British Standard

A single copy of this British Standard is licensed to

Geoff Leech
on December 07, 2001
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This is an uncontrolled copy. Ensure use of the most

current version of this document by searching British
Standards Online at bsonline.techindex.co.uk
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 BS ISO 6974-5:2000

National foreword
This British Standard reproduces verbatim ISO 6974-5:2000 and implements it as the
UK national standard. Together with BS ISO 6974 parts 1-4 and 6 it supersedes BS
3156-11.1.1:1986 which is withdrawn.
The UK participation in its preparation was entrusted to Technical Committee
PTI/15, Natural gas and gas analysis, which has the responsibility to:

Ð aid enquirers to understand the text;

Ð present to the responsible international/European committee any enquiries
on the interpretation, or proposals for change, and keep the UK interests
informed;
Ð monitor related international and European developments and promulgate
them in the UK.

A list of organizations represented on this committee can be obtained on request to

its secretary.
Cross-references
The British Standards which implement international or European publications
referred to in this document may be found in the BSI Standards Catalogue under the
section entitled ªInternational Standards Correspondence Indexº, or by using the
ªFindº facility of the BSI Standards Electronic Catalogue.
A British Standard does not purport to include all the necessary provisions of a
contract. Users of British Standards are responsible for their correct application.
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

Compliance with a British Standard does not of itself confer immunity

from legal obligations.

Summary of pages
This document comprises a front cover, an inside front cover, the ISO title page,
pages ii to v, a blank page, pages 1 to 14, an inside back cover and a back cover.
The BSI copyright notice displayed in this document indicates when the document
was last issued.

This British Standard, having Amendments issued since publication

been prepared under the
direction of the Sector Amd. No. Date Comments
Committee for Materials and
Chemicals, was published under
the authority of the Standards
Committee and comes into effect
on 15 August 2000

 BSI 08-2000

ISBN 0 580 36318 X

 INTERNATIONAL ISO
STANDARD 6974-5

First edition
2000-04-01

Natural gas — Determination of

composition with defined uncertainty by
gas chromatography —
Part 5:
Determination of nitrogen, carbon dioxide
and C1 to C5 and C6+ hydrocarbons for a
laboratory and on-line process application
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

using three columns

Gaz naturel — Détermination de la composition avec une incertitude
définie par chromatographie en phase gazeuse —
Partie 5: Détermination de l'azote, du dioxyde de carbone et des
hydrocarbures (C1 à C5 et C6+) pour l'application du processus en continu
employant trois colonnes

Reference number
ISO 6974-5:2000(E)
 Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

ii
ISO 6974-5:2000(E)
 ISO 6974-5:2000(E)

Contents Page

Foreword.....................................................................................................................................................................iv
Introduction .................................................................................................................................................................v
1 Scope ..............................................................................................................................................................1
2 Normative references ....................................................................................................................................2
3 Principle..........................................................................................................................................................2
4 Materials .........................................................................................................................................................3
5 Apparatus .......................................................................................................................................................3
6 Procedure .......................................................................................................................................................4
6.1 Gas chromatographic operating conditions...............................................................................................4
6.2 Calibration ......................................................................................................................................................5
6.3 Performance requirements ...........................................................................................................................6
6.3.1 Peak resolution ..............................................................................................................................................6
6.3.2 Chromatogram ...............................................................................................................................................6
6.4 Determination.................................................................................................................................................6
6.4.1 Sample valve purge .......................................................................................................................................6
6.4.2 Analysis ..........................................................................................................................................................7
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7 Expression of results ....................................................................................................................................7

7.1 Precision and accuracy.................................................................................................................................7
8 Test report ......................................................................................................................................................7
Annex A (informative) Procedure for setting valve timings and restrictor setting ............................................11
Annex B (informative) Typical precision values ....................................................................................................13
Bibliography ..............................................................................................................................................................14

iii
 ISO 6974-5:2000(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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.

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.

Attention is drawn to the possibility that some of the elements of this part of ISO 6974 may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.

