ISO 19723-1:2025

Road vehicles - Liquefied natural gas (LNG) fuel systems - Part 1: Safety requirements

This document specifies the minimum safety requirements applicable to liquefied natural gas (LNG) on-board fuel system intended for use on the types of motor vehicles defined in ISO 3833. This document is applicable to vehicles (mono-fuel, bi-fuel or dual-fuel applications) using LNG according to the ISO 15403 series. It is applicable to original-production and converted vehicles. All matters relating to the skills of installers and converters have been excluded from this document. This document is only applicable to the components in the LNG system which is an assembly of components (tanks, valves, flexible fuel lines, etc., see Annex B) and connecting parts (fuel lines, fittings, etc.) fitted on motor vehicles using LNG in their propulsion system and related components up to and including the vaporizer. Other parts downstream from the vaporizer are considered as compressed natural gas (CNG) components covered by the ISO 15501 series.

Véhicules routiers — Systèmes à carburant gaz naturel liquéfié (GNL) — Partie 1: Exigences de sécurité

General Information

• Status: Published
• Publication Date: 11-Dec-2025
• ICS: 43.060.40 - Fuel systems
• Technical Committee: ISO/TC 22/SC 41 - Specific aspects for gaseous fuels
• Drafting Committee: ISO/TC 22/SC 41 - Specific aspects for gaseous fuels
• Current Stage: 6060 - International Standard published
• Start Date: 12-Dec-2025
• Due Date: 17-Feb-2026
• Completion Date: 12-Dec-2025

Relations

• Revises: ISO 19723-1:2018/Amd 1:2021 - Road vehicles - Liquefied natural gas (LNG) fuel systems - Part 1: Safety requirements - Amendment 1
• Effective Date: 18-Feb-2023
• Revises: ISO 19723-1:2018 - Road vehicles - Liquefied natural gas (LNG) fuel systems - Part 1: Safety requirements
• Effective Date: 18-Feb-2023

Overview

ISO 19723-1:2025 - Road vehicles - Liquefied natural gas (LNG) fuel systems - Part 1: Safety requirements defines the minimum safety requirements for on-board LNG fuel systems fitted to motor vehicles (ISO 3833 vehicle types). The standard covers assembly components (tanks, valves, flexible fuel lines, fittings, etc.) and connecting parts up to and including the vaporizer. It applies to mono‑fuel, bi‑fuel and dual‑fuel vehicles, both original‑production and converted vehicles, and references the ISO 15403 series for LNG quality.

Key topics and technical requirements

ISO 19723-1:2025 focuses on safety design and functional requirements for LNG on-board fuel systems. Major topics include:

• System design principles - Components must conform to ISO 12614/12617 series and be designed based on declared service pressure.
• Component requirements - Tanks, pressure relief valves (PRVs), check valves, fuel pumps, fittings and flexible lines are covered; materials and placement must resist environmental, chemical and mechanical damage.
• Refuelling interfaces - Receptacle location, mounting and minimum clearance requirements to ensure safe and accessible refuelling.
• Leakage control and venting - Requirements for venting systems, PRV ventilation lines and vent line closures to safely manage LNG boil‑off and relieve pressure.
• Mounting, heat protection & corrosion - Rules for tank mounting, protection from heat and road debris, and minimizing galvanic corrosion.
• Ignition risk mitigation - Measures for electrical conductivity, grounding and minimizing sources of gas ignition.
• Instrumentation and controls - Inclusion of electronic control unit (ECU) requirements for engine demand control and automatic valve cut‑off on pipe break or engine stall; gas temperature sensing requirements.
• Documentation - Instructions for use, marking and inspection access requirements.
• Scope limit - Standard applies only up to and including the vaporizer; downstream components are treated as CNG components (see ISO 15501 series).

Annex A provides technical solution examples; Annex B lists typical LNG on‑board system components.

Practical applications and users

ISO 19723-1:2025 is used to:

• Guide OEMs and vehicle system designers integrating LNG propulsion systems.
• Define compliance criteria for component manufacturers (tanks, valves, vaporizers, connectors).
• Support conversion companies and engineering teams responsible for converted LNG vehicles (design compliance; installer skills excluded).
• Inform safety engineers, certifying bodies and regulators evaluating on‑board LNG system safety and conformity.
• Complement test methods in ISO 19723-2 for verification.

Related standards

• ISO 12614 (LNG fuel system components)
• ISO 12617 (LNG refuelling connector)
• ISO 15403 (LNG fuel quality)
• ISO 15500 / ISO 15501 series (CNG components)
• ISO 12991 (LNG tanks)
• ISO 19723-2 (Test methods)
• ISO 20653 / IEC 60529 (IP protection)

Keywords: ISO 19723-1:2025, LNG fuel systems, road vehicles, safety requirements, LNG on‑board fuel system, vaporizer, pressure relief valve, ECU, venting system.

