ISO 21903:2020

Refrigerated hydrocarbon fluids — Dynamic measurement — Requirements and guidelines for the calibration and installation of flowmeters used for liquefied natural gas (LNG) and other refrigerated hydrocarbon fluids

This document specifies the metrological and technical requirements for flowmeters intended to be used for the dynamic measurement of liquefied natural gas (LNG) and other refrigerated hydrocarbon fluids. For LNG static volume measurement used in custody transfer, see ISO 10976. This document sets the best practice for the proper selection and installation of flowmeters in cryogenic applications and identifies the specific issues that can affect the performance of the flowmeter in use. Moreover, it offers a calibration guideline for laboratory and on-site conditions (mass or volume) by either using LNG or other reference fluids. The choice of calibration fluid will depend on the capabilities of the available flow calibration facilities and the ability to achieve the required overall measurement uncertainty demanded by the intended application. This document is applicable, but is not limited, to the use of Coriolis and ultrasonic flowmeters for dynamic measurements of LNG. In principle, LNG and other refrigerated liquid hydrocarbons are considered in this document. Recommendations in this document are based on the available test results with LNG. These results are probably applicable to other cryogenic fluids.

Hydrocarbures liquides réfrigérés — Mesurage dynamique — Exigences et lignes directrices pour l’étalonnage et l’installation de débitmètres utilisés pour le gaz naturel liquéfié (GNL) et autres hydrocarbures liquides réfrigérés

General Information

• Status: Published
• Publication Date: 09-Feb-2020
• ICS: 75.180.30 - Volumetric equipment and measurements
• Technical Committee: ISO/TC 28 - Petroleum and related products, fuels and lubricants from natural or synthetic sources
• Drafting Committee: ISO/TC 28 - Petroleum and related products, fuels and lubricants from natural or synthetic sources
• Current Stage: 6060 - International Standard published
• Start Date: 10-Feb-2020
• Due Date: 13-Oct-2019
• Completion Date: 10-Feb-2020

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 21903
First edition
2020-02
Refrigerated hydrocarbon
fluids — Dynamic measurement —
Requirements and guidelines for
the calibration and installation of
flowmeters used for liquefied natural
gas (LNG) and other refrigerated
hydrocarbon fluids
Hydrocarbures liquides réfrigérés — Mesurage dynamique —
Exigences et lignes directrices pour l’étalonnage et l’installation
de débitmètres utilisés pour le gaz naturel liquéfié (GNL) et autres
hydrocarbures liquides réfrigérés
Reference number
ISO 21903:2020(E)
©
ISO 2020

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ISO 21903:2020(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
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Published in Switzerland
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ISO 21903:2020(E)

Contents  Page
Foreword .v
Introduction .vi
1 Scope . 1
2  Normative references . 1
3  Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 3
4  Flowmeter selection . 3
4.1 Considerations of meters specific to LNG metering. 3
4.2 Coriolis flowmeter . 4
4.3 Ultrasonic flowmeter . 4
5  Process conditions . 5
5.1 Temperature effects . 5
5.1.1 Loading procedures. 5
5.1.2 Temperature effects on CMF measurements . 5
5.1.3 Temperature effects on USM measurements. 6
5.2 Pressure effects . 6
5.2.1 Coriolis flowmeter . 6
5.2.2 Ultrasonic flowmeter . 7
5.3 Mechanical vibrations . 7
5.3.1 Coriolis flowmeter . 7
5.3.2 Ultrasonic flowmeter . 8
5.4 Cavitation . 8
5.4.1 Coriolis flowmeter . 8
5.4.2 Ultrasonic flowmeter . 9
5.5 Thermodynamic properties of LNG . 9
6  Installation . 9
6.1 Valves . 9
6.2 Swirl and non-uniform profiles . 9
6.2.1 Coriolis flowmeter . 9
6.2.2 Ultrasonic flowmeter .10
6.3 Flow conditioners .10
6.4 Pipe stress and torsion .10
6.4.1 Coriolis flowmeter .10
6.4.2 Ultrasonic flowmeter .11
6.5 Flowmeter installation recommendations .11
6.5.1 Coriolis flowmeter .11
6.5.2 Ultrasonic flowmeter .12
6.6 Crosstalk and sensitivity to noise .12
6.6.1 Coriolis flowmeter .12
6.6.2 Ultrasonic flowmeter .12
6.7 Zero offset — Verification and adjustment procedures .13
6.7.1 Coriolis flowmeter .13
6.7.2 Ultrasonic flowmeter .15
6.8 Temperature management .15
6.8.1 Thermal insulation .15
6.8.2 Cooling procedure .16
6.8.3 Warming procedure .17
7  Calibration .18
7.1 General considerations .18
7.2 Calibration in a laboratory .18
7.2.1 Gravimetric method .18
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ISO 21903:2020(E)