International Standard ISO 6974-5 was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee
SC 1, Analysis of natural gas.

This part as well as the other five parts of ISO 6974 cancel and replace ISO 6974:1984 which specified only one
method.
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

ISO 6974 consists of the following parts, under the general title Natural gas — Determination of composition with
defined uncertainty by gas chromatography :

— Part 1: Guidelines for tailored analysis

— Part 2: Measuring-system characteristics and statistics for data treatment

— Part 3: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to C8 using
two packed columns

— 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

— Part 5: Determination of nitrogen, carbon dioxide and C1 to C5 and C6+ hydrocarbons for a laboratory and on-
line process application using three columns

— Part 6: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to C8 using
three capillary columns

Annexes A and B of this part of ISO 6974 are for information only.

iv
 ISO 6974-5:2000(E)

Introduction
This part of ISO 6974 describes a precise and accurate method for the determination of the composition of natural
gas. The compositional data obtained are used for the calculation of the calorific value, the relative density and the
Wobbe index.

This method is based on a automatic column-switching technique in which multiple columns, chosen for their
separating ability for particular groups of components, are switched automatically.

This part of ISO 6974 provides one of the methods that may be used for determining the composition of natural gas
in accordance with parts 1 and 2 of ISO 6974.
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

v
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI
 INTERNATIONAL STANDARD ISO 6974-5:2000(E)

Natural gas — Determination of composition with defined

uncertainty by gas chromatography —
Part 5:
Determination of nitrogen, carbon dioxide and hydrocarbons C1
up to C5 and C6+ for a laboratory and on-line process application
using three columns

1 Scope
This part of ISO 6974 describes a gas chromatographic method for the quantitative determination of natural gas
constituents using a three-column system. This method is applicable to natural gases of limited range, on-line and
automatically calibrating on a regular basis for gas samples not containing any hydrocarbon condensate and/or
water. It is applicable to the analysis of gases containing constituents within the mole fraction ranges given in
Table 1. 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 detected present, the method can still
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

be applicable.

This part of ISO 6974 is only applicable if used in conjunction with parts 1 and 2 of ISO 6974.

Table 1 — Application ranges

Component Mole fraction range

%
Nitrogen 0,001 to 15,0
Carbon dioxide 0,001 to 8,5
Methane 75 to 100
Ethane 0,001 to 10,0
Propane 0,001 to 3,0
iso-Butane (2-methylpropane) 0,001 to 1,0
n-Butane 0,001 to 1,0
neo -Pentane (2,2-dimethylpropane) 0,001 to 0,5
iso-Pentane (2-methylbutane) 0,001 to 0,5
n-Pentane 0,001 to 0,5
Hexanes + sum of all C6 and higher hydrocarbons 0,001 to 1,0

NOTE 1 Hydrocarbons higher than n-pentane are expressed as the "pseudo-component" C6+ which is measured as one
composite peak and calibrated as such. The properties of C6+ are calculated from detailed analyses of the individual C6 and
higher hydrocarbons by extended analysis or from historical data.

NOTE 2 Oxygen is not a normal constituent of natural gas and would not be expected to be present in gas sampled for an
on-line instrument. If any oxygen is present as a result of air contamination, it will be measured with the nitrogen. The resulting
(nitrogen + oxygen) value will be in error to a small extent because of the slight difference between detector responses for

1
 ISO 6974-5:2000(E)

oxygen and nitrogen. Nonetheless, the result for the natural gas/air mixture will be reasonably accurate since neither component
contributes to the calorific value.

NOTE 3 The content of helium and argon are assumed to be negligible and unvarying such that helium and argon need not
be determined.

2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this part of ISO 6974. For dated references, subsequent amendments to, or revisions of, any of these publications
do not apply. However, parties to agreements based on this part of ISO 6974 are encouraged to investigate the
possibility of applying the most recent editions of the normative documents indicated below. For undated
references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain
registers of currently valid International Standards.

ISO 6142, Gas analysis — Preparation of calibration gas mixtures — Gravimetric method.

