol)
H
h
Mass basis calorific value (MJ/kg)
H
-
Volumetric basis calorific value (MJ/m3)
H
LDS Local distribution system
M Mass per mole (kglkmol)
NG Natural gas
(Absolute) pressure (kPa)
P
SNG Substitute (synthetic) natural gas
t Celsius temperature ( “C)
T Thermodynamic (absolute) temperature (K)
V (Gas) volume (m3)
W Wobbe index (number) (MJ/m3)
Z Compression factor
d Density ( kg/m3)
Subscripts
d (Gas volume) dry
I Inferior (calorific value)
S (Gas volume) saturated
S Superior (calorific value)
(Gas volume) wet
Calorific value
Superior calorific value denoted by Hs; inferior calorific value denoted by HI.
The calorific value shall be specified under the combustion conditions. The
volumetric calorific value shall be specified under standard reference
conditions. The calorific value is normally stated as “dry ”.
Example:
Superior calorific value, specified on a volumetric basis, at standard reference
conditions and stated as wet. For simplicity, the combustion conditions are not
specified.
fi s,w (ps, T,)
Wobbe index
The Wobbe index, denoted by W, is expressed on a volumetric basis and
given in MJ/m3, where the volume is stated at standard reference conditions.
The Wobbe index can be specified as superior or inferior, depending on the
calorific value and as dry or wet, depending on the calorific value and the
corresponding density.
7

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SIST EN ISO 13686:2005
0 IS0
IS0 13686: 1998(E)
Example:
Wobbe index, superior, specified on a volumetric basis, at standard reference
conditions and stated as “wet”
&w (Ps 9 7;)
wsw (Ps9Ts) = &i&-y)
5 Quality designation parameters
This section deals with the various parameters which may be referred to in a
designation of the quality of natural gas. The parameters actually selected will
depend upon the purpose for which the designation is required and it is
unlikely that all the parameters listed in this International Standard will be
used.
51 . Gas composition
Natural gas is composed primarily of methane with smaller amounts of higher
hydrocarbons and non combustible gases. Major and minor components and
trace constituents may be determined as follows.
Limits are not given in this document, but analysis to determine the natural-gas
properties may be specified in contracts and state and federal codes in some
countries. (See informative annexes.)
5.1 .I Major components
-
Test methods
Constituent Units
Methane
Ethane
Propane
IS0 6568
Butanes
mol % IS0 6974
Pentanes
IS0 6975
Hexanes plus
Nitrogen
Carbon dioxide
5.1.2 Minor components
Units Test methods
Constituent
Hydrogen
IS0 6975
Oxygen
mol % IS0 6974
Carbon monoxide
Helium
8

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SIST EN ISO 13686:2005
0 IS0
IS0 13686:1998(E)
5.1.3 Trace constituents
Constituent Units
Test methods
Hydrogen sulfide
IS0 6326-l
Mercaptan sulfur
IS0 6326-2
Dialkyl (di) sulfide mglm3
IS0 6326-3
Carbonyl sulfide IS0 6326-4
Total sulfur IS0 6326-5
52 . Gas properties
5.2.1 Physical properties
Property Units Test methods
Molar calorific value H
M J/mol
Mass-basis calorific value k M J/kg
Volumetric-basis calorific value i?
M J/m3
IS0 6976
Density
d
Wobbe index
W M J/m3
Water dew point
IS0 6327
OC 0
Water liquid content
mg/m3 IS0 10101-1
IS0 10101-2
IS0 10101-3
IS0 11541
Hydrocarbon dew point
OC (K)
Hydrocarbon liquid content
mg/m3
IS0 6570-I
IS0 6570-2
IS0 6570-3
5.2.2 Other properties
Natural gas shall be technically free of:
Water and hydrocarbons in liquid form;
Solid particulate substances in amounts deleterious to the materials
normally encountered in transportation and utilisation;
Other gases that could adversely affect the transportation or utilisation of
the gas.
Note:
Technically free means that there are no visible traces of the components mentioned under actual
conditions.
6
Sampling
Natural gas shall be sampled at agreed upon points, using routines
representing established good practice, for the purpose of applying the test
methods required. See IS0 10715 for guidance on sampling.

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SIST EN ISO 13686:2005
0 IS0
IS0 13686:1998(E)
Annex A
(informative)
Introduction to informative annexes
Al Quality specification
German regulations Code of Practice DVGW G 26011: 1983;
A 1.1
G 260111: 1990
(Relevant parts for natural gases, see Annex B)
NOTE Deutscher Verein des Gas- und Wasserfaches (DVGW) is a scientific
association whose prime task is the production of codes of practice for
the entire gas and water industry. It is a member of DIN.
A 1.2 French regulations concerning gas quality
In France, gas quality is principally defined by two government regulatory texts
(Arretes Ministeriels) the first of which specifies the superior calorific value and the
second the water and sulfur contents. All other gas quality specifications should be
defined if necessary by contractual documents signed between the companies
involved in gas transportation, which are now Gaz de France, Elf Aquitaine
Production and Societe Nationale des Gaz du Sud Quest. The two governmental
documents can be summed up as follows:
1 Arr& du 16 septembre 1977
Limits of variations of superior calorific value of natural gas. Reference
conditions called normal conditions (n) are:
T : 0 degree C
...