ISO 18172-2:2007

Gas cylinders - Refillable welded stainless steel cylinders - Part 2: Test pressure greater than 6 MPa

ISO 18172-2:2007 specifies minimum requirements concerning material, design, construction and workmanship, manufacturing processes and testing of refillable transportable welded stainless steel gas cylinders, of water capacities from 0,5 l up to and including 150 l, for compressed and liquefied gases. ISO 18172-2:2007 is applicable only to cylinders manufactured from stainless steels with a maximum tensile strength of less than 1 100 MPa and a test pressure above 6 MPa (60 bar).

Bouteilles à gaz — Bouteilles soudées en acier inoxydable rechargeables — Partie 2: Pression d'épreuve supérieure à 6 MPa

General Information

• Status: Published
• Publication Date: 20-Feb-2007
• ICS: 23.020.30 - Pressure vessels, gas cylinders
• ICS: 23.020.35 - Gas cylinders
• Technical Committee: ISO/TC 58/SC 3 - Cylinder design
• Drafting Committee: ISO/TC 58/SC 3 - Cylinder design
• Current Stage: 9093 - International Standard confirmed
• Start Date: 09-Jun-2021
• Completion Date: 13-Dec-2025

Overview

ISO 18172-2:2007 - "Gas cylinders - Refillable welded stainless steel cylinders - Part 2: Test pressure greater than 6 MPa" specifies minimum requirements for the design, materials, manufacture and testing of refillable, transportable welded stainless steel gas cylinders. It applies to cylinders with water capacities from 0.5 L to 150 L, manufactured from stainless steels with maximum tensile strength below 1 100 MPa, and having a test pressure greater than 6 MPa (60 bar). The standard supports safe use for compressed and liquefied gases and is written for regulatory conformity and transport safety.

Key topics and technical requirements

• Materials and heat treatment: requirements for stainless steel categories, solution annealing, cryoforming and heat-treatment controls.
• Design: calculation of cylindrical shell wall thickness, design of convex or other end shapes, bungs, openings and minimum permitted thicknesses; use of a design stress factor (F) and related symbols.
• Construction and workmanship: welding qualifications, cryoforming and cold-forming procedures, welded joints, valve protection, neck threads and visual examination.
• Testing regime:
  • New design tests and certification requirements for prototype designs.
  • Batch tests: tensile, bend, impact, hardness, macroscopic examination of weld cross-sections, plus acceptance criteria.
  • Tests on every cylinder: hydraulic/pressure testing, leakage and hardness checks.
• Non-destructive testing: radiographic and visual testing of welds (annexes and references specify techniques and acceptance).
• Marking and certification: mandated markings and documentation for conformity with transport regulations.

Practical applications

• Ensures safety and reliability of refillable welded stainless steel cylinders used in industry, medical gas supply, specialty gases, and laboratory settings.
• Guides manufacturers through material selection, welding procedures, production controls and quality assurance for cylinders intended for high-pressure service (>60 bar).
• Provides the technical basis for type approval, batch certification and conformity assessment, and for demonstrating compliance with transport of dangerous goods rules.

Who uses this standard

• Cylinder manufacturers and designers
• Welding and quality-control engineers
• Testing and inspection laboratories
• Regulatory authorities, certification bodies and transport safety organizations
• Purchasers and specifiers of high-pressure gas cylinders

Related standards

Commonly used with other ISO documents referenced in the standard, e.g.:

• ISO 11114-1 (compatibility of materials with gas contents)
• ISO 13769 (stamp marking)
• ISO 17636 / ISO 17637 (radiographic and visual testing of welds)
• ISO 6892 (tensile testing)

Keywords: ISO 18172-2, gas cylinders, refillable welded stainless steel cylinders, test pressure greater than 6 MPa, high-pressure cylinders, cylinder design, welding, batch tests, pressure testing.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 18172-2
First edition
2007-03-01
Gas cylinders — Refillable welded
stainless steel cylinders —
Part 2:
Test pressure greater than 6 MPa
Bouteilles à gaz — Bouteilles soudées en acier inoxydable
rechargeables —
Partie 2: Pression d'épreuve supérieure à 6 MPa

Reference number
©
ISO 2007
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2007
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 2007 – All rights reserved

