Ok, I'll backtrack to say that for 1/4" and smaller, higher pressures
are likely to be fine *if* you are swaging correctly and using normal
hardness tubing. The forces are not as crazy. But still, the advantage
of "cone and thread" and AN fittings is that if they are not tightened
they just leak, rather than having the tube go shooting out of the
fitting like an un- or under-swaged tube.
The Swagelok FK medium pressure system in half inch and larger with
thicker and harder tubing is trouble waiting to happen. I've seen it
go off twice, it's very violent and loud. Fortunately in neither case
was anyone injured.
And for folks new to this, "medium pressure" and "high pressure" are
terms of art for specific fitting systems, where medium is up to
20,000 psi and high is up to 60,000. Most rocketeers will never get
into medium pressure, and the highest you'll ever see is a 6000 psi
bottle that can be worked with using normal AN fittings.
And "Swagelok" is a company and a brand, but generally people use it
specifically to mean two ferrule compression fittings in the
Swagelok/Yor-Lok/A-lok line.
On Sat, Mar 28, 2020 at 2:55 PM Michael Clive <zeinin@xxxxxxxxx> wrote:
However, to follow up with Ben:
Medium, high pressure and UHP fittings are PHENOMENAL. If you do them right
(cone-and-thread fittings), they never, ever leak. And they are very
repeatable. Not good for vibe, but for test stands, they are great! Maximator
makes the best tools for them. Peter Duffy at High Pressure Technologies is
about the best sales guy you will ever talk to, and he will sell you anything
you need for 10k+ work. Tubes, fittings, valves, anything.
I agree with Ben on not using bite type fittings EVER for above 10k work.
Cone on thread is the best!
On Fri, Mar 27, 2020 at 8:55 PM Ben Brockert <wikkit@xxxxxxxxx> wrote:
I kinda hate Swagelok fittings over 3/4". They just suck to get leak
tight. 1/4" and 1/8" swagelok fittings are great, they are perfectly
suited to the task. Work fine with LOX. Follow the assembly
instructions! Buy a preswaging die if you use them a lot. 1/4"
swagelok fittings are the easiest way to build weird assemblies like
temperature/pressure ports in pipes or coaxial heat exchangers. And
work well for things like gauge boxes, test benches, etc. They're also
called "instrumentation fittings" and that is a good description of
what they are good for.
They are not great with vibe. Well suited to ground equipment, less so
to flight.
If you use them with peroxide the nuts should be processed in acid
first. There is silver plating on the threads to act as antiseize, if
you have a peroxide leak through a silver fitting then the fitting
ends up getting hot and the leak escalates itself to asking to speak
to the manager. Once you strip the silver antiseize off, then you need
to replace it with something else on assembly, like krytox.
Any use of any swage fitting over 3000 psi is a mistake. Go straight
from AN to cone and thread. Do not believe the marketing around medium
pressure swage fittings. When a 5000 psi fitting decides to dissociate
next to you, the best case is a startling.
Ben
On Fri, Mar 27, 2020 at 5:05 PM Rick Wills <willsrw@xxxxxxxxx> wrote:
Hi
Has anyone on the list used Swagelok compression fittings in service with
LOX (3/4 or 1 in)? I’ve used AN (37 deg flare) and NPT fittings
successfully for LOX. NPT fittings are wrapped with PTFE tape. My entire
LOX side on my test stand is all AN expect for the NPT adapters used on
the main LOX valve and LOX load valve. Any LOX experience with any other
brand of compression fittings (Parker, etc). I’m looking at changing the
stand plumbing and am considering Swagelok. Check out our Facebook page
https://www.facebook.com/midwestpropulsiongroup to see the stand in action.
The Army missile spec forbids NPT fittings. Most likely reason is they
could vibrate loose. I’ve seem amateur liquid systems using NPT
fittings successfully. Any advice would be appreciated. On my first test
stand, I welded the NPT fittings together. It worked but looked pretty
bad. An A&P I know suggested safety wiring the NPTs so they would come
loose. That is a lot of work.
I’m lucky. My outbuilding is also my rocket workshop. When I want to
escape I run out back for a few hours. Thanks and everyone take care.
Be Safe,
Rick Wills
Huber Heights, Ohio