ISO 15112:2007

INTERNATIONAL ISO
STANDARD 15112
First edition
2007-12-01
Natural gas — Energy determination
Gaz naturel — Détermination d'énergie

Reference number
©
ISO 2007
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ii © ISO 2007 – All rights reserved

Contents Page
Foreword. v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols and units. 6
5 General principles. 6
6 Gas measurement. 8
6.1 General. 8
6.2 Volume measurement. 9
6.3 Calorific value measurement. 9
6.4 Volume conversion. 10
6.5 Calibration . 11
6.6 Data storage and transmission . 11
7 Energy determination. 12
7.1 Interfaces . 12
7.2 Methods of energy determination. 14
8 Strategy and procedures . 16
8.1 General. 16
8.2 Strategies for energy determination. 18
8.3 Plausibility checks. 22
9 Assignment methods . 23
9.1 Fixed assignment. 23
9.2 Variable assignment. 25
9.3 Determination of the representative calorific value. 26
10 Calculation of energy quantities . 28
10.1 General equations for energy. 28
10.2 Calculation of averaged values — Calculation from average calorific values and
cumulative volumes. 29
10.3 Volume and volume-to-mass conversions . 30
10.4 Energy determination on the basis of declared calorific values . 30
11 Accuracy on calculated energy. 31
11.1 Accuracy. 31
11.2 Calculation of uncertainty. 31
11.3 Bias. 32
12 Quality control and quality assurance. 33
12.1 General. 33
12.2 Check of the course of the measuring data. 33
12.3 Traceability . 34
12.4 Substitute values . 34
Annex A (informative)  Main instruments and energy-determination techniques. 36
Annex B (informative) Different possible patterns in the change of the calorific value . 40
Annex C (informative) Volume conversion and volume-to-mass conversion . 43
Annex D (informative) Incremental energy determination . 44
Annex E (informative) Practical examples for volume conversion and energy quantity calculation . 46
Annex F (informative) Practical examples for averaging the calorific value due to different
delivery situations. 50
Annex G (informative) Ways of determining substitute values . 55
Annex H (informative) Plausibility check graphical example. 57
Annex I (informative) Uncorrected data, bias correction and final result graphical example . 58
Annex J (informative) Single-reservoir calorific value determination. 60
Bibliography . 61

iv © ISO 2007 – All rights reserved

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 2.
The main task of technical committees is to prepare International Standards. 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 document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 15112 was prepared by Technical Committee ISO/TC 193, Natural gas.
Introduction
Since the early 1800s, it has been general practice for manufactured gas and, subsequently, natural gas to be
bought and sold on a volumetric basis. Much time and effort has therefore been devoted to developing the
means of flow measurement.
Because of the increasing value of energy and variations in gas quality, billing on the basis of thermal energy
has now become essential between contracting partners and the need to determine calorific value by
measurement or calculation has lead to a number of techniques. However, the manner in which calorific value
data are applied to flow volume data to produce the energy content of a given volume of natural gas has been
far from a standardized procedure.
Energy determination is frequently a necessary factor wherever and whenever natural gas is metered, from
production and processing operations through to end-user consumption. This International Standard has been
developed to cover aspects related to production/transmission and distribution/end user. It provides guidance
to users of how energy units for billing purposes are derived, based either on measurement or calculation or
both, to increase confidence in results for contracting partners.
Other standards relating to natural gas, flow measurement, calorific value measurement, calculation
procedures and data handling in regard to gas production, transmission and distribution involving purchase,
sales or commodity transfer of natural gas may be relevant to this International Standard.
ISO 15112 contains ten informative annexes.

vi © ISO 2007 – All rights reserved

INTERNATIONAL STANDARD ISO 15112:2007(E)

