Refrigerated hydrocarbon liquids — Static measurement — Calculation procedure

ISO 6578:2017 specifies the calculation procedure to convert the volume of liquefied petroleum gas (LPG) and liquefied natural gas (LNG) under the conditions at the time of measurement to the equivalent volume of liquid or vapour at the standard condition, i.e. 15 °C and 101,325 kPaA, or to the equivalent mass or energy (calorific content). It applies to the quantities of refrigerated hydrocarbon liquids stored in or transferred to/from tanks and measured under static storage conditions. Calculation of pressurized gases is out of the scope of ISO 6578:2017.

Hydrocarbures liquides réfrigérés — Mesurage statique — Procédure de calcul

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Published
Publication Date
12-Oct-2017
Current Stage
6060 - International Standard published
Start Date
13-Oct-2017
Completion Date
13-Oct-2017
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INTERNATIONAL ISO
STANDARD 6578
Second edition
2017-10
Refrigerated hydrocarbon liquids —
Static measurement — Calculation
procedure
Hydrocarbures liquides réfrigérés — Mesurage statique — Procédure
de calcul
Reference number
ISO 6578:2017(E)
ISO 2017
---------------------- Page: 1 ----------------------
ISO 6578:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form

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ii © ISO 2017 – All rights reserved
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ISO 6578:2017(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms, definitions and symbols ............................................................................................................................................................ 1

3.1 Terms and definitions ....................................................................................................................................................................... 1

3.2 Symbols ......................................................................................................................................................................................................... 2

4 Outline of calculation ....................................................................................................................................................................................... 3

4.1 LPG .................................................................................................................................................................................................................... 3

4.2 LNG .................................................................................................................................................................................................................... 4

4.3 Data for calculation ............................................................................................................................................................................. 4

5 Mass ................................................................................................................................................................................................................................... 5

5.1 Mass of liquid phase ........................................................................................................................................................................... 5

5.2 Correction for vapour phase ....................................................................................................................................................... 6

5.3 Mass in vacuum to mass in air ................................................................................................................................................... 8

6 Energy content (calorific content) ..................................................................................................................................................... 8

7 Inter-conversion of liquid mass and vapour volume at standard conditions ....................................11

8 Calculation of liquid density from composition................................................................................................................12

8.1 General ........................................................................................................................................................................................................12

8.2 LPG .................................................................................................................................................................................................................12

8.3 LNG .................................................................................................................................................................................................................13

9 Calculation of calorific value from composition ...............................................................................................................14

9.1 Volumetric basis..................................................................................................................................................................................14

9.2 Mass basis.................................................................................................................................................................................................14

Annex A (informative) Characteristics of static measurement of refrigerated

hydrocarbon liquids .......................................................................................................................................................................................16

Annex B (normative) Molar volume of individual component ...............................................................................................17

Annex C (normative) Correction factors for volume reduction of LNG mixtures ................................................18

Annex D (normative) Gross calorific values for individual components .....................................................................19

Annex E (normative) Molar mass, compression factor and summation factor of

individual component ..................................................................................................................................................................................20

Annex F (informative) Boiling point of individual component ..............................................................................................21

Annex G (informative) Alternative procedure to calculate liquid density from composition ...............22

Bibliography .............................................................................................................................................................................................................................25

© ISO 2017 – All rights reserved iii
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ISO 6578:2017(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 on 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 the following

URL: 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, Subcommittee SC 5, Measurement of refrigerated

hydrocarbon and non-petroleum based liquefied gaseous fuels.

This second edition cancels and replaces the first edition (ISO 6578:1991), which has been technically

revised.
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ISO 6578:2017(E)
Introduction

Large quantities of refrigerated hydrocarbon liquids such as liquefied natural gas (LNG), liquefied

petroleum gas (LPG), etc. are transported by marine carriers dedicated for these applications. These

gases are traded based on static measurement on board marine carriers rather than the measurement

at shore tanks or pipelines due mainly to the nature of the tank operation.

The measurement on board involves determination of liquid/vapour interface, i.e. liquid level, average

temperatures of liquid and vapour, and vapour pressure in the tanks of marine carriers. The volumetric

quantity of the liquid and gas is then computed with the tank capacity tables.

