Double vs. Single Ferrule Fittings

Did you know that there are far more than 50 brands of tube fittings of various designs, dimensions, and performance on the market today? When choosing the right tube fitting, it is critical that you understand the advantages and shortfalls of various fitting designs.

Tube fittings generally come in two varieties; single or double ferrule. Tube fitting ferrules serve multiple purposes, two of which include creating a seal and holding the tubing in place (aka “grip”). Single ferrule fittings use a lone ferrule to create a seal and grip the tubing. Double ferrule fitting split these functions between two separate ferrules. The front ferrule is typically designed to provide a seal between the ferrule, tubing, and fitting body, while the rear ferrules grips the tubing and holds the front ferrule in the proper position. The gripping action of the rear ferrules prevents tubing “blow-out” when the system is pressurized.

The development of the single ferrule tube fitting provided many advantages over other connections of the time. Its primary advantage was that it required no special tools for installation. Unlike threaded pipe, a tube fitting could be quickly and easily installed using only a couple of standard wrenches.

Two ferrule fittings share the “no special tools required” advantage of their single ferrule counterparts, while providing several other important benefits. The specialization of the front and rear ferrules resulted in better sealing performance and greater tube grip than single ferrule designs. This meant that a two ferrule fitting was capable of higher pressures and better media containment than a fitting with a single ferrule.

It is important to note that the number of ferrules in a fitting is just one of many tube fitting design variables. From the brass plumbing fitting at the local hardware store, to a high performance fitting capable of containing 20,000psig, there are a lot of choices. When making a selection, it is important to be aware how a fitting’s ferrule design will impact its performance. At a minimum, consider the following:

• Is the fitting compatible with the system media?
• Will the fitting provide an adequate seal and keep the system media where it’s intended to be?
• Will the fitting provide the tube grip necessary to contain system pressure and pressure and temperature fluctuations?
• How well will the fitting perform when faced with system vibration?

Want to learn more? Our Tube Fitting Essentials training course provides an in-depth look at the advantages and disadvantages of various tube fitting designs and manufacturing processes, as well as a number of other connection types commonly encountered in today’s fluid systems.

Questions on choosing the right fitting for your application? Email or call 866.901.0151.