An Interesting HPA Failure

[rsterne]

I got some pics today of an imploded plenum spacer that was between a regulator and valve in a Disco.... Thanks to lorraine and Norm_m for the photos.... Here is a side view, the pellet is to keep it from rolling over....



Here is the other side after a few shots.... The velocity dropped by over 400 fps in about 3 shots as this happened....



End view, showing the imploded section is stretched all the way across the ID of the tube.... Note how a small portion of the circumference of the wall has extruded to about twice it's original width....



Another view of this puzzling failure.... It would appear that the material is about 1/32" wall aluminium tubing....




I think the initial failure was probably a small buckle failure of the tube wall from end compression, combined with a small amount of air in the space between the spacer and the reservoir wall.... Once it collapsed even slightly (the initial collapse may have only moved the wall in 1/32" or so), it had a larger volume of HPA behind it for the next shot, and imploded slightly.... That now much larger space then filled with HPA and on the next shot the much larger volume then expanded and extruded the wall inwards, stretching it into the current shape.... If you look at how much of the tube wall has stretched into the much longer length of the folded in portion, the wall must now be very thin.... One more shot would likely have ruptured it....

When I first heard about this "implosion" I thought it unlikely and suspected to see a buckle failure in the tube wall when I got the photos, but in fact the spacer was within about 1/16" of it's original length.... It would still slide out of the Disco reservoir.... I am not sure of the regulator setpoint, but if we assume it was 1500 psi, the end force on the tube spacer was about 650 lbs. when the reservoir was full, and would have increased during the shot cycle as the pressure in the plenum dropped.... There would not have been much volume of air initially between the spacer and the reservoir wall, but enough that combined with the end load it likely caused an initial buckle failure which then filled with air, and within a few shots imploded the tube spacer to produce the results shown.... It certainly shows the power of HPA, and how completely unexpected things can happen....

Bob

[TF89]

Is this spacer just that, a spacer that keeps the regulator a set distance or does it do something else to increase or contain pressure?  I must admit that I did not think that a tube inside a tube would have different pressure and forces applied to it.  Is this tube sealed against the regulator and valve? If that is the case then it makes sense, otherwise my brain is being stretched.

[sixshootertexan]

Only thing I can think of is that the pressure is even on both sides of the tube. Once a shot was fired and the pressure dropped inside the tube the air on the outside could not get around the end of the tube fast enough and therefore pushed the tube inwards.

[rsterne]

Absolutely correct, this is a simple spacer between the regulator and the valve, and there is no perfect seal between the ends of the tube and the regulator and valve, but if the ends are smooth and square, with all that end force, there would be no significant flow past the ends, but they would leak enough to fill the (initially very small) space between the outside of the tube and the inside of the reservoir with air at the same pressure as inside the spacer (2000 psi).... When the gun is fired, the pressure inside the spacer drops suddenly, and the shot is of such short duration (about 2 milliseconds) that the regulator can't replenish the air before the valve closes (exactly the reason a plenum is required).... The air outside the tube expands, pushing inwards on the tube wall.... On the first shot, there is very little trapped air, so the tube likely didn't collapse much, but whatever amount it did collapse created a larger volume for the second shot, and so on, and by the third shot this was the result....

Bob

[TF89]

Thanks for the info.  After I posted I found the beginning of the discussion in this thread.  http://www.gatewaytoairguns.org/GTA/index.php?topic=89785.20 

Messing around with HPA requires a lot of thought towards safety margins.

[rsterne]

You likely have way more hammer strike than you need for the Cothran valve, and are wasting a lot of air.... That valve design requires a much lighter hammer strike to operate efficiently, and is likely operating as a dump valve, with the pressure in the plenum dropping to near zero if the pellet is leaving the muzzle with the valve still open.... That much greater drop in pressure would explain why you never had a problem before....

I would suggest drastically reducing the hammer spring preload, plotting velocity vs. preload, and look for the "knee" in the curve.... You can likely save at least half the air per shot without losing any velocity at all....

Bob

[sixshootertexan]

Perforating the tube or going to a thicker wall should help.

[rsterne]

If adding preload increases the velocity, then you don't have too much.... you can just tune for the velocity you want and the air used will follow inversely.... However, that leaves me at a loss as to why your spacer failed, maybe it just sealed better on the ends in this tube.... Anyway, vent holes should prevent a repeat....

Bob

[Pellethuntr]

Very interesting to finally see the pics Lorraine.

I still think you should go up to .30 caliber and see what that Disco can do

[Rocko]

When I put a Robert Lane reg in my AT44, he said to be sure to drill a few holes in the spacer to equalize pressure on both sides of the tube.

[miksatx]

You likely have way more hammer strike than you need for the Cothran valve, and are wasting a lot of air.... That valve design requires a much lighter hammer strike to operate efficiently, and is likely operating as a dump valve, with the pressure in the plenum dropping to near zero if the pellet is leaving the muzzle with the valve still open.... That much greater drop in pressure would explain why you never had a problem before....

I would suggest drastically reducing the hammer spring preload, plotting velocity vs. preload, and look for the "knee" in the curve.... You can likely save at least half the air per shot without losing any velocity at all....

Bob

I use 2 o rings in front of the valve stem on my cothran valves. Also use a HS .30x1.5x.041 oe hammer and no power adjuster. Tames the valve an bit better shot count.

[rsterne]

I've never used a Cothran valve, but from the limited understanding I have, I can see that the pressure, and the duration of that pressure, in the exhaust port of the valve could have a drastic effect on how fast the valve closes.... This may lead to drastic changes in dwell not only with changes to the transfer port diameter, but also the pellet weight.... I would imagine they are a different beast to tune, for sure....

Bob