ISO/TS 18226:2006

TECHNICAL ISO/TS
SPECIFICATION 18226
First edition
2006-10-01
Plastics pipes and fittings — Reinforced
thermoplastics pipe systems for the
supply of gaseous fuels for pressures up
to 4 MPa (40 bar)
Tubes et raccords en matières plastiques — Systèmes de canalisations
en matière thermoplastique renforcée pour la distribution de
combustibles gazeux à des pressions allant jusqu'à 4 MPa (40 bar)

Reference number
©
ISO 2006
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ii © ISO 2006 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviations. 2
3.1 General terms and definitions . 2
3.2 Temperature- and pressure-related definitions. 4
3.3 Abbreviations . 5
4 Performance requirements . 6
4.1 Materials . 6
4.2 Pipes and fittings. 7
4.3 Re-qualification . 8
5 Process and quality control. 8
6 Dimensions and marking . 8
6.1 Dimensions. 8
6.2 Marking . 8
7 Handling, storage and installation. 8
Annex A (informative) Description of RTP Products . 9
Annex B (informative) Liner material durability considerations . 12
Annex C (informative) Rationale for the elevated temperature test. 14
Annex D (normative) Test procedures . 17
Annex E (normative) Qualification protocol. 19
Annex F (informative) Process and quality control requirements . 32
Bibliography . 35

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.
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of normative document:
⎯ an ISO Publicly Available Specification (ISO/PAS) represents an agreement between technical experts in
an ISO working group and is accepted for publication if it is approved by more than 50 % of the members
of the parent committee casting a vote;
⎯ an ISO Technical Specification (ISO/TS) represents an agreement between the members of a technical
committee and is accepted for publication if it is approved by 2/3 of the members of the committee casting
a vote.
An ISO/PAS or ISO/TS is reviewed after three years in order to decide whether it will be confirmed for a
further three years, revised to become an International Standard, or withdrawn. If the ISO/PAS or ISO/TS is
confirmed, it is reviewed again after a further three years, at which time it must either be transformed into an
International Standard or be withdrawn.
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/TS 18226 was prepared by Technical Committee ISO/TC 138, Plastics pipes, fittings and valves for the
transport of fluids, Subcommittee SC 4, Plastics pipes and fittings for the supply of gaseous fuels.
iv © ISO 2006 – All rights reserved

Introduction
A reinforced thermoplastics pipe (RTP) comprises a thermoplastics liner with continuous reinforcement and a
thermoplastics outer cover. An RTP “system” comprises runs of RTP, along with the fittings required to
connect them to each other and to the other components of a conventional gas transmission system.
This Technical Specification is applicable for operating pressures up to 4 MPa (40 bar). However it may be
used for guidance in the development of RTP systems for higher operating pressures. It is intended to
accommodate the upgrading of the performance of RTPs and to provide a framework within which future
development can take place.
RTP can be used in both new pipe systems and in the replacement of corroded metallic pipes.
The principal load-bearing components of the RTP are high-strength reinforcing members in the form of fibres,
yarns, tapes or wire, which generally carry load only in tension. The reinforcing element may take the form of
helically-wound yarns or fibre-reinforced tapes, in which the matrix may be a thermoplastics resin.
In the most frequently employed configuration of reinforcement, dry (non-impregnated) aramid-fibre yarns are
encapsulated in a tape of polymer resin or adhesive. It is also possible to employ other classes of
reinforcement, such as glass, carbon or textile fibres, or metallic wire or strip.
The reinforcement may or may not be bonded to the liner or to the outer cover.
Several types of fitting design are possible, with joints made by mechanical means, electrofusion or other
methods of bonding or welding.

TECHNICAL SPECIFICATION ISO/TS 18226:2006(E)

Plastics pipes and fittings — Reinforced thermoplastics pipe
systems for the supply of gaseous fuels for pressures up to
4 MPa (40 bar)
1 Scope
This Technical Specification describes the use of reinforced thermoplastics pipe (RTP) systems for
1)
transmission of gaseous fuels at maximum operating pressures up to and including 4 MPa (40 bar) , and
service temperatures in the region − 50 °C to 120 °C, depending on the liner and cover materials.