Standards Content (Sample)

International
Standard
ISO 19723-1
Second edition
Road vehicles — Liquefied natural
2025-12
gas (LNG) fuel systems —
Part 1:
Safety requirements
Véhicules routiers — Systèmes à carburant gaz naturel
liquéfié (GNL) —
Partie 1: Exigences de sécurité
Reference number
© ISO 2025
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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Requirements . 4
4.1 Design .4
4.1.1 General .4
4.1.2 Components .5
4.2 Refuelling .8
4.2.1 General .8
4.2.2 Receptacle location .8
4.2.3 Receptacle mounting .8
4.2.4 Minimum receptacle clearance .8
4.3 Leakage control .8
4.4 Mounting of the tank(s) .9
4.5 Heat protection .9
4.6 Minimizing risk of gas ignition .9
4.7 Venting system .10
4.7.1 General .10
4.7.2 Ventilation lines for PRVs .10
4.7.3 Vent line closures .11
4.8 Gas temperature sensor .11
5 Instructions for use . .11
6 Marking . .11
Annex A (informative) Technical solutions to functional requirements .12
Annex B (informative) Liquefied natural gas (LNG) on-board fuel systems .13
Bibliography . 14

iii
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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 22, Road vehicles, Subcommittee SC 41, Specific
aspects for gaseous fuels.
This second edition cancels and replaces the first edition (ISO 19723-1:2018), which has been technically
revised. It also incorporates the Amendment ISO 19723-1:2018/Amd 1:2021.
The main changes are as follows:
— updated Clause 2, Normative references;
— added requirements for the electronic control unit (ECU);
— updated requirements for minimum receptacle clearance;
— added requirements for vent line closures.
A list of all parts in the ISO 19723 series can be found on the ISO website.
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.

iv
Introduction
All fuel system components in contact with liquid natural gas have been considered suitable for natural gas
as defined in the ISO 15403 series.
A safety device to prevent overfilling the vehicle's fuel system is part of the refuelling station. The pressure
gauge has not been considered as a safety component.
Technical solutions regarding functional requirements are given in Annex A.

v
International Standard ISO 19723-1:2025(en)
Road vehicles — Liquefied natural gas (LNG) fuel systems —
Part 1:
Safety requirements
1 Scope
This document specifies the minimum safety requirements applicable to liquefied natural gas (LNG) on-
board fuel system intended for use on the types of motor vehicles defined in ISO 3833.
This document is applicable to vehicles (mono-fuel, bi-fuel or dual-fuel applications) using LNG according to
the ISO 15403 series. It is applicable to original-production and converted vehicles.
All matters relating to the skills of installers and converters have been excluded from this document.
This document is only applicable to the components in the LNG system which is an assembly of components
(tanks, valves, flexible fuel lines, etc., see Annex B) and connecting parts (fuel lines, fittings, etc.) fitted
on motor vehicles using LNG in their propulsion system and related components up to and including the
vaporizer. Other parts downstream from the vaporizer are considered as compressed natural gas (CNG)
components covered by the ISO 15501 series.
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 7637-1, Road vehicles — Electrical disturbances from conduction and coupling — Part 1: Vocabulary and
general considerations
ISO 7637-2, Road vehicles — Electrical disturbances from conduction and coupling — Part 2: Electrical transient
conduction along supply lines only
ISO 11451-1, Road vehicles — Vehicle test methods for electrical disturbances from narrowband radiated
electromagnetic energy — Part 1: General principles and terminology
ISO 11451-2, Road vehicles — Vehicle test methods for electrical disturbances from narrowband radiated
electromagnetic energy — Part 2: Off-vehicle radiation sources
ISO 11451-4, Road vehicles — Vehicle test methods for electrical disturbances from narrowband radiated
electromagnetic energy — Part 4: Harness excitation methods
ISO 11452-1, Road vehicles — Component test methods for electrical disturbances from narrowband radiated
electromagnetic energy — Part 1: General principles and terminology
ISO 11452-2, Road vehicles — Component test methods for electrical disturbances from narrowband radiated
electromagnetic energy — Part 2: Absorber-lined shielded enclosure
ISO 11452-3, Road vehicles — Component test methods for electrical disturbances from narrowband radiated
electromagnetic energy — Part 3: Transverse electromagnetic (TEM) cell
ISO 11452-4, Road vehicles — Component test methods for electrical disturbances from narrowband radiated
electromagnetic energy — Part 4: Harness excitation methods

ISO 11452-5, Road vehicles — Component test methods for electrical disturbances from narrowband radiated
electromagnetic energy — Part 5: Stripline
ISO 12614 (all parts), Road vehicles — Liquefied natural gas (LNG) fuel system components
ISO 12617:2015, Road vehicles — Liquefied natural gas (LNG) refuelling connector — 3,1 MPa connector
ISO 12991, Liquefied natural gas (LNG) — Tanks for on-board storage as a fuel for automotive vehicles
ISO 15500 (all parts), Road vehicles — Compressed natural gas (CNG) fuel system components
ISO 19723-2, Road vehicles — Liquefied natural gas (LNG) fuel systems — Test methods
ISO 20653, Road vehicles — Degrees of protection (IP code) — Protection of electrical equipment against foreign
objects, water and access
IEC 60529, Degrees of protection provided by enclosures (IP Code)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
liquefied natural gas
LNG
cryogenic liquid, produced by reducing the temperature of natural gas to about −162 °C at atmospheric
pressure
3.2
pressure relief valve
device that prevents a pre-determined upstream pressure being exceeded
3.3
working pressure
maximum pressure to which a component is designed to be subjected to and which is the basis for
determining the strength of the component under consideration
Note 1 to entry: For a liquefied natural gas (LNG) (3.1) tank, the primary relief valve setting prevents tank pressures
from exceeding the tank’s working pressure.
3.4
service pressure
intended settled pressure of the tank in use as declared by the manufacturer
3.5
check valve
automatic valve (3.11) which allows gas or liquid to flow in only one direction
3.6
fuel pump
device to establish the supply of liquefied natural gas (LNG) (3.1) to the engine by increasing the pressure of
the fluid (liquid or vapour)
3.7
category M
power-driven vehicles having at least four wheels and used for the carriage of passengers

