ISO 19723-1:2018

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 liquefied natural gas in accordance with the ISO 15403 series. It is not 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 on the components in the "LNG system" meaning 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 CNG components covered by ISO 15501.

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

General Information

• Status: Published
• Publication Date: 16-Jul-2018
• 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/WG 7 - General safety requirements for gaseous fuelled vehicles and terminology
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 12-Dec-2025
• Completion Date: 27-Dec-2025

Relations

• Amended By: ISO 19723-1:2018/Amd 1:2021 - Road vehicles - Liquefied natural gas (LNG) fuel systems - Part 1: Safety requirements - Amendment 1
• Effective Date: 23-Apr-2020
• Revised: ISO 19723-1:2025 - Road vehicles - Liquefied natural gas (LNG) fuel systems - Part 1: Safety requirements
• Effective Date: 18-Feb-2023

Overview

ISO 19723-1:2018 - Road vehicles - Liquefied natural gas (LNG) fuel systems - Part 1: Safety requirements - specifies minimum safety requirements for LNG on-board fuel systems fitted to motor vehicles defined in ISO 3833. The standard covers systems used in mono‑fuel, bi‑fuel and dual‑fuel applications that use LNG in accordance with the ISO 15403 series. It applies to the LNG system components up to and including the vaporizer and excludes downstream CNG components (covered by ISO 15501). The document also excludes matters relating to installer/converter skills and - as stated in the scope - is not applicable to original‑production and converted vehicles.

Key topics and technical requirements

• Scope of coverage: Components in the LNG system (tanks, valves, flexible fuel lines, fittings, vaporizer, etc.) and connecting parts fitted on motor vehicles.
• Design requirements:
  • Components must withstand operational and environmental temperatures (LNG ≈ −162 °C at atmospheric pressure).
  • Location, mounting and protection against impact, corrosion, road debris, heat and chemical splash.
  • Minimize galvanic corrosion and ensure proper electrical conductivity and grounding to avoid electrostatic charge.
• Component-specific rules:
  • Tanks and pressure systems designed per service pressure and ISO 12614 series.
  • Pressure relief valves (PRV): primary PRV must vent via a pipe‑away to a high level; secondary PRV outlets must be protected and not share the same pipe‑away.
  • Excess flow limiting devices: allowed inside or directly on the tank and may be integrated with LNG/CNG automatic valves.
  • Automatic valves: must be normally closed when deactivated and only open when LNG operation is selected and the engine is cranking or running.
  • Refuelling receptacle: must comply with ISO 12617, include a cap and nearby marking (fuel type and service pressure).
• Safety systems: venting system, gas‑tight housings or natural gas detectors for certain vehicle categories, isolation valves, check valves, and provisions up to the vaporizer.
• Documentation and marking: instructions for use and component marking requirements.

Practical applications - who uses this standard

• Vehicle manufacturers (OEMs) designing LNG‑powered vehicles (M and N categories per ISO 3833).
• Component and system suppliers (tanks, PRVs, receptacles, valves, vaporizers).
• System integrators and safety engineers for vehicle-level safety design and routing of LNG lines.
• Test laboratories and certification bodies conducting compliance and type‑approval testing.
• Fleet operators and specifiers seeking safe LNG vehicle procurement and maintenance guidance.

Related standards

• ISO 12614 (LNG fuel system components)
• ISO 12617 (LNG refuelling connector)
• ISO 12991 (LNG on‑board tanks)
• ISO 19723‑2 (refuelling connector test methods)
• ISO 15500 (CNG fuel system components)
• Relevant IEC standards for explosive atmospheres (e.g., IEC 60079 series)

ISO 19723-1:2018 is essential reading for anyone responsible for the safe design, manufacture or certification of on‑board LNG fuel systems for road vehicles.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 19723-1
First edition
2018-07
Road vehicles — Liquefied natural 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 2018
© ISO 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
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Phone: [phone]
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Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 3
4.1 Design . 3
4.1.1 General. 3
4.1.2 Components . 5
4.2 Refuelling . 6
4.2.1 General. 6
4.2.2 Receptacle location . 7
4.2.3 Receptacle mounting . 7
4.2.4 Minimum receptacle clearance . 7
4.3 Leakage control . 7
4.4 Mounting of the tank(s). 8
4.5 Heat protection . 8
4.6 Minimizing risk of gas ignition . 9
4.7 Venting system . 9
5 Instruction for use .10
6 Marking .10
Annex A (informative) Technical solutions to functional requirements .11
Annex B (informative) Liquefied natural gas (LNG on board fuel systems) .12
Bibliography .13
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
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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
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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 ISO/TC 22, Road vehicles, Subcommittee SC 41, Specific aspects for
gaseous fuels.
A list of all the parts in the ISO 19723 series, can be found on the ISO website.
iv © ISO 2018 – All rights reserved

Introduction
For the purposes of this document, all fuel system components in contact with liquid natural gas have
been considered suitable for natural gas as defined in the ISO 15403 series.
When applying this document, it is understood that a safety device to prevent overfilling the vehicle's
fuel system is part of the refueling station. The pressure gauge has not been considered as a safety
component.
When necessary, technical solutions regarding functional requirements are given in Annex A.
INTERNATIONAL STANDARD ISO 19723-1:2018(E)
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 liquefied
natural gas in accordance with the ISO 15403 series. It is not 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 on the components in the "LNG system" meaning 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 CNG
components covered by ISO 15501.
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 12614 (all parts), Road vehicles — Liquefied natural gas (LNG) fuel system components
ISO 12617, 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 19723-2, Road vehicles — Liquefied natural gas (LNG) refueling connector — Test methods
ISO 15500 (all parts), Road vehicles — Compressed natural gas (CNG) fuel system components
IEC 60079-1, Explosive atmospheres — Part 1: Equipment protection by flameproof enclosures “d”
IEC 60079-10, Electrical apparatus for explosive gas atmospheres — Part 10: Classification of
hazardous areas
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 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/
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 that an LNG refuelling connector can be expected to withstand in actual service,
equal to the setting of the primary relief valve of the vehicle on board fuel tank
Note 1 to entry: An LNG refuelling connector is understood to be a joined assembly of LNG refuelling nozzle and
receptacle.
Note 2 to entry: An LNG refuelling nozzle is understood to be a device which permits quick connection and
disconnection of fuel supply hose to the LNG receptacle in a safe manner.
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 LNG 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
2 © ISO 2018 – All rights reserved

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 criteria
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
4 Requirements
4.1 Design
4.1.1 General
The LNG on-board fuel system components shall comply with ISO 12614 (all parts) and ISO 12617, as
applicable.
The pressure system, specified in ISO 12614-1, shall be designed on the basis of the service pressure.
For bi-fuel vehicles, provision 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 fulfill 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.), or by 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 assure the proper electrical conductivity throughout the fuel system in order to avoid
electrostatic charges. This provision does not apply to gas-tight housing 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;
— LNG pressure relief valve(s);
— LNG 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 pressure indicator or fuel indicator;
...