ISO/TC 193/SC 1 - Analysis of natural gas

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.

This document surveys the quality designation of CBM all around the world, and analyses whether ISO/TC 193 standards for sampling, test and calculation methods are applicable to CBM.

This document describes the precision that can be expected from the gas chromatographic method that is set up in accordance with ISO 6974-1. The stated precision provides values for the magnitude of variability that can be expected between test results when the method described in ISO 6974-1 is applied in one or more competent laboratories. This document also gives guidance on the assessment of bias.

ISO 20729:2017 applies to the determination of total sulfur content in natural gas expressed as sulfur mass concentration ranging from 1 mg/m3 to 200 mg/m3. Natural gas with sulfur contents above 200 mg/m3 can be analysed after dilution with a suitable sulfur-free solvent.

ISO/TR 29922:2017 acts as a repository for those manifold technical details which justify and explain the methods presented in the third edition of ISO 6976 but which are not directly needed in the everyday routine implementation of the standard. Each main clause addresses a specific aspect of the calculational method described in ISO 6976:2016, and is intended to be self-sufficient and essentially independent of each other clause. For this reason, the user should not expect the whole to be accessible to study as a sequentially coherent narrative.

ISO 6976:2016 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. Mole fractions by definition sum to unity. Guidance on the achievement of this requirement by chromatographic analysis is available in ISO 6974‑1 and ISO 6974‑2. 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 standard uncertainties of calculated properties. The methods of calculation of the values of properties on either a molar, mass or volume basis are applicable to any natural gas, natural gas substitute or other combustible fuel that is normally gaseous, except that for properties on the volume 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 document, 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 There are no explicit limits of composition to which the methods described in this document are applicable. However, the restriction of volume-basis calculations to mixtures with a compression factor greater than 0,9 at reference conditions sets implicit limits on composition. NOTE 3 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/TR 29922. NOTE 4 For aliphatic hydrocarbons of carbon number 7 or above, any isomer present is included with the normal isomer of the same carbon number. NOTE 5 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 is 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. Any so-called "spectator water" and "non-combustible hydrogen sulfide" are treated as pseudo-components by setting the appropriate enthalpy of combustion values to zero.

ISO 20765-2:2015 specifies a method to calculate volumetric and caloric properties of natural gases, manufactured fuel gases, and similar mixtures, at conditions where the mixture may be in either the homogeneous (single-phase) gas state, the homogeneous liquid state, or the homogeneous supercritical (dense-fluid) state.

ISO 16960:2014 specifies a method for the determination of total sulfur in the range from 1 mg/m3 to 200 mg/m3 in pipeline natural gas by oxidative microcoulometry. Natural gas with sulfur contents above 200 mg/m3 can be analysed after dilution with a suitable sulfur-free solvent.

ISO 6974-5:2014 describes a gas chromatographic method for the quantitative determination of the content of nitrogen, carbon dioxide and C1 to C5 hydrocarbons individually and a composite C6+ measurement, which represents all hydrocarbons of carbon number 6 and above in natural gas samples.

ISO 10723:2012 specifies a method of determining whether an analytical system for natural gas analysis is fit for purpose. It can be used either to determine a range of gas compositions to which the method can be applied, using a specified calibration gas, while satisfying previously defined criteria for the maximum errors and uncertainties on the composition or property or both, or to evaluate the range of errors and uncertainties on the composition or property (calculable from composition) or both when analysing gases within a defined range of composition, using a specified calibration gas.

This part of ISO 6974 describes the process required to determine the uncertainty associated with the mole fraction for each component from a natural gas analysis in accordance with ISO 6974‑1.

