CEN/TR 15281:2022
(Main)Potentially explosive atmospheres - Explosion prevention and protection - Guidance on inerting for the prevention of explosions
Potentially explosive atmospheres - Explosion prevention and protection - Guidance on inerting for the prevention of explosions
Inerting is a preventive measure to avoid explosions or fire to happen. By feeding inert gas into a system, which is to be protected against an explosion or a fire, the oxygen content is reduced below a certain limit or completely replaced by an inert gas, depending on the inert gas, on the fuel and the process until no explosion or fire can occur or develop.
Inerting can be used to prevent fire and explosion by reducing the O2 content.
NOTE Inerting can also be used to prevent and to extinguish smouldering nests and glowing fires which are a primary source of ignition in pulverized fuel storage and handling facilities, substituting air by sufficient inert gas inside the equipment.
The following cases are not covered by the guideline:
- admixture of an inert solid powder to a combustible dust;
- inerting of flammable atmospheres by wire mesh flame traps in open spaces of vessels and tanks;
- firefighting;
- avoiding an explosive atmosphere by exceeding the upper explosion limit of a flammable substance;
- anything related to product quality (oxidation or ingress of humidity) or product losses;
- any explosive atmosphere caused by other oxidizing agents than oxygen.
Other technologies might be used in combination with inerting such as floating screens made of independent collaborative floaters consisting of an array of small floaters non-mechanically linked but overlapping each other in order to form a continuous layer covering the liquid surface.
Product oxidation or evaporation reduction is directly proportional to the surface area covering ratio and quality of the inerting.
Explosionsfähige Atmosphären - Explosionsschutz - Leitsätze für die Inertisierung zum Explosionsschutz
Inertisieren ist eine Maßnahme zur Vermeidung von Explosionen oder Bränden. Durch das Einbringen eines Inertgases in eine gegen Explosionen oder Brände zu schützende Anlage wird der Sauerstoffgehalt unter eine bestimmte Konzentration gesenkt oder je nach Inertgas, Brennstoff und Prozess vollständig durch ein Inertgas gesetzt, bis keine Explosion und kein Brand mehr möglich ist.
Inertisieren kann ebenfalls eingesetzt werden, um Brände und Explosionen durch die Reduzierung des O2-Gehalts zu verhindern.
ANMERKUNG Inertisieren kann außerdem angewendet werden, um Glutnester und Schwelbrände, die Hauptzündquellen in Pulverbrennstofflagern und -handhabungseinrichtungen sind, durch das Ersetzen der Luft durch eine ausreichende Menge an Inertgas in der Einrichtung zu verhindern.
Die folgenden Fälle sind durch die Leitsätze nicht abgedeckt:
festes Inertpulver wird einem brennbaren Staub beigemischt;
Verhinderung der Explosion einer explosionsfähigen Atmosphäre durch flammenlöschende Drahtgewebe in offenen Räumen von Behältern oder Tanks;
Brandbekämpfung;
Vermeiden einer explosionsfähigen Atmosphäre durch Überschreiten der oberen Explosionsgrenze eines brennbaren Stoffes;
alles in Verbindung mit der Produktqualität (Oxidation oder Eindringen von Feuchte) oder Produktverluste;
alle durch andere oxidierende Wirkstoffe, als Sauerstoff, verursachten explosionsfähigen Atmosphären.
Andere Technologien könnten in Kombination mit Inertisieren verwendet werden, wie etwa schwimmende Abschirmungen aus unabhängigen kollaborativen Schwimmern, die aus einer Anordnung kleiner Schwimmer bestehen, die nicht mechanisch verbunden sind, aber sich gegenseitig überlappen, um eine durchgängige Schicht zu bilden, die die Oberfläche der Flüssigkeit abdeckt.
Die Oxidation des Produkts oder Reduktion durch Verdampfung ist direkt proportional mit dem Oberflächen-Abdeckungsverhältnis und der Qualität des Inertisierens verbunden.
Atmosphères explosibles - Prévention des explosions et protection contre celles ci - Guide de l’inertage pour la prévention des explosions
L’inertage est une mesure préventive visant à éviter les explosions ou les incendies. En introduisant un gaz inerte dans un système qui doit être protégé contre une explosion ou un incendie, la teneur en oxygène est réduite en dessous d’une certaine limite ou complètement remplacée par un gaz inerte, en fonction du gaz inerte, du combustible et du processus de telle manière qu’aucune explosion ou qu’aucun incendie ne puisse se produire ou se développer.
L’inertage peut être utilisé pour prévenir les incendies et les explosions en réduisant la teneur en O2.
NOTE L’inertage peut également être utilisé pour prévenir et éteindre les nids couvants et les feux incandescents qui sont la principale source d’inflammation dans les installations de stockage et de manutention de combustible pulvérisé, en remplaçant l’air par suffisamment de gaz inerte à l’intérieur de l’équipement.
