Big bore air tubes

[Aerotulz]

I've  built my second big bore air rifle and was able to find 7075T6 tubing from a national supplier who supplied material certification as to material alloy. The tube I  got has .156" wall and using several of the popular strength of pressure vessel formulas, this tube should be good to over 15000 psi internal pressure. I personally tested it to 4200 psi, filled with water and pressurized with my nitrogen tank and micro hose and my end plugs with 70a buna orings and Teflon backup ring. I left if for four hours at this pressure. The test was to prove performance of my design for plugs and retention. It's not threaded.  I have since completed and shot gun with fair accuracy results. Then I thought it would be comforting to know just how strong this vessel really is. After contacting several aviation hydro test shops and two hydraulic shops and three discusions with mechanical engineers in the hi pressure hydraulic world, the consensus is that the seals will fail somewhere around 5000+ psi, way before the vessel rating. Also. The highest testing facility I talked to could only go to 10000psi, and they where not thrilled to try either!So after this search for how to destructive test this air tube in its as used configuration, I have given up on doing a destructive test of my airgun air cylinder. Talked to several big bore Mfg's and they have never tried a destructive test. Any of you engineer gurus got any ideas ?
John Hagan

[michaelthomas]

Try calling Galiso in Montrose, CO.  They have tested many reservoirs for me.  The seals will probably not fail...I have had my 6061 cylinders tested well over 10,000psi with no seal failure.  Matter of fact, they looked just like new after the test.

Mike

[Rob M]

thats great to hear, I love threads like this. I have not seen anyone using 7075 as a pcp tube , usually 2024 if they decide on aluminum at all( correction now 2 people using aluminum tubes). Sounds like you used a fairly thick wall for this application.. One idea I believe lloyd executed was a hydraulic to pneumatic booster( he made a video).. Essentially letting the ease of the hydraulic compression boost the precharged cylinder to very high pressures. I forget the ratio but i know 5:1 is to be expected ..

[MJP]

You need to measure the stretch and expansion of the tube, with and without pressure. Before and after the test.
I have test pump that goes to 800bar to test my own tubes. I would definitely advice to buy or make a suitable pump to test the pressure vessels.
Tube stretch can be measured with dial gauge just chuck the tube in to a lathe on one end and put the gauge to the other.
Can be very revealing.

Marko

[Aerotulz]

Thank you all for your imput.
  Mike, it's nice to know there are builders in the Rocky State! When you had the hydraulic shop test done, were there any different connectors required. How did that work? I would be happy to pass 10,000 psi with my tubes. What seal configuration are you using if I may ask?

  Marko, thanks for the suggestion, I did do a before and after diameter measurement and on my 25" long tube (internal length) 1-1/2" OD. I measure .0013" expansion at pressure (4200) and back to original size at ambient.  So, what do you do with those dimensions?  How do you determine what they reveal as to tube durability. Is there any info available to a layman that can interperate them? As for building my own pump I just am to busy with a full time job and a very busy home machine shop making AG parts.
Again, Thanks for the info to all!
John Hagan

[michaelthomas]

It's been so long since I made the fitting, I cannot remember if it was 1/2 npt or 3/4 npt that they needed.  They manufacture the hydro test equptment for the worldwide market...great company. 

I use an oring inboard of the threads.  The tube always breaks at the root of the first thread on mine.

The expansion of the tube is not necessarily bad...as long as the tube returns to within a small percentage of its original volume.  They monitor all that stuff and can give you the report. 

When they saw my tubes....they predicted it would not take even 2000 psi to failure....yet it was still intact after sitting at 10500 for 1/2 hour.  Point is....  Even experienced people cannot predict a vessels performance by looks or calculation.  Something to keep in mind.  Best to get it tested.

Mike