ISO/TR 12148:2009 describes the principles of, and general requirements for, the traceable calibration of automatic hydrocarbon-dew-point chilled-mirror instruments using the indirect automatic weighing method (method B) described in ISO 6570:2001 to determine the potential hydrocarbon liquid content of natural gas, or similar gas. The calibration procedure is intended for use by chilled-mirror instruments in downstream applications transferring processed natural gas. If the gas composition is constant, the manual weighing method (method A) described in ISO 6570:2001 is also applicable. The application of this calibration procedure in the upstream area is not excluded a priori, however, currently there is no experience using this procedure in an upstream environment. The usability of data on the potential hydrocarbon liquid content of natural gas for verification, adjustment or calibration of hydrocarbon-dew-point chilled-mirror instruments is based on the condensation behaviour of natural gases. ISO/TR 12148:2009 provides information on the condensation behaviour of natural gases and the various measuring techniques to determine properties, like hydrocarbon dew point and potential hydrocarbon liquid content, related to the condensation behaviour of natural gases.

ISO/TR 11150:2007 describes the various means of estimating hydrocarbon dew point and hydrocarbon content of natural gas.

ISO 12213 specifies methods for the calculation of compression factors of natural gases, natural gases containing a synthetic admixture and similar mixtures at conditions under which the mixture can exist only as a gas. It is divided into three parts: this part, ISO 12213-2:2006, specifies a method for the calculation of compression factors when the detailed composition of the gas by mole fractions is known, together with the relevant pressures and temperatures. The method is applicable to pipeline quality gases within the ranges of pressure p and temperature T at which transmission and distribution operations normally take place, with an uncertainty of about +/- 0,1 %. It can be applied, with greater uncertainty, to wider ranges of gas composition, pressure and temperature.

ISO 12213 specifies methods for the calculation of compression factors of natural gases, natural gases containing a synthetic admixture and similar mixtures at conditions under which the mixture can exist only as a gas. It is divided into three parts: this part, ISO 12213-1:2006, gives an introduction and provides guidelines for the methods of calculation described in Parts 2 and 3.

ISO 12213 specifies methods for the calculation of compression factors of natural gases, natural gases containing a synthetic admixture and similar mixtures at conditions under which the mixture can exist only as a gas. It is divided into three parts: this part, ISO 12213-3:2006, specifies a method for the calculation of compression factors when the superior calorific value, relative density and carbon dioxide content are known, together with the relevant pressures and temperatures. If hydrogen is present, as is often the case for gases with a synthetic admixture, the hydrogen content also needs to be known. The method is primarily applicable to pipeline quality gases within the ranges of pressure p and temperature T at which transmission and distribution operations normally take place, with an uncertainty of about +/-0,1 %. For wider-ranging applications the uncertainty of the results increases.

ISO 23874:2006 describes the performance requirements for analysis of treated natural gas of transmission or pipeline quality in sufficient detail so that the hydrocarbon dewpoint temperature can be calculated using an appropriate equation of state. ISO 23874:2006 can be applied to gases that have maximum dewpoint temperatures (cricondentherms) between 0 °C and - 50 °C. The pressures at which these maximum dewpoint temperatures are calculated are in the range 2 MPa (20 bar) to 5 MPa (50 bar). The procedure given in ISO 23874:2006 covers the measurement of hydrocarbons in the range C5 to C12. n-Pentane, which is quantitatively measured using ISO 6974 (all parts), is used as a bridge component and all C6 and higher hydrocarbons are measured relative to n-pentane. Major components are measured using ISO 6974 (all parts) and the ranges of components that can be measured are as defined in ISO 6974-1.

ISO/TR 24094:2006 describes the validation of the calorific value and density calculated from current practice natural gas analysis by statistical comparison with values obtained by measurement using a reference calorimeter and a density balance.

This part of ISO 20765 specifies a method of calculation for the volumetric and caloric properties of natural gases, natural gases containing synthetic admixture and similar mixtures, at conditions where the mixture can exist only as a gas. The method is applicable to pipeline-quality gases within the ranges of pressure and temperature at which transmission and distribution operations normally take place. For volumetric properties (compression factor and density), the uncertainty of calculation is about ± 0,1 % (95 % confidence interval). For caloric properties (for example enthalpy, heat capacity, Joule-Thomson coefficient, speed of sound), the uncertainty of calculation is usually greater.

