SIST EN ISO 13686:2013

SLOVENSKI STANDARD
01-september-2013
1DGRPHãþD
SIST EN ISO 13686:2005
Zemeljski plin - Specificiranje kakovosti (ISO 13686:2013)
Natural gas - Quality designation (ISO 13686:2013)
Erdgas - Bestimmung der Beschaffenheit (ISO 13686:2013)
Gaz naturel - Désignation de la qualité (ISO 13686:2013)
Ta slovenski standard je istoveten z: EN ISO 13686:2013
ICS:
75.060 Zemeljski plin Natural gas
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 13686
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2013
ICS 75.060 Supersedes EN ISO 13686:2005
English Version
Natural gas - Quality designation (ISO 13686:2013)
Gaz naturel - Désignation de la qualité (ISO 13686:2013) Erdgas - Bestimmung der Beschaffenheit (ISO
13686:2013)
This European Standard was approved by CEN on 29 May 2013.

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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
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COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2013 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 13686:2013: E
worldwide for CEN national Members.

Contents Page
Foreword .3
Foreword
This document (EN ISO 13686:2013) has been prepared by Technical Committee ISO/TC 193 "Natural gas".
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 December 2013, and conflicting national standards shall be withdrawn
at the latest by December 2013.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 13686:2005.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech
Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.
Endorsement notice
The text of ISO 13686:2013 has been approved by CEN as EN ISO 13686:2013 without any modification.
INTERNATIONAL ISO
STANDARD 13686
Second edition
2013-06-15
Natural gas — Quality designation
Gaz naturel — Désignation de la qualité
Reference number
ISO 13686:2013(E)
©
ISO 2013
ISO 13686:2013(E)
© ISO 2013
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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Published in Switzerland
ii © ISO 2013 – All rights reserved

ISO 13686:2013(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols, abbreviations and units . 6
4.1 Symbols . 6
4.2 Abbreviations . 6
4.3 Subscripts . 6
5 Quality designation parameters. 7
5.1 General . 7
5.2 Gas composition . 7
5.3 Gas properties . 8
6 Sampling . 9
Annex A (informative) Introduction to informative annexes .10
Annex B (informative) German Regulation Code of Practice DVGW G 260:2008, Extract of the
relevant parts for natural gases .18
Annex C (informative) European Standard EN 437 “Test gases, test pressures and categories of
appliances” .22
Annex D (informative) Interchangeability AGA index method .25
Annex E (informative) British Gas Hydrocarbon Equivalence Method .32
Annex F (informative) Weaver index method .37
Annex G (informative) French method for determining gas interchangeability (Delbourg method)
(guide for determining the interchangeability of second family gases) .39
Annex H (informative) Spanish regulation code (Detail Protocol-01 — Measurement) — Extract of
the relevant parts for natural gases .45
Annex I (informative) Harmonization of gas property data for cross-border transportation .46
Bibliography .48
ISO 13686:2013(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. 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. 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.
The committee responsible for this document is ISO/TC 193, Natural gas.
This second edition cancels and replaces the first edition (ISO 13686:1998), which has been
technically revised.
iv © ISO 2013 – All rights reserved

ISO 13686:2013(E)
Introduction
The need for an International Standard concerning the designation of natural gas quality was a basic
reason for the establishment of ISO/TC 193 in 1989. Standardization of the designation of quality is
specifically stated in the scope of ISO/TC 193. 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) recommended 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 majority of the natural gas that is
sold in international trade and transmitted for custody transfer to local distribution systems.
INTERNATIONAL STANDARD ISO 13686:2013(E)
Natural gas — Quality designation
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 International 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 (see Clause A.2).
