EN 746-3:1997+A1:2009

SLOVENSKI STANDARD
01-september-2009
Industrijska termoprocesna oprema - 3. del: Varnostne zahteve za pridobivanje in
uporabo atmosferskih plinov
Industrial thermoprocessing equipment - Part 3: Safety requirements for the generation
and use of atmosphere gases
Industrielle Thermoprozessanlagen - Teil 3: Sicherheitsanforderungen für die Erzeugung
und Anwendung von Schutz- und Reaktionsgasen
Equipements thermiques industriels - Partie 3: Prescriptions de sécurité pour la
génération et l'utilisation des gaz d'atmosphère
Ta slovenski standard je istoveten z: EN 746-3:1997+A1:2009
ICS:
25.180.01 ,QGXVWULMVNHSHþLQDVSORãQR Industrial furnaces in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 746-3:1997+A1
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2009
ICS 25.180.01 Supersedes EN 746-3:1997
English Version
Industrial thermoprocessing equipment - Part 3: Safety
requirements for the generation and use of atmosphere gases
Equipements thermiques industriels - Partie 3: Industrielle Thermoprozessanlagen - Teil 3:
Prescriptions de sécurité pour la génération et l'utilisation Sicherheitsanforderungen für die Erzeugung und
des gaz d'atmosphère Anwendung von Schutz- und Reaktionsgasen
This European Standard was approved by CEN on 19 March 1997 and includes Amendment 1 approved by CEN on 21 May 2009.

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. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN Management Centre or to any CEN member.

This European Standard exists 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 Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2009 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 746-3:1997+A1:2009: E
worldwide for CEN national Members.

Contents Page
Foreword .3
Introduction .5
1 Scope .5
2 Normative references .6
3 Definitions .7
4 List of hazards . 10
5 Safety requirements for the use of atmosphere gases. 16
5.1 Purging . 16
5.2 Safety control equipment for atmosphere gases . 17
5.3 Thermo-processing equipment requirements . 18
6 Safety requirements for atmosphere gas generators . 22
6.1 Reaction gas supplies . 22
6.2 Fuel gas supplies . 23
6.3 Air supplies . 23
6.4 Mixture supplies. 24
6.5 Electrical supplies . 24
6.6 Cooling water supplies and discharge . 24
6.7 Atmosphere gas supplies and distribution . 25
6.8 System requirements . 25
7 Verification of the safety requirements and/or measures . 27
8 Information for use . 27
8.1 Instruction handbook . 27
8.2 Marking . 30
Annex A (informative) Typical Atmosphere Gases . 31
Annex B (informative) Explosion Hazards of Gas Mixtures Containing Combustibles . 32
Annex C (informative) Inert Gas Purging . 34
Annex D: Bibliography . 36
Annex E (informative) Used Definitions . 37
Annex ZA (informative) !Relationship between this European Standard and the Essential
Requirements of EU Directive 98/37/EC" . 40
Annex ZB (informative) !Relationship between this European Standard and the Essential
Requirements of EU Directive 2006/42/EC" . 41

Foreword
This document (EN 746-3:1997+A1:2009) has been prepared by Technical Committee CEN/TC 186
“Industrial thermoprocessing - Safety”, the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by December 2009, and conflicting national standards shall be withdrawn
at the latest by December 2009.
This document includes Amendment 1, approved by CEN on 2009-05-21.
This document supersedes EN 746-3:1997.
The start and finish of text introduced or altered by amendment is indicated in the text by tags ! ".
The working group that drafted this Part of EN 746 comprised experts from the following countries: France,
Germany, Italy, Switzerland and the United Kingdom.
This standard forms one part of safety standards covering Industrial Thermoprocessing Equipment.
The full list of parts of this standard is given below:
EN 746 Industrial Thermoprocessing Equipment
Part 1: Common Safety Requirements for Industrial Thermoprocessing Equipment
Part 2: Safety Requirements for Combustion and Fuel Handling Systems
Part 3: Safety Requirements for the Generation and Use of Atmosphere Gases
Part 4: Particular Safety Requirements for Hot Dip Galvanising Thermoprocessing Equipment
Part 5: Particular Safety Requirements for Salt Bath Thermoprocessing Equipment
Part 6: Particular Safety Requirements for Material Melting, Remelting and Liquid Phase Maintaining
Thermoprocessing Equipment
Part 7: Particular Safety Requirements for Vacuum Thermoprocessing Equipment
Part 8: Particular Safety Requirements for Quenching Equipment
This document has been prepared under a mandate given to CEN by the European Commission and the
European Free Trade Association, and supports essential requirements of EU Directive(s).
