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Coupling vs Union – The Definitive Guide

Coupling Vs Union

When you need to set up the system for piping, a number of connections assist in smoothening the flow. In such a situation, you cannot deny the importance of coupling and Union.

This guide highlights parts, types, materials, applications, and more. Continue to read to uncover insights.

What is Coupling vs Union?


A coupling is a tube or pipe with a short length containing a socket at both ends. This, in turn, allows the connection between tubes and pipes. Depending on the coupling material, you can connect by soldering, brazing, and welding. Please note the joint you make with the coupling is permanent.

A union is fitting containing threads that help you connect or separate a piping system. You can do this without causing horizontal pipe movement. Union fitting refers to a standalone type of fitting or a fitting that serves as an integral part. The connection you make using Union is semi-permanent. You can separate the fitting upon desire.

Types Of Coupling And Union in Plumbing

Coupling and Union have different types when it comes to piping systems. Each type is suitable for a particular application and requirement. Let’s uncover these types.

Coupling Types

1. Compression Coupling

You can join or connect any two pipes or tubes with this coupling. It makes use of clamp force for sealing. Compression coupling consists of a seat, a ring, and a nut. Another name for a compression ring is the ferrule.

Compression Coupling
Compression Coupling

It lies on tubing having a nut on behind. This tube you need to insert this in the seat and then follow screwing on it. Doing so tightens the nut and results in ring compression, which circles the tubing and leads to very tight sealing.

2. Slip Coupling

When you are facing a leakage in the pipe, or a burst pipe, choose Slip coupling. With a slip coupling, you can join two pipes without cutting or soldering phenomenon. Sealing the joint by compressing the O ring/gasket right between two flanges is an option.

Slip Coupling
Slip Coupling

Union Types

1. Ground Joint Union

The ground union has an angle cut right in the union. This is where two ends meet or join. Doing so increases the surface area, which leads to good seating.

Ground Union
Ground Union

2. Flange Union

A Flange union is a union where two flanges play a role in connecting two pipes. It employs screwing on pipe ends which ensures leak free seal.

 Flange Union
Flange Union

Main Parts of Pipe Coupling vs Union in Plumbing System

Coupling Parts

The components which make up coupling include:

  • Compression nut/screw

The nut in the pipe coupling refers to the fastener, which has a hole containing thread. You can use it to tighten or screw the coupling with the connection.

  • One or more Ferrules

A ferrule is a cap or ring that helps strengthen the tube, stick, and handle end. It also prevents the end from separating. You can use it to seal the compression fitting assembly.

  • Fitting body

The fitting body is the main structure that supports nuts and ferrules in a coupling. The fitting body assists in smoother connection.

Union Parts

The union comprises three main parts:

  • Male End

The male end is a part of the union, which contains thread on the outer side. You need to screw it to achieve proper fastening.

  • Nut

A nut is an important part of the union, and it aids in joining two pipe sections together. The nut structure allows you to experience greater grip with the help of a wrench to facilitate tightening.

  • Female End

A female end in a union is a part that contains a thread on the inner side. By screwing, you can achieve a proper fix. In general, you need to screw the male end into the female end.

Materials Used In Coupling vs Union Systems

There are different materials that you can use in coupling and Union. The key materials include:


  1. Brass
  2. Cast Iron
  3. Copper
  4. Carbon Steele
  5. Aluminum
  6. Bronze
  7. Stainless Steel
  8. PVC (Polyvinyl Chloride)


  1. Stainless Steel
  2. Carbon Steel
  3. Alloy Steel
  4. PVC (Polyvinyl Chloride)
  5. PE
  6. Rubber
  7. Nickle
  8. Duplex Stainless Steel
  9. Copper
  10. Brass
  11. Bronze
  12. Aluminum
  13. Cast Iron

Applications Of Union & Coupling

Coupling Applications

There are multiple applications exist when it comes to pipe coupling. Some applications are:

1. Irrigation Systems

An irrigation system is a system that comprises pipelines (main and sub-main), pumps, etc. This system helps irrigate various crops and boosts their growth, especially in drought areas. The coupling has wide adaptability in irrigation systems to prevent leakage and ensure a permanent fit.

2. Water lines

The public water lines are spread over an entire city for a clean and safe water supply. To make strong connections in water lines, you can use couplings.

