Compression fitting

Compression fittings are used in plumbing and electrical conduit systems to join two tubes or thin-walled pipes together. In instances where two pipes made of dissimilar materials are to be joined (most commonly PVC and copper), the fittings will be made of one or more compatible materials appropriate for the connection. Compression fittings for attaching tubing (piping) commonly have ferrules (or olives in the UK) in them.

Compression fittings are also used extensively for hot and cold water faucets (taps) and toilet stop valves; compression fittings are well suited to this application, as these valves are usually located in confined spaces where copper pipe would be difficult to solder without creating a fire hazard.

Compression fittings are the industry standard for chemical, oil and gas, R&D, Bio-tech, and the semiconductor industry. They are used because of their ability to provide leak-tight seals. These fittings can be remade.

In small sizes, the compression fitting is composed of an outer compression nut and an inner compression ring or ferrule (sometimes referred to as an "olive") that is typically made of brass or copper. Ferrules vary in shape and material but are most commonly in the shape of a ring with beveled edges. To work properly, the ferrule must be oriented correctly—usually the ferrule is fitted such that the longest sloping face of the ferrule faces away from the nut.

When the nut is tightened, the ferrule is compressed between the nut and the receiving fitting; the ends of the ferrule are clamped around the pipe, and the middle of the ferrule bows away from the pipe, making the ferrule effectively thicker. The result is that the ferrule seals the space between the pipe, nut, and receiving fitting, thereby forming a tight joint.

Larger sizes of compression fitting do not have a single nut to compress the ferrule/olive but a flange with a ring of bolts that performs this task. The bolts have to be tightened down evenly.

Thread sealants such as joint compound (pipe dope or thread seal tape such as PTFE tape) are unnecessary on compression fitting threads, as it is not the thread that seals the joint but rather the compression of the ferrule between the nut and pipe. However, a small amount of plumber's grease or light oil applied to the threads will provide lubrication to help ensure a smooth, consistent tightening of the compression nut.

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Wikipedia