!For relationship with EU Directive(s), see informative Annexes ZA and ZB, which are integral parts of this
document."
An assessment of the foreseeable risks arising from the use of the equipment was carried out when this
standard was prepared.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and United Kingdom.

Introduction
This standard has been prepared to be a harmonised standard to provide one means of conforming to the
Essential requirements of the Machinery Directive and associated EFTA Regulations.
The extent to which hazards are covered is indicated in the scope of this standard. In addition, machinery
shall comply as appropriate with !EN ISO 12100-1 and EN ISO 12100-2" for hazards which are not
covered by this standard.
This European Standard is a type-C standard as defined in !EN ISO 12100-1 and EN ISO 12100-2".
!EN 746-1 contains the common safety provisions for all types of industrial thermoprocessing equipment.
This part of the standard details in addition those extra safety requirements for the equipment listed in the
scope, which need special attention."
The equipment dealt with and the extent to which hazards are covered are indicated in the scope of this part
of EN 746.
Where for clarity an example of a preventative measure is given in the text, this should not be
considered as the only possible solution. Any other solution leading to the same risk reduction is
permissible if an equivalent level of safety is achieved.
This part of EN 746 assumes that the installations are operated and maintained by trained personnel.
1 Scope
This part of EN 746 specifies safety requirements for atmosphere gas systems and their use in industrial
thermo-processing equipment and associated plant, including systems for the production of atmosphere
gases by reaction inside the thermo-processing equipment.
It applies to the supply of atmosphere gases, gaseous and liquid additions to, and their removal from industrial
thermo-processing equipment and associated plant, confined to equipment integrated in the thermo-
processing and associated plant.
This part of EN 746 also details the anticipated significant hazards associated with atmosphere gas systems
and their use in industrial thermo-processing equipment and specifies the appropriate preventative measures
for the reduction or elimination of these hazards.
This part of EN 746 does not apply to atmosphere process gases, essential safety equipment, start-up,
operation and shut-down of thermo-processing plant for semi-conductor devices for which special additional
engineering requirements are necessary.
This part of EN 746 specifies the requirements to be met to ensure the safety of persons and property during
commissioning, start up, operation, shut down and maintenance, as well as in the event of foreseeable faults
or malfunctions which can occur in the equipment. It specifies the safety requirements at stages in the life of
the equipment, and its design, ordering, construction and use.
This part of EN 746 applies to equipment which is placed on the market after the date of issue of this
standard.
The hazards covered by this Part of EN 746 are listed in clause 4.
A table of typical atmosphere gases is given in Annex A.
2 Normative references
!The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies."
!EN 88-1, Pressure regulators and associated safety devices for gas appliances — Part 1: Pressure
regulators for inlet pressures up to and including 500 mbar"
EN 161, Automatic shut-off valves for gas burners and gas appliances
!deleted text"
EN 298, Automatic gas burner control systems for gas burners and gas burning appliances with or without
fans
EN 746-1, Industrial thermoprocessing equipment — Part 1: Common safety requirements for Industrial
Thermoprocessing Equipment
EN 746-2, Industrial thermoprocessing equipment — Part 2: Safety requirements for combustion and fuel
handling systems
EN 60204-1, Safety of machinery — Electrical equipment of machines — Part 1: General requirements
!(IEC 60204-1:2005, modified)"
EN 60519-1, Safety in electroheat installations — Part 1: General requirements !(IEC 60519-1:2003)"
EN 60519-2, Safety in electroheat installations — Part 2: Particular requirements for resistance heating
equipment !(IEC 660519-2:2006)"
!EN ISO 12100-1:2003, Safety of machinery — Basic concepts, general principles for design — Part 1:
Basic terminology, methodology (ISO 12100-1:2003)"
!EN ISO 12100-2:2003, Safety of machinery — Basic concepts, general principles for design — Part 2:
Technical principles (ISO 12100-2:2003)"
!IEC 60364-4-41, Low-voltage electrical installations — Part 4-41: Protection for safety — Protection
against electric shock"
!IEC 60364-43, Electrical installations of buildings — Part 4-43: Protection for safety — Protection against
overcurrent"
!deleted text"
!IEC 60364-4-44, Low-voltage electrical installations — Part 4-44: Protection for safety — Protection
against voltage disturbances and electromagnetic disturbances"
!deleted text"
!IEC 60519-3, Safety in electroheat installations — Part 3: Particular requirements for induction and
conduction heating and induction melting installations"
3 Definitions
For the purposes of this standard the following definitions apply:
NOTE An alphabetic listing of the definitions, as well as their cross-references in German, French and English are
given in informative Annex E.
