CEN/TR 17924:2023

Safety and control devices for burners and appliances burning gaseous and/or liquid fuels - Guidance on hydrogen specific aspects

This document gives guidance on hydrogen specific safety, design, construction, and performance requirements and testing of safety, control or regulating devices (hereafter referred to as controls) for burners and appliances burning gases with hydrogen content.
The following hydrogen concentrations are covered in this document:
- H2NG (hydrogen in natural gas) blends of 20 % hydrogen; or
- 100 % hydrogen; or
- varying blends / admixtures to natural gas.
Furthermore, it identifies the expected revision needs of the existing CEN/TC 58 standards as well as the need of potential further new standardization deliverables.

Sicherheits- und Regeleinrichtungen für Brenner und Brennstoffgeräte für gasförmige und/oder flüssige Brennstoffe - Leitfaden zu wasserstoffspezifischen Aspekten

Varnostne in nadzorne naprave za gorilnike in aparate na plin in/ali tekoča goriva - Navodilo o posebnih vidikih, značilnih za vodik

Ta dokument vsebuje navodilo za varnostne, zasnovne in konstrukcijske zahteve v zvezi z vodikom ter zahteve glede zmogljivosti in preskušanja varnostnih, nadzornih ali regulacijskih naprav (v nadaljnjem besedilu: kontrolniki) za gorilnike in aparate na plin z vsebnostjo vodika.
V tem dokumentu so zajete naslednje koncentracije vodika:
– mešanice H2NG (vodik v zemeljskem plinu) z 20-odstotnim vodikom; ali
– 100-odstotni vodik; ali
– različne mešanice/dodatki v zemeljskem plinu.
Prav tako določa pričakovane potrebe glede revizije obstoječih standardov CEN/TC 58 ter potrebo po nadaljnjih novih standardizacijskih dokumentih.

General Information

• Status: Withdrawn
• Publication Date: 25-Apr-2023
• Withdrawal Date: 13-Apr-2025
• ICS: 23.060.40 - Pressure regulators
• Technical Committee: CEN/TC 58 - Safety and control devices for burners and appliances burning gaseous or liquid fuels
• Drafting Committee: CEN/TC 58/WG 11 - Generics
• Current Stage: 9960 - Withdrawal effective - Withdrawal
• Start Date: 05-Feb-2025
• Completion Date: 14-Apr-2025
• Ref Project: SIST-TP CEN/TR 17924:2023 - Safety and control devices for burners and appliances burning gaseous and/or liquid fuels - Guidance on hydrogen specific aspects

Relations

• Replaced By: CEN/TR 17924:2025 - Safety and control devices for burners and appliances burning gaseous and/or liquid fuels - Guidance on hydrogen specific aspects
• Effective Date: 19-Jul-2023

Standards Content (Sample)

SLOVENSKI STANDARD
01-julij-2023
Varnostne in nadzorne naprave za gorilnike in aparate na plin in/ali tekoča goriva -
Navodilo o posebnih vidikih, značilnih za vodik
Safety and control devices for burners and appliances burning gaseous and/or liquid
fuels - Guidance on hydrogen specific aspects
Sicherheits- und Regeleinrichtungen für Brenner und Brennstoffgeräte für gasförmige
und/oder flüssige Brennstoffe - Leitfaden zu wasserstoffspezifischen Aspekten
Ta slovenski standard je istoveten z: CEN/TR 17924:2023
ICS:
23.060.40 Tlačni regulatorji Pressure regulators
27.060.01 Gorilniki in grelniki vode na Burners and boilers in
splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TR 17924
TECHNICAL REPORT
RAPPORT TECHNIQUE
April 2023
TECHNISCHER REPORT
ICS 23.060.40
English Version
Safety and control devices for burners and appliances
burning gaseous and/or liquid fuels - Guidance on
hydrogen specific aspects
Sicherheits- und Regeleinrichtungen für Brenner und
Brennstoffgeräte für gasförmige und/oder flüssige
Brennstoffe - Leitfaden zu wasserstoffspezifischen
Aspekten
This Technical Report was approved by CEN on 9 January 2023. It has been drawn up by the Technical Committee CEN/TC 58.