7.2.2 Master meter method .20
7.3 Calibration in situ .22
7.3.1 Gravimetric method using a weighbridge .22
7.3.2 Road tanker temporarily on weighbridge .23
7.3.3 Measurement uncertainty .23
7.4 Interconnected pipe volume .23
Annex A (informative) Working principle Coriolis flowmeter .27
Annex B (informative) Working principle of the ultrasonic flowmeter .30
Annex C (normative) Hardware for an LNG calibration facility .33
Annex D (informative) Examples of calibration data .36
Annex E (normative) Alternative calibration procedure based on alternative liquids .39
Annex F (informative) Thermodynamic properties of LNG .41
Bibliography .47
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ISO 21903:2020(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 28, Petroleum and related products, fuels
and lubricants from natural or synthetic sources.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 21903:2020(E)

Introduction
Reliable, accurate and commonly agreed measurement methods are a first requirement for the trade of
goods. In the LNG distribution chain, there is a commonly agreed measurement practice, as described
[10]
in various International Standards and in the GIIGNL Custody transfer handbook . The LNG industry
is committed to improve measurement accuracy to reduce financial risks and to optimize mass and
energy balances throughout the LNG measurement chain. Dynamic measurement technologies have
the potential to reduce measurement uncertainty. As an extension of the traditional distribution chain
for LNG, a new market of professional consumers for LNG is developing related to transport fuel and
metrological infrastructure. In this respect, the availability of the following tools for dynamic flow
measurement is essential:
— primary standards for the determination of the amount of an LNG substance and calibration of
working standards;
— LNG test and calibration facilities (for volume and mass flow) for the calibration of equipment for
custody transfer, allocation or process control under operational conditions;
— stable meters for the determination of volume and mass flow under cryogenic conditions;
— guidelines for the selection and installation of cryogenic flowmeters;
— guidelines for zeroing and adjusting cryogenic flowmeters, including tips and traps;
— guidelines for the further dissemination of traceability by (master meter) calibration techniques,
including correction methods for parasitic metrological effects;
— guidelines for the calibration of volume and mass flowmeters with alternative fluids such as water.
This document provides designers of metering stations and end-users with a set of valuable guidelines
to enable a better performance of liquid flowmeters under cryogenic operating conditions. The
document focuses on LNG as a medium, however, it is assumed that much of the information is also
directly applicable to other cryogenic fluids.
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INTERNATIONAL STANDARD  ISO 21903:2020(E)
Refrigerated hydrocarbon fluids — Dynamic measurement
— Requirements and guidelines for the calibration and
installation of flowmeters used for liquefied natural gas
(LNG) and other refrigerated hydrocarbon fluids
1 Scope
This document specifies the metrological and technical requirements for flowmeters intended to be
used for the dynamic measurement of liquefied natural gas (LNG) and other refrigerated hydrocarbon
fluids. For LNG static volume measurement used in custody transfer, see ISO 10976.
This document sets the best practice for the proper selection and installation of flowmeters in cryogenic
applications and identifies the specific issues that can affect the performance of the flowmeter in use.
Moreover, it offers a calibration guideline for laboratory and on-site conditions (mass or volume) by
either using LNG or other reference fluids. The choice of calibration fluid will depend on the capabilities
of the available flow calibration facilities and the ability to achieve the required overall measurement
uncertainty demanded by the intended application.
This document is applicable, but is not limited, to the use of Coriolis and ultrasonic flowmeters for
dynamic measurements of LNG.
In principle, LNG and other refrigerated liquid hydrocarbons are considered in this document.
Recommendations in this document are based on the available test results with LNG. These results are
probably applicable to other cryogenic fluids.
2  Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 10790, Measurement of fluid flow in closed conduits — Guidance to the selection, installation and use of
Coriolis flowmeters (mass flow, density and volume flow measurements)
ISO 12242, Measurement of fluid flow in closed conduits — Ultrasonic transit-time meters for liquid
3  Terms, definitions and abbreviated terms
3.1  Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
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ISO 21903:2020(E)