3.1
industrial thermo-processing and associated equipment
Any equipment through or into which the atmosphere gases flow
NOTE This includes atmosphere gas distribution systems, safety control equipment and the furnace or other
enclosures in which atmosphere gases are used.
3.2
safe ignition temperature
the minimum temperature at which spontaneous, safe auto-ignition of flammable gases occurs
NOTE The safe ignition temperature has been established at 750 °C.
3.3
purging
the general displacement of one type of atmosphere within a thermal processing plant by another
3.4
flammable atmosphere gas
any gas mixture that is capable of forming flammable mixtures with air or oxygen under the conditions of
temperature and pressure used in the process
NOTE 1 Typically any gas mixture containing more than 5% (V/V) combustibles (H +CO+CH ) of which CH is not
2 4 4
more than 1% (V/V), where the remainder of the mixture is non-flammable, is considered to be flammable. Any gas
mixture which contains more than 1% (V/V) C H or 2,5% (V/V) or 2,5% (V/V) NH where the remainder of the mixture is
n m 3
non-flammable is also considered to be flammable (see Annexes A and B).
NOTE 2 A flammable gas which contains 1 % (V/V) or less oxygen cannot in itself form an explosive or flammable
mixture.
3.5
non-flammable atmosphere gas
any gas mixture which is not capable of forming flammable mixtures with air or oxygen under the conditions of
temperature and pressure used in the process
NOTE Typically any gas mixture containing 5% (V/V) or less of combustibles (H +CO+CH ) of which CH is not more
2 4 4
than 1% of the 5% (V/V) and the remainder is non-flammable and which will not support combustion.
3.6
inert gas
a non-flammable gas which will not support combustion and does not react to produce a flammable gas
NOTE 1 An inert gas can safely be used for pre- and post-purging of cold and hot enclosures of thermo-processing
equipment.
NOTE 2 Inert gas can be produced by appropriate adjustment of an exothermic generator. In the event of malfunction
such as gas can be flammable and provisions should be taken to ensure that adjustment is correct.
3.7
toxic atmosphere gas
a gas which, in addition to having asphyxiating properties, also acts as a poison
3.8
fluid
a liquid or a gas
NOTE If specific reference is made to "liquid" or "gas" this means the physical state of the fluid being considered.
3.9
safe level of vacuum
the absolute pressure of an evacuated furnace chamber/enclosure corresponding to safe atmosphere
conditions.
NOTE The safe level of vacuum has been established as a maximum of 45 mbar absolute (1 bar = 10 Pa).
3.10
safety purge volume
the volume of inert purge gas needed to displace either air or a flammable gas from a furnace
chamber/enclosure to achieve 1% (V/V) or less oxygen and/or a non-flammable atmosphere gas (as defined
in 3.5) and/or 25% of the lower flammability limit.
NOTE Typically this will be a volume equal to five times the volume of the thermo-processing equipment chamber to
be purged, see Annex C.
3.11
atmosphere gas generating system
equipment that converts or modifies a mixture of fluids (gaseous or liquid) into a gas which can be utilised as
the controlled atmosphere within the thermo-processing equipment.
3.12
automatic re-start
automatic repetition of the starting up sequence without manual intervention.
3.13
flame instability
undesirable variation in the size, shape and position of a flame.
3.14
flame sensor
that part of a flame safeguard or flame monitoring system that is responsive to flame properties and which
signals the presence of flame.
3.15
sensor output drift
a condition occurring within the flame sensor whereby it responds to flame characteristics outside the safety
range for which it was designed.
3.16
flame failure
loss of flame from the normally detected position by any cause other than the action of de-energising the
safety shut-off valves system (EN 746-2).
3.17
flame safeguard
a device responsive to flame properties, detecting the presence of a nominated flame and, in the event of
ignition failure or subsequent flame failure, causing safety shut-down or lock out.
It consists of a flame sensor, an amplifier and a relay for signal transmission. These parts, with the possible
exception of the actual flame sensor, may be assembled in a single housing for use in conjunction with a
programming unit [EN 746-2].
