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. Though inert gases have a variety of applications, they are generally used to prevent u ...

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 , bibcode = 1997JCPro...5..115A , 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.

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 a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

. Other inert gases, e.g.

argon Argon is a chemical element; it has 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 abu ...

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

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