Potentially explosive atmospheres - Explosion prevention and protection - Guidance on inerting for the prevention of explosions

Inerting is a measure to prevent explosions. By feeding inert gas into a system which is to be protected against an explosion, the oxygen content is reduced below a certain concentration until no explosion can occur. The addition of sufficient inert gas to make any mixture non-flammable when mixed with air (absolute inerting) is only required in rare occasions. The requirements for absolute inerting will be discussed. Inerting may also be used to influence the ignition and explosion characteristics of an explosive atmosphere.
The guidance given on inerting is also applicable to prevent an explosion in case of a fire.
The following cases are not covered by the guideline:
-   admixture of an inert dust to a combustible dust;
-   inerting of flammable atmospheres by wire mesh flame traps in open spaces of vessels and tanks;
-   fire fighting;
-   avoiding an explosive atmosphere by exceeding the upper explosion limit of a flammable substance.
Inerting which is sufficient to prevent an explosion is not a protective measure to prevent fires, self-ignition, exothermic reactions or a deflagration of dust layers and deposits.

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, comme 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

Status
Published
Publication Date
22-Nov-2022
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
26-Oct-2022
Due Date
31-Dec-2022
Completion Date
23-Nov-2022

Relations

Buy Standard

Technical report
SIST-TP CEN/TR 15281:2023 - BARVE na PDF-str 25,26,27,60,63
English language
63 pages
sale 10% off
Preview
sale 10% off
Preview
e-Library read for
1 day

Standards Content (Sample)

SLOVENSKI STANDARD
SIST-TP CEN/TR 15281:2023
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
SIST-TP CEN/TR 15281:2023 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST-TP CEN/TR 15281:2023
---------------------- Page: 2 ----------------------
SIST-TP CEN/TR 15281:2023
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.
---------------------- Page: 3 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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

---------------------- Page: 4 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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.
---------------------- Page: 5 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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
---------------------- Page: 6 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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
defined value at which the process controller initiates a shut-down trip
3.4
set point
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
---------------------- Page: 7 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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.
---------------------- Page: 8 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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.

---------------------- Page: 9 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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).
---------------------- Page: 10 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)

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
---------------------- Page: 11 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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

---------------------- Page: 12 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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 %
---------------------- Page: 13 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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.
---------------------- Page: 14 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)

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.
---------------------- Page: 15 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)

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

---------------------- Page: 16 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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

---------------------- Page: 17 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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.
---------------------- Page: 18 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:2022 (E)
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.
---------------------- Page: 19 ----------------------
SIST-TP CEN/TR 15281:2023
CEN/TR 15281:
...

Questions, Comments and Discussion

Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.