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.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 17924:2023 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 7
3 Terms and definitions . 8
4 Classification. 9
4.1 Classes of control . 9
4.2 Classification of hydrogen . 9
5 Common properties . 11
6 General considerations regarding design and construction . 13
6.1 Mechanical parts of the control . 13
6.1.1 Theoretical background . 13
6.1.2 Holes . 15
6.1.3 Breather holes . 15
6.2 Materials . 25
6.2.1 General. 25
6.2.2 Housing . 26
6.2.3 Zinc alloys . 28
6.2.4 Springs . 28
6.2.5 Resistance to corrosion and surface protection . 28
6.3 Electrical parts of the control . 28
6.3.1 Electrical components . 28
7 Performance . 28
7.1 Leak-tightness . 29
7.1.1 Flow calculations . 29
7.1.2 Leakage rate measurements . 29
7.1.3 Conclusions on leakage rate measurements and calculations . 30
7.1.4 Considerations based on a risk assessment . 31
7.2 Durability . 36
7.2.1 Elastomers in contact with gas . 36
7.2.2 Lubricants in contact with gas . 36
8 Marking, instructions . 36
8.1 Instructions . 36
Annex A (informative) Modifications and/or additions to subclauses of CEN/TC 58/WG 11
standards due to introduction of Cat Hy as combustible gas . 37
Annex B (informative) Modifications and/or additions to subclauses of CEN/TC 58/WG 12
standards due to introduction of Cat Hy as combustible gas . 38
Annex C (informative) Modifications and/or additions to subclauses of CEN/TC 58/WG 13
standards due to introduction of Cat Hy as combustible gas . 41
Annex D (informative) Modifications and/or additions to subclauses of CEN/TC 58/WG 14
standards due to introduction of Cat Hy as combustible gas . 42
Annex E (informative) Risk assessment, standardization, certification and operation of gas
appliances with up to 20 vol.-% H fluctuating admixtures . 43
Annex F (informative) Risk assessment, standardization, certification and operation of gas
appliances using hydrogen (ISO 14687:2019, Type I, Grade A) . 48
Annex G (informative) Proposal for leakage rate requirements and tests for gas pipe work
including controls (e.g. valves, regulators, pressure switches) used in gas appliances (e.g.
forced draught gas-burners or industrial thermo-processing equipment) and the impact
on the installation room size . 52
Annex H (informative) Breather hole leakage rate mitigation measures . 65
Annex I (informative) Leakage rate mitigation measures . 68
Bibliography . 72
European foreword
This document (CEN/TR 17924:2023) has been prepared by Technical Committee CEN/TC 58 “Safety
and control devices for burners and appliances burning gaseous or liquid fuels”, the secretariat of which
is held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
Introduction
The use of hydrogen as a renewable fuel next to biomethane is seen as a promising alternative to natural
gas in the near future. As soon as the according regulations and standards are in force, the use of hydrogen
can be expected on a more regular basis.
For this reason, the heating and combustion business have to provide suitable solutions based on
standardized safety, construction, and performance requirements.
This document will provide a first summary of considerations regarding safety and performance aspects
for safety and control devices which will in some cases require further research and which is not
exhaustive.
There are ongoing research projects on the use of hydrogen as an admixture with natural gas in various
percentages or as hydrogen like the European THyGA project (up to 60 vol.-% hydrogen admixtures to
natural gas) which results could have an influence on these first considerations.
Therefore, this document is written in preparation of future revisions of CEN/TC 58 documents and will
describe findings pointing at potential changes, give the according research backgrounds and provide
literature sources.
The first edition of this document includes theoretical evaluations regarding different gases, comparing
their different characteristics, properties, behaviours, and their impact on the risk assessment for gas
appliances. These theoretical evaluations will be complemented by laboratory measurements, which will
be then included in a future revision of this document.
For the future implementation of hydrogen in the whole value chain co-operation with other CEN/TCs is
necessary like e.g. CEN/TC 234 “Gas infrastructure”, CEN/TC 109 “Central heating boilers using gaseous
fuels”, CEN/TC 131 “Gas burners using fans” and CEN/TC 186 “Industrial thermoprocessing -Safety”.
This document up to Annex A is based on the structure of EN 13611:2019, which means that clauses and
subclauses including their designations are aligned to this standard.
In this document only those clauses of EN 13611:2019 are referred to, which may be affected by using
hydrogen or hydrogen admixtures as gaseous fuels. All other clauses, which may be not affected, are not
listed in this document.
1 Scope
This document is written in preparation of future revision of standards dealing with the general safety,
design, construction, and performance requirements and testing of safety, control or regulating devices
(hereafter referred to as controls) for burners and appliances burning:
• H NG (hydrogen in natural gas) fluctuating admixture of no more than 20 vol.-% hydrogen content;
or
• hydrogen according to ISO 14687:2019, at least Type I, Grade A; or
• fluctuating admixtures to natural gas from 0 vol.-% to above 20 vol.-% hydrogen (e.g. 0 vol.-% to
10 vol.-% or 0 vol.-% to 40 vol.-%).