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 6326-1, Natural gas — Determination of sulfur compounds — Part 1: General introduction
ISO 6326-3, Natural gas — Determination of sulfur compounds — Part 3: Determination of hydrogen sulfide,
mercaptan sulfur and carbonyl sulfide sulfur by potentiometry
ISO 6326-5, Natural gas — Determination of sulfur compounds — Part 5: Lingener combustion method
ISO 6327, Gas analysis — Determination of the water dew point of natural gas — Cooled surface
condensation hygrometers
ISO 6570, Natural gas — Determination of potential hydrocarbon liquid content — Gravimetric methods
ISO 6974-1, Natural gas — Determination of composition and associated uncertainty by gas
chromatography — Part 1: General guidelines and calculation of composition
ISO 6974-2, Natural gas — Determination of composition and associated uncertainty by gas
chromatography — Part 2: Uncertainty calculations
ISO 6974-3, Natural gas — Determination of composition with defined uncertainty by gas chromatography —
Part 3: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to C8
using two packed columns
ISO 6974-4, Natural gas — Determination of composition with defined uncertainty by gas chromatography —
Part 4: Determination of nitrogen, carbon dioxide and C1 to C5 and C6+ hydrocarbons for a laboratory and
on-line measuring system using two columns
ISO 6974-5, Natural gas — Determination of composition and associated uncertaint y by gas chromatography —
Part 5: Isothermal method for nitrogen, carbon dioxide, C1 to C5 hydrocarbons and C6+ hydrocarbons
ISO 6974-6, Natural gas — Determination of composition and associated uncertainty by gas
chromatography — Part 6: Determination of helium, oxygen, nitrogen, carbon dioxide and C1 to C10
hydrocarbons using capaillary columns
ISO 6975, Natural gas — Extended analysis — Gas-chromatographic method
ISO 13686:2013(E)
ISO 6976:1995, Natural gas — Calculation of calorific values, density, relative density and Wobbe index
from composition
ISO 6978-1, Natural gas — Determination of mercury — Part 1: Sampling of mercury by chemisorption on iodine
ISO 6978-2, Natural gas — Determination of mercury — Part 2: Sampling of mercury by amalgamation on
gold/platinum alloy
ISO 10101-1, Natural gas — Determination of water by the Karl Fischer method — Part 1: Introduction
ISO 10101-2, Natural gas — Determination of water by the Karl Fischer method — Part 2: Titration procedure
ISO 10101-3, Natural gas — Determination of water by the Karl Fischer method — Part 3: Coulometric procedure
ISO 11541, Natural gas — Determination of water content at high pressure
ISO 13443, Natural gas — Standard reference conditions
ISO 14532, Natural gas — Vocabulary
ISO 15970:2008, Natural gas — Measurement of properties — Volumetric properties: density, pressure,
temperature and compression factor
ISO 15971:2008, Natural gas — Measurement of properties — Calorific value and Wobbe index
ISO 18453, Natural gas — Correlation between water content and water dew point
ISO 19739, Natural gas — Determination of sulfur compounds using gas chromatography
ISO 23874, Natural gas — Gas chromatographic requirements for hydrocarbon dewpoint calculation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 14532 and the following apply.
3.1
natural gas
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
Note 1 to entry: It also includes some inert gases, such as nitrogen and carbon dioxide, plus minor amounts of
trace constituents.
Note 2 to entry: 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 can then be transmitted within a local distribution
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 Clause A.1).
3.2
liquefied natural gas
natural gas which, after processing, has been liquefied for storage or transportation purposes
Note 1 to entry: Liquefied natural gas is revaporized 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
Note 1 to entry: Substitute natural gas is sometimes called synthetic natural gas.
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ISO 13686:2013(E)
Note 2 to entry: This also includes gases manufactured by thermal process from biomass.