Les cas suivants ne sont pas couverts par ces lignes directrices :
— adjonction de poudre solide inerte à une poussière combustible ;
— inertage d’atmosphères explosibles par des arrête-flammes de treillis métallique dans les espaces libres des cuves et des réservoirs ;
— lutte contre l’incendie ;
— prévention d’une atmosphère explosible par le dépassement de la limite supérieure d’explosivité d’une substance inflammable ;
— tout ce qui concerne la qualité des produits (oxydation ou pénétration d’humidité) ou les pertes de produits ;
— toute atmosphère explosible causée par des agents oxydants autres que l’oxygène.
D’autres technologies peuvent être utilisées en combinaison avec l’inertage, telles que les écrans flottants constitués de flotteurs collaboratifs indépendants, qui consistent en un ensemble de petits flotteurs non reliés mécaniquement mais qui se chevauchent les uns les autres afin de former une couche continue couvrant la surface du liquide.
L’oxydation du produit ou la réduction de l’évaporation est directement proportionnelle au taux de couverture de la surface et à la qualité de l’inertage.
Potencialno eksplozivna atmosfera - Preprečevanje eksplozij in zaščita - Vodilo o inertizaciji za preprečitev eksplozij
Inertizacija je ukrep za preprečevanje eksplozij. Z dovajanjem inertnega plina v sistem, ki ga je treba zaščititi pred eksplozijo, vsebnost kisika pade pod določeno koncentracijo, pri kateri eksplozija ni več mogoča. Le v redkih primerih je treba dodati zadostno količino inertnega plina, da postane zmes z zrakom negorljiva (absolutna inertizacija). Obravnavane so zahteve za absolutno inertnost. Z inertizacijo je mogoče vplivati tudi na lastnosti eksplozivne atmosfere pri vžigu in eksploziji.
Vodilo za inertizacijo velja tudi za preprečevanje eksplozije v primeru požara.
To vodilo ne zajema naslednjih primerov:
– mešanica inertnega prahu z vnetljivim prahom;
– inertizacija vnetljive atmosfere z lovilci plamena iz žične mreže v odprtih prostorih plovil in rezervoarjev;
– gašenje požarov;
– preprečevanje eksplozivne atmosfere s preseženo zgornjo mejo eksplozije vnetljive snovi.
Inertizacija, ki zadostuje za preprečitev eksplozije, ni zaščitni ukrep za preprečevanje požarov, samovžiga, eksotermnih reakcij ali deflagracije prašnih plasti in usedlin.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-januar-2023
Nadomešča:
SIST-TP CEN/TR 15281:2006
Potencialno eksplozivna atmosfera - Preprečevanje eksplozij in zaščita - Vodilo o
inertizaciji za preprečitev eksplozij
Potentially explosive atmospheres - Explosion prevention and protection - Guidance on
inerting for the prevention of explosions
Atmosphères explosibles - Prévention des explosions et protection contre celles ci -
Guide de l’inertage pour la prévention des explosions
Ta slovenski standard je istoveten z: CEN/TR 15281:2022
ICS:
13.230 Varstvo pred eksplozijo Explosion protection
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
CEN/TR 15281
TECHNICAL REPORT
RAPPORT TECHNIQUE
October 2022
TECHNISCHER REPORT
ICS 13.230 Supersedes CEN/TR 15281:2006
English Version
Potentially explosive atmospheres - Explosion prevention
and protection - Guidance on inerting for the prevention of
explosions
Atmosphères explosibles - Prévention des explosions
et protection contre celles ci - Guide de l'inertage pour
la prévention des explosions
This Technical Report was approved by CEN on 9 October 2022. It has been drawn up by the Technical Committee CEN/TC 305.
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
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 15281:2022 E
worldwide for CEN national Members.
Contents Page
European foreword . 3
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 4
4 Inerting process and methods . 6
4.1 General . 6
4.2 Inerting system design and operation . 6
4.3 Establishing inert atmosphere . 8
4.4 Advanced preventive inerting (Blend inerting). 19
Annex A (informative) Formulae for pressure/vacuum-swing inerting . 38
Annex B (informative) Calculations for flow-through inerting . 41
Annex C (informative) Displacement inerting for low pressure storage tanks . 43
Annex D (informative) Prevention of diffusion of air down vent pipes . 48
Annex E (informative) Sensor technology . 50
Annex F (informative) Advanced preventive inerting method for pulverized coal grinding,
handling and storage facilities . 57
Annex G (informative) Advanced preventive inerting method applied to biomass handling and
storage facilities . 59
Bibliography . 62
European foreword
This document (CEN/TR 15281:2022) has been prepared by Technical Committee CEN/TC 305
“Potentially explosive atmospheres – Explosion prevention and protection”, the secretariat of which is
held by DIN.
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.
This document supersedes CEN/TR 15281:2006.
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.
1 Scope
Inerting is a preventive measure to avoid explosions or fire to happen. By feeding inert gas into a system,
which is to be protected against an explosion or a fire, the oxygen content is reduced below a certain limit
or completely replaced by an inert gas, depending on the inert gas, on the fuel and the process until no
explosion or fire can occur or develop.
Inerting can be used to prevent fire and explosion by reducing the O content.
