A pipe coupling is any fitting or device used to join two or more pipes in such a way that continuity is maintained between them. The body of a pipe coupling is typically made of the same or similar materials as those used to construct the pipes it joins. They may be rigid or flexible depending on the amount of movement the pipe is subjected to and may per permanent or removable fixtures. A pipe coupling can be stepped or reduced in internal diameter to accept pipes of different sizes, be T or cross shaped to join more than two pipes, or angled to form bends. Pipe couplings may also include peripheral features or equipment such as inspection openings, flow meters, or valves.
Most pipe installations require that several lengths of pipe be joined together or cut to facilitate changes in direction and crossing of obstacles. This requires a fairly quick method of connecting the sections while maintaining the integrity of the pipes in the process. These couplings may be permanent by nature or, if possible disassembly is desirable, of a removable design. Permanent pipe couplings generally make use of soldering or brazing in the case of steel or copper pipes or adhesives in the case of PVC pipes. These permanent joints, if correctly installed, offer excellent rigidity and sealing characteristics where no future changes in the piping are foreseen.
Removable pipe couplings are most often of a threaded design which allows them to be screwed onto the pipes to be joined. The most simple of these is a basic pipe section slightly larger than the pipes to be joined and cut with an internal thread. The ends of the pipes are also threaded, and the coupling is simply sealed with hemp or sealing tape and screwed onto both pipes. More complex removable pipe couplings are of a two-part design where one part is attached to each pipe and then screwed together to complete the joint. These types of couplings are often used on high pressure copper gas pipes and flexible hydraulic or pneumatic hoses.
A pipe coupling is also often used to facilitate changes of pipe routing, crossing of obstacles, and penetration of walls. These couplings are typically elbow or U shaped but may include several variants featuring different angles of deflection. Step-down or reduction pipe couplings featuring different end diameters are used to join pipes of dissimilar sizes. Adaptor fitting may also be used to join pipes of different materials and feature separate end fittings suitable for both materials. Where more than two pipes are to be joined Y, T, or cross shaped couplings are common methods of achieving the joints.
Pipe couplings may also include other features such as inspection or maintenance access points. These are typically screw-on caps that allow for visual inspections or the insertion of rods used to remove blockages. Flow meters and other analytical attachments are frequently built into a pipe coupling to ease installation. Pipe couplings may also include mechanisms such as shut-off and pressure release valves.