3.18
flame trap
a device capable of arresting a flame while allowing the passage of gas
3.19
start-up interlock
the safety shut-down condition of the control system such that re-start cannot be accomplished without
manual re-set.
3.20
multiturn valve
a valve which, in order to operate from the fully closed to the fully open position, requires a number of
revolutions of the operating key or handwheel to be completed.
3.21
non-return valve
a device to prevent the reversal of flow of air, fuel, oxygen etc. [746-2].
3.22
pilot flame
pilot burner flame that is used to ignite the main flame [EN 746-2].
3.23
interrupted pilot
a pilot which is ignited each time the burner is started up and which is extinguished at the end of the main
flame establishment period.
3.24
pilot shrinkage
unintended reduction in the length of the pilot flame.
3.25
safe start check
a means of providing safety shut-down or start-up interlock on start-up if a fault or flame simulating condition is
present.
3.26
safe shut-down
the shutting off of all reaction gas and reaction air supplies to the atmosphere generator.
NOTE 1 This can be accomplished either manually or automatically
NOTE 2 This should not be confused with system of safety shut-down (see 3.27).
3.27
system of safety shut-down
a system that provides the automatic shutting off of all gas and ignition energy.
3.28
safety shut-off valve system
a system of valves with associated circuits which enables the supply of gas to be admitted or shut off.
3.29
slam shut valve
a valve which automatically closes when it is actuated by an increase in the downstream line pressure above
a set limit. Manual intervention is required to re-open the valve.
3.30
start gas flame
a flame established at the start gas rate either at the main burner or at a separate pilot burner
4 List of hazards
The anticipated significant hazards are detailed in table 1. For ease of reference this table also indicates the
hazardous situations and corresponding preventative measures and should be used in conjunction with part 1
and 2 of EN 746 and clauses 5, 6 and 8 of this part of EN 746, as identified in the reference column.
Table 1 — List of Hazards, Hazardous Situations and Preventative Measures
1 2 3 4 5 6
Clause Hazards !!deleted Hazardous situation Preventative Measures References
!!
text""
""
1 MECHANICAL
1.1 General  EN 746-1
2 ELECTRICAL
2.1 General
2.2 Thermal • Breakdown/reduction of • System design EN 746-1
radiation and
electrical insulation
5.3.2.18
• Correct placement of cables
other
• Cable damage/Short
6.5
• Protection of cables
phenomena
circuits:
• Fire fighting equipment (suitable for
 Fire
electrical systems)
 Electric shock
2.3 External • Corruption of control • System design EN 746-1
influences on circuits, particularly
5.3.2.18
• Protection of hardware
electrical
software systems
6.5
• Monitoring systems
equipment
• Failure of safety control:
• First aid:
 Fire/Explosion
 Equipment
 Toxicity/Asphyxiation
 Training
• Fire fighting equipment
3 THERMAL
3.1 General
3.2 Flames, EN 746-1
• Personal injuries such • System design
Explosions,
as:
EN 746-2
• Monitoring/inspection/maintenance
Radiation
 Flash/radiation, burns
• Operator:
 Dehydration
 Instruction 6
 Eyesight damage
 Training
8.1
• deterioration of joints
• Personal protection equipment
and pipework and/or
distortion of structures:
• First aid:
 Escaping of flammable
 Equipment
or toxic fluids
 Training
(gases/liquids)
• Safety devices in the work shop
• Ejection of parts
• Fire fighting equipment
4 HAZARDS GENERATED BY MATERIALS AND SUBSTANCES PROCESSED/ USED/ EXHAUSTED
4.1 General
(continued)
Table 1: (continued)
1 2 3 4 5 6
Clause Hazards !!!!deleted Hazardous situation Preventative Measures References
text""""
4.2 • Contact • Personal injuries such as: • Regulations for storage/use/disposal EN 746-1
with
 Skin effects
5.3.1
• System design
harmful
 Poisoning
5.3.2.1
• Monitoring/inspection/maintenance
liquids
 Physiological effects
5.3.2.2
• Personal protection equipment
• Contact  Respiratory/asphyxiating
effects 5.3.2.13
with or
• First aid:
inhalation
5.3.2.14
 Equipment
of harmful
 Training
6.7.3
gases/
• Consider:
6.8.9
vapours/fu
 Permit-to-work system
mes
8.1
 Gas analysing system
4.3 Contact with or • Personal injuries such as: • Regulations for storage/use/disposal 8.1.6
inhalation of
 Skin effects
8.1.7
• Procedures for assembling, maintenance
dusts
 Respiratory effects
and dismantling
 Poisoning
• Provisions for disposal
Carcinogenic effects from
nickel-bearing dusts
5 FIRE AND EXPLOSION
5.1 General
5.2 Fire hazard • Property damage and/or EN 746-1
• System
personal injury caused by
 Design
EN 746-2
flammable fluids
 Safety systems
5.3
(gases/liquids)
• Fire fighting equipment
6.8
• Monitoring/inspection/maintenance
8.1
• Operator
 Instruction
 Training
• Plant housekeeping procedures
• First aid:
 Equipment
 Training
5.3 Explosion 5
• Property damage and/or • System
hazard personal injury caused by
 Design
flammable fluids
 Safety systems
8.1
(gases/liquids)
• Fire fighting equipment
• Monitoring/inspection/maintenance
• Operator
 Instruction
 Training
• Explosion relief devices
(continued)
Table 1: (continued)
1 2 3 4 5 6
Clause Hazards !!deleted Hazardous situation Preventative Measures References
!!
