Network Code
Gas Quality
GAS QUALITY
1) INTRODUCTION .......................................................................................................................................... 2
2) GAS QUALITY PARAMETERS ................................................................................................................. 2
2.1)
2.2)
2.3)
2.4)
PARAMETERS FOR CALCULATING ENERGY .............................................................................................. 2
PARAMETERS FOR CONTROLLING NATURAL GAS QUALITY ...................................................................... 2
INFORMATION OBLIGATIONS ................................................................................................................... 3
COMPLIANCE WITH THE QUALITY SPECIFICATION ................................................................................... 3
3) DETERMINATION OF THE PARAMETERS FOR THE CALCULATION OF ENERGY ................ 3
3.1)
DELIVERY POINTS ................................................................................................................................... 3
3.1.1)
Imports........................................................................................................................................... 4
3.1.2)
Regassification plants .................................................................................................................... 4
3.1.3)
Storage fields ................................................................................................................................. 4
3.1.4)
National production....................................................................................................................... 4
3.2)
REDELIVERY POINTS ............................................................................................................................... 4
4) DETERMINING THE PARAMETERS TO CONTROL THE QUALITY OF NATURAL GAS......... 5
4.1)
4.2)
4.3)
4.4)
IMPORTS.................................................................................................................................................. 6
REGASSIFICATION PLANTS ...................................................................................................................... 7
STORAGE FIELDS ..................................................................................................................................... 7
NATIONAL PRODUCTION ......................................................................................................................... 8
5) DATA QUALITY VALIDATION ................................................................................................................ 8
5.1)
OPERATIONAL AND FISCAL VALUES ........................................................................................................ 9
6) PROCEDURE FOR VALIDATION REQUESTS .................................................................................... 10
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1)
Gas Quality
INTRODUCTION
The objective is to assure the integrity and safety of the transportation system - e.g.
protecting the pipelines from corrosion – as well as the technical compatibility with the
utilization of the gas by the Final Customer. Gas flowing in the transportation network
must comply with a quality specification indicating the maximum and minimum
permitted values for natural gas quality parameters.
As a necessary condition to inject gas in the transportation network, the Shipper must
comply with this quality specification. There is a single quality specification for the
entire Snam Rete Gas pipeline network, thereby allowing the interchangeability of gas
flows throughout the pipeline.
2)
GAS QUALITY PARAMETERS
The parameters characterising the quality of the gas can be divided into chemicalphysical parameters to calculate energy (Gross Calorific Value) and parameters
controlling the quality of the natural gas.
2.1) Parameters for calculating energy
The main parameter used to calculate energy is the Gross Calorific Value (“PCS”),
determined on the basis of the chemical composition of the natural gas pursuant to ISO
6976 standard, taking at least the following elements into consideration:
Methane – C1
Ethane – C2
Propane – C3
IsoButane – iC4
NormalButane – nC4
IsoPentane – iC5
NormalPentane – nC5
Hexanes and superior – C6+
Nitrogen – N2
Carbon Dioxide – CO2
2.2) Parameters for controlling natural gas quality
In order to assure the safety of the transportation system, and the interchangeability
and transportability of the natural gas in the network, the following natural gas quality
parameters are monitored:
1. Gross Calorific Value
2. Relative Density
3. Wobbe Index
4. Carbon Dioxide – CO2
5. Oxygen – O2
6. Hydrogen sulphide – H2S
7. Sulphur in mercaptans – SRSH
8. Total Sulphur - STOT
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Gas Quality
9. Water Dew Point
10. Hydrocarbon Dew Point
11. Temperature
2.3) Information obligations
Each month Snam Rete Gas publishes on its website the average monthly PCS value of
natural gas for each Delivery Point, related to the twelve months prior to the current
one as well as, for each Redelivery Point, the relevant AOP and the average monthly
PCS value related to the month prior to the current one.
2.4) Compliance with the quality specification
Shippers are obliged to inject gas at the Delivery Points of the natural gas pipeline
network in compliance with Quality Specifications as stated in Attachment 11/A.
In cases of non-compliance with the quality specification, the provisions of the chapter
“Parties’ Liabilities” will be applied.
3)
DETERMINATION OF THE PARAMETERS FOR THE CALCULATION OF
ENERGY
The following methods are used to determine the composition of the natural gas to
calculate the PCS (and therefore energy):

continuous determination through the fixed installation of automatic gas
chromatographs;