ISO 18453:2004 specifies a method to provide users with a reliable mathematical relationship between water content and water dew point in natural gas when one of the two is known. The calculation method, developed by GERG; is applicable to both the calculation of the water content and the water dew point. ISO 18453:2004 gives the uncertainty for the correlation but makes no attempt to quantify the measurement uncertainties.

ISO 19739:2004 specifies the determination of hydrogen sulfide, carbonyl sulfide, C1 to C4 thiols, sulfides and tetrahydrothiophene (THT) using gas chromatography (GC). Depending on the method chosen from those given in its annexes, the application ranges for the determination of sulfur compounds can vary, but whichever of the methods is used, its requirements apply.

ISO 6978-1:2003 specifies a method for the determination of total mercury content in natural gas using a sampling method at pressures up to 40 MPa by chemisorption on iodine impregnated silica gel. This sampling method is suitable for the determination of mercury contents within the range of 0,1 microgram/m3 to 5 000 micrograms/m3 in natural gas. This method is applicable to sampled gas volumes containing less than 20 mg hydrogen sulfide and less than a total liquid hydrocarbon condensate of 10 g/m3 under the sampling conditions. The collected mercury is determined by measuring the absorbance or fluorescence of mercury vapour at 253,7 nm.

This ISO 6978-2:2003 specifies a method for the determination of total mercury content of pipeline quality natural gas using a sampling method by amalgamation on gold/platinum (Au/Pt) alloy thread. This method is applicable to the sampling of raw natural gas when no condensation is present. At atmospheric pressure, this method is suitable for the determination of mercury content within the range of 0,01 microgram /m3 to 100 micrograms/m3 in natural gas samples. At higher pressures (up to 8 MPa), this sampling method is suitable for the determination of mercury contents within the range of 0,001 microgram/m3 to 1 microgram/m3. The collected mercury is determined by measuring the absorbance or fluorescence of mercury vapour at 253,7 nm.

ISO 6974-6:2002 describes a gas chromatographic method for the quantitative determination of the content of helium, hydrogen, oxygen, nitrogen, carbon dioxide and C1 to C8 hydrocarbons in natural gas samples using three capillary columns. This method is applicable to the determination of these gases within the mole fraction ranges varying from 0,000 1 % to 40 %, depending on the component analysed, and is commonly used for laboratory applications. However, it is only applicable to methane within the mole fraction range of 40 % to 100 %. These ranges do not represent the limits of detection, but the limits within which the stated precision of the method applies. Although one or more components in a sample may not be present at detectable levels, the method can still be applicable. ISO 6974-6:2002 is only applicable if used in conjunction with ISO 6974-1:2000 and ISO 6974-2:2001. This method can also be applicable to the analysis of natural gas substitutes. Additional information on the applicability of this method to the determination of natural gas substitutes is also given where relevant.

This International Standard describes the principles of, and general requirements for, two gravimetric methods for the determination of the potential hydrocarbon liquid content of natural gas, or similar gas, at a given pressure and temperature. Two methods are specified in this International Standard to determine the amount of condensate in a sample gas: _ Method A: a manual weighing method; _ Method B: an indirect automatic weighing method based on the indication of the pressure difference caused by the accumulation of condensate in a vertical tube. The manual weighing method is a reference method for the indirect automatic method (Method B). The indirect automatic method (Method B) is suitable for semi-continuous control. NOTE Unless otherwise specified, gas volumes are in cubic metres at 273,15 K and 101,325 kPa.

Describes a method of determining the water content of natural gas under pressure of more than 1 MPa. Also applicable to natural gas, containing hydrogen sulfide with a water concentration of 10 mg/m^3 or more.

The principle of the method specified is passing a measured volume of gas through a cell containing a relatively small volume of absorbent solution. Water in the gas is extracted by the absorbent solution and subsequently titrated with Karl Fischer reagent. The design of the cell and the absorbent solution are chosen so as to ensure efficient collection of the water at the high flowrates necessary. Is applicable to water concentrations between 5 mg/cm^3 and 5000 mg/cm^3.

Specifies general requirements for the determination. The principle of the method is reaction of water present in the test sample with iodine and sulfur-dioxide in a pyridine/methanol mixture (Karl Fischer reagent). ISO 10101-2 and ISO 10101-3 specify two individual methods of determination.

The principle of the method specified is passing a measured volume of gas through the titration cell, where the water is absorbed by the anodic solution. The iodine required for the determination of water by the Karl Fischer reaction is generated coulometrically from iodide. The quantity of electricity is directly proportional to the mass of iodine generated and hence to the mass of water determined. Applies to natural gas and other gases which do not react with Karl Fischer reagents. Is applicable to water concentrations between 5 mg/cm^3 and 5000 mg/cm^3.

Describes hygrometers which determine the water content of a gas by detecting water vapour condensation occurring on a cooled surface or by checking the stability of the condensation on this surface. The hygrometers considered here may be used for determining water vapour pressure, without requiring calibration, in a system operating under total pressures greater than or equal to atmospheric pressure.

ISO 6326-1:2007 gives a brief description of standardized methods that can be used for the determination of sulfur compounds in natural gas. The principle of each method is described generally, the range of concentrations for which the method is suitable is indicated, and the analytical range and precision of each method is given. It should enable the user to select judiciously the proper method for the application being considered. Sulfur analysis is performed in order to determine total sulfur, sulfur contained in specific groups [e.g. thiols (mercaptans)], individual sulfur compounds and specific groups of sulfur compounds. The available standardized methods in the field of sulfur analysis are the Wickbold combustion method for total sulfur determination (ISO 4260); the Lingener combustion method for total sulfur determination (ISO 6326-5); gas chromatography for determination of individual sulfur compounds (ISO 19739); potentiometry for determination of hydrogen sulfide, carbonyl sulfide and thiol compounds (ISO 6326-3).

Describes a method of assessing whether an analytical system for natural gas is satisfactory provided that the analytical requirements have been clearly defined and the analytical and calibration procedures have been fully described.

Specifies methods for the calculation of the superior calorific value and the inferior calorific value, density, relative density and Wobbe index of dry natural gas and other combustible gaseous fuels, when the composition of the gas by mole fraction is known. Replaces the first edition, which has been technically revised.

The method specified uses a single temperature-programmed column, and a sulfur-selective flame photometric detector. It is applicable to the determination of hydrogen sulfide, carbonyl sulfide, C1 - C4 thiols and sulfides and tetrahydrothiophene, generally in the range 0,5 µmol/mol to 50 µmol/mol. It is also applicable to the quantitative determination of sulfur compounds other than hydrogen sulfide, when hydrogen sulfide is present at concentrations up to 5000 µmol/mol.

Two methods are specified: method A for higher mercury content (> 0,5 µg/m^3) based on sampling at atmospheric pressure, absorption in potassium permangante solution, reduction of mercury ions, analysis by flameless atomic absorption spectrometry; method B for lower mercury content (10^- 3 µg/m^3 to 1 µg/m^3) based on sampling at atmospheric or higher pressure, adsorption on silver/gold, desorption, analysis by flameless atomic absorption spectrometry.

Specifies a method for the determination of total sulphur in natural gas. It is applicable to gases with sulphur contents between 0,5 mg/m3 and 1 000 mg/m3. With a total sulphur content of more than 0,1 mg sulphur in the absorption solution, visual titration with an indicator can be chosen, whereas for lower contents turbidimetric titration is preferable.

Specifies a potentiometric method for components in natural gas in the concentration range equal to or above 1 mg/m3. The gas must be free of dust, mist, oxygen, hydrogen cyanide and carbon disulfide. The method is not recommended for gases containing more than approximately 1,5 % (V/V) carbon dioxide.