Contents Page
Foreword. v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols. 2
3.1 Terms and definitions. 2
3.2 Symbols . 3
4 Materials and heat treatment . 4
4.1 General. 4
4.2 Categories . 4
4.3 Heat treatment. 5
4.4 Test requirements. 5
5 Design . 5
5.1 General requirements. 5
5.2 Calculation of cylindrical shell wall thickness . 5
5.3 Design of convex ends. 6
5.4 Minimum wall thickness. 9
5.5 Ends of other shapes . 9
5.6 Design of openings. 9
5.7 Bungs. 9
6 Construction and workmanship. 9
6.1 General. 9
6.2 Welding . 10
6.3 Cryoforming procedure. 10
6.4 Welded joints of pressure-containing parts . 11
6.5 Non-pressure-containing attachments.11
6.6 Valve protection. 11
6.7 Neck threads . 11
6.8 Visual examination . 11
7 New design tests. 12
7.1 General requirements. 12
7.2 Verifications and tests. 13
7.3 Descriptions of tests . 14
7.4 Design testing certificate . 15
8 Batch tests. 15
8.1 General. 15
8.2 Information . 15
8.3 Tensile test . 15
8.4 Bend test. 18
8.5 Impact test . 20
8.6 Macroscopic examination of weld cross-sections. 21
8.7 Checks and verifications . 21
9 Tests on every cylinder. 22
9.1 Pressure test . 22
9.2 Hardness test . 22
9.3 Leakage test . 22
10 Failure to meet test requirements. 22
11 Markings. 22
12 Certificate. 23
Annex A (normative) Radiographic examination of welds. 24
Annex B (normative) Description and evaluation of manufacturing defects and conditions
for rejection of welded stainless steel gas cylinders at time of visual inspection . 26
Annex C (informative) Examples of design and batch testing certificates. 28
Bibliography . 33

iv © ISO 2007 – All rights reserved

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 18172-2 was prepared by Technical Committee ISO/TC 58, Gas cylinders, Subcommittee SC 3, Cylinder
design.
ISO 18172 consists of the following parts, under the general title Gas cylinders — Refillable welded stainless
steel cylinders:
⎯ Part 1: Test pressure 6 MPa and below
⎯ Part 2: Test pressure greater than 6 MPa

Introduction
The purpose of this part of ISO 18172 is to provide a specification for the design, manufacture and testing of
refillable, transportable, welded stainless steel gas cylinders with a test pressure above 6 MPa (60 bar).
The specifications given are based on knowledge of, and experience with, materials, design requirements,
manufacturing processes and control during manufacture of cylinders in common use.
ISO 18172 has been prepared to address the general requirements in Section 6.2.1 of the UN model
regulations for the transportation of dangerous goods ST/SG/AC.10/1/Rev.13. It is intended to be used under
a variety of regulatory regimes, but has been written so that it is suitable for use with the conformity
assessment system in paragraph 6.2.2.5 of the above-mentioned model regulations.

vi © ISO 2007 – All rights reserved

INTERNATIONAL STANDARD ISO 18172-2:2007(E)