discontinuous determination through gas chromatograph analysis of a gas sample
taken in the field (instant sampling) carried out in SINAL or SIT accredited
laboratories.
The PCS is accurately determined at each Delivery Point of Snam Rete Gas pipeline
network, while in the Redelivery Points, in the absence of an accurate determination,
the PCS is calculated in correspondence to the measurement points of the
Homogeneous Off-take Area identified according to the “Methodology relating to the
Homogeneous Off-take Areas” published on the Snam Rete Gas website.
3.1) Delivery Points
As part of Resolution n. 185/05 and its subsequent amendments, in each Delivery
Point of Snam Rete Gas pipeline network, the PCS is accurately determined through
the installation of measurement devices or via instant sampling. When the PCS
measuring devices at a Delivery Point are not owned by Snam Rete Gas, the
measurement recorded will be used by Snam Rete Gas on the condition that the owner
of the measuring device has sent Snam Rete Gas, by 31st October each year, a written
communication containing:
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Gas Quality

a declaration stating that the suitable checks and periodic calibrations of the
devices have been carried out in compliance with current legislation;

the documentation proving the reasons for any unavailability of the time
measurements relating to the previous thermal year and the state of repair of the
measurement devices at 30 September of the previous year.
In the absence of written agreements between Snam Rete Gas and the owner of the
measurement devices, with which the latter undertakes to fulfil the obligations
mentioned above, Snam Rete Gas will equip the Delivery Point with its own
measurement devices and will use the measurement value recorded with them.
3.1.1) Imports
In each Delivery Point from imports, the PCS is calculated continuously by two gas
chromatographs: the PCS daily value is calculated as the average of the analyses run
during the day.
3.1.2) Regassification plants
In each Delivery Point from regassification plants, the PCS is calculated continuously
by two gas chromatographs. The PCS daily value is calculated as the average of the
analyses run during the day.
3.1.3) Storage fields
In each Delivery Point from storage fields, the PCS is determined continuously by a gas
chromatograph: and the PCS daily average value is calculated on the basis of the
analyses carried out in the daily off-take/injection period.
3.1.4) National production
In each Delivery Point from national production with daily flow rates greater or equal
to 100,000 Sm3, the PCS is measured continuously by a gas chromatograph: the PCS
daily value is calculated as the average of the analyses run during the day.
In each Delivery Point from national production with daily flow rates lower than
100,000 Sm3, the PCS is measured discontinuously on a quarterly basis via gas
chromatograph laboratory analysis of a gas sample taken in the field. Should the
specification limits be exceeded, the calculations will be carried out each month,
starting from the month following the one of excess; if these parameters are within the
norm for at least two consecutive recordings, the quarterly frequency will be restored.
3.2) Redelivery points
In the Redelivery points, when there is no accurate calculation, the PCS is calculated in
correspondence with the measurement points of the Homogeneous Off-take Areas
(AOP) identified according to the “Methodology relating to the Homogeneous Off-take
Areas” published on Snam Rete Gas' website.
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In each AOP the PCS value is measured continuously via a gas chromatograph. The
PCS daily value is calculated as the average of the instruments’ analyses during the
day. The PCS value measured as so is associated to all the Redelivery points belonging
to the AOP.
In the AOPs with a single power point from a network of gas pipelines managed by
another transport company or in the AOPs with a single power point other than an
import point, characterised by daily gas volumes lower than 100,000 Sm3, the PCS is
calculated discontinuously with a gas chromatograph laboratory analysis of a gas
sample taken in the field with a quarterly frequency.
If the Final Customer wants to install a gas chromatograph directly, the procedures
and the requirements indicated in Attachment 11/B should be followed. The Shipper is
responsible for guaranteeing compliance by the Final Customer. The request must be
sent to Snam Rete Gas by the Shipper or directly by the Final Customer in case the
Shipper’s approval is attached.
As part of Resolution n. 185/05 and its subsequent amendments, when the PCS
measurement devices in correspondence to a Redelivery Point is not owned by Snam
Rete Gas, the measurement recorded will be used by Snam Rete Gas on the condition
that the owner of the measurement device has sent Snam Rete Gas, by 31st October
each year, a written communication containing:

a declaration stating that the suitable checks and periodic calibrations of the
devices have been carried out in compliance with current legislation;

the documentation proving the reasons for any unavailability of the time
measurements relating to the previous thermal year and the state of repair of the
measurement devices at 30 September of the previous year.
In addition the measurement recorded through this device will be used by Snam Rete
Gas on the condition that the requirements and procedures described in Attachment
11/B are respected.
In the absence of this communication by the owner of the measurement device or in
case the requirements and procedures described in Attachment 11/B are not respected,
Snam Rete Gas aggregates the Redelivery Point concerned with an AOP, based on the
criteria in the “Methodology relating to the Homogeneous Off-take Areas” published
on Snam Rete Gas' website.
If the procedures and requirements described in Attachment 11/B are not satisfied, the
data taken from the gas chromatograph will not be considered for the purposes of
energy measurement and validation.
4)
DETERMINING THE PARAMETERS TO CONTROL THE QUALITY OF
NATURAL GAS
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The parameters to control the gas quality under points 2. to 10. of paragraph 2.2 are
determined in accordance with the provisions of Resolution n.185/05 and subsequent
amendments, according to the following methods:

