Double contained tubing or pipe is mandated in some cases. The use of double-walled piping for many below-ground transport systems in wastewater treatment plants and at sanitary or hazardous-waste landfills is common. Regulations affect below-grade transport of hazardous materials in fuel storage systems, tank farms, drainage or runoff from process plants, and some food-processing applications.
Double-walled containment systems are also used for certain classes of toxic or corrosive chemicals, mostly gases, used in semiconductor fabrication, which I’ll focus on here:
Welding of these system is a simple process for the inner tube or pipe or the “Carrier or Process” pipe. Common practice is to orbitally weld the inner process pipe, a very effective way to insure the integrity of the “carrier” line. An opening in the “outer” jacket or containment pipe must allow for the width of the Orbital Weld Head to clamp onto the inner pipe. Once this weld is made, a sleeve, typically some 4 inches in length can be slid over the jacket material and welded in place using a “fillet” style weld, also utilizing the orbital tube welding systems.
A good example is a 1/4″ OD inner pipe or tube can be contained by a 1/2″ OD outer “Jacket” pipe. Using a 5/8″ OD, sleeve that is made of the same base materials as the outer jacket and is approximately 4 inches in length can by slide over the opening in the 1/2″ OD outer jacket and held in a position that completely covers the opening. The tungsten electrode in the Orbital Weld Head is aligned with the edge of the 5/8″ OD sleeve and positioned to perform a simple fillet weld on the sleeve to the jacket. This is not a full penetration weld, only a leak tight weld, for containment purposes only. Alignment of this weld is simple as the orbital weld head “collets” can be affixed to both the 1/2″ OD Jacket and the 5/8″ OD sleeve making for sound positioning of the orbital process. The trick to this process is to make sure you have a well-developed “sleeve weld” program in the orbital systems that does not over penetrate the outer jacket.