This Technical Specification relates to transmission systems in which wear and damage to the liner are
restricted by limiting pigging operations to soft pigging only.
The recommendations in this Technical Specification are confined to RTP and its associated in-line fittings
and end-fittings. Where the other system components (elbows, tees, valves, etc.) are of conventional
construction, they will be governed by existing standards and codes of practice.
This Technical Specification specifies a qualification testing procedure for RTP systems. It also provides a
procedure for reconfirmation of the design basis that may be used for product variants where changes have
been made in design, materials or the manufacturing process.
This Technical Specification provides informative annexes relating to quality assurance, product marking,
handling and storage.
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 4433-1:1997, Thermoplastics pipes — Resistance to liquid chemicals — Classification —
Part 1: Immersion test method
ISO 4433-2:1997, Thermoplastics pipes — Resistance to liquid chemicals — Classification —
Part 2: Polyolefin pipes
ISO 4433-3:1997, Thermoplastics pipes — Resistance to liquid chemicals — Classification —
Part 3: Unplasticized poly(vinyl chloride) (PVC-U), high-impact poly(vinyl chloride) (PVC-HI) and chlorinated
poly (vinyl chloride) (PVC-C) pipes
ISO 4433-4:1997, Thermoplastics pipes — Resistance to liquid chemicals — Classification — Part 4: Poly
(vinylidene fluoride) (PVDF) pipes
ISO 4437, Burried polyethylene (PE) pipes for the supply of gaseous fuels — Metric series — Specifications

1) 1 bar = 0,1 MPa = 10 Pa.
ISO 9080:2003, Plastics piping and ducting systems — Determination of the long-term hydrostatic strength of
thermoplastics materials in pipe form by extrapolation
ISO 12162:1995, Thermoplastics materials for pipes and fittings for pressure applications — Clarification and
designation — Overall service (design) coefficient
ISO 12176-1:1998, Plastics pipes and fittings — Equipment for fusion jointing polyethylene systems —
Part 1: Butt fusion
ISO 14531-1, Plastics pipes and fittings — Crosslinked polyethylene (PE-X) pipe systems for the conveyance
of gaseous fuels — Metric series — Specifications — Part 1: Pipes
ISO 14531-2, Plastics pipes and fittings — Crosslinked polyethylene (PE-X) pipe systems for the conveyance
of gaseous fuels — Metric series — Specifications — Part 2: Fittings for heat-fusion jointing
ISO 14531-3, Plastics pipes and fittings — Crosslinked polyethylene (PE-X) pipe systems for the conveyance
of gaseous fuels — Metric series — Specifications — Part 3: Fittings for mechanical jointing (including
PE-X/metal transitions)
ISO 14531-4, Plastics pipes and fittings — Crosslinked polyethylene (PE-X) pipe systems for the conveyance
of gaseous fuels — Metric series — Specifications — Part 4: System design and installation guidelines
ASTM D2992-01, Standard Practice for Obtaining Hydrostatic or Pressure Design Basis for “Fiberglass”
(Glass-Fiber-Reinforced Thermosetting-Resin) Pipe and Fittings
3 Terms, definitions and abbreviations
For the purpose of this document, the following terms, definitions and abbreviations apply.
3.1 General terms and definitions
3.1.1
aramid
class of high-strength organic fibre “aromatic amide”
2) 2)
EXAMPLES Twaron , Kevlar .
3.1.2
application-related service factor(s)
multiplication factor(s) applied to the manufacturer's nominal pressure rating, to allow for effects such as
cyclicity
3.1.3
ballooning
inflation of the cover of an RTP, by pressurised gas, that has accumulated in the reinforcing layer
3.1.4
blistering
damage in polymer materials caused by the release of absorbed gas on sudden decompression
3.1.5
carbon fibre
class of high-strength graphite-based reinforcing fibre

2) Twaron and Kevlar are examples of suitable products available commercially. This information is given for the
convenience of users of this document and does not constitute an endorsement by ISO of these products.
