ISO 6578:2017

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 2017
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

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
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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electrotechnical standardization.
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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).
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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.
iv © ISO 2017 – All rights reserved

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.
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.]
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 ad
...