This document refers to controls with declared maximum inlet pressure up to and including 500 kPa and
of nominal connection sizes up to and including DN 250.
The purpose of this document is to provide guidance on hydrogen specific topics, which need to be
considered in the future standardization of controls covered by CEN/TC 58 documents including:
• automatic shut-off valves;
• automatic burner control systems;
• flame supervision devices;
• gas/air ratio controls;
• pressure regulators;
• manual taps;
• mechanical thermostats;
• multifunctional controls;
• pressure sensing devices;
• valve proving systems;
• automatic vent valves.
Hydrogen will play significant role in the future in several energy segments and requirements and test
methods need to be verified and adapted, if necessary.
The main target of this document is to lay the ground for defining requirements and tests for controls
used for safety related functions (e.g. safety valves, automatic burner control systems, gas/air ratio
controls) or regulating devices.
Summaries of subclauses to be addressed in the respective standards of each CEN/TC 58 WG are given in
• Annex A: Specific considerations to CEN/TC 58 WG 11 standards,
• Annex B: Specific considerations to CEN/TC 58 WG 12 standards,
• Annex C: Specific considerations to CEN/TC 58 WG 13 standards, and
• Annex D: Specific considerations to CEN/TC 58 WG 14 standards.
Aspects to be included for gas appliances (e.g. boilers, forced draught gas-burners, or industrial
thermoprocessing equipment) covering e.g. risk assessment, standardization, certification and operation
are listed in
• Annex E: Risk assessment, standardization, certification and operation of gas appliances with 20 vol.-
% H fluctuating admixtures, and
• Annex F: Risk assessment, standardization, certification and operation of gas appliances using
hydrogen (ISO 14687:2019, Type I, Grade A).
Proposals for leakage rate requirements and tests for gas pipe work including controls (e.g. valves,
regulators, pressure switches) used in gas appliances (e.g. forced draught gas-burners or industrial
thermoprocessing equipment) and the impact on the installation room size are shown in Annex G.
Considerations to be taken to stay below possible lower explosion limits in gas appliances (e.g. boilers,
forced draught gas-burners, or industrial thermoprocessing equipment) and its installation rooms are
shown in
• Annex H: Examples of mitigation measures in case of fracture of non-metallic parts for each
combustible gas to stay below 25 % of its LEL, based on calculation, and
• Annex I: Examples of mitigation measures in case of leakages for each combustible gas to stay below
25 % of its LEL, based on calculation.
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.
EN 88-1:2022, Safety and control devices for gas burners and gas burning appliances - Part 1: Pressure
regulators for inlet pressures up to and including 50 kPa
EN 88-2:2022, Safety and control devices for gas burners and gas burning appliances - Part 2: Pressure
regulators for inlet pressures above 50 kPa up to and including 500 kPa
EN 88-3:2022, Safety and control devices for gas burners and gas burning appliances - Part 3: Pressure
and/or flow rate regulators for inlet pressures up to and including 500 kPa, electronic types
EN 126:2012, Multifunctional controls for gas burning appliances
EN 161:2022, Automatic shut-off valves for gas burners and gas appliances
EN 377:1993, Lubricants for applications in appliances and associated controls using combustible gases
except those designed for use in industrial processes
EN 437:2021, Test gases - Test pressures - Appliance categories
EN 549:2019, Rubber materials for seals and diaphragms for gas appliances and gas equipment
EN 751-1:1996, Sealing materials for metallic threaded joints in contact with 1st, 2nd and 3rd family gases
and hot water - Part 1: Anaerobic jointing compounds
EN 751-2:1996, Sealing materials for metallic threaded joints in contact with 1st, 2nd and 3rd family gases
and hot water - Part 2: Non-hardening jointing compounds
EN 751-3:1996, Sealing materials for metallic threaded joints in contact with 1st, 2nd and 3rd family gases
and hot water — Part 3: Unsintered PTFE tapes
EN 1854:2022, Safety and control devices for burners and appliances burning gaseous and/or liquid fuels -
Pressure sensing devices for gas burners and gas burning appliances
EN 13611:2019, Safety and control devices for burners and appliances burning gaseous and/or liquid fuels
- General requirements
EN 14394:2005, Heating boilers — Heating boilers with forced draught burners — Nominal heat output
not exceeding 10 MW and maximum operating temperature of 110 °C
ISO 14687:2019, Hydrogen fuel quality — Product specification
EN 16726:2015+A1:2018, Gas infrastructure — Quality of gas — Group H
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 13611:2019 and the following
apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• ISO Online browsing platform: available at https://www.iso.org/obp/ui/#home
• IEC Electropedia: available at https://www.electropedia.org/
3.1
lower explosion limit
LEL
lowest concentration of the explosion range at which an explosion can occur
[SOURCE: EN 13237:2012, 3.19.1]
3.2
hydrogen
gaseous hydrogen with a purity of at least Type I, Grade A
Note 1 to entry: Purity according to ISO 14687:2019.
3.3
hydrogen admixture
hydrogen mixed with gaseous fuels in a fluctuating percentage from 0 to a maximum value
Note 1 to entry: According to EN 16726:2015+A1:2018, Annex E.

