ISO 21903:2020
(Main)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
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
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 2020
ISO 21903:2020(E)
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ii © ISO 2020 – All rights reserved
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
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
iv © ISO 2020 – All rights reserved
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.
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.
vi © ISO 2020 – All rights reserved
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/
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)
2 © ISO 2020 – All rights reserved
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.
ISO 21903:2020(E)
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
4 © ISO 2020 – All rights reserved
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 employed to increase the likelihood of maintaining a cryogenic
single-phase liquid flow. One effective way to accomplish this is by keeping the meter cooled down, not
only during loading operations, but at all times, e.g. by using a proper circulating loop. A disadvantage
is the increased cost of cooling the cryogenic medium as the circulation will increase the heat gain. In
general, a low-flow velocity, large pipe diameter, and poorly insulated meter and flow lines should be
avoided as this will add to the probability of boiling and two-phase flow.
Maintaining the temperature of the meter at cryogenic conditions will minimize stresses on the pipe
material, which is desirable.
Depending on the loading product, it is common practice to cool down the meter and pipes from ambient
to cryogenic conditions prior to transfer. For LNG application, gas and liquid nitrogen are often used for
this purpose. Starting from an ambient temperature, cold nitrogen gas can be introduced to gradually
lower the temperature and avoid stress from temperature shock. Small amounts of liquid nitrogen are
then injected to boil off and further cool down the system.
For some applications (e.g. in a small-scale LNG transfer), it is not possible to cool the meter and pipes
with liquid nitrogen because it is not accessible at the location. In this case, purging the system with
cold natural gas is allowed.
After loading, the temperature will have to change from cryogenic to ambient conditions. It is common
to let the remaining liquid boil off from meter/pipes and this can cause a two-phase metering condition.
Depending on the conditions, at loading, both the CMF and USM can apply compensation to account
for changes in process conditions such as temperature. Any such compensation can increase the
measurement uncertainty and shall be considered specifically for the actual application. Therefore, it is
advisable to consult the flowmeter manufacturer.
5.1.2 Temperature effects on CMF measurements
One fundamental design parameter for CMFs at cryogenic temperatures is the consideration of the
measuring tube’s material properties and its behaviour at very low temperatures. This is quite relevant,
since the Young’s modulus of elasticity of tube material at standard conditions (e.g. water at laboratory
ISO 21903:2020(E)
temperature) is significantly different from the cryogenic conditions, and, more importantly, its
value is defined by a nonlinear relationship with the temperature. Further, because of the cryogenic
temperatures, the volume of the measuring tubes changes significantly. Disregarding these effects can
cause a shift in the calibration curve and thus a measurement bias.
Since CMF manufacturers are aware of these effects, a dedicated algorithm is implemented in CMF
software to correct for the Young’s modulus dependence on temperature, thermal contraction and any
other relevant parameters, if applicable.
In general, straight-measuring tube CMFs are more sensitive to cryogenic temperatures, as the axial
stress created in the tube can be very high and can exceed the material strength. A bent-measuring
tube CMF is a more robust sensor, since the axial stress generated at cryogenic temperatures is much
smaller, i.e. within the allowable material strength, and thus it gives a better zero-point stability.
Some independent studies on the performance of CMFs under cryogenic conditions indicate that most
meters are suitable for cryogenic flow measurements. However, the closeness of the flow measurement
to the reference value will vary according to the correction algorithm developed by the manufacturer,
and the data concerning the sensor’s material properties obtained from a fitted polynomial curve.
It is worth noting that, despite having a reliable source of data, the tube material properties and/or
the influence of the manufacturing process can cause a shift from the reference data, thus causing
unaccountable axial stress on the sensor.
5.1.3 Temperature effects on USM measurements
For all ultrasonic meters, the flow correction factor due to changes in meter geometry at cryogenic
temperatures can be given as a straightforward analytical solution, see ISO 12242. Owing to this, the
correction has a very small uncertainty and the only uncertainties related to this correction are those
associated with the material constants.
The flow correction factor due to a change in the meter body temperature, ΔT, is shown by Formula (1):
32 3
KT=+11ααΔΔ=+33TT+ ααΔΔ+ T (1)
() () ()
()
bt
where
K is the thermal correction factor;
bt
ΔT is T – T ;
operating calibration
α is the thermal expansion coefficient.
