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.

Purge into service

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 An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...

to reduce the concentration of oxygen so that when the flammable gas is admitted, an ignitable mixture cannot form. In NFPA 56,NFPA 56. Standard for Fire and Explosion Prevention During Cleaning and Purging of Flammable Gas Piping Systems. National Fire Protection Association 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

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

to reduce the concentration of the flammable gas so that when air is introduced, an ignitable mixture cannot form. In NFPA 56 this is known as ''purge-out-of-service''.

Benefits of having two purging terms

It is useful with two terms for purging because purge-out-of-service requires much larger quantities of inert agent than purge-into-service. The terminology of German standardsTRBS 2152 Teil 2 / TRGS 722. Vermeidung oder Einschränkung gefährlicher explosionsfähiger Atmosphäre. Technische Regeln für Betriebssicherheit. Ausgabe: März 2012 art 2: Prevention or reduction of explosive atmospheres/ref> refers to purge-into-service as ''partial inerting'', and purge-out-of-service as ''total inerting'', clearly indicating the difference between the two purging practices, although the choice of the term ''inerting'', rather than ''purging'', can be confusing, see below.

Comparison with other explosion prevention practices

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, {{cite journal , last1 = Ashford , first1 = NA , title = Industrial safety: the neglected issue in industrial ecology , journal = J Clean Prod , volume = 5 , issue = 1–2 , pages = 115–21 , publisher = Elsevier , date = 1997 , url = , doi = 10.1016/S0959-6526(97)00024-3 , hdl = 1721.1/115912 , hdl-access = free reducing the possibility of an explosion, whereas control of sources of ignition relies on secondary prevention, reducing the probability of an explosion. Primary prevention is also known as

inherent safety In the chemical industry, chemical and process industries, a process has inherent safety if it has a low level of danger even if things go wrong. Inherent safety contrasts with other processes where a high degree of hazard is controlled by protect ...

Confusion with inerting

The purge gas is inert, i.e. by definition non-combustible, or more precisely, ''non-reactive''. The most common purge gases commercially available in large quantities are

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

. Other inert gases, e.g.

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

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.

External links

* Fighting Smoldering Fires in Silos – A Cautionary Note on Using Carbon Dioxide. Guest post at
www.mydustexplosionresearch.com blog
Nov 27, 2017

References

Explosion protection Fire Safety