Swagelok Fittings - Low Pressure Applications - Leaks? 7

[tommo]

I've used a lot of Swaglok fittings over the years and really never had a problem, always using a tube cutter, and always marking the fitting at finger tight, and doing the standard 1 1/4 turn tightening procedure.

Always use an appropriate tube wall thickness to the tube diameter. e.g 0.035" for 1/4 - 3/8", 0.049" for 1/2" etc.

My boss swears that in his experience Swagelok always come loose and leak.

There are a number of posts saying about using Swagelok fittings at very high pressures with no problems. I can see that high pressure may help the fittings bite into the tube, and lock the whole thing together.


My question is this -

Has anyone noticed on low pressure < 100psi that ferrule fittings can leak over time, through tube creep or distortion, vibration, temperature cycling, time etc.?

The gasses we are using are Hydrogen, Methane, Oxygen.

I would be interested to hear of your opinions and perhaps we can settle the argument!

Thanks for any help on this.

 

[MikeHalloran]

>>>...Swagelok always come loose and leak.<<<

Not if you follow the directions, in my experience.



Mike Halloran
Pembroke Pines, FL, USA

 

[icecool]

I used Swagelok for Vacuum applications in semiconductor industry. The pressure was with in 100 torr with 1/4" - 3/8" connector. I have tested with different inert gases (He, Ar, SF6, N2). I did not found immediate leakage sound but when I checked it with helium leak detector the leak rate was high in the range of 1e-3 atm-cc/sec. Also, over the period of time the leak rate was very high even my fingers feel it.
I think that it is good enough for the Liquid but for the Gases it is not good.

I hope this will helps..

 

[pmover]

contact the Swagelok experts and consult with them.

build a test unit and conduct your own tests/analysis.

for natural gas systems upwards of 1000-psig, no known leaks. then again, the leaks that are found are usually due to faulty or incorrect installation.

good luck!
-pmover

 

[IRstuff]

We used exclusively Swageloks for all our process gas piping. We ran the usual hydrogen, oxygen, and nitrogen with no problems. The facility was in operation for more than a decade.

TTFN
faq731-376

 

[tommo]

Missed out on the critical information - the material of the tubing, we use 316 Stainless Steel tubing.

Interesting point from Icecool, about leaking under vacuum which sort of backs my point of higher pressures assisting to lock the assembly together - the tube reacting under system pressure, tries to escape the fitting.

i.e. The tube tries to blow out of the fitting and thus forces the ferrules to bite into the tubing, increasing the sealing pressures from the ferrules...

Possibly, under low pressure, you really only have the mechanical deformation of the tube to lock it into place.

On a material like 316, with a low yield strength, it may deform under stress and reduce the force exerted by the ferrule on the tube - and thus leak?

A higher pressure system may not have this problem...?

 

[tommo]

I'm not sure if I got a wrap over the knuckles for mentioning the brand name Swagelok.

This post wasn't supposed to recommend or defame Swagelok - I used their name as they are one of the biggest names in the business - like 'Band-Aid' is for sticking plasters...

So my question is open to any experiences of ferrule type tube fittings! Of which there are many brands...

My boss is a big lover of the Face Seal type fittings, but for me it's an extravagance best reserved for High Purity systems, he feels they give a more secure, leak proof and therefore safer connection...

You can't argue against safety...but I think ferrule systems are adequate for many applications.

 

[moltenmetal]

We use 2-ferrule compression fittings, made by Swagelok and by others, to sizes up to 3/4", to close to the limits of the tubing.

We use these successfully in all kinds of services, including hydrogen service at high pressures and relatively high temperatures.

There is no problem using them at low pressures. They do not leak at low pressure.

We don't use them for ultahigh vacuum or where the client gives a tight helium leak spec which is basically a code word for orbital welding and face seal fittings.

Note that sometimes it's necessary to use a wrench to get the fitting to the "hand tight" point- you need to get to the point where all the looseness is out of the system and the torque starts to rise steeply, before you start counting turns for swaging.

