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ISO DIS 6976 2015 Edition, March 19, 2015 NATURAL GAS - CALCULATION OF CALORIFIC VALUES, DENSITY, RELATIVE DENSITY AND WOBBE INDICES FROM COMPOSITION
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Description / Abstract:
This International Standard specifies methods for the
calculation of gross calorific value, net calorific value, density,
relative density, gross Wobbe index and net Wobbe index of natural
gases, natural gas substitutes and other combustible gaseous fuels,
when the composition of the gas by mole fraction is known. The
methods specified provide the means of calculating the properties
of the gas mixture at commonly used reference conditions
For the purpose of this International Standard the input mole
fractions shall sum to unity exactly. Guidance on the achievement
of this requirement is available in ISO 6974-1 and ISO 6974-2. All
components with mole fractions greater than 0,000 05 shall be
accounted for.
If the composition of the gas is known by volume fractions,
these shall first be converted to mole fractions in accordance with
ISO 14912 (subclause 5.1.2). Note, however, that these derived mole
fractions will have uncertainties greater than those of the
original volume fractions.
The methods of calculation require values for various physical
properties of the pure components; these values, together with
associated uncertainties, are provided in tables and their sources
are identified.
Methods are given for estimating the uncertainties of calculated
properties.
The methods of calculation of the values of properties on either
a molar, mass or volumetric basis are applicable to any natural
gas, natural gas substitute or other combustible fuel that is
normally gaseous, except that for properties on the volumetric
basis the method is restricted to mixtures for which the
compression factor at reference conditions is greater than 0,9.
Example calculations are given in annex D for the recommended
methods of calculation
NOTE 1 The qualifiers "superior", "higher", "upper" and "total"
are, for the purposes of this International Standard, synonymous
with "gross"; likewise, "inferior" and "lower" are synonymous with
"net". The term "heating value" is synonymous with "calorific
value"; "mass density" and "specific density" are synonymous with
"density"; "specific gravity" is synonymous with "relative
density"; "Wobbe number" is synonymous with "Wobbe index";
"compressibility factor" is synonymous with "compression factor".
The dimensionless quantity molecular weight is numerically equal to
the molar mass in kg·kmol-1.
NOTE 2 For aliphatic hydrocarbons of carbon number 7 or above,
any isomer present at a mole fraction greater than 0,000 05 should
be included with the normal isomer of the same carbon number.
NOTE 3 If the user's requirement includes the replacement of,
for example, a C6+ or C7+ grouping of analytically unresolved
components by a single pseudo-component, then it shall be the
user's own task to set the mole fraction composition, and hence
properties, of this pseudo-component so as to be fit for purpose in
the particular application. Socalled ‘spectator water' and
‘non-combustible hydrogen sulfide' may also be treated as
pseudo-components by setting the appropriate enthalpy of combustion
values to zero.
NOTE 4 There are no explicit limits of composition to which the
methods described in this International Standard are applicable.
However, the restriction of volumetric basis calculations to
mixtures with compression factor greater than 0,9 at reference
conditions sets implicit limits on composition.
NOTE 5 Because the mole fraction of any water present is not
normally available from chromatographic analysis, it is common
practice to calculate the physical properties on a dry gas basis
and to allow for the effects of water vapour in a separate
procedure. However, if the mole fraction of water vapour is known
then the property calculations can be carried out completely in
accordance with the procedures described herein. The effects of
water vapour on calorific value, whether the latter is directly
measured or calculated, are discussed in ISO Technical Report TR
29922 [1].