text""
""
6 HAZARDS GENERATED BY FAILURE OF SERVICES
EN 746-1
6.1 Failure of • Failure of normal control • System
energy supply:
EN 746-2
 Design
• Damage to property and
 Safety systems 5.2.4
• Electrical personnel
supplies
• Inert gas supply 5.3.2.17
• Control fluid 5.3.2.18
• Failure mode system
supplies
6.3.4.2
• Operator
• Cooling 6.5
 Instruction
system
6.6
 Training
supplies
6.8.1
• Service/repair management
6.8.9
• Consider:
8.1
 Standby energy system
6.2 Failure of • Formation of explosion • Automatic inert gas purging 5.2.4
atmosphere gas/air mixtures in
5.2.5
• Standby supply of atmosphere gas
gas supply
industrial
8.1
• Failure mode system
thermoprocessing plant
• Operator
 Instruction
 Training
• Service/repair management
6.3 Failure of 6.1.5
• Breakdown of • Safe shut-down procedure
supplies for atmosphere gas supply
6.1.6
• Automatic inert gas purging
atmosphere
6.2
• Generating plant/failure
• Failure mode system
gas generators
of normal control
6.3
• Service/repair management
6.4
8.1
7 HAZARDS GENERATED BY FUNCTIONAL DISORDERS, MISSING OR INCORRECTLY POSITIONED/ SET SAFETY
DEVICES
EN 746-1
7.1 Unexpected
• Unexpected escape of • Avoidance of/ protection from uncontrolled
escape of fluids
high temperature gases draught effects
5.1
through furnace
5.2.3
• Precautions during maintenance
openings
5.3.2.1
• Protective devices
• Flashes of burning
5.3.2.12
gases
5.3.2.13
5.3.2.14
5.3.2.15
5.3.2.16
5.3.2.17
7.2 Failure/disorder • Unexpected start-up • System design 5
of control
• Unexpected overrun • Safety shut-down procedure
systems
8.1
• Inspection/maintenance
• Operator
 Instruction
 Training
(continued)
Table 1: (continued)
1 2 3 4 5 6
Clause Hazards !!deleted Hazardous situation Preventative Measures References
!!
text""
""
7.3 Errors of fitting
• Damage to property • System design
and/or personal injuries 6
• Marking/identification of components
8.1
• Supplier information/components
8.2
• Spare parts maintenance management
• Instruction for maintenance
• Consider:
 PERMIT-TO-WORK system
7.4 Loading and EN 746-1
• Personal injuries such • System design
unloading
as:
5.1
• Sequence logic
means for
 Burns
5.2.7
• Personal protection
workpieces
 Poisoning
being 5.3.2.16
• Inspection/maintenance
• Damage to property
processed
8.1
caused by:
• Operator:
 Overheating  Instruction
 Incorrect purging  Training
 Fire
7.5 5.2
• Pressure or • Incorrect atmosphere • System design
flow control gas production or use,
5.2.6
• Monitor/inspection/maintenance
devices resulting in:
5.3.2.7
• Operator:
 Fire/explosion
• Low or high
 Instruction 6.1.1
 Poisoning
temperature
 Training
6.8
detectors
• Explosion relief devices
• Fire fighting equipment
• First aid:
 Equipment
 Training
• Consider:
 Gas analysing system
7.6 Explosion • Leakage of atmosphere • Inspection/maintenance 5.3.2.17
reliefs (if fitted) gas into the workshop,
8.1
• General ventilation of workshop
causing:
• Operator
 Fire/explosion
 Instruction
 Poisoning/asphyxiation
 Training
• Fire fighting equipment
• Consider:
 PERMIT-TO-WORK system
(continued)
Table 1: (continued)
1 2 3 4 5 6
Clause Hazards !!deleted Hazardous situation Preventative Measures References
!!