The most frequent causes of leakage with compression fittings in our experience are the use of poor quality or damaged tubing, tubing with inadequate wall thickness, fittings put through too many make/remake cycles, a mismatch of tubing material to fitting material (i.e. brass ferrules swaged on stainless tube), and tubing swaged too close to the end (improperly inserted into the fitting).

 

[JJPellin]

We use fittings of this type (both Swagelok and Parker) in services from 2000 psi hydrogen to 20 psi lube oil. Our largest use is probably low pressure (<15 psi) air and nitrogen. If they are correctly assembled and not abused, I have never seen any problem. I agree with moltenmetal on the pitfalls. Pressure is not necessary to lock the fitting and get it to seal. Proper tightening accomplishes that.

Johnny Pellin

 

[btrueblood]

I too have been forced to change some tubing connections from 2-ferrule swage type fittings to face-seal o-ring fittings, over concern that the swage fittings could loosen under conditions where the tube would see torsion loads. I have never seen or heard of this happening, but others apparently have. The larger thread size of the face seal fittings allow for higher preload of the threaded joint, and we also apply a locking compound. I still wonder if it is a belt and suspenders fix for a nonexistant problem.

 

[hacksaw]

in all lab and operating plant environments I've dealt with, they are fine, but some applications require flared fittings on basis of actual service history

 

[tommo]

Can hardly remember the last time I used a flared fitting?

...but you are generally restricted to the softer, malleable alloys? I think if you flared stainless steel tubing - you would just get a cracked end?

Anyway - thats the value of these forums, you never know what folks will come up with to pass on.

 

[Duwe6]

Got you beat on vacuum pulled on Swagelok; 10E-6 Torr on some 1/4" and a bunch of 3/8" tubing. No special precautions used on making the connections, just the usual:
Cut with tubing cutter with a reasonably sharp wheel.
Debur the ID of the cut
Bottom out the tubing in the fitting and tighten 1-1/4 turns past finger-tight.

IMHO the pipe specs that require flared ends are 'legacy documents' that don't recognize that something better was invented about 60 years ago. Flares are fine on copper, but Swagelok is just as good or better. Flaring steel tubing is stupid. Too much time & effort to try to equal Swagelok.

 

[StoneCold]

We use the swagelok type fittings all the time on low pressure stuff. That being said it really depends on who makes up the fitting. If you are not carefull you can get leaks. I have also had better luck with seamless tubing than with ERW tubing because it seems to be closer to perfectly round. All of my tubing is 316L 1/4" to 3/4" . I know that we use 0.035" wall for 1/2" and 0.049" wall for 3/4". I don't know the wall thickesses of the smaller tubing.

Regards
StoneCold

 

[btrueblood]

I'd have to go double check, but I think our sister company uses swage fittings for helium leak check connections, holding vacuum to the millitorr range. At least one of their tests goes down to 1x10^-7 scc/sec, though I don't know if the swage joint itself is being checked (i.e. the swage fitting may not have one side immersed in He). In that particular case, the swage fitting is an easy test connection to make, and leakage of the fitting easy to repair, so not sure if a leak would even get reported.

 

[tommo]

Just to wrap this thread up, I'll finish it.

The boss's experience was 30+ years ago, perhaps they didn't have 2-piece ferrules back then, or was using some other brand, although he is definite it was Swagelok, but like a description like Band-Aid will cover many brands..could be something like that...

Anyway, I used Swagelok in our application and it has tested fine - the boss isn't happy, but the job is finished, leak-tight, and now moving onto other things.

I think through my experience, and the contributions here clearly show - it's very important to do the joint up as per manufacturers specifications, including marking the nut and fitting, etc. etc. The manufacturers have done the work so you don't have to work out how best to do it! Amazing how many people just tighten the crap out of these fittings.

Thanks for all that contributed to my seemingly frivolous post - it was most interesting to find out some of the pressures and materials getting pumped around out there that make our lives possible!