In fire and explosion prevention engineering, purging refers to the introduction of an inert (i.e. non-combustible) purge gas into a closed system (e.g. a container or a process vessel) to prevent the formation of an ignitable atmosphere. Purging relies on the principle that a combustible (or flammable) gas is able to undergo combustion (explode) only if mixed with air in the right proportions. The flammability limits of the gas define those proportions, i.e. the ignitable range.
Assume a closed system (e.g. a container or process vessel), initially containing air, which shall be prepared for safe introduction of a flammable gas, for instance as part of a start-up procedure. The system can be flushed with an inert gas to reduce the concentration of oxygen so that when the flammable gas is admitted, an ignitable mixture cannot form. In NFPA 56,[1] this is known as purge-into-service. In combustion engineering terms, the admission of inert gas dilutes the oxygen below the limiting oxygen concentration.
Purge out of serviceEdit
Assume a closed system containing a flammable gas, which shall be prepared for safe ingress of air, for instance as part of a shut-down procedure. The system can be flushed with an inert gas to reduce the concentration of the flammable gas so that when air is introduced, an ignitable mixture cannot form. In NFPA 56[1] this is known as purge-out-of-service.
Benefits of having two purging termsEdit
It is useful with two terms for purging because purge-out-of-service requires much larger quantities of inert agent than purge-into-service.[2] The terminology of German standards[3] refers to purge-into-service as partial inerting, and purge-out-of-service as total inerting,[2] clearly indicating the difference between the two purging practices, although the choice of the term inerting, rather than purging, can be confusing,[2] see below.
Comparison with other explosion prevention practicesEdit
Prevention of accidental fires and explosions can also be achieved by controlling sources of ignition. Purging with an inert gas provides a higher degree of safety however, because the practice ensures that an ignitable mixture never forms. Purging can therefore be said to rely on primary prevention,[4] reducing the possibility of an explosion, whereas control of sources of ignition relies on secondary prevention,[4] reducing the probability of an explosion. Primary prevention is also known as inherent safety.[4]
Confusion with inertingEdit
The purge gas is inert, i.e. by definition[1] non-combustible, or more precisely, non-reactive. The most common purge gases commercially available in large quantities are nitrogen and carbon dioxide. Other inert gases, e.g. argon or helium may be used. Nitrogen and carbon dioxide are unsuitable purge gases in some applications, as these gases may undergo chemical reaction with fine dusts of certain light metals.
Because an inert purge gas is used, the purge procedure may (erroneously) be referred to as inerting in everyday language. This confusion may lead to dangerous situations. Carbon dioxide is a safe inert gas for purging. Carbon dioxide is an unsafe inert gas for inerting, as it may ignite the vapors and result in an explosion.[2]
Fighting Smoldering Fires in Silos – A Cautionary Note on Using Carbon Dioxide. Guest post at www.mydustexplosionresearch.com blog, Nov 27, 2017
ReferencesEdit
^ abcNFPA 56. Standard for Fire and Explosion Prevention During Cleaning and Purging of Flammable Gas Piping Systems. National Fire Protection Association
^ abcd Hedlund, FH (2018). "Carbon dioxide not suitable for extinguishment of smouldering silo fires: static electricity may cause silo explosion" (PDF). Biomass and Bioenergy. Elsevier. 108: 113–119. doi:10.1016/j.biombioe.2017.11.009. S2CID 33522226.
^TRBS 2152 Teil 2 / TRGS 722. Vermeidung oder Einschränkung gefährlicher explosionsfähiger Atmosphäre. Technische Regeln für Betriebssicherheit. Ausgabe: März 2012 [Part 2: Prevention or reduction of explosive atmospheres]
^ abc Ashford, NA (1997). "Industrial safety: the neglected issue in industrial ecology". J Clean Prod. Elsevier. 5 (1–2): 115–21. doi:10.1016/S0959-6526(97)00024-3. hdl:1721.1/115912.
