prEN 18397

• Abstract:

  This document specifies requirements for the composition of CO2 streams relevant to the integrity of carbon steel pipeline systems, including limits on non-condensable components. It establishes the framework conditions for the supply, transportation and distribution of CO2 streams. Considerations related to the health effects of the CO2 stream (e.g. toxicity), environmental impact, or other properties of impurities are outside the scope of this document.

• Status: Not Published
• Publication Date: 13-Jul-2027
• ICS: 13.020.40 - Pollution, pollution control and conservation
• ICS: 23.040.10 - Iron and steel pipes
• Technical Committee: TC 474 - CO2 capture, transportation, utilization, storage and carbon accounting
• Drafting Committee: WG 01 - CO2 Streams and Quality
• Current Stage: 4020 - Submission to enquiry - Enquiry
• Start Date: 13-Aug-2026
• Due Date: 29-Jun-2025
• Completion Date: 13-Aug-2026
• Ref Project: oSIST prEN 18397:2026 - Properties of carbon dioxide streams in carbon steel pipelines

Overview

prEN 18397 is a draft European Standard developed by CEN that specifies the requirements for the composition of carbon dioxide (CO₂) streams transported through carbon steel pipelines. The primary focus of this standard is on the attributes of CO₂ streams that directly affect the structural integrity and operational safety of carbon steel pipeline systems. It addresses aspects such as allowable concentrations of non-condensable gases and other impurities that may be present in the CO₂ stream. Importantly, considerations related to toxicity, environmental impact, or other impurity properties fall outside the scope of this document.

Key Topics

• CO₂ Stream Composition
  The standard defines minimum purity levels for transported CO₂ (typically greater than 95 mol-%) and places particular emphasis on controlling impurity levels, with specific focus on those that could compromise pipeline integrity, such as water, hydrogen sulfide, oxygen, nitrogen, other non-condensables, and select reactive or hygroscopic substances.

• Thermodynamic and Phase Behavior
  The document highlights the significant impact of impurities on the physical and chemical behavior of CO₂ streams. Impurities can alter the phase envelope, affect density, viscosity, and create multi-phase conditions, which must be considered in pipeline design and operation.

• Corrosion and Material Compatibility
  The standard addresses how impurities, particularly water and acid-forming components, can result in corrosion processes in carbon steel pipelines. It outlines the necessity for impurity limits to prevent carbonic and strong acid corrosion, hydrate formation, and the formation of corrosive aqueous phases.

• Operating Conditions
  Operating a CO₂ pipeline in dense or gaseous phases requires careful attention to pressure and temperature ranges to avoid accidental phase changes and ensure safe containment. The standard covers guidance to avoid operation near critical points or phase boundaries.

Applications

prEN 18397 is essential for organizations involved in carbon dioxide transportation as part of carbon capture, utilization, and storage (CCUS) projects. Its key practical values include:

• Safe Pipeline Operation
  By specifying allowable impurity concentrations and minimum CO₂ purity, prEN 18397 helps operators design and manage pipeline systems that minimize corrosion risk, maintain mechanical integrity, and support long-term operational reliability.

• Design and Engineering
  Engineering firms and project designers can use the standard to define material requirements and assess the technical feasibility of proposed CO₂ pipeline networks, ensuring alignment with industry best practices and regulatory expectations.

• Quality Control and Compliance
  The standard provides a clear framework for the analytical assessment of CO₂ stream composition (often referencing prEN 18329 for sampling and analysis), supporting traceability and compliance for operators and regulators.

• Integration into CCUS Value Chains
  As industries increase their focus on CCUS technologies to meet environmental targets, prEN 18397 enables safe and efficient integration of CO₂ capture, transportation, and storage infrastructure by harmonizing pipeline supply quality requirements.

Related Standards

• EN ISO 27913:2025 - Carbon dioxide capture, transportation, and geological storage - Pipeline transportation systems. This standard offers broader requirements for pipeline transport of CO₂ and is explicitly referenced by prEN 18397 for additional design guidance.

• prEN 18329 - Measurement of CO₂ streams - Sampling and analysis for pipeline transportation. It supports the implementation of prEN 18397 by detailing methods for compositional analysis of CO₂ streams.

