Installation instructions for pp compression fitting va […]
Installation instructions for pp compression fitting vary depending on the manufacturer and the specific fitting design. Care should be taken to follow the assembly procedures recommended by the manufacturer. There are however, several common elements to the installation procedure for nearly all compression fittings:
Distance vs. Torque
Typically the procedure for tightening a compression fitting is specified using the rotation of the nut as the key metric, rather than torque. Compression nuts are threaded, meaning that the number of rotations of the nut is directly correlated to the linear compression of the ferrule, based on the thread’s pitch. Torque, on the other hand, can vary greatly, depending on fitting and ferrule material, lubrication, degree of galling (for stainless fittings), and other factors.
Compression fitting assembly using a pre-swage tool in a vise
Tighter is Not “Better”
Often when assembling fittings, assemblers and engineers assume that “tighter is better”. This does not hold true for compression fittings. As described in the “Ferrules” section above, the optimal seal is created by a line contact between the ferrule and the tubing. Insufficient tightening of the compression nut will not deform the ferrule enough to create this contact, but over-tightening will deform the ferrule too much, creating a surface contact. A surface contact weakens the seal and often results in leaks
Pre-swaged ferrules and compression screw.
Disassembly and Re-Assembly
The ease of assembly and disassembly is one of the reasons why compression fittings are so widely used. Disassembly involves simply loosening the compression nut or screw. Re-assembly can be accomplished in a similar manner to the initial assembly, although typically fewer turns of the nut are required, since the ferrule is already swaged to the tubing. It is important to note however, that compression fittings can only be disassembled and reassembled a few times before either the ferrules and/or the fitting body should be replaced, to ensure a leak-tight seal.
Mixing and Matching Components
Even if components from two different manufacturers look similar externally, they will often differ in key internal dimensions, such as ferrule/body taper, ferrule length, and thread size and pitch. Mixing and matching components from different manufacturers therefore, can provide variable results at best, and is not usually recommended.
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