ISO 15501-1:2016

Road vehicles - Compressed natural gas (CNG) fuel systems - Part 1: Safety requirements

ISO 15501-1:2016 specifies the minimum safety requirements applicable for the functionality of CNG on-board fuel systems intended for use on the types of motor vehicles defined in ISO 3833. This document is applicable to vehicles using compressed natural gas in accordance with ISO 15403, including mono-fuel, bi-fuel or dual-fuel applications, original-production and converted vehicles. All matters relating to the skills of installers and converters have been excluded from this document.

Véhicules routiers — Systèmes d'alimentation en gaz naturel comprimé (GNC) — Partie 1: Exigences de sécurité

General Information

• Status: Published
• Publication Date: 07-Nov-2016
• 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: 01-Dec-2025
• Completion Date: 13-Dec-2025

Relations

• Amended By: ISO 15501-1:2016/Amd 1:2021 - Road vehicles - Compressed natural gas (CNG) fuel systems - Part 1: Safety requirements - Amendment 1
• Effective Date: 25-Apr-2020
• Revised: ISO 15501-1:2025 - Road vehicles - Compressed natural gas (CNG) fuel systems - Part 1: Safety requirements
• Effective Date: 18-Feb-2023
• Revises: ISO 15501-1:2012 - Road vehicles - Compressed natural gas (CNG) fuel systems - Part 1: Safety requirements
• Effective Date: 05-Nov-2015

Overview

ISO 15501-1:2016 - Road vehicles - Compressed natural gas (CNG) fuel systems - Part 1: Safety requirements specifies minimum safety requirements for CNG on-board fuel systems used in road vehicles (as defined in ISO 3833). It applies to mono‑fuel, bi‑fuel and dual‑fuel vehicles, both original production and conversions, using compressed natural gas in accordance with ISO 15403. Installer competency and training are excluded. The document is based on a nominal service pressure of 20 MPa (200 bar).

Key topics and technical requirements

ISO 15501-1 sets out functional and safety design requirements for CNG fuel systems with emphasis on the following areas:

• System components and design

  • Requirement to comply with component standards such as ISO 11439 (cylinders) and ISO 15500 (CNG components).
  • Components must withstand environmental temperatures, mechanical damage, automotive fluids and UV exposure (for fibre‑reinforced cylinders).
  • Ensure electrical conductivity to avoid electrostatic charging (where applicable).

• Cylinder and valve arrangements

  • Mandatory automatic valve on every cylinder plus a manual isolating valve.
  • Pressure‑relief devices (PRDs) installed per cylinder and additional PRDs as required by ISO 11439.
  • Excess‑flow valves or functionally equivalent systems to limit abnormal gas flow.

• Refuelling connection

  • Receptacle must comply with ISO 14469, include a protective cap and permanent markings showing “CNG”, cylinder inspection date and service pressure.

• Pressure control and protection

  • Downstream components protected from regulator failure (pressure relief inside regulator or equivalent).
  • PRD configuration guidance: parallel or thermal relief devices recommended; series PRDs limited to type 1 steel cylinders.

• Installation, mounting and safety features

  • Cylinder mounting, heat protection (clearances or shields), venting, leakage control, minimization of ignition risk, and accessibility for inspection.
  • Automatic valves normally closed when deactivated; considerations for start‑stop systems noted.

Applications and who uses this standard

ISO 15501-1 is used by:

• Vehicle manufacturers (OEMs) designing factory CNG systems
• CNG system integrators and component suppliers (cylinders, regulators, PRDs, receptacles)
• Conversion companies producing aftermarket CNG conversions
• Test laboratories and homologation bodies for vehicle approval
• Fleet operators and safety engineers defining procurement, inspection and maintenance criteria

Practical uses include system design verification, safety audits, procurement specifications, homologation documentation and maintenance guidance.

