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Compression fittings consist of three core components: a compression nut, a ferrule or ferrules, and a body. Cullen Crawford of the Crawford Fitting Company introduced the first flareless fitting design in the 1940s with a two-ferrule design. Thus, the invention of the double ferrule, flareless fitting was born.
One ferrule fittings and valves have a similar make-up, supplementing one large ferrule in place of two smaller ones. Tube fittings with one ferrule are called single ferrule fittings. Comparably, fittings with two ferrules are called double ferrule fittings.
To this day, there are two options to choose from when deciding on the best ferrule design for your application. We’re going to dig further into these two options to help inform your decision.
Initially, the shoulder inside a compression nut engages with the back-end of the ferrule as it's tightened. Under this force, the angled front-end of the ferrule compresses into the body and grips the stainless steel tube. This back-end ferrule compression action protects the front of the single ferrule in an attempt to isolate tube vibration from affecting the front end tube bite.
In the double ferrule, or twin ferrule design, the front ferrule is the larger piece that sits deepest in the body. The back ferrule, is smaller and engages with the nut. The front ferrule is a softer metal and bites down between the tube and the fitting body creating the seal for the double ferrule technology. The back ferrule has a chemically hardened lead edge. This hardened portion is then able to bite down into the softer metal of the tubing, providing grip. The gripping action keeps the tubing from slipping under pressure, which could be dangerous.
Much care goes into the design of both single and double ferrule designs. Both styles are highly-engineered and designed with metal property science, called metallurgy, to provide accurate grip and seal. These actions, provided by the ferrule(s), are the most important functions when it comes to surviving pressure, temperature, and vibration. Loss of seal and tube blowout are two of the main concerns if ferrules don’t perform correctly.
Double ferrule fittings have become the preferred option over single ferrule fittings in recent years. However, there are a few advantages of each to be aware of before choosing between the two.
The two-piece, double ferrule product incorporates a better sealing technology than the single ferrule system. As discussed earlier, both pieces have their specified roles to create the ultimate sealing product. Because of this, double ferrule fittings are the more advanced product of the two. The “double seal” advantage on stainless steel tubing creates a highly-efficient seal that the single ferrule product can’t compete with.
In addition to their better design, the double ferrule system is the preferred choice for high-pressure applications. Because of their ability to provide tight seals, double ferrule designs are ideal for tasks such as sealing off high-pressure gas systems.
Because of the piece count, using one-piece compared to using two pieces is an advantage, in itself. When installing a single ferrule to the body, there's only one way a person can install it incorrectly. However, if you’re using the double ferrule method, there are more than 11 ways a person can incorrectly install the ferrules. One of these ways includes a chance that one of the two-part pieces could go missing, leaving only one piece to be installed into the body incorrectly. Unfortunately, back ferrules commonly go missing. They're also easy to insert facing the wrong direction. Without both pieces inserted correctly, double ferrule fittings will not grip or seal correctly.
Many compression fitting applications have high-vibration and pulsation factors to consider.
The single ferrule has a compressed, light back-end grip which dampens vibration by isolating the seal point on the front end of the ferrule. Vibration can then flow from the back of the ferrule and through the nut/body of the assembly with less interference on the gripping action on the front end of the ferrule. Single ferrule handles these applications well because of this specific back-end compression.
In double ferrule fittings, vibration and pulsation can affect the seals by allowing stress from these instances to concentrate at the non-isolated back ferrule grip. Subsequently, double ferrule fittings have a slight disadvantage compared to the single ferrule for these vibration and pulsation applications.
Mako Products offers both single and double ferrule tube fitting options. Our Superlok tube fittings use a patented technology that’s not available on any other tube fitting. Superlok’s built-in gap gauge technology will work with both single and double ferrule designs! Utilizing this patented technology, we guarantee Superlok tube fittings to be leak-free for life. Click here to learn more about this amazing patented solution.
If you’ve had a chance to review and choose the best ferrule product option for your tube fitting application, contact us to purchase your fittings today.
Resources
Instrumentation Tubing and Their Connections
Fluid Flow – 3 Common Causes of Compressed Air Leaks and a Single (Ferrule) Solution
Fluid Flow – How Compression Fittings Can Make or Break A Compressed Air System
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