continuous determination through the fixed installation of automatic gas
chromatographs and specific measurement devices;

discontinuous determination through an analysis of a gas sample taken in the field
(instant sampling) carried out in SINAL or SIT accredited laboratories or via
measurements in the field.
The gas quality parameters under points from 2. to 4. of paragraph 2.2 (Relative
Density, Wobbe Index and CO2) are calculated in compliance with ISO 6976, based on
the chemical composition of the natural gas recorded according to the methods
described in the previous paragraph “Determination of the parameters for the
calculation of energy”. In particular, the gas quality parameters under points 2. to 4. of
paragraph 2.2 are determined accurately at each Delivery Point of the Snam Rete Gas
pipeline network, while in the Redelivery Points, in the absence of an accurate
determination, these parameters are calculated in correspondence to the measurement
points of the Homogeneous Off-take Areas identified according to the “Methodology
relating to the Homogeneous Off-take Areas” published on the Snam Rete Gas website.
The parameters to control the gas quality under points 5. to 10. of paragraph 2.2 are
only determined at the Delivery Points of the Snam Rete Gas pipeline network
according to the methods reported below.
4.1) Imports
In each Delivery Point from imports, the gas quality control parameters under points 5.
to 10. of paragraph 2.2 are determined according to the following methods:

The oxygen content is monitored discontinuously on a quarterly basis via lab
analyses of samples taken in the field.

The sulphur compounds content is monitored discontinuously on a monthly basis
through lab analyses of samples taken in the field.

The water Dew Points are monitored continuously via specific measurement
devices.

The hydrocarbon Dew Points are monitored discontinuously on a monthly basis,
through samples taken in the field.
For the quality parameters not measured continuously, should the specification limits
be exceeded, the calculations will be carried out each fortnight, starting from the month
following the one of excess; in case these parameters are within the norm for at least
two consecutive recordings, the ordinary frequency will be restored.
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4.2) Regassification plants
In each Delivery Point from regassification plants, the gas quality control parameters
under points 5. to 10. of paragraph 2.2 are determined according to the following
methods:

The oxygen content is monitored discontinuously on a quarterly basis via lab
analyses of samples taken in the field.

The measurement of hydrogen sulphide under point 6. of paragraph 2.2 is carried
out continuously through specific measurement devices. The monitoring of
sulphur compounds, under points 7. and 8. of paragraph 2.2, is carried out
discontinuously via lab analyses of a gas sample taken in the field, at least once a
year. Each time the analyses produced continuously by the analyzer of hydrogen
sulphide give concentration values exceeding 1 mg/Sm3, an additional calculation
of the sulphur compounds is carried out.

The water and hydrocarbon Dew Points are monitored discontinuously on an
annual basis through samples taken in the field.
For the quality parameters not measured continuously, should the specification limits
be exceeded, the calculations will be carried out each fortnight, starting from the month
following the one of excess; in case these parameters are within the norm for at least
two consecutive recordings, the ordinary frequency will be restored.
4.3) Storage fields
In each Delivery Point from storage fields, the gas quality control parameters under
points 5. to 10. of paragraph 2.2 are determined according to the following methods:

The oxygen content is monitored discontinuously on a quarterly basis via lab
analyses of samples taken in the field.

The sulphur compounds content is monitored discontinuously on a monthly basis
via lab analyses of samples taken in the field.

The water Dew Points are monitored continuously via specific measurement
devices.

The hydrocarbon Dew Points are monitored discontinuously on a monthly basis
through samples taken in the field.
For the quality parameters not measured continuously, should the specification limits
be exceeded, the calculations will be carried out each fortnight, starting from the month
following the one of excess; in case these parameters are within the norm for at least
two consecutive recordings, the ordinary frequency will be restored.
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4.4) National production
In each Delivery Point from national production with a daily flow greater or equal to
100,000 Sm3, the gas quality control parameters under points 5. to 10. of paragraph 2.2
are determined according to the following methods:

The oxygen content is monitored discontinuously on a quarterly basis via lab
analyses of samples taken in the field.

The sulphur compounds content is monitored discontinuously on a monthly basis
via lab analyses of samples taken in the field, on a monthly basis or on a semi
annual basis if the annual average values are stably lower than 20% of the specific
limit.