3. Gas lines

Gas lines connect the natural gas main supply to the office, apartment or building. In a smooth and uninterruptible gas supply, you cannot underestimate the importance of coupling.

4. Oil Pipelines

An oil pipeline you can use to move a large amount of oil to refineries for further processing. They can be above 180 miles in length. Coupling ensures durable and longevity connections in this pipeline at multiple sections.

Union Applications

1. Wastewater piping systems

This piping system is a system that you can use for transferring wastewater (domestic/commercial/industrial) to the wastewater treatment plant. As these pipelines are prone to clogging. Union serves best in their connection because you can separate the line for cleaning purposes anytime you want.

2. Piping systems for fire protection

These systems serve best when you experience a horrible fire. For instance, in the oil industry, building or factory. The piping systems for fire protection transfer fire extinguisher fluid or material to the main point. Union helps connect these pipes and helps you experience a longer, seamless pipeline.

3. Process Piping system

A process piping system is a system that transports processing fluids (chemicals, fuel, gases) to a certain vicinity. Union helps you prevent leakage of these fluids in the atmosphere by ensuring a good connection.

4. Plumbing

Plumbing is essential in every housing/factory/industrial and commercial sector. It supplies and delivers clean water through the vicinity. Several connections contribute to effective plumbing systems. Union assists best in this regard. It also allows you to separate the connections and perform cleaning when needed.

Advantages of Union and Coupling


Please find below the number of advantages of coupling and union which makes them worthy.

Coupling Advantages

  • You can use it with any pipeline, even on dissimilar piping materials
  • It resists extra pressure and prevents leakage from occurring
  • No maintenance requirement
  • Offers great resistance to misalignment, both angular as well as axial
  • No need for exhaust ventilation or gas-free purging
  • Great sealing effect along with long service life

Union Advantages

  • You can create a leak-free connection with pipe union
  • With Union, you can place metering or regulation device in the piping system
  • You can make a connection between vessel to piping systems
  • With Union, you can connect two pipes of different sizes
  • The union can assist you in separating two pipes
  • You don’t need to put effort into the installation of Union. It offers super convenient installation.
  • You can get a union at a low cost.

Joining Methods for Pipe Coupling and Union

Coupling Joining Methods

There are three main types of joining methods that exist for coupling. They are:

Joining Methods
Joining Methods
  • Threaded: One or two ends contain threads you can screw to tighten them together for leakproof sealing.
  • Butt Welded: In this, you bring the two-end close together and then perform welding for a permanent joint.
  • Socket Welding: These welds require a fillet to process welding. First, you insert the smaller diameter part into the larger diameter part and then perform welding.

Union Joining Methods

The union supports only two methods of joining. They include:

  • Threaded

The term threaded means the union ends contain threads. You can achieve fastening by screwing. The threading configuration is maybe clockwise or anticlockwise.

  • Socket Welding

These unions you cannot open up. They belong to the permanent connection type. It works similarly to stock welding as in coupling. First, insert a small diameter side into a bigger opening and then weld them together. These ends do not contain threads.

Things To Consider While Choosing Coupling and Union

First, consider the application to which you subject your coupling and Union. If it involves chemicals or liquids with severe reactions, then choose accordingly. PVC, Stainless steel, resists chemical reactions in this regard.

The type of pressure and temperature your coupling and Union will experience matters. The material of the coupling and Union decides what pressure and temperature range it can bear easily.

If your piping systems need maintenance frequently, choose Union instead of coupling because coupling offers permanent joints.

Your budget also matters. Consider the cost of the material. In general, the cost of Union is higher than that of coupling.

Consider the frequency of piping system operation and the entire service life. Choosing based on these factors will help you get the most out of coupling and Union in the long run.


We have presented everything regarding pipe coupling and Union in the above article. This will, in the end, assists you in selecting the best one as per your needs. Let us know if you find the above article useful and helpful in the comment section below.

Feel free to reach out if you need assistance for any couplings and union from China.

More Resources:

Sheet Metal Fabrication – Source: KDM

Difference between Pipe Union and Coupling – Source: Pipes and Fittings

Coupling – Source: Wikipedia

Pipe Fittings – Source: Hard Hat Engineer

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