FprCEN/TR 17924
(Main)Safety and control devices for burners and appliances burning gaseous and/or liquid fuels - Guidance on hydrogen specific aspects
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
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
kSIST-TP FprCEN/TR 17924:2022
01-november-2022
Varnostne in nadzorne naprave za gorilnike in aparate na plin in/ali tekoča goriva -
Navodilo o posebnih vidikih, značilnih za vodikSafety and control devices for burners and appliances burning gaseous and/or liquid
fuels - Guidance on hydrogen specific aspectsSicherheits- und Regeleinrichtungen für Brenner und Brennstoffgeräte für gasförmige
und/oder flüssige Brennstoffe - Leitfaden zu wasserstoffspezifischen AspektenTa slovenski standard je istoveten z: FprCEN/TR 17924
ICS:
27.060.01 Gorilniki in grelniki vode na Burners and boilers in
splošno general
kSIST-TP FprCEN/TR 17924:2022 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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kSIST-TP FprCEN/TR 17924:2022
FINAL DRAFT
TECHNICAL REPORT
FprCEN/TR 17924
RAPPORT TECHNIQUE
TECHNISCHER REPORT
September 2022
ICS
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 draft Technical Report is submitted to CEN members for Vote. 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.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 Technical Report. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a Technical Report.EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. FprCEN/TR 17924:2022 E
worldwide for CEN national Members.---------------------- Page: 3 ----------------------
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FprCEN/TR 17924:2022 (E)
Contents
Page
European foreword ...................................................................................................................................................... 4
Introduction .................................................................................................................................................................... 5
1 Scope .................................................................................................................................................................... 6
2 Normative references .................................................................................................................................... 7
3 Terms and definitions ................................................................................................................................... 8
4 Classification..................................................................................................................................................... 8
4.1 Classes of control ............................................................................................................................................ 8
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 ........................................................................................................................................................ 27
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 Consideration regarding performance ................................................................................................. 28
7.1 Leak-tightness ................................................................................................................................................ 28
7.1.1 Flow calculations .......................................................................................................................................... 28
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
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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
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FprCEN/TR 17924:2022 (E)
European foreword
This document (FprCEN/TR 17924:2022) 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.This document is currently submitted to the Vote on TR.
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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 admixture 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 the 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”, and CEN/TC 131 “Gas burners using fans”.This document up to Annex A is based on the structure of EN 13611:2019 + AC:2021, which means that
clauses and subclauses including their designations are aligned to this standard.
In this document only those clauses of EN 13611:2019+AC:2021 are referred to, which may be affected
by using hydrogen or hydrogen admixtures as gaseous fuel. All other clauses, which may be not affected,
are not listed in this document.---------------------- Page: 7 ----------------------
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1 Scope
This document is written in preparation of future revision of standards dealing with 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;
• 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 an important 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 proposal 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
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• 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 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 GG.
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.
FprEN 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 kPaFprEN 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 kPaFprEN 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 appliancesFprEN 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 processesEN 437:2021, Test gases - Test pressures - Appliance categories
EN 549:2019, Rubber materials for seals and diaphragms for gas appliances and gas equipment
FprEN 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 appliancesEN 13611:2019, Safety and control devices for burners and appliances burning gaseous and/or liquid fuels
- General requirements---------------------- Page: 9 ----------------------
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EN 14394:2005, Heating boilers — Heating boilers with forced draught burners — Nominal heat output
not exceeding 10 MW and maximum operating temperature of 110 °CISO 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 E4 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 likely 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.---------------------- Page: 10 ----------------------
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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:Cat NG: gaseous fuels of 2nd family according to EN 437:2021, or their admixtures with an overall
hydrogen content of up to and including 20 vol.-%;Cat Hy: gaseous fuels of 2nd 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:Extract of PAS 4444:2020 + A1:2021:
Table 1 — Hydrogen test gas characteristics — gas dry at 15 °C and 1 013,25 mbar
W H W H d
l l s s
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 H = 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.pdfExtract of ISO 14687:2019:
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
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
1
applications
2 Hydrogen-based fuel; high power applications
3 Gaseous hydrogen; high power/high efficiency applications
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.Grade D may be used for PEM fuel cell stationary appliances alternative to grade E category 3.
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Table 3 — Fuel quality specification for applications other than PEM fuel cell road vehicle and
stationary applicationsConstituents Type I Type II Type III
(assay) Grade A Grade B Grade C Grade C
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, %)
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 (N ) 400 µmol/mol
Helium (He) 39 µmol/mol 39 µmol/mol
e e
Carbon dioxide (CO )
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
Density
NOTE 1 NS: Not specified.
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FprCEN/TR 17924:2022 (E)
Constituents Type I Type II Type III
(assay) Grade A Grade B Grade C Grade C
The hydrogen fuel index is determined by subtracting the “total non-hydrogen gases” expressed in
molepercent, from 100 mol percent.
Combined water, oxygen, nitrogen and argon: maximum mole fraction of 1,9 %.
Combined nitrogen, water and hydrocarbon: max. 9 μmol/mol.
Combined oxygen and argon: max. 1 μmol/mol.
Total CO and CO: max. 1 μmol/mol.
To be agreed between supplier and customer.
The hydrogen shall not contain dust, sand, dirt, gums, oils, or other substances in an amount sufficient
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-cbpsFurthermore, 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 admixture.---------------------- Page: 13 ----------------------
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Table 4 — Gas properties
butane admixture
methane propane hydrogen
properties unit air (isobutane) (20 vol.-% H ,
(CH4) (C3H8) (H2)
(C4H10) 80 vol.-% CH4)
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)
3 d d d d d d
density ρ (at 15 °C) [kg/m ] 1,220 0,680 1,893 2,527 0,084 0,560
gas
relative density dV
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 Hi
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
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 W
3 f f f d d
[MJ/m ] — 50,72 76,84 87,33 45,65 48,20
(superior)
Source:
IEC 80079-20-1:2017 “Explosive atmospheres — Part 20-1: Material characteristics for gas and vapour
classification — Test methods and data”Scholten Dörr Wersky “Mögliche Beeinflussung von Bauteilen der Gasinstallation bei
Wasserstoffanwendungen”Calculation by CEN/TC 58/WG 15/PG 1, 2022–02
VDI-Wärmeatlas: 2013. 11th edition; Mason, E. A. u. S. C. Saxena: Phys. Fluids 1 (1958). 361
Günter Cerbe: “Grundlagen der Gastechnik – Gasbeschaffung – Gasverteilung – Gasverwendung”
EN 437:2021Based 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 7.2
2) breather holes and housings – see 6.2.3 and 6.3.2
3) materials – see 6.3
4) safety aspects (risk assessment – see Annexes EE and FF)
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6 General considerations regarding design and construction
6.1 Mechanical parts of the control
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 under 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 the 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).---------------------- Page: 15 ----------------------
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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 – calculation results with software “Druckverlust”
Very small pinholes 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 and CEN/TC 109 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 7.2.2).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 6.2.3.2.
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