ISO 12991:2012

Liquefied natural gas (LNG) - Tanks for on-board storage as a fuel for automotive vehicles

ISO 12991:2012 specifies the construction requirements for refillable fuel tanks for liquefied natural gas (LNG) used in vehicles as well as the testing methods required to ensure that a reasonable level of protection from loss of life and property resulting from fire and explosion is provided. It is applicable to fuel tanks intended to be permanently attached to land vehicles but can be used as a guide for other modes of transport.

Gaz naturel liquéfié (GNL) — Réservoirs pour le stockage à bord comme carburant pour véhicules automobiles

General Information

• Status: Published
• Publication Date: 12-Nov-2012
• ICS: 23.020.40 - Cryogenic vessels
• ICS: 75.200 - Petroleum products and natural gas handling equipment
• Technical Committee: ISO/TC 220 - Cryogenic vessels
• Drafting Committee: ISO/TC 220 - Cryogenic vessels
• Current Stage: 9093 - International Standard confirmed
• Start Date: 17-Oct-2024
• Completion Date: 13-Dec-2025

Overview

ISO 12991:2012 - "Liquefied natural gas (LNG) - Tanks for on‑board storage as a fuel for automotive vehicles" defines construction and testing requirements for refillable LNG fuel tanks permanently attached to land vehicles. The standard aims to ensure a reasonable level of protection against loss of life and property from fire, explosion and cryogenic hazards. It covers design, materials, insulation, manufacturing, type tests and routine inspection for on‑board cryogenic storage systems.

Key topics and technical requirements

• Scope and application: Applicable to fuel tanks permanently mounted on land vehicles; can be used as guidance for other transport modes.
• Terminology: Defines critical terms such as inner tank, outer jacket, MAWP (maximum allowable working pressure) and holding time (insulation performance metric).
• Mechanical requirements:
  • Inner and outer pressure design and test levels (inner tank designed for pressures related to MAWP; outer jacket to resist 1 bar outer pressure).
  • Dynamic loads and crash/acceleration criteria by vehicle category (e.g., M1/N1: 20 g forward, 8 g lateral; M2/N2: 10 g forward, 5 g lateral; M3/N3: 6.6 g forward, 5 g lateral).
• Thermal requirements:
  • Design temperature baseline 20 °C; ambient design range −40 °C to 85 °C (or 120 °C for engine compartments if applicable).
  • Components in contact with LNG must tolerate cryogenic temperatures (down to −196 °C).
• Materials:
  • Compatibility with LNG and common vehicle fluids (coolant, brake fluid, battery acid).
  • Cryogenic toughness per ISO 21028‑1 for low‑temperature materials; outer jackets typically austenitic stainless steel to preserve insulation integrity.
• Design and insulation:
  • Requirements for insulation performance measured by holding time, and design rules for fittings, supports and accessories (pressure relief valves, level gauges).
• Testing and inspection:
  • Type tests: burst pressure, holding time, maximum fuelling level and accessory tests.
  • Routine tests: pressure, leak checks, dimensional verification, destructive/non‑destructive weld tests, visual inspection.
• Marking and labelling: Mandatory markings for inner tank, outer jacket and temporary markings for first fuelling.

Applications and users

ISO 12991:2012 is essential for:

• Vehicle OEMs (cars, buses, trucks) integrating LNG fuel tanks
• Cryogenic tank designers and fabricators
• Component suppliers (pressure‑relief devices, valves, sensors)
• Testing laboratories, certification bodies and regulatory authorities
• Fleet operators converting vehicles to LNG

Practical uses include tank design validation, supplier specifications, type approval, factory inspection protocols and safety case documentation.