October 23, 2023
purging, confused, with, inerting, fire, explosion, prevention, engineering, purging, refers, introduction, inert, combustible, purge, into, closed, system, container, process, vessel, prevent, formation, ignitable, atmosphere, purging, relies, principle, that. Not to be confused with Inerting gas In fire and explosion prevention engineering purging refers to the introduction of an inert i e non combustible purge gas into a closed system e g a container or a process vessel to prevent the formation of an ignitable atmosphere Purging relies on the principle that a combustible or flammable gas is able to undergo combustion explode only if mixed with air in the right proportions The flammability limits of the gas define those proportions i e the ignitable range Contents 1 Purge into service 2 Purge out of service 3 Benefits of having two purging terms 4 Comparison with other explosion prevention practices 5 Confusion with inerting 6 See also 7 External links 8 ReferencesPurge into service EditAssume a closed system e g a container or process vessel initially containing air which shall be prepared for safe introduction of a flammable gas for instance as part of a start up procedure The system can be flushed with an inert gas to reduce the concentration of oxygen so that when the flammable gas is admitted an ignitable mixture cannot form In NFPA 56 1 this is known as purge into service In combustion engineering terms the admission of inert gas dilutes the oxygen below the limiting oxygen concentration Purge out of service EditAssume a closed system containing a flammable gas which shall be prepared for safe ingress of air for instance as part of a shut down procedure The system can be flushed with an inert gas to reduce the concentration of the flammable gas so that when air is introduced an ignitable mixture cannot form In NFPA 56 1 this is known as purge out of service Benefits of having two purging terms EditIt is useful with two terms for purging because purge out of service requires much larger quantities of inert agent than purge into service 2 The terminology of German standards 3 refers to purge into service as partial inerting and purge out of service as total inerting 2 clearly indicating the difference between the two purging practices although the choice of the term inerting rather than purging can be confusing 2 see below Comparison with other explosion prevention practices EditPrevention of accidental fires and explosions can also be achieved by controlling sources of ignition Purging with an inert gas provides a higher degree of safety however because the practice ensures that an ignitable mixture never forms Purging can therefore be said to rely on primary prevention 4 reducing the possibility of an explosion whereas control of sources of ignition relies on secondary prevention 4 reducing the probability of an explosion Primary prevention is also known as inherent safety 4 Confusion with inerting EditThe purge gas is inert i e by definition 1 non combustible or more precisely non reactive The most common purge gases commercially available in large quantities are nitrogen and carbon dioxide Other inert gases e g argon or helium may be used Nitrogen and carbon dioxide are unsuitable purge gases in some applications as these gases may undergo chemical reaction with fine dusts of certain light metals Because an inert purge gas is used the purge procedure may erroneously be referred to as inerting in everyday language This confusion may lead to dangerous situations Carbon dioxide is a safe inert gas for purging Carbon dioxide is an unsafe inert gas for inerting as it may ignite the vapors and result in an explosion 2 See also EditATEX Flammability limits Limiting oxygen concentration Inerting gas External links EditFighting Smoldering Fires in Silos A Cautionary Note on Using Carbon Dioxide Guest post at www mydustexplosionresearch com blog Nov 27 2017References Edit a b c NFPA 56 Standard for Fire and Explosion Prevention During Cleaning and Purging of Flammable Gas Piping Systems National Fire Protection Association a b c d Hedlund FH 2018 Carbon dioxide not suitable for extinguishment of smouldering silo fires static electricity may cause silo explosion PDF Biomass and Bioenergy Elsevier 108 113 119 doi 10 1016 j biombioe 2017 11 009 S2CID 33522226 TRBS 2152 Teil 2 TRGS 722 Vermeidung oder Einschrankung gefahrlicher explosionsfahiger Atmosphare Technische Regeln fur Betriebssicherheit Ausgabe Marz 2012 Part 2 Prevention or reduction of explosive atmospheres a b c Ashford NA 1997 Industrial safety the neglected issue in industrial ecology J Clean Prod Elsevier 5 1 2 115 21 doi 10 1016 S0959 6526 97 00024 3 hdl 1721 1 115912 Retrieved from https en wikipedia org w index php title Purging gas amp oldid 1110808738, wikipedia, wiki, book, books, library,
