Compression fitting (ferrule tube fitting)

01

Understand how the ferrules seal and how to make up the joint

Swagelok's gaugeable tube fitting catalogue (MS-01-140), covering tube sizes from 1/16 to 2 in. and 2 to 50 mm, describes a two-ferrule mechanical-grip design in which the two ferrules separate sealing and gripping: as the nut is turned, the front ferrule is driven into the fitting body and onto the tube outside diameter to make the primary seals, while the back ferrule advances the front ferrule and hinges inward to grip the tube, with no torque transmitted to the tubing during installation. [1]

Its installation instructions for fittings up to 1 in./25 mm call for inserting the tube fully against the shoulder, turning the nut finger-tight, marking it at the 6 o'clock position and tightening one and one-quarter turns while holding the body (three-quarters of a turn for 1/16, 1/8 and 3/16 in. and 2, 3 and 4 mm sizes), with a gap inspection gauge to confirm sufficient pull-up on first installation; for 5/8–1 in. and 16–25 mm sizes it recommends preswaging ferrules with a hydraulic swaging unit, which is required above 1 in./25 mm. Fittings may be remade by returning the nut to the marked position and tightening slightly; the gauge is not used on remade joints. [1]

The same catalogue states that no industrial design standard exists for two-ferrule tube fittings, that each company makes components to its own design, and that intermixing or interchanging components of different makers can cause unpredictable performance and releases. ASTM F1387 instead specifies performance for mechanically attached fittings — including flared, flareless (bite-type) and grip-type separable designs — with qualification tests such as pneumatic and hydrostatic proof, impulse, flexure fatigue, tensile, burst, repeated assembly and rotary flexure, grouped into ten pressure classes. [1] [3]

Give the supplier the fitting system already used on site, tube OD and wall in inch or metric units, materials, fitting shapes and end connections (NPT, ISO, SAE threads), installation tools needed (gap gauges, preswaging units), cleaning level and quantity; ask which performance standard and test reports the fittings are qualified to.

02

Rate the joint by the tubing it grips

Swagelok states that its tube fitting ends are rated to the working pressure of the tubing listed in its Tubing Data (MS-01-107). That document calculates allowable working pressures from ASME B31.3 stress values — 138 MPa (20,000 psi) for stainless tubing at −28 to 37 °C — and its metric seamless stainless table gives, for 6 mm OD tube, 320 bar at 0.8 mm wall, 430 bar at 1.0 mm, 550 bar at 1.2 mm and 720 bar at 1.5 mm, and for 12 mm OD, 200 bar at 1.0 mm rising to 480 bar at 2.0 mm. [1] [2]

The tube must be softer than the fitting: stainless tubing is to be fully annealed with hardness not exceeding 90 HRB or 200 HV, free of scratches, and within the wall thickness range tabulated for each size; for gas service it advises heavier walls, because thin walls resist the ferrules less and are less able to coin out surface scratches. Welded and drawn tube is derated by 0.85 (double-welded) or 0.80 (single-welded), and elevated-temperature factors apply — its own example takes 316 stainless 1/2 in. × 0.035 in. tube from 2,600 psig at room temperature to 2,600 × 0.76 = 1,976 psig at 1,000 °F (537 °C). Over 1 in./25 mm stainless fittings above 450 °F (232 °C) need silver-plated front ferrules. [2] [1]

Editorial calculation: 12 mm × 1.5 mm stainless seamless tube is listed at 330 bar; the same size in double-welded tube would be about 330 × 0.85 ≈ 280 bar, before any temperature factor. [2]

Ask for the allowable working pressure of the actual tube OD, wall, material and seam type at your maximum temperature, the hardness and tolerance of the tube to be supplied, and any third-party approvals needed (marine, gas, pressure equipment).

03

Two-ferrule, cutting-ring, bite-type or grip-type

Swagelok, DIN 2353 and ASTM F1387 name four designs; confirm which one your system uses before ordering spares.

04

One linked supplier: ask which fitting system it follows

One company is linked to this page so far and shows compression fittings on its own website. Ask it whether its parts are two-ferrule or cutting-ring, and which standard they are tested to, before fitting them into an installed system.

05

Body, nut, ferrules, cutting ring and tube

The tube is listed as a part because it, not the fitting, sets the pressure rating.

06

Tube OD, wall, material and service values for the quotation

Tube and service values come from your system. Where Swagelok or ASTM F1387 states a value, it is in the middle column.

07

Send the tubing, service and fitting data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Tubing: [OD; wall; material; seamless or welded; hardness]
Service: [fluid; gas or liquid; pressure; temperature]
Fittings: [types; quantities; end threads]
System: [existing brand or standard; cutting ring or two-ferrule]
Quality: [cleaning; certificates; approvals]
Tools: [gap gauges; preswaging]

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