text""
""
EN 746-2
7.7 Flame failure
• Incorrect production or • System design
control (main
use of atmosphere gas, 5.1.3.3
• Self checking flame safeguard
burner(s) and
resulting in:
5.3.2.14
• Interlocks preventing sequence continuation
pilot(s))
 Unreliable
5.3.2.15
ignition/explosion • Monitoring/inspection/maintenance

5.3.2.16
 Risk of personal
• Operator
6.8.5
injuries (burns)
 Instruction
6.8.6
 Training
6.8.9
• Safety shut-down procedure
8.1
• First aid:
 Equipment
 Training
7.8 Inert purging • Incorrect purge of air or 5.1
• System design
gas supplies
atmosphere gases from
5.2.4
• Monitoring/inspection/maintenance
thermal processing
8.1
• Operator
equipment, resulting in:
 Instruction
 Fire/explosion
 Training
 Personal injuries
(burns) • Operating manual
• First aid:
 Equipment
 Training
• Consider:
 PERMIT-TO-WORK system
7.9 Starting and • Inability to operate plant 5.1
• System design
stopping
correctly causing:
5.2.4
• Start & stop logic
devices:
 Fire/explosion
8.1
• Inspection/maintenance
 Asphyxiation
• Electrical
• Operator
circuits
 Instruction
• Fluids
 Training
(gas/air/
liquids) • Operating manual
• Consider:
 PERMIT-TO-WORK system
7.10 Energy supply EN 746-1
• Inability to isolate plant: • System design
disconnecting
 Electrically 5.2
• Start and stop logic
devices:
 From fluids supply
6.2
• Inspection/maintenance
• Electrical
6.3.2
• Operator
supplies
6.5
 Instruction
• Control
 Training
8.1
fluids
• Operating manual
(gas/air/
liquids)
• Remote isolation system
• Consider:
 PERMIT-TO-WORK system
(continued)
Table 1: (concluded)
1 2 3 4 5 6
Clause Hazards !!deleted Hazardous situation Preventative Measures References
!!
text""
""
5.2
7.11 Emergency stopping
• Inability to stop plant • System design
devices
operation under 5.3
• Start and stop logic
emergency conditions
6.8
• Inspection/maintenance
8.1
• Operator
 Instruction
 Training
• Operating manual
• Remote isolation system
• Consider:
 PERMIT-TO-WORK system
7.12 Essential 5.3
• Over or under supply • System design
equipment/accessories
of fluids, due to
6.1
• Limitation of flow
for safe adjusting incorrect
6.2
• Separate low flow system
and/or maintenance of settings/pressure
fluid supplies. 6.3
variations resulting in:
• Reliable pressure control
 Fire/explosion
6.4
• Flow measurement
• Monitor/inspection/maintenance
 Toxicity/Asphyxiation
6.6
• Pressure
• Operator:
measurement
8.1
 Instruction
 Training
• Regular check of settings
• Consider:
 PERMIT-TO-WORK system
7.13 Equipment evacuating 5.3.2.13
• Increased toxicity, • System design
gases:
asphyxiation, fire,
5.3.2.14
• No interconnection of vents
explosion if:
• Natural/forced
5.3.2.15
• Monitoring/inspection/maintenance
 Insufficient
draught chimneys
5.3.2.16
evacuation of gases • Explosion relief devices
• Fume extraction
 Flammable deposits, 6.7.3
• Fire fighting equipment
ducts
vapours and ignition
6.8.3
• Consider:
• Vents
sources
6.8.4
 Gas analysing system
8.1
NOTE Effect can be
delayed in time
8 HAZARDS GENERATED BY NOISE
Health damaging effects
8.1 General EN 746-1
• System design
• Personal protective devices
9 HAZARD COMBINATIONS
9.1 General Property damage and/or 5
• System design
personal injuries caused
• Failure mode
by uncontrolled
8.1
• Safe shut-down procedures, based on
voluntary/involuntary
risk analysis
action
• Monitoring/inspection/maintenance
• Regular testing of safety systems
• Operator:
 Instruction
 Training
5 Safety requirements for the use of atmosphere gases
5.1 Purging
The following subclauses detail established process requirements for purging safety controls for atmosphere
gas and thermo-processing equipment that can be used in the design of equipment for atmosphere gases.