3.4
raw gas
unprocessed gas taken from well heads through gathering lines to processing facilities
3.5
local distribution system
gas mains and services which supply natural gas directly to consumers
3.6
gas quality
attribute of natural gas by its composition (major components, minor components and trace components)
and its physical properties (calorific value, Wobbe index, compression factor, relative density and dew points)
3.7
reference conditions
standard reference conditions of temperature, pressure and humidity (state of saturation) to be used for
measurements and calculations carried out on natural gases, natural gas substitutes and similar fluids
in the gaseous state
Note 1 to entry: Standard reference conditions are denoted by the subscript “s”: p = 101,325 kPa; T = 288,15 K.
s s
Note 2 to entry: Adapted from ISO 13443.
3.8
calorific value
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
Note 1 to entry: It is divided into two types: superior calorific value and inferior calorific value.
Note 2 to entry: 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 temperature shall be stated at standard reference conditions.
Note 3 to entry: Calorific values can also be stated as dry or wet, depending on the water vapour content of the
gas prior to combustion.
Note 4 to entry: The effect of water vapour on the calorific values, either directly measured or calculated, is
described in Annex F of ISO 6976:1995.
Note 5 to entry: Normally, the calorific value is expressed as the superior, dry value specified as a volumetric basis
under standard reference conditions.
Note 6 to entry: Adapted from ISO 6976.
3.8.1
superior calorific value
amount of heat that would be released by the complete combustion with oxygen of a specified quantity
of gas, in such a way that the pressure p at which the reaction takes place remains constant, and all the
products of combustion are returned to the same specified temperature t as that of the reactants, all
of these products being in the gaseous state except for water, which is condensed to the liquid state at t
Note 1 to entry: Adapted from ISO 6976.
ISO 13686:2013(E)
3.8.2
inferior calorific value
amount of heat that would be released by the complete combustion with oxygen of a specified quantity
of gas, in such a way that the pressure p at which the reaction takes place remains constant, and all the
products of combustion are returned to the same specified temperature t as that of the reactants, all of
these products being in the gaseous state
Note 1 to entry: Adapted from ISO 6976.
3.9
density
mass of a gas divided by its volume at specified pressure and temperature
3.10
relative density
density of a gas divided by the density of dry air of standard composition at the same specified conditions
of pressure and temperature
Note 1 to entry: The term “ideal relative density” applies when both gas and air are considered as fluids which
obey the ideal gas law; the term “real relative density” applies when both gas and air are considered as real fluids.
For the standard composition of dry air.
Note 2 to entry: Adapted from ISO 6976.
3.11
Wobbe index
volumetric-basis superior (inferior) calorific value, at specified reference conditions, divided by the
square root of the relative density at the same specified metering reference conditions
Note 1 to entry: 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.
Note 2 to entry: Adapted from ISO 6976.
3.12
compression factor
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
Note 1 to entry: The terms “compressibility factor” and “Z-factor” are synonymous with compression factor.
Note 2 to entry: Adapted from ISO 12213-1.
3.13
water dew point
temperature above which no condensation of water occurs at a specified pressure
Note 1 to entry: For any pressure lower than the specified pressure there is no condensation at this temperature.
Note 2 to entry: Adapted from ISO 6327.
3.14
hydrocarbon dew point
temperature above which no condensation of hydrocarbons occurs at a specified pressure
Note 1 to entry: At a given dew point, there is a pressure range within which condensation occurs except at one
point, the cricondentherm (see A.3.2).
3.15
gas composition
concentrations of the major and minor components and trace components in natural gas as analysed
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ISO 13686:2013(E)
3.16
molar composition
gas composition expressed as a molar (or mole) fraction, or molar (mole) percentage
Note 1 to entry: The mole fraction, x, of component i is the quotient of amount of substance of this component and
amount of substance of the whole mixture. The unit of amount of substance is mole. The mass of one mole of any
chemical species, in grams, is numerically equal to its relative molecular mass. A table of recommended values of
molar masses is given in ISO 6976. For an ideal gas, the mole fraction is identical to the volume fraction, but this
relationship cannot in general be assumed to apply to real gas behaviour.