NOTE Inerting can also be used to prevent and to extinguish smouldering nests and glowing fires which are a
primary source of ignition in pulverized fuel storage and handling facilities, substituting air by sufficient inert gas
inside the equipment.
The following cases are not covered by the guideline:
— admixture of an inert solid powder to a combustible dust;
— inerting of flammable atmospheres by wire mesh flame traps in open spaces of vessels and tanks;
— firefighting;
— avoiding an explosive atmosphere by exceeding the upper explosion limit of a flammable substance;
— anything related to product quality (oxidation or ingress of humidity) or product losses;
— any explosive atmosphere caused by other oxidizing agents than oxygen.
Other technologies might be used in combination with inerting such as floating screens made of
independent collaborative floaters consisting of an array of small floaters non-mechanically linked but
overlapping each other in order to form a continuous layer covering the liquid surface.
Product oxidation or evaporation reduction is directly proportional to the surface area covering ratio and
quality of the inerting.
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 13237:2012, Potentially explosive atmospheres - Terms and definitions for equipment and protective
systems intended for use in potentially explosive atmospheres
EN ISO 28300:2008, Petroleum, petrochemical and natural gas industries - Venting of atmospheric and low-
pressure storage tanks (ISO 28300:2008)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 13237:2012 and the following
apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at https://www.electropedia.org/
— ISO Online browsing platform: available at https://www.iso.org/obp
3.1
limiting oxygen concentration
LOC
maximum oxygen concentration in a mixture of a flammable substance and air and an inert gas, in which
an explosion will not occur, determined under specified test conditions
3.2
maximum allowable oxygen concentration
MAOC
maximum oxygen concentration in a mixture of a combustible substance and air and an inert gas, in which
an explosion will not occur, determined under specified test conditions
3.3
trip point
TP
defined value at which the process controller initiates a shut-down trip
3.4
set point
ST
defined value at which the process controller maintains the gas concentration
3.5
lower explosion limit
LEL
concentration of flammable gas or vapour in air, below which the mixture is not explosible
3.6
upper explosion limit
UEL
concentration of flammable gas or vapour in air above which the gas atmosphere is not explosible
3.7
inert gas
non-combustible gas which will not support combustion and does not react at all that avoid explosion to
occur mainly by reducing the oxygen-concentration in the protected space
Note 1 to entry: Inert can be argon, nitrogen, carbon dioxide or mixtures of these gases.
3.8
blanketing
replacement of air by an inert gas in an equipment in order to achieve inert conditions
3.9
blanketing regulator
pressure regulators used to introduce inert gas in an equipment to be inerted
3.10
breathing valve
pressure/vacuum valve
device to relieve the pressure or vacuum formed inside the cargo tanks by opening the valves at the
designated setting value to protect the tank from over-pressure or vacuum exceeding the design
parameters of the tanks
3.11
flame arrester
device fitted to the opening of an enclosure, or to the connecting pipe work of a system of enclosures, and
whose intended function is to allow flow but prevent the transmission of flame
3.12
back pressure regulator
device used to control/maintain gas pressure immediately upstream of its installed position
Note 1 to entry: It has the ability to maintain a near constant inlet pressure within design parameters, regardless
of pressure or flow fluctuations in other parts of the system.
3.13
smouldering nets
exothermic oxidation, without flaming, that is self-propagating, i.e. independent of the ignition source
Note 1 to entry: It might or might not be accompanied by incandescence.
3.14
Programmable Logic Control
PLC
electronic device designed for control of the logical sequence of events
4 Inerting process and methods
4.1 General
Many processes are routinely inerted to avoid the presence of explosive atmosphere by reducing O
content, when potential ignition sources can occur or become active. Inerting should not replace but
complement the control of ignition sources to reduce risk to an acceptable level.
Inerting requires design, procedure, maintenance and control to achieve its objective of reducing the risk
of explosive atmospheres and hence potential fires and explosions. Inerting may also introduce additional
risk to personnel through the creation of asphyxiating atmospheres in case of leakage of inerting gas in
the atmosphere, and further more environmental hazards due to entrained gases and dusts in exhausted
gas. Such risks should be taken into consideration during engineering and design phase of inerting
systems.
,
Inerting systems are a preventive measure and differ from firefighting systems (e.g. using liquid CO2
gaseous N or Argon or a dedicated mixture of gases to extinguish a fire) and curative explosion
protection systems (like suppression systems, explosion vents, etc.) that are used to minimize and reduce
the consequences or severity of a fire or an explosion that already happened.
4.2 Inerting system design and operation
4.2.1 General
To achieve adequate levels of risk reduction from an inerting system, certain design and appropriate
maintenance procedures should be followed depending on the selected technology described below.
When designing or increasing the automation of a plant or process, it is important to define safe operating
conditions. It is recommended to estimate safety levels.
Figure 1 — Oxygen concentrations to be observed when inerting equipment
Effectiveness of inert gas used decreases usually in the following order:
1) CO ;
2) Steam;
3) Flue gases;
4) N ;
5) Noble gases.