ISO 6974-1:2000, Natural gas — Determination of composition with defined uncertainty by gas chromatography —
Part 1: Guidelines for tailored analysis.

ISO 6974-2:—1), Natural gas — Determination of composition with defined uncertainty by gas chromatography —
Part 2: Measuring-system characteristics and statistics for data treatment.

ISO 7504:1984, Gas analysis — Vocabulary.

Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

3 Principle
Determination of nitrogen, carbon dioxide and hydrocarbons from C1 to C6+ by gas chromatography using a three-
column switching/backflush arrangement, configured as shown in Figure 1. The three chromatographic columns
are connected by two six-port valves for handling sample injection and backflushing operations (or alternatively a
single ten-port valve is used) to a thermal conductivity detector (TCD) which is used for quantification.

A single sample is injected first onto a boiling-point separation column, divided into short and long sections. The C6
and heavier hydrocarbons are initially retained on the short section of this column. The long section of this column
retains C3 to C5 hydrocarbons. The lighter components (nitrogen, methane, carbon dioxide and ethane) pass
rapidly and unresolved through the boiling-point separation column onto a porous polymer-bead column, suitable
for their retention and separation. Following an accelerated backflush of the short column situated closest to the
detector, the heavier C6+ hydrocarbons (determined as a recombined "pseudo component" rather than by the
summation of individual component measurements) elute first and are quantified as a single peak. Next, from the
longer section of this column situated farther from the detector, the C3 to C5 hydrocarbons are separated then
quantified by TCD. Finally, by redirecting carrier gas onto the porous polymer-bead column, the lighter
components, i.e. nitrogen, carbon dioxide, methane and ethane, are separated then quantified by the detector. A
six-port valve either connects this column to the carrier-gas flow or by-passes it during measurement of C3 to C5
components.

The separations that occur in the columns are as follows:

Column 1 Retains C6+ components ready for backflushing as one composite peak.

Column 2 Separates propane, iso-butane, n-butane, neo-pentane, iso-pentane and n-pentane, (which elute after
C6+ has left column 1).

1) To be published.

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 ISO 6974-5:2000(E)

Column 3 Retains and separates nitrogen, methane, carbon dioxide and ethane which elute after n-pentane has
left column 2.

4 Materials
4.1 Helium carrier gas, > 99,99 % pure, free from oxygen and water.

4.2 2-Dimethyl-butane, used to check complete backflushing of C6+, 95% pure.

4.3 Working-reference gas mixtures (WRM), the composition of which shall be chosen to be similar to that of
the anticipated sample.

A cylinder of distributed natural gas, containing all the components measured by this method is to be used by the
laboratory as a control gas. The working-reference gas mixtures shall be prepared in accordance with ISO 6142.
The working-reference gas mixture shall contain at least nitrogen, carbon dioxide, methane, ethane, propane,
n-butane, iso-butane and possibly iso-pentane, n-pentane, neo-pentane and n-hexane.

5 Apparatus
5.1 Laboratory gas chromatographic (GC) system, consisting of the following components:

5.1.1 Gas chromatograph (GC), capable of isothermal operation and equipped as follows:

a) column oven, capable of being maintained to within
0,1 °C over the temperature range: 70 °C to 105 °C;
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

b) valve oven, capable of being maintained over the temperature range 70 °C to 105 °C or alternatively having
the capacity to fit the valves in the column oven;

c) flow regulators, capable of regulating the carrier gas flowrates.

5.1.2 Injection device, consisting of a ten-port sample-injection valve V1 and also used for backflushing C6+
components (two six-port valves may be used for these duties using the same operating principle). See Figure 1.

5.1.3 Column isolation valve, six-port, to by-pass the porous polymer bead column (column 3). See valve 2 in
Figure 1.

5.1.4 Metal columns packed with either 28 % DC-200/500 on Chromosorb PAW or 15 % DC-200/500 on
Porapak N, satisfying the performance requirements given in clause 6.3 and consisting of the following packing
materials and column dimensions, given as examples, for use with conventional and readily available injection
valves and TCD.