Formula (1) may be simplified without a significant loss of accuracy to Formula (2):
KT=+13αΔ (2)
bt
5.2 Pressure effects
5.2.1 Coriolis flowmeter
Operating a CMF at fluid pressures higher than the calibration reference conditions will lead to changes
in the mechanical characteristics of the measuring tube, thus modifying the CMF fundamental vibration
frequency, and, if not corrected, could create significant flow measurement errors.
The fluid pressure effect may be interpreted in mechanical terms as an additional axial stress acting on
the measuring tube. From manufacturers’ data and independent tests, it has been found that pressure
effects can differ with measuring tube geometry. For most bent-tube CMFs, the sign of the pressure
sensitivity (percentage of error per bar) is negative, while for most straight-tube CMFs it is positive.
6 © ISO 2020 – All rights reserved
ISO 21903:2020(E)
Currently, the majority of CMFs have a relatively small sensitivity to pressure changes. However, if
there is a need to quantify the CMF’s impact on the measurand, then the end-user is advised to follow
the manufacturer’s recommendations. Alternatively (if applicable), a pressure sensor may be employed
to make a real-time correction to the measurand, thus minimizing the CMF’s pressure sensitivity. The
latter shall be taken into consideration only if the CMF pressure-induced error exceeds the maximum
error tolerated by the process measurement, or if the operational pressure is so significantly high that
the manufacturer advises using an auxiliary pressure sensor.
5.2.2 Ultrasonic flowmeter
The influence of pressure on the performance of a USM, if operated at fluid pressures different than
the calibration pressure, is almost negligible. Only an expansion caused by the meter body due to a
pressure difference will affect the internal diameter, and hence will cause an under- or over-reading.
This will only be significant if the pressure difference is substantial. In this case, the flowmeter should
have the capability to correct for it. A general formula to calculate the pressure correction factor is
shown by Formula (3):
C
pb
K =+1 ×−PP (3)
()
pb processcal
where
K is the correction factor used for the pressure expansion;
pb
C is the linear pressure coefficient, in %/kPa;
pb
P is the process pressure, in kPa;
process
P is the reference pressure, in kPa.
cal
A typical pressure correction factor for a pressure difference is C = 0,000 04 %/kPa.
pb
NOTE A generic formula to calculate the effect of volume increase due to pressure is shown by Formula (4):
ΔA Dw+
P
i i
= × (4)
A w E
i
where
ΔA is the difference of the internal cross-sectional area;
i
A is the internal cross-sectional area;
i
D is the internal pipe diameter;
i
w is the wall thickness;
P is the pressure;
E is the elasticity modulus.
5.3 Mechanical vibrations
5.3.1 Coriolis flowmeter
In some cases, CMFs are exposed to external vibrations or pulsations. Such vibrations can be induced
by mechanical means (i.e. a pumping system), the environment or by the fluid dynamics in the pipeline.
ISO 21903:2020(E)
In general, a CMF is designed in such a way so that the effect of the external vibration is minimized,
whereby it has no relevant impact on the CMF measurement.
In cases where the end-user deals with a severe vibration application, it is recommended to use flexible
piping or isolation pipe supports to minimize the vibration, or to contact the manufacturer for further
assistance.
5.3.2 Ultrasonic flowmeter
USMs are built out of a robust metal body and the principle of operation is based on measuring the time
differences between two ultrasonic pulses travelling across the pipe diameter in opposite directions.
Currently, there is no proof of sensitivity of the meter reading to mechanical vibration.
5.4 Cavitation
5.4.1 Coriolis flowmeter
Cavitation in CMFs is defined as the process of formation of the vapour phase of a liquid within the
measuring tube. This phenomenon occurs when the hydrostatic pressure is decreased, which is caused
by a reduction in the cross-sectional area. The decrease in hydrostatic pressure causes a decrease in the
boiling point, which can cause the liquid to start boiling. This phenomenon is also known as “cavitation”.
As the liquid cavitates within the measuring tube, it forms tiny vapour bubbles, which grow as they
collapse on one another (implosion). If the fluid velocity at the measuring tube continues to increase
(the pressure continues to fall), then the vapour bubbles will continue growing in the same manner
(see Figure 1).
In terms of linearity, the cavitation effect plays a significant role, since the two-phase condition can
generate a significant measurement error depicted by a nonlinear response at the upper flow range of
the CMF.
