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.

When approaching stainless steel tube fittings for instrumentation, a systematic, multi-step selection framework is essential to prevent costly leaks, pressure drops, or premature corrosion. Based on industrial experience, here is a breakdown of how to tackle these key factors:

1. Dimensional & Sourcing Strategy (OD vs. Wall Thickness): Yes, you should always start with the tube Outer Diameter (OD) and wall thickness. The wall thickness dictates the maximum allowable working pressure (MAWP) for a given temperature according to code standards (e.g., ASME B31.1/B31.3). When handling inch versus metric tubing, the golden rule is never to mix fitting and tubing systems (e.g., do not use metric tubes with fractional inch fittings or vice versa), even if the nominal size seems close. Doing so will inevitably lead to improper grip by the ferrule, dimensional mismatch, and catastrophic leakage under pressure.

2. Material Selection (304 vs. 316/316L & Beyond):

• 304/304L: Suitable for non-corrosive, clean instrument air or general plant water utilities. However, it has lower resistance to pitting and stress corrosion cracking.
• 316/316L: The industry baseline for process lines. The lower carbon content in 316L makes it superior for welded applications by preventing carbide precipitation at grain boundaries.
• Severe/Corrosive Environments: If your process contains chlorides, hydrogen sulfide, or acids, or operates in marine atmospheres, standard 316 may suffer from localized pitting. In such cases, stepping up to higher alloys like Hastelloy C276 or specialized stainless steels is non-negotiable despite the cost.

3. Connection Types & Leak Prevention:

• Compression Fittings (e.g., Swagelok style): The industry favorite for standard instrumentation because they offer reliable mechanical grip, ease of installation, and vibration resistance. However, as noted by others, ensure that solids or dust do not lodge in the mouth of the fitting prior to make-up.
• Cone & Thread Fittings (e.g., Autoclave/Butech style): Essential for high-pressure applications (typically above 5,000 to 10,000 psi) where traditional twin-ferrule compression fittings reach their limits.
• Welded Connections: Ideal for permanent, zero-leakage joints in hazardous or highly toxic gas/fluid services.

4. Pre-Order Checklist: Before issuing a purchase order, always verify:

• Exact tube OD, wall thickness, and material specification (mill test reports/MTRs).
• Pressure and temperature derating curves (ratings drop as temperature increases).
• Compliance with applicable codes (ASME, NACE MR0175 for sour service if applicable).

Hope this helps streamline your instrumentation design!