Any alternative combination of columns which provide similar separations and satisfy the performance
requirements may be used. Micro-packed or even capillary columns can be chosen, with appropriately-sized
injection and detector systems, in which case packing or coating details would be different.

Columns shall satisfy the following requirements:

a) metal tubing:

 column 1: 0,75 m (2,5 ft) long, 2 mm internal diameter (i.d.) (1/8 in o.d.)

 column 2: 5,2 m (17 ft) long, 2 mm i.d. (1/8 in o.d.)

 column 3: 2,4 m (8 ft) long, 2 mm i.d. (1/8 in o.d.)

b) packing:

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 ISO 6974-5:2000(E)

 column 1: 28 % DC-200/500 on Chromosorb PAW (45 ASTM mesh to 60 ASTM mesh)

 column 2: 28 % DC-200/500 on Chromosorb PAW (45 ASTM mesh to 60 ASTM mesh)

 column 3: 15 % DC-200/500 on Porapak N (50 ASTM mesh to 80 ASTM mesh) for the separation of
nitrogen, methane, carbon dioxide and ethane

NOTE Columns 1 and 2 are boiling-point separation columns for the separation of propane, iso-butane, n-butane,
neo-pentane, iso-pentane, n-pentane. They effectively are a single column, divided into short and long sections for rapid
backflush of C6+ components to the detector.

c) method of packing: packed by any suitable packing method providing uniform column packing and
performance characteristics in accordance with 6.2. If purchased individually, as part of a system, or packed
individually, their performance shall comply with the specification. When packed individually it is assumed that
this will be according to a recognized technique.

NOTE The following packing method is suitable:

Close the column outlet with a sintered disc or glass wool plug. Connect a reservoir containing rather more packing than is
needed to fill the column to the inlet and apply a pressure of 0,4 MPa of nitrogen to this reservoir. The flow of packing into
the column is assisted by vibration. When the column is full, allow the pressure to decay slowly before disconnecting the
reservoir.

d) conditioning: with freshly prepared columns, more stable baselines can be obtained by conditioning them
overnight, with carrier gas flowing, at a temperature of 50 °C higher than that at which they are intended to
operate. If this is necessary, but not easily achieved in the gas chromatograph in which the columns are to be
used, they can be conditioned after being installed in another unit.
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5.1.5 Thermal conductivity detector (TCD), with a time constant of not greater than 0,1 s, and internal volume
appropriate for the column sizes and flowrate used.

5.1.6 Controller/peak-measurement system, having a wide range of sensitivity (0 V to 1 V), capable of

measuring peaks on a sloping baseline and able to control automatic operation of the valves according to a
sequence selected by the operator.

5.1.7 Auxiliary equipment, consisting of valves, tubing and any other accessories, to control the flow of sample
gas to the chromatograph and for shutting off this flow for a defined period of time before injection.

6 Procedure

6.1 Gas chromatographic operating conditions

If the apparatus has been used for previous determinations, ensure that it is returned to the starting conditions
before injecting a sample or calibration gas mixture.

Set the operating conditions for the apparatus (5.1) as follows.

a) Oven column: 70 °C to 105 °C, capable of being maintained to within
0,1 °C

b) Carrier gas flowrate: dependent upon the column diameter. With the carrier gas flowing through all columns in
the order column 1 to column 2 to column 3 [Figure 1 b)], set the flowrate so that the average linear velocity for
nitrogen (total column length divided by hold-up time) is between 10 cm/s and 15 cm/s.

c) Valves: if the valves are not fitted in the column oven, set them to a temperature in the range of 70 °C to
105 °C (no less than the column temperature), isothermal and stable to
2 °C.

4
 ISO 6974-5:2000(E)

d) Detector: TCD

¾ temperature setting: between 70 °C and 105 °C;

¾ set the bridge current according to manufacturer's instructions.

e) Controller/peak-measurement system: set up in accordance with the manufacturer's instructions.

Set up the gas chromatograph according to the manufacturer's instructions.