3.1.1
master meter
MM
flowmeter calibrated against a primary standard with sufficiently low uncertainty and used to calibrate
the meter under test
3.1.2
measurement error
measured quantity value (3.1.3) minus a reference quantity value
3.1.3
measured quantity value
quantity value representing a measurement result
3.1.4
measurement uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used
Note 1 to entry: A list of metrological definitions can be found in ISO/IEC Guide 99.
3.1.5
stored zero value
S
ZV
value stored in the flowmeter transmitter representing a meter reading at a no flow condition
3.1.6
turndown ratio
ratio of maximum and minimum flow rates
3.1.7
zero adjustment
dedicated procedure to set a new stored zero value (3.1.5), with the aim to keep the flowmeter within its
zero offset limit (3.1.9)
3.1.8
zero offset
Z
O
average mass or volume flow rate reading observed under zero (no) flow conditions
Note 1 to entry: In this instance, the (Coriolis) flowmeter’s low flow cut-off filter is disabled, and the flow
direction in the electronics is set to bi-directional.
3.1.9
zero offset limit
Z
OL
maximum permissible zero offset (3.1.8) specified by the manufacturer
Note 1 to entry: Some Coriolis mass flowmeter manufacturers also state a specific zero offset for verification and
adjustment.
3.1.10
zero verification
procedure to check that the actual zero offset (3.1.8) of the flowmeter has not exceeded the zero offset
limit (3.1.9)
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ISO 21903:2020(E)

