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.
Carefully controlled purging of air from pipelines by direct displacement with natural gas has been safely practised for many years.
A perfect purge is one in which the replacement of gas or air by a purge gas is effected entirely by displacement and only one volume of purge gas is needed.
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 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.
When welding stainless steel, titanium and other corrosion-resistant materials, creating a perfect weld environment is of the utmost importance, because the desired corrosion resistance depends on it. During welding, the heated weld seam is exposed to air and oxidizes if it is not protected. This oxidation must either be prevented or treated later. Successful welding depends in part on a combination of proper purging equipment and techniques, and these are the focus of this article.
In purging from combustible gas to air, especially when old piping is being purged, it should be remembered that purging removes only gases and volatile materials. Undetected liquid combustibles can be ignited by sparks carried back into a purged line when the line is cut. It is possible that solid combustible material remains int he lines after purging is completed and that pyrophoric or auto-ignition can take place as soon as an adequate air supply is available.
An acceptable purging medium is products of combustion produced by carefully controlled combustion of various fuels. These combustion products can be prepared in inert gas generators. These inert gas generators can be designed to produce practically perfect combustion of fuel gases or oil. Since nitrogen constitutes approximately 80% of air, the product of an inert gas generator is predominantly nitrogen.