Fit Matters: What You Need to Know about
When it comes
to connecting metal or plastic tubing, it’s hard to beat compression fittings. Known
to stand up to tough pressure and temperature conditions while handling a
diverse array of fluids, compression fittings are found in everything from
refinery gas lines to residential plumbing. You’ll even find miniature versions
living in all kinds of applications: printers, chromatography, and even medical
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
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).
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.
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
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
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.
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
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
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.
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.
Shop Our Selection of Stainless Tube Fittings Here.