These techniques can be applied singly or in combination depending on the application.
5.1.1 General
Specific instructions on the procedure for admitting atmosphere gas into, and removing atmosphere gas from,
thermo-processing equipment and enclosures shall be provided by the manufacturer in accordance with the
purging method(s) adopted (see also Annexes A, B and C).
The effects of doors (operational speed and sequencing) and/or other openings on safe purging shall be taken
into account in the selection and use of purging procedures.
5.1.2 Purging during start-up
The following purging methods shall be used, either singly or in combination.
The normal start-up of thermo-processing equipment or enclosure which uses a flammable atmosphere gas
involves replacing the air with the flammable atmosphere gas.
In some cases evacuation or an intermediate purge with inert gas can be necessary.
5.1.2.1 Purge without ignition
If a flammable atmosphere gas is introduced into thermo-processing equipment or enclosures which operate
below 750 °C, then its introduction shall be preceded by an inert gas purge to remove air until the oxygen
content is 1 % (V/V) or less.
NOTE 1 If the atmosphere gas supply source can be adjusted to provide a non-flammable atmosphere gas, this gas
can be utilized as the purge inert gas.
NOTE 2 The inert gas purge can be omitted provided that the air has been evacuated to a predetermined safe level of
vacuum (see 3.9).
5.1.2.2 Purge with spontaneous ignition
If a flammable atmosphere gas is introduced into a furnace zone where the temperature is maintained at
750 °C or more then, providing that ignition occurs immediately upon the introduction of the flammable
atmosphere gas, purging of the furnace chamber may continue until all the air has been consumed and/or
displaced by the incoming flammable atmosphere gas.
5.1.2.3 Purge with induced ignition
If the thermo-processing equipment zone or enclosure into which the flammable atmosphere gas is introduced
is at a temperature below 750 °C, purging without the use of inert gas is only permitted if a reliable source of
ignition is provided at the position where the flammable atmosphere gas is introduced.
This method of purging shall not be used when atmosphere gas circulation fans are operating in the zone or
enclosure.
5.1.3 Purging during normal shut-down
The following purging methods shall be used, either singly or in combination.
The normal shut-down of a furnace or enclosure containing a flammable atmosphere gas can involve
replacing the flammable atmosphere gas with air.
In some cases evacuation or an intermediate purge with inert gas can be necessary.
5.1.3.1 Purge without ignition
Prior to shutting off the supply of flammable atmosphere gas to a furnace or enclosure operating below 750 °C
an inert gas purge shall be initiated and the flow adjusted to the required value. The flammable atmosphere
gas supply valve shall then be closed and the inert gas purge continued until all the flammable atmosphere
gas has been displaced. The inert gas can then be replaced by air.
NOTE 1 If the atmosphere gas supply source can be adjusted to provide a non flammable atmosphere gas this gas
can be utilised as the inert gas purge (5.1.3.2).
NOTE 2 The inert gas purge volume can be reduced to a backfill requirement, i.e. sufficient to raise the pressure to the
prevailing atmospheric value, provided that the flammable atmosphere gas has been evacuated to a predetermined safe
level of vacuum (see 3.9)
5.1.3.2 Purge with spontaneous ignition
If it is required to purge a flammable atmosphere gas from thermo-processing equipment which is maintained
above 750 °C, then providing that the flammable atmosphere gas will burn safely, the flammable atmosphere
gas supply valve can be closed, thus allowing the flammable atmosphere gas within the thermo-processing
equipment to be removed by combustion.
NOTE Where furnace doors are fitted, they should be opened before the flammable atmosphere gas supply valve is
closed.
5.1.3.3 Purge with induced ignition
If it is required to purge flammable atmosphere gas from a furnace or enclosure which is maintained at a
temperature below 750 °C purging without the use of inert gas shall be allowed only if a reliable source of
ignition is provided at every position where air is admitted and combustion of the flammable atmosphere gas
is maintained.
Where furnace or enclosure doors are fitted they shall be opened fully before the flammable atmosphere gas
supply valve is closed.
Provided that the flammable atmosphere gas is burning safely the flammable atmosphere gas supply valve
may be closed, allowing the flammable atmosphere gas within the thermo-processing equipment or enclosure
to be removed by combustion.