Table 2 summarizes column data and operating conditions for typical implementation of this method.

Table 2 — Typical chromatographic conditions

Column characteristics Column 1 Column 2 Column 3

Silicone oil Silicone Oil Silicone oil
Stationary phase
DC-200/500 DC-200/500 DC-200/500
Loading % 28 % 28 % 15 %
Support Chromosorb PAW Chromosorb PAW —
Active solid — — Porapak N
ASTM mesh size 45/60 45/60 50/80
Column length 0,75 m 5,2 m 2,4 m
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

Column i.d. 2 mm 2 mm 2 mm
Material Stainless steel
Gas chromatographic conditions
Oven temperature 100 °C
Carrier gas Helium
Pressure 400 kPa (4 bar)
Flow 28 ml/min
Detector TCD
Detector temperature 100 °C
Injection device Valve
Injection device temperature 100 °C
Sample volume 1,0 ml

6.2 Calibration

Calibrate the equipment in accordance with the procedures described in parts 1 and 2 of ISO 6974.

The use of a single calibration standard is consistent either with the assumption that instrument response to a
component is represented by a straight line through the origin or that it is some other function which has been well
defined. If the response differs from that which is assumed then the use of a single calibration standard will
introduce an error. The scale of such an error can be assessed by testing the linearity using the method given in
ISO 10723[1] with the wide-ranging test gases prepared as described in ISO 6142. The nature of such testing is
outside the scope of this International Standard.

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 ISO 6974-5:2000(E)

6.3 Performance requirements

6.3.1 Peak resolution

It is important that all components be measured with as less interference from others as possible. Possible
interference can be assessed by measuring peak resolution in accordance with 3.3.4.2 of ISO 7504:1984. Although
the resolution of all peaks is important, there are no particular pairs of peaks which are critical although satisfactory
resolution of one pair can ensure that of other pairs of peaks.

Furthermore, the resolution required is likely to vary with respect to component uncertainty although it may be
deemed acceptable for particular applications. If the procedure is implemented correctly, the acceptable resolution
values indicated in Table 3 are to be expected. Higher resolution may require modification of column dimensions,
temperature and flowrate, and would likely require longer analysis time.

Each value of resolution shall be tested as part of the normal analytical cycle, not by some alternative procedure
designed only to measure these parameters.

Table 3 — Peak resolution

Component 1 Component 2 Acceptable resolution High resolution

iso-Butane n-Butane 1,0 1,5
Nitrogen Methane 0,75 1,5
Carbon dioxide Ethane 2,0 2,3
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6.3.2 Chromatogram

A typical chromatogram is shown in Figure 2.

The order of elution of components should not change, but actual retention times shall be determined for each
individual system.

Measure the areas or heights of component peaks from the detector with an integrator or data system. (Use areas
for all major components, but it may be found that peak heights give better repeatability for some minor
components, such as the pentanes, where peaks are both small and relatively wide.) Set the integration
parameters so as to correctly allocate baselines, and so that there is no interference with peak measurement from
valve-switching disturbances.

6.4 Determination

6.4.1 Sample valve purge

Purge the sample valve with the gas to be analysed, using at least 20 times the volume of the valve and associated
pipework.

Stop the purge to enable the gas to reach the temperature of the valve and ambient pressure, then start the
analytical cycle, injecting the sample and switching the valves as required.

If this volume of sample is not enough to purge the valve, contamination by air or by the previous sample will
interfere with the determination. If either occur, then use a larger volume of sample for purging.

NOTE The sample loop should be purged with gas for a precise time period, at a defined rate, and the sample should then
be allowed to equilibrate to ambient pressure before injection. In the absence of equipment which can confirm the latter, there
should be a defined time between sample-valve shut-off and injection.

6
 ISO 6974-5:2000(E)

6.4.2 Analysis

The analytical system shown in Figure 1 comprises one ten-port sample-injection/backflush valve, V1, and one six-
port by-pass valve V2. Restrictor A maintains the pneumatic balance of the system when column 3 is isolated. The
detailed setting-up procedure is given in annex A. (Two six-port valves may be used in place of the 10-port valve
V1, one for controlling sample injection and the other backflushing column 1. If these operations are simultaneous,
their timings may be taken to be the same as those for a single 10-port valve.)