3.7.1
M1
vehicles used for the carriage of passengers and comprising no more than eight seats in
addition to the driver's seat
3.7.2
M2
vehicles used for the carriage of passengers and comprising more than eight seats in
addition to the driver's seat and having a maximum authorized mass not exceeding 5 000 kg
3.7.3
M3
vehicles used for the carriage of passengers and comprising more than eight seats in
addition to the driver's seat and having a maximum authorized mass exceeding 5 000 kg
3.8
category N
power-driven vehicles having at least four wheels and used for the carriage of goods
3.8.1
N1
vehicles used for the carriage of goods and having a maximum authorized mass not
exceeding 3 500 kg
3.8.2
N2
vehicles used for the carriage of goods and having a maximum authorized mass exceeding
3 500 kg but not exceeding 12 000 kg
3.8.3
N3
vehicles used for the carriage of goods and having a maximum authorized mass exceeding
12 000 kg
3.9
protected position
area where components are installed to be protected against impact or collision
3.10
excess flow limiting device
device that automatically shuts off, or limits, the gas or liquid flow when the flow exceeds a certain defined
criterion
3.11
automatic valve
valve or combination of valves that is/are not operated manually
Note 1 to entry: A manual valve is a valve which is operated manually.
3.12
gas-tight housing
device which vents gas leakage to outside the vehicle including the gas ventilation hose, the clear opening of
which is at least 450 mm
3.13
electronic control unit
ECU
device which controls the liquefied natural gas (LNG) (3.1) demand of the engine and establishes the cut-off of
the automatic valve (3.11) in case of a broken fuel supply pipe or in case of stalling of the engine, or during a crash

4 Requirements
4.1 Design
4.1.1 General
The LNG on-board fuel system components shall conform to the ISO 12614 series and ISO 12617, as applicable.
The LNG fuel system components, specified in ISO 12614-1, shall be designed on the basis of the service
pressure. For bi-fuel vehicles, provisions shall be made to avoid accelerated deterioration of the non-LNG
fuel system as a result of sustained operation on natural gas. Such measures shall be as recommended by the
original vehicle manufacturer (e.g. fuel hoses).
All fuel system components shall fulfil the following conditions:
a) they shall withstand the environmental temperatures and other environmental conditions safely during
their operational life;
b) they shall be located with full regard for anticipated damage while the vehicle is being used safely. Such
damage can be caused by the vehicle itself, by extraneous factors such as heat, road debris, automotive
chemical splash (brake liquid, oil, petrol, cooling liquid, etc.), rust, etc.;
c) they shall not exceed the outermost, highest or lowest parts of the vehicles without further protection;
d) they shall be fitted so as not to affect ground clearance, approach angle, ramp (break-over) angle or
departure angles as defined by the vehicle manufacturer;
e) they shall be located so that they will not suffer corrosion damage by accumulation of water or cargo
chemicals;
f) they shall ensure the proper electrical conductivity throughout the fuel system in order to avoid
electrostatic charges. This provision does not apply to gas-tight housings and ventilation hoses;
g) all connections shall be made in locations where access is possible for inspection.
The system shall be so designed that galvanic corrosion is minimized.
The system shall be so designed that using a grounding point protects it against electrical discharge.
The following tank components, when used, shall be placed in a safe place or suitably protected against
damage, such as that arising from moving vehicle components, collision and grit or that due to loading or
unloading of the vehicle or the shifting of those loads:
— manual valve(s);
— pressure relief valve(s) (PRV);
— automatic shut-off valve;
— excess flow limiting device;
— fill check valve.
The manual valves shall be easily accessible.
The LNG on-board fuel system shall include:
— LNG tank(s) or vessel(s);
— LNG heat exchanger/vaporizer;
— pressure relief valve(s);
— venting system;
— LNG receptacle;
— LNG excess flow limiting device;
— LNG manual valve or means to isolate the tank;
— LNG/CNG automatic valve or means to isolate the tank;
— LNG fuel line;
— LNG couplings;
— LNG check valve or non-return valve;
— LNG fuel indicator or pressure indicator;
— electronic control unit (ECU) (no separate ECU is necessary – it can be integrated);
— natural gas detector or gas-tight housing for category M vehicles.
The LNG on-board fuel system may include:
— LNG pressure control regulator;
— LNG pressure and/or tempera
...