2 © ISO 2006 – All rights reserved

3.1.6
cyclic
fatigue
service conditions where the internal pressure fluctuates
3.1.7
dynamic
service condition involving external time-dependent loads
3.1.8
elevated temperature test
constant-pressure survival test aimed at verifying that no undesirable failure mode can occur between the end
of the qualification test period and the end of the design life
3.1.9
end-fitting
joint that occurs at either end of a run of RTP, where it is connected to other parts of the system
3.1.10
fitting
coupler
pipe joint
3.1.11
glass fibre
high-strength inorganic reinforcement based on E-glass or S-glass
3.1.12
in-line fitting
pipe joint between adjacent lengths of RTP
3.1.13
lower prediction limit
97,5 % lower prediction limit of the mean regression curve
3.1.14
minimum required strength
lower prediction limit at 20°C in a thermoplastics pipe at 50 years in accordance with ISO 9080:2003, rounded
down in accordance with ISO 12162:1995
3.1.15
Principal
party that initiates and pays for a project, or his agent
NOTE The Principal will generally specify the technical requirements of a project.
3.1.16
principal mode
only failure mode that shall be permitted in the pressure testing of RTP
3.1.17
product family
group of RTP products having certain similarity characteristics
3.1.18
product-family representative
member of a product family, chosen for full qualification
3.1.19
product variability factor
factor, allowing for product variability, applied to the Lower Prediction Limit (LPL) pressure, to give the
Manufacturer's Nominal Pressure Rating (MNPR)
3.1.20
product variant
member of the same product family, to which certain permissible changes have been made
3.1.21
rapid crack propagation
undesirable fracture mode, in which a crack propagates along a pipeline at very high speed
3.1.22
regression analysis
statistical procedure to establish a design rating from pressure test results carried out over a period of 104 h
(or a number of pressure cycles)
3.1.23
safety class
classification associated with a particular probability of failure
3.1.24
stress rupture
static fatigue
failure, as a result of a period under steady stress or pressure
3.1.25
survival test
constant-pressure test, to demonstrate that a product performs at least as well as the qualified product
3.2 Temperature- and pressure-related definitions
3.2.1
design temperature
maximum operating temperature of the RTP system
3.2.2
FAT pressure
Factory Acceptance Test pressure
3.2.3
LPL pressure
pressure obtained by extrapolating the LPL to the design life
3.2.4
long-term hydrostatic pressure
pressure obtained by extrapolating the mean regression curve to the design life
3.2.5
manufacturer's nominal pressure rating
pressure obtained by multiplying the LPL pressure by the product variability factor
3.2.6
maximum service pressure
pressure obtained by multiplying the manufacturer's nominal pressure rating by application-related service
factors
4 © ISO 2006 – All rights reserved

3.2.7
maximum operating temperature
maximum temperature to which the piping is expected to be exposed during normal operational activities,
including start-up and shut-down operations, but excluding abnormal situations such as a fire
3.2.8
minimum operating temperature
minimum temperature to which the piping is expected to be exposed during normal operational activities,
including start-up and shut-down operations and controlled blow-out, but excluding abnormal situations such
as piping rupture
3.2.9
qualification test temperature
temperature at which pressure tests are carried out to establish the lower prediction limit
NOTE The design temperature shall not exceed this temperature.
3.2.10
short-term hydrostatic pressure
pressure corresponding to the LPL pressure at a prescribed time of 100 h or less
3.2.11
short-term burst pressure
burst pressure measured in a short-term test, where pressure is increased at a prescribed rate at
Standard Laboratory Temperature (SLT)
3.2.12
standard laboratory temperature
temperature of 23 °C ± 2 °C
3.2.13
survival test pressure
pressure for a 1 000 h survival test
NOTE This is the pressure of the LPL line at 1 000 h.