The water Dew Points are monitored continuously via specific measurement
devices.

The hydrocarbon Dew Points are monitored discontinuously through samples
taken in the field, on a monthly basis or on a quarterly basis if the annual average
values are stably lower than -15°C in the pressure field 100 – 7.000 kPa.
For the quality parameters not measured continuously, should the specification limits
be exceeded, the calculations will be carried out each fortnight, starting from the month
following the one of excess; in case these parameters are within the norm for at least
two consecutive recordings, the ordinary frequency will be restored.
In each Delivery Point from national production with a daily flow lower to 100,000
Sm3, the gas quality control parameters under points 5. to 10. of paragraph 2.2 are
determined according to the following methods:

The oxygen content is monitored discontinuously on a quarterly basis via lab
analyses of samples taken in the field.

The sulphur compounds content is monitored discontinuously on a monthly basis
via lab analyses of samples taken in the field, on a quarterly basis or on a semi
annual basis if the annual average values are stably lower than 20% of the specific
limit.

The water and hydrocarbon Dew Points are monitored discontinuously on a
monthly basis through samples taken in the field.
For the quality parameters measured quarterly, should the specification limits be
exceeded, the calculations will be carried out each month, starting from the month
following the one of excess; in case these parameters are within the norm for at least
two consecutive recordings, the ordinary frequency will be restored.
5)
DATA QUALITY VALIDATION
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As part of the provisions of Resolution n. 185/05 and its subsequent amendments,
Snam Rete Gas validates the gas quality data used to calculate the energy as reported
below:
 acquisition and examination of the data automatically received by each gas
chromatograph and of the data related to the lab chemical analysis on previously
taken gas samples;
 checking and validation of the above data on the basis of:
1) standard message errors recorded directly by gas chromatographs;
2) consistency of the data in the analysis;
3) limit values of concentration taken from historical trends from the field.
The identification of erroneous data triggers the production of reports which can be
used for diagnostic purposes.
The daily measurement of PCS in a measurement point will be considered available by
Snam Rete Gas if, for the Gas-day, the hourly measurements related to at least 12
hours, even when not consecutive, are available, considering each hourly measurement
as valid if at least half of the measurements recorded in the hour are validated.
If at a Delivery Point of the transmission network the daily measurement of the PCS for
natural gas is not available, Snam Rete Gas will use the latest valid measurement
recorded in the same point.
If, at a measurement point of an AOP, the daily measurement of the PCS for natural
gas is not available, Snam Rete Gas will use the measurement recorded in the same
Gas-day in an alternative AOP, identified pursuant to the “Methodology relating to the
Homogeneous Off-take Areas” published on the Snam Rete Gas website.
If an alternative AOP cannot be identified, Snam Rete Gas will use a daily PCS equal to
the average monthly PCS of the previous month at that same point.
Starting from the tenth gas day of the measurement being unavailable at a
measurement point of an AOP, Snam Rete Gas will use for the AOP concerned the PCS
value measured daily through gas chromatograph laboratory analysis of a gas sample
taken in the field.
Snam Rete Gas will restore the daily PCS measurement within 15 days starting from the
date when the data became unavailable.
5.1) Operational and fiscal values
On the basis of the validated data, operational and fiscal gas quality values are
calculated and archived in the “Archive of the operational data” and in the “Archive of
fiscal values”: the former is made up by operational values which have passed the
various checks and automatic system controls, and is therefore suitable for further
processing. The latter is made up by fiscal values that have been fully validated.
For invoicing purposes, the valid data is that collected, processed and validated by
Snam Rete Gas.
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6)
Gas Quality
PROCEDURE FOR VALIDATION REQUESTS
The Shipper may forward Snam Rete Gas a request to verify the data in question. Such
request must include at least the following elements:
 ID code of the Redelivery Point concerned;
 the queried data and the related time period;
 any other technical details supporting the query.
Snam Rete Gas, based on the elements reported above and/or other elements deemed
necessary, carries out, if this is within its responsibilities under these specifications, the
necessary checks and assessments by the technical terms necessary for the execution of
these activities.
As soon as available, the results of the checks in question are communicated to the
requesting party and combined, upon the Shipper's specific request, with the
information relating to the adjacent areas.
If the verifications are well founded, Snam Rete Gas will provide a new measurement
of the quality parameters and issue a new measurement report as well as make the
concerning economic adjustment, where applicable, in compliance with the provisions
of paragraph 5 of the Chapter 9 “Balancing”.
If the verifications prove that the reported data is correct, Snam Rete Gas will charge
the requesting party for the costs incurred for the verification in question.
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