A ferrule is an essential part of a junction of pipes or tubing which carry liquids or gases under pressure. The pipe or tube on one end of the connection is molded or machined to have a concave threaded opening. A compression screw is slipped over the end of the other tube. This is essentially a hollowed out screw that is flat on the end. The hole through the screw has a diameter that matches the outer diameter of the tube passing through it. The ferrule is then slipped over the tube between the compression screw and the end of the second tube. A ferrule resembles a hollowed out cone. The flat base of the cone sits against the compression screw, and the convex side fits into the concavity in the first tube. In some cases the ferrule and compression screw are manufactured as a single integrated part. The screw is threaded into the first tube and tightened, compressing the ferrule and resulting in a connection between the tubes which can withstand high internal pressures without leaking.

             ___________                     ___________|   |
            | | | | | |             | | | | |               |
           /  | | | | |         _   | | | | |               |
          /                    / |  |_|_|_|_|               |
_________/                  __/  |__|_______|_______________|___
______|                    |__|  |______________________________
                ^             |  |
                |             |  |
tube          threads         |  |     tube interior
                |             |  |
_______         v           __|  |______________________________
______|__                  |__|  |______________________________
         \                    \  |  |_______|               |
          \                    \_|  | | | | |               |
           \                        | | | | |               |
            |  | | | | |        ^   | | | | |___________    |
            |__|_|_|_|_|        |                       |___|
                              ferrule        ^
                                          cross section
                                          of compression screw
                                          fitted over end of
                                          tube behind ferrule

Ferrules are commonly seen on pressure regulators where they connect to the valve on a cylinder of compressed gas. Fittings for this type of connection are typically made of brass. The threading of the connection is reversed (turned counterclockwise to tighten) for some gases including acetylene and oxygen.

Ferrules are an essential part of tubing connections for high-performance liquid chromatography. Standard HPLC tubing is made of stainless steel or polyetherletherketone (PEEK) and has an external diameter of 1/16 inch. This tubing is designed to withstand pressures of up to 10,000 PSI, but in the absence of fatigue or chemical degradation of the tubing material, the fitting will always succumb to pressure and leak before the tubing bursts. Like the tubing, ferrules and screws for HPLC applications are also made of stainless steel or PEEK. It is typical to use fittings of the same material as the tubing, but is not necessary. PEEK ferrules and screws (especially one-piece combinations) are often used with steel tubing, but the reverse is not very common. Steel ferrules permanently bind to steel and PEEK tubing when the compression screw is tightened. If it is not seated correctly or the end of the tube becomes damaged, the tube must be cut behind the ferrule and a new fitting made. PEEK ferrules do not bind to either steel or PEEK, so they are easily interchanged. One-piece PEEK ferrule/screw combinations offer the further convenience of providing a finger-tight connection, so no tools are necessary to change a connection. This convenience comes at the price of reduced durability and lower pressure limits before a leak occurs.

Other applications for this type of ferrule probably exist, but are beyond the scope of my knowledge.