This document is applicable to calculate the volume at standard condition, liquid density from

chemical composition, mass and energy content of fully refrigerated hydrocarbon liquids at a vapour

pressure near to atmospheric pressure from the results of custody transfer measurement. This

document is also applicable to ascertain the inventory in shore tanks. Calculation procedures for

refrigerated hydrocarbon liquids consisting predominantly of ethane or ethylene, or for partially

refrigerated hydrocarbon liquids at pressures substantially above atmospheric, are not included. No

recommendations are given for the measurement of small parcels of refrigerated liquids, which are

directly weighed.

Aspects of safety are not dealt with in this document. It is the responsibility of the user to ensure that

the procedure of measurement meets applicable safety regulations.

Basic data and source references used in the calculation procedures are given in annexes.

Annexes A to G form an integral part of this document.
© ISO 2017 – All rights reserved v
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INTERNATIONAL STANDARD ISO 6578:2017(E)
Refrigerated hydrocarbon liquids — Static measurement
— Calculation procedure
1 Scope

This document specifies the calculation procedure to convert the volume of liquefied petroleum gas

(LPG) and liquefied natural gas (LNG) under the conditions at the time of measurement to the equivalent

volume of liquid or vapour at the standard condition, i.e. 15 °C and 101,325 kPaA, or to the equivalent

mass or energy (calorific content). It applies to the quantities of refrigerated hydrocarbon liquids

stored in or transferred to/from tanks and measured under static storage conditions. Calculation of

pressurized gases is out of the scope of this document.
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 91, Petroleum and related products — Temperature and pressure volume correction factors (petroleum

measurement tables) and standard reference conditions
3 Terms, definitions and symbols
3.1 Terms and definitions

For the purposes of this document, the following terms, definitions and symbols apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at http://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
3.1.1
compression factor

actual (real) volume of a given amount of gas at a specified pressure and temperature divided by its

volume, under the same conditions as calculated from the ideal gas law
[SOURCE: ISO 6976:2016, 3.10]
3.1.2
gross calorific value

amount of heat that would be released by the complete combustion with oxygen 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, which is condensed to the liquid state at t

Note 1 to entry: t and p are combustion reference temperature and combustion reference pressure, respectively.

1 1
[SOURCE: ISO 6976:2016, 3.1, modified — Note 1 to entry has been replaced.]
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ISO 6578:2017(E)
3.1.3
liquefied natural gas
LNG
liquid composed predominantly of methane
3.1.4
liquefied petroleum gas
LPG

liquid composed predominantly of any of the following hydrocarbons or mixtures thereof: propane,

propene, butanes and butene
3.1.5
refrigerated hydrocarbon liquid

liquid composed predominantly of hydrocarbons, which are stored in a fully refrigerated condition at

pressures near atmospheric
3.1.6
volumetric basis (ideal)
volume calculated on the basis that the vapour behaves like an ideal gas
3.1.7
volumetric basis (real)

volume calculated on the basis that the vapour behaves like a super-compressible gas

3.2 Symbols

The following symbols are defined here for use in this document, but additionally, some symbols are

given more restricted meanings when used in some formulae. The restricted meaning is then given

after the formulae.

H gross (superior) calorific value on a mass basis, in megajoules per kilogram, of component i

s,m,i
(see Table D.1)

H gross (superior) calorific value on a mass basis, in megajoules per kilogram, of the liquid

s,m

H gross (superior) calorific value on a volumetric basis (ideal), in megajoules per cubic metre, of

s,V,i
component i (see Table D.1)

H gross (superior) calorific value on a volumetric basis, in megajoules per cubic metre, of the

s,vol
vapour at standard condition
m mass, in kilograms, of product transferred, i.e. liquid plus vapour
m mass, in kilograms, of liquid
liq
M molar mass, in kilograms per kilomole, of component i (see Table E.1)
M relative molar mass, in kilograms per kilomole, of the vapour mixture
mix
P standard reference pressure, i.e. 101,325 kPaA (kilopascal absolute)
P pressure, in kilopascals absolute, of the vapour in the container
vap
Q net energy, in megajoules, transferred, based on gross calorific value
Q energy (calorific) content, in megajoules, of the liquid
liq
−1 −1
R molar gas constant, 8,314 462 1 J·mol ·K , see ISO 6976:2016, A.1
t temperature, in degrees Celsius, of the liquid
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ISO 6578:2017(E)
T standard reference temperature, i.e. 288,15 K (15 °C)
T temperature, in kelvins, of the vapour in the container
vap
V molar volume, in cubic metres per kilomole, of component i, as a liquid at t
V volume, in cubic metres, of the liquid at t
liq