Related standards (if applicable)

• ISO 21028‑1 (toughness at cryogenic temperatures)
• ISO 21014 (insulation performance)
• ISO 21013‑3 (pressure‑relief accessory sizing)
• ISO 23208 (cleanliness for cryogenic service)
• ISO 12617 (fuelling system interface - under preparation)

Keywords: ISO 12991:2012, LNG tanks, on‑board storage, vehicle fuel tanks, cryogenic vessels, MAWP, holding time, pressure testing, insulation performance.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 12991
First edition
2012-11-15
Liquefied natural gas (LNG) — Tanks
for on-board storage as a fuel for
automotive vehicles
Gaz naturel liquéfié (GNL) — Réservoirs pour le stockage à bord
comme carburant pour véhicules automobiles
Reference number
©
ISO 2012
© ISO 2012
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any
means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the
address below or ISO’s member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO 2012 – 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 General requirements . 3
4.2 Mechanical stresses . 3
4.3 Thermal stresses . 4
4.4 Materials . 5
4.5 Design . 5
4.6 Insulation .
General requirements . 6
4.7 Accessories . 6
4.8 Manufacturing and assembly. 7
5 Type tests .7
5.1 Approval of new designs . 7
5.2 Inner tank burst pressure test . 8
5.3 Holding time test . 8
5.4 Maximum fuelling level test . 8
5.5 Accessory type tests . 8
6 Routine tests and inspection .8
6.1 General . 8
6.2 Pressure test . 9
6.3 Leak test . 9
6.4 Verification of the dimensions . 9
6.5 Destructive and non-destructive tests of welded joints . 9
6.6 Visual inspection .10
7 Marking and labelling .10
7.1 Marking method .10
7.2 Inner tank markings .10
7.3 Outer jacket markings . .10
7.4 Temporary markings for first fuelling .10
Annex A (normative) Fuel tank type tests .12
Annex B (normative) Accessory type tests .14
Bibliography .16
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International
Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies
casting a vote.
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.
ISO 12991 was prepared by Technical Committee ISO/TC 220, Cryogenic vessels.
iv © ISO 2012 – All rights reserved

Introduction
The fuel tanks described in this International Standard are intended to be used in conjunction with the
fuelling system interface described in ISO 12617, which is under preparation.
INTERNATIONAL STANDARD ISO 12991:2012(E)
Liquefied natural gas (LNG) — Tanks for on-board storage
as a fuel for automotive vehicles
1 Scope
This International Standard specifies the construction requirements for refillable fuel tanks for liquefied
natural gas (LNG) used in vehicles as well as the testing methods required to ensure that a reasonable
level of protection from loss of life and property resulting from fire and explosion is provided.
This International Standard is applicable to fuel tanks intended to be permanently attached to land
vehicles but can be used as a guide for other mode of transport.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 1176, Road vehicles — Masses — vocabulary and codes
ISO 1431-1, Rubber, vulcanized or thermoplastic — Resistance to ozone cracking — Part 1: Static and
dynamic strain testing
ISO 2768-1, General tolerances — Part 1: Tolerances for linear and angular dimensions without individual
tolerance indications
ISO 6957, Copper alloys — Ammonia test for stress corrosion resistance
ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests
ISO 21013-3, Cryogenic vessels — Pressure-relief accessories for cryogenic service — Part 3: Sizing and
capacity determination
ISO 21014, Cryogenic vessels — Cryogenic insulation performance
ISO 21028-1, Cryogenic vessels — Toughness requirements for materials at cryogenic temperature — Part 1:
Temperatures below -80 degrees C
ISO 21029-1:2004, Cryogenic vessels — Transportable vacuum insulated vessels of not more than 1 000
litres volume — Part 1: Design, fabrication, inspection and tests
ISO 23208, Cryogenic vessels — Cleanliness for cryogenic service
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
accessory
device fixed directly to the inner tank or outer jacket of a fuel tank such as a pressure relief valve, shut-
off valve, non-return valve or level gauge
3.2
boil-off management system
system that controls the boil-off of gas under normal conditions
3.3
burst pressure
pressure that causes the rupture of a pressure vessel subjected to a constant increase of pressure during
a destructive test
3.4
design temperature
temperature of the inner tank, the outer jacket and all other accessories to which fabrication drawings,
inspections and physical measurements such as volume are referred
3.5
fuel tank
vessel used for the storage of cryogenic liquefied natural gas (LNG) fuel
3.7
impermissible fault range
pressure range within which an unwanted event is to be expected
3.8
inner tank
part of the fuel tank that contains liquefied natural gas (LNG)
3.9
level gauge
device that measures the level of liquefied natural gas (LNG) in the fuel tank
3.10
maximum allowable working pressure
MAWP
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
3.11
normal operating range
range planned for the process values
NOTE In the case of inner tanks, the normal operating range of the inner tank pressure is from 0 bar to the
set pressure of the primary pressure relief valve, which is lower than or equal to the maximum allowable working
pressure (MAWP) of the inner tank.
3.12
outer jacket
part of the fuel tank that encases the inner tank(s) and its insulation system
3.13
outer pressure
pressure acting on the outside of the inner tank or outer jacket
3.14
permissible fault range
range between the normal operating range and the impermissible fault range
3.15
pressure
pressure for which the value is equal to the algebraic difference between the absolute pressure and the
atmospheric pressure
NOTE This is also known as gauge pressure.
2 © ISO 2012 – All rights reserved