Related standards

• ISO 11439 - High pressure cylinders for on‑board storage
• ISO 14469 - CNG refuelling connector
• ISO 15500 (all parts) - CNG fuel system components
• ISO 15501-2 - Test methods for CNG on‑board systems
• ISO 15403 - Natural gas fuel quality
• IEC 60079-10-1 - Explosive atmosphere classification
• ISO 3833, ISO 1176 - Vehicle definitions and terminology

Keywords: ISO 15501-1, CNG fuel systems, compressed natural gas, road vehicles, safety requirements, CNG on-board, PRD, refuelling receptacle, ISO 11439, ISO 15500.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 15501-1
Third edition
2016-11-15
Road vehicles — Compressed natural
gas (CNG) fuel systems —
Part 1:
Safety requirements
Véhicules routiers — Systèmes d’alimentation en gaz naturel
comprimé (GNC) —
Partie 1: Exigences de sécurité
Reference number
©
ISO 2016
© ISO 2016, Published in Switzerland
All rights reserved. Unless otherwise specified, 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
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
ii © ISO 2016 – 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 . 4
4.2 Refuelling connection . 5
4.2.1 General. 5
4.2.2 Receptacle location . 5
4.2.3 Receptacle mounting . 5
4.2.4 Minimum receptacle clearance . 6
4.3 Leakage control . 6
4.4 Mounting of the cylinders . 7
4.5 Heat protection . 8
4.6 Minimizing risk of gas ignition . 8
4.7 Venting system . 8
5 Instruction for use . 9
6 Marking . 9
Annex A (informative) Technical solutions to functional requirements .10
Annex B (informative) Compressed natural gas (CNG) on-board fuel systems .11
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
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 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.
The committee responsible for this document is ISO/TC 22, Road vehicles, Subcommittee SC 41, Specific
aspects for gaseous fuels.
This third edition cancels and replaces the second edition (ISO 15501-1:2012), which has been
technically revised.
A list of all parts in the ISO 15501 series can be found on the ISO website.
iv © ISO 2016 – All rights reserved

Introduction
For the purposes of this document, all fuel system components in contact with natural gas have been
considered suitable for natural gas as defined in ISO 15403.
When applying this document, it is to be understood that 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.
When necessary, technical solutions regarding functional requirements are given in Annex A.
This document refers to a service pressure of 20 MPa (200 bar).
5 2
NOTE 1 1 bar = 0,1 MPa = 10 Pa. 1 MPa = 1 N/mm .
NOTE 2 This document is based upon a service pressure for natural gas as fuel of 20 MPa (200 bar) settled at
15 °C. Other service pressures can be accommodated by adjusting the pressure by the appropriate factor (ratio).
For example, a 25 MPa (250 bar) service pressure system will require pressures to be multiplied by 1,25.
INTERNATIONAL STANDARD ISO 15501-1:2016(E)
Road vehicles — Compressed natural gas (CNG) fuel
systems —
Part 1:
Safety requirements
1 Scope
This document specifies the minimum safety requirements applicable for the functionality of CNG on-
board fuel systems intended for use on the types of motor vehicles defined in ISO 3833. This document
is applicable to vehicles using compressed natural gas in accordance with ISO 15403, including mono-
fuel, bi-fuel or dual-fuel applications, original-production and converted vehicles.
All matters relating to the skills of installers and converters have been excluded from 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 11439, Gas cylinders — High pressure cylinders for the on-board storage of natural gas as a fuel for
automotive vehicles
ISO 14469 (all parts), Road vehicles — Compressed natural gas (CNG) refuelling connector
ISO 15500 (all parts), Road vehicles — Compressed natural gas (CNG) fuel system components
ISO 15501-2, Road vehicles — Compressed natural gas (CNG) fuel systems — Part 2: Test methods
IEC 60079-10-1, Explosive atmospheres — Part 10-1: Classification of areas — Explosive gas atmospheres
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1176, ISO 15500-1 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
3.1
service pressure
settled pressure of 20 MPa (200 bar) at a uniform gas temperature of 15 °C
3.2
CNG on-board fuel system
compressed natural gas fuel system comprising cylinder, or cylinders according to ISO 11439, mounting,
one or more refuelling receptacles according to ISO 14469 (all parts), and the components described in
ISO 15500-3 to ISO 15500-20
3.3
main shut-off valve
automatic valve designed to isolate a high-pressure source
3.4
mono-fuel NGV
vehicle which operates on natural gas only
Note 1 to entry: Also known as “Dedicated Natural Gas Vehicle”.
Note 2 to entry: In Europe and in India the term Mono-Fuel also applies to a light duty NGV with max 15 l
gasoline tank.
3.5
bi-fuel NGV
vehicle that has two independent fuel systems (one of them for natural gas) and can run alternatively
on either fuel, but only on one at a time
Note 1 to entry: The term bi-fuel also applies to vehicles that run on both fuels simultaneously in limited amount
or duration.
3.6
dual-fuel NGV
vehicle that has two independent fuel systems (one of them for natural gas) and can run on both fuels
simultaneously
Note 1 to entry: Vehicle also may run on one fuel alone.
3.7 Vehicle mass
3.7.1
kerb mass
complete shipping mass of a vehicle fitted with all equipment necessary for normal operation plus the
mass of the following elements for M1, N1 and M2 having a maximum authorized mass not exceeding
3 500 kg:
— lubricants, coolant (if needed), washer fluid;
— fuel (tank filled to at least 90 % of the capacity specified by the manufacturer);
— other equipment if included as basic parts for the vehicle, such as spare wheels, wheel chocks, fire
extinguishers, spare parts and tool kit
Note 1 to entry: The definition of kerb mass may vary from country to country, but in this document it refers to
the definition contained in ISO 1176.
3.7.2
maximum authorized mass
kerb mass plus the maximum allowable payload
3.8 Vehicle categories
3.8.1
category M
power-driven vehicles having at least four wheels and used for the carriage of passengers
3.8.1.1
category M1
vehicles used for the carriage of passengers and comprising no more than eight seats in addition to the
driver’s seat
2 © ISO 2016 – All rights reserved