4.2.2 Design Features
a) The oxygen content of the inert gas supply (LOC is a function of the type of inert gas used and the
type of combustible used) and the target oxygen at the end of a purging process should be known
(for pressure/vacuum swing).
b) A suitable method for inerting should be chosen, and parameters selected (O analysis).
c) Calculation notes should be provided or the system should be commissioned to show it can reach
theoretical design.
4.2.3 Operational Features
a) Inert atmosphere is established as per design and before processing or handling of materials start.
b) System is maintained to keep oxygen levels within design parameters and safety parameters.
c) Cause of system failure should be defined and/or detected and corrective, or protective, actions
taken.
d) Personnel are protected, informed and trained on the potential risk of asphyxia, including for
operations planned after processing where the inerted system can be made safe for entry.
4.2.4 Information on inert gas to be taken into consideration
a) LOC depends on inert gas used thus variability of gas on an industrial site should be taken into
consideration.
b) Oxygen content of the inert gas itself can vary from few ppm to several percent, depending of the
source of supply.
c) For some onsite inert gas production methods, the oxygen concentration may vary with the rate of
production. This should be taken into consideration in the inerting procedures.
d) Availability of inert gas supply and emergency capacity with appropriate procedures in case of
supply failure.
e) Industrial sites that generate their own inert gas should be equipped with an emergency cryogenic
storage or compressed cylinder supply as backup.
f) Define the maximum simultaneous demands on the inert gas supply for an industrial site and define
priority of supply (process shut down or reduced supply to non-safety related inerting).
4.3 Establishing inert atmosphere
4.3.1 General
There are four basic methods of establishing an inert atmosphere. These methods are described below.
— Pressure swing inerting is a common method for process (reaction) vessels and batch production
methods. The choice of such technology depends on the pressure rating of the vessel as well as
normal practice on a site and process requirements. Pressure swing is mainly used for small steel
tanks with simple geometry that are resistant to pressure. Some vessels with complex shapes and
dead ends might be difficult to inert.
— Vacuum swing inerting is another common method for process (reaction) vessels and batch
production methods. The choice of such technology depends on the vacuum rating of the vessel as
well as practice on a site and process requirements. Vacuum swing is a preferred technology for glass
equipment that resist to elevated vacuum conditions and of complex geometry with dead ends that
need to be inerted.
— Flow through inerting is used for continuous production process or when products need to be
introduced in the process vessels during the production or for non-pressure rated vessels. Such
methods imply a circulating flow technique to avoid high consumption of inert gas, asphyxia risks for
the personnel and environmental impact.
— Liquid displacement (replacement by inert gas) is commonly used for inerting of storage vessels of
various capacities.
4.3.2 Pressure swing inerting
4.3.2.1 Principle
The pressure system is tightly closed and pressurized using an inert gas. The system is then vented to
atmosphere, and the process repeated until the required reduction in the oxygen content is achieved. The
theoretical oxygen content after a given number of pressure and relieve cycles will be integrated in the
control unit functionality. Three inerting trips or cycles are generally used to achieve an acceptable inert
condition (to reach LOC).
Where a system is large and contains branches, the gas in the closed ends of the system will be
compressed by the inert gas, but it is unlikely to mix well. Thus, when the pressure is released, the gas
will simply expand, and the oxygen content in the branches will remain similar to that before it was
compressed. Therefore, it will be necessary to take account of this branching when calculating the final
oxygen content. Thus, the particular shape of the vessels should be considered as important information
for defining pressure swing inerting.
Where the system is very complex, a vacuum purging system may be better and ensure that there is a
homogeneous mixture.
With such a method, continuous oxygen monitoring should be used.
Where a system is operated at over pressure, any leaks will be of inert gas into the workplace. Therefore,
adequate precautions should be taken to ensure that personnel cannot be asphyxiated by any escape of
inert gas. Where systems are located in the open air, asphyxiation will only present a risk under
conditions of massive leakage. In closed workplaces, adequate ventilation has to be provided.
Key features for pressure swing inerting are shown in Table 1.
Table 1 — Key features for pressure swing inerting
Description Vessel is pressurized with inert gas to target pressure, and then vented back to
atmospheric pressure.
Pressure swing is repeated the required number of times to reduce oxygen
content to the required level (see annex for appropriate levels).
Suitable for Vessels that can withstand pressure and can be isolated and vented. Small
vessels of steel construction with simple geometry
NOT suitable for Low pressure process vessel which cannot withstand the overpressure cycles
Vessels which are difficult to seal
Large vessels of complicated geometry
Glass equipment
How to design The appropriate theoretical number of pressure cycles can be found in
Figures 2, 3 and 4 and formulae in Annex A
Piloted pressure and back pressure regulators and valves for the inerting
process. Control of oxygen content.
Batch production process
Requirements Minimum of 2 cycles
Verification during commissioning that procedure achieves targeted limit
oxygen concentration
‘adequate’ monitoring
Good practice Pressure test: included when target pressure reached (first swing only)
Benefit Allows good mixing to effectively reduce the oxygen content
Disadvantage Leaks
— Leaking material is not necessarily inert when mixed with air.