3.17
gas analysis
use of test methods and other techniques for determining the gas composition, as stated in this
International Standard
3.18
interchangeability
measure of the degree to which the combustion characteristics of one gas resemble those of another gas
Note 1 to entry: 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
addition of odorants, in most cases intensively smelling organic sulfur compounds, to natural gas to
allow the recognition of gas leaks by smell at very low concentration (before a build up to a dangerous
gas in air concentration can occur)
Note 1 to entry: Natural gas is normally odourless. It is necessary to add an odorant to the gas for safety reasons.
It permits the detection of the gas by smell at very low concentrations.
Note 2 to entry: Odorants in use for gas odorization are specified in ISO 13734.
3.20
methane number
rating indicating the knocking characteristics of a fuel gas
Note 1 to entry: It is comparable to the octane number for petrol.
Note 2 to entry: The methane number expresses the volume percentage of methane in a methane/hydrogen mixture
which, in a test engine under standard conditions, has the same tendency to knock as the fuel gas to be examined.
ISO 13686:2013(E)
4 Symbols, abbreviations and units
4.1 Symbols
Symbol Meaning and units
d Relative density
Molar basis calorific value (kJ/mol)
H
ˆ Mass basis calorific value (MJ/kg)
H

Volumetric basis calorific value (MJ/m )
H
M Mass per mole (kg/kmol)
p (Absolute) pressure (kPa)
t Celsius temperature (°C)
T Thermodynamic (absolute) temperature (K)
V (Gas) volume (m )
W Wobbe index (number) (MJ/m )
Z Compression factor
D Density (kg/m )
4.2 Abbreviations
Abbreviation Meaning
LDS Local distribution system
NG Natural gas
SNG Substitute (synthetic) natural gas
4.3 Subscripts
d (Gas volume) dry
l Inferior (calorific value)
s (Gas volume) saturated
S Superior (calorific value)
w (Gas volume) wet
Superior calorific value is denoted by Hs; inferior calorific value is denoted by H . The calorific value
I
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”.
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ISO 13686:2013(E)
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.

Hp(,T )
S,ws s
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.
EXAMPLE Wobbe index, superior, specified an a volumetric basis, at standard reference conditions and
stated as “wet”.

Hp(,T )
S,ws s
Wp(,T )=
S,ws s
dp(,T )
ws s
5 Quality designation parameters
5.1 General
This clause 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.
5.2 Gas composition
5.2.1 General
Natural gas is composed primarily of methane with smaller amounts of higher hydrocarbons and of non
combustible gases. Major and minor components and trace constituents may be determined as given in
Tables 1, 2 and 3.
Limits are not given in this International Standard, but analysis to determine the natural-gas properties
may be specified in contracts and state and federal codes in some countries (see informative annexes).
5.2.2 Major components
Table 1 — Major components of natural gas
Constituent Units Relevant standard
Methane mol % ISO 6974 (parts 1 to 6)
Ethane mol % ISO 6974 (parts 1 to 6)
Propane mol % ISO 6974 (parts 1 to 6)
Butanes mol % ISO 6974 (parts 1 to 6)
Pentanes mol % ISO 6974 (parts 1 to 6)
Hexanes plus mol % ISO 6974 (parts 1 to 6), ISO 6975
Nitrogen mol % ISO 6974 (parts 1 to 6)
Carbon dioxide mol % ISO 6974 (parts 1 to 6), ISO 6975
ISO 13686:2013(E)
5.2.3 Minor components
Table 2 — Minor components of natural gas
Constituent Units Relevant standard
Hydrogen mol % ISO 6974-3 and ISO 6974−6,
ISO 6975
Oxygen mol % ISO 6974-3 and ISO 6974−6,
ISO 6975
Carbon monoxide mol % ISO 6974-3
Helium mol % ISO 6974-3 and ISO 6974−6,
ISO 6975
5.2.4 Trace constituents
Table 3 — Trace constituents of natural gas
Constituent Units Relevant standard
Hydrogen sulfide mg/m ISO 6326-1and ISO 6326−3,
ISO 19739
Mercaptan sulfur mg/m ISO 6326-3, ISO 19739
Dialkyl (di) sulfide mg/m ISO 19739
Carbonyl sulfide mg/m ISO 6326-3, ISO 19739
Total sulfur mg/m ISO 6326-5, ISO 19739
Mercury μg/m ISO 6978-1 and ISO 6978−2
5.3 Gas properties
5.3.1 General
Physical properties may be determined as specified in Table 4.