3.3 Abbreviations
ASTM American Society for Testing and Materials
API American Petroleum Institute
BS British Standard
CEN Comité Européen de Normalisation
COV Coefficient of Variation
DVS German Standard
EN European Standard
ESC Environment-Sensitive Cracking
FAT Factory Acceptance Test
F Regression relationship constant
G Regression line gradient
IGE Institution of Gas Engineers
ISO International Standard Organization
LPL Lower Prediction Limit
LTHP Long-Term Hydrostatic Pressure
MNPR Manufacturer's Nominal Pressure Rating
MRS Minimum Required Strength
MSP Maximum Service Pressure
3)
PA11 Polyamide 11 (Trade name Rilsan )
PE Polyethylene
PE-X Cross-linked polyethylene (also referred to as XLPE)
PM Principal Mode of failure
PVDF Polyvinylidene fluoride
PVF Product Variability Factor
QA Quality Assurance
RCP Rapid Crack Propagation
RTP Reinforced Thermoplastic Pipe
SLT Standard Laboratory Temperature
STBP Short-Term Burst Pressure
STHP Short-Term Hydrostatic Pressure
UV Ultraviolet
WIS Water Industry Specification
4 Performance requirements
4.1 Materials
4.1.1 Liner materials
Liner materials shall conform to an appropriate Standard for gas applications (i.e. ISO 4437 and EN 1555 in
the case of polyethylene, and ISO 14531 for PE-X). For polyethylene and PE-X liners, the MRS shall be at
least 8 MPa.
3) Rilsan is an example of a suitable product available commercially. This information is given for the convenience of
users of this document and does not constitute an endorsement by ISO of this product.
6 © ISO 2006 – All rights reserved

Other thermoplastics materials (for example, PVDF and PA11) may be used, provided they conform to the
material requirement of a relevant ISO pipe standard and that fitness for the purpose has been established. In
all cases, materials shall be evaluated and classified in accordance with ISO 12162:1995 (see Annex E, E.2).
The liner shall possess RCP resistance at a stress equal to a minimum of 1,5 times the stress induced at the
MSP and minimum operating temperature (see E.3.2).
The liner material shall have adequate resistance to blistering. A suitable procedure is described in
API Spec 17J, Section 6.2.3.2.
4.1.2 Cover materials
Cover materials shall conform to the material requirements of a relevant ISO pipe standard, for example
ISO 4437 or ISO 14531, and fitness for the purpose shall be established.
In the case of pipes that may be exposed to sunlight during storage or service the manufacturer shall
demonstrate that the cover possesses adequate resistance to UV and to UV-transmissions when the
reinforcement is susceptible to UV-damage.
4.1.3 Reinforcements
The manufacturer shall provide the data required to demonstrate the short-term and long-term load-bearing
capability of the reinforcement, as described in Annex A.
The manufacturer shall ensure that the tape supplier operates an effective quality plan relating to all aspects
of tape manufacture. The following characteristics shall be considered in the quality plan: reproducible
strength, dimensional consistency, evenness and reproducibility of cord spacing.
4.2 Pipes and fittings
Each type of RTP pipe body shall be qualified by means of the regression procedure described in Annex E.
The fittings used for these tests may be fittings as used in the field or re-usable test end-fittings. At least one
regression point shall be measured in excess of 10 000 h, with field end-fittings attached to both ends of the
pipe body.
The regression test results shall be used to determine the regression-line gradient, the LTHP and the LPL for
the RTP system, using the statistical procedure described in ISO 9080:2003.
In addition to the regression tests, every field fitting/pipe body combination shall pass an elevated temperature
test, as described in Annex C, to verify the integrity of the fitting/pipe body connection.
The manufacturer shall inform the Principal of any substantial change to the fittings and/or pipe body.
The manufacturer shall prove and guarantee that any change to the field fittings or to the re-usable test
end-fittings does not invalidate the results of qualification tests.
RTP products shall be divided into product families, as described in Annex E. Each product family shall have a
representative named the product-family representative. Other products within the family are termed “product
variants”.