— An asphyxiating atmosphere can be formed outside vessel during leak.
Large systems with branches may have dead ends which are not inerted.
Defined Upper pressure, lower pressure (atmospheric usually), number of swings, target
parameters oxygen, oxygen in inert gas, flow rate of inert gas
4.3.2.2 Necessary swings for pressure swing inerting
Key
X pressure at peak (barg)
Y final concentration of oxygen in vessel (%)
A 2
B 3
C 4
D 5
Figure 2 — Final concentration of oxygen achieved in vessel for different numbers of pressure
swings (2 to 5) given a concentration of oxygen in the nitrogen supply of 200 ppm
Key
X pressure at peak (barg)
Y final concentration of oxygen in vessel (%)
A 2
B 3
C 4
D 5
Figure 3 — Final concentration of oxygen achieved in vessel for different numbers of pressure
swings (2 to 5) given a concentration of oxygen in the nitrogen supply of 1 %
Key
X pressure at peak (barg)
Y final concentration of oxygen in vessel (%)
A 2
B 3
C 4
D 5
Figure 4 — Final concentration of oxygen achieved in vessel for different numbers of pressure
swings (2 to 5) given a concentration of oxygen in the nitrogen supply of 3 %
4.3.3 Vacuum swing inerting
4.3.3.1 Principle
This method can be used where a vessel cannot be subjected to internal pressure, but will withstand full
vacuum. Examples in this category are glass vessels.
The procedure is similar to that for pressure swing purging, but, since the vessel is under vacuum, it is
possible that air ingress may occur, thus the system integrator needs to take that into consideration.
3 inerting trips or cycles are generally used to achieve an acceptable inert condition (to reach LOC).
Where oxygen or air is likely to leak in because the inerted system is held at a sub-atmospheric pressure,
then the oxygen concentration should be measured continuously.
For a system operating under vacuum, any leaks will allow air to enter the system and this will gradually
destroy any inert atmosphere. The ingress of air can be detected by two methods:
— The inferential method relies on the vacuum source being isolated and the rate of pressure-rise being
monitored. Thus, it is possible to estimate the maximum oxygen concentration that would occur with
time in the system at a given vacuum.
— The most efficient method to monitor the oxygen level would be to have a continuous measuring
system, which would provide adequate warning that the oxygen level in the atmosphere of the
system is rising.
Leaks may be complicated to monitor especially for large volumes, positive method of control will be
preferred. A combination of both methods (vacuum and pressure swing) can be used taking into
consideration the correct safety parameters of each method.
Key features for vacuum swing inerting are shown in Table 2.
Table 2 — Key features for vacuum swing inerting
Description Vessel is put into target vacuum. Inert gas is introduced to bring back vessel to
ambient pressure
Vacuum swing is repeated to reduce oxygen content to the required level
Suitable for Vessels that can withstand vacuum and that can be isolated. Glass equipment
NOT suitable for Low pressure storages (or non-vacuum rated vessels)
How to design The appropriate number of vacuum cycles can be found in Figures 5, 6 and 7
and formulae in Annex A
Requirements At least 2 cycles
Leak test – leak rate should not exceed 10 % of either vacuum phase or rate of
pressure rise during inert gas addition
Verification during commissioning that procedure achieves required limit
oxygen concentration
Break the vacuum with inert gas
Good practice — Vacuum phase to lowest achievable vacuum condition
— Isolate vacuum system
— Monitor pressure rise of the vacuum
Benefit Less inert gas needed than for pressure swings
Disadvantage Air is drawn in due to driving force (pressure differential)
Under vacuum conditions explosive mixtures can occur even at temperatures
well below the flash point so care should be taken
Defined Upper pressure (atmospheric; absolute), lower pressure (absolute), number of
Parameters swings, target oxygen, percentage of oxygen in inert gas
4.3.3.2 Necessary swings for vacuum swing inerting
The number of swings required can be calculated from well-established formulae or can be read from
Figure 5, Figure 6 and Figure 7 for 3 different oxygen concentrations.
A maximum 500 mbar (a) vacuum is recommended.
NOTE Using much higher degrees of vacuum is possible but can mean that it is difficult to establish the seal
sufficiently to make the pressure test meet the necessary criteria, particularly in solids handling systems where
contamination of the seal with particles can be an issue.
4.3.3.3 Charts of final theoretical concentration of oxygen given varying conditions
Key
X absolute pressure mbar(a) at lowest vacuum
Y final concentration of oxygen in vessel (%)
A 2
B 3
C 4
D 5
E 6
Figure 5 — Final concentration of oxygen achieved in vessel for different numbers of vacuum
swings (2 to 6) at different maximum levels of vacuum for nitrogen containing 200 ppm oxygen
Key
X absolute pressure mbar(a) at lowest vacuum
Y final concentration of oxygen in vessel (%)
A 2
B 3
C 4
D 5
E 6
Figure 6 — Final concentration of oxygen achieved in vessel for different numbers of vacuum
swings (2 to 6) at different maximum levels of vacuum for nitrogen containing 1 % oxygen
Key
X absolute pressure mbar(a) at lowest vacuum
Y final concentration of oxygen in vessel (%)
A 2
B 3
C 4
D 5
E 6
Figure 7 — Final concentration of oxygen achieved in vessel for different numbers of vacuum
swings (2 to 6) at different maximum levels of vacuum for nitrogen containing 3 % oxygen
4.3.4 Flow through inerting
4.3.4.1 Principle
Flow through purging assumes perfect back-mixing of the air and the inert gas in the system, i.e. the
concentration of oxygen at all points within the system is the same at any time and is the same as in the
gas leaving.