• ISO/TR 27921:2020 - Provides examples of impurity sources in CO₂ streams, offering context for the necessity of impurity control highlighted in prEN 18397.

Summary

By providing a clear set of requirements for CO₂ stream composition, prEN 18397 supports the safe, reliable, and standardized design and operation of carbon steel pipelines used in carbon dioxide transport. This standard is crucial for those developing or operating CCUS projects throughout Europe, ensuring pipeline integrity and facilitating the growth of a low-carbon infrastructure. For CO₂ transportation project success, adherence to prEN 18397 and related standards is essential.

SLOVENSKI STANDARD
01-oktober-2026
Lastnosti tokov ogljikovega dioksida v cevovodih iz ogljikovega jekla
Properties of carbon dioxide streams in carbon steel pipelines
Eigenschaften von Kohlenstoffdioxidströmen in Kohlenstoffstahlleitungen
Propriétés des flux de dioxyde de carbone dans les conduites en acier carbone
Ta slovenski standard je istoveten z: prEN 18397
ICS:
13.020.40 Onesnaževanje, nadzor nad Pollution, pollution control
onesnaževanjem in and conservation
ohranjanje
23.040.10 Železne in jeklene cevi Iron and steel pipes
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
EUROPEAN STANDARD
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2026
ICS 23.040.10
English Version
Properties of carbon dioxide streams in carbon steel
pipelines
Propriétés des flux de dioxyde de carbone dans les Eigenschaften von Kohlenstoffdioxidströmen in
conduites en acier carbone Kohlenstoffstahlleitungen
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 474.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 18397:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
Introduction . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Symbols and units . 6
5 Properties of CO streams . 7
5.1 General. 7
5.2 Thermodynamics . 7
5.3 Multiphase range . 7
5.4 Solubility of water in CO . 8
5.5 Mixing different CO streams . 8
6 Further design and operating parameters for CO stream pipelines . 8
6.1 Expansion behaviour of CO and CO streams . 8
2 2
6.2 Impact of CO stream composition on density . 9
6.3 Effects of water content . 10
6.4 Effects of selected impurities. 10
6.5 Chemical reactions . 12
6.6 Operating ranges . 12
6.7 Requirements for the CO stream to maintain line integrity from EN ISO 27913 . 12
6.8 Limit values for the CO stream . 12
Annex A (informative) Background information on CO properties . 14
A.1 Properties of pure CO . 14
A.2 Thermodynamics . 14
Annex B (informative) Limits for impurities . 20
B.1 CO minimum purity . 20
B.2 Impurities forming strong acids . 20
B.3 Glycols . 22
B.4 Amines . 23
B.5 H . 23
B.6 Total non-condensables (e.g. N , Ar, CH , O , H , CO). 23
2 4 2 2
B.7 CO . 25
Bibliography . 26