3.16
holding time
time of the pressure increase in the inner tank measured from a starting pressure of 0 bar at the
corresponding boiling point of liquefied natural gas (LNG) (−164° C) up to the maximum allowable
working pressure (MAWP) of the inner tank
NOTE The holding time is a measure of the insulation performance of the fuel tank.
3.17
batch inner tank heads
number (no more than 100) of tank heads produced by the same manufacturer, from the same material,
and having the same dimensions
3.18
maximum mass
a maximum authorized total mass, as defined in ISO 1176
4 Requirements
4.1 General requirements
The fuel tank and its accessories shall function in a correct and safe way. It shall withstand and
remain gas tight when subjected to the mechanical, thermal and chemical stresses specified in this
International Standard.
4.2 Mechanical stresses
4.2.1 Inner/outer pressure
4.2.1.1 Inner tank
The inner tank shall be designed to resist the following inner test pressure:
p =+13,(MAWP 1)
test
pM=+13,(MAWP 01,)()Pa 
 test 
where
p is the test pressure, expressed in bar
test
MAWP is the maximum allowable working pressure of the inner tank, expressed in bar. The
inner tank and its accessories shall be designed to resist an outer pressure of 1 bar.
4.2.1.2 Outer jacket
The outer jacket shall be designed to resist an outer pressure of 1 bar (see 4.5.2).
4.2.2 Accelerations
4.2.2.1 General
The fuel tank and its accessories shall be mounted and protected so that the accelerations shown in
Table 1 can be absorbed without structural damage to the fuel tank and its accessories. No uncontrolled
release of LNG is permitted.
Table 1 — Accelerations
Vehicle categories Accelerations
Vehicles of categories M1 and N1 20 g in the direction of travel
8 g horizontally perpendicular to the direction of travel
Vehicles of categories M2 and N2 10 g in the direction of travel
5 g horizontally perpendicular to the direction of travel
Vehicles of categories M3 and N3 6,6g in the direction of travel
5 g horizontally perpendicular to the direction of travel
The vehicle categories include the following:
— Category M1: Vehicles used for the transportation of passengers and comprising not more than eight seats
in addition to the driver’s seat.
— Category M2: Vehicles used for the transportation of passengers, comprising more than eight seats in addi-
tion to the driver’s seat, and having a maximum mass that does not exceed 5 000 kg.
— Category M3: Vehicles used for the transportation of passengers, comprising more than eight seats in addi-
tion to the driver’s seat, and having a maximum mass exceeding 5 000 kg.
— Category N1: Vehicles used for the transportation of goods and having a maximum mass that does not
exceed 3 500 kg
— Category N2: Vehicles used for the transportation of goods and having a maximum mass exceeding
3 500 kg, but not exceeding 12 000 kg.
— Category N3: Vehicles used for the transportation of goods and having a maximum mass exceeding
12 000 kg.
4.2.2.2 Inner and outer support
When exposed to the accelerations described in Table 1, the stress in the support elements shall not
exceed the minimum ultimate tensile strength of the material (R , calculated according with the linear
m
stress model).
The allowable stress in the support elements may not have to be calculated if it can be demonstrated
that the fuel tank supports the accelerations given in Table 1 without any structural damage to the inner
tank or its supports.
Acceptable calculation methods include
— finite element,
— finite difference,
— boundary element, and
— established calculation method.
In these calculations static loads shall be substituted for static plus dynamic loads.
4.3 Thermal stresses
4.3.1 Design temperature
The design temperature of the inner tank, the outer jacket and the accessories shall be 20°C. In addition,
the inner tank, the outer jacket and the accessories shall be designed to withstand a temperature range
from the lowest to the highest possible operating temperatures that will be encountered in service.
4 © ISO 2012 – All rights reserved