V ideal gaseous molar volume, in cubic metres per kilomole, at standard conditions:

i.e. V = (R × T )/P = 23,644 8 m /kmol at 15 °C and 101,325 kPaA (kilopascal absolute)

m s s
V vapour volume, in cubic metres, in the container
vap
V vapour volume at standard condition
vap,s
x ; x mole fractions of the components i and j, respectively
i j
x mole fraction of methane in the LNG
x mole fraction of nitrogen in the LNG
Z compression factor for component i at the required pressure and temperature

Z compression factor for the vapour mixture under known conditions of temperature and

mix
pressure
ρ density, in kilograms per cubic metre, of the liquid at T
s s
ρ density, in kilograms per cubic metre, of the liquid at t

NOTE Additional subscripts F and I indicate, respectively, the final and initial measurements or product

properties in either of the two containers used for a transfer.
4 Outline of calculation
4.1 LPG
Figure 1 outlines the calculation of mass of LPG from liquid volume at t °C.
Figure 1 — Calculation flow (LPG)
© ISO 2017 – All rights reserved 3
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ISO 6578:2017(E)

The procedure for converting the volume of refrigerated LPG to its equivalent volume at a standard

temperature and corresponding equilibrium pressure includes the following aspects.

a) Very large factors may have to be applied for the correction of observed density to density at

standard temperature, e.g. a correction for the effect of a temperature difference of 60 °C may be

necessary for refrigerated propane. Provided that the LPG does not contain more than 20 % of

unsaturated hydrocarbons, the correction tables introduced in ISO 91 shall be used for volume

corrections. Mass of LPG is calculated by multiplying its volume at standard temperature by its

density at standard temperature.

b) The equivalent liquid content in the vapour space of a container holding refrigerated LPG

is significantly less than the liquid in the container if the tank and contents are at ambient

temperature. Therefore, any error in accounting for the equivalent liquid content in the vapour

space will be of lesser significance.

NOTE The following examples illustrate the magnitude of errors that can be introduced by using the tables

referred to in ISO 91.

a) Pure butene or propene: the maximum error will be approximately 2 % for a correction from −60 °C to +20 °C.

b) Mixtures containing approximately 20 % of unsaturated hydrocarbons: a typical error will be approximately

0,1 % for a temperature difference of 20 °C.

A condition in which a liquid has a vapour pressure significantly higher than atmospheric pressure at

a standard temperature of 15 °C can only be considered as a pseudo-condition, and the volume of the

liquid in this condition may be used only when convenient in a procedure for obtaining the density at

refrigerated temperatures by means of pressure hydrometer measurement at ambient conditions (see

ISO 3993).
4.2 LNG

Figure 2 outlines the calculation energy content of LNG from liquid volume at t °C.

Figure 2 — Calculation flow (LNG)

Energy content of LNG is the product of its volume at observed temperature, the density at that

temperature and the calorific value per unit mass. This calculation does not involve conversion of

volume at observed temperature to the equivalent volume at standard temperature.
4.3 Data for calculation

Values specified in the normative annexes (Annexes B, C, D and E), such as physical properties of

components of refrigerated hydrocarbon liquids, constants, factors, etc., shall be normatively applied in

the use of this document.
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ISO 6578:2017(E)
5 Mass
5.1 Mass of liquid phase
5.1.1 The mass of liquid (m ), in kilograms, is calculated from Formula (1):
liq
mV= ρ (1)
liqliq
where V and ρ are for the same temperature.
liq
EXAMPLE 1
Measured volume of liquid LNG in a container = 45 550 m at −159,5 °C.
Calculated density at −159,5 °C = 462,4 kg/m
6 3
Mass of LNG (m ) = 45 550 × 462,4 = 21,06 × 10 kg or 21,06 × 10 t
liq

5.1.2 The density of refrigerated LPG may be determined at the standard temperature of 15 °C by use

of the pressure hydrometer method (see ISO 3993) or suitable densimeter. The liquid sample drawn into

a suitable container is allowed to approach ambient temperature under pressure, without loss of vapour,

before it is introduced into the hydrometer cylinder.

The density of liquid may also be calculated from a composition analysis (see Clause 8).