Continuous oxygen monitoring should be used in order to achieve correct flow through inerting method.
Key features for flow through inerting are shown in Table 3.
Table 3 — Key features for flow through inerting
Description Inert gas is allowed to flow through the equipment for a predetermined time to
reduce the oxygen content to the required level of LOC
Suitable for Low pressure equipment
Long thin vessels
The miscibility of the gases should be assured. The miscibility is for instance not
assured for low density inerting gases like helium
NOT suitable for Large valves and complex branched vessels as these are difficult to be purged
adequately
How to design See formulae in Annex B + use safety factors
Safety factors:
× 2 for small vessel (with no branches and diametrically opposite inlet and
outlets) and pipework based systems
× 5 for a vessel where inlet and outlet are not diametrically opposite
Requirements Nozzle entries and exit should be arranged to allow efficient inerting of the
(Should) process.
Good practice Inlet velocity and directions optimized to allow good gas mixing
(should)
Multiple inlets can help mixing for some complex geometry vessels
Benefit Simple principle
Disadvantage Poor mixing conditions; appropriate mixing conditions need to be established
Defined Vessel volume, purge gas flowrate, safety factor, duration, target oxygen,
Parameters percentage of oxygen in inert gas
4.3.4.2 Flow to provide sufficient inerting
Formulae to calculate the time needed for a given flow of inert gas to reach the target of limit oxygen
concentration can be found in Annex B.
4.3.5 Displacement inerting with liquid removal
4.3.5.1 Principle
Displacement inerting using Nitrogen is mainly used for inerting of storage vessels of different capacity.
The inerted system should be tight enough to avoid any leak of inert gas as such inerting method is
requiring a minimum pressure above atmospheric conditions. The pressure required to operate the tank
blanketing method is recommended between 5 mbar to 30 mbar. Piloted valves system based on Oxygen
Monitoring can be used to achieve the correct inerting of the system, however, it is recommended to base
such method on the liquid removal principal. Use of self-regulated mechanical devices bring a higher level
of control and safety for this inerting method. Displacement with liquid removal also called “blanketing
method” provides a permanent inert atmosphere during the storage operations.
A blanketing regulator specially designed for nitrogen is used to reduce the high pressure of the inlet pipe
to the desire pressure for inerting. The material used for construction of the blanketing regulator should
be in stainless steel as minimum. Blanketing regulator can reduce inlet pressure of 6 bars or more to
few mbar in one stage. In order to reduce nitrogen consumptions, such device should close tightly below
10 mbar. The flow capacity of the blanketing regulator needs to compensate any liquid removal and
thermal contraction of the gas volume above liquid.
Pressure regulators are fail safe equipment and should go to full opening in case of malfunction. The
maximum flow rate of the pressure regulator that mainly depends of the inlet pressure should be known
in order to be taken into consideration by the pressure safety valve or the breathing valve directly
installed on the tank.
It is necessary to use a breathing valve that is adequately sized to relieve the pressure that is the sum of
the flow rate of the filling pump, the thermal expansion of the gas volume above the liquid media and the
maximum flow rate of the pressure regulator. EN ISO 28300:2008 gives methods of calculation of
required pressure flow rate. A vacuum valve is needed in order to protect the tank from collapsing due
to shortage of Nitrogen or malfunctioning of the pressure regulator. It is sized to compensate any liquid
removal and thermal contraction of the gas volume above the liquid. EN ISO 28300:2008 gives methods
of calculation of required vacuum flow rates.
In some occasion a secondary pressure safety device might be used for specific purposes, e.g. emergency
vent or other specific requirements for tank vapour recovery systems or to direct the flow to a VOC
treatment unit. If several pressure relief devices are used, the pressure settings of such devices should be
different in order to avoid fluttering effects of the valves (rapid opening/closing of the pressure valves
that create pressure loss of the system and advanced damage of the valve seats and diaphragms).
If achievable, a minimum of 5 mbar pressure difference should be observed between the different
pressure settings of the equipment. The vacuum setting of the vacuum valve should be close to the
atmospheric pressure.
The different vacuum and pressure settings of the pressure regulators and pressure/vacuum valves with
their operating overpressure should always remain below the maximum test pressure of the tank as
defined in EN 14015:2004.
Nitrogen is a dangerous gas that present risks of anoxia. Thus, a specific care needs to be observed
concerning tightness of equipment. It is important to keep the operating pressure below 50 mbar above
atmospheric pressure when achievable to avoid leaks to atmosphere. Any tank located in a close working
place should be vented outside the building with a piped away valve.
