Things to Consider When Selecting Tubing for Your Application
We generally work in a world where there is a\ specification or governing body somewhere that oversees most aspects of our day-to-day work. ASTM, ANSI, ISO, Mil Spec, UL, CSA and FM are just some of the governing bodies overseeing most aspects of what we do. Add in more specialist trade associations such as API, ISA and others and we can generally rely on someone to provide guidance on what we do and specify.
Strangely enough, cleanliness and internal surface finish of tubing are not generally covered. Each mill (and sometimes each production line within a mill) selects and uses whatever method they choose. This can result in widely varying internal surface finishes and internal cleanliness on the tubing you purchase. As an example, ASTM specifications that govern the manufacture of 316/316L tubing do not specify any internal finish requirements. ASTM merely states, under “Surface Condition” that tubing “shall be pickled free of scale. When bright annealing is used, pickling is not necessary.” Most internal surface finishes of tubing run from 32 to 64 Ra micro inches. Cleaning standards are not addressed. The more reputable mills typically utilize a cleaning process to degrease tubing, using repeated rinsing with caustic or citric acid solutions and then repeated rinsing, with hot air drying. Felt plugs are then blown through the tube to see if the bore is clean. I’ve personally worked on jobs where tapping tubing on the floor caused a shower of black scale to fall out of the tube.
Tubing can, for a premium, be purchased that meets supplier defined surface finishes and cleanliness. Swagelok®, as an example, offers Ultra High Purity tubing with an electropolished interior and cleaning to specific levels.
When you purchase tubing consider your application. Avoiding cheap tubing from unknown mills might save you cleaning out your system or having stray shavings caught in small orifices.