European foreword
This document (prEN 18397:2026) has been prepared by Technical Committee CEN/TC 474 “Carbon
dioxide Capture, transportation, Utilisation, and Storage (CCUS)”, the secretariat of which is held by NEN.
This document is currently submitted to the CEN Enquiry.
Introduction
This document describes the characteristics of carbon dioxide (CO ) streams and gives requirements and
recommendations for the properties of CO streams for transportation in carbon steel pipelines that have
the potential to influence pipeline integrity. It includes matters related to corrosion, ductile fracture and
thermophysical effects. It also contains information on the effects of CO streams on the design and
operation of CO pipeline transportation systems.
For the purposes of this document, a CO stream is predominantly composed of CO , which either results
2 2
from a CO2 capture process or from the purification of a CO2-rich stream, and are transported to storage
sites or for further utilization. Storage and utilization might impose additional requirements concerning
CO stream composition not covered in this document.
The different sources of CO streams result in different impurity contents, which in turn, apart from
integrity considerations, also have a major influence on the design and operation of CO transport
pipelines. In the following, the possible influences and consequences of impurities for transportation in
carbon steel pipelines are described in more detail.
The behaviour of impurities within CO streams in pipeline transport described in this document has,
where it is available, been based on experimental or operational experience.
Where this experience is not currently available, the current state of knowledge is presented and no firm
statements on limit values have been made. Reference is further made to required research. Once
appropriate experience has been gained definition of limit values is expected.
1 Scope
This document specifies requirements for the composition of CO streams relevant to the integrity of
carbon steel pipeline systems, including limits on non-condensable components. It establishes the
framework conditions for the supply, transportation and distribution of CO streams. Considerations
related to the health effects of the CO2 stream (e.g. toxicity), environmental impact, or other properties of
impurities are outside the scope of this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
prEN 18329, Measurement of CO2 streams - Sampling and analysis for pipeline transportation
EN ISO 27913:2025, Carbon dioxide capture, transportation and geological storage — Pipeline
transportation systems (ISO 27913:2024)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
3.1
carbon dioxide stream
CO stream
stream consisting overwhelmingly of CO (usually > 95 mol-%)
3.2
dense phase
CO or CO streams in the single-phase fluid state above a density of 500 kg/m
2 2
[SOURCE: EN ISO 27913:2025, 3.9]
3.3
aqueous phase
liquid water-containing phase
3.4
non-condensable
impurity which, as pure substance, cannot be liquefied by increasing the pressure at the operating
temperatures of the pipelines considered in this document
Note 1 to entry: When pure, they are in gaseous or supercritical state at possible CO2 equilibrium conditions
throughout the CO value chain. This is true for impurities with a critical temperature below the minimum operating
temperature. The presence of a non-condensable results in an increase of the bubble point pressure of a CO stream
containing this impurity.
3.5
hygroscopic impurity
chemical constituent present as impurity in a CO stream that have the ability to absorb, adsorb or retain
water from the surrounding CO phase or from trace moisture, potentially leading to the formation of an
aqueous phase and thereby affecting corrosion behaviour, hydrate formation, phase stability or the
integrity and operability of transport systems
3.6
chemically reactive impurity
chemical constituent present as impurity in a CO stream that can participate in chemical reactions under
transport conditions, potentially affecting corrosion behaviour, phase stability or the integrity and
operability of transport systems
3.7
transport system
point where the CO stream enters the inlet valve of the pipeline, where the composition, temperature
and pressure of the CO stream is within a certain specified range to meet the requirements for
transportation, until the point where the CO stream leaves the transportation pipeline infrastructure
Note 1 to entry: As described by EN ISO 27913:2025, Figure 1.
3.8
cricondenbar
maximum pressure at which a CO stream can exist in several phases
3.9
cricondentherm
maximum temperature at which a CO stream can exist in several phases
4 Symbols and units
Used symbols and units of this document are listed in Table 1.
Table 1 — Symbols and units
Symbol Characteristic Unit
P Pressure bar or bar
a g
t Temperature °C
t Temperature at critical point °C
c
ν Specific volume m /kg
a
Parts per million molar ppm-mol
Molar mass kg / mol
Specific heat capacity J / (kg K)
a
ppm-mol deviates from ppm volume at typical CO transport conditions [1]
5 Properties of CO streams
5.1 General
The behaviour of a CO stream is essentially determined by its main component CO . The behaviour of
2 2
pure CO is described further in Annex A. Since impurities can have a significant influence on the physical,