For a pressurized system, ingress of air is not possible. Thus, only a loss of inerting pressure resulting to
the opening of the vacuum breaker used to protect the tank against damageable vacuum conditions will
allow air to enter the system and gradually destroy the inert atmosphere. The ingress of air can be
detected by two methods.
The inferential method will be the preferred solution when achievable. It relies on the pressure/vacuum
being monitored. Thus, if the tank enters into vacuum conditions, ingress of air will be possible. Another
method consists of equipping the vacuum valve with a proximity switch that will give information of valve
opening and potential air ingress into the tank. It is possible to estimate the maximum oxygen
concentration that would occur with time in the system at a given vacuum.
An alternative solution could be to monitor the oxygen level continuously, which would provide adequate
warning that the oxygen level in the atmosphere of the system is rising.
If possible, self-regulating mechanical solution will be preferred to instrumented methods. Thus, for
liquid displacement inerting, inferential control method will be preferred.
Key features for displacement inerting with liquid removal are shown in Table 4. Annex C describes a
practical application of displacement inerting for low pressure storage tanks.
Table 4 — Key features for displacement inerting with liquid removal
Description System is fully filled with liquid. Inert gas is introduced as liquid is removed at a
rate that matches the liquid discharge rate including any thermal contraction
and vacuum effects on the gas volume above liquid (cooling effects)
Suitable for Liquid storage vessels and large systems (particularly initial purge at
commissioning, or re-commissioning). Most commonly used to maintain the
inert conditions on vessel breathing or liquid removal
NOT suitable for Non tight vessels
How to design Fill liquid and replace with inert gas when emptying or due to contraction of gas
volume above liquid
Requirements Gas supply rate should match liquid emptying rate / vacuum created by gas
(Should) volume above liquid
Good practice Self-regulated mechanical devices do not need instrumentation to control the
(should) efficiency of the inerting. As the system operate to a minimum pressure above
the atmospheric pressure, pressure gauge or limit switch to check opening of
vacuum breaker is sufficient to control the effectiveness of the inerting.
Benefit Reduction of Inert gas consumption
Disadvantage Vacuum can be created if inert gas flow rate is insufficient
Defined Maximum emptying flow rate / thermal vacuum effect and gas supply flow rate
parameters
4.4 Advanced preventive inerting (Blend inerting)
4.4.1 General
Advanced preventive inerting method is an inerting practice that is activated in an emergency situation.
This method is applicable only for process, handling and storing of pulverized combustible materials.
When under normal operating conditions deviation of specific parameters announce a dangerous
situation (e.g. increase of CO-CH -O level), automatic inerting trip needs to be started in order to reduce
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the oxygen concentration to an acceptable level based on the LOC value. Due to response time and human
failure, manual inerting systems are not recommended.
The first phase of the Blending method consists of a flow through method. Once the desired O level based
on the LOC value is reach the flow of inert gas is reduced or stopped in order to maintain a safe level of
oxygen concentration.
The injection point of the inert gas shall be determined according to the geometry of the installation and
injection points should be positioned from the inlet to the outlet in order to optimize the gas dilution and
specific nozzle shape needs to be used relying on manufacturer knowledge.
1 to 2 inerting trips or cycles are generally used to achieve the inert conditions and reach the MAOC and
avoid explosive atmosphere. The time to achieve the adequate inerting is an important parameter of the
safety of the installation. The time frame to reach the safe conditions at MAOC value is defined within 30
to 60 min depending on user’s risk analysis and standards. In order to eliminate smouldering nest and
smouldering fires present in the storage, it might be necessary to empty the volume, thus foreseeing
sufficient inert gas capacity to bring this process into safe condition. The storage needs to be equipped
with fast emptying system and rate of emptying should be compensated by injection of inert gas. To
achieve this, an extra volume of inert gas equal to the full volume of the capacity should be stored on site.
2 to 3 or more inerting trips or cycles might be necessary if the aim is to try to extinguish smouldering
nests and glowing fires. A continuous process monitoring (temperature, O CO, CH ) should be operated
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during all time of the procedure in order to check effectiveness of suppression of the smouldering nests
and glowing fires. This procedure might need several hours or days to be achieved and sufficient inert
gas capacity should be foreseen. Success of extinguishing smouldering nests and glowing fires is not
guaranteed and emptying of the storage vessels needs to be foreseen with sufficient remaining inert gas
capacity to bring it into safe conditions. The storage needs to be equipped with fast emptying system and
rate of emptying should be compensated by injection of inert gas. To achieve this, an extra volume of inert
gas equal to the full volume of the capacity should be stored on site.
Continuous oxygen monitoring should be used to achieve effective inerting.
4.4.2 Inerting system technology
An inerting system is a preventative measure avoiding smouldering nests or glowing fires to become an
active source of ignition of an explosion or degenerating to an open fire by creating an inert atmosphere.