3.8.1.2
category 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
3.8.1.3
category 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
3.8.2
category N
power-driven vehicles having at least four wheels and used for the carriage of goods
3.8.2.1
category N1
vehicles used for the carriage of goods and having a maximum authorized mass not exceeding 3 500 kg
3.8.2.2
category N2
vehicles used for the carriage of goods and having a maximum authorized mass exceeding 3 500 kg but
not exceeding 12 000 kg
3.8.2.3
category N3
vehicles used for the carriage of goods and having a maximum authorized mass exceeding 12 000 kg
4 Requirements
4.1 Design
4.1.1 General
The CNG on-board fuel system components shall comply with ISO 11439, ISO 14469 (all parts) and
ISO 15500 (all parts), as applicable.
For bi-fuel vehicles, provision shall be made to avoid accelerated deterioration of the non-CNG 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 may be caused by the vehicle itself, by extraneous factors such as heat, road debris,
automotive fluids (brake liquid, oil, petrol, cooling liquid, etc.), or by rust, etc.
c) They shall be fitted so that they are not the outermost, highest or lowest parts of the vehicles;
otherwise they shall be protected.
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
the electrostatic charges. This provision does not apply to gas-tight housing and ventilation hose.
g) All connections shall be made in locations where access is possible for inspection.
The CNG system shall be installed in such a way that it has suitable protection against damage, such
as damage due to moving vehicle components, collision, grit or due to the loading or unloading of the
vehicle or the shifting of those loads.
The CNG system shall include automatic valves designed to close when the engine is not running on CNG,
and shall be able to be manually opened or closed in case of failure of the automatism (see Annex B).
The CNG on-board fuel system shall include the following:
— an automatic valve to be installed directly on every CNG cylinder with a manual valve rigidly fixed
to the CNG cylinder, which may be integrated into the automatic valve. The manual valve shall be
able to isolate the cylinder content from the automatic valve;
— a PRD installed on each cylinder, functionally independent from any other component;
— one or more additional PRD as applicable to the approval of the cylinder according to ISO 11439; and
— an excess flow valve inside every cylinder or a functionally equivalent system to control the gas
leakage in the event of an abnormal flow (see informative Annex A).
The automatic valve shall be closed when
— the vehicle is not operating on CNG, and
— the engine is not running.
The valve may remain open when the engine stops during the stop phase in start-stop systems where
th
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