The qualification test temperature shall be greater than or equal to the design temperature.
Other qualification issues are examined in Annex E.
4.3 Re-qualification
Re-qualification may be required when any change to the RTP system is made by the manufacturer. The
manufacturer shall inform the Principal if any changes to the previous qualified RTP product family have
occurred.
Depending on the level of change, the following re-qualification options are defined:
⎯ unimportant – previous qualification remains valid;
⎯ minor – (partial) re-qualification will be required in accordance with Annex E;
⎯ major – (full) re-qualification will be required in accordance with Annex E.
The manufacturer and Principal shall agree on the classification of each change.
NOTE Currently, major or minor changes cannot be defined with greater precision.
5 Process and quality control
The manufacturer shall produce a quality plan relating to all aspects of the manufacturing process. The quality
assurance procedure for RTP is described in Annex F. It requires that either batch tests or a hydrotest be
carried out on the product or, where required by the application, both types of test.
6 Dimensions and marking
6.1 Dimensions
The nominal size of the pipe shall be the internal diameter of the liner expressed in millimetres (mm). The
preferred nominal size shall be a multiple of 25 mm, enabling an approximate correspondence to be
maintained with inch sizes.
6.2 Marking
The required information shall be permanently marked on the pipe body, in a colour that contrasts that of the
pipe, the height of the characters being at least 5 mm (10 mm on pipes larger than 150 mm in diameter).
The required markings should be repeated at reasonable intervals to be agreed with the Principal.
The following information shall be given on the RTP pipe body:
⎯ Manufacturer's name or trademark.
⎯ The word, “GAS” or “GAZ”.
⎯ ISO/TS 18226.
⎯ Nominal pipe size in mm.
⎯ Product identification code.
Markings shall be durable and non-damaging.
The Principal may request additional markings if necessary.
7 Handling, storage and installation
The manufacturer shall provide the Principal with written instructions on the handling, storage and installation
requirements of the RTP system.
8 © ISO 2006 – All rights reserved

Annex A
(informative)
Description of RTP Products
A.1 General
An RTP ‘system' comprises runs of RTP, along with the fittings required to connect them to each other and to
the other components of a conventional gas transmission system. The essential components of such a system
are
⎯ a thermoplastics liner, the main function of which is to contain the fluid being transmitted,
⎯ an even number of balanced helical windings of continuous reinforcement, to resist the applied pressure
and other loads; these can be applied using a number of possible processes, including helical tape-
wrapping, filament winding and braiding,
⎯ an outer protective thermoplastics cover, and
⎯ a system of fittings to enable runs of RTP to be connected to one another and to other components.
A.2 Liner
The thermoplastics liner may be manufactured in-line with the RTP production process or supplied as a
separate component. It may, on occasion, be necessary to join lengths of liner by butt fusion. When this is
done, it should be carried out according to a recognised standard, for example EN 1555-1, EN 1555-2,
EN 1555-3, EN 1555-4 or EN 1555-5, using butt fusion equipment meeting ISO 12176-1. The procedure
should be documented and a QA system should be in place to ensure that the properties of the joint are equal
to those of the parent pipe.
To fulfil its function of containing the transported fluid, the liner material should have adequate resistance to
degradation from all the components of the fluid. Resistance to degradation includes
⎯ resistance to physical interaction, which may cause leaching, excessive swelling, plasticisation and
consequent loss of properties,
⎯ resistance to chemical attack, and
⎯ resistance to wear and abrasion by suspended solids.
The liner should also possess sufficient ductility to enable it to withstand the strains imposed upon it during
RTP manufacture, storage and deployment (which may involve reeling or axial loads). It should also be able to
resist long-term loads imposed upon it by joints and fittings without excessive creep. Furthermore, it should be
capable of withstanding the strains imposed during pressurisation and, where appropriate, cyclic
pressurisation.