Gas cylinders — Refillable welded stainless steel cylinders —
Part 2:
Test pressure greater than 6 MPa
1 Scope
This part of ISO 18172 specifies minimum requirements concerning material, design, construction and
workmanship, manufacturing processes and testing of refillable transportable welded stainless steel gas
cylinders, of water capacities from 0,5 l up to and including 150 l, for compressed and liquefied gases. This
part of ISO 18172 is applicable only to cylinders manufactured from stainless steels with a maximum tensile
strength of less than 1 100 MPa and a test pressure above 6 MPa (60 bar).
2 Normative references
The following referenced documents are indispensable for the application 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 2504:1973, Radiography of welds and viewing conditions for films — Utilization of recommended patterns
of image quality indicators (I.Q.I.)
ISO 3651-2, Determination of resistance to intergranular corrosion of stainless steels — Part 2: Ferritic,
austenitic and ferritic-austenitic (duplex) stainless steels — Corrosion test in media containing sulfuric acid
ISO 5817, Welding — Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding
excluded) — Quality levels for imperfections
ISO 6506-1, Metallic materials — Brinell hardness test — Part 1: Test method
ISO 6892, Metallic materials — Tensile testing at ambient temperature
ISO 7438, Metallic materials — Bend test
ISO 9328-7:2004, Steel flat products for pressure purposes — Technical delivery conditions — Part 7:
Stainless steels
ISO 9606-1, Approval testing of welders — Fusion welding — Part 1: Steels
ISO 9956-1, Specification and approval of welding procedures for metallic materials — Part 1: General rules
for fusion welding
ISO 9956-3, Specification and approval of welding procedures for metallic materials — Part 3: Welding
procedure tests for arc welding of steels
ISO 11114-1, Transportable gas cylinders — Compatibility of cylinder and valve materials with gas
contents — Part 1: Metallic materials
ISO 11117, Gas cylinders — Valve protection caps and valve guards — Design, construction and tests
ISO 13769, Gas cylinders — Stamp marking
ISO 14556, Steel — Charpy V-notch pendulum impact test — Instrumented test method
ISO 14732, Welding personnel — Approval testing of welding operators for fusion welding and of resistance
weld setters for fully mechanized and automatic welding of metallic materials
ISO 17636, Non-destructive testing of welds — Radiographic testing of fusion-welded joints
ISO 17637, Non-destructive testing of welds — Visual testing of fusion-welded joints
ISO 20807, Non-destructive testing — Qualification of personnel for limited application of non-destructive
testing
3 Terms, definitions and symbols
For the purposes of this document, the following terms, definitions and symbols apply.
3.1 Terms and definitions
3.1.1
yield stress
value corresponding to 0,2 % proof stress (R ) or, for austenitic steels in the solution annealed condition,
p0,2
1 % proof stress (R )
p1,0
3.1.2
solution annealing
softening heat treatment for austenitic steels in which a cylinder is heated to a uniform temperature above the
solid solution temperature, followed by rapid cooling
3.1.3
cryoforming
process where the cylinder is subjected to a controlled low temperature deformation treatment that results in a
permanent increase in strength
3.1.4
cold working
plastic deformation treatment given to sheet material at ambient temperature, with the aim of permanently
increasing the material strength
3.1.5
cold forming
final deformation treatment at ambient temperature given to the prefabricated cylinder, known as the preform,
which results in a permanent increase in the material strength
3.1.6
batch
quantity of cylinders made consecutively by the same manufacturer, using the same manufacturing
techniques, to the same design, size and material, from the same cast on the same type of welding machines
and welding procedures
3.1.7
design stress factor
F
ratio of equivalent wall stress at test pressure (p ) to guaranteed minimum yield stress (R )
h e
2 © ISO 2007 – All rights reserved

3.2 Symbols
a calculated minimum thickness, in mm, of the cylindrical shell
a′ guaranteed minimum thickness, in mm, of the cylindrical shell, including any corrosion allowance
(see 7.1.1)
a calculated value of a used in the calculation of b (see 5.3.2)
a mean wall thickness, in mm, of the cylindrical shell of the cylinder preform

m
A percentage elongation after fracture
b calculated minimum thickness, in mm, of the cylinder end
b′ guaranteed minimum thickness, in mm, of the cylinder end (see 7.1.1)
C shape factor of dished ends
D outside diameter, in mm, of the cylinder (see Figure 1)
D diameter of former, in mm (see 8.4.3 and Figure 10)
f
D mean diameter of the cylindrical shell of a cylinder preform, in mm
m
F design stress factor (see 3.1.7)
f cryoforming factor established by the manufacturer for each batch of cylinders
c
h height, in mm, of the cylindrical part of the end (see Figure 1)
H outside height, in mm, of the domed part of the end (see Figure 1)
J stress reduction factor
L length, in mm, of the cylinder
n ratio of diameter of bend test former (D ) to the thickness of the test piece (t)
f
1)
p measured burst pressure, in MPa (bar) , above atmospheric pressure, in the burst test
b
p cryoforming or cold forming pressure in MPa (bar), above atmospheric pressure
c
p hydraulic test pressure, in MPa (bar), above atmospheric pressure
h
p observed yield pressure, in MPa (bar), above atmospheric pressure
y
r inside radius of the knuckle end, in mm (see Figure 1)
R inside radius of the dished end, in mm (see Figure 1)
R yield stress, in MPa, as defined in 3.1.1 and used for design calculation
e
R value of the actual yield stress, in MPa, determined by the tensile test
ea
1) 0,1 MPa = 10 Pa = 1 bar
R minimum value of 0,2 % proof stress, in MPa, guaranteed by the cylinder manufacturer for the finished

p0,2
cylinder, in accordance with ISO 6892 (see Note)
R minimum value of 1,0 % proof stress, in MPa, guaranteed by the cylinder manufacturer for the finished
p1,0
cylinder, in accordance with ISO 6892 (see Note)
R minimum value of tensile strength, in MPa, guaranteed by the cylinder manufacturer for the finished

