Choosing the Right Stainless Steel Tube Fittings for Instrumentation

Having worked in Power Stations and Coal Mines the tube used was 316 SS and the fittings were Swagelock for most process connections and hydraulic connections.

Control air and water was either copper or Nylon with the correct compression fittings.

The tube was specified by the ID to suit the pressure but the 316 SS was standardised with Swagelock.

I am very experienced with the question. I worked 26 years testing Solid Rocket propellant. I did design, assembly, maintenance, repair and did the testing. The work is called strand burning. Where a 1/4 in square propellant strand 3 to 6 inches long is put in a pressure vessel and pressurized to the required (vacuum to 5000 or even 10000 Psi). As the Oxidizer is Ammonium Perchlolate, which combusts the binder and Aluminum. So the result is Pure chlorine gas if burning in Nitrogen or Hydrochloride Acid if burned under water at pH1. The Aluninum combusts to Aluminum Oxide (Corumdum) which is destructive to valve seats. Even micro diamonds can be formed. For critical plumbing only Hastalloy C276 is the choice in spite of the cost. For welded joints SS316L is the best choice with chloride and critical uses since it is a truly formulated SS. 304SS is a poor choice since it has a a wide formula variation and is mostly just scrap SS.

For clean gases/fluids Swagelock is fine. But if there is dust or solids switch to Autoclave/Butech 5000psi rated joints. Why? The seal line in almost at the junction of the fittings and the seal line while Swaglock has a 1/4 inch open area before the seal line where solids lodge (Thus difficult to separate). Chloride causes pinhole corrosion following the inter crystal voids and carbon tracks from welding.