The liner acts as a barrier to limit the diffusion of gas or vapour. The accumulation of gas at the interface
between the liner and the reinforcing layer must not lead to blistering of the cover, or to collapse of the liner, if
the RTP is suddenly depressurised. Certain corrosive gaseous species may also have an undesirable effect
on the reinforcement. In situations where significant diffusion takes place through the liner, the RTP system
may be equipped with a means of venting the gas, for instance at the fitting.
The liner does not normally contribute to the strength of an RTP except under rare loading conditions; for
instance, if the RTP is subjected to external pressure.
With certain designs of fitting, the liner may form part of the load path from the reinforcement to the fitting. In
these cases, the material may be subjected to significant local stresses, which it must resist without failure or
undue deformation.
This procedure applies only to thermoplastics liner materials (including cross-linked thermoplastics, such as
PE-X). In the majority of cases, the liner will be a single component, but multi-layer liners, containing for
instance a thermoplastics barrier layer, are permitted.
Typical thermoplastics materials that may be used in RTP manufacture are: polyethylene (PE), cross-linked
polyethylene (PE-X), polyamide 11 (PA-11) and polyvinylidene fluoride (PVDF).
The liner material should contain no filler, only appropriate additives, well-dispersed in the parent polymer.
A.3 Reinforcement
The principal load-bearing components of the RTP are high-strength reinforcing members in the form of fibres,
yarns, tapes or wire. These generally carry load only in tension. The reinforcing element may take the form of
helically wound fibre-reinforced tapes, in which the resin may be either a thermoplastics or a hot-melt
adhesive.
The most frequently employed reinforcement comprises dry (non-impregnated) aramid fibre yarns, which may
be encapsulated in a polymer resin or adhesive to form a tape. It is also possible to employ other
reinforcements that have been fully or partially impregnated by thermoplastics resin, metallic strip or wire.
Factors to be considered in relation to the reinforcement include
⎯ the effects of the void content in the reinforcement on gas accumulation,
⎯ fibre-fibre friction wear and damage in the dry fibre case, and
⎯ tape/tape friction wear and damage.
It is also necessary to consider possible effects of environment on the reinforcement. Environment-induced
failure can arise through the diffusion of corrosive or sensitising agents through the liner, penetration of agents
along the reinforcement (having entered in the region of the end-fitting or through external damage) and, in
rare cases, diffusion of agents through the cover.
The response of the reinforcement to all possible external environments (water, air, chemicals or
photo-oxidation) as a result of cover damage (or at the cut ends of pipe during storage) also needs to be taken
into account. This should preferably include long-term stress rupture data in the appropriate environment.
Reinforcements should preferably run continuously from one end of the pipe to the other. If reinforcements do
require to be joined (for instance, tape joints in the case of tape reinforcement) this needs to be specified, and
a well-defined jointing procedure laid down. Pipes with such discontinuities are given special consideration in
the qualification procedure.
A.4 Cover
The purpose of the cover is to protect the internal components, most especially the reinforcement, from
damage. Depending on the field of application (e.g. above ground, buried, inside an existing pipe, or subsea)
there are several potential sources of damage. These include abrasion, compression or gouging during coiling
and deployment, environmental attack from chemical species or photo-oxidation, external damage during
trenching and back-filling, external interference and the effects of land movement.
10 © ISO 2006 – All rights reserved

The cover will generally be applied to the RTP by a process of extrusion and may or may not be welded to the
thermoplastics component of the reinforcement.
Although the cover does not contribute significantly to the load-bearing capacity of the RTP under normal
working conditions, it is subject to significant strains that arise from the deformation of the underlying
components when the RTP is pressurised. These strains may be magnified in the vicinity of the end-fittings,
due to the restraining effect of the latter.
The cover is also subject to flexural strains during deployment and to thermal strains during its lifetime.
With certain designs of end-fitting, the cover may form part of the load path from the reinforcement to the
fitting. In this case, the cover material may be subjected to significant local stresses, which it must be shown
to resist without failure or undue deformation.
A.5 Fittings
The function of the fittings is to connect RTP runs to one another and to other components, allowing free
passage of fluid along the line without leakage, while permitting the transmission of loads from the RTP to the
other system components. In certain applications, fittings may be required to allow pigging of the flowline.