5.1.3 If the actual temperature t , at which the density is measured, does not differ by more than 5 °C

from the temperature t of the main bulk of liquid in the container, then the observed density may be

corrected to the required bulk temperature by using Formula (2). The density at t shall be measured or

calculated if the difference of the temperatures exceeds 5 °C.
ρρ=+Ft −t (2)
tt,,12 21
where
ρ and ρ are the densities at temperatures t and t , respectively;
t,1 t,2 1 2

F is the density correction factor applicable to the particular liquid. The units of F shall

be compatible with the units of ρ, e.g. when ρ is expressed in kilograms per cubic

metre, F is expressed in kg/(m ∙°C).
Product F
kg/(m ∙°C)
LNG [>80 % (m/m) methane] 1,4
Liquid propanes [>60 % (m/m) propane] 1,2
Liquid butanes [>60 % (m/m) butane] 1,1
© ISO 2017 – All rights reserved 5
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ISO 6578:2017(E)
EXAMPLE 2
The density of the LNG is 463,1 kg/m at t = −160,0 °C.
What is the density of the LNG at - 159,5 °C?
Substituting into Formula (2) gives:
ρ = 463,1 + 1,4[−160,0 − (−159,5)]
t,1
= 463,1 − 0,7
= 462,4 kg/m
5.2 Correction for vapour phase

5.2.1 When a quantity of refrigerated hydrocarbon liquid is transferred, it will be necessary to make

a correction for the mass of vapour occupying the volume into which, or from which, the liquid is

transferred.

Assuming that all measurements have been made under liquid equilibrium conditions, Formula (3) can

be applied to measurements made in either the delivery or the receiving container.

Mass transferred = |Final mass − Initial mass|
 
vap,F
mix,F
VVρ +× ×× −
 
liq,FF vap,F
T P VZ
 
vap,F s mmix,F
 
∴=m (3)
 P 
vap,I
mix,I
VVρ +× ×× 
liq,II vap,I
T P VZ
 vap,I s mmix,I 
 
where V and ρ are at the storage temperature t.
liq

If it is impractical to measure the density of the liquid in a container, ρ and ρ cannot be determined. By

F I

using the measured density of the liquid being transferred, however, the simplified Formulae (3a) and

(3b) may be employed to calculate the mass of product transferred.
 P 
vap,F
mix,F
At delivery container: mV=−ρ V ×× × (3a)
 
liqliq
 
T P VZ
vap,F s mmix,F
 
 
P M
vap,I mix,I
At receiving container: mV=−ρ V ×× ×  (3b)
liqliq
 
T P VZ
vap,I s mmix,I
 
where
V = |V − V |;
liq liq,F liq,I
ρ is the average density of the liquid which is transferred.
6 © ISO 2017 – All rights reserved
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ISO 6578:2017(E)

For a receiving container which does not already contain hydrocarbon liquid or vapour, Formula (3)

becomes
 P 
T M
vap
s mix
mV=+ρ V ×× ×  (3c)
liq,FFvap,
 
T P VZ
vap s mmix
 

If the vapour space is negligibly small in comparison with the liquid volume or the liquid volume is

negligibly small in comparison with the vapour space in the initial or final condition in the tanks, the

simplified Formula (3a) or (3b) may be used in practice.

Because the mass of vapour is small compared with the mass of liquid transferred, the accurate

knowledge of vapour composition and the use of a compression factor are not essential and the

ideal gaseous molar volume may be used without correction, and typical values may be used for the

temperature and pressure of the vapour space (T , P ) and for the molar mass and compression

vap vap
factor of the vapour mixture (M , Z ).
mix mix

NOTE For measurements in a receiving container, Formula (3b) is strictly valid only if the temperature of

the incoming liquid is the same as that already contained in the tank. The error involved in this assumption is at

a maximum when equal volumes of liquid are involved and is then of the order of 0,004 % per kelvin for LNG.

EXAMPLE 1
LNG transfer from a container

Calculate the mass of LNG transferred from a container under the following conditions:

Volume of liquid LNG transferred at temperature t = 45 550 m
Measured temperature of liquid, t = −159,5 °C
Liquid density at −159,5 °C = 462,4 kg/m
Average temperature of vapour after transfer = −118 °C = 155,15 K
Pressure of vapour after transfer = 110,0 kPaA
It may be assumed that the molar mass of the vapour mixture is that of pure
methane (see Table E.1). = 16,042 kg/kmol

The compression factor for the vapour can be taken as unity, with a resultant error of less than 0,05 %.