3.2  Abbreviated terms
CMF Coriolis mass flowmeter
LNG liquefied natural gas
MM master meter
MUT meter under test
USM ultrasonic flowmeter
4  Flowmeter selection
4.1  Considerations of meters specific to LNG metering
Table 1 gives an overview of the considerations for the selection of the appropriate flowmeter for a
specific situation.
Table 1 — Flowmeter selection considerations
Parameter Coriolis flowmeter Ultrasonic flowmeter
Type of measurement Mass flow measurement, Volumetric flow measurement
density measurement. (at actual conditions).
a b
Diameter of the meter Limited line size. Availability for larger lines.
c
Required space to Relative large meter body dimensions. Relative small meter body dimensions.
install the meter
Pressure drop Considerable pressure drop at high flow Low pressure drop.
rates. Possibility of LNG flashing.
Turndown ratio Large rangeability; the flowmeter can be Large rangeability; the flowmeter can be
applied to a large range of flow rates. applied to a large range of flow rates.
Diagnostics Density, gain of excitation (gas detection), Multiple paths flow profile, speed of
tube temperature. sound, gain, signal to noise ratio (gas
detection).
Straight length Not required to have a straight length For meters with a small number of paths
requirements upstream of the flowmeter. This is (< 4) a significant straight length up and
(flow profile) because CMFs are typically not affected downstream of the meter is required to
by swirling and non-uniform flow achieve sufficient accuracy. This is
velocity profiles induced by upstream because meters with small number of
or downstream piping configurations. paths may be sensitive to swirl and
non-uniform flow velocity profiles
induced by upstream or downstream
piping configurations.
Multipath types may not be sensitive to
swirling and non-uniform flow velocity
profiles induced by upstream or down-
stream piping configurations.
Bi-directional flow Suitable for bi-directional flow. Suitable for bi-directional flow.
a
Typically meters with a diameter up to 12" are available.
b
Typically meters with a diameter up to 36" are available.
c
The total setup could be relatively large due to a long upstream straight pipe length.
d
The stiffness change of the vibrating tube due to cryogenic temperatures has a significant impact, however, it can be
corrected for by the temperature model.
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Table 1 (continued)
Parameter Coriolis flowmeter Ultrasonic flowmeter
Reynolds number Generally low sensitivity to Reynolds Depending on the number of paths there
sensitivity number for low viscosity fluids such as is a moderate to high sensitivity on the
LNG. For very high viscosity fluids the Reynolds number.
flowmeter error is dependent on the
The viscosity changes due to changes in
Reynolds number, especially for
the composition are anticipated to be
laminar-turbulent transition.
negligible.
The viscosity changes due to changes in
the composition are anticipated to be
negligible.
Sensitivity to vibrations Could be affected by vibrations when the Insensitive to vibrations.
frequency is near the vibration frequency
of the tube.
Mechanical stress Sensitive to mechanical stress. Impact of Insensitive to mechanical stress.
mechanical stress can be monitored for
zero flow conditions.
Pressure Small effect for pressures up to roughly Smaller effect, can be corrected for based
30 bar. Can be corrected for based on on available correction models and
available correction models and internal internal or external pressure
or external pressure measurement. measurement.
Temperature Thermal expansion of the meter body Thermal expansion of the meter body
may be compensated for based on inter- may be compensated for by an internal/
d
nal/external temperature measurement. external temperature measurement.
Others Measured flow and density can be Measured flow can be influenced by
influenced by bubbles caused by (local) bubbles caused by (local) boiling and/or
boiling and/or cavitation in the flow. cavitation in the flow. Consider velocity
limits to prevent cavitation around
transducers.
a
Typically meters with a diameter up to 12" are available.
b
Typically meters with a diameter up to 36" are available.
c
The total setup could be relatively large due to a long upstream straight pipe length.
d
The stiffness change of the vibrating tube due to cryogenic temperatures has a significant impact, however, it can be
corrected for by the temperature model.
4.2  Coriolis flowmeter
The CMF is a device that measures mass flow rate as well as fluid density. Its fundamental operational
principle is based on vibration mechanics and its interaction with the fluid dynamics. Because of its
working principle, the flowmeter is capable of determining the density of the fluid when it matches a
resonance frequency that corresponds to the fluid mass enclosed in the measuring tube’s finite volume.
The mass flow rate is directly linked to the Coriolis force that is present when the fluid moves at a certain
velocity and in combination with the measuring tube’s angular motion. As this occurs, a secondary
oscillation mode will take place, thus generating a phase shift in the measuring tube displacement. Such
a phase shift is proportional to the mass flow rate, and is therefore used as a primary output signal to
determine flow.
NOTE More information on the CMF is given in Annex A.
4.3  Ultrasonic flowmeter
The ultrasonic transit-time flowmeter is a sampling device that measures discrete path velocities using
one or more pairs of transducers. Each pair of transducers is located at a known distance apart such
that one is upstream of the other. The upstream and downstream transducers send and receive pulses
of ultrasound alternately. The times of arrival are used in the calculation of average axial velocity. At
any given instant, the difference between the apparent speed of sound in a moving liquid and the speed
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ISO 21903:2020(E)

of sound in that same liquid at rest is directly proportional to the liquid’s instantaneous velocity. As
a consequence, a measure of the average axial velocity of the liquid along a path can be obtained by
transmitting an ultrasonic signal along the path in both directions and subsequently measuring the
transit-time difference.
The volumetric flow rate of a liquid flowing in a completely filled closed conduit is defined as the
average velocity of the liquid over a cross section multiplied by the area of the cross section. Thus, by
measuring the average velocity of a liquid along one or more ultrasonic paths (i.e. lines, not the area)
and combining the measurements with knowledge of the cross-sectional area and the velocity profile
over the cross section, it is possible to obtain an estimate of the volumetric flow rate of the liquid in the
conduit.
NOTE More information on the ultrasonic flowmeter is given in Annex B.
5  Process conditions
5.1  Temperature effects
5.1.1  Loading procedures
Both CMF and USM applications require a stable and consistent single-phase flowing medium in order
to correctly measure the flow. It is particularly important to consider this requirement when loading
at cryogenic temperatures as potentially large temperature variations and heat gain increase the
likelihood of a two-phase flow. This will at least be the case if the meter/pipes connecting the meter are
at ambient temperature prior to loading.
Several mitigating actions may be em
...