Impacted by EN 13611:2019/prA1:2021
4 Classification
4.1 Classes of control
The use of hydrogen would require the categorization based on the used concentration. There will be
fluctuations and variation in concentration which will be limited and will be described in future revisions
of EN 16726:2015+A1:2018.
There are research and considerations on the use of hydrogen as an admixture with natural gas in various
percentages or as hydrogen. The admixture up to 10 vol.-% hydrogen is already mentioned in
EN 16726:2015+A1:2018, Annex E.
An admixture of 20 vol.-% hydrogen is considered from legal authorities and many gas appliances
manufactures as a next step as well as the use of hydrogen. As a consequence, controls can be categorized
as follows:
nd
Cat NG: gaseous fuels of 2 family according to EN 437:2021, or their admixtures with an overall
hydrogen content of up to and including 20 vol.-%;
nd
Cat Hy: gaseous fuels of 2 family according to EN 437:2021, with an overall hydrogen content from 0 to
above 20 vol.-%.
4.2 Classification of hydrogen
Hydrogen gas is currently not yet defined in EN 437:2021. Based on literature hydrogen gas properties
and purity are:
Table 1 is an extract of PAS 4444:2020+A1:2021, Table 1:
Table 1 — Hydrogen test gas characteristics — gas dry at 15 °C and 1 013,25 mbar
Wl Hl Ws Hs d
Composition
Gas family Test gases Designation
by volume
3 3 3 3
MJ/m MJ/m MJ/m MJ/m
Gases of the fourth family
Reference gas G40 H2 = 99,9 38,67 10,2 45,88 12,1 0,0696
Group Y
Limit gases To be defined
Purity report from Hy4Heat:
https://static1.squarespace.com/static/5b8eae345cfd799896a803f4/t/5e58ebfc9df53f4eb31f7cf8/15
82885917781/WP2+Report+final.pdf
Table 2 is an extract of ISO 14687:2019, Table 1. Table3 is an extract of ISO 14687:2019, Table 4:
Table 2 — Hydrogen and hydrogen-based fuel classification by application
Type Grade Category Applications Clause
Gaseous hydrogen; internal combustion engines for
A — transportation; residential/commercial combustion 7
appliances (e.g. boilers, cookers and similar applications)
Gaseous hydrogen; industrial fuel for power generation
B — 7
and heat generation except PEM fuel cell applications
Gaseous hydrogen; aircraft and space-vehicle ground
C — 7
support systems except PEM fuel cell applications
I
a,b
Gas
D — Gaseous hydrogen; PEM fuel cells for road vehicles 5
E  PEM fuel cells for stationary appliances 6
Hydrogen-based fuel; high efficiency/low power
applications
2 Hydrogen-based fuel; high power applications
3 Gaseous hydrogen; high power/high efficiency applications
a
Grade D may be used for other fuel cell applications for transportation including forklifts and other industrial
trucks if agreed upon between supplier and customer.
b
Grade D may be used for PEM fuel cell stationary appliances alternative to grade E category 3.
Table 3 — Fuel quality specification for applications other than PEM fuel cell road vehicle and
stationary applications
Type II
Constituents Type I   Type III
Grade C
Grade A Grade B Grade C
a
Hydrogen fuel index
(minimum mole 98,0 % 99,90 % 99,995 % 99,995 % 99,995 %
fraction, %)
Para-hydrogen
(minimum mole NS NS NS 95,0 % 95,0 %
fraction, %)
Impurities
(maximum content)
Total gases 20 000 µmol/mol 1 000 µmol/mol 50 µmol/mol 50 µmol/mol
Non-condensing Non-condensing
Water (H20)
c c
at all ambient at all ambient
(mole fraction, %)
b
conditions conditions
Non-condensing
c c
Total hydrocarbon 100 µmol/mol at all ambient
conditions
b d d
Oxygen (O2) 100 µmol/mol
b d d
Argon (Ar)
b c c
Nitrogen (N2) 400 µmol/mol
Helium (He)   39 µmol/mol 39 µmol/mol
Type II
Constituents Type I   Type III
Grade C
Grade A Grade B Grade C
e e
Carbon dioxide (CO2)
e e
Carbon monoxide (CO) 1 µmol/mol
Mercury (Hg)  0,004 µmol/mol
Sulfur (S) 2,0 µmol/mol 10 µmol/mol
Permanent
g f f f
particulates
f
Density
NOTE 1 NS: Not specified.
a
The hydrogen fuel index is determined by subtracting the “total non-hydrogen gases” expressed in mole percent,
from 100 mol percent.
b
Combined water, oxygen, nitrogen and argon: maximum mole fraction of 1,9 %.
c
Combined nitrogen, water and hydrocarbon: max. 9 μmol/mol.
d
Combined oxygen and argon: max. 1 μmol/mol.
e
Total CO and CO: max. 1 μmol/mol.