Unlike other nonlinear effects upon CMFs, this is perhaps the easiest to handle, since cavitation can
be prevented by setting the operating process conditions properly. In this respect, there are six
fundamental recommendations to avoid cavitation:
a) use the correct size of the CMF according to the process conditions;
b) avoid low upstream pressure;
c) avoid fluid velocities out of the manufacturer’s specifications;
d) use the correct operational window and pressure/temperature relation (see the phase envelope);
e) ensure proper venting at a high point to prevent vapour pockets growing over time;
f) ensure enough recirculation (cooling) over the line piece to minimize the influence of ambient
heat gain.
Figure 1 depicts a sudden spread of the measurement error in the CMF response, due to the early
presence of cavitation in the measuring tube(s).
8 © ISO 2020 – All rights reserved
ISO 21903:2020(E)
Key
1 early presence of cavitation
X flow, in kg/h
Y relative error, in %
Figure 1 — Early presence of cavitation in a Coriolis flowmeter
5.4.2 Ultrasonic flowmeter
Cavitation in the USM is unlikely to happen when the transducers are built-in in such a way that no
cavities are present (the transducers are flush with the inner pipe wall). If cavitation takes place in the
meter, the readout will be unstable and, in some cases, will completely stop due to the attenuation of
the sound pulse.
5.5 Thermodynamic properties of LNG
The fluid properties for density and viscosity for LNG can be found in the tables in Annex F.
6 Installation
6.1 Valves
For both CMFs and USMs, it is recommended to use valves that are fully leak-tight. Flow regulating
valves (control valves) should be installed downstream to ensure that the fluid remains in a single
phase and no flashing or cavitation occurs.
6.2 Swirl and non-uniform profiles
6.2.1 Coriolis flowmeter
The performance of a CMF in single-phase flow is not affected by swirls or non-uniform velocity profiles
induced by upstream or downstream-piping configurations.
Caution shall be exercised with respect to inducing external stress or vibration on the flow tubes when
planning and carrying out the installation of Coriolis meters as explained in ISO 10790.
ISO 21903:2020(E)
6.2.2 Ultrasonic flowmeter
Ultrasonic meters can be sensitive to hydraulic influences. In other words, distortions of the axial
velocity profile or the introduction of non-axial flow components or swirling flow resulting from bends,
valves and other pipe fittings upstream of the meter can result in measurement errors if not addressed
in either the meter design or the system configuration.
It is possible to make some general statements about factors affecting the magnitude of such
hydraulic installation effects. For example, it is generally true that USMs employing a larger number of
measurement paths are less sensitive to hydraulic effects than meters with fewer paths. It can also be
shown that meters with measurement paths arranged in crossed pairs can be effective in reducing the
effects of non-axial and swirling flows.
Ultimately, type evaluation data are required to properly judge the sensitivity of a given meter design to
hydraulic disturbances. It is recommended for LNG applications that the uncertainty component owing
to hydraulic effects for a given meter design and upstream pipe configuration is based on laboratory
test data in accordance with the type testing requirements of ISO 12242.
6.3 Flow conditioners
Flow conditioners may be installed upstream to achieve the desired hydraulic conditions for ultrasonic
meters. However, a flow conditioner creates a pressure drop, which could result in flashing of the LNG
upstream of the meter. Therefore, if it is demonstrated, by reference to ISO 12242, that a flowmeter
design achieves the uncertainty required for custody transfer without a flow conditioner, upstream
flow conditioners should not be used.
The pressure loss owing to the use of a flow conditioner can be calculated as shown by Formula (5):
ΔPk=05, ρ v (5)
where
ΔP is the pressure loss, in bar;
v is the fluid velocity in the pipe, in m/s;
k is the loss coefficient for a given type of conditioner (−);
ρ is the fluid density, in kg/m .
Typical loss coefficients for flow conditioners are in the range of 2 to 5. Once the pressure loss has been
calculated, pressures and temperatures can be calculated to ensure that the LNG is kept comfortably
below the boiling point in order to avoid cavitation, see Figure F.1.
6.4 Pipe stress and torsion
6.4.1 Coriolis flowmeter
Depending on the type and construction of the meter, the CMF can be sensitive to pipe stresses. Consult
the manufacturer and consider the use of bellows.
The flow sensor is subjected to axial, bending and torsional forces during operation. Changes in these
forces, resulting from variations in the process temperature and/or pressure, can affect the Coriolis
mass flow measurement. Care should be taken to ensure that no forces are exerted on the CMF from the
clamping arrangement
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