4.3.2 Ambient temperature
The fuel tank shall be designed to withstand ambient temperatures ranging from −40°C to 85°C. If the
fuel tank is to be installed in areas of internal heat sources such as the internal combustion engine
compartment of a vehicle, the fuel tank shall be designed for an ambient temperature of 120°C, or a
lower value if substantiated by calculations.
4.3.3 Operating temperature
The thermal stresses produced by the operating conditions shall be considered. The inner vessel and the
other components that may be in contact with LNG shall be designed to operate at –196 °C.
4.4 Materials
The materials of the fuel tank and its accessories shall be compatible, as applicable, with
a) LNG, and
b) other media and fluids found in a vehicle environment, such as coolants, brake fluid and battery acid.
Materials used at low temperatures shall meet the toughness requirements of ISO 21028-1. For non-
metallic materials, low temperature suitability shall be validated by an experimental method, taking
into account the service conditions.
The materials used for the outer jacket shall ensure the integrity of the insulation system and shall be
made of austenitic stainless steel
For the inner vessel if materials other than austenitic stainless steels are used it shall be ensured that
the materials will resist all the in service fatigue load
A corrosion allowance does not need to be added for the inner tank. A corrosion allowance does not need
to be added on other surfaces, if they are protected against corrosion.
For welded vessels, welds shall have properties equivalent to those specified for the parent material for
all temperatures that the material may encounter.
4.5 Design
4.5.1 Design validation
Each type of vessel shall be validated in accordance with the design options specified in 10.1 of
ISO 21029-1:2004 and in accordance with one of options given in 4.5.1.1 and 4.5.1.2.
4.5.1.1 Validation by calculation
This option requires calculation in accordance with 10.3 of ISO 21029-1:2004. Additional calculations
are required to validate the design for accelerations in 4.2.2.
4.5.1.2 Validation by experiment method
This option requires validation in accordance with 10.4 of ISO 21029-1:2004 with the following
modification; drop tests shall include a 9 m drop test of the fuel tank on the most critical area of the tank
(other than the piping end) and a 3 m drop test on the piping end.
4.5.2 Inner tank and outer jacket
For the design of inner and outer tank the only acceptable options are
— the design is validated as per option 10.1.3 of ISO 21029-1:2004, especially the requirements of Table 3;
— the design of the inner tank and the outer jacket shall meet all the design rules specified in 10.3
of ISO 21029-1:2004. In this case if the minimum thickness is less than per 10.1.3 Table 3 an
experimental validation shall be performed (see Annex A).
Unless indicated otherwise, the general tolerances of ISO 2768-1 shall apply.
4.6 Insulation – General requirements
The insulation system installed on a fuel tank shall be evaluated in accordance with ISO 21014 and stated.
4.7 Accessories
4.7.1 General requirements
Accessories shall have a minimum working pressure equal to the MAWP of the inner tank (see ISO 21011
and ISO 21013-1.
4.7.2 Pressure relief valves for the inner tank
The inner tank shall be protected by a pressure relief valve set to open at MAWP with a full flow at no
more than 110 % of MAWP.
The inner tank shall also be protected by a second pressure relief valve that shall not operate below
115 % of the MAWP and that limit the pressure inside the inner tank to no more than the test pressure.
The inner tank pressure relief valves shall, after discharge, close at a pressure higher than 90 % of the
set pressure of the pressure relief valve and remain closed at all lower pressures.
The sizing of each inner tank pressure relief valves shall be don
...