f
To be agreed between supplier and customer.
g
The hydrogen shall not contain dust, sand, dirt, gums, oils, or other substances in an amount sufficient to damage
the fuelling station equipment or the vehicle (engine) being fuelled.
EASEE-Gas also published a common business practice about the hydrogen quality specification for
dedicated hydrogen pipelines. Link: https://easee-gas.eu/latest-cbps
Furthermore, a CEN Technical Specification for the quality of hydrogen used in converted/rededicated
gas systems is under preparation, which is proposing a minimum hydrogen concentration of 98 mol-%
within CEN/TC 234.
5 Common properties
The intention of this document is to enable the use of the controls with hydrogen and hydrogen
admixtures. Controls are already used with several fuels like biomethane or natural gas. Therefore, it is
reasonable to summarize the properties of different common fuels like methane (natural gas), propane,
and butane in comparison to hydrogen admixtures and hydrogen. Based on similarities and differences
further conclusions, consequences, risk assessments, and impacts on materials are derived in this
document.
Table 4 summarizes the properties of air (as common reference), methane (natural gas), propane, butane,
hydrogen, and 20 vol.-% hydrogen admixtures.
Table 4 — Gas properties
admixture
butane
methane propane hydrogen (20 vol.-% H ,
properties unit air (isobutane)
(CH4) (C3H8) (H2) 80 vol.-%
(C4H10)
CH )
lower explosion limit
a a a a b
[%] — 4,4 1,7 1,3 4,0 4,2
LEL (20 °C)
flammability
a a a a c
[°C] — 595 445 460 560 588
temperature (air)
admixture
butane
methane propane hydrogen (20 vol.-% H ,
properties unit air (isobutane)
(CH4) (C3H8) (H2) 80 vol.-%
(C H )
4 10
CH )
3 d d d d d d
density ρgas (at 15 °C) [kg/m ] 1,220 0,680 1,893 2,527 0,084 0,560
relative density d
V
f f f d d
— 1 0,555 1,550 2,075 0,069 0,457
related to air
dynamic viscosity η
d d d d d d
[kg⋅m/s] 17,97 E-6 10,87 E-6 7,95 E-6 7,32 E-6 8,65 E-6 10,98 E-6
(at 15 °C)
3 3 e e e e c
minimum air supply AS [m air]/[m gas] — 9,52 23,80 30,94 2,38 8,12
calorific value H
i
3 f f f e c
(inferior) (at 15 °C; [MJ/ m ] — 34,02 88.00 116,09 10,22 29,27
101,3 kPa)
calorific value H
s
3 f f f d d
(superior) (at 15 °C; [MJ/m ] — 37,78 95,65 125,81 11,97 32,56
101,3 kPa)
Wobbe Index Wi
3 f f f d d
[MJ/m ] — 45,67 70,69 80,58 38,62 43,24
(inferior)
Wobbe Index Ws
3 f f f d d
[MJ/m ] — 50,72 76,84 87,33 45,65 48,20
(superior)
Source:
a
IEC 80079-20-1:2017 “Explosive atmospheres — Part 20-1: Material characteristics for gas and vapour
classification — Test methods and data”
b
Scholten Dörr Wersky “Mögliche Beeinflussung von Bauteilen der Gasinstallation bei
Wasserstoffanwendungen”
c
Calculation by CEN/TC 58/WG 15/PG 1, 2022–02
d
VDI-Wärmeatlas: 2013. 11th edition; Mason, E. A. u. S. C. Saxena: Phys. Fluids 1 (1958). 361
e
Günter Cerbe: “Grundlagen der Gastechnik – Gasbeschaffung – Gasverteilung – Gasverwendung”
f
EN 437:2021
Based on the common properties and research the following aspects demand further considerations with
respect to subclauses of EN 13611:2019 + AC:2021:
1) leak-tightness – see 7.1
2) breather holes and housings – see 6.1.3 and 6.2.2
3) materials – see 6.2
4) safety aspects (risk assessment – see Annexes E and F)
6 General considerations regarding design and construction
6.1 Mechanical parts of the control
NOTE 6.1 Refers to EN 13611:2019 , 6.2
6.1.1 Theoretical background
To avoid too much damping on the regulator, breather holes need to have a certain minimum size. That
is, the flow may turn from laminar to turbulent, because breather hole sizes are bigger than
internal/external leakage hole sizes.
Figure 1 — Leakage models by theory
Figure 1 summarizes the flow characteristics which need to be taken into consideration if leakage of
gaseous fuel is to be expected.
Depending on the design and the leakage rate a molecular, laminar, or turbulent flow of the gaseous fuel
have to be differentiated.
Based on the common configuration of a control a molecular flow can be excluded.
Also calculations in the area between a molecular and laminar flow showed that the “Knudsen flow” is
not relevant (see also Figure 2).