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ISO 13686:2013(E)
5.3.2 Physical properties
Table 4 — Physical properties of natural gas
Constituent Units Relevant standard
MJ/mol ISO 6976, ISO 15971
Molar calorific value, H
ˆ
MJ/kg ISO 6976, ISO 15971
Mass-basis calorific value, H

MJ/m ISO 6976, ISO 15971
Volumetric-basis calorific value H
Relative density, d - ISO 6976, ISO 15970
Wobbe index, W MJ/m ISO 6976, ISO 15971
Water dew point °C (K) ISO 6327, ISO 18453
ISO 10101-1, ISO 18453
ISO 10101-2
Water content mg/m
ISO 10101-3
ISO 11541
Hydrocarbon dew point °C (K) ISO 23874
Hydrocarbon liquid content mg/m ISO 6570
5.3.3 Other parameters
Content of:
— water and hydrocarbons in liquid form;
— solid particulate substances;
— other gases.
NOTE Usually, the above substances are not present in the natural gas in an amount that could adversely
affect the transportation, distribution or utilization of the gas.
6 Sampling
For the control of natural gas quality, sampling is necessary. Natural gas is generally sampled at agreed
upon points, using routines representing established good practice, applying the relevant standards.
See ISO 10715 for guidance on sampling.
ISO 13686:2013(E)
Annex A
(informative)
Introduction to informative annexes
A.1 Quality specification
A.1.1 General
Gas quality specifications are originating from legislation, codes of practice and/or contractual
agreement and are generally nationally based. Within the European Union all infrastructure operators
publish the gas quality parameters to grant access to their systems. As examples, information on some
national situations is given in the following.
A.1.2 German regulations
Code of Practice DVGW G 260:2008 (Relevant parts for natural gases, see Annex B)
NOTE Deutsche Vereinigung 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.1.3 French regulations concerning gas quality
In France, gas quality is principally defined by two governmental 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 shall be published by the infrastructure operators. The two governmental
documents can be summed up as follows:
a) Arrête du 16 septembre 1977
Limits of variations of superior calorific value of natural gas. Reference conditions called normal
conditions (n) are:
P: 1,013 bar                 T: 0 °C
The superior calorific value of natural gas must be between 10,7 and 12,8 kWh/m3 (n) in areas fed by
high cal. gas (H Gas) and between 9,5 and 10,5 kWh/m3 (n) in areas fed by low cal. gas (B Gas). In the
actual regulatory text calorific values are expressed in thermie (th)/m3 (n).
b) Arrête du 28 janvier 1981
Sulfur and sulfur components in natural gases:
The gas must not corrode the pipelines i.e. no component capable of reacting chemically with materials
used in construction of the pipelines or which modifies physical characteristics of these material can be
allowed in natural gas.
— Hydrogen sulfide
Instantaneous content of hydrogen sulfide must be less than 15 milligrams per cubic metre (n).
Hydrogen sulfide content must not exceed 12 milligrams per cubic meter (n) for more than 8
consecutive hours.
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ISO 13686:2013(E)
The average content of hydrogen sulfide for any period of 8 days must be less than 7 milligrams per
cubic metre (n).
— Sulfur
Instantaneous total sulfur content must be less than 150 milligrams per cubic metre (n).
— Water
Water dew point must be less than - 5 °C at the maximum service pressure of the gas pipeline.