chemical and thermodynamic behaviour of CO streams, their effects on the transport of CO are
2 2
explained in more detail in this EN Standard.
CO streams are mixtures with the main component CO and other impurities, e.g. from the capture
2 2
processes. The exact composition depends on the source and the capture process used.
Impurities that can be present in addition to CO include:
— oxygen (O );
— carbon monoxide (CO);
— water (H O);
— nitrogen (N );
— particulates;
— hydrogen (H );
— sulphur and nitrogen oxides (SO und NO );
x x
— hydrogen sulfide (H S).
Other impurities can also be present which may need to be limited as well. Table 2 lists some of these as
well, along with their possible impacts. ISO/TR 27921:2020 [2] shows examples of impurities from
different capture processes.
Impurities can have effects on the phase behaviour of a CO stream, such that it can no longer be
determined solely from the properties of the pure CO . This changed phase behaviour shall be considered
when designing and operating CO pipelines. Impurities can also change other physical properties of the
CO stream, e.g.; viscosity.
In addition, certain impurities can trigger chemical reactions that form strong acids, induce or stabilize
aqueous phases causing corrosion, and can lead to the deposition of solids. These effects shall be taken
into account during both design and operation according to EN ISO 27913.
5.2 Thermodynamics
The thermodynamic properties of CO streams can differ from the properties of pure CO . Even small
2 2
amounts of impurities can lead to significant changes in the phase diagram. A suitable equation of state
shall be used, see [3,4,5].
5.3 Multiphase range
A fundamental influence of impurities is the change in phase behaviour. In the pressure-temperature
diagram, the bubble point line and the dew point line no longer coincide. The following changes shall be
taken into account:
— formation of a multi-phase area;
— the maximum pressure at which two phases can coexist shifts from the critical pressure to the
cricondenbar;
— the maximum temperature at which two phases can coexist shifts from the critical temperature to the
cricondentherm;
B.6 further illustrates the impact of impurities on phase behaviour, individually and in combination.
5.4 Solubility of water in CO
Important factors influencing solubility are pressure and temperature. In a binary system, the solubility
will decrease significantly when CO evaporates, but will rise again for constant temperature and
decreasing pressure [6].
Impurities and reaction products in the CO stream can significantly reduce water solubility, especially
polar compounds (e.g. glycols, alcohols, amines, aldehydes), salts and acids (e.g. H SO and HNO formed
2 4 3
by reaction of H S, SO , NO , O )
2 x x 2
5.5 Mixing different CO streams
When CO streams of different origins and different impurities are fed into a transport network a different
bulk composition will occur. In this situation there is a possibility that the resultant levels of impurity in
the CO stream could affect the phase behaviour and possibly also the corrosion potential. When mixing
different CO streams, possible physical changes, potential chemical reactions and those of their reaction
products shall be taken into account.
6 Further design and operating parameters for CO stream pipelines
6.1 Expansion behaviour of CO and CO streams
2 2
The expansion behaviour of liquid CO , or CO in the dense phase, differs from that of most other fluids.
2 2
During expansion the pressure of CO drops down very quickly to the saturation vapour pressure
compared to, for instance, methane (see Figure 1 or [2] as an example). This means that the scenario of
Running Ductile Fracture (RDF) will become a relevant design consideration, in many cases determining
the required wall thickness.
Key
1 decompression wave CO stream
2 decompression wave natural gas
3 fracture propagation
4 saturation vapour pressure of CO
5 running fracture
6 no running fracture
X-axis velocity
Y-axis pressure
Figure 1 — Representation of expansion behaviour of CO based on [7]
In a pressure-velocity diagram (Figure 1) this phase behaviour results in a plateau, meaning that the
internal pressure will be higher than that of methane and, crucially, higher than the pressure required to
maintain the fracture propagation. The fracture will thus propagate indefinitely, where in the case of
methane it will be arrested. The saturation vapour pressure thus becomes the determining cause for
crack growth in a pipe wall. This pressure depends strongly on the impurities of a CO stream and on the
current operating conditions at the point of CO2 release and shall therefore be taken into account when
designing the CO line. See EN ISO 27913 for design guidance on RDF, also for the case of gaseous CO .
2 2
6.2 Impact of CO2 stream composition on density
Impurities can have a significant impact on the density of a CO stream, particularly close to the critical
point. When designing and using e.g. pressurization equipment, these changes shall be considered.
6.3 Effects of water content
For CO streams, the maximum permissible water content (taking into account possible chemical
reactions with water as a product) should be defined to manage unwanted effects which include:
— Hydrate formation:
CO hydrates are solids which can block devices exposed to flow (such as safety valve stubs) and
inhibit the closure of valves. Besides the water content, hydrate formation also depends on