Impacted by EN 13611:2019/prA1:2021
Key
X pinhole diameter (mm)
Y air flow rate (cm /h)
Figure 2 — Pinhole calculations derived from leakage rate measurements at ΔP = 15 kPa
Justification that Knudsen flow is not applicable is supported by calculation results.
Very small pinholes are needed for EN 13611:2019+AC:2021 and EN 15502-1:2021 limits:
• ~ 35 μm for 40 cm /h at ΔP = 15 kPa air;
• ~ 50 μm for 140 cm /h at ΔP = 15 kPa air.
Calculation with Knudsen number:
The diameter of a pinhole leakage rate needs to be:
• < 230 nm for pure molecular flow type;
• > 11,5 μm for pure laminar flow type.
Conclusion: 35 μm > 11,5 μm, therefore, molecular flow and Knudsen flow should not be relevant.
The internal and external leakage rates are limited by the CEN/TC 58 CEN/TC 109, CEN/TC 186, and
CEN/TC 235 standards. A summary of these values is given in Table 5. Based on the design of the control
(the leakage path is assumed to have a greater length than cross-section) a laminar flow can be assumed
and was also confirmed by initial measurements (see also 7.1.2).
For breather holes, where the leakage rate limit value in case of fracture of a diaphragm is 70 dm /h of
air, and for housings, where the leakage rate limit value in case of fracture of non-metallic parts is
30 dm /h of air, turbulent flow can be assumed according to the model given in 6.1.3.2.
Table 5 — Summary of internal and external leakage rate requirements in current standard
editions
Internal leakage rate at External leakage rate at
Parameter
1,5 ⋅ p /minimum 1,5 ⋅ p /minimum
max max
Test criteria Nominal inlet size
15 kPa 15 kPa
Unit
3 3
[cm /h] [cm /h]
DN < 10 ≤ 20 ≤ 20
EN 13611:2019 + AC:2021
10 ≤ DN ≤ 25 ≤ 40 ≤ 40
Safety and control devices
for burners and appliances
25 < DN ≤ 80 ≤ 60
burning gaseous and/or
liquid fuels — General
80 < DN ≤ 150 ≤ 100 ≤ 60
requirements
150 < DN ≤ 250 ≤ 150
EN 126:2012 DN < 10 EN 13611 + AC:2021 ≤ 60
Multifunctional controls
10 ≤ DN EN 13611 + AC:2021 ≤ 120
for gas burning appliances
EN EN 161:2022; EN 88-1:2022; EN 88-2:2022; EN 88-3:2022; EN 1854:2022;
refer to the requirements and tests of EN 13611:2019 .
EN 15502-1:2021 Gas-
fired heating boilers — ≤ 140 (at 5 kPa or 15 kPa
— —
Part 1: General only)
requirements and tests
6.1.2 Holes
NOTE 6.1.2 refers to EN 13611:2019 , 6.2.2.
There are no specific considerations to holes.
6.1.3 Breather holes
NOTE 6.1.3 refers to EN 13611:2019 , 6.2.3.
6.1.3.1 Alternative requirements for breather holes
For breather holes the leakage rate limit values in case of diaphragm fracture is 70 dm /h of air, and
turbulent flow was confirmed by measurements (see 6.2.3.3).
In case of transition flow, where the flow rates are between the values of
• laminar flow according the flow model in 7.2.1, and
• turbulent flow according the flow model in 6.2.3.2,
the leakage rate limit values are to be calculated according to the turbulent flow model as given in 6.2.3.2.
This should cover the higher leakage rate values to stay on the safe side.
Table 6 shows the breather hole and housing leakage rate limits (referring to air as test gas), which are
only to be considered if a diaphragm fracture or fracture of non-metallic parts has to be assumed as
failure, and the possibility to use an alternative requirement and test.
Table 6 — Breather hole and housings leakage rates referring to air as test gas
Parameter Scope limit of standards Housing at Breather hole at
Test criteria p p p
max max max
3 3
Unit kPa dm /h dm /h
EN 13611 + AC:2021: General up to and including 500 30 70
EN 126:2012: Multifunctional
up to and including 50 — —
controls
EN 161:2022: Automatic
up to and including 500 EN 13611 + AC:2021 —
shut-off valves
EN 13611 + AC:2021 or
EN 88-1:2022: Pressure
up to and including 50 EN 13611 + AC:2021 alternative requirement
regulators, pneumatic type
and test
EN 88-2:2022: Pressure above 50 up to and
EN 13611 + AC:2021 EN 13611 + AC:2021
regulators, pneumatic type including 500
EN 13611 + AC:2021 or
EN 88-3:2022: Pressure
up to and including 500 EN 13611 + AC:2021 alternative requirement
regulators, electronic type
and test
EN 13611 + AC:2021 or EN 13611 + AC:2021 or
EN 1854:2022: Pressure
up to and including 500 alternative requirement alternative requirement
switches
and test and test
If all controls used in applications fulfil the alternative requirements and tests according to
• EN 88-1:2022, 6.2.3, or
• EN 88-3:2022, 6.2.3, or
• EN 1854:2022, 6.2.3 and 6.3.2,
the leakage rate requirements and tests are the same as given in Table 5.
In this case there is no need to apply the deduced considerations given in 6.1.3.2 to 6.1.3.5 concerning
breather holes.
6.1.3.2 Turbulent flow model and calculations
For a turbulent flow characteristic, the Bernoulli equation is applicable:

V
ρ
leak, i
i
∆p=ξ×


2 A
leak

where
∆p is the pressure difference between two pressure zones;
ξ is the pressure drop coefficient;
ρ is the density of the gas;
is the type of gas;
i

V is the volume flow rate of the leak;
leak
A is the dimension of the leak.
leak
This leads to:

V
air,leak

V =
gas,leak, i
d
V,gas,i
where
is the relative density (see Table 4).
d
V
Table 7 shows the calculation of leakage rates according to the turbulent flow model.
Table 7 — Calculation of leakage rates and its relations for different combustible gases in case of
diaphragm fracture or fracture of non-metallic housing parts
20 vol.-%
methane propane butane hydrogen
air H2
(CH4) (C3H8) (C4H10) (H2)
admixture
density ρ at 15 °C [kg/m ] 1,220 0,680 1,893 2,527 0,084 0,560
gas
breather hole volume
ρ
air
=
flow rate factor related 1 1,34 0,80 0,69 3,81 1,48
ρ
to air
gas
breather hole volume
ρ
methane
=
flow rate factor related — 1 0,60 0,52 2,85 1,10
ρ
to methane
gas
lower explosion limit (LEL) [%] — 4,4 1,7 1,3 4 4,2
LEL factor considered
LEL
methane
=
as an additional — 1,00 2,59 3,38 1,10 1,05
LEL
hazard/safety factor
gas
breather hole gas

