Pressure tube Epoxy question

[Tweaker]

I'm thinking of starting some pcp airgun mods/builds involving OEM pressure tubes rated for 3000 PSI.

 My question is if you had two ADEQUATE tubes where one was a slide fit onto the other and used a high psi rated epoxy like 4500 psi tensile strength or higher to secure them with a large overlap. Would that have a good safety margin if the overlap is quite a few inches long ? and be leak resistant.

A typical pcp valve's backwards force on the tube is handled by rated screws, which while adequate doesnt seem by design overly robust either (low contact area). Im curious how the above situation would compare in terms of holding strength, thoughts/feedback appreciated.

[Goose]

I'd weld them together and do everything I could to ensure good fusion.  I just couldn't trust any form of glue when it come to that high of pressure.  Glues and plastic degrade over time.

Luck,

J~

[JuryRigger]

In a word-NO! 
There is just really no way to know that the bond is adequate for the stress; or how long it will stay that way-personally  I would stick with a method that has/is used in such types of applications, EG welding, pinning, threading (have read that there are tubes with brazed plugs/valves, cannot remember which-but it was either a Benji or Sheridan pumper, IIRC)...
Jesse

[Tweaker]

Thanks for that , I figured it wasnt too viable but had to ask since there would have been quite a few instances where that technique (if it were safe) could have made things a lot easier.

[Madd Hatter]

If you used certain adhesives that just aren't available to the public and had the right clearances in order to have the right bond line sure. But for John Q public to do that, no! Forgot to mention a sufuric/chromic acid etch with maybe a phosporic anodize also.

[Glem Chally]

Sounds like a dangerous and unnecessary experiment. 

[subscriber]

The greater the tube overlap, the higher the velocity one of the tubes would reach when the glue failed.   A one square inch ID tube containing 3000 PSI with a 3" overlap would release 750 FPE when it let go.

Stronger epoxy is harder and less flexible.  Each time tube is toped up with air, the metal tubes would swell slightly.  Would that crack the rigid glue or separate it from the metal?  A slow leak would be a nice way for it to fail...

If you can't thread the tubes together, or make one long tube; then use pins of sufficient area to carry the load with an appropriate factor of safety, to lock the overlapped joint together.  So, the epoxy is only uses as a sealant; in which case I would use a flexible epoxy.

[BC81]

The greater the tube overlap, the higher the velocity one of the tubes would reach when the glue failed.   A one square inch ID tube containing 3000 PSI with a 3" overlap would release 750 FPE when it let go.

Stronger epoxy is harder and less flexible.  Each time tube is toped up with air, the metal tubes would swell slightly.  Would that crack the rigid glue or separate it from the metal?  A slow leak would be a nice way for it to fail...

If you can't thread the tubes together, or make one long tube; then use pins of sufficient area to carry the load with an appropriate factor of safety, to lock the overlapped joint together.  So, the epoxy is only uses as a sealant; in which case I would use a flexible epoxy.

Thats what I would've recommended, or tack welds if you can't ensure an air tight weld.

I've tried to use loctite hydraulic high pressure sealant and RTV silicone on very small leaks around the gauge threads on some guns and nothing ever works. 3000 psi is ALOT of pressure.

Tig welds would be the way to go if making a custom tube, or threaded ends with o-rings.

Or adapting other tubes from other guns to your application.

[subscriber]

Performing an actual hydraulic pressure test on the assembly would be a good thing.  Of course, that is a single test, even if it is done to 3X working pressure.  What a single test fails to do is predict fatigue.  To make that moot, use materials and methods that have sufficient fatigue resistance.

Or build a mock-up of the design, then perform a destructive test:












Hydraulic testing is used because compressible gasses such as air store too much energy.  The latter turning something a little scary into something very dangerous.

[subscriber]

You could use multiple screws, as shown in the top video, above to join the two tubes.  If the inner tube wall is on the thin side, screw into a third thick walled innermost tube, to mimic retaining a valve body.  Use square countersunk holes for the cap screw heads to bear against.  Use enough high grade screws of sufficient diameter.

Learn from the way people in the know have solved similar problems; and use their methods to prove the method and materials good for your design.

Don't work on a pressurized system, that was poorly modified.  Don't be this guy:

[subscriber]

Worth reading as background for the above videos:  https://www.gatewaytoairguns.org/GTA/index.php?topic=97121.msg907672#msg907672

First of all, I'm glad that the guy wasn't killed, or injured more seriously than he was.... That could have been a fatal wound if it was over just a little bit.... WOW !!!

Now, let's analyze some of the things in the video.... There appear to have been (at least) TWO separate failures here....

1. They disassembled the back of the gun, removing the rear plug and the hammer.... The threaded end of the HiPac was still in the front of the tube.... Therefore, the Disco valve MUST have come out the back of the tube.... Why?.... See #2....

2. The tube does NOT have any side holes in it for the side screws in the Disco Valve.... Therefore, we can assume that only a single screw was used in the bottom.... They said that the gun let go when they were taking off the stock.... The stock screw sits in the slot in the Disco valve just behind the O-rings.... My guess is that the stock screw was taking part of the load, and when they undid it, all the load went on the bottom valve screw.... and BOOM, out the back of the gun it went, and through his leg....

3. I'm not a fan of HiPacs, but HOW in heaven's name do you use one with a Disco valve?.... They are designed to work with a stock 22XX valve, and seal against the valve, so that the tube is not under any pressure.... Did these guys cobble up some kind of seal between the HiPac and the Disco valve?.... or did they just use the HiPac to pressurize the tube (something it was never designed to do), and seal the Disco valve into the tube with O-rings?.... If they had a seal of some kind between the front of the Disco valve and the HiPac, and the tube wasn't under pressure.... then when the valve moved back a fraction, BANG the tube was pressurized, and that stressed the HiPac in ways it was never designed for....

4. Despite #3, how in heaven's name did the HiPac fracture at the threads, leaving the threads in the tube, and the HiPac imbedded in the ceiling?.... Is this a completely separate failure from the valve shooting out the back?....

Too many questions, and not enough answers.... However, I would say that whoever built that gun, using a Disco valve with apparently only one screw, must bear some of the blame.... If they had the valve sealed to the HiPac, the way it was intended to be, then did they have O-rings on the valve, so that it sealed to the tube?.... If they didn't, and the HiPac was only pressurizing the valve and not the tube.... then how did it develop enough pressure to shear the valve screw and fire the valve into his leg?....

Bob

[Rob M]

it would be cool to wwatch this experiement in a safe setting with proper hydro testing.. But agreed with above , its a bomb waiting to go off. hydrostatic failures with air are incredibly scary , even when you anticipate them      ..Thats why testing is done with liquids like oil and water, incompressable ...Needless to say  Ive blown up some stuff

[subscriber]

To illustrate why a liquid is used for pressure testing of gas tanks, here is a destructive test of a scuba tank using air.  The cage to contain the tank held, but was not "happy" afterwards: