What is a Compression Fitting and How Does it Work?

Posted by
John Woodard on February 05, 2019

Compression fittings are the old-school version of the quick-connect fitting. While these fittings may require more work upon installation, they still provide strong, tight, and reliable seals for any of your water treatment configurations. John, our Master Water Specialist, offers his knowledge on compression fittings to help you better understand how a compression fitting works and how to install it properly.

What is a compression fitting?

A compression fitting compresses a component of the fitting into another component, usually some type of tubing, to create a watertight seal between two different lines. With the introduction of quick-connect style fittings to the water treatment industry, the compression fitting was used less frequently, but it's still a great fitting that can handle high pressure and give you a reliable, tight seal. A compression fitting is comprised of a sleeve, a nut, and the fitting body itself. The sleeve acts as the seal when the nut compresses it into the fitting body.

How do you install and remove a compression fitting?

Compression fittings are simple to install but require a wrench in most cases. They consist of three components: the sleeve, nut, and fitting body. All components work together to create a watertight seal. To create this seal, you must:

  1. Slide the nut onto your tubing.
  2. Slide the sleeve up to the nut on its threaded side.
  3. Put the tubing into the connector on the fitting body, and pull the nut, and then the sleeve, down so that the threads on the nut meet the threads on the fitting body.
  4. At that point, you will screw the nut onto the fitting body.
  5. Use your wrench to tighten the connection.

To remove the fitting, simply do the same thing in reverse. When you pull the sleeve out of the fitting body, the sleeve will stick to the tubing. If you're just putting the sleeve right back into the fitting body, or you're connecting the same tubing to a new angle-stop configuration, you can simply reuse the sleeve. If you are not reconnecting the tubing to another form of a compression fitting, you will have to cut the sleeve off to remove it from your tubing.

What are compression fittings made from?

Compression fittings are made from a variety of materials. The most common material for a compression fitting is brass, but you must make sure to buy lead-free brass fittings because it's illegal to use any fitting with lead content in the U.S. They are also commonly made from plastic and stainless steel. Depending on the material or your fitting, the amount of pressure or temperature that the fitting can withstand will vary.

How are compression fittings used?

Compression fittings are used in a variety of applications. Anywhere you're connecting a waterline or air pneumatic lines you will see a compression fitting used. If you're just hooking up drinking water systems, you're probably going to use a quick-connect or a quick-disconnect type fitting.

The angle-stop underneath your sink is an example of a compression thread. These types of flex lines often will use compression type threads to hook up to a coffee brewer, for example. Some have a faucet connection on one side and then the angle-stop connection, which is your compression thread, on the other. You can use just a regular compression nut and sleeve to attach tubing to the top of an angle-stop. These fittings all connect the same way.

How much pressure can a compression fitting withstand?

The amount of pressure a compression fitting can withstand is simple to understand. Usually, a brass fitting will withstand more pressure than a plastic fitting, and so forth. Typically, you can find a fitting’s pressure rating online or on the packaging that you receive it in.

What do you do if your compression fitting is leaking?

If a compression fitting is leaking, you should determine where the leak is coming from. Usually, if you have a sound connection, you've put the fitting together properly, and you've wrenched down to where it's good and tight, the fitting is not likely to leak. Unless a piece of tubing is moved and puts some strain or torque on the fitting itself, the seal should be tight and complete. 

Plastic fittings might be more prone to start leaking once they've been installed, but again, usually that's because something has happened externally to alter or stress the component. Areas of excess vibration may stress plastic components. If you're using a fitting on an appliance or a piece of equipment with a pump or a motor that causes vibration, it is possible that the nut might back off of the fitting body.

The best way to stop a leak with this type of fitting is to turn the water off. Then, you would need to take the fitting apart and reassemble it once you’ve inspected it for any malfunction. Hopefully, after reassembling, your leak will stop. If not, you may have to replace the fitting, or a piece of it, and the tubing that was connected to it.

Learn more about quick-connect and quick-disconnect fittings from our Master Water Specialist.

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