The time settings of the valve switching operations shall ensure that

a) V1 is returned to the backflush configuration (configuration 1) after all the n-pentane leaves column 1 but
before the lowest C6 isomer leaves column 1 on its way to column 2,

b) V2 is switched to isolate column 3 (configuration 2) before any propane leaves column 2 (on its way to
column 3) and after all the ethane has left column 2 and entered column 3,

c) V2 is not returned to reconnect column 3 (configuration 1) until all the n-pentane has been detected, having
emerged from column 2 via column 1.

7 Expression of results
Refer to ISO 6974-1.

7.1 Precision and accuracy

Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

Refer to ISO 6974-2.

See annex B for typical precision values.

8 Test report
Report the results in accordance with clause 14 of ISO 6974-1:2000.

7
 ISO 6974-5:2000(E)

a) Valve 1 (V1) in configuration 1 and valve 2 (V2) in configuration 1

Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

b) Valve 1 (V1) in configuration 2 and valve 2 (V2) in configuration 1

8
 ISO 6974-5:2000(E)

c) Valve 1 (V1) in configuration 1 and valve 2 (V2) in configuration 2

Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

d) Valve 1 (V1) in configuration 2 and valve 2 (V2) in configuration 2

Key
1 Carrier gas 5 Column 1
2 Sample introduction 6 Column 2
3 Sample loop 7 Column 3
4 TCD detector 8 Restrictor A

Figure 1 — Valve configurations for natural gas analyser

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 Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

10
ISO 6974-5:2000(E)

Figure 2 — Example of a typical chromatogram

 ISO 6974-5:2000(E)

Annex A
(informative)

Procedure for setting valve timings and restrictor setting

A.1 Set all the valves to configuration 1 [Figure 1 a)], so that the carrier gas flows from column 2 (long boiling-
point separation column) to column 3 (porous polymer-bead column) to column 1 (short boiling-point separation
column) to the detector. Set the column temperature and the TCD analytical gas-flowrate to manufacturer's
suggested values. In the absence of manufacturer's values, use 95 °C and 28 ml/min, for a system using 2 mm i.d.
columns.

A.2 Switch valve 2 to configuration 2 [Figure 1 c)] to by-pass column 3. Allow the carrier flow to stabilize and then
adjust restrictor A so that the TCD analytical flowrate is identical to that measured in A.1.

A.3 Set the TCD reference flowrate to the value measured in A.1.

A.4 With valve 2 in configuration 2, inject a sample of natural gas by switching valve 1 to configuration 2
[Figure 1 d)]. Record the chromatogram as components elute from column 2. Ensure that the retention time for
n-pentane is about two-thirds of the anticipated analysis cycle time. If it is significantly different, repeat A.1 after
adjusting the flow accordingly. Then repeat A.2 to A.4.

A.5 If no switching time is provided by the manufacturer, measure the time from injection to the peak valley
minimum between ethane and propane (tfirst,cut). This will be the initial time used for retaining the lighter
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

components on column 3.

A.6 Backflushing:

A.6.1 The backflush operation allows all of the latest-eluting C5 (n-C5) to be measured by forward elution and all
of the lightest C6 (2,2-dimethyl butane) to be backflushed. Use a gas mixture containing n-C5 and 2,2-dimethyl
butane with no other C6 or heavier components present.

A.6.2 Set an initial time of 1 min (or as recommended by the manufacturer) after injection at which time valve V1
returns to configuration 1. Switch V2 to configuration 2. Inject the gas mixture and record the chromatogram.

2,2-Dimethyl butane should appear as a backflushed component (C6+) shortly after valve V1 returns to
configuration 1, and n-C5 should appear as a normally eluted peak with a slightly longer retention time than that
measured in A.4. (It has to travel through column 1 twice). If no C6+ peak is seen, reduce the initial time setting and
repeat the operations described in this subclause.

