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Purging is an inerting method commonly used in safety-critical process chambers such as reactors to eliminate oxygen and moisture when operations are started up or shut down. In other areas such as the additive manufacturing (AM) industry, it also plays a key role in maintaining the precise atmosphere composition required in powder storage cabinets and in 3D metal printer chambers.
A differentiation is made between displacement, dilution and vacuum purging. With displacement purging, an inert gas is injected into an open apparatus to displace a dangerous or harmful gas. Dilution purging involves injecting an inert gas to lower the concentration of a harmful or dangerous gas. The diluted gas, composed of the harmful gas and the inert gas, can then often be discharged to the atmosphere. In the case of vacuum purging, harmful gases are extracted with a vacuum pump and an inert gas is then fed into the evacuated apparatus. This method is particularly suited to vessels with several areas of dead space.
In refining, petrochemical and related industries, hydrocarbon and air mixtures from vessels, gas lines, transfer lines and equipment can often present an explosion hazard. Inert gases such as nitrogen provide a safe, precise way to control the atmosphere in these areas. In addition, purging eliminates the need for pumps, and thus the risk of sparks, ignition and cross-contamination with the next step in the process flow. Purging can also be deployed to control the adverse effects of oxidation during welding or additive manufacturing. The ability to measure and control the atmosphere in AM storage cabinets and printer chambers has a major impact on the final quality, structural stability and surface finish of 3D-printed parts. The precise oxygen concentration in powder cabinets and printer chambers can be carefully monitored so the air can be automatically purged to keep the atmosphere at the requisite purity level. In the food industry, purging is used to displace atmospheric gases in mixing/grinding vessels and food packaging.
What industries rely on purging?
Any industry that handles flammable gases, vapors and mixtures will have a need for purging. Typical examples include the refining and chemicals industries, which rely heavily on purging for process safety. In fine and specialty chemicals manufacturing, some process stages – such as reactions in stirring tanks – may require the manual charging of solids. Charging ports are purged with inert gases via inert gas locks to stop air/oxygen from entering the vessel along with the bulk material. Shielding gases are also used in welding. Here the weld root and adjacent parent material are purged with a shielding gas in order to protect the penetration bead from becoming contaminated by coming into contact with atmospheric gases during and after welding. The additive manufacturing industry relies on purging for ultra-precise atmosphere control. Purging is also common in the food and beverage industry, where inert gases are flushed into modified atmosphere packages and mixing/grinding vessels to purge the air.
What gases are used for purging?
In the refining and chemicals industry, nitrogen and carbon dioxide are used to safely purge potentially explosive hydrocarbon and air mixtures from vessels and pipes. In the food industry, gases such as nitrogen, carbon dioxide and argon are commonly deployed. Looking at welding, argon and nitrogen act as inert gases during root protection, with helium and carbon dioxide sometimes added to improve pitting corrosion resistance. The AM industry typically relies on nitrogen, argon and carbon dioxide plus mixtures of those gases.
Help from Linde
We provide a full-service offering for all purging applications, spanning secure gas supplies, tailored gas metering equipment, state-of-the-art nozzles plus installation and start-up support.Talk to our experts to find out more: contact us