Different types of fitting design are permissible, in which a joint is made by mechanical means, electrofusion or
other methods of bonding or welding.
Since the reinforcement takes most of the loads in an RTP system, the fitting design must provide a load path
from the reinforcement into the fitting. This load path may be achieved by directly gripping or bonding the
reinforcement or by frictional or shear transfer involving other components of the RTP.
In addition to the loads mentioned above, the fittings shall also be capable of resisting loads due to
deployment, ground movement, thermal stress and external interference.
The manufacturer should provide the necessary documentation and training to enable fittings to be installed in
a consistent and reproducible manner.
At least one set of fittings shall be specified and qualified for each RTP product.
The construction of the fitting shall be fluid-tight, to prevent the pressurising medium from leaking into the
surrounding environment or into the reinforcing layer. In certain designs of RTP, however, the fitting may also
fulfil the function of allowing diffused volatiles to escape. In these circumstances, the rate of flow of diffused
volatiles should be estimated and taken account of in the system design.
A.6 Bonded and non-bonded construction
Different types of RTP design are possible, in which the liner, reinforcement and cover may or may not be
bonded together. Bonding can influence several aspects of performance, including flexibility, response to
permeated gas and load transfer in fittings.
Annex B
(informative)
Liner material durability considerations
B.1 Ageing
Ageing of thermoplastics polymers is temperature-dependent and occurs on exposure to particular
environments. For liner materials, the behaviour is highly dependent on the composition of the fluid being
carried. Ageing may result in changes to properties such as strength and ductility and can involve
embrittlement, cracking or softening. The mechanisms may be different for different polymers. Typically they
may involve
⎯ environment-sensitive cracking (ESC),
⎯ absorption of species from the carried fluid,
⎯ leaching of low-molecular-weight material or plasticiser from the polymer, or
⎯ chemical changes to the molecular structure of the polymer.
ESC is an embrittlement process that can be activated by specific fluid components. In polyethylene,
susceptibility to ESC is decreased by increasing molecular weight or lowering crystallinity.
Absorption of species from the fluid carried results in plasticisation, which reduces strength and stiffness.
These species may also react chemically with the polymer, often resulting in a loss of molecular weight
through chain scission. In the special case of polyamides, this can occur through hydrolysis, a reaction that is
strongly influenced by the water content of the fluid.
The first requirement for consideration for use as a liner is that the material have “satisfactory resistance” to
the fluids carried, in accordance with ISO 4433. In addition, the polymer manufacturer should provide detailed
information relating to the degradation mechanisms that operate in the presence of the particular fluids to be
transported. This information should be in a form that can be used to predict lifetime and residual integrity. For
example, if the polymer is subject to hydrolysis, as is the case for Polyamide 11, ageing models should be
available to predict the residual lifetime and integrity as a function of time, temperature and fluid composition.
B.2 Retention of properties
The liner needs to retain a minimum level of strength over the design life.
The failure mode of the polymer, when tested in tension, shall always be ductile, i.e. there should be yield
before break. There shall be no local cracking or crazing. This applies across the range of temperatures and
fluids under consideration.
The grade of polymer used for the liner should have documented creep and stress rupture characteristics at a
range of temperatures encompassing the qualification temperature, and for a time period of at least 10 000 h.
This documented behaviour needs to be in a form that can be used to estimate a time/temperature
equivalence factor for the polymer.
The stress rupture regression characteristic of the polymer, log (failure stress) versus log (time to failure),
should be documented and examined for linearity according to ISO 9080:2003. Certain polymers are known to
display two-stage stress rupture curves, in which there is a transition in failure mechanism at moderate times
or high temperature. The data and characteristics produced should be examined for the presence of “Knees”,
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and used to determine the potential of the material to fail prematurely within the time and temperature
conditions specified for use. Only materials which do not exhibit a knee in ISO 9080 datasets before 1 year
should be used. In the case of polyethylene, only established ‘pipe' grades of material with well-documented
performance (such as PE80 and PE100) should be used.
