Figure 3. Vaporizing pressure
Glass-coated stainless steel often is an attractive option for highly reactive samples such as multiple-component reduced sulfur samples that are part of many facilities’ EPA-mandated pollution-monitoring requirements. The coating process imparts inertness and corrosion resistance to the substrate; 316L stainless is the most widely treated material but the process suits most stainless steels. Besides tubing, many components including valves, pressure control regulators and various other parts can be coated.
The maximum allowable working pressure of the material selected should significantly exceed the maximum expected operating pressure — 20% is a reasonable minimum safety factor but 200% is a desirable target. With most wall thicknesses of 300-series stainless steels, this generally isn’t an issue as their working pressure ratings can be as high as 6,000 psig. However, with copper the typical installation is limited to 250 psig.
Installation is the last major issue. Use only qualified and certified professionals. The procedures and testing required to install and certify a high-purity or hazardous process line are somewhat unique. Where practical, compression-type tube fittings speed installation and obviate purging the piping during installation. However, welded connections sometimes are used for higher leak integrity. Any welding or brazing operation demands special care and purging the piping to prevent oxide and contaminant formation. High-purity argon is needed to shield the weld area and continuously purge the piping interior during and until the weld area has cooled; nitrogen typically is the purge gas when brazing copper lines. Thoroughly leak-test the system after installation — ideally with a helium leak detector at each joint. (Using liquid solutions to observe bubbles is a subjective test that only can detect rather large leaks.) Failure to properly install and test the system can lead to extensive additional costs and damage to valuable instrumentation.
Following these simple guidelines and general considerations should result in a system that is delivered under budget, is safe and provides extended service life while delivering high-purity gas and consistent process results.
Larry Gallagher is specialty gas products manager for CONCOA, Virginia Beach, Va. E-mail him at firstname.lastname@example.org.