 
 288,15 110,0 16,042 
m=×45550 462,44−×5550 ××
 
 
155,15 101,325 23,6444 8
 
 
 
=−21062320 62309
=×21000 10 kg
or 21000t
EXAMPLE 2
LPG transfer from a container

Calculate the mass of LPG transferred from a container under the following conditions:

Initial Final
Volume of liquid in container at 15 °C (m ) 45 550 850
Liquid density at 15 °C (kg/m ) 507 507
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ISO 6578:2017(E)
Vapour space in container (m ) 950 40 000
Temperature of vapour in container (K) 233,15 250,15
Pressure in container vapour space (kPaA) 108,0 112,0

It may be assumed that the molar mass of the vapour mixture is the same as that of the liquid and that

the compression factor is unity, i.e. M = 44,153 kg/kmol.
mix
Substituting into Formula (3) gives:
 
 
288,15 108,0 44,153
m=×45550 507 +×950 ×× −
  
233,15 101,325 23,644 8
 
 
 
 
 288,15 112,0 444,153 
850×507 +×40000 ××
 
 
250,15 101,325 23,644 8
 
 
 
=+23093850 2337 −+430 950 95105
() ()
==×22570 10 kg
or 22570t

5.2.2 Similarly, if the energy measurements are required for stock purposes, take into consideration

the liquid equivalent of the vapour in the total ullage space.
5.3 Mass in vacuum to mass in air

The current practice for measurement of LPG is by apparent mass in air. The tables for kilogram per

cubic metre at 15 °C and cubic metres at 15 °C per metric ton against density at 15 °C introduced in

ISO 91 may be used to convert mass into apparent mass in air.
6 Energy content (calorific content)
6.1 The energy content of the liquid is calculated by using Formula (4):
Qm= H (4)
liqliq s,m

6.2 When a quantity of refrigerated hydrocarbon liquid is transferred, it will be necessary to make a

correction for the calorific content of the vapour occupying the volume into which, or from which, the

liquid is transferred.

Assuming that all measurements have been made under liquid equilibrium conditions, Formula (5)

applies to measurements made in either the delivery or the receiving container.
Energy transferred = |Final energy content − Initial energy content|
 P 
vap,F
VHρ +×V ××H −
 
liq,FF s,mF,vap,F s,vol,F
T P
 
vap,F s
 
∴=Q (5)
 P 
vap,I
VHρ +×V ××H 
liq,II s,mI,vap,I s,vol,I
T P
 vap,I s 
 
where
8 © ISO 2017 – All rights reserved
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ISO 6578:2017(E)
= the gross calorific value on volumetric basis, in megajoules per cubic
mix
H =×H
s,vol s,m
metre, of the vapour at standard condition.
mmix

If it is impractical to measure the density of the liquid in a container, ρ and ρ cannot be determined. By

F I

using the measured density of the liquid being transferred, however, the simplified Formulae (5a) and

(5b) may be employed to calculate the net energy transferred.
vap,F
At delivery container: QV=−ρHV ×× ×H (5a)
liqs,lm iq s,vol
T P
vap,F s
vap,I
At receiving container: QV=−ρHV ×× ×H (5b)
liqs,lm iq s,vol
T P
vap,I s
where
V = |V − V |;
liq liq,F liq,I
ρ is the average density of the liquid which is transferred;

H is the estimated gross calorific value of the transferred gas at standard condition.

s,vol

For a receiving container which does not already contain hydrocarbon liquid or vapour, Formula (5)

becomes
vap
QV=+ρHV ×× ×H (5c)
liqs,vm ap s,vol
T P
vap s

If the vapour space is negligibly small in comparison with the liquid volume or the liquid volume is

negligibly small in comparison with the vapour space in the initial or final condition in the tanks, the

simplified Formula (5a) or (5b) may be used in practice.

NOTE For measurements in a receiving container, Formula (5b) is strictly valid only if the temperature of

the incoming liquid is the same as that already contained in the tank. The error involved in this assumption is at

a maximum when equal volumes of liquid are involved and is then of the order of 0,004 % per kelvin for LNG.

EXAMPLE 1
LNG transfer from a container

Calculate the calorific content of LNG transferred from a container under the following conditions:

Volume of liquid LNG transferred at temperature t = 45 550 m
Liquid temperature, t = −159,5 °C
Liquid density at
...

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