temperature and pressure. See Figure A.5. for details.
— Formation of an aqueous phase:
Even below the solubility limit, water can also form an aqueous phase in association with other
impurities, such as glycols. The presence of this aqueous phase will enable corrosion mechanisms.
The impurity content of the relevant compounds shall be such that aqueous phases will not form.
— Carbonic acid corrosion:
To avoid this, the water content of the CO2 stream shall be such that carbonic acid will not form.
— Strong acid corrosion:
Water is also known potentially to react with other impurities within the CO stream to form strong
acids. The level of these impurities in combination shall be specified to limit the extent of acid
formation so that acid drop-out is avoided. See B.2 for details.
The water content in the CO stream should be specified on a molar basis.
6.4 Effects of selected impurities
Table 2 shows the influences of impurities that can occur in a CO stream. Other impurities can be present
as well. Chemical reactions are not covered exhaustively, according to current knowledge reactions
mentioned may produce increased concentrations of impurities or new impurities having an effect.
Some impurities have an influence on the thermodynamic properties of the CO stream (e.g. density) and
on the fluid dynamic properties (e.g. viscosity). Furthermore, certain impurities can cause or intensify
various corrosion processes on pipe or sealing materials or promote hydrate formation. All these effects
depend on the given operating parameters, the concentration of the impurity and possibly the presence
of other impurities.
Table 2 — Possible effects of impurities in a CO stream
Impurity Effects
Ar Influence on thermophysical properties.
CH4 Hydrate formation possible, influence on thermophysical
properties.
CO Involved in a stress corrosion cracking mechanism
requiring a CO matrix and an aqueous phase.
HCl Corrosive in the presence of an aqueous phase.
HF Corrosive in the presence of an aqueous phase.
H Influence on thermophysical properties, can lead to
metallurgical problems (hydrogen induced stress corrosion
cracking, other grain boundary phenomena).
H O Promotes the formation of aqueous phases, corrosive,
hydrate formation possible.
H S Hydrate formation possible, corrosive in the presence of an
aqueous phase.
Can form solids (S ) in the presence of oxygen.
NH Can react with CO to form solid ammonium carbamate
3 2
which is hygroscopic and can form a corrosive aqueous
phase.
Can react with H SO to form solid ammonium sulphate.
2 4
NO Promotes the formation of an aqueous phase. Readily
oxidized to NO2 in the presence of an oxidizing agent like
e.g. oxygen.
NO Corrosive in the presence of an aqueous phase, promotes
the formation of an aqueous phase, strong oxidizing agent.
N Influence on thermophysical properties.
O2 Corrosion-promoting, influence on thermophysical
properties, promotes the formation of an aqueous phase,
strong oxidizing agent.
SO Corrosive in the presence of an aqueous phase, promotes
the formation of an aqueous phase.
SO More corrosive than SO2 in the presence of an aqueous
phase, promotes the formation of an aqueous phase.
Other chemically reactive Depending on the impurity and concentration level, other
impurities (e.g. formaldehyde, chemically reactive impurities can impact solid formation
methanol, carboxylic acids, and acid formation reactions. Reaction products can be
amides, amines) corrosive in the presence of an aqueous phase.
Hygroscopic impurities (e.g. Dependent on the impurity level and pipeline conditions,
glycols, alcohols, amines, condensation and subsequent formation of an aqueous
ammonium carbamate) phase can occur at different limits.
Further requirements on the composition of CO streams and their effects can be found in
EN ISO 27913:2025, 6.6.2.
6.5 Chemical reactions
The potential exists for impurities to react with each other. Notably, this includes H S, NO , SO , O and
2 x x 2
H O, which can form strong acids. These impurities should therefore be carefully controlled in CO
2 2
streams to minimize the possibility of the effects listed in Table 2.
A more detailed discussion of strong acid formation can be found in B.2.
Generally, the concentrations at which impurities undergo chemical reactions depend not only on their
individual concentrations, but also on the presence of condensed phases and on surfaces on which there
are existing corrosion products, as well as on pressure and temperature. This implies that a CO stream
specification only safeguards integrity in a specific operating window (pressure and temperature) and
pipeline internal surface conditions.
The presence of corrosion products on surfaces will influence corrosion rates, but not whether or not
corrosion takes place.
6.6 Operating ranges
CO stream pipelines can be operated in either dense or gas phase. Dense phase pipelines are likely to be
operated at pressures of up to 250 bar and with a sufficient margin above the cricondenbar, and
g
temperatures between - 10 and + 50 °C. For gas phase operation, the same temperature range applies,
while operating pressures can vary with temperature but will typically be below 40 bar .
g
A change between these two phases can lead to considerable and sudden changes in the density of the
fluid. Operating parameters close to the phase boundary should therefore be
...