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Learn Ferrules Of A PP Compression Fitting

                       
Update:18-08-2020
Summary:

The ferrule is the primary sealing component of a pp co […]

The ferrule is the primary sealing component of a pp compression fitting assembly, and they can be found in a wide variety of materials ranging from stainless steel to graphite. The majority of ferrules in the marketplace however, are manufactured from metals. Metal ferrules are attractive because they are stable over a wide temperature range, and they can bear compressive loads without relaxing. Typically, metal-to-metal seals are considered leak-prone (for example, metal pipe threads are often reinforced with pipe tape). Ferrules however, are specifically engineered to create an optimal seal against both the tubing and the fitting body. Some of the key factors of the ferrule design include:

Ferrule Shape
The shape of both the ferrule and the mating angle of the fitting body are critical factors in the reliability of a compression seal. Both components must be tapered in a manner that allows the ferrule to compress properly as the nut is tightened, while maintaining axial alignment with the tubing. Additionally, the relative angles of the ferrule and the fitting body will determine how much linear motion is converted into radial compression, and what level of contact (“line contact” or “surface contact”) is made with the tubing. A uniform, “line contact” compression around the entire circumference of the ferrule will create the most reliable seal. To this end, it is also important that the ferrule have a sharp forward edge.

One piece symmetrical and asymmetrical ferrules (brass) and stainless steel front (cone) and rear (disk) ferrules
One Piece vs. Two Piece Ferrules
Most basic compression fittings contain a single ferrule. Single ferrule designs minimize the total number of components, and work reliably when fabricated from softer materials (plastic or brass for example). With harder materials like steel however, torque is often transferred from the compression nut to the ferrule as the nut is tightened. The resulting rotation can cause the ferrule to compress asymmetrically, or to shift over time due to residual torque. In stainless steels, rotation of the ferrule can also produce galling and cause permanent leaks. Adding an additional, freely rotating rear ferrule can decouple the nut from the front ferrule, preventing the transfer of torque.

Stainless steel compression fitting with two piece ferrule design
Asymmetrical vs. Symmetrical Ferrules
One piece ferrules are often available as asymmetrical or symmetrical. Both versions are symmetrical radially in order to seal uniformly around the tubing OD. Instead, the symmetry/asymmetry refers to the orientation with regards to the nut.

An asymmetrical ferrule is cone shaped, and can only be placed into the fitting body in one direction (usually with the tip of the cone facing into the fitting body). Symmetrical ferrules look like two cones back-to-back, and can be placed into the fitting body in either direction. This increases the ease of assembly in applications where many fittings are used, and assembly time is critical.

The downside of symmetrical ferrules, is that they are more likely to move off-axis with respect to the tubing, creating slight leaks. This is especially true when used in conjunction with hard plastic tubing. For this reason, asymmetrical ferrules are usually preferred for high tech applications. Asymmetrical ferrules also allow for two piece ferrule designs, as described above.

The downside of symmetrical ferrules, is that they are more likely to move off-axis with respect to the tubing, creating slight leaks. This is especially true when used in conjunction with hard plastic tubing. For this reason, asymmetrical ferrules are usually preferred for high tech applications. Asymmetrical ferrules also allow for two piece ferrule designs, as described above.

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