The value of the elongation at yield of the liner material, measured in a tensile test, according to
ISO 527-2:1993, 1BA (or ASTM D638), should be provided at both the maximum and minimum operating
temperature. Where appropriate, the polymer should be saturated in the fluid to be transported.
Under all conditions, the maximum liner strain at the LTHP should be less than 80 % of the strain to yield of
the liner polymer. In the case of polyethylene liners, this should be no more than 6 %, as stipulated in
API Spec 17J.
Annex C
(informative)
Rationale for the elevated temperature test
It is necessary to establish that no failure mode, associated with thermoplastics components of the RTP, can
occur at times between the end of the qualification test period and the end of the design life. Such a failure
mode could, for example, involve
⎯ strain rupture of the liner,
⎯ failure of part of the liner in or near the coupling as a result of local stresses, or
⎯ failure of the axial load capacity of the coupling as a result of stress relaxation of the thermoplastics
components.
To accelerate undesirable failure modes into the region where they would be observed during a reasonable
qualification testing period requires knowledge of the failure modes of the thermoplastics polymer and the
time-temperature equivalence of these failure modes. The grade of polymer used for the liner should therefore
possess well-documented creep and stress rupture characteristics over a range of temperatures exceeding
the qualification temperature, and over a time period that is long enough to allow any possible undesirable
failure modes to be observed. This period should be at least 10 000 h, or possibly more in the case of longer
design lives (50 years or more).
The pressure at which the elevated temperature test should be carried out should relate directly to the
regression curve at the qualification temperature. This pressure should therefore be the LPL.
The most relevant data are stress rupture measurements on pressurised pipe samples covering a range of
temperatures, fitted in accordance with the standard extrapolation method laid down in ISO 9080:2003. Under
these conditions, thermoplastics can display two types of failure mode:
⎯ ductile failure, associated with prolonged creep and gross deformation, or
⎯ brittle failure, associated with crack propagation at long times or high temperatures, sometimes
associated with chemical effects.
Each of these modes is characterised by a different value of activation energy and a different form of
temperature dependence. This needs to be borne in mind when considering the requirements for an
accelerated test at elevated temperature. Ductile failure processes, in general, require a smaller temperature
change to produce a given shift in time-scale than brittle processes.
Figure C.1 shows schematically two sets of ductile failure data at different temperatures. A time/temperature
equivalence factor, α , in decades/°C, can be found by comparing the horizontal distance between the two
curves. It should be noted that this factor may vary somewhat with timescale, since the curves often have
different values of slope. It is recommended that, when comparing curves, this be done over a logarithmic
timescale, with 1 000 h taken as the median point, as shown. For polyethylenes showing ductile stress rupture
behaviour, equivalence factors, in the range 0,2 to 0,3 decades/°C, are usually found, as shown in Table C.1.
When brittle failure is encountered, α is generally much lower, in the range 0,05 to 0,075 decades/°C, as
shown in Table C.1.
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Table C.1 — Time/temperature equivalence factors for different processes in PE
Equivalence factor,
Type of process
α
decades/°C
Pipe stress rupture Stage 1 (ductile failure) 0,2 to 0,3
Pipe stress rupture Stage 2 (brittle failure) 0,05 to 0,075
ISO 9080:2003 gives recommendations concerning “acceleration factors” for use in elevated temperature
testing. These are based conservatively on the lowest observed values of α, namely 0,05 decades/°C. In the
case of thermoplastics materials for RTP, it is reasonable to use such a factor only when it is possible that a
brittle failure mode may occur. However, it is generally undesirable to use material that may display such a
characteristic in RTP if it can be avoided.

Key
X log (time)
Y log (hoop stress)
Figure C.1 — Schematic pipe stress rupture data for a polymer showing ductile behaviour at two
temperatures, and calculation of the time/temperature equivalence factor, α
The activation energy corresponding to the ISO 9080:2003 default value is close to th
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