Compression fittings are used to create leak-free tube connections easily and quickly. Double ferrule compression fittings feature an outer nut, two deformable ferrules in soft alloy or non-ferrous metals and, a tube fitting body and are available for metric and fractional tube sizes. When the fitting nut is screwed on the tube, the ferrules get deformed to create a tight seal. Swagelok and Parker are the two main players in the compression fittings market.
COMPRESSION FITTINGS
Compression fittings allow the connection of two imperial tubes (or fractional tubes), two metric tubes or, also, the connection of a metric tube at one end and a fractional/imperial tube at the other end.
A common size range is between 1/16″ and 2″ for fractional sizes and between 2 and 50 mm for metric sizes).
Compression tube fittings are rated to the maximum working pressure of the tubing to be installed.
Double ferrule compression tube fittings are largely used across multiple industries, such as downstream petrochemical, pulp & paper, laboratories, aeronautical and shipbuilding, defense, power generation, semiconductors manufacturing, and heavy industries.
HOW TO INSTALL COMPRESSION FITTINGS
Ferrule fittings are easy to assemble and disassembly, as shown in this online tutorial from Swagelok tube fittings:
The tube has to be inserted into the fitting and the nut has to be screwed. By doing this, the back ferrule advances axially towards the front ferrule and applies an effective grip on the tube.
The ferrule is deformed, compressed and clamped around the tube creating a very tight seal able to withstand the nominal pressure of the tubing.
The two-ferrule design compensates the tolerances in tube diameter, wall, thickness, and material hardness, giving the connection exceptional leak-free performance (without impacting the mechanical properties of the tubing: the back ferrule moves according to a predefined path without reducing or stressing the inner diameter of the tube).
The connecting tube, at the outlet of the fitting, can be screwed or welded on the compression tube fittings (depending on fitting end type). Multiple options are available in terms of screwed ends (NPT, ISO, etc).
A joint made with a ferrule fitting can be disassembled for maintenance (spare ferrules and nuts are available).
By using some basic tools, provided by compression tube fittings manufacturers, the installation is smooth and quick. No welding is required.
As shown in the video above, a proper force shall be applied to the nut when screwing it on the tube otherwise the fitting may get damaged and the connection might leak. It is recommended to tighten the nut manually at first and then use an appropriate wrench.
COMPRESSION FITTINGS TYPES
Compression tube fittings (ferrule) can be grouped into the following families:
- Connectors (male connector, female connector, bulkhead male connector, bulkhead female connector, butt weld pipe connector, socket weld tube connectors)
- Male and female elbows
- End closures (tube, fitting)
- Reducers
- Tees (male run tee, female-run tee, male branch tees, female branch tees)
- Unions
All these tube fittings are shown in the image below:
FERRULE FITTINGS PRESSURE RATING
Compression fittings are available in standard (up to 11 kPSiG), medium (11 to 20kpsig) and high-pressure (up to 60kpsig) ratings:
STANDARD TUBE FITTINGS (< 11 Kpsig)
| Pressure Ratings | Up to 11.000 PSIG (760 bar) |
| Materials | Stainless steel |
| End Connections | NPT, ISO/BP, SAE/MS, AN, weld ends, port connectors, sanitary flanges, tube adapters |
| Dimensional Range | 1/16 to 2 inches (imperial); 2 to 50 mm (metric) |
| Configurations | Straights, elbows, tees, crosses, caps, plugs, nuts and ferrules Chromatograph and Column End |
MEDIUM PRESSURE TUBE FITTINGS (11 to 20kpsig)
| Pressure Ratings | Up to 20.000 PSIG (1378 bar) |
| Temperature Ratings | Up to 1000°F (537°C) |
| Materials | 316 Stainless Steel or Alloy 625 |
| End Connections | Unions, NPT, medium-pressure cone and thread adapters, male SAE/MS, reducers, port connectors, tube adapters, caps, plugs |
| Dimensional Range | 1/4, 3/8, 1/2, 9/16, 3/4 in.; 6, 10, 12 mm |
| Configurations | Straights, elbow, tees, cross |
| Other medium-pressure products available | Medium-pressure ball valves, medium-pressure tubing |
CONE AND THREAD TUBE FITTINGS (MEDIUM PRESSURE)
Cone and thread connection types are suited for medium and high-pressure performance and can be equipped with antivibration components. They can meet NACE MR0175/ISO 15156.
| Pressure Ratings | Up to 20.000 PSIG (1378 bar) |
| Temperature Ratings | Up to 1000°F (537°C) |
| Materials | 316 Stainless Steel Alloy 2507 Alloy 625 |
| End Connections |
1/4 to 1-1/2 in. |
| Configurations | Straights, elbow, tees |
CONE AND THREAD TUBE FITTINGS (HIGH PRESSURE)
| Pressure Ratings | Up to 60.000 PSIG (4134 bar) |
| Temperature Ratings | Up to 1000°F (537°C) |
| Materials | 316 Stainless Steel Alloy 2507 Alloy 625 |
| End Connections | 1/4 to 9/16 in. |
| Configurations | Straights, elbow, tees |
For more information about compression tube fittings, we suggest the Swagelok Essential Guide or the Parker Fittings Tubing Guide.
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8 Responses
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You have a picture of the make-up of a Superlok fitting with a built-in gap gauge, but no link to that particular fitting. Check out Superlok World, https://superlokworld.com/ to source the i-Fitting tube fittings from the first picture in the article and watch this Tube Fitting Basics video on How to Properly Install Tube Fittings: https://www.youtube.com/watch?v=HwmkNiTNGeU
Nice summary, we are in Australia and have used all 3 of the brands mentioned (Swage-lok, Parker, and DK-Lok) and never had an issue with these top 3 quality wise or even mixing on selected parts. We did find the service from Valve and Flow Control Specialists on the DK-Lok to be better although Swagelok seemed to have a better availability when it came to above 1″ (25mm). The larger sizes though are not as easy to install.
Your blog so good and also helpful for me.keep it up