Natural gas — Energy determination
1 Scope
This International Standard provides the means for energy determination of natural gas by measurement or by
calculation, and describes the related techniques and measures that are necessary to take. The calculation of
thermal energy is based on the separate measurement of the quantity, either by mass or by volume, of gas
transferred and its measured or calculated calorific value. The general means of calculating uncertainties are
also given.
Only systems currently in use are described.
NOTE Use of such systems in commercial or official trade can require approval of national authorization agencies,
and compliance with legal regulations is required.
This International Standard applies to any gas-measuring station from domestic to very large high-pressure
transmission.
New techniques are not excluded provided their proven performance is equivalent to, or better than, that of
those techniques referred to in this International Standard.
Gas-measuring systems are not the subject of this International Standard.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 6976, Natural gas — Calculation of calorific values, density, relative density and Wobbe index from
composition
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
accuracy of measurement
closeness of the agreement between a measurement result and a true value of the measurand
[ISO 14532:2001]
3.2
adjustment
〈of a measuring instrument〉 operation of bringing a measuring instrument into a state of performance suitable
for its use
[VIM:1996, 4.30]
NOTE Adjustment may be automatic, semi-automatic or manual.
3.3
assignment method
〈energy determination〉 method to derive a calorific value to be applied to the gas passing specified interfaces
having only volume measurements
3.4
availability
probability, at any time, that the measuring system, or a measuring instrument forming part of the measuring
system, is functioning according to specifications
[EN 1776]
3.5
bias
systematic difference between the true energy and the actual energy determined of the gas passing a
gas-measuring station
3.6
calibration
set of operations that establish, under specified conditions, the relationship between values of quantities
indicated by a measuring instrument or measuring system, or values represented by a material measure or a
reference material, and the corresponding values obtained using working standards
[ISO 14532:2001, 2.5.2.2]
3.7
superior calorific value
energy released as heat by the complete combustion in air 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 formed by combustion, which is condensed to the liquid state at T
[ISO 14532:2001, 2.6.4.1]
3.8
inferior calorific value
energy released as heat by the complete combustion in air 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
[see ISO 14532:2001, 2.6.4.2]
3.9
calorific value station
installation comprising the equipment necessary for the determination of the calorific value of the natural gas
in the pipeline
2 © ISO 2007 – All rights reserved