ρ LEL
methane methane
1/⋅
leakage rate factor — 100 64 57 32 87

ρ LEL
related to methane [%]
gas gas

related to methane, the limit values for breather hole reduced reduced reduced reduced by
— —
leakage rates measured with air should be (see Table 6): by 36 % by 43 % by 68 % 13 %
NOTE The values do not consider the result of application risk assessments, which can lead to lower leakage
rates.
6.1.3.3 Measurements and calculations
The laboratory measurement data will be included in a future revision of this document. Table 8 shows
calculations of leakage rate ratios which will be completed by the laboratory measurements.
=
Table 8 — Measurements and calculations of leakage rate ratios for different combustible gases,
related to methane, in case of diaphragm fracture
minimum required air
breather hole gas time to reach 25 % of exchange flow rate
time to reach 1 m
leakage rate BGL to be the LEL in a volume of BACmin for methane to
methane emitted
considered 1 m stay below 25 % of LEL
combustible gas
in a volume of 1 m
3 3
[dm /h] [h] [h] [m /h]
3 3 3 3
at 70 dm /h air at 70 dm /h air at 70 dm /h air at 70 dm /h air
measurement calculation measurement calculation measurement calculation measurement calculation
methane (CH4)  94  11  0,117  8,55
minimum required air
time ratio to be
breather hole gas time ratio to be exchange rate ratio to
combustible gas
considered to reach
leakage ratio to be considered to reach be considered to stay
ratios compared to
25 % of the LEL in a
considered 1 m emitted gas below 25 % of LEL in a
values of methane 3
volume of 1 m
volume of 1 m
above
[–] [–] [–] [–]
measurement calculation measurement calculation measurement calculation measurement calculation
propane (C H )/
3 8
0,60  1,67  0,645  1,55
methane
butane (C4H10)/
0,52  1,93  0,570  1,76
methane
hydrogen (H2)/
2,85  0,35  0,320  3,13
methane
20 vol.-%
hydrogen
1,10  0,91  0,866  1,15
admixture /
methane
6.1.3.4 Conclusions on leakage rate measurements and calculations in case of diaphragm
fracture
The conclusions based on the flow rate calculations are preliminary, because laboratory measurements
are to be conducted:
• initial measurements confirmed a turbulent flow according to the Bernoulli equation;
• the measured and calculated values for hydrogen are about 3 times higher than the limit values for
methane;
• for 20 vol.-% hydrogen admixtures the values are about 20 % higher.
For Cat NG and Cat Hy the requirements should stay at 70 dm /h, and a risk assessment for the
application in the real foreseeable use should be conducted. Examples are given in 6.1.3.5.
Limit values will be discussed after laboratory measurement results are available.
The higher leakage rate values for propane and butane need to be considered (see Table 7) in a risk
assessment.
6.1.3.5 Considerations based on a risk assessment
6.1.3.5.1 Design – failure mode conditions
The existing design configurations have been reviewed under failure mode conditions as shown in
Figure 3. Under these configurations a leakage to the environment would be possible and would lead to
the following consequences.
a) Gas circuit pressurized during operation b) Gas circuit pressurized permanently
only (configuration A) (configuration B)
Under a failure two different leakage scenarios Under a failure two different leakage scenarios
occur: occur:
A.1 Damage of a diaphragm with one closed B.1 Damage of a diaphragm → leakage to
upstream gas safety shut-off valve → no leakage environment
A.2 Damage of a diaphragm with open upstream gas B.2 Damage of a diaphragm with open downstream
safety shut-off valve→ leakage to environment and gas safety shut-off valve → leakage to environment
flow to combustion chamber and flow to combustion chamber
Key
V automatic shut-off valve (EN 161:2022)
M non-metallic housing part (EN 13611:2019 + AC:2021)
R regulator, where a diaphragm rupture leads to external or internal leakage
(EN 88-1:2022, EN 88-3:2022)
S pressure sensing device (EN 1854:2022)
Figure 3 — Example of risk scenarios in case of diaphragm fracture or fracture of non-metallic
parts
Since in case of diaphragm fracture or fracture of non-metallic parts a certain amount of gas can leak to
the environment (see Table 8 and Table 14), it is important to consider the accumulated gas within a
defined room volume and time. Therefore, the LEL values of each combustible gas are considered.
Two scenarios for different combustible gases were considered:
• calculation of minimum required air exchange rate BAC in case of diaphragm fracture or fracture
min
of non-metallic parts for each combustible gas to stay below 25 % of its LEL at certain leakage rate
limits (see Table 9);
• calculation of minimum required room volume BVmin in case of diaphragm fracture or fracture of non-
metallic parts at a typical air exchange rate of 0,3 1/h for each combustible gas to stay below 25 % of
its LEL at certain leakage rate limits (see Table 10).
Table 9 — Calculation of minimum required breather hole air exchange rate BAC for each
min
combustible gas to stay below 25 % of its LEL at certain leakage rate limits
minimum
required air
breather hole gas
breather hole time to reach 1 m time to reach exchange flow
leakage rate BGL
leakage rate with combustible gas of the emitted 25 % of the LEL in rate BAC to stay
min
calculated with
air fuel a volume of 1 m below 25 % of
Bernoulli
LEL in a volume of
1 m
3 3 3
[dm /h] [dm /h] [h] [h] [m /h]
methane (CH ) 1,3 747 8,212 0,12
propane (C3H8) 0,8 1246 5,294 0,19
butane (C H ) 0,7 1439 4,677 0,21
4 10
hydrogen (H ) 3,8 262 2,624 0,38
20 vol.-% H2
1,5 678 7,114 0,14
admixture
methane (CH ) 40 25 0,274 3,65
propane (C3H8) 24 42 0,176 5,67
butane (C4H10) 21 48 0,156 6,41
hydrogen (H ) 114 9 0,087 11,43
20 vol.-% H2
44 23 0,237 4,22
admixture
methane (CH ) 94 11 0,117 8,5
propane (C H ) 56 18 0,076 13,2
3 8
butane (C4H10) 49 21 0,067 15,0
hydrogen (H2) 267 4 0,037 26,7
20 vol.-% H
103 10 0,102 9,8
admixture
For Table 9 the worst case approach consists of:
• no air exchange;
• appliance is in stand-by → only external leakage rate is considered for a gas circuit under permanent
pressure;
• internal leakage rate is not connected to the room and therefore of no concern.
For each combustible gas the time to reach 25 % of an explosive concentration in a certain room volume
can be calculated.
Also for each combustible gas the minimum required breather hole air exchange rate BAC to stay below
min
25 % of an explosive concentration can be calculated (~ 1/room size).
Table 10 — Calculation of minimum required room volume BV at a typical air exchange rate of
min
0,3 1/h for each combustible gas to stay below 25 % of its LEL at certain leakage rate limits
breather hole gas
minimum required
breather hole leakage leakage rate BGL minimum required
air exchange flow rate
combustible gas
rate with air calculated with volume BVmin
BAC
min
Bernoulli
3 3 3 3
[dm /h]  [dm /h] [m /h] [m ]
methane (CH ) 1,3 0,12 0,41
propane (C H ) 0,8 0,19 0,63
3 8
1 butane (C4H10) 0,7 0,21 0,71
hydrogen (H2) 3,8 0,38 1,27
20 vol.-% H admixture 1,5 0,14 0,47
methane (CH ) 40 4 12
propane (C3H8) 24 6 19
30 butane (C4H10) 21 6 21
hydrogen (H ) 114 11 38
20 vol.-% H2 admixture 44 4 14
methane (CH ) 94 9 28
propane (C3H8) 56 13 44
70 butane (C H ) 49 15 50
4 10
hydrogen (H ) 267 27 89
20 vol.-% H2 admixture 103 10 33
The gas leakage rate BGL caused by diaphragm fracture or fracture of non-metallic parts is calculated
gas
based on the Bernoulli equation (turbulent flow).
ρ
air
BGL = A ·
gas
ρ
gas
where
A is the leakage rate with air;
are the different gas densities.
ρ , ρ
air gas
The minimum required air exchange rate BAC is
min
BGL
gas
BAC =
min
S·LEL
gas
The minimum required volume BV is
min
BAC
min
BV =
min
typical air exchange rate
Depending on the room size and the air exchange rate a safety margin BSM can be calculated.
BSM= room size /BV
min
The safety margin indicates the difference between the gas concentration caused by the leakage rate and
the safety factor of 25 % of its LEL.
6.1.3.5.2 Application specific requirements in case of diaphragm fracture or fracture of non-
metallic parts
Another risk approach in case of diaphragm fracture or fracture of non-metallic parts can be derived by
calculating a minimum required d
...