How to Assemble Tube Fitting Caps and Plugs
Proper assembly of tube fitting caps and plugs is essential for maintaining fluid system integrity, enabling pressure testing, and preparing systems for future expansion or modification. While caps and plugs may appear similar, they serve different purposes within a tubing system. Caps are designed to seal the end of a tube and include a complete fitting assembly with a nut and ferrules that can be preswaged onto tubing. Plugs, on the other hand, are used to seal an unused port and feature integrated machined ferrules for quick installation.
In this video, learn the key differences between tube fitting caps and plugs and discover the most effective methods for assembling each. You'll see how proper installation techniques help ensure leak-tight performance and allow systems to be safely pressure tested to their full rated pressure. Whether you are building a new fluid system or modifying an existing one, understanding when and how to use caps and plugs can improve efficiency, reliability, and overall system performance.
Video Transcript
We’re frequently asked, “What’s the difference between caps and plugs?”
This is a cap.
This is a plug.
Caps are used to cap off a piece of tubing. A cap is a complete Swagelok assembly. You have a nut, front ferrule, back ferrule, and a fitting body. But the fitting body is blank. This allows you to install the nut and ferrules onto the tubing and actually preswage them onto the tubing. So you simply slide the tubing up tight against the shoulder of the fitting, pull up this nut one and a quarter turns, and it’s completely done. Then you can fully pressurize your system.
Later, when you’re ready to use the fitting, you simply remove the blank fitting body. When you’re ready to install it, you remove the nut and ferrules from the Swagelok tube fitting, assemble the tubing with the preswaged nut and ferrules on it, and follow the reassembly instructions. Simply tighten the nut finger-tight, tighten the nut with a wrench until you feel significant resistance, and then snug it a little bit more. The job’s all done. It’s ready to go.
A plug plugs off a port. So, if you want to use a tee but you only want to use two ends of the tee, you simply remove the nut and ferrules, or the piece of tubing, take the plug, and it’s got machined ferrules already inside. Tighten the nut finger-tight. Because the machined ferrules are already there, you don’t have to pull it up one and a quarter turns. You simply tighten that nut one quarter turn, and the job’s all done. The beauty of the plug is that you can again fully pressurize the system and test it, ship it, or do anything you need to do.
So, in review, caps cap off a piece of tubing. Plugs plug a port. Both can be fully pressurized so you can pressure test your system to the full pressure rating of your specifications.