3.10
adjusted calorific value
calorific value measured at a measuring station compensated for the time taken for the gas to travel to the
respective volume-measuring station
3.11
corrected calorific value
result of correcting a measurement to compensate for systematic error
3.12
declared calorific value
calorific value that is notified in advance of its application to interfaces for the purpose of energy determination
3.13
representative calorific value
calorific value which is accepted to sufficiently approximate the actual calorific value at an interface
3.14
charging area
set of interfaces where the same method of energy determination is used
3.15
conversion
determination of the volume at reference conditions from the volume at operating conditions
3.16
correction
value added algebraically to the uncorrected result of a measurement to compensate for systematic error
NOTE 1 The correction is equal to the negative of the estimated systematic error.
NOTE 2 Since the systematic error cannot be known perfectly, the correction cannot be complete (see Annex I).
3.17
correction factor
numerical factor by which the uncorrected result of a measurement is multiplied to compensate for systematic
error object
NOTE Since the systematic error cannot be known perfectly, the correction cannot be complete (see Annex I).
3.18
determination
set of operations that are carried out on an object in order to provide a qualitative or quantitative information
about this object
NOTE In this International Standard, the term “determination” is only used quantitatively.
3.19
direct measurement
measurement of a property from quantities that, in principle, define the property
NOTE For example, the determination of the calorific value of a gas using the thermoelectric measurement of the
energy released in the form of heat during the combustion of a known amount of gas.
[ISO 14532:2001, 2.2.1.2]
3.20
energy
product of gas quantity (mass or volume) and calorific value at given conditions
NOTE 1 The energy may be called energy amount.
NOTE 2 Energy is usually expressed in units of megajoules.
3.21
energy determination
quantitative determination of the amount of energy of a quantity of gas based either on measurement or
calculation using measured values
3.22
energy flow rate
energy of gas passing through a cross-section divided by time
NOTE Energy flow rate is usually expressed in units of megajoules per second.
3.23
fixed assignment
application without modification of the calorific value measured at one specific calorific-value-measuring
station, or the calorific value declared in advance, to the gas passing one, or more, interfaces
3.24
gas transporter
company that conveys gas from one place to another through pipelines
3.25
grid simulation
calculation of a set of pressures and flow rates in a pipeline or a grid on the basis of given topology data,
values of the flow rates at the inlet and outlet points and of the pressure and temperature at various points of
the pipeline(s) by means of a mathematical model
NOTE The objective of any grid simulation is to yield information about a future state of gas pressures and flows. The
result of the simulation is a more or less accurate estimation of the state of the gas flow.
3.26
interface
place on a pipe used for the transportation or supply of gas at which there is a change of ownership or
physical custody of gas
NOTE Generally, an interface has an associated measuring station.
3.27
local distribution company
LDC
company that delivers gas to industrial, commercial and/or residential customers
3.28
measuring station
installation comprising all the equipment, including the inlet and outlet pipework as far as the isolating valves
and structure within which the equipment is housed, used for gas measurement in custody transfer
[EN 1776]
3.29
measuring system
complete set of measuring instruments and auxiliary equipment assembled to carry out specified
measurements
[VIM:1996, 4.5]
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3.30
measuring instrument
device intended to be used for measurements, alone or in conjunction with supplementary device(s)
[VIM:1996, 4.1]
3.31
plausibility
property of a value to be within reasonable limits
3.32
producer
company that extracts raw natural gas from reservoirs which, after processing and (fiscal) measurement, is
supplied as dry natural gas to the transportation system
3.33
regional distributor
company that delivers gas to local distribution companies and/or industrial, commercial or residential
customers
3.34
residential customer
person whose occupied premises are supplied with gas, wholly or in part, such gas not being used for any
business purpose, commercial or industrial
3.35
systematic error
mean that would result from an infinitive number of measurements of the same measurand carried out under
repeatability conditions minus a true value of the measurand
[VIM:1996, 3.14]
3.36
traceability
property of the result of a measurement or the value of a standard whereby it can be related to stated
references, usually national or international standards, through an unbroken chain of comparisons all having
stated uncertainties
[VIM:1996, 6.10]
NOTE This chain of comparisons is called a traceability chain.
3.37
uncertainty of measurement
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
could reasonably be attributed to the measurand
[VIM:1996, 3.9]
3.38
variable assignment
application of a calorific value for an assignment procedure based on measurement(s) at calorific value
station(s) to the gas passing one, or more, interfaces
NOTE That applied calorific value may take into account the time taken for the gas to travel from the calorific value
station to the respective volume-measuring stations and other factors, to derive an average calorific value for a network, a
state reconstruction of the variation of calorific values through a network, etc.
3.39
zero floating point
position in a grid conveying gas where there is a boundary with different gas qualities on either side
4 Symbols and units
Symbol Meaning SI unit USC unit
E energy MJ kWh
e energy flow rate MJ/s kWh/h
3 3
H calorific value
MJ/m ; MJ/kg kWh/m
NOTE 1 Where the calorific value is in megajoules per cubic metre and the gas volume is in cubic meters, or where the
calorific value is in megajoules per kilogram and the gas mass is in kilograms, then the calculated energy is in megajoules.
Where the calorific value is in kilowatt-hours per cubic meter and the gas volume is in cubic meters, or where the calorific
value is in kilowatt-hours per kilogram and the gas mass is in kilograms, then the calculated energy is in kilowatt-hours.
To convert the number of megajoules to the number of kilowatt-hours divide the number by 3,6.
M mass kg t
p pressure (absolute) Pa, kPa bar, mbar
Q quantity of gas t
m , kg
NOTE 2 When the quantity is given in cubic metres, it should be qualified by temperature and pressure.
3 3
q volume flow rate
m /h, m /s
v
q mass flow rate kg/s, kg/h
m
T temperature (absolute) K
t time s, h, d s, h, d
V
volume (gas)
m
Z compression factor
ρ density
kg/m
temperature °C °F
ϑ
Subscripts:
I inferior calorific value
n
number of time intervals
nc normal reference conditions (273,15 K; 101,325 kPa)
r ISO-recommended standard reference conditions (288,15 K; 101,325 kPa)
S superior calorific value
wa weighted average
5 General principles
The quantity of energy, E, contained in a given quantity of gas, Q, is given by the multiplication of the calorific
value, H, by the respective quantity of gas.
Energy may be either measured directly (see Figure 1) or calculated from the quantity and the calorific value
of the gas (see Figure 2).
6 © ISO 2007 – All rights reserved

Figure 1 — Energy-measurement scheme

Figure 2 — Energy-determination scheme
Generally, the quantity of gas is expressed as a volume and the calorific value is on a volumetric basis. In
order to achieve accurate determinations of energy, it is necessary that both the gas volume and calorific
value be at the same reference conditions. The determination of energy is based either on the accumulation
over time of calculation results from consecutive sets of calorific values and the concurrent flow rate values or
on the multiplication of the total volume and the representative (assigned) calorific value for that period.
Especially in situations of varying calorific values and when flow rates are determined at a place different from
that of the (representative) calorific value, the effect on the accuracy caused by the difference in time between
the determination of the flow rate and the calorific value shall be considered (see Clause 11).
The gas volume may either be measured and reported as the volume at the ISO-recommended standard
reference conditions or be measured at some other conditions and converted to an equivalent volume at the
ISO-recommended standard reference conditions, using an appropriate method of volume conversion. The
method of volume conversion used at a specific gas-volume-measuring station may require gas quality data
determined at other places. For the purpose of this International Standard, the ISO-recommended standard
reference conditions of 288,15 K and 101,325 kPa as defined in ISO 13443 should be used.
NOTE For gas supply, other conditions can be used, corresponding to national standards or laws. Methods for
conversion between different conditions for dry natural gases are given in ISO 13443.
The calorific value may be measured at the gas-measuring station or at some other representative point and
assigned to the gas-measuring station. It is also possible for the quantity of gas and the calorific value to be
expressed on a mass basis.
This general principle of energy determination is extended in Clause 10 to those cases when the quantity of
gas is expressed on either a volumetric or a mass basis.
To achieve the calculation of the quantity of energy of the gas passing a gas-measuring station over a period
of time, the methods of energy determination in Clauses 7 to 10 are used. Such methods involve an
integration over the time period; that integration may be
⎯ of the energy flow, or
⎯ of the gas flow rate over time to obtain the quantity of gas, which is then multiplied by the representative
calorific value.
The method of integration may depend on contractual agreements or national legislation.
The general principles of energy determination in Clauses 7 to 10 are independent of the method with which
the integrations are carried out. The method of integration influences the uncertainty of the determined
energy; these effects are considered in Clause 11.
6 Gas measurement
6.1 General
The type of measuring devices and methods used in real measuring stations depends among other things on
⎯ the respective national requirements,
⎯ the flow rate,
⎯ the commercial value of the gas,
⎯ the gas quality variations,
⎯ the need for redundancy, and
⎯ the instrument specification.
Only proven methods and measuring devices/products used at the respective interfaces should be used. An
overview of the techniques and procedures currently used in different countries is shown in Annex A.
Methods used for flow and calorific value measurement shall be in accordance with standards, contractual
agreements and/or national legislation, as appropriate.
Action should be taken to identify and reconcile systematic effects. For example, use of different national
standards, regulations and/or operating procedures can introduce systematic differences; contract partners
should determine the appropriate means to overcome these differences.
The quality of the measurement results in general depends on the following factors:
⎯ operating conditions;
⎯ maintenance frequency and quality;
⎯ calibration standards;
8 © ISO 2007 – All rights reserved