g
cylinder
R actual value of tensile strength, in MPa, determined by tensile test (see 8.3)
m
t actual thickness of the test specimen, in mm (see Figure 7)
NOTE For cryoformed and cold formed cylinders, the minimum value guaranteed by the manufacturer refers only to
the cylindrical part of the finished cylinder.
4 Materials and heat treatment
4.1 General
4.1.1 Materials of shells and end pressings shall be stainless steels in a condition suitable for pressing or
drawing and welding, and shall conform to ISO 9328-7.
4.1.2 Grades of steel used for the cylinder manufacture shall be compatible with the intended gas service
(e.g. corrosive gases, embrittling gases), in accordance with ISO 11114-1. Due to the scope of this part of
ISO 18172, which permits tensile strengths up to 1 100 MPa, particular care shall be taken when designing
hydrogen gas cylinders.
4.1.3 There is a risk of sensitization to intergranular corrosion resulting from the hot processing of austenitic
and duplex stainless steels. If any heat treatment was done during manufacturing, an intergranular corrosion
test in accordance with 7.3.3 shall be carried out.
4.1.4 The manufacturer shall be able to guarantee cylinder steel casting traceability for each pressure-
retaining part of the cylinder.
4.1.5 All parts welded to the cylinder shall be made of compatible material with respect to the weldability.
4.1.6 The cylinder manufacturer shall obtain and provide material certificates of the ladle analysis from the
steel manufacture of the steel supplied for the construction of the pressure-retaining parts of the cylinder and
for the welding consumables.
4.1.7 Some grades of stainless steel may be susceptible to environmental stress corrosion cracking (SCC).
A check shall be made of the material standard to ensure that the selection of material is compatible with the
intended service. Special precautions shall be taken, e.g. by carrying out a post-processing SCC test or by
using a grade of material more resistant to SCC. No special precautions shall compromise any other
requirements in this part of ISO 18172.
4.1.8 The welding consumables shall be such that they are capable of giving consistent welds. The strength
characteristics of the welds shall not be less than those considered in the design and/or calculations.
4.2 Categories
The following three broad categories of stainless steels are recognised:
⎯ ferritic;
⎯ austenitic;
4 © ISO 2007 – All rights reserved

⎯ ferritic/austenitic (duplex).
The steels used shall be in accordance with ISO 9328-7.
4.3 Heat treatment
4.3.1 For cylinders subjected to cold forming or cryoforming processes, heat treatment of the preform
component part is not required. Cryoformed cylinders shall not be subjected to any subsequent heat treatment
or to additional heat application, such as welding.
4.3.2 Raw materials used for the manufacture of pressure-retaining parts of the cylinders shall be annealed
for ferritic steels, or solution annealed for austenitic and duplex steels (see ISO 9328-7:2004, Annex C).
4.3.3 The cylinder manufacturer shall obtain and provide certificates for the heat treatment of all parts
covered by 4.3.2 that are used for the construction of the gas cylinders.
4.3.4 The cylinder manufacturer shall maintain records of any heat treatment carried out.
4.4 Test requirements
The material of the finished cylinders shall satisfy the requirements of Clause 7.
5 Design
5.1 General requirements
5.1.1 The calculation of the wall thickness of the pressure-containing parts shall be related to the yield
stress of the parent material.
5.1.2 For calculation purposes, the value of the yield stress R is limited to a maximum of 0,85 R .
e g
5.1.3 The internal pressure upon which the calculation of gas cylinders is based shall be the test pressure
p .
h
5.1.4 A fully dimensioned drawing of the material, including the specification, shall be produced.
5.2 Calculation of cylindrical shell wall thickness
The wall thickness, a, of the cylindrical shell shall not be less than that calculated using the following formula:
⎛⎞
D 10⋅⋅F JR⋅ − 3⋅p
eh
⎜⎟
a=−1 (1)
⎜⎟
210⋅⋅FJR⋅
e
⎝⎠
where
F = 0,77
J = 1 for circumferential welds
J = 0,9 for longitudinal welds.
The minimum wall thickness shall also satisfy the requirements of 5.4.
5.3 Design of convex ends
5.3.1 The shape of ends of gas cylinders shall be such that the following conditions are fulfilled:
⎯ for torispherical shaped ends [see Figure 1 a)]: R u D; r W 0,1 D; h W 4 b
⎯ for ellipsoidal shaped ends [see Figure 1 b)]: H W 0,192 D; h W 4 b
5.3.2 The wall thickness, b, of the ends of gas cylinders shall be not less than that calculated using the
formula
b = a × C (2)
where
a is the value of a calculated in accordance with 5.2, using J = 1,0
C is a shape factor, whose value shall be obtained from the graph given in Figure 2 or Figure 3.
5.3.3 For cryoformed cylinders, convex ends shall be hemispherically shaped. The shape factor C shall be
equal to 1.
The minimum 1,0 % proof stress to be achieved in the hemispherical ends shall be equal to
R = R × (a ÷ 2b) (3)
p p1,0
6 © ISO 2007 – All rights reserved

