A Handy Guide to Stainless Tube Compression Fittings
How Well Do You Know: Stainless Steel Tube Fittings
But no matter the application, you’ll always find some common denominators: they last a long time, they’re easy to put together, and if you do it right, they simply don’t leak.
And speaking of the old adage that great things come in threes, you’ll find the same thing is true when it comes to the way they’re assembled. Stainless steel tubing fittings can differ a bit in design, depending on who builds them, but you can generally point to three consistent elements: a compression fitting body, a compression screw (or nut), and at least one ferrule. This is also key to how compression fitting gets its name: with the tubing attached into the fitting and the screw or nut tightened, the ferrule is moved into the fitting body, and the angled body shape compresses the end of the ferrule onto the exterior part of the tubing, creating a leak-resistant seal. How about that for connection?
A Closer Look at Ferrules
As the number one sealing component in the assembly of stainless steel tube fittings, they run the gamut of materials, from graphite to steel. The latter are preferable, thanks to a high degree of stability over a broad temperature band. They can also hold up without relaxing while under compressive loads. With metal-to-metal seals, leaks can be expected, but ferrules are designed to create an ideal connection when joined with the fitting and tubing.
Shape Is Everything
Want to ensure the reliability of your compression seal? Pay close attention to the ferrule’s shape and the fitting body’s mating angle. They’ve both got to be tapered to allow proper compression when the screw or nut is tightened. And don’t forget: axial alignment with the tubing has to be maintained at the same time. Those relative angles will determine the amount of linear motion turned into radial compression as well as the amount of surface contact you’ll get with the tubing. A line contact – or uniform compression around the entire ferrule – will deliver the best seal. Pro tip: you’ll also want to make sure your ferrule has a sharp forward edge.
By the Numbers
In most, basic compression fittings, you’ll find just one ferrule. In softer fabrications, this keeps things simple with fewer components. When you move up to a stainless steel tube fitting, you’ll find that torque is transferred to the ferrule as you tighten the screw. Over time, that rotation can make the ferrule shift, produce galling, and cause long-term leaks. Put another freely rotating rear ferrule into the mix, and you’ll see the nut decoupled from the front – meaning a prevention of torque transfer.
A Question of Symmetry
When looking at one-piece ferrules, you’ll see asymmetrical and symmetrical options. The cone-shaped asymmetrical ferrule can only go into the fitting body in one direction while symmetrical versions resemble two back-to-back cones. The latter can go into the fitting in both directions, making your assembly just a little bit easier when it comes to applications with a lot of fittings, and rapid required assembly time.
What’s not to love about symmetrical ferrules? They tend to drift off-axis, and that can cause small leaks. The problem’s amplified when you’re working with hard plastic tubing. That’s why asymmetrical ferrules, which give you the flexibility of a two-piece design, are generally the way to go with tech-forward applications.
Talking about Tubing
You’ll find compression fittings typically used with hard tubing – that wall has to be durable against all the compression coming from the ferrule. These could include stainless steel or copper and stiff plastic, such as nylon, polyethylene, or Teflon®.
Soft tubing is usually avoided due to potential tubing wall collapse or separation from the ferrule. As you might imagine, this wouldn’t be good for the ferrule’s holding power, and could also keep you from achieving a leak-proof seal. If you do end up using softer tubing, be sure to reinforce that tubing wall with inserts (some manufacturers carry these).
Do your homework when choosing your tubing, and don’t skimp on expertise. A tubing supplier can offer invaluable tech support and help you to consider factors like environment and vibration conditions, minimum bend radius, compatibility of fluid, and temp and pressure variations. And guess what? Your checklist still isn’t complete without these:
Also known as Teflon tubing, it offers a high degree of resistance to most chemicals, stands up to high pressure, is low out-gassing, and delivers better flexibility than metal. Special note: it tends to “cold flow,” causing leaks over time as the tubing wall starts to move away from the ferrule. And this problem gets worse when the tubing then hits high temperatures. You can meet this problem head-on by utilizing a redundant seal (like an o-ring) in the fitting body, creating a radial seal against outside diameter.
How it’s produced can make all the difference in the way it performs. You’ll want to use smooth tubing (no roughness that can cause a leak path) that’s been stored in coils, preventing even compression. Pro tip: an elastomeric seal can deform to match variations, but a metal-to-metal seal is less adaptable. Be sure that each section is cut squarely to allow the tubing to rest symmetrically in the body of the fitting.
A Winning Installation
We all know that instructions can vary by manufacturer and design, so be sure to follow the recommended procedures. That said, here are a few things to consider on nearly any compression fitting installation:
A Word about Torque
Usually, you tighten a compression fitting using the nut rotation instead of torque as your key metric. Those nuts are threaded, and that means your number of rotations correlates to the ferrule’s linear compression (based on the pitch of the thread). With torque, you can be all over the place, depending on factors like fitting and material, lubrication, and degree of galling.
Not too Tight
Sometimes when it comes to fitting assembly, it’s easy to assume that “tighter is righter.” Not so with compression fittings. You get that optimal seal with a line contact between the tubing and ferrule. Yes, you can insufficiently tighten the compression nut, and it won’t deform the ferrule enough to create this contact. But too much tightening will deform the ferrule too much, eventually weakening the seal and causing leaks.
Take Apart. Put Back Together.
Compression fittings are popular because they’re easy to assembly and then disassemble. You just loosen the compression nut or screw, and you’re on your way. Likewise, re-assembly is just like the first time (don’t lose those instructions!), albeit with fewer nut turns to get to your optimal position. Pro tip: compression fittings can only handle a few dis-assemblies before the ferrules or fitting body require replacing.
Just Say No to Component Buffets
Even though two components from different makers might look the same, they probably differ in some key areas, from thread size and pitch to ferrule length. Mixing and matching between different manufacturers can often lead to poor results down the road.
Your Application. Your Options.
There are a lot of reasons to choose a specific compression fitting. One application might require completely different capabilities than another. Here are some application-specific considerations to look at:
Feeling the Pressure
Your ideal choice for high pressure jobs: compression fittings. Used in concert with high pressure tubing, these can meet the needs of applications requiring performance exceeding pressures above 10,000 psig, such as hydrogen fuel cells and aerospace.
In industries from medical to semiconductor to instrumentation, your application needs inert, contamination-free components. That’s where compression fittings really shine. With stainless steel or an inert plastic tubing, engineers can avoid contaminants that occur as a result of high out-gassing tubing materials. They also have the ability to autoclave, clean, and sterilize metal compression nuts and ferrules more easily.
When working with high purity applications, keep an eye out for galling, which can happen between the threads or between the ferrule and body. It happens most often when fittings have been deep cleaned (losing residual oils), and can first show up as small notching or scraping. Be sure to use ferrules specifically designed to resist galling, and don’t forget to employ a volatile lubricant (like isopropyl alcohol) during assembly.
So many industries. So many applications across almost any fluid power design. That’s the power of compression fittings. Thanks for taking the time to brush up on your expertise with us, and we hope you’ll make us a regular stop on your internet journeys.
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