⎯ sampling and clean-up;
⎯ changes in gas composition;
⎯ ageing of measurement devices.
A high accuracy can be achieved if the requirements fixed by the manufacturers and by officials are met and
all operating procedures for operating, calibration and maintenance are strictly observed.
6.2 Volume measurement
The volume flow-metering system of a natural-gas-measuring station consists of one or more meter runs.
Generally, the meters measure the gas volume flow at actual operating conditions. Standards for orifice
meters (ISO 5167-1) and turbine meters (ISO 9951) exist.
The selection of a flow-metering system for a specific application depends as a minimum on the following:
⎯ conditions of flow;
⎯ flow-measuring range;
⎯ operating conditions, especially operating pressure;
⎯ acceptable pressure loss;
⎯ required accuracy.
For natural-gas volume flow measurement, the instruments mostly used at the interfaces 1 to 6 (see 7.1) are
shown in Annex A.
6.3 Calorific value measurement
6.3.1 Measurement techniques and sampling
A calorific-value measuring system consists of a sampling system and a measurement device taken from one
of the following groups:
⎯ direct measurement (e.g. by combustion calorimeters);
⎯ inferential measurement [e.g. by a gas chromatograph (GC)];
⎯ correlation techniques.
To achieve a high accuracy of calorific value measurement, representative sampling is required. Guidelines
are given in ISO 10715.
Depending on
⎯ the measuring system,
⎯ the operating procedures,
⎯ the fluctuation of composition of the gas, and/or
⎯ the quantity of gas delivered,
one of the following sampling techniques can be used:
a) continuous direct sampling;
b) periodical spot sampling;
c) incremental sampling.
Samples are taken for either online analysis or offline analysis.
6.3.2 Direct measurement — Calorimetry
With direct measurement, natural gas at a constant flow rate is burned in an excess of air and the energy
released is transferred to a heat-exchange medium resulting in an increase in its temperature. The calorific
value of the gas is directly related to the temperature increase.
Calorimetry is used for interfaces 1 to 3 and 5. ISO 15971 gives details of the measurement of combustion
properties.
6.3.3 Inferential measurement
With inferential measurement, the calorific value is calculated from the gas composition in accordance with
ISO 6976.
The most widely used analytical technique is gas chromatography. Procedures for the determination of the
composition with defined uncertainty by gas chromatography are given in ISO 6974 (all parts). GC
measurement is used at interfaces 1 to 3 and 5.
6.3.4 Correlation techniques
Correlation techniques make use of the relationships between one or more physical properties and the
calorific value of the gas. Also, the principle of stoichiometric combustion can be used.
6.4 Volume conversion
6.4.1 General
Conversion of a volume of natural gas measured at operating conditions to a volume at reference or base
conditions is based either on a gas pressure, temperature and compression factor (pTZ-conversion) or on gas
densities at operating and base conditions (density conversion).
For details, see Annex C, Clauses E.1 and E.2, ISO 12213 (all parts) and EN 12405.
6.4.2 Density
The density at reference conditions (sometimes referred to as normal, standard or base density) can be
required for conversion of volume data. Density at operating conditions may be measured for mass-flow
determination and volume conversion.
Details are given in ISO 15970, which, at the date of publication of this International Standard, was under
development.
6.4.3 Pressure and temperature
Pressure and temperature measurements can be necessary for the conversion of the gas volume at operating
conditions to a volume at standard reference or normal conditions. Details are given in ISO 15970, which, at
the date of publication of this International Standard, was under development.
10 © ISO 2007 – All rights reserved