Outside Inside Outside Inside
a)  Torispherical shaped end b)  Ellipsoidal shaped end
NOTE For torispherical shaped ends, the height H can be calculated using
DD
H=+(Rb)− [(Rb+ )− ]⋅[(Rb+ )+ − 2(r+b)] (4)
Figure 1 — Illustration of cylinder ends
Figure 2 — Values of shape factor C for H/D between 0,2 and 0,25

Figure 3 — Values of shape factor C for H/D between 0,25 and 0,5
8 © ISO 2007 – All rights reserved

5.4 Minimum wall thickness
5.4.1 The minimum wall thickness of the cylindrical shell a and of the end b shall be not less than the value
derived from the following formulae, as appropriate:
for D u 100 mm: a = b = 1,1 mm (5)
for 100 mm < D u 150 mm: a = b = 1,1 + 0,008 (D − 100) mm (6)
D
for D > 150 mm: a = b = + 0,7 mm, with an absolute minimum of 1,5 mm (7)
These formulae apply to cylindrical shells and ends irrespective of whether they are designed by calculation
under 5.2 and 5.3, or by the pressure cycling test described in 7.3.2.
5.4.2 Apart from the requirements of 5.3, 5.4 and 5.5, any cylindrical part integral with an end shall, except
as qualified by 5.4.3, also satisfy the requirements given in 5.2 for the cylindrical shell.
5.4.3 Where the length of the cylindrical portion of the gas cylinder, measured between the beginning of the
domed parts of the two ends, is not more than 2bD , the wall thickness shall be not less than that of the
domed part (see 5.3.2).
5.5 Ends of other shapes
Ends of shapes other than those covered by 5.3 may be used, provided that the adequacy of their design is
demonstrated by a pressure cycling test in accordance with 7.3.2, or by stress analysis.
5.6 Design of openings
5.6.1 The location of all openings shall be restricted to the end(s) of cylinders.
5.6.2 Each opening in the cylinder shall be reinforced either by a valve boss or pad, of weldable and
compatible steel, securely attached by welding and so designed as to be of adequate strength and to result in
no harmful stress concentrations. This shall be confirmed by design calculations or a pressure cycling test in
accordance with 7.3.2.
5.6.3 The welds of the openings shall be separated from longitudinal and circumferential joints by a
distance not less than 3a.
5.7 Bungs
Bungs shall be designed such that they are welded in position with the shoulder of the bung on the inside of
the domed end.
6 Construction and workmanship
6.1 General
The cylinder or cylinder preform shall be produced by using one of the following:
⎯ seamless or longitudinally welded tube with forged or deep drawn ends circumferentially welded,
⎯ longitudinally welded tube with spun ends,
⎯ a seamless tube, followed by hot forming where the base is sealed with added weld metal,
⎯ cold worked tube or plate,
⎯ welded deep drawn parts,
⎯ cold forming or cryoforming of welded cylinder preforms, or
⎯ plate material rolled and formed with longitudinal weld, with forged or deep drawn ends circumferentially
welded.
6.2 Welding
6.2.1 General
In view of the thicker walls required for this specification, attention shall be given to the welding processes
(see 6.4 and 6.8.2)
6.2.2 Procedures
Before proceeding with the production of a given design of cylinder, each manufacturer shall qualify all
welding procedures to ISO 9956-1 and ISO 9956-3, and welders to ISO 14732 and ISO 9606-1. Records of
such qualification shall be kept on file by the manufacturer.
6.2.3 Quality
For embrittling gases, e.g. hydrogen, particular care shall be taken during the welding to ensure that all
defects conform to ISO 5817 level B. The effects of typical welding defects, e.g. porosity and undercutting,
should not be underestimated when dealing with such gases.
6.3 Cryoforming procedure
6.3.1 Cryoforming, as defined in 3.1.3, is a method to increase the mechanical properties of especially
metastable austenitic steels by applying a high internal pressure to the vessel preform at cryogenic
temperatures.
The relevant process parameters are the cryoforming pressure p and the cryoforming temperature.
c
The design of the finished cylinder shall be in accordance with Clause 5.
6.3.2 The cryoforming operation shall be carried out at a cryoforming pressure p , which shall be calculated
c
according to
f××Ra20×
cp1,0 m
p = (8)
c
D
m
f depends on the cryoforming temperature, the cast analysis for each batch of cylinders and the yield strength
c
to be achieved, and shall be established by the manufacturer such that the finished cylinders satisfy the
requirements as specified in Clauses 5 and 8. Between batches of one design of cylinder (as defined in 3.1.6),
f shall not vary by more than ± 10 %.
c
6.3.3 For each cryoformed cylinder, the manufacturer shall record a pressure-time diagram of the
cryoforming process, where the rate of pressure increase, the achieved cryoform pressure and the holding
time are shown. The rate of pressure increase shall not be more, and the holding time shall not be less, than
that established during the design testing for the cylinder.
6.3.4 The maximum permanent deformation of the vessel’s perimeter due to the cryoforming shall be no
more than 15 %. Measurement may be carried out by using a tape measure, for example.
10 © ISO 2007 – All rights reserved