A.6.3 Continue to inject the gas mixture, increasing the backflush time (V1 to configuration 1) by 0,05 min for
each successive injection until the backflushed C6+ peak area (in fact 2,2-dimethyl butane) starts to decrease.

A.6.4 Continue to inject the gas mixture, now reducing the backflush time by 0,05 min for each successive
injection. Note the time at which the area of the C6+ peak first becomes constant (tback,high).

A.6.5 Continue to inject the gas mixture with further incremental reductions in the backflush time until the size of
the n-pentane peak starts to decrease, with a corresponding increase in the size of the C6+ peak. Note the latest
backflush time at which the areas of both peaks are still constant (tback,low)

A.6.6 Determine the value of tback for the backflush of column 1 (V1  configuration 1) using the following
equation

tback = (tback,low  tback,high) / 2

11
 ISO 6974-5:2000(E)

A.7 Valve 2 (V2) time setting:

A.7.1 In the absence of manufacturer's suggested time setting, set the time of tback for backflush
(V1  configuration 1) and tfirst,cut for isolation of column 3 (V2  configuration 2).

Switch both valves initially to configuration 1. Inject a sample of natural gas and, after the elution of n-pentane,
switch valve 2 to configuration 1. Note this time as tV2,off and use it for the operations given in the remainder of this
clause.

Measure the peak area for the propane peak eluted from column 2 (via column 1) and that for the ethane peak
eluted from column 3 (also via column 1).

A.7.2 Repeat the analysis, reducing tfirst,cut successively in increments of 0,05 min until the ethane peak eluted
from column 3 decreases in size.

A.7.3 Continue to repeat the analysis, now increasing the tfirst,cut in 0,05 min increments until a constant value is
obtained for ethane eluted from column 3. Note the lowest time setting value at which this occurs as tfirst,low.

A.7.4 Continue to repeat the analysis until the value for propane eluted from column 2 starts to decrease. Note
the time setting value at which this starts to occur as tfirst,high.

A.7.5 Determine the value of tV2,on as the time necessary to initially isolate column 3

tV2,on = (tfirst,low  tfirst,high) / 2

A.8 Final time setting

Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

Implement the method with the time settings given in Table A.1.

Table A.1 — Time settings

Time Action Valve Position Configuration

0,01 min Inject V1  configuration 2 Figure 1 b)
tback Backflush V1  configuration 1 Figure 1 a)
tV2,on By-pass column 3 V2  configuration 2 Figure 1 c)
tV2,off Re-connect column 3 V2  configuration 1 Figure 1 a)

12
 ISO 6974-5:2000(E)

Annex B
(informative)

Typical precision values

Repeatability of normalized results is given by the relationship:

ln r = - 4,5  0,25  ln x

where

r is the repeatability expressed as a mole fraction in percent;

x is the mole fraction of the component in percent.

Typical precision values for repeatability and reproducibility are given in Table B.1

Table B.1 — Repeatability and reproducibility of measurement results

Mole fraction
Repeatability Reproducibility
x
mole fraction (%) mole fraction (%)
%
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

Absolute Absolute
x  0,1 0,006 0,012
0,1  x  1 0,006 to 0,011 0,012 to 0,022
1  x  50 0,011 to 0,03 0,022 to 0,06
50  x  100 0,03 to 0,035 0,06 to 0,07
NOTE These values have been obtained from practical experience and indicate the performance of the
method. They cannot be compared as such with precision values mentioned in informative annexes of other
parts of ISO 6974 because they result from the quality of the calibration gases and laboratory skills
employed.

13
 ISO 6974-5:2000(E)

Bibliography

[1] ISO 10723, Natural gas — Performance evaluation for on-line analytical systems.

[2] ISO 6976, Natural gas — Calculation of calorific values, density, relative density and Wobbe index from
composition.
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI

14
Licensed Copy: Geoff Leech, December 07, 2001, Uncontrolled Copy, (c) BSI
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