A Short Article on Coupling as Pipe Fitting

A Pipe Coupling, used in piping or plumbing, is a very short length of pipe or tube with either socket or female pipe threads at one or both ends that allows two pipes or tubes of equal or different sizes to be joined together. Couplings are pipe fittings that help to extend or terminate pipe runs. These fittings are also used to change pipe size. It’s also used to repair a broken or leaking pipe.

Most pipe installations require several lengths of pipe to be joined together or cut to facilitate changes in direction and crossing of obstacles. This requires a fairly quick method of connecting the pipe sections while maintaining the integrity of the pipes in the process. A pipe coupling is a pipe fitting 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 either the same or similar materials as that of the pipes it joins. They may be rigid or flexible depending on the amount of movement the pipe is subjected to and may also be permanent or removable. A pipe coupling can be increased or reduced in internal diameter to join 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.

Categories of Pipe Couplings

Couplings under the scope of pipe fittings can be divided into two main categories.

• Permanent Coupling
• Removable Coupling

Permanent Coupling

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 Coupling

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.

Types of Pipe Couplings

• Full Coupling
• Half Coupling
• Reducing Coupling
• Compression Coupling
• Slip Coupling / Repair Coupling

Full Coupling

Full Coupling is used for connecting small bore pipes. It used to connect pipe to pipe or pipe to swage or nipple. It can be threaded or socket ends types.

A socket weld full coupling is used to join small bore plain end pipes where the pipe spec requirement is socket weld. A threaded full coupling is used to join small bore pipes with threaded ends. If the two ends of a coupling are different (e.g. one BSP threaded and one NPT threaded), then it is usually referred to as an adapter.

Half Coupling

Half Coupling is used for small bore branching from a vessel or large bore pipe. It can be threaded or socket type. It has a socket or thread end on only one side.

A socket weld half coupling can be directly welded to large bore pipe, to make a branch connection. It is used to take a small bore pipe branch-off from a large bore pipe where the pipe spec requirement is socket weld in small bore size. A threaded half coupling only has one thread end and another end of the coupling should be butt welding end with either a plain end or a bevel end.

Reducing Coupling

Reducing coupling is used to connect pipes of dissimilar diameters. A reducing coupling has two different sizes of threads on each side. Reducing couplings are typically used where small process feeder lines are joined into large supply circuits or where small diameter fittings are installed. Reducing couplings will normally feature a simple, stepped down profile and screw onto the two lengths of pipe in the same way a standard pipe joint does. Welded reducing coupling designs are similar to threaded  design but feature no threads.

Compression Coupling

A compression coupling connects two perfectly aligned pipes in which a slotted tapered sleeve is placed over the junction and two flanges are drawn over the sleeve so that they automatically center the pipes and provide sufficient contact pressure.

Slip Coupling / Repair Coupling

A slip coupling (sometimes also called a repair coupling) includes two pipes, one of which slides out of the other pipe to a various length. Slip coupling is deliberately made without any internal stop, to allow it to be slipped into place in tight locations, such as the repair of a pipe that has a small leak due to corrosion or freeze bursting, or which had to be cut temporarily for some reason. Since the alignment stop is missing, it is up to the installer to carefully measure the final location of the slip coupling to ensure that it is located correctly.

Couplings in Rotating Shafts (wikipedia link)

A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power  while permitting some degree of misalignment or end movement or both. Couplings under the scope of connecting rotating shafts can be of following types.

• Rigid Coupling
• Flexible Coupling

This article has been restricted delicately for coupling under the scope of pipe fittings. Couplings under the scope of connecting rotating shafts will be dealt separately.

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Now it’s Your Turn to Contribute

What do you think about the above article – informative or needs improvements? What are further types of pipe couplings commonly used in piping industry? Any question or query about pipe coupling in your mind? Share your feedback and experience in the comment box below for making this article more meaningful. 

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