6.3.5 The cryoforming temperature shall remain constant during the process (e.g. by submerging the
cylinder preform in a suitable cryogenic liquid) and shall be established during the design test.
6.4 Welded joints of pressure-containing parts
6.4.1 The longitudinal joint, of which there shall be not more than one, shall be butt-welded.
6.4.2 Circumferential joints, of which there shall be no more than two in the cylindrical part, shall be butt-
welded in all cases for cryoformed cylinders, and for all cylinders used in corrosive gas applications.
6.5 Non-pressure-containing attachments
6.5.1 Parts which are not submitted to pressure, such as foot rings, handles and neck rings, shall be made
of steel compatible with that of the cylinder, e.g. selected to avoid corrosion couples.
6.5.2 Each attachment shall be clear of longitudinal and circumferential joints, and so designed as to permit
inspection of the attachment welds and to avoid trapping water.
6.5.3 A foot ring or other support shall be fitted to the cylinder when required to provide stability, and
attached so as to permit inspection of the bottom circumferential weld. Permanently attached foot rings shall
be drained and the space enclosed by the foot ring shall be ventilated.
6.5.4 In the case of cylinders subjected to a cryoforming process, the non-pressure-retaining attachments
shall be welded to the cylinder preformed component before cryoforming.
6.6 Valve protection
6.6.1 Valves of cylinders of more than 5 l water capacity shall be protected from damage which could cause
release of gas, either by the design of the cylinder (e.g. protective shroud) or by a valve protection device (in
accordance with ISO 11117).
6.6.2 When a protective shroud is used, it shall fulfil the requirements of the drop test, as described in
ISO 11117.
6.6.3 The requirements of 6.6.1 and 6.6.2 may be waived when the cylinders are intended to be conveyed
in bundles or cradles or when, transported as single cylinders, some other effective valve protection is
provided and it can be demonstrated that the valve can withstand damage without leakage of product (see
ISO 10297).
6.7 Neck threads
The internal neck threads shall conform to a recognized standard to permit the use of a corresponding valve,
thus minimizing neck stresses following the valve torquing operation. Internal neck threads shall be checked
using gauges corresponding to the specific neck thread, or by an alternative method. Particular care shall be
taken to ensure that neck threads are accurately cut, are of full form and free from any sharp profiles, e.g.
burrs.
NOTE For example, where the neck thread is specified to be in accordance with ISO 10920, the corresponding
gauges are specified in ISO 11191.
6.8 Visual examination
6.8.1 Unacceptable defects
Before assembly, the pressure envelope parts of the cylinder shall be examined for uniform quality and
freedom from unacceptable defects, examples of which are given in Annex B.
6.8.2 Welds
6.8.2.1 Before the cylinders are closed, longitudinal welds shall be visually examined from both sides.
Permanent backing strips shall not be used with longitudinal welds.
6.8.2.2 All welds shall have a smooth finish without concavity, and shall merge into the parent material
without under-cutting or abrupt irregularity.
6.8.2.3 All butt welds shall have full penetration (see Figures 8 and 9). The excess thickness shall be
such that the weld integrity is not compromised. Joggled butt welds shall have their penetration verified by
macro etch, bend testing or tensile testing. Lap welds shall have their penetration verified by macro etch and
bend testing. The tests shall be carried out as specified in Clause 8.
6.8.2.4 Radiographic examination, radioscopic examination, or non-destructive examination (NDE)
carried out using another suitable method shall be as specified in Annex A.
6.8.3 Out of roundness
The out of roundness of the cylindrical shell shall be limited such that the difference between the maximum
and the minimum outside diameter in the same cross-section is not more than 1 % of the mean of these
diameters for two-piece cylinders, and not more than 1,5 % of the mean of these diameters for three-piece
cylinders.
6.8.4 Straightness
Unless otherwise specified on the manufacturing drawing, the maximum deviation of the cylindrical part of the
shell from a straight line shall not exceed 0,3 % of the cylindrical length.
6.8.5 Verticality
When the cylinder is standing on its base, the cylindrical shell and concentric opening shall be vertical to
within 1 % of the cylindrical length.
7 New design tests
7.1 General requirements
7.1.1 Testing shall be carried out for each new design of cylinder.
A cylinder shall be considered to be of a new design, compared with an existing design, in the case of one of
the following:
⎯ it is manufactured in a different factory;
⎯ it is manufactured by a different welding or manufacturing process or there is a radical change in an
existing process, e.g. change in heat treatment, change in cold working or cryoforming operation;
⎯ it is manufactured from a steel of different specified chemical composition range;
⎯ it is given a different heat treatment, as specified in 4.3;
⎯ there is a change in base profile, e.g. concave, convex, hemispherical, or there is a change in the base
thickness/cylinder diameter ratio;
⎯ the guaranteed minimum yield stress (R ) and/or the guaranteed minimum tensile strength (R ) has
e g
changed;
12 © ISO 2007 – All rights reserved