6.4.4 Compression factor
For gas volume conversion, the compression factor is
⎯ calculated from the gas composition using a molar analysis (see Clause E.2 and ISO 12213-2),
⎯ calculated using physical properties and some constituents (see Clause E.1 and ISO 12213-3) or
⎯ measured by a Z-meter.
Details are given in ISO 15970, which, at the date of publication of this International Standard, was under
development.
The compression factor at reference conditions may also be calculated according to ISO 6976. Depending on
the quantity of gas delivered and variations in pressure, temperature and gas composition at the specific
metering point, either the compression factor may be set constant or shall be calculated from time to time.
The user of this International Standard shall take account of the gas composition, especially with respect to
the molar relationships of the higher hydrocarbons to each other and at high pressure. Depending on the gas
composition and the pressure, methods for calculating the Z-factor on the basis of ISO 12213-2, rather than
on the basis of ISO 12213-3, should be considered to avoid systematic errors.
6.5 Calibration
Quality of calibrations has a significant impact on the trueness of a measurement result. The frequency of the
calibrations shall be determined according to the stability of the measurement devices. Calibrations should be
traceable to appropriate standards and reference materials.
A representative calibration should be performed at conditions close to those at which the meter operates. For
calorific measurement devices, calibration gases that are close to the expected calorific value or composition
of the gas to be measured (see, for example, ISO 15971) should be used.
If, upon verification of any measuring instrument used for energy-determination purposes, an agreed deviation
between the instrument reading and the corresponding value realized by a standard is exceeded, a calibration
of the measuring instrument shall be carried out in order either
⎯ to make adjustments to the instrument that establishes the smallest possible difference between the
measured value and the value given by the standard, or
⎯ to derive a correction that is applied to the measured value for subsequent periods to produce the correct
value.
The actual process of adjustment or correction may be either manual or automatic, depending on the type of
instrument.
If, at the calibration of the calorific-value-measuring device, a difference between measured and certified
values occurs, for subsequent periods a correction of the measured values or adjustment shall be performed.
6.6 Data storage and transmission
All relevant data for determining energy shall be securely stored. The length of storage time and place of
storage shall take into account national regulations and/or contractual conditions.
The data incorporate
⎯ information contributing to and/or consisting of the amount of energy supplied and, where available,
⎯ information on the data validity or the functioning of the metering station (hardware and software).
For data transfer, safe procedures shall be taken to ensure the integrity of the data.
7 Energy determination
7.1 Interfaces
Natural gas custody transfer between contract parties is, in general, performed from the producer(s) or gas
storages to the end user via intermediate stages involving some or all of the following:
⎯ gas transporter(s);
⎯ regional distributors;
⎯ local distributions company(ies).

Key
1 to 6 interfaces
a
If this entity exists.
Figure 3 — Possible interfaces for energy determination from producer(s) to end users
12 © ISO 2007 – All rights reserved

Key
1 to 6 interfaces
a
If this entity exists.
Figure 4 — Possible interfaces for energy determination from producer(s) to end users
including gas storages
The boxes numbered 1 to 6 in Figures 3 and 4 represent the different interfaces within a delivery chain; they
may consist physically of a real measuring station or may be regarded only as virtual interfaces without any
measurement to define the point of delivery or redelivery within contractual situations. Energy determination in
a delivery chain between contract parties is performed at interfaces 1 to 6 (see Figures 3 and 4), often also
named points of delivery and/or points of redelivery. Figure 3 shows the delivery chain from the producer to
the end user; Figure 4 includes, additionally, a storage operator, who usually stores gas for producers, gas
transporters or regional distributors for future take-off. The kinds of interfaces may differ within the different
countries. They may be used as gas-billing interfaces, if they are actual measuring stations.
Three different models of different delivery situations are given as examples.
a) The gas transporter supplies gas directly to an industrial customer.
For energy determination at interface 5, the gas volume is measured at interface 2 or 5; because there is
no regional distributor/storage company or local distribution company (LDC) involved, the calorific value
measured at interface 2 can be used if nearly constant gas quality (see Figure B.1) can be expected.
b) The gas producer supplies gas directly to an industrial customer.
The pipeline system is used by several gas transporters and regional distributors for transportation, LDCs
are not involved. On its way to the industrial customer, no gas quality changes occur. For energy
determination at interface 5, the gas volume is measured at interface 5 and the calorific value at, for
example, interfaces 5, 3 or 2.
c) The LDC supplies gas to the end user, commercial and industrial customer.
The LDC is supplied by a regional distributor, or gas transporter or storage company. For energy
determination, volume measurement is performed at interfaces 4 to 6. Due to different gas qualities (see
Figure B.3), the regional distributor operates a state reconstruction program for calorific-value determination
at interface 3; that calorific value is taken by the LDC for energy determination at interfaces 4 to 6.
The method of energy determination depends on a number of important factors; they shall be taken into
account for the suitable energy-determination strategy to support the user of this International Standard to
perform a correct energy determination. They include
⎯ grid topology,
⎯ flow directions,
⎯ take-off structure or consumption profile,
⎯ course of calorific value,
⎯ technical equipment,
⎯ contractual requirements,
⎯ national regulations.
It is the main goal of the methods given in 7.2
a) to support a satisfactory energy balancing within the transportation grid, and
b) to provide a justified energy determination at interfaces,
taking into account economic aspects.
7.2 Methods of energy determination
7.2.1 Direct determination of energy
For direct measurement (see Figure 5), individual physical parameters (e.g. Q, H) are not measured. The
energy flow and energy quantity are calibrated and shown at the measuring point. At the time of preparation of
this International Standard, direct energy-measurement instruments have entered the marketplace, but they
are not yet proven technology for custody transfer. No international standards exist at the moment.

