High purity gases meet high tech demands: to optimize performance, new technologies, like laser cutting, require high purity levels and special equipment. These requirements have shifted metal fab cutting and welding from industrial to specialty gas applications.
More Stringent Requirements
Metal fabrication processes have always had certain purity levels that have been less stringent and fairly easy to accomplish. But newer technologies, like laser cutting and welding, have drastically changed these requirements. In reality, unique high-purity gas and system requirements have elevated metal fab to the realm of a hybrid market requiring a specialist on the distributor sales team.
Carbon dioxide (C[O.sub.2]) lasers used for material processing require lasing or resonator gases, purging gases and process or assist gases. The resonator, the location in which the laser beam is generated, requires extremely high purities to generate the laser beam. The primary lasing gases are nitrogen, helium and carbon dioxide. Laser manufacturers have established minimum purity levels required to allow the laser to operate to full potential.
In addition to the traditional terminology for specifying gas grade or percentage of purity, laser manufacturers have gone a step further, specifying maximum levels of specific impurities.
Considering the purity requirements, it is easy to see that while the application may be metal fabrication, the requirements are definitely high purity. General principles, common to specialty gas and high-purity metal fab applications, are now required in high-purity metal fabrication processes.
The main goal of the gas distribution system is to get the gas from the source to the use point at the same level of purity as it is at the source.
The first part of any gas distribution system is a pressure regulator or automatic switchover system. When considering a regulator for any application, it is important to consider the materials of construction, level of cleanliness and required leak integrity. The typical regulator used for traditional metal fabrication processes is a forged brass regulator with a neoprene diaphragm. While these regulators are suitable for most welding and cutting applications, they are not suitable for laser and other high tech metal fab processes.
There are several principles that must be observed in a pressure regulator to make it suitable for high-purity service. First, the leak integrity must be extremely high. Second, the materials of construction must be compatible with the gases to be used. Perhaps the most critical component of a regulator is the diaphragm.
With industrial regulators, the most common material is neoprene, a hydrocarbon-based rubber. By virtue of the structure of the material, it is naturally permeable, allowing air and moisture to permeate through the diaphragm and contaminate the gas flowing through to the balance of the system. In addition, because it is a hydrocarbon-based material, the diaphragm can off-gas hydrocarbons into the gas stream. Both conditions are undesirable. The preferred material for the diaphragm on a high-purity regulator is 316 stainless steel. Stainless steel diaphragms are non-permeable and will prevent this contamination.
Also to be considered are the methods used to make the regulator body. The first method is a forged-body regulator, which typically starts out as a slug of brass that is heated and forced under high pressure into a mold that is the basic shape of the regulator body. The raw forging is then machined to add the bonnet threads, inlet and outlet connections and threads to install the seat parts. The balance of the body is untouched. Because the surface texture of a forged body regulator is relatively rough, cleaning is difficult.
The second, more desirable method is to make the body from barstock. These bodies start out as a bar of material and all surfaces are machined. This gives the manufacturer more control of the internal surface finish allowing easier cleaning and maintenance of higher purity levels.
High purity metal fabrication processes also demand leak integrity. Leak integrity is measured in the amount of helium that can leak in or out of the completed regulator in a given period of time (cc of helium per second). Helium is used as the leak detection gas because the molecule is extremely small, easy to detect and safe to use. High-purity regulators have a minimum leak integrity rating of 1 x 1[0.sup.-8] cc/sec helium. This means that 1 c[m.sup.3] of helium (about the volume of a sugar cube) could leak into or out of the regulator every 3.5 years.
In comparison, the rating of most industrial regulators would be around 1 x 1[0.sup.-6] cc/sec of helium. In other words, the 1 c[m.sup.3] of helium would leak out every 11 days. This means that a high-purity regulator will protect the gas 100 times better than an industrial regulator.
While many laser systems have been installed using specialty gas equipment, gas use equipment specifically designed for laser applications is a wiser choice. This equipment adheres to the principles used for specialty gas equipment, but also has features that make it better suited for laser applications, such as integral purges, psi/bar pressure gauges and stainless steel compression fittings.
The principles of leak integrity apply to every component of the gas distribution system, including the most overlooked area--the piping system. In most metal fab processes, the gas is run to the application in a rubber hose because it is flexible and inexpensive--an unacceptable method for high-purity applications. The best material to use is stainless steel tubing, which should be either orbital welded or connected with instrumentation quality compression tube fittings.
A second choice for the gas piping is cleaned copper tubing. A copper system should not be brazed because of the contamination added by residual flux left in the lines. Again, compression tube fittings are the preferred connection method.
Understanding the principles of designing a high-purity system will go a long way toward success in the new hybrid market of high purity metal fabrication. A specialist on the distributor sales team can help to accomplish that. CONCOA
[Editor's Note: Special thanks to David Durkin, manager of product enhancement, CONCOA (Virginia Beach, VA) for assistance with this article.]
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Typical Laser Requirements Resonator Gas Grade Purity Pressure Helium (He) 4.5 99.995% 25-70psig Nitrogen 4.5 99.995% 25-70psig ([N.sub.2]) Carbon Dioxide 4 99.990% 25-70psig (C[O.sub.2]) Assist Gas Oxygen 3.5 99.950% 5-250psig ([O.sub.2]) Nitrogen 4.6 99.996% 110-500psig ([N.sub.2]) Air Clean/Dry Dew Point<-40[degrees]F 20-220psig Beam Purge Nitrogen 4.5 99.995% 20-80psig ([N.sub.2]) Air Clean/Dry Dew Point<-40[degrees]F 20-100psig Resonator Gas Flow Helium (He) 0.46scfh Nitrogen 0.21scfh ([N.sub.2]) Carbon Dioxide 0.04scfh (C[O.sub.2]) Assist Gas Oxygen 10-1000scfh ([O.sub.2]) Nitrogen 300-5300scfh ([N.sub.2]) Air 10-1150scfh Beam Purge Nitrogen 100-350scfh ([N.sub.2]) Air N/A Typical Maximum Impurities By Manufacturer Resonator Gases Beam Purge Manufacturer [H.sub.2]O (THC) [O.sub.2] [H.sub.2]O (THC) A <3ppm <1ppm <100ppm <5ppm <1ppm B <15ppm <10ppm <10ppm Dew Point Oil Free <-40[degrees]F C <5ppm <1ppm <3ppm N/A N/A THC = Total Hydrocarbons ppm = Parts Per Million
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|Publication:||Modern Applications News|
|Date:||Apr 1, 2006|
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