⎯ the overall length of the cylinder has increased by more than 50 % (cylinders with a length/diameter ratio
less than 3 shall not be used as reference cylinders for any design with this ratio greater than 3);
⎯ the outside diameter has changed by more than ± 2 %;
⎯ the guaranteed minimum wall thickness (a′) or the guaranteed minimum end thickness (b′) has been
decreased;
⎯ the hydraulic test pressure has been changed (where the cylinder is to be used for a lower pressure duty
than that for which the cylinder was approved, it shall not be deemed a new design);
⎯ the cryoforming factor (f ) varies by more than ± 10 % for cryoformed cylinders; or
c
⎯ a different cryoforming temperature is used.
7.1.2 A technical specification of the cylinder, including design drawing, design calculations, material details,
welding and manufacturing process and heat treatment, shall be prepared by the manufacturer and attached
to the design test certificate (see Annex C).
7.1.3 A minimum of 25 cylinders, which shall be guaranteed by the manufacturer to be representative of a
new design, shall be made available for design testing. However, if the total production is less than 50
cylinders, enough cylinders shall be made to complete the tests required, in addition to the production quantity.
In this case, the design test certificate is limited to the particular batch.
7.1.4 The testing process shall include the verifications and tests listed in 7.2.1 and 7.2.2 respectively.
7.2 Verifications and tests
7.2.1 Verifications
It shall be verified that:
⎯ the requirements of Clause 4 (materials) are fulfilled;
⎯ the design conforms to the requirements of Clause 5;
⎯ the thickness of the walls and domed ends of two cylinders meet the requirements of 5.2 to 5.5, the
measurements being taken at a minimum of three transverse sections of the cylindrical part and on a
longitudinal section of the base and head;
⎯ the requirements of Clause 6 and Annex A are fulfilled for all cylinders selected.
7.2.2 List of tests
The following tests shall be performed on cylinders selected after the welds of the cylinders have been visually
inspected:
⎯ the tests specified in 8.3 (tensile test), 8.4 (bend test), 8.5 (impact test) where applicable, and 8.6
(macroscopic examination of weld cross-sections) on two cylinders, the test pieces being identifiable to
the batch;
⎯ the test specified in 7.3.1 (hydraulic burst test) on two cylinders, the cylinders bearing representative
stamp marking;
⎯ radiographic examination, radioscopic examination or non-destructive examination carried out using
another suitable method, in accordance with Annex A;
⎯ the test specified in 7.3.2 (pressure cycling) on one cylinder, the cylinder bearing representative stamp
marking;
⎯ the test specified in 7.3.3 (corrosion test) on one cylinder when the cylinders are intended for use in
corrosive gas service (see ISO 11114-1) and are manufactured from austenitic or duplex stainless steels.
These tests shall be performed with finished cylinders after all manufacturing processes, including cold
forming or cryoforming steps, have been completed.
7.3 Descriptions of tests
7.3.1 Hydraulic burst test
7.3.1.1 Cylinders subjected to this test shall bear markings in accordance with the complete stamp
markings required for the finished cylinder. The hydraulic burst test shall be carried out with equipment which
enables the pressure to be increased at a controlled rate, not more than 0,1 MPa/s (1 bar/s), until the cylinder
bursts and the change in pressure with time is recorded.
7.3.1.2 The burst pressure (p ) shall be at least 1,6 times the test pressure. The observed yield pressure
b
(p ) shall be equal to or greater than the value calculated by the following formula:
y
p W p /F (9)
y h
The burst test shall not cause any fragmentation of the cylinder.
The main fracture shall not show any brittleness, i.e. the edges of the fracture shall not be radial but shall be
at an angle to a diametral plane, and shall display a reduction of area throughout their thickness.
7.3.2 Pressure cycling test
7.3.2.1 The pressure cycling test shall be carried out on three cylinders bearing the required stamp
markings. See Clause 11 for the particular requirements concerning the stamp marking on the dome ends.
7.3.2.2 This test shall be carried out with a non-corrosive liquid, subjecting the cylinder to successive
reversals at an upper cyclic pressure which is equal to the hydraulic test pressure (p ). The value of the lower
h
cyclic pressure shall not exceed 10 % of the upper cyclic pressure. The frequency of reversals of pressure
shall not exceed 0,25 Hz (15 cycles/min). The temperature measured on the outside surface of the cylinder
shall not exceed 50 °C during the test.
7.3.2.3 The cylinder shall be subjected to 12 000 cycles without leakage or failure.
7.3.2.4 The thickness of the centre of the cylinder bases shall be measured and recorded on the design
test certificate. The centre of the cylinder base of production cylinders shall not be more than 15 % thinner
than the recorded thickness for the prototypes.
7.3.3 Corrosion test
An intergranular corrosion test shall be carried out in accordance with ISO 3651-2 on one cylinder for design
testing for cylinders which are intended to be used for corrosive applications and are manufactured from
steels specified in 4.2. The specimens shall be taken from a part of the cylinder providing a specimen
geometry suitable for bend testing.
The mechanical properties may depend on the location of the samples, therefore the exact location of the
samples shall be identified and this location shall be as indicated in Figure 4 or Figure 5. Two specimens shall
be taken from the location shown in Figure 4 or Figure 5, as appropriate.
Corrosive gases are listed in ISO 11114-1, and cylinders for such gases shall be stamp-marked “H”, as
specified in ISO 13769.
14 © ISO 2007 – All rights reserved