Figure 5 — Direct determination of energy
7.2.2 Indirect determination of energy
For indirect energy determination, the energy is determined on the basis of previously measured or calculated
values for volume/mass, calorific value and other entities.
7.2.2.1 Measurement of volume or mass and calorific value at the same station
For indirect determination of energy, the volume or mass, calorific value and additional physical entities, such
as CO , density, etc., of the gas are measured separately in a metering station (see Figure 6); the
14 © ISO 2007 – All rights reserved

measurement devices are individually calibrated. The volume flow rate and energy quantity are typically
displayed at the measuring point. For large gas quantities Q and Q , for example at border crossings, it can
1 2
be necessary to determine the calorific values H and H by means of two calorific-value measurement
S1 S2
devices at each station (see Figure 15).
Another method is to collect the calorific value and volume data in the measuring station and to transmit the
data to a different central energy-determination station where energy flow and energy quantity are determined.

Figure 6 — Local online calorific value measurement
Due to the local gas-quality situation and for economical reasons, it is sometimes of use to take samples of
gas (time- or flow-controlled) within the measuring station and to determine the calorific value at a different
place (see Figure 7).
Figure 7 — Local offline calorific-value measurement
7.2.2.2 Measurement of volume or mass and calorific value at different stations
Whereas the gas volume is measured at every delivery point between contract parties, it can be too expensive
to operate a calorific-value measurement device there, too. Thus, the most common method (especially in
extensive supply systems) is to assign a representative calorific value (see Clause 9) to the volume. The
calorific values assigned to those interfaces (volume-measuring points) are values measured elsewhere or a
value formed from several representative measured values (see Figure 8). These values are the basis for
energy determination. The kind of assignment is determined by the location of the input/output stations in the
grid and the conditions of gas flow (see Clause 9).

Figure 8 — Remote calorific value measurement (example)
8 Strategy and procedures
8.1 General
“Design” in the context of this International Standard encompasses the requirements of what information is
necessary and how it should be obtained to fulfil the needs of the energy-determination strategy, taking into
account the expected course of data.
Energy determination starts with an assessment of a reasonable energy-determination strategy, followed by a
plausibility check of the measured data. The next steps are the assignment of the representative calorific
value and the combination of the data (calculation procedures). Finally, a quality control procedure is
performed.
An energy-determination scheme, including “start” and “end” points, is shown in Figure 9.
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a
Strategy, action and procedures.
Figure 9 — Strategy for an indirect energy determination
Principally, elaborated strategy and the applicable methods and procedures for energy determination shall be
applied without changes. They may be c
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