“Pipe Coupling”-A short Introduction for the piping professionals - What is Piping

“Pipe Coupling”-A short Introduction for the piping professionals

Most pipe installations require several lengths of pipe to be joined together or cut to facilitate changes in direction and crossing of obstacles. A Pipe Coupling or Coupler plays an important role for connecting the pipe sections while maintaining the integrity of the pipes in the process.  It is a very useful pipe fitting for the piping and plumbing industry. Coupling 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. The various functions that a Coupling can serve are as follows:

  • It helps to extend or terminate pipe runs.
  • It can be used to change pipe size.
  • It can be used to repair a broken or leaking pipe.

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 can be categorized into two main groups.

  • Permanent Coupling and
  • 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: There are three types of coupling available

  1. Full Coupling
  2. Half Coupling
  3. Reducing Coupling
  4. Compression Coupling
  5. 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.
Fig. 1: Typical Images of Full Coupling

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.
Fig. 2: Typical Images of Half Coupling

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.

Fig. 3: Various Types of Couplings

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.

Some important features of couplings are as follows:

  • Couplings come in all standard pipe sizes and are available for almost all types of pipe
  • Mostly cylindrical in design.
  • Available in several finishes.
  • Couplings compensate for axial, lateral and angular shaft misalignment.
  • Couplings operate in quiet and smooth way.
  • There is a great degree of torsional stiffness.
  • There is the right angular transmission of angular motion and torque.
  • Usually long life.
  • Easy mounting and dismounting.
  • Low lateral stiffness etc.

Materials Used: Pipe couplings are made of various materials and the buyers should always consider the material used for making them. Some of the materials used for making couplings are:

  • Brass
  • Aluminum
  • Cast Iron
  • Stainless Steel
  • Copper
  • Bronze etc.

How to evaluate the couplings? There is a long list of evaluation factors that affect the type of coupling best suited for an application. It is not necessary that all couplings will have all the features. The factors to be considered are as follows:

  • Design Adaptability
  • Alignment Capabilities
  • Axial Freedom
  • Backlash
  • Chemical Resistance
  • Damping Capacity
  • Ease of Installation
  • Fail Safe or Fusible Link
  • Field Repairable
  • High Speed Capacity
  • Inherent Balance
  • Maintenance Required
  • Number of Component Parts
  • Reciprocating Drivers and Loads
  • Temperature Sensitivity
  • Torque Capacity to Diameter
  • Torque Overload Capacity
  • Torsional Stiffness
Print Friendly, PDF & Email

Leave a Comment

Your email address will not be published. Required fields are marked *


Subscribe to the Piping Engineering CommunityAnd get instant notification on new articles publish, job updates, and much more!