7.4 Design testing certificate
If the results of the checks are satisfactory, a design testing certificate shall be issued, a typical example of
which is given in Annex C.
8 Batch tests
8.1 General
For the purpose of carrying out the batch testing, a random sample of cylinders, as indicated in Table 1, shall
be taken from each batch, as defined in 3.1.6. A batch shall consist of a maximum of 3 000 cylinders. All batch
tests shall be carried out on finished cylinders.
Table 1 — Batch sampling
Number of cylinders to be tested
Number of cylinders taken
Batch size
as samples
a b
Mechanical tests Burst tests Radiography
up to 200 2 1 1
201 to 500 3 1 2
See Annex A
501 to 1 500 9 2 7
1 501 to 3 000 18 3 15
a
Mechanical tests comprise tensile tests (in accordance with 8.3), bend tests (in accordance with 8.4), impact tests where applicable
(in accordance with 8.5), and macroscopic examination of weld cross-sections (in accordance with 8.6).

b
In accordance with 7.3.1.
8.2 Information
For the purpose of batch testing, the manufacturer shall provide the following:
⎯ the design test certificate;
⎯ the certificates for the material of construction, as required in 4.1.6, stating the cast analyses of the steel
supplied for the construction of the cylinders;
⎯ a list of cylinders, stating serial numbers and stamp markings as required;
⎯ a statement of the thread checking method used, and the results thereof.
8.3 Tensile test
8.3.1 General
The tensile test on parent material shall be carried out on a test sample taken from a finished cylinder in
accordance with the requirements of ISO 6892. The two faces of the test sample formed by the inside and the
outside surfaces of the cylinder shall not be machined. Only the ends may be flattened, by cold pressing, for
gripping in the test machine. The tensile test on welds shall be carried out in accordance
...