Advanced preventive inerting systems are standby systems mainly used in handling and storage facilities
of pulverized fuel and their related equipment like in silos, mills, bag house filters dryers and their
interconnected pipework. In case of a CH , CO-, O - or temperature alarm, the inerting process is initiated
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automatically by the PLC. Continuous and reliable CH , CO-, O – or temperature measuring is necessary
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to achieve correct inert condition.
The reduction of the oxygen concentration to the LOC (Limited Oxygen Concentration) value respectively
MAOC (Maximum Allowed Oxygen Concentration) implies refill of inert gas inside the inerted vessel and
permanent monitoring and control of the oxygen level. The effectiveness of advanced preventive inerting
is controlled by the monitored and measured oxygen level in the system. The maximum allowed O
concentration (MAOC) is an operational parameter that is set at 2 % to 3 % below the LOC. It is a safety
margin set below the LOC.
Advanced preventive inerting is initiated by a flow through method. The necessary inerting time and inert
gas volumes is theoretically calculated during engineering and design phase taking into consideration the
individual volume of the connected vessels and should be checked and corrected if necessary after
commissioning by practical tests based on the measured MAOC value. For individual aggregates with
variable geometrical volumes like storage silos, empty volumes of the aggregates should be taken into
consideration unless it is proven that a minimum volume of product will always remain inside the storage
equipment.
The flow through method of inerting combined to a homogeneous blending of the inert gas with the air
contained in the volumes to be inerted, halves the oxygen concentration every time a volume equal to the
gas space in the volume to be inerted is introduced. For each cycle, the oxygen concentration is again
divided by two, etc. until it reaches safe operating conditions avoiding any resulting explosive
atmosphere.
In most case, a single inerting cycle might be sufficient to reduce the oxygen concentration from 21 % to
the MAOC value at approximatively 10 % to 8 %. However, extinguishing smouldering nests or glowing
fires of combustible dusts and powders is only possible at Oxygen concentrations of max. 2 % to 3%. The
inert gas concentration and related oxygen level should be kept up over a longer period (several hours
or days) until the smouldering fire is suffocated or extinguished. Monitoring and control of effectiveness
of Advanced preventive inerting is only possible with a combination of CO/CH and O measurement.
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Single CO measurement isn’t indicating any effectiveness, since the inert gas is diluting the CO level
without giving any reliable information about the remaining oxygen concentration.
By installing the correct monitoring equipment on e.g. temperature, CO or CH level, early warning is
achieved, furthermore the Advanced preventive inerting could be activated when it is needed.
4.4.3 Advanced preventive inerting process
Emergency inerting is initiated either manually or automatically based on readings taken from sample
points positioned on the equipment being protected. In an automated system the sample points deliver a
sample of the gas within the equipment to analysers which then transmit the readings to a standalone or
a PLC for interpretation. If the readings are outside ‘safe’ parameters, then a signal is given to start the
inerting sequence. The signal opens automated valve(s) which allows gas to flow to the affected area.
These valve(s) are connected between the inert medium storage and the injection nozzles at each piece
of equipment, it distributes the flow of gas and regulates the pressure to each area.
In the case of abnormal levels of carbon monoxide (CO), methane (CH ), oxygen or heat the inerting
process is initiated automatically through a process-control system. The goal at all times is to reduce the
oxygen concentration below the MAOC so that an explosion can no longer take place. The LOC is the
highest oxygen concentration at which explosion is not possible regardless of the reactivity of dust. The
LOC is dependent on the ratio of the fuel and inert gas used and needs to be predetermined. EN 1839:2017
gives information about the determination of LOC.
Extinguishing smouldering or glowing fires is only possible at an oxygen concentration as low as 2 % to
3 %. To achieve this, the inerting process should be repeated up to three or four times depending on the
LOC when inerting is first started.
The objective is to ensure that the alarm is triggered on time. The alarm level prevents the Oxygen level
from exceeding the MAOC. Different levels of Oxygen, Carbon Monoxide and Temperature should be fixed
in the explosion protection document (according to ATEX Directive 1999/92/CE). The Alarm Level
settings are safety critical parameters that must be determined during the engineering phase and
maintained during the life of the process.
4.4.4 Advanced preventive inerting processes in practice
4.4.4.1 Storage capacity, design and flow rates of advanced preventive inerting systems
Several technologies of advanced preventive inerting systems are available. Most used are storage tank
systems for CO and N which are designed to store sufficient liquefied or compressed inert gas capacities
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for repeating a number of inerting cycles. Depending on the overall geometrical volumes of single
aggregates, advanced preventive inerting systems are designed to repeat inerting cycles 3 to 4 times, then
refilling of the tank is necessary. The storage capacity is also influenced by local infrastructure of gas
companies the delivery time for refilling the tank. Also, these aspects should be considered during the
design and engineering phase.
1) The maximum inert gas volume needed should be stored 3 to 5 times for additional reserve capacity
(Figure 8 is mainly depending on characteristics of combustible dusts and local infrastructure in
terms of emergency inert gas supply).
2) The maximum inert gas volume should be discharged within 30 min to 60 min in relation to overall
geometrical volumes of up to 3 000 m . For larger volumes longer disc
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