A.1.4 UK. Statutory Legislation with respect to gas quality
Within the UK there are certain statutory requirements with respect to gas quality. This legislation
stipulates standards of purity and odourosity that must be met by any supplier of gas through pipes.
These standards are as follows:
— Purity
No person shall supply through pipes any gas which contains more than 5 mg of hydrogen sulphide
per cubic metre.
— Odour
No person shall supply through pipes any gas which does not possess a distinctive odour.
A.2 Interchangeability
A.2.1 General
The interchangeability of natural gases in a given LDS is not only dependent on the relevant gas
parameters, but is also strongly dependent on the characteristics of the appliances used in the LDS and
on the end use pressure of the gas.
Interchangeability can be defined as the ability of a distributed natural gas to be substituted by another
without the need for adjustment at the customer’s equipment. The appliances will continue to operate
safely and satisfactorily.
The criteria to be considered for interchangeability are as follows:
— Thermal input: Flow of gas through an orifice at constant pressure, a function of Wobbe
index.
— Flash back: The tendency for the flame to contract towards the port and for the combus-
tion to take place inside the burner.
— Lifting: Burning surface expands to the point where burning ceases at the port and
burns above it.
— Yellow tipping: Incomplete combustion where excess hydrocarbons could, but do not always,
result in unacceptable levels of carbon monoxide. May result in soot deposi-
tion and a continuing deterioration of combustion.
The substituted gas may be deemed to be interchangeable when, without the need for adjustment of the
appliances, it provides a thermal input comparable with that provided by the gas previously distributed,
without the occurrence of flash back, lifting or yellow tipping.
For the examination of interchangeability there are two routes which can be followed, namely Wobbe
index or gas composition-based prediction methods.
ISO 13686:2013(E)
A.2.2 Wobbe index (see Annexes B, C)
Natural gases are included in the second gas family. Inside the second family different gas groups can
be identified.
Each gas group is a collection of gases characterized by:
— a reference gas with which the appliances operate under nominal conditions, when supplied at the
corresponding normal pressure;
— limit gases representative of the extreme variations in the characteristics of the usable gases;
— test pressures representative of the extreme variations in the appliance supply conditions.
Appliances adjusted on the reference gas, at the normal pressure, and judged to perform satisfactorily
with the limit gases at the test pressures, are approved for use within this gas group. In this approach
the Wobbe index is the primary gas parameter, whose range identifies the gas group.
A.2.3 AGA index method (see Annex D)
In this prediction method for interchangeability, the measured appliance characteristics in the LDS are
translated to defined relevant gas parameters, based on gas composition. Wobbe index is basically a
measure of heat input to the appliance. It is indicative of interchangeability, but not conclusive. When
kept within the established limits as determined by appliance certification procedures, control of the
Wobbe index provides a satisfactory measure.
However, where no such appliance certification regime exists, or for borderline cases of gas composition,
alternative methods for determining interchangeability exist.
A.2.4 British Gas Hydrocarbon Equivalence method (see Annex E)
The British Gas method is a composition and Wobbe index-based prediction method for determining gas
interchangeability within the UK.
A.2.5 Weaver index method (see Annex F)
The Weaver index method introduces the flame speed into the equations particularly for lifting and
flash back.
A.2.6 French method for determining gas interchangeability (Delbourg method) (see
Annex G)
The French method for determining gas interchangeability essentially continues to be the Delbourg
method. The latter is based on the definition of interchangeability indices indicating the limits of gas
combustion. In an appliance at reference conditions, the occurrence of a malfunction (incomplete
combustion, flame lift, flashback, sooting, ignition at the injector) corresponds to a precise index value.
The ranges deemed satisfactory for different indices were suggested to operators in 1963 after studying
a sample of representative appliances available then.
