Technical Foundation
What Is a Compression Tube Fitting?
A compression tube fitting seals a rigid or semi-rigid tube by driving a metal ferrule down onto the tube's outer diameter as the nut is tightened against the fitting body, biting into the tube wall to form a metal-to-metal, leak-tight joint without soldering, welding, or flaring the tube end. Single-ferrule (two-piece) designs use one ferrule that both bites and seals; double-ferrule (three-piece) designs, popularized by Swagelok- and Parker-style construction, split the job between a front ferrule that seals and a back ferrule that grips and resists pull-out, giving noticeably better resistance to vibration and thermal cycling at a higher parts cost.
Fittings are sized to a specific tube OD — commonly 1/8″ to 1″ in fractional-inch sizes, or millimeter sizes such as 6mm, 8mm, 10mm and 12mm — and the tube must be cut square and free of scratches for the ferrule to bite evenly. Body material is chosen for the media and pressure: brass for general air, water and low-pressure service; 316 stainless steel for corrosive media, higher pressure, or process/instrumentation lines; and PTFE-lined or all-plastic bodies for high-purity or aggressive chemical service. Correct assembly matters as much as material selection: most double-ferrule designs are tightened finger-tight and then turned a further 1–1¼ turns with a wrench (less on tubing under 1/8″ or on soft tube), and a fitting that has already been swaged onto a tube can typically be disassembled and re-tightened to the same pre-swaged position without replacing the ferrules, provided the original bite marks aren't disturbed.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Ferrule Configuration | Single-ferrule (two-piece) vs. double-ferrule (three-piece) | Double-ferrule designs resist vibration and pull-out better and generally re-torque more reliably after disassembly |
| Tube OD | Fractional inch (1/8″–1″) or metric (6mm–28mm) | The fitting must match the tube's actual outer diameter exactly — it is not adjustable |
| Body Material | Brass, 316 stainless steel, PTFE-lined/plastic | Determines pressure rating, corrosion resistance and media compatibility |
| Assembly Torque | Finger-tight, then 1–1¼ additional turns (typical) | Under-tightening leaks; over-tightening can over-swage the ferrule and damage the tube or seal |
| Reusability | Re-tighten to original ferrule position after disassembly | Lets a joint be broken for maintenance without cutting the tube or replacing the ferrule in most cases |
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Applications
Where Compression Tube Fittings Get Used
Vibration exposure and media compatibility decide ferrule count and body material more than tube size alone.
Instrumentation & process control tubing
Double-ferrule stainless fittings on small-bore impulse and gauge lines feeding pressure, temperature and flow transmitters.
Pneumatic control systems
Brass compression fittings joining rigid air lines on machine-mounted valve manifolds and panel wiring runs.
Hydraulic sampling & instrumentation lines
Stainless fittings rated for higher pressure on sample lines drawn off hydraulic circuits for monitoring.
Analytical & gas chromatography sample lines
Small-OD stainless tubing with double-ferrule fittings, chosen for leak-tight seals at the low flows analytical instruments require.
OEM machine building
Compression fittings specified into skid and machine builds where field technicians need to service tube runs without welding.
Semiconductor & high-purity gas delivery
Electropolished stainless double-ferrule fittings minimizing particle generation and outgassing in high-purity gas panels.
Food & beverage sanitary tubing
Plastic or stainless compression fittings on wash-down and process lines where corrosion resistance and cleanability matter.
Brands
Compression Tube Fittings Brands in the Catalogue
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FAQ
Compression Tube Fittings Questions, Answered
Should I choose a single-ferrule or double-ferrule compression fitting?
Single-ferrule (two-piece) fittings cost less and work well for general air and water service with limited vibration. Double-ferrule (three-piece) designs split sealing and gripping between two ferrules, giving better resistance to vibration, pressure cycling and pull-out — the standard choice for instrumentation, process and high-pressure lines.
Brass or 316 stainless steel — which body material should I use?
Brass is suitable for general compressed air, water and low-pressure service where the media isn't corrosive. 316 stainless steel is specified for corrosive or high-purity media, higher pressure ratings, and outdoor or washdown environments where brass would corrode or discolor over time.
What's the correct way to tighten a compression fitting?
Insert the tube fully to the shoulder, then tighten the nut finger-tight before wrenching it a further 1 to 1¼ turns past finger-tight for most double-ferrule designs (less for thin-wall or small-OD tubing). Under-tightening leaves the joint prone to leaks; over-tightening can over-swage the ferrule and damage the tube.
Can a compression fitting be reused after it's disassembled?
In most cases yes — if the ferrule's bite mark on the tube is undisturbed, the fitting can be reassembled and re-tightened to its original swaged position without replacing the ferrule. If the tube is removed and a different tube inserted, or the ferrule is damaged, replace the ferrule to guarantee the seal.
Does tube wall thickness affect the pressure rating of a compression fitting?
Yes — the fitting's pressure rating assumes a specific tube wall thickness and material for a given OD; thinner-wall tubing lowers the safe working pressure and can affect how well the ferrule bites. Always match the fitting's rated wall thickness range to the actual tube being used, not just the OD.
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