Compression tube fittings hold and seal tubing without welding, and at the heart of every one is the ferrule: the ring that grips the tube and forms the seal when you tighten the nut. Fittings come in two families, single ferrule and double ferrule, and the difference between them affects pressure capability, vibration resistance, and how the fitting behaves over its life. This article compares the two so you can choose the right one for your application.
What a ferrule does
A compression tube fitting is made of a body, a nut, and one or two ferrules. As you tighten the nut, the ferrule or ferrules are driven onto the tube, where they perform two jobs at once: they grip the tube so it cannot pull out under pressure, and they form a leak-tight seal against the tube and the fitting body. How those two jobs are shared is exactly what separates single ferrule from double ferrule designs.
Single ferrule fittings
In a single ferrule fitting, one ferrule does both jobs. It grips the tube and forms the seal on its own as the nut is tightened. The appeal is simplicity: there is one less part, the fitting is quick to install, and there is less that can go wrong during assembly. Single ferrule fittings are generally a good fit for lower-pressure and less demanding service, and they can be the more economical choice. The trade-off is that asking one component to grip and seal at the same time can concentrate stress on the tube, and single ferrule designs typically carry a lower pressure rating than their double ferrule counterparts.
Double ferrule fittings
A double ferrule fitting, also called a twin ferrule fitting, splits the two jobs between two separate components. The front ferrule forms the seal against the tube and the fitting body, while the back ferrule grips the tube, usually with a hardened leading edge that bites into the tube wall to hold it against pressure and prevent blow-out. Because each ferrule is optimised for a single job, a double ferrule fitting generally seals better, grips more securely, and handles higher pressures than a single ferrule design. Splitting the functions also helps the fitting cope with vibration and thermal cycling, and it supports reliable make-and-break, so the connection can be taken apart and remade during maintenance.
Performance and safety compared
For most instrumentation and process applications, the double ferrule design is the more capable choice. The dedicated back ferrule gives a firmer grip and better protection against tube blow-out, which is a safety consideration wherever a connection is pressurised, and the separation of grip and seal helps the fitting resist the vibration fatigue that can loosen or crack lesser connections. Single ferrule fittings remain a sound, simpler option in lower-pressure service where their ease of installation and lower part count are an advantage. The number of ferrules is only one design variable, so the right answer depends on your pressure, your media, your vibration environment, and your cost target.
Two points apply to both designs. First, follow the manufacturer’s make-up instructions, including the correct number of turns past finger-tight, so the ferrules set correctly. Second, do not intermix ferrules, nuts, and bodies from different brands. The components are engineered as a matched set, and mixing them can compromise the grip and the seal.
Where FITOK fits
FITOK tube fittings are available in both single and double ferrule designs, so you can specify the right one for the job rather than settle for whatever a supplier happens to stock. The double ferrule range, including the FITOK 6D series, is built for leak-tight connections with strong resistance to vibration fatigue and high working pressures, while single ferrule options remain available for suitable applications. If you are not sure which design your system needs, we can help you choose and supply it from stock.
Not sure which ferrule design fits your system? Our engineers can help you compare the options for your pressure and service, and prepare a tailored quotation. Contact us to get started.