This method of purging shall not be used when furnace atmosphere gas circulation fans are operating in the
zone or enclosure.
5.2 Safety control equipment for atmosphere gases
5.2.1 Manual shut-off valve
Manual shut-off valves shall be fitted in the atmosphere gas supply and in the gaseous/liquid additions
supplies to each furnace or point of use.
5.2.2 Automatic shut-off valve
If the supply of atmosphere gas or gaseous/liquid additions to the furnace is under the control of an electrically
operated automatic shut-off valve (e.g. as described in 5.2.4 and 5.2.6), this valve shall comply with EN 161.
5.2.3 Safety interlocks
If an automatic shut-off valve is used it shall be provided with safety interlocks (e.g. for temperature, pressure,
supplies) to prevent the atmosphere gas, including any gaseous/liquid additions, from being admitted to the
furnace if this could give rise to an unsafe condition.
5.2.4 Emergency inert gas supply
If the atmosphere gas used in the furnace is flammable and if interruption of its supply could cause a hazard,
a supply of inert gas of sufficient capacity to effect a safety purge (see 3.10) shall be provided and its
availability shall be proved before the atmosphere gas is admitted.
Loss of an adequate supply of inert gas at any time shall be indicated by means of a suitable alarm.
5.2.5 Atmosphere gas supply sensor
If safety is dependent upon the atmosphere gas flow rate then a device shall be fitted to prove that an
adequate supply of that atmosphere gas is available. If, for any reason, the supply of atmosphere gas or of
gaseous/liquid fluids for furnace-reacted atmospheres, is not sufficient, an alarm condition shall be indicated.
In the case of a flammable atmosphere gas, closure of the atmosphere gas supply valve shall be preceded by,
or be simultaneous with, the admittance of inert gas, or the furnace or enclosure doors shall be opened, where
appropriate, to prevent any hazardous situation arising.
5.2.6 Temperature sensor
A temperature sensor, interlock and alarm shall be fitted where the safe use of thermo-processing equipment
in which a flammable atmosphere gas is used is dependent upon the operating temperature being maintained
above a safe ignition temperature. This system shall prevent the introduction of flammable atmosphere gas
before the safe ignition temperature has been reached and shall cause an alarm condition if the temperature
subsequently falls below that safe value. If this situation is not corrected within a predetermined time, it shall
be followed by:
 Admittance of inert gas or the opening of thermo-processing equipment doors to prevent any hazardous
situation arising; and
 Closure of the flammable atmosphere gas supply valve(s).
5.2.7 Processing chamber door interlocks
Door interlocks (e.g. Ignition flame door gas curtain) shall be fitted where the safe use of thermo-processing
equipment in which a flammable atmosphere gas is used is dependent upon the automatic operation of
processing chamber doors.
5.3 Thermo-processing equipment requirements
5.3.1 Construction
The following requirements are in addition to those detailed in EN 746-1.
Atmosphere gases shall only be used in thermo-processing equipment that has been constructed to avoid the
leakage of either flammable or toxic gases into the working environment at any point other than at vents or
other exits designed for the purpose.
5.3.2 Distribution system for atmosphere gases, including fluid (gaseous or liquid) additions
5.3.2.1 General
All pipework, valves and controls shall be designed, installed and marked in accordance with EN 746-2.
All pipework shall be installed and jointed in such a manner as to be gas tight and shall be securely supported.
In addition pipework shall be located where mechanical, thermal or corrosion damage cannot occur, or shall
be suitably protected.
The system shall be designed to prevent any pipework or control equipment being subjected to pressures in
excess of the maximum intended pressure.
5.3.2.2 Purging
All distribution pipework and storage vessels for atmosphere gas shall be purged using procedures in
accordance with 5.1.2.1 or in such a manner which excludes the simultaneous presence of flammable gas/air
mixtures and an ignition source. Each venting point shall be provided with a valve which shall either:
 Be fitted with a device to prevent unauthorised operation; or
 Be blanked off during normal operation of the plant.
5.3.2.3 Drainage points
If drainage points are provided to be prevent the accumulation of moisture in pipework they shall either:
 Be fitted with a valve incorporating a device designed to prevent unauthorised operation;
or
 Be sealed during normal operation of the plant.
5.3.2.4 Flexible pipework
If flexible gas pipes are necessary they shall be suitable for the duty intended and be of the metal or metal
armoured type having flanged or screwed connections. They shall be kept
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