Both double and single ferrule fittings are used to join together lengths of tubing or piping in such a way that creates a seal while preventing any vibration or movement of the tube or fitting, effectively holding the created joint in place.
The primary difference between the two types of fittings is that double ferrule fittings have two ferrules - one front ferrule and one back ferrule. By comparison, single ferrules only feature one ferrule.
In double ferrule fittings - sometimes called twin or two ferrule fittings - the functions of the ferrule are distributed across a pair of twinned ferrules. One ferrule (the sealing ferrule) seals the fitting and helps to hold it in place while the other (the locking ferrule) only helps to hold the fitting in place. The two ferrules are designed to grasp tightly around the tube without causing any damage to the tube wall.
These fittings are sometimes also designed with a longer thread area, which will help to evenly distribute the pressure placed on the fitting among the two ferrules.
In more basic, single ferrule fittings, the nose of just one ferrule will perform both of those functions, both sealing the fitting and locking it in place.
Ferrules in single ferrule fittings may sometimes be asymmetrical, unlike the symmetrical fittings found in double ferrule fittings. This is because in a compression fitting with a single symmetrical ferrule, it's possible for the nut to transfer torque to the ferrule as it is tightened. This torque can cause the ferrule to compress asymmetrically or shift over time. Both types of movement aren't particularly desirable and can cause various issues - like galling of the ferrule, which can result in permanent leaks or components that are stuck together.
Double ferrule fittings are the most familiar type of fitting and are generally more popular across industries than single fittings. As a result, double ferrules are often recommended over single ferrule compression tube fittings, regardless of the conditions these fittings will face. However, while double ferrule fittings have their advantages, both single and double ferrule compression tube fittings can be a better choice than the other in certain situations.