The interchangeability diagram drawn then shows the range in a system of coordinates (corrected
Wobbe number, combustion potential) within which all appliances will function satisfactorily. Any
gas of a different composition is positioned on the basis of the 1963 reference values. The method of
calculation and the interchangeability diagram are shown in Annex G.
12 © ISO 2013 – All rights reserved

ISO 13686:2013(E)
Whenever gas conversion becomes necessary, the likely scenario can be determined with the aid of the
interchangeability indices. Deschamps defined in a general manner the indices for second-family gases.
This new method was employed during the 1970s during the changeover from Groningen to Lacq gas.
NOTE Existing approaches to interchangeability are based essentially on experience and studies with
atmospheric burner, natural draft appliances. The technology of gas appliances and equipment is changing
rapidly. Many advanced efficiency units incorporate power burners with much less excess air allowance. Internal
combustion engines used for cogeneration systems are growing in numbers. Natural gas vehicles, fuel cells, and
other end-use applications are coming into use. Thus, interchangeability parameters and techniques must be
constantly reviewed and updated as natural gas utilization becomes more complex and sophisticated with time.
The European test gas procedures, as embodied in EN 437, provide continuous interchangeability proof for
equipment by means of appliance.
A.3 Condensation curves
A.3.1 Water
Key
1 solid
2 liquid
3 vapour
4 triple point
5 D dew point
6 critical point
ISO 13686:2013(E)
A.3.2 Hydrocarbons
Key
1 liquid
2 vapour
3 critical point
4 cricondenbar
5 dew point
6 cricondentherm
7 dew point
8 retrograde region
9 two phase area
A.4 Odorization
Distributed natural gas is always odorized to provide information about any possible leakage. The level
of odorization is generally chosen so that the information is perceived before the gas concentration in
air reaches 20 % (warning level). The following different categories of odorant blends are generally used
to odorize natural gases.
a) Blends of mercaptans, consisting predominantly of Tertiary Butyl Mercaptan (TBM) with lower
concentrations of Iso Propyl Mercaptan (IPM) and Normal Propyl Mercaptan (NPM).
b) Blends of mercaptans with alkyl sulfides, where Dimethyl Sulfide (DMS) and Methyl Ethyl Sulfide
(MES) are the most commonly used alkyl sulfides.
c) Tetrahydrothiophene (THT): cyclic sulfide used in the gas industry as single component odorant.
d) Blends of THT with mercaptans.
e) Some countries introduced sulfur free or low sulfur odorants based on acrylates.
Odorant used for the odorization of natural gas have to meet the requirements mentioned in ISO 13734.
Odorization guidelines are described in ISO/TS 16922.
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ISO 13686:2013(E)
A.5 Nominal range of natural gas components
A.5.1 European market
As relevant to the European market, ‘Natural gas, dried’ is determined by the components (all
concentrations on a mass-to-mass basis) given in Table A.1.
Table A.1 — Natural gas components
Methane 70,0 % to 98,0 % (w/w)
Ethane 0,3 % to 18,0 % (w/w)
Propane < 8,0 % (w/w)
Butane < 2,0 % (w/w)
Pentane < 0,2 % (w/w)
Nitrogen < 30,0 % (w/w)
Carbon dioxide < 15,0 % (w/w)
The content of each of all other components and constituents is less than 0,1 % (w/w).
NOTE Existing Substances Regulation No 793/93 /EEG of 23 March 1993, Natural gas, dried, EINECS no 270-
085-9, CAS no 68410-63-9).
A.5.2 United States
A.5.2.1 National overview
Natural gas composition to end-use customers in the US is a complex issue, with no particularly ‘correct’
answer. There are certainly differences in the chemical constituents present in natural gas as well
as in the key indices used to measure natural gas ‘quality’ and value: heating value, specific gravity,
and Wobbe index. Existing gas industry practices acquired over the years provide a measure of self-
regulating control and are complemented by contract terms for